Merge pull request #1461 from MoonchildProductions/ubbo

Back out unboxed array/object removals

83 files changed, 8482 insertions, 803 deletions

diff --git a/js/src/builtin/ModuleObject.cpp b/js/src/builtin/ModuleObject.cpp
index 575bab0b0..b275cb968 100644
--- a/js/src/builtin/ModuleObject.cpp
+++ b/js/src/builtin/ModuleObject.cpp
@@ -927,7 +927,7 @@ ModuleObject::evaluate(JSContext* cx, HandleModuleObject self, MutableHandleValu
 ModuleObject::createNamespace(JSContext* cx, HandleModuleObject self, HandleObject exports)
 {
     MOZ_ASSERT(!self->namespace_());
-    MOZ_ASSERT(exports->is<ArrayObject>());
+    MOZ_ASSERT(exports->is<ArrayObject>() || exports->is<UnboxedArrayObject>());
 
     RootedModuleNamespaceObject ns(cx, ModuleNamespaceObject::create(cx, self));
     if (!ns)
diff --git a/js/src/frontend/BytecodeEmitter.cpp b/js/src/frontend/BytecodeEmitter.cpp
index f4574b248..8cd06feac 100644
--- a/js/src/frontend/BytecodeEmitter.cpp
+++ b/js/src/frontend/BytecodeEmitter.cpp
@@ -6495,8 +6495,8 @@ ParseNode::getConstantValue(ExclusiveContext* cx, AllowConstantObjects allowObje
         }
         MOZ_ASSERT(idx == count);
 
-        ArrayObject* obj = ObjectGroup::newArrayObject(cx, values.begin(), values.length(),
-                                                       newKind, arrayKind);
+        JSObject* obj = ObjectGroup::newArrayObject(cx, values.begin(), values.length(),
+                                                    newKind, arrayKind);
         if (!obj)
             return false;
 
@@ -9623,7 +9623,7 @@ BytecodeEmitter::emitCallOrNew(ParseNode* pn, ValueUsage valueUsage /* = ValueUs
                 return false;
         }
 
-        if (!emitArray(args, argc))
+        if (!emitArray(args, argc, JSOP_SPREADCALLARRAY))
             return false;
 
         if (optCodeEmitted) {
@@ -10138,11 +10138,11 @@ BytecodeEmitter::emitArrayLiteral(ParseNode* pn)
         }
     }
 
-    return emitArray(pn->pn_head, pn->pn_count);
+    return emitArray(pn->pn_head, pn->pn_count, JSOP_NEWARRAY);
 }
 
 bool
-BytecodeEmitter::emitArray(ParseNode* pn, uint32_t count)
+BytecodeEmitter::emitArray(ParseNode* pn, uint32_t count, JSOp op)
 {
 
     /*
@@ -10153,6 +10153,7 @@ BytecodeEmitter::emitArray(ParseNode* pn, uint32_t count)
      * to avoid dup'ing and popping the array as each element is added, as
      * JSOP_SETELEM/JSOP_SETPROP would do.
      */
+    MOZ_ASSERT(op == JSOP_NEWARRAY || op == JSOP_SPREADCALLARRAY);
 
     uint32_t nspread = 0;
     for (ParseNode* elt = pn; elt; elt = elt->pn_next) {
@@ -10173,7 +10174,7 @@ BytecodeEmitter::emitArray(ParseNode* pn, uint32_t count)
 
     // For arrays with spread, this is a very pessimistic allocation, the
     // minimum possible final size.
-    if (!emitUint32Operand(JSOP_NEWARRAY, count - nspread))         // ARRAY
+    if (!emitUint32Operand(op, count - nspread))                    // ARRAY
         return false;
 
     ParseNode* pn2 = pn;
@@ -11314,8 +11315,8 @@ BytecodeEmitter::setSrcNoteOffset(unsigned index, unsigned which, ptrdiff_t offs
         /* Maybe this offset was already set to a four-byte value. */
         if (!(*sn & SN_4BYTE_OFFSET_FLAG)) {
             /* Insert three dummy bytes that will be overwritten shortly. */
-            if (MOZ_UNLIKELY(notes.length() + 3 > MaxSrcNotesLength)) {

-                ReportAllocationOverflow(cx);

+            if (MOZ_UNLIKELY(notes.length() + 3 > MaxSrcNotesLength)) {
+                ReportAllocationOverflow(cx);
                 return false;
             }
             jssrcnote dummy = 0;
diff --git a/js/src/frontend/BytecodeEmitter.h b/js/src/frontend/BytecodeEmitter.h
index 8ad409c11..77f77599c 100644
--- a/js/src/frontend/BytecodeEmitter.h
+++ b/js/src/frontend/BytecodeEmitter.h
@@ -543,7 +543,7 @@ struct MOZ_STACK_CLASS BytecodeEmitter
     MOZ_MUST_USE bool emitAtomOp(ParseNode* pn, JSOp op);
 
     MOZ_MUST_USE bool emitArrayLiteral(ParseNode* pn);
-    MOZ_MUST_USE bool emitArray(ParseNode* pn, uint32_t count);
+    MOZ_MUST_USE bool emitArray(ParseNode* pn, uint32_t count, JSOp op);
     MOZ_MUST_USE bool emitArrayComp(ParseNode* pn);
 
     MOZ_MUST_USE bool emitInternedScopeOp(uint32_t index, JSOp op);
diff --git a/js/src/gc/Marking.cpp b/js/src/gc/Marking.cpp
index 262fc8cbc..43e325394 100644
--- a/js/src/gc/Marking.cpp
+++ b/js/src/gc/Marking.cpp
@@ -18,7 +18,6 @@
 #include "builtin/ModuleObject.h"
 #include "gc/GCInternals.h"
 #include "gc/Policy.h"
-#include "gc/StoreBuffer-inl.h"
 #include "jit/IonCode.h"
 #include "js/SliceBudget.h"
 #include "vm/ArgumentsObject.h"
@@ -29,6 +28,7 @@
 #include "vm/Shape.h"
 #include "vm/Symbol.h"
 #include "vm/TypedArrayObject.h"
+#include "vm/UnboxedObject.h"
 #include "wasm/WasmJS.h"
 
 #include "jscompartmentinlines.h"
@@ -37,6 +37,7 @@
 
 #include "gc/Nursery-inl.h"
 #include "vm/String-inl.h"
+#include "vm/UnboxedObject-inl.h"
 
 using namespace js;
 using namespace js::gc;
@@ -1394,6 +1395,14 @@ js::ObjectGroup::traceChildren(JSTracer* trc)
     if (maybePreliminaryObjects())
         maybePreliminaryObjects()->trace(trc);
 
+    if (maybeUnboxedLayout())
+        unboxedLayout().trace(trc);
+
+    if (ObjectGroup* unboxedGroup = maybeOriginalUnboxedGroup()) {
+        TraceManuallyBarrieredEdge(trc, &unboxedGroup, "group_original_unboxed_group");
+        setOriginalUnboxedGroup(unboxedGroup);
+    }
+
     if (JSObject* descr = maybeTypeDescr()) {
         TraceManuallyBarrieredEdge(trc, &descr, "group_type_descr");
         setTypeDescr(&descr->as<TypeDescr>());
@@ -1427,6 +1436,12 @@ js::GCMarker::lazilyMarkChildren(ObjectGroup* group)
     if (group->maybePreliminaryObjects())
         group->maybePreliminaryObjects()->trace(this);
 
+    if (group->maybeUnboxedLayout())
+        group->unboxedLayout().trace(this);
+
+    if (ObjectGroup* unboxedGroup = group->maybeOriginalUnboxedGroup())
+        traverseEdge(group, unboxedGroup);
+
     if (TypeDescr* descr = group->maybeTypeDescr())
         traverseEdge(group, static_cast<JSObject*>(descr));
 
@@ -1469,6 +1484,23 @@ CallTraceHook(Functor f, JSTracer* trc, JSObject* obj, CheckGeneration check, Ar
         return nullptr;
     }
 
+    if (clasp == &UnboxedPlainObject::class_) {
+        JSObject** pexpando = obj->as<UnboxedPlainObject>().addressOfExpando();
+        if (*pexpando)
+            f(pexpando, mozilla::Forward<Args>(args)...);
+
+        UnboxedPlainObject& unboxed = obj->as<UnboxedPlainObject>();
+        const UnboxedLayout& layout = check == CheckGeneration::DoChecks
+                                      ? unboxed.layout()
+                                      : unboxed.layoutDontCheckGeneration();
+        if (layout.traceList()) {
+            VisitTraceList(f, layout.traceList(), unboxed.data(),
+                           mozilla::Forward<Args>(args)...);
+        }
+
+        return nullptr;
+    }
+
     clasp->doTrace(trc, obj);
 
     if (!clasp->isNative())
@@ -2261,6 +2293,18 @@ static inline void
 TraceWholeCell(TenuringTracer& mover, JSObject* object)
 {
     mover.traceObject(object);
+
+    // Additionally trace the expando object attached to any unboxed plain
+    // objects. Baseline and Ion can write properties to the expando while
+    // only adding a post barrier to the owning unboxed object. Note that
+    // it isn't possible for a nursery unboxed object to have a tenured
+    // expando, so that adding a post barrier on the original object will
+    // capture any tenured->nursery edges in the expando as well.
+
+    if (object->is<UnboxedPlainObject>()) {
+        if (UnboxedExpandoObject* expando = object->as<UnboxedPlainObject>().maybeExpando())
+            expando->traceChildren(&mover);
+    }
 }
 
 static inline void
@@ -2504,6 +2548,8 @@ js::TenuringTracer::moveObjectToTenured(JSObject* dst, JSObject* src, AllocKind
         InlineTypedObject::objectMovedDuringMinorGC(this, dst, src);
     } else if (src->is<TypedArrayObject>()) {
         tenuredSize += TypedArrayObject::objectMovedDuringMinorGC(this, dst, src, dstKind);
+    } else if (src->is<UnboxedArrayObject>()) {
+        tenuredSize += UnboxedArrayObject::objectMovedDuringMinorGC(this, dst, src, dstKind);
     } else if (src->is<ArgumentsObject>()) {
         tenuredSize += ArgumentsObject::objectMovedDuringMinorGC(this, dst, src);
     } else if (src->is<ProxyObject>()) {
diff --git a/js/src/gc/Tracer.cpp b/js/src/gc/Tracer.cpp
index 3416464dd..63cd9b08a 100644
--- a/js/src/gc/Tracer.cpp
+++ b/js/src/gc/Tracer.cpp
@@ -201,15 +201,80 @@ gc::TraceCycleCollectorChildren(JS::CallbackTracer* trc, Shape* shape)
     } while (shape);
 }
 
+// Object groups can point to other object groups via an UnboxedLayout or the
+// the original unboxed group link. There can potentially be deep or cyclic
+// chains of such groups to trace through without going through a thing that
+// participates in cycle collection. These need to be handled iteratively to
+// avoid blowing the stack when running the cycle collector's callback tracer.
+struct ObjectGroupCycleCollectorTracer : public JS::CallbackTracer
+{
+    explicit ObjectGroupCycleCollectorTracer(JS::CallbackTracer* innerTracer)
+        : JS::CallbackTracer(innerTracer->runtime(), DoNotTraceWeakMaps),
+          innerTracer(innerTracer)
+    {}
+
+    void onChild(const JS::GCCellPtr& thing) override;
+
+    JS::CallbackTracer* innerTracer;
+    Vector<ObjectGroup*, 4, SystemAllocPolicy> seen, worklist;
+};
+
+void
+ObjectGroupCycleCollectorTracer::onChild(const JS::GCCellPtr& thing)
+{
+    if (thing.is<BaseShape>()) {
+        // The CC does not care about BaseShapes, and no additional GC things
+        // will be reached by following this edge.
+        return;
+    }
+
+    if (thing.is<JSObject>() || thing.is<JSScript>()) {
+        // Invoke the inner cycle collector callback on this child. It will not
+        // recurse back into TraceChildren.
+        innerTracer->onChild(thing);
+        return;
+    }
+
+    if (thing.is<ObjectGroup>()) {
+        // If this group is required to be in an ObjectGroup chain, trace it
+        // via the provided worklist rather than continuing to recurse.
+        ObjectGroup& group = thing.as<ObjectGroup>();
+        if (group.maybeUnboxedLayout()) {
+            for (size_t i = 0; i < seen.length(); i++) {
+                if (seen[i] == &group)
+                    return;
+            }
+            if (seen.append(&group) && worklist.append(&group)) {
+                return;
+            } else {
+                // If append fails, keep tracing normally. The worst that will
+                // happen is we end up overrecursing.
+            }
+        }
+    }
+
+    TraceChildren(this, thing.asCell(), thing.kind());
+}
+
 void
 gc::TraceCycleCollectorChildren(JS::CallbackTracer* trc, ObjectGroup* group)
 {
     MOZ_ASSERT(trc->isCallbackTracer());
 
-    group->traceChildren(trc);
-}
+    // Early return if this group is not required to be in an ObjectGroup chain.
+    if (!group->maybeUnboxedLayout())
+        return group->traceChildren(trc);
+
+    ObjectGroupCycleCollectorTracer groupTracer(trc->asCallbackTracer());
+    group->traceChildren(&groupTracer);
 
+    while (!groupTracer.worklist.empty()) {
+        ObjectGroup* innerGroup = groupTracer.worklist.popCopy();
+        innerGroup->traceChildren(&groupTracer);
+    }
+}
 
+
 /*** Traced Edge Printer *************************************************************************/
 
 static size_t
diff --git a/js/src/jit-test/tests/basic/unboxed-object-clear-new-script.js b/js/src/jit-test/tests/basic/unboxed-object-clear-new-script.js
new file mode 100644
index 000000000..f55456222
--- /dev/null
+++ b/js/src/jit-test/tests/basic/unboxed-object-clear-new-script.js
@@ -0,0 +1,49 @@
+
+function Foo(a, b) {
+    this.a = a;
+    this.b = b;
+}
+
+function invalidate_foo() {
+    var a = [];
+    var counter = 0;
+    for (var i = 0; i < 50; i++)
+        a.push(new Foo(i, i + 1));
+    Object.defineProperty(Foo.prototype, "a", {configurable: true, set: function() { counter++; }});
+    for (var i = 0; i < 50; i++)
+        a.push(new Foo(i, i + 1));
+    delete Foo.prototype.a;
+    var total = 0;
+    for (var i = 0; i < a.length; i++) {
+        assertEq('a' in a[i], i < 50);
+        total += a[i].b;
+    }
+    assertEq(total, 2550);
+    assertEq(counter, 50);
+}
+invalidate_foo();
+
+function Bar(a, b, fn) {
+    this.a = a;
+    if (b == 30)
+        Object.defineProperty(Bar.prototype, "b", {configurable: true, set: fn});
+    this.b = b;
+}
+
+function invalidate_bar() {
+    var a = [];
+    var counter = 0;
+    function fn() { counter++; }
+    for (var i = 0; i < 50; i++)
+        a.push(new Bar(i, i + 1, fn));
+    delete Bar.prototype.b;
+    var total = 0;
+    for (var i = 0; i < a.length; i++) {
+        assertEq('a' in a[i], true);
+        assertEq('b' in a[i], i < 29);
+        total += a[i].a;
+    }
+    assertEq(total, 1225);
+    assertEq(counter, 21);
+}
+invalidate_bar();
diff --git a/js/src/jit-test/tests/basic/unboxed-object-convert-to-native.js b/js/src/jit-test/tests/basic/unboxed-object-convert-to-native.js
new file mode 100644
index 000000000..691fe166c
--- /dev/null
+++ b/js/src/jit-test/tests/basic/unboxed-object-convert-to-native.js
@@ -0,0 +1,47 @@
+
+// Test various ways of converting an unboxed object to native.
+
+function Foo(a, b) {
+    this.a = a;
+    this.b = b;
+}
+
+var proxyObj = {
+  get: function(recipient, name) {
+    return recipient[name] + 2;
+  }
+};
+
+function f() {
+    var a = [];
+    for (var i = 0; i < 50; i++)
+        a.push(new Foo(i, i + 1));
+
+    var prop = "a";
+
+    i = 0;
+    for (; i < 5; i++)
+        a[i].c = i;
+    for (; i < 10; i++)
+        Object.defineProperty(a[i], 'c', {value: i});
+    for (; i < 15; i++)
+        a[i] = new Proxy(a[i], proxyObj);
+    for (; i < 20; i++)
+        a[i].a = 3.5;
+    for (; i < 25; i++)
+        delete a[i].b;
+    for (; i < 30; i++)
+        a[prop] = 4;
+
+    var total = 0;
+    for (i = 0; i < a.length; i++) {
+        if ('a' in a[i])
+            total += a[i].a;
+        if ('b' in a[i])
+            total += a[i].b;
+        if ('c' in a[i])
+            total += a[i].c;
+    }
+    assertEq(total, 2382.5);
+}
+f();
diff --git a/js/src/jit-test/tests/basic/unboxed-object-getelem.js b/js/src/jit-test/tests/basic/unboxed-object-getelem.js
new file mode 100644
index 000000000..b30b8325a
--- /dev/null
+++ b/js/src/jit-test/tests/basic/unboxed-object-getelem.js
@@ -0,0 +1,20 @@
+
+function f() {
+    var propNames = ["a","b","c","d","e","f","g","h","i","j","x","y"];
+    var arr = [];
+    for (var i=0; i<64; i++)
+	arr.push({x:1, y:2});
+    for (var i=0; i<64; i++) {
+        // Make sure there are expandos with dynamic slots for each object.
+        for (var j = 0; j < propNames.length; j++)
+            arr[i][propNames[j]] = j;
+    }
+    var res = 0;
+    for (var i=0; i<100000; i++) {
+	var o = arr[i % 64];
+	var p = propNames[i % propNames.length];
+	res += o[p];
+    }
+    assertEq(res, 549984);
+}
+f();
diff --git a/js/src/jit-test/tests/basic/unboxed-object-set-property.js b/js/src/jit-test/tests/basic/unboxed-object-set-property.js
new file mode 100644
index 000000000..fdcfcf6b2
--- /dev/null
+++ b/js/src/jit-test/tests/basic/unboxed-object-set-property.js
@@ -0,0 +1,31 @@
+
+// Use the correct receiver when non-native objects are prototypes of other objects.
+
+function Thing(a, b) {
+    this.a = a;
+    this.b = b;
+}
+
+function foo() {
+    var array = [];
+    for (var i = 0; i < 10000; i++)
+        array.push(new Thing(i, i + 1));
+
+    var proto = new Thing(1, 2);
+    var obj = Object.create(proto);
+
+    Object.defineProperty(Thing.prototype, "c", {set:function() { this.d = 0; }});
+    obj.c = 3;
+    assertEq(obj.c, undefined);
+    assertEq(obj.d, 0);
+    assertEq(obj.hasOwnProperty("d"), true);
+    assertEq(proto.d, undefined);
+    assertEq(proto.hasOwnProperty("d"), false);
+
+    obj.a = 3;
+    assertEq(obj.a, 3);
+    assertEq(proto.a, 1);
+    assertEq(obj.hasOwnProperty("a"), true);
+}
+
+foo();
diff --git a/js/src/jit-test/tests/basic/unboxed-property-enumeration.js b/js/src/jit-test/tests/basic/unboxed-property-enumeration.js
new file mode 100644
index 000000000..142d932dd
--- /dev/null
+++ b/js/src/jit-test/tests/basic/unboxed-property-enumeration.js
@@ -0,0 +1,24 @@
+function O() {
+    this.x = 1;
+    this.y = 2;
+}
+function testUnboxed() {
+    var arr = [];
+    for (var i=0; i<100; i++)
+	arr.push(new O);
+
+    var o = arr[arr.length-1];
+    o[0] = 0;
+    o[2] = 2;
+    var sym = Symbol();
+    o[sym] = 1;
+    o.z = 3;
+    Object.defineProperty(o, '3', {value:1,enumerable:false,configurable:false,writable:false});
+    o[4] = 4;
+
+    var props = Reflect.ownKeys(o);
+    assertEq(props[props.length-1], sym);
+
+    assertEq(Object.getOwnPropertyNames(o).join(""), "0234xyz");
+}
+testUnboxed();
diff --git a/js/src/jit-test/tests/ion/unboxed-objects-invalidate.js b/js/src/jit-test/tests/ion/unboxed-objects-invalidate.js
new file mode 100644
index 000000000..02e27614f
--- /dev/null
+++ b/js/src/jit-test/tests/ion/unboxed-objects-invalidate.js
@@ -0,0 +1,16 @@
+
+var a = [];
+for (var i = 0; i < 2000; i++)
+    a.push({f:i});
+
+function f() {
+    var total = 0;
+    for (var i = 0; i < a.length; i++)
+        total += a[i].f;
+    return total;
+}
+assertEq(f(), 1999000);
+
+var sub = Object.create(a[0]);
+
+assertEq(f(), 1999000);
diff --git a/js/src/jit/AliasAnalysisShared.cpp b/js/src/jit/AliasAnalysisShared.cpp
index 400626b33..99c23d2a3 100644
--- a/js/src/jit/AliasAnalysisShared.cpp
+++ b/js/src/jit/AliasAnalysisShared.cpp
@@ -91,6 +91,10 @@ GetObject(const MDefinition* ins)
       case MDefinition::Op_Elements:
       case MDefinition::Op_MaybeCopyElementsForWrite:
       case MDefinition::Op_MaybeToDoubleElement:
+      case MDefinition::Op_UnboxedArrayLength:
+      case MDefinition::Op_UnboxedArrayInitializedLength:
+      case MDefinition::Op_IncrementUnboxedArrayInitializedLength:
+      case MDefinition::Op_SetUnboxedArrayInitializedLength:
       case MDefinition::Op_TypedArrayLength:
       case MDefinition::Op_SetTypedObjectOffset:
       case MDefinition::Op_SetDisjointTypedElements:
@@ -108,6 +112,8 @@ GetObject(const MDefinition* ins)
       case MDefinition::Op_GuardObjectGroup:
       case MDefinition::Op_GuardObjectIdentity:
       case MDefinition::Op_GuardClass:
+      case MDefinition::Op_GuardUnboxedExpando:
+      case MDefinition::Op_LoadUnboxedExpando:
       case MDefinition::Op_LoadSlot:
       case MDefinition::Op_StoreSlot:
       case MDefinition::Op_InArray:
diff --git a/js/src/jit/BaselineCacheIR.cpp b/js/src/jit/BaselineCacheIR.cpp
index 67c80473b..bf96932d1 100644
--- a/js/src/jit/BaselineCacheIR.cpp
+++ b/js/src/jit/BaselineCacheIR.cpp
@@ -16,7 +16,7 @@ using namespace js;
 using namespace js::jit;
 
 // OperandLocation represents the location of an OperandId. The operand is
-// either in a register or on the stack.
+// either in a register or on the stack, and is either boxed or unboxed.
 class OperandLocation
 {
   public:
@@ -787,6 +787,9 @@ BaselineCacheIRCompiler::emitGuardClass()
       case GuardClassKind::Array:
         clasp = &ArrayObject::class_;
         break;
+      case GuardClassKind::UnboxedArray:
+        clasp = &UnboxedArrayObject::class_;
+        break;
       case GuardClassKind::MappedArguments:
         clasp = &MappedArgumentsObject::class_;
         break;
@@ -815,6 +818,36 @@ BaselineCacheIRCompiler::emitGuardSpecificObject()
 }
 
 bool
+BaselineCacheIRCompiler::emitGuardNoUnboxedExpando()
+{
+    Register obj = allocator.useRegister(masm, reader.objOperandId());
+
+    FailurePath* failure;
+    if (!addFailurePath(&failure))
+        return false;
+
+    Address expandoAddr(obj, UnboxedPlainObject::offsetOfExpando());
+    masm.branchPtr(Assembler::NotEqual, expandoAddr, ImmWord(0), failure->label());
+    return true;
+}
+
+bool
+BaselineCacheIRCompiler::emitGuardAndLoadUnboxedExpando()
+{
+    Register obj = allocator.useRegister(masm, reader.objOperandId());
+    Register output = allocator.defineRegister(masm, reader.objOperandId());
+
+    FailurePath* failure;
+    if (!addFailurePath(&failure))
+        return false;
+
+    Address expandoAddr(obj, UnboxedPlainObject::offsetOfExpando());
+    masm.loadPtr(expandoAddr, output);
+    masm.branchTestPtr(Assembler::Zero, output, output, failure->label());
+    return true;
+}
+
+bool
 BaselineCacheIRCompiler::emitLoadFixedSlotResult()
 {
     Register obj = allocator.useRegister(masm, reader.objOperandId());
@@ -841,6 +874,26 @@ BaselineCacheIRCompiler::emitLoadDynamicSlotResult()
 }
 
 bool
+BaselineCacheIRCompiler::emitLoadUnboxedPropertyResult()
+{
+    Register obj = allocator.useRegister(masm, reader.objOperandId());
+    AutoScratchRegister scratch(allocator, masm);
+
+    JSValueType fieldType = reader.valueType();
+
+    Address fieldOffset(stubAddress(reader.stubOffset()));
+    masm.load32(fieldOffset, scratch);
+    masm.loadUnboxedProperty(BaseIndex(obj, scratch, TimesOne), fieldType, R0);
+
+    if (fieldType == JSVAL_TYPE_OBJECT)
+        emitEnterTypeMonitorIC();
+    else
+        emitReturnFromIC();
+
+    return true;
+}
+
+bool
 BaselineCacheIRCompiler::emitGuardNoDetachedTypedObjects()
 {
     FailurePath* failure;
@@ -951,6 +1004,19 @@ BaselineCacheIRCompiler::emitLoadInt32ArrayLengthResult()
 }
 
 bool
+BaselineCacheIRCompiler::emitLoadUnboxedArrayLengthResult()
+{
+    Register obj = allocator.useRegister(masm, reader.objOperandId());
+    masm.load32(Address(obj, UnboxedArrayObject::offsetOfLength()), R0.scratchReg());
+    masm.tagValue(JSVAL_TYPE_INT32, R0.scratchReg(), R0);
+
+    // The int32 type was monitored when attaching the stub, so we can
+    // just return.
+    emitReturnFromIC();
+    return true;
+}
+
+bool
 BaselineCacheIRCompiler::emitLoadArgumentsObjectLengthResult()
 {
     Register obj = allocator.useRegister(masm, reader.objOperandId());
diff --git a/js/src/jit/BaselineCompiler.cpp b/js/src/jit/BaselineCompiler.cpp
index d254b9826..ae5a2e666 100644
--- a/js/src/jit/BaselineCompiler.cpp
+++ b/js/src/jit/BaselineCompiler.cpp
@@ -2050,7 +2050,13 @@ BaselineCompiler::emit_JSOP_NEWARRAY()
     return true;
 }
 
-typedef ArrayObject* (*NewArrayCopyOnWriteFn)(JSContext*, HandleArrayObject, gc::InitialHeap);
+bool
+BaselineCompiler::emit_JSOP_SPREADCALLARRAY()
+{
+    return emit_JSOP_NEWARRAY();
+}
+
+typedef JSObject* (*NewArrayCopyOnWriteFn)(JSContext*, HandleArrayObject, gc::InitialHeap);
 const VMFunction jit::NewArrayCopyOnWriteInfo =
     FunctionInfo<NewArrayCopyOnWriteFn>(js::NewDenseCopyOnWriteArray, "NewDenseCopyOnWriteArray");
 
@@ -4181,14 +4187,14 @@ BaselineCompiler::emit_JSOP_REST()
 {
     frame.syncStack(0);
 
-    ArrayObject* templateObject =
+    JSObject* templateObject =
         ObjectGroup::newArrayObject(cx, nullptr, 0, TenuredObject,
                                     ObjectGroup::NewArrayKind::UnknownIndex);
     if (!templateObject)
         return false;
 
     // Call IC.
-    ICRest_Fallback::Compiler compiler(cx, templateObject);
+    ICRest_Fallback::Compiler compiler(cx, &templateObject->as<ArrayObject>());
     if (!emitOpIC(compiler.getStub(&stubSpace_)))
         return false;
 
diff --git a/js/src/jit/BaselineCompiler.h b/js/src/jit/BaselineCompiler.h
index a200f7ab9..7b1af092a 100644
--- a/js/src/jit/BaselineCompiler.h
+++ b/js/src/jit/BaselineCompiler.h
@@ -100,6 +100,7 @@ namespace jit {
     _(JSOP_BITNOT)             \
     _(JSOP_NEG)                \
     _(JSOP_NEWARRAY)           \
+    _(JSOP_SPREADCALLARRAY)    \
     _(JSOP_NEWARRAY_COPYONWRITE) \
     _(JSOP_INITELEM_ARRAY)     \
     _(JSOP_NEWOBJECT)          \
diff --git a/js/src/jit/BaselineIC.cpp b/js/src/jit/BaselineIC.cpp
index e65f10aac..9c8cd9835 100644
--- a/js/src/jit/BaselineIC.cpp
+++ b/js/src/jit/BaselineIC.cpp
@@ -44,8 +44,8 @@
 #include "jit/shared/Lowering-shared-inl.h"
 #include "vm/EnvironmentObject-inl.h"
 #include "vm/Interpreter-inl.h"
-#include "vm/NativeObject-inl.h"
 #include "vm/StringObject-inl.h"
+#include "vm/UnboxedObject-inl.h"
 
 using mozilla::DebugOnly;
 
@@ -741,6 +741,11 @@ LastPropertyForSetProp(JSObject* obj)
     if (obj->isNative())
         return obj->as<NativeObject>().lastProperty();
 
+    if (obj->is<UnboxedPlainObject>()) {
+        UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando();
+        return expando ? expando->lastProperty() : nullptr;
+    }
+
     return nullptr;
 }
 
@@ -1157,6 +1162,56 @@ TryAttachNativeOrUnboxedGetValueElemStub(JSContext* cx, HandleScript script, jsb
 
     ICStub* monitorStub = stub->fallbackMonitorStub()->firstMonitorStub();
 
+    if (obj->is<UnboxedPlainObject>() && holder == obj) {
+        const UnboxedLayout::Property* property = obj->as<UnboxedPlainObject>().layout().lookup(id);
+
+        // Once unboxed objects support symbol-keys, we need to change the following accordingly
+        MOZ_ASSERT_IF(!keyVal.isString(), !property);
+
+        if (property) {
+            if (!cx->runtime()->jitSupportsFloatingPoint)
+                return true;
+
+            RootedPropertyName name(cx, JSID_TO_ATOM(id)->asPropertyName());
+            ICGetElemNativeCompiler<PropertyName*> compiler(cx, ICStub::GetElem_UnboxedPropertyName,
+                                                            monitorStub, obj, holder,
+                                                            name,
+                                                            ICGetElemNativeStub::UnboxedProperty,
+                                                            needsAtomize, property->offset +
+                                                            UnboxedPlainObject::offsetOfData(),
+                                                            property->type);
+            ICStub* newStub = compiler.getStub(compiler.getStubSpace(script));
+            if (!newStub)
+                return false;
+
+            stub->addNewStub(newStub);
+            *attached = true;
+            return true;
+        }
+
+        Shape* shape = obj->as<UnboxedPlainObject>().maybeExpando()->lookup(cx, id);
+        if (!shape->hasDefaultGetter() || !shape->hasSlot())
+            return true;
+
+        bool isFixedSlot;
+        uint32_t offset;
+        GetFixedOrDynamicSlotOffset(shape, &isFixedSlot, &offset);
+
+        ICGetElemNativeStub::AccessType acctype =
+            isFixedSlot ? ICGetElemNativeStub::FixedSlot
+                        : ICGetElemNativeStub::DynamicSlot;
+        ICGetElemNativeCompiler<T> compiler(cx, getGetElemStubKind<T>(ICStub::GetElem_NativeSlotName),
+                                            monitorStub, obj, holder, key,
+                                            acctype, needsAtomize, offset);
+        ICStub* newStub = compiler.getStub(compiler.getStubSpace(script));
+        if (!newStub)
+            return false;
+
+        stub->addNewStub(newStub);
+        *attached = true;
+        return true;
+    }
+
     if (!holder->isNative())
         return true;
 
@@ -1320,7 +1375,7 @@ IsNativeDenseElementAccess(HandleObject obj, HandleValue key)
 static bool
 IsNativeOrUnboxedDenseElementAccess(HandleObject obj, HandleValue key)
 {
-    if (!obj->isNative())
+    if (!obj->isNative() && !obj->is<UnboxedArrayObject>())
         return false;
     if (key.isInt32() && key.toInt32() >= 0 && !obj->is<TypedArrayObject>())
         return true;
@@ -1404,7 +1459,7 @@ TryAttachGetElemStub(JSContext* cx, JSScript* script, jsbytecode* pc, ICGetElem_
     }
 
     // Check for NativeObject[id] and UnboxedPlainObject[id] shape-optimizable accesses.
-    if (obj->isNative()) {
+    if (obj->isNative() || obj->is<UnboxedPlainObject>()) {
         RootedScript rootedScript(cx, script);
         if (rhs.isString()) {
             if (!TryAttachNativeOrUnboxedGetValueElemStub<PropertyName*>(cx, rootedScript, pc, stub,
@@ -1424,6 +1479,20 @@ TryAttachGetElemStub(JSContext* cx, JSScript* script, jsbytecode* pc, ICGetElem_
         script = rootedScript;
     }
 
+    // Check for UnboxedArray[int] accesses.
+    if (obj->is<UnboxedArrayObject>() && rhs.isInt32() && rhs.toInt32() >= 0) {
+        JitSpew(JitSpew_BaselineIC, "  Generating GetElem(UnboxedArray[Int32]) stub");
+        ICGetElem_UnboxedArray::Compiler compiler(cx, stub->fallbackMonitorStub()->firstMonitorStub(),
+                                                  obj->group());
+        ICStub* unboxedStub = compiler.getStub(compiler.getStubSpace(script));
+        if (!unboxedStub)
+            return false;
+
+        stub->addNewStub(unboxedStub);
+        *attached = true;
+        return true;
+    }
+
     // Check for TypedArray[int] => Number and TypedObject[int] => Number accesses.
     if ((obj->is<TypedArrayObject>() || IsPrimitiveArrayTypedObject(obj)) &&
         rhs.isNumber() &&
@@ -1816,6 +1885,14 @@ ICGetElemNativeCompiler<T>::generateStubCode(MacroAssembler& masm)
     Register holderReg;
     if (obj_ == holder_) {
         holderReg = objReg;
+
+        if (obj_->is<UnboxedPlainObject>() && acctype_ != ICGetElemNativeStub::UnboxedProperty) {
+            // The property will be loaded off the unboxed expando.
+            masm.push(R1.scratchReg());
+            popR1 = true;
+            holderReg = R1.scratchReg();
+            masm.loadPtr(Address(objReg, UnboxedPlainObject::offsetOfExpando()), holderReg);
+        }
     } else {
         // Shape guard holder.
         if (regs.empty()) {
@@ -1866,6 +1943,13 @@ ICGetElemNativeCompiler<T>::generateStubCode(MacroAssembler& masm)
         if (popR1)
             masm.addToStackPtr(ImmWord(sizeof(size_t)));
 
+    } else if (acctype_ == ICGetElemNativeStub::UnboxedProperty) {
+        masm.load32(Address(ICStubReg, ICGetElemNativeSlotStub<T>::offsetOfOffset()),
+                    scratchReg);
+        masm.loadUnboxedProperty(BaseIndex(objReg, scratchReg, TimesOne), unboxedType_,
+                                 TypedOrValueRegister(R0));
+        if (popR1)
+            masm.addToStackPtr(ImmWord(sizeof(size_t)));
     } else {
         MOZ_ASSERT(acctype_ == ICGetElemNativeStub::NativeGetter ||
                    acctype_ == ICGetElemNativeStub::ScriptedGetter);
@@ -2015,6 +2099,56 @@ ICGetElem_Dense::Compiler::generateStubCode(MacroAssembler& masm)
 }
 
 //
+// GetElem_UnboxedArray
+//
+
+bool
+ICGetElem_UnboxedArray::Compiler::generateStubCode(MacroAssembler& masm)
+{
+    MOZ_ASSERT(engine_ == Engine::Baseline);
+
+    Label failure;
+    masm.branchTestObject(Assembler::NotEqual, R0, &failure);
+    masm.branchTestInt32(Assembler::NotEqual, R1, &failure);
+
+    AllocatableGeneralRegisterSet regs(availableGeneralRegs(2));
+    Register scratchReg = regs.takeAny();
+
+    // Unbox R0 and group guard.
+    Register obj = masm.extractObject(R0, ExtractTemp0);
+    masm.loadPtr(Address(ICStubReg, ICGetElem_UnboxedArray::offsetOfGroup()), scratchReg);
+    masm.branchTestObjGroup(Assembler::NotEqual, obj, scratchReg, &failure);
+
+    // Unbox key.
+    Register key = masm.extractInt32(R1, ExtractTemp1);
+
+    // Bounds check.
+    masm.load32(Address(obj, UnboxedArrayObject::offsetOfCapacityIndexAndInitializedLength()),
+                scratchReg);
+    masm.and32(Imm32(UnboxedArrayObject::InitializedLengthMask), scratchReg);
+    masm.branch32(Assembler::BelowOrEqual, scratchReg, key, &failure);
+
+    // Load obj->elements.
+    masm.loadPtr(Address(obj, UnboxedArrayObject::offsetOfElements()), scratchReg);
+
+    // Load value.
+    size_t width = UnboxedTypeSize(elementType_);
+    BaseIndex addr(scratchReg, key, ScaleFromElemWidth(width));
+    masm.loadUnboxedProperty(addr, elementType_, R0);
+
+    // Only monitor the result if its type might change.
+    if (elementType_ == JSVAL_TYPE_OBJECT)
+        EmitEnterTypeMonitorIC(masm);
+    else
+        EmitReturnFromIC(masm);
+
+    // Failure case - jump to next stub
+    masm.bind(&failure);
+    EmitStubGuardFailure(masm);
+    return true;
+}
+
+//
 // GetElem_TypedArray
 //
 
@@ -2318,8 +2452,8 @@ CanOptimizeDenseOrUnboxedArraySetElem(JSObject* obj, uint32_t index,
                                       Shape* oldShape, uint32_t oldCapacity, uint32_t oldInitLength,
                                       bool* isAddingCaseOut, size_t* protoDepthOut)
 {
-    uint32_t initLength = obj->as<NativeObject>().getDenseInitializedLength();
-    uint32_t capacity = obj->as<NativeObject>().getDenseCapacity();
+    uint32_t initLength = GetAnyBoxedOrUnboxedInitializedLength(obj);
+    uint32_t capacity = GetAnyBoxedOrUnboxedCapacity(obj);
 
     *isAddingCaseOut = false;
     *protoDepthOut = 0;
@@ -2328,6 +2462,10 @@ CanOptimizeDenseOrUnboxedArraySetElem(JSObject* obj, uint32_t index,
     if (initLength < oldInitLength || capacity < oldCapacity)
         return false;
 
+    // Unboxed arrays need to be able to emit floating point code.
+    if (obj->is<UnboxedArrayObject>() && !obj->runtimeFromMainThread()->jitSupportsFloatingPoint)
+        return false;
+
     Shape* shape = obj->maybeShape();
 
     // Cannot optimize if the shape changed.
@@ -2409,8 +2547,8 @@ DoSetElemFallback(JSContext* cx, BaselineFrame* frame, ICSetElem_Fallback* stub_
     uint32_t oldCapacity = 0;
     uint32_t oldInitLength = 0;
     if (index.isInt32() && index.toInt32() >= 0) {
-        oldCapacity = obj->as<NativeObject>().getDenseCapacity();
-        oldInitLength = obj->as<NativeObject>().getDenseInitializedLength();
+        oldCapacity = GetAnyBoxedOrUnboxedCapacity(obj);
+        oldInitLength = GetAnyBoxedOrUnboxedInitializedLength(obj);
     }
 
     if (op == JSOP_INITELEM || op == JSOP_INITHIDDENELEM) {
@@ -2618,6 +2756,18 @@ BaselineScript::noteArrayWriteHole(uint32_t pcOffset)
 // SetElem_DenseOrUnboxedArray
 //
 
+template <typename T>
+void
+EmitUnboxedPreBarrierForBaseline(MacroAssembler &masm, T address, JSValueType type)
+{
+    if (type == JSVAL_TYPE_OBJECT)
+        EmitPreBarrier(masm, address, MIRType::Object);
+    else if (type == JSVAL_TYPE_STRING)
+        EmitPreBarrier(masm, address, MIRType::String);
+    else
+        MOZ_ASSERT(!UnboxedTypeNeedsPreBarrier(type));
+}
+
 bool
 ICSetElem_DenseOrUnboxedArray::Compiler::generateStubCode(MacroAssembler& masm)
 {
@@ -2736,6 +2886,29 @@ ICSetElem_DenseOrUnboxedArray::Compiler::generateStubCode(MacroAssembler& masm)
         masm.loadValue(valueAddr, tmpVal);
         EmitPreBarrier(masm, element, MIRType::Value);
         masm.storeValue(tmpVal, element);
+    } else {
+        // Set element on an unboxed array.
+
+        // Bounds check.
+        Address initLength(obj, UnboxedArrayObject::offsetOfCapacityIndexAndInitializedLength());
+        masm.load32(initLength, scratchReg);
+        masm.and32(Imm32(UnboxedArrayObject::InitializedLengthMask), scratchReg);
+        masm.branch32(Assembler::BelowOrEqual, scratchReg, key, &failure);
+
+        // Load obj->elements.
+        masm.loadPtr(Address(obj, UnboxedArrayObject::offsetOfElements()), scratchReg);
+
+        // Compute the address being written to.
+        BaseIndex address(scratchReg, key, ScaleFromElemWidth(UnboxedTypeSize(unboxedType_)));
+
+        EmitUnboxedPreBarrierForBaseline(masm, address, unboxedType_);
+
+        Address valueAddr(masm.getStackPointer(), ICStackValueOffset + sizeof(Value));
+        masm.Push(R0);
+        masm.loadValue(valueAddr, R0);
+        masm.storeUnboxedProperty(address, unboxedType_,
+                                  ConstantOrRegister(TypedOrValueRegister(R0)), &failurePopR0);
+        masm.Pop(R0);
     }
 
     EmitReturnFromIC(masm);
@@ -2929,6 +3102,40 @@ ICSetElemDenseOrUnboxedArrayAddCompiler::generateStubCode(MacroAssembler& masm)
         BaseIndex element(scratchReg, key, TimesEight);
         masm.loadValue(valueAddr, tmpVal);
         masm.storeValue(tmpVal, element);
+    } else {
+        // Adding element to an unboxed array.
+
+        // Bounds check (key == initLength)
+        Address initLengthAddr(obj, UnboxedArrayObject::offsetOfCapacityIndexAndInitializedLength());
+        masm.load32(initLengthAddr, scratchReg);
+        masm.and32(Imm32(UnboxedArrayObject::InitializedLengthMask), scratchReg);
+        masm.branch32(Assembler::NotEqual, scratchReg, key, &failure);
+
+        // Capacity check.
+        masm.checkUnboxedArrayCapacity(obj, RegisterOrInt32Constant(key), scratchReg, &failure);
+
+        // Load obj->elements.
+        masm.loadPtr(Address(obj, UnboxedArrayObject::offsetOfElements()), scratchReg);
+
+        // Write the value first, since this can fail. No need for pre-barrier
+        // since we're not overwriting an old value.
+        masm.Push(R0);
+        Address valueAddr(masm.getStackPointer(), ICStackValueOffset + sizeof(Value));
+        masm.loadValue(valueAddr, R0);
+        BaseIndex address(scratchReg, key, ScaleFromElemWidth(UnboxedTypeSize(unboxedType_)));
+        masm.storeUnboxedProperty(address, unboxedType_,
+                                  ConstantOrRegister(TypedOrValueRegister(R0)), &failurePopR0);
+        masm.Pop(R0);
+
+        // Increment initialized length.
+        masm.add32(Imm32(1), initLengthAddr);
+
+        // If length is now <= key, increment length.
+        Address lengthAddr(obj, UnboxedArrayObject::offsetOfLength());
+        Label skipIncrementLength;
+        masm.branch32(Assembler::Above, lengthAddr, key, &skipIncrementLength);
+        masm.add32(Imm32(1), lengthAddr);
+        masm.bind(&skipIncrementLength);
     }
 
     EmitReturnFromIC(masm);
@@ -4061,7 +4268,18 @@ TryAttachSetValuePropStub(JSContext* cx, HandleScript script, jsbytecode* pc, IC
         return true;
 
     if (!obj->isNative()) {
-        return true;
+        if (obj->is<UnboxedPlainObject>()) {
+            UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando();
+            if (expando) {
+                shape = expando->lookup(cx, name);
+                if (!shape)
+                    return true;
+            } else {
+                return true;
+            }
+        } else {
+            return true;
+        }
     }
 
     size_t chainDepth;
@@ -4209,6 +4427,40 @@ TryAttachSetAccessorPropStub(JSContext* cx, HandleScript script, jsbytecode* pc,
 }
 
 static bool
+TryAttachUnboxedSetPropStub(JSContext* cx, HandleScript script,
+                            ICSetProp_Fallback* stub, HandleId id,
+                            HandleObject obj, HandleValue rhs, bool* attached)
+{
+    MOZ_ASSERT(!*attached);
+
+    if (!cx->runtime()->jitSupportsFloatingPoint)
+        return true;
+
+    if (!obj->is<UnboxedPlainObject>())
+        return true;
+
+    const UnboxedLayout::Property* property = obj->as<UnboxedPlainObject>().layout().lookup(id);
+    if (!property)
+        return true;
+
+    ICSetProp_Unboxed::Compiler compiler(cx, obj->group(),
+                                         property->offset + UnboxedPlainObject::offsetOfData(),
+                                         property->type);
+    ICUpdatedStub* newStub = compiler.getStub(compiler.getStubSpace(script));
+    if (!newStub)
+        return false;
+    if (compiler.needsUpdateStubs() && !newStub->addUpdateStubForValue(cx, script, obj, id, rhs))
+        return false;
+
+    stub->addNewStub(newStub);
+
+    StripPreliminaryObjectStubs(cx, stub);
+
+    *attached = true;
+    return true;
+}
+
+static bool
 TryAttachTypedObjectSetPropStub(JSContext* cx, HandleScript script,
                                 ICSetProp_Fallback* stub, HandleId id,
                                 HandleObject obj, HandleValue rhs, bool* attached)
@@ -4291,6 +4543,12 @@ DoSetPropFallback(JSContext* cx, BaselineFrame* frame, ICSetProp_Fallback* stub_
         return false;
     RootedReceiverGuard oldGuard(cx, ReceiverGuard(obj));
 
+    if (obj->is<UnboxedPlainObject>()) {
+        MOZ_ASSERT(!oldShape);
+        if (UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando())
+            oldShape = expando->lastProperty();
+    }
+
     bool attached = false;
     // There are some reasons we can fail to attach a stub that are temporary.
     // We want to avoid calling noteUnoptimizableAccess() if the reason we
@@ -4363,6 +4621,15 @@ DoSetPropFallback(JSContext* cx, BaselineFrame* frame, ICSetProp_Fallback* stub_
 
     if (!attached &&
         lhs.isObject() &&
+        !TryAttachUnboxedSetPropStub(cx, script, stub, id, obj, rhs, &attached))
+    {
+        return false;
+    }
+    if (attached)
+        return true;
+
+    if (!attached &&
+        lhs.isObject() &&
         !TryAttachTypedObjectSetPropStub(cx, script, stub, id, obj, rhs, &attached))
     {
         return false;
@@ -4445,7 +4712,20 @@ GuardGroupAndShapeMaybeUnboxedExpando(MacroAssembler& masm, JSObject* obj,
 
     // Guard against shape or expando shape.
     masm.loadPtr(Address(ICStubReg, offsetOfShape), scratch);
-    masm.branchTestObjShape(Assembler::NotEqual, object, scratch, failure);
+    if (obj->is<UnboxedPlainObject>()) {
+        Address expandoAddress(object, UnboxedPlainObject::offsetOfExpando());
+        masm.branchPtr(Assembler::Equal, expandoAddress, ImmWord(0), failure);
+        Label done;
+        masm.push(object);
+        masm.loadPtr(expandoAddress, object);
+        masm.branchTestObjShape(Assembler::Equal, object, scratch, &done);
+        masm.pop(object);
+        masm.jump(failure);
+        masm.bind(&done);
+        masm.pop(object);
+    } else {
+        masm.branchTestObjShape(Assembler::NotEqual, object, scratch, failure);
+    }
 }
 
 bool
@@ -4484,7 +4764,13 @@ ICSetProp_Native::Compiler::generateStubCode(MacroAssembler& masm)
     regs.takeUnchecked(objReg);
 
     Register holderReg;
-    if (isFixedSlot_) {
+    if (obj_->is<UnboxedPlainObject>()) {
+        // We are loading off the expando object, so use that for the holder.
+        holderReg = regs.takeAny();
+        masm.loadPtr(Address(objReg, UnboxedPlainObject::offsetOfExpando()), holderReg);
+        if (!isFixedSlot_)
+            masm.loadPtr(Address(holderReg, NativeObject::offsetOfSlots()), holderReg);
+    } else if (isFixedSlot_) {
         holderReg = objReg;
     } else {
         holderReg = regs.takeAny();
@@ -4621,17 +4907,31 @@ ICSetPropNativeAddCompiler::generateStubCode(MacroAssembler& masm)
     regs.add(R0);
     regs.takeUnchecked(objReg);
 
-    // Write the object's new shape.
-    Address shapeAddr(objReg, ShapedObject::offsetOfShape());
-    EmitPreBarrier(masm, shapeAddr, MIRType::Shape);
-    masm.loadPtr(Address(ICStubReg, ICSetProp_NativeAdd::offsetOfNewShape()), scratch);
-    masm.storePtr(scratch, shapeAddr);
+    if (obj_->is<UnboxedPlainObject>()) {
+        holderReg = regs.takeAny();
+        masm.loadPtr(Address(objReg, UnboxedPlainObject::offsetOfExpando()), holderReg);
+
+        // Write the expando object's new shape.
+        Address shapeAddr(holderReg, ShapedObject::offsetOfShape());
+        EmitPreBarrier(masm, shapeAddr, MIRType::Shape);
+        masm.loadPtr(Address(ICStubReg, ICSetProp_NativeAdd::offsetOfNewShape()), scratch);
+        masm.storePtr(scratch, shapeAddr);
 
-    if (isFixedSlot_) {
-        holderReg = objReg;
+        if (!isFixedSlot_)
+            masm.loadPtr(Address(holderReg, NativeObject::offsetOfSlots()), holderReg);
     } else {
-        holderReg = regs.takeAny();
-        masm.loadPtr(Address(objReg, NativeObject::offsetOfSlots()), holderReg);
+        // Write the object's new shape.
+        Address shapeAddr(objReg, ShapedObject::offsetOfShape());
+        EmitPreBarrier(masm, shapeAddr, MIRType::Shape);
+        masm.loadPtr(Address(ICStubReg, ICSetProp_NativeAdd::offsetOfNewShape()), scratch);
+        masm.storePtr(scratch, shapeAddr);
+
+        if (isFixedSlot_) {
+            holderReg = objReg;
+        } else {
+            holderReg = regs.takeAny();
+            masm.loadPtr(Address(objReg, NativeObject::offsetOfSlots()), holderReg);
+        }
     }
 
     // Perform the store.  No write barrier required since this is a new
@@ -4663,6 +4963,70 @@ ICSetPropNativeAddCompiler::generateStubCode(MacroAssembler& masm)
 }
 
 bool
+ICSetProp_Unboxed::Compiler::generateStubCode(MacroAssembler& masm)
+{
+    MOZ_ASSERT(engine_ == Engine::Baseline);
+
+    Label failure;
+
+    // Guard input is an object.
+    masm.branchTestObject(Assembler::NotEqual, R0, &failure);
+
+    AllocatableGeneralRegisterSet regs(availableGeneralRegs(2));
+    Register scratch = regs.takeAny();
+
+    // Unbox and group guard.
+    Register object = masm.extractObject(R0, ExtractTemp0);
+    masm.loadPtr(Address(ICStubReg, ICSetProp_Unboxed::offsetOfGroup()), scratch);
+    masm.branchPtr(Assembler::NotEqual, Address(object, JSObject::offsetOfGroup()), scratch,
+                   &failure);
+
+    if (needsUpdateStubs()) {
+        // Stow both R0 and R1 (object and value).
+        EmitStowICValues(masm, 2);
+
+        // Move RHS into R0 for TypeUpdate check.
+        masm.moveValue(R1, R0);
+
+        // Call the type update stub.
+        if (!callTypeUpdateIC(masm, sizeof(Value)))
+            return false;
+
+        // Unstow R0 and R1 (object and key)
+        EmitUnstowICValues(masm, 2);
+
+        // The TypeUpdate IC may have smashed object. Rederive it.
+        masm.unboxObject(R0, object);
+
+        // Trigger post barriers here on the values being written. Fields which
+        // objects can be written to also need update stubs.
+        LiveGeneralRegisterSet saveRegs;
+        saveRegs.add(R0);
+        saveRegs.add(R1);
+        saveRegs.addUnchecked(object);
+        saveRegs.add(ICStubReg);
+        emitPostWriteBarrierSlot(masm, object, R1, scratch, saveRegs);
+    }
+
+    // Compute the address being written to.
+    masm.load32(Address(ICStubReg, ICSetProp_Unboxed::offsetOfFieldOffset()), scratch);
+    BaseIndex address(object, scratch, TimesOne);
+
+    EmitUnboxedPreBarrierForBaseline(masm, address, fieldType_);
+    masm.storeUnboxedProperty(address, fieldType_,
+                              ConstantOrRegister(TypedOrValueRegister(R1)), &failure);
+
+    // The RHS has to be in R0.
+    masm.moveValue(R1, R0);
+
+    EmitReturnFromIC(masm);
+
+    masm.bind(&failure);
+    EmitStubGuardFailure(masm);
+    return true;
+}
+
+bool
 ICSetProp_TypedObject::Compiler::generateStubCode(MacroAssembler& masm)
 {
     MOZ_ASSERT(engine_ == Engine::Baseline);
@@ -5135,6 +5499,13 @@ GetTemplateObjectForSimd(JSContext* cx, JSFunction* target, MutableHandleObject
     return true;
 }
 
+static void
+EnsureArrayGroupAnalyzed(JSContext* cx, JSObject* obj)
+{
+    if (PreliminaryObjectArrayWithTemplate* objects = obj->group()->maybePreliminaryObjects())
+        objects->maybeAnalyze(cx, obj->group(), /* forceAnalyze = */ true);
+}
+
 static bool
 GetTemplateObjectForNative(JSContext* cx, HandleFunction target, const CallArgs& args,
                            MutableHandleObject res, bool* skipAttach)
@@ -5166,7 +5537,10 @@ GetTemplateObjectForNative(JSContext* cx, HandleFunction target, const CallArgs&
             // With this and other array templates, analyze the group so that
             // we don't end up with a template whose structure might change later.
             res.set(NewFullyAllocatedArrayForCallingAllocationSite(cx, count, TenuredObject));
-            return !!res;
+            if (!res)
+                return false;
+            EnsureArrayGroupAnalyzed(cx, res);
+            return true;
         }
     }
 
@@ -5191,7 +5565,10 @@ GetTemplateObjectForNative(JSContext* cx, HandleFunction target, const CallArgs&
                     return true;
                 }
                 res.set(NewFullyAllocatedArrayTryReuseGroup(cx, obj, 0, TenuredObject));
-                return !!res;
+                if (!res)
+                    return false;
+                EnsureArrayGroupAnalyzed(cx, res);
+                return true;
             }
         }
     }
@@ -5208,7 +5585,10 @@ GetTemplateObjectForNative(JSContext* cx, HandleFunction target, const CallArgs&
         }
 
         res.set(NewFullyAllocatedArrayForCallingAllocationSite(cx, 0, TenuredObject));
-        return !!res;
+        if (!res)
+            return false;
+        EnsureArrayGroupAnalyzed(cx, res);
+        return true;
     }
 
     if (native == StringConstructor) {
@@ -5421,7 +5801,7 @@ TryAttachCallStub(JSContext* cx, ICCall_Fallback* stub, HandleScript script, jsb
             if (!thisObject)
                 return false;
 
-            if (thisObject->is<PlainObject>())
+            if (thisObject->is<PlainObject>() || thisObject->is<UnboxedPlainObject>())
                 templateObject = thisObject;
         }
 
@@ -5521,24 +5901,15 @@ TryAttachCallStub(JSContext* cx, ICCall_Fallback* stub, HandleScript script, jsb
 }
 
 static bool
-CopyArray(JSContext* cx, HandleArrayObject arr, MutableHandleValue result)
+CopyArray(JSContext* cx, HandleObject obj, MutableHandleValue result)
 {
-    uint32_t length = arr->length();
-    ArrayObject* nobj = NewFullyAllocatedArrayTryReuseGroup(cx, arr, length, TenuredObject);
+    uint32_t length = GetAnyBoxedOrUnboxedArrayLength(obj);
+    JSObject* nobj = NewFullyAllocatedArrayTryReuseGroup(cx, obj, length, TenuredObject);
     if (!nobj)
         return false;
-    
-    MOZ_ASSERT(arr->isNative());
-    MOZ_ASSERT(nobj->isNative());
-    MOZ_ASSERT(nobj->as<NativeObject>().getDenseInitializedLength() == 0);
-    MOZ_ASSERT(arr->as<NativeObject>().getDenseInitializedLength() >= length);
-    MOZ_ASSERT(nobj->as<NativeObject>().getDenseCapacity() >= length);
-
-    nobj->as<NativeObject>().setDenseInitializedLength(length);
-
-    const Value* vp = arr->as<NativeObject>().getDenseElements();
-    nobj->as<NativeObject>().initDenseElements(0, vp, length);
-    
+    EnsureArrayGroupAnalyzed(cx, nobj);
+    CopyAnyBoxedOrUnboxedDenseElements(cx, nobj, obj, 0, 0, length);
+
     result.setObject(*nobj);
     return true;
 }
@@ -5569,22 +5940,26 @@ TryAttachStringSplit(JSContext* cx, ICCall_Fallback* stub, HandleScript script,
     RootedValue arr(cx);
 
     // Copy the array before storing in stub.
-    if (!CopyArray(cx, obj.as<ArrayObject>(), &arr))
+    if (!CopyArray(cx, obj, &arr))
         return false;
 
     // Atomize all elements of the array.
-    RootedArrayObject arrObj(cx, &arr.toObject().as<ArrayObject>());
-    uint32_t initLength = arrObj->length();
+    RootedObject arrObj(cx, &arr.toObject());
+    uint32_t initLength = GetAnyBoxedOrUnboxedArrayLength(arrObj);
     for (uint32_t i = 0; i < initLength; i++) {
-        JSAtom* str = js::AtomizeString(cx, arrObj->getDenseElement(i).toString());
+        JSAtom* str = js::AtomizeString(cx, GetAnyBoxedOrUnboxedDenseElement(arrObj, i).toString());
         if (!str)
             return false;
 
-        arrObj->setDenseElementWithType(cx, i, StringValue(str));
+        if (!SetAnyBoxedOrUnboxedDenseElement(cx, arrObj, i, StringValue(str))) {
+            // The value could not be stored to an unboxed dense element.
+            return true;
+        }
     }
 
     ICCall_StringSplit::Compiler compiler(cx, stub->fallbackMonitorStub()->firstMonitorStub(),
-                                          script->pcToOffset(pc), str, sep, arrObj);
+                                          script->pcToOffset(pc), str, sep,
+                                          arr);
     ICStub* newStub = compiler.getStub(compiler.getStubSpace(script));
     if (!newStub)
         return false;
@@ -6472,7 +6847,7 @@ ICCallScriptedCompiler::generateStubCode(MacroAssembler& masm)
     return true;
 }
 
-typedef bool (*CopyArrayFn)(JSContext*, HandleArrayObject, MutableHandleValue);
+typedef bool (*CopyArrayFn)(JSContext*, HandleObject, MutableHandleValue);
 static const VMFunction CopyArrayInfo = FunctionInfo<CopyArrayFn>(CopyArray, "CopyArray");
 
 bool
@@ -7949,6 +8324,19 @@ ICGetElem_Dense::Clone(JSContext* cx, ICStubSpace* space, ICStub* firstMonitorSt
     return New<ICGetElem_Dense>(cx, space, other.jitCode(), firstMonitorStub, other.shape_);
 }
 
+ICGetElem_UnboxedArray::ICGetElem_UnboxedArray(JitCode* stubCode, ICStub* firstMonitorStub,
+                                               ObjectGroup *group)
+  : ICMonitoredStub(GetElem_UnboxedArray, stubCode, firstMonitorStub),
+    group_(group)
+{ }
+
+/* static */ ICGetElem_UnboxedArray*
+ICGetElem_UnboxedArray::Clone(JSContext* cx, ICStubSpace* space, ICStub* firstMonitorStub,
+                              ICGetElem_UnboxedArray& other)
+{
+    return New<ICGetElem_UnboxedArray>(cx, space, other.jitCode(), firstMonitorStub, other.group_);
+}
+
 ICGetElem_TypedArray::ICGetElem_TypedArray(JitCode* stubCode, Shape* shape, Scalar::Type type)
   : ICStub(GetElem_TypedArray, stubCode),
     shape_(shape)
@@ -8324,8 +8712,8 @@ static bool DoRestFallback(JSContext* cx, BaselineFrame* frame, ICRest_Fallback*
     unsigned numRest = numActuals > numFormals ? numActuals - numFormals : 0;
     Value* rest = frame->argv() + numFormals;
 
-    ArrayObject* obj = ObjectGroup::newArrayObject(cx, rest, numRest, GenericObject,
-                                                   ObjectGroup::NewArrayKind::UnknownIndex);
+    JSObject* obj = ObjectGroup::newArrayObject(cx, rest, numRest, GenericObject,
+                                                ObjectGroup::NewArrayKind::UnknownIndex);
     if (!obj)
         return false;
     res.setObject(*obj);
diff --git a/js/src/jit/BaselineIC.h b/js/src/jit/BaselineIC.h
index e1ad12559..5600f816a 100644
--- a/js/src/jit/BaselineIC.h
+++ b/js/src/jit/BaselineIC.h
@@ -22,6 +22,7 @@
 #include "jit/SharedICRegisters.h"
 #include "js/GCVector.h"
 #include "vm/ArrayObject.h"
+#include "vm/UnboxedObject.h"
 
 namespace js {
 namespace jit {
@@ -891,6 +892,54 @@ class ICGetElem_Dense : public ICMonitoredStub
     };
 };
 
+class ICGetElem_UnboxedArray : public ICMonitoredStub
+{
+    friend class ICStubSpace;
+
+    GCPtrObjectGroup group_;
+
+    ICGetElem_UnboxedArray(JitCode* stubCode, ICStub* firstMonitorStub, ObjectGroup* group);
+
+  public:
+    static ICGetElem_UnboxedArray* Clone(JSContext* cx, ICStubSpace* space,
+                                         ICStub* firstMonitorStub, ICGetElem_UnboxedArray& other);
+
+    static size_t offsetOfGroup() {
+        return offsetof(ICGetElem_UnboxedArray, group_);
+    }
+
+    GCPtrObjectGroup& group() {
+        return group_;
+    }
+
+    class Compiler : public ICStubCompiler {
+      ICStub* firstMonitorStub_;
+      RootedObjectGroup group_;
+      JSValueType elementType_;
+
+      protected:
+        MOZ_MUST_USE bool generateStubCode(MacroAssembler& masm);
+
+        virtual int32_t getKey() const {
+            return static_cast<int32_t>(engine_) |
+                  (static_cast<int32_t>(kind) << 1) |
+                  (static_cast<int32_t>(elementType_) << 17);
+        }
+
+      public:
+        Compiler(JSContext* cx, ICStub* firstMonitorStub, ObjectGroup* group)
+          : ICStubCompiler(cx, ICStub::GetElem_UnboxedArray, Engine::Baseline),
+            firstMonitorStub_(firstMonitorStub),
+            group_(cx, group),
+            elementType_(group->unboxedLayoutDontCheckGeneration().elementType())
+        {}
+
+        ICStub* getStub(ICStubSpace* space) {
+            return newStub<ICGetElem_UnboxedArray>(space, getStubCode(), firstMonitorStub_, group_);
+        }
+    };
+};
+
 // Accesses scalar elements of a typed array or typed object.
 class ICGetElem_TypedArray : public ICStub
 {
@@ -1066,7 +1115,9 @@ class ICSetElem_DenseOrUnboxedArray : public ICUpdatedStub
           : ICStubCompiler(cx, ICStub::SetElem_DenseOrUnboxedArray, Engine::Baseline),
             shape_(cx, shape),
             group_(cx, group),
-            unboxedType_(JSVAL_TYPE_MAGIC)
+            unboxedType_(shape
+                         ? JSVAL_TYPE_MAGIC
+                         : group->unboxedLayoutDontCheckGeneration().elementType())
         {}
 
         ICUpdatedStub* getStub(ICStubSpace* space) {
@@ -1174,7 +1225,9 @@ class ICSetElemDenseOrUnboxedArrayAddCompiler : public ICStubCompiler {
         : ICStubCompiler(cx, ICStub::SetElem_DenseOrUnboxedArrayAdd, Engine::Baseline),
           obj_(cx, obj),
           protoChainDepth_(protoChainDepth),
-          unboxedType_(JSVAL_TYPE_MAGIC)
+          unboxedType_(obj->is<UnboxedArrayObject>()
+                       ? obj->as<UnboxedArrayObject>().elementType()
+                       : JSVAL_TYPE_MAGIC)
     {}
 
     template <size_t ProtoChainDepth>
@@ -1822,7 +1875,8 @@ class ICSetProp_Native : public ICUpdatedStub
         virtual int32_t getKey() const {
             return static_cast<int32_t>(engine_) |
                   (static_cast<int32_t>(kind) << 1) |
-                  (static_cast<int32_t>(isFixedSlot_) << 17);
+                  (static_cast<int32_t>(isFixedSlot_) << 17) |
+                  (static_cast<int32_t>(obj_->is<UnboxedPlainObject>()) << 18);
         }
 
         MOZ_MUST_USE bool generateStubCode(MacroAssembler& masm);
@@ -1927,6 +1981,7 @@ class ICSetPropNativeAddCompiler : public ICStubCompiler
         return static_cast<int32_t>(engine_) |
               (static_cast<int32_t>(kind) << 1) |
               (static_cast<int32_t>(isFixedSlot_) << 17) |
+              (static_cast<int32_t>(obj_->is<UnboxedPlainObject>()) << 18) |
               (static_cast<int32_t>(protoChainDepth_) << 19);
     }
 
@@ -1951,7 +2006,10 @@ class ICSetPropNativeAddCompiler : public ICStubCompiler
             newGroup = nullptr;
 
         RootedShape newShape(cx);
-        newShape = obj_->as<NativeObject>().lastProperty();
+        if (obj_->isNative())
+            newShape = obj_->as<NativeObject>().lastProperty();
+        else
+            newShape = obj_->as<UnboxedPlainObject>().maybeExpando()->lastProperty();
 
         return newStub<ICSetProp_NativeAddImpl<ProtoChainDepth>>(
             space, getStubCode(), oldGroup_, shapes, newShape, newGroup, offset_);
@@ -2816,10 +2874,10 @@ class ICCall_StringSplit : public ICMonitoredStub
     uint32_t pcOffset_;
     GCPtrString expectedStr_;
     GCPtrString expectedSep_;
-    GCPtrArrayObject templateObject_;
+    GCPtrObject templateObject_;
 
     ICCall_StringSplit(JitCode* stubCode, ICStub* firstMonitorStub, uint32_t pcOffset, JSString* str,
-                       JSString* sep, ArrayObject* templateObject)
+                       JSString* sep, JSObject* templateObject)
       : ICMonitoredStub(ICStub::Call_StringSplit, stubCode, firstMonitorStub),
         pcOffset_(pcOffset), expectedStr_(str), expectedSep_(sep),
         templateObject_(templateObject)
@@ -2846,7 +2904,7 @@ class ICCall_StringSplit : public ICMonitoredStub
         return expectedSep_;
     }
 
-    GCPtrArrayObject& templateObject() {
+    GCPtrObject& templateObject() {
         return templateObject_;
     }
 
@@ -2856,7 +2914,7 @@ class ICCall_StringSplit : public ICMonitoredStub
         uint32_t pcOffset_;
         RootedString expectedStr_;
         RootedString expectedSep_;
-        RootedArrayObject templateObject_;
+        RootedObject templateObject_;
 
         MOZ_MUST_USE bool generateStubCode(MacroAssembler& masm);
 
@@ -2867,13 +2925,13 @@ class ICCall_StringSplit : public ICMonitoredStub
 
       public:
         Compiler(JSContext* cx, ICStub* firstMonitorStub, uint32_t pcOffset, HandleString str,
-                 HandleString sep, HandleArrayObject templateObject)
+                 HandleString sep, HandleValue templateObject)
           : ICCallStubCompiler(cx, ICStub::Call_StringSplit),
             firstMonitorStub_(firstMonitorStub),
             pcOffset_(pcOffset),
             expectedStr_(cx, str),
             expectedSep_(cx, sep),
-            templateObject_(cx, templateObject)
+            templateObject_(cx, &templateObject.toObject())
         { }
 
         ICStub* getStub(ICStubSpace* space) {
diff --git a/js/src/jit/BaselineInspector.cpp b/js/src/jit/BaselineInspector.cpp
index 3b852debf..c9e09bed7 100644
--- a/js/src/jit/BaselineInspector.cpp
+++ b/js/src/jit/BaselineInspector.cpp
@@ -96,19 +96,32 @@ VectorAppendNoDuplicate(S& list, T value)
 
 static bool
 AddReceiver(const ReceiverGuard& receiver,
-            BaselineInspector::ReceiverVector& receivers)
+            BaselineInspector::ReceiverVector& receivers,
+            BaselineInspector::ObjectGroupVector& convertUnboxedGroups)
 {
+    if (receiver.group && receiver.group->maybeUnboxedLayout()) {
+        if (receiver.group->unboxedLayout().nativeGroup())
+            return VectorAppendNoDuplicate(convertUnboxedGroups, receiver.group);
+    }
     return VectorAppendNoDuplicate(receivers, receiver);
 }
 
 static bool
 GetCacheIRReceiverForNativeReadSlot(ICCacheIR_Monitored* stub, ReceiverGuard* receiver)
 {
-    // We match:
+    // We match either:
     //
     //   GuardIsObject 0
     //   GuardShape 0
     //   LoadFixedSlotResult 0 or LoadDynamicSlotResult 0
+    //
+    // or
+    //
+    //   GuardIsObject 0
+    //   GuardGroup 0
+    //   1: GuardAndLoadUnboxedExpando 0
+    //   GuardShape 1
+    //   LoadFixedSlotResult 1 or LoadDynamicSlotResult 1
 
     *receiver = ReceiverGuard();
     CacheIRReader reader(stub->stubInfo());
@@ -117,6 +130,14 @@ GetCacheIRReceiverForNativeReadSlot(ICCacheIR_Monitored* stub, ReceiverGuard* re
     if (!reader.matchOp(CacheOp::GuardIsObject, objId))
         return false;
 
+    if (reader.matchOp(CacheOp::GuardGroup, objId)) {
+        receiver->group = stub->stubInfo()->getStubField<ObjectGroup*>(stub, reader.stubOffset());
+
+        if (!reader.matchOp(CacheOp::GuardAndLoadUnboxedExpando, objId))
+            return false;
+        objId = reader.objOperandId();
+    }
+
     if (reader.matchOp(CacheOp::GuardShape, objId)) {
         receiver->shape = stub->stubInfo()->getStubField<Shape*>(stub, reader.stubOffset());
         return reader.matchOpEither(CacheOp::LoadFixedSlotResult, CacheOp::LoadDynamicSlotResult);
@@ -125,13 +146,40 @@ GetCacheIRReceiverForNativeReadSlot(ICCacheIR_Monitored* stub, ReceiverGuard* re
     return false;
 }
 
+static bool
+GetCacheIRReceiverForUnboxedProperty(ICCacheIR_Monitored* stub, ReceiverGuard* receiver)
+{
+    // We match:
+    //
+    //   GuardIsObject 0
+    //   GuardGroup 0
+    //   LoadUnboxedPropertyResult 0 ..
+
+    *receiver = ReceiverGuard();
+    CacheIRReader reader(stub->stubInfo());
+
+    ObjOperandId objId = ObjOperandId(0);
+    if (!reader.matchOp(CacheOp::GuardIsObject, objId))
+        return false;
+
+    if (!reader.matchOp(CacheOp::GuardGroup, objId))
+        return false;
+    receiver->group = stub->stubInfo()->getStubField<ObjectGroup*>(stub, reader.stubOffset());
+
+    return reader.matchOp(CacheOp::LoadUnboxedPropertyResult, objId);
+}
+
 bool
-BaselineInspector::maybeInfoForPropertyOp(jsbytecode* pc, ReceiverVector& receivers)
+BaselineInspector::maybeInfoForPropertyOp(jsbytecode* pc, ReceiverVector& receivers,
+                                          ObjectGroupVector& convertUnboxedGroups)
 {
     // Return a list of the receivers seen by the baseline IC for the current
     // op. Empty lists indicate no receivers are known, or there was an
-    // uncacheable access.
+    // uncacheable access. convertUnboxedGroups is used for unboxed object
+    // groups which have been seen, but have had instances converted to native
+    // objects and should be eagerly converted by Ion.
     MOZ_ASSERT(receivers.empty());
+    MOZ_ASSERT(convertUnboxedGroups.empty());
 
     if (!hasBaselineScript())
         return true;
@@ -143,7 +191,8 @@ BaselineInspector::maybeInfoForPropertyOp(jsbytecode* pc, ReceiverVector& receiv
     while (stub->next()) {
         ReceiverGuard receiver;
         if (stub->isCacheIR_Monitored()) {
-            if (!GetCacheIRReceiverForNativeReadSlot(stub->toCacheIR_Monitored(), &receiver))
+            if (!GetCacheIRReceiverForNativeReadSlot(stub->toCacheIR_Monitored(), &receiver) &&
+                !GetCacheIRReceiverForUnboxedProperty(stub->toCacheIR_Monitored(), &receiver))
             {
                 receivers.clear();
                 return true;
@@ -151,12 +200,14 @@ BaselineInspector::maybeInfoForPropertyOp(jsbytecode* pc, ReceiverVector& receiv
         } else if (stub->isSetProp_Native()) {
             receiver = ReceiverGuard(stub->toSetProp_Native()->group(),
                                      stub->toSetProp_Native()->shape());
+        } else if (stub->isSetProp_Unboxed()) {
+            receiver = ReceiverGuard(stub->toSetProp_Unboxed()->group(), nullptr);
         } else {
             receivers.clear();
             return true;
         }
 
-        if (!AddReceiver(receiver, receivers))
+        if (!AddReceiver(receiver, receivers, convertUnboxedGroups))
             return false;
 
         stub = stub->next();
@@ -538,7 +589,7 @@ BaselineInspector::getTemplateObjectForNative(jsbytecode* pc, Native native)
 
 bool
 BaselineInspector::isOptimizableCallStringSplit(jsbytecode* pc, JSString** strOut, JSString** sepOut,
-                                                ArrayObject** objOut)
+                                                JSObject** objOut)
 {
     if (!hasBaselineScript())
         return false;
@@ -649,12 +700,14 @@ bool
 BaselineInspector::commonGetPropFunction(jsbytecode* pc, JSObject** holder, Shape** holderShape,
                                          JSFunction** commonGetter, Shape** globalShape,
                                          bool* isOwnProperty,
-                                         ReceiverVector& receivers)
+                                         ReceiverVector& receivers,
+                                         ObjectGroupVector& convertUnboxedGroups)
 {
     if (!hasBaselineScript())
         return false;
 
     MOZ_ASSERT(receivers.empty());
+    MOZ_ASSERT(convertUnboxedGroups.empty());
 
     *holder = nullptr;
     const ICEntry& entry = icEntryFromPC(pc);
@@ -666,7 +719,7 @@ BaselineInspector::commonGetPropFunction(jsbytecode* pc, JSObject** holder, Shap
         {
             ICGetPropCallGetter* nstub = static_cast<ICGetPropCallGetter*>(stub);
             bool isOwn = nstub->isOwnGetter();
-            if (!isOwn && !AddReceiver(nstub->receiverGuard(), receivers))
+            if (!isOwn && !AddReceiver(nstub->receiverGuard(), receivers, convertUnboxedGroups))
                 return false;
 
             if (!*holder) {
@@ -698,19 +751,21 @@ BaselineInspector::commonGetPropFunction(jsbytecode* pc, JSObject** holder, Shap
     if (!*holder)
         return false;
 
-    MOZ_ASSERT(*isOwnProperty == (receivers.empty()));
+    MOZ_ASSERT(*isOwnProperty == (receivers.empty() && convertUnboxedGroups.empty()));
     return true;
 }
 
 bool
 BaselineInspector::commonSetPropFunction(jsbytecode* pc, JSObject** holder, Shape** holderShape,
                                          JSFunction** commonSetter, bool* isOwnProperty,
-                                         ReceiverVector& receivers)
+                                         ReceiverVector& receivers,
+                                         ObjectGroupVector& convertUnboxedGroups)
 {
     if (!hasBaselineScript())
         return false;
 
     MOZ_ASSERT(receivers.empty());
+    MOZ_ASSERT(convertUnboxedGroups.empty());
 
     *holder = nullptr;
     const ICEntry& entry = icEntryFromPC(pc);
@@ -719,7 +774,7 @@ BaselineInspector::commonSetPropFunction(jsbytecode* pc, JSObject** holder, Shap
         if (stub->isSetProp_CallScripted() || stub->isSetProp_CallNative()) {
             ICSetPropCallSetter* nstub = static_cast<ICSetPropCallSetter*>(stub);
             bool isOwn = nstub->isOwnSetter();
-            if (!isOwn && !AddReceiver(nstub->receiverGuard(), receivers))
+            if (!isOwn && !AddReceiver(nstub->receiverGuard(), receivers, convertUnboxedGroups))
                 return false;
 
             if (!*holder) {
diff --git a/js/src/jit/BaselineInspector.h b/js/src/jit/BaselineInspector.h
index 1ed4b5547..961df18aa 100644
--- a/js/src/jit/BaselineInspector.h
+++ b/js/src/jit/BaselineInspector.h
@@ -95,7 +95,8 @@ class BaselineInspector
   public:
     typedef Vector<ReceiverGuard, 4, JitAllocPolicy> ReceiverVector;
     typedef Vector<ObjectGroup*, 4, JitAllocPolicy> ObjectGroupVector;
-    MOZ_MUST_USE bool maybeInfoForPropertyOp(jsbytecode* pc, ReceiverVector& receivers);
+    MOZ_MUST_USE bool maybeInfoForPropertyOp(jsbytecode* pc, ReceiverVector& receivers,
+                                             ObjectGroupVector& convertUnboxedGroups);
 
     SetElemICInspector setElemICInspector(jsbytecode* pc) {
         return makeICInspector<SetElemICInspector>(pc, ICStub::SetElem_Fallback);
@@ -113,7 +114,7 @@ class BaselineInspector
     bool hasSeenNonStringIterMore(jsbytecode* pc);
 
     MOZ_MUST_USE bool isOptimizableCallStringSplit(jsbytecode* pc, JSString** strOut,
-                                                   JSString** sepOut, ArrayObject** objOut);
+                                                   JSString** sepOut, JSObject** objOut);
     JSObject* getTemplateObject(jsbytecode* pc);
     JSObject* getTemplateObjectForNative(jsbytecode* pc, Native native);
     JSObject* getTemplateObjectForClassHook(jsbytecode* pc, const Class* clasp);
@@ -130,10 +131,12 @@ class BaselineInspector
 
     MOZ_MUST_USE bool commonGetPropFunction(jsbytecode* pc, JSObject** holder, Shape** holderShape,
                                             JSFunction** commonGetter, Shape** globalShape,
-                                            bool* isOwnProperty, ReceiverVector& receivers);
+                                            bool* isOwnProperty, ReceiverVector& receivers,
+                                            ObjectGroupVector& convertUnboxedGroups);
     MOZ_MUST_USE bool commonSetPropFunction(jsbytecode* pc, JSObject** holder, Shape** holderShape,
                                             JSFunction** commonSetter, bool* isOwnProperty,
-                                            ReceiverVector& receivers);
+                                            ReceiverVector& receivers,
+                                            ObjectGroupVector& convertUnboxedGroups);
 
     MOZ_MUST_USE bool instanceOfData(jsbytecode* pc, Shape** shape, uint32_t* slot,
                                      JSObject** prototypeObject);
diff --git a/js/src/jit/CacheIR.cpp b/js/src/jit/CacheIR.cpp
index d184ea40c..f1061af70 100644
--- a/js/src/jit/CacheIR.cpp
+++ b/js/src/jit/CacheIR.cpp
@@ -10,7 +10,8 @@
 #include "jit/IonCaches.h"
 
 #include "jsobjinlines.h"
-#include "vm/NativeObject-inl.h"
+
+#include "vm/UnboxedObject-inl.h"
 
 using namespace js;
 using namespace js::jit;
@@ -59,6 +60,10 @@ GetPropIRGenerator::tryAttachStub(Maybe<CacheIRWriter>& writer)
             return false;
         if (!emitted_ && !tryAttachNative(*writer, obj, objId))
             return false;
+        if (!emitted_ && !tryAttachUnboxed(*writer, obj, objId))
+            return false;
+        if (!emitted_ && !tryAttachUnboxedExpando(*writer, obj, objId))
+            return false;
         if (!emitted_ && !tryAttachTypedObject(*writer, obj, objId))
             return false;
         if (!emitted_ && !tryAttachModuleNamespace(*writer, obj, objId))
@@ -158,9 +163,19 @@ GeneratePrototypeGuards(CacheIRWriter& writer, JSObject* obj, JSObject* holder,
 }
 
 static void
-TestMatchingReceiver(CacheIRWriter& writer, JSObject* obj, Shape* shape, ObjOperandId objId)
+TestMatchingReceiver(CacheIRWriter& writer, JSObject* obj, Shape* shape, ObjOperandId objId,
+                     Maybe<ObjOperandId>* expandoId)
 {
-    if (obj->is<TypedObject>()) {
+    if (obj->is<UnboxedPlainObject>()) {
+        writer.guardGroup(objId, obj->group());
+
+        if (UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando()) {
+            expandoId->emplace(writer.guardAndLoadUnboxedExpando(objId));
+            writer.guardShape(expandoId->ref(), expando->lastProperty());
+        } else {
+            writer.guardNoUnboxedExpando(objId);
+        }
+    } else if (obj->is<UnboxedArrayObject>() || obj->is<TypedObject>()) {
         writer.guardGroup(objId, obj->group());
     } else {
         Shape* shape = obj->maybeShape();
@@ -173,7 +188,8 @@ static void
 EmitReadSlotResult(CacheIRWriter& writer, JSObject* obj, JSObject* holder,
                    Shape* shape, ObjOperandId objId)
 {
-    TestMatchingReceiver(writer, obj, shape, objId);
+    Maybe<ObjOperandId> expandoId;
+    TestMatchingReceiver(writer, obj, shape, objId, &expandoId);
 
     ObjOperandId holderId;
     if (obj != holder) {
@@ -196,6 +212,9 @@ EmitReadSlotResult(CacheIRWriter& writer, JSObject* obj, JSObject* holder,
                 lastObjId = protoId;
             }
         }
+    } else if (obj->is<UnboxedPlainObject>()) {
+        holder = obj->as<UnboxedPlainObject>().maybeExpando();
+        holderId = *expandoId;
     } else {
         holderId = objId;
     }
@@ -247,6 +266,51 @@ GetPropIRGenerator::tryAttachNative(CacheIRWriter& writer, HandleObject obj, Obj
 }
 
 bool
+GetPropIRGenerator::tryAttachUnboxed(CacheIRWriter& writer, HandleObject obj, ObjOperandId objId)
+{
+    MOZ_ASSERT(!emitted_);
+
+    if (!obj->is<UnboxedPlainObject>())
+        return true;
+
+    const UnboxedLayout::Property* property = obj->as<UnboxedPlainObject>().layout().lookup(name_);
+    if (!property)
+        return true;
+
+    if (!cx_->runtime()->jitSupportsFloatingPoint)
+        return true;
+
+    writer.guardGroup(objId, obj->group());
+    writer.loadUnboxedPropertyResult(objId, property->type,
+                                     UnboxedPlainObject::offsetOfData() + property->offset);
+    emitted_ = true;
+    preliminaryObjectAction_ = PreliminaryObjectAction::Unlink;
+    return true;
+}
+
+bool
+GetPropIRGenerator::tryAttachUnboxedExpando(CacheIRWriter& writer, HandleObject obj, ObjOperandId objId)
+{
+    MOZ_ASSERT(!emitted_);
+
+    if (!obj->is<UnboxedPlainObject>())
+        return true;
+
+    UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando();
+    if (!expando)
+        return true;
+
+    Shape* shape = expando->lookup(cx_, NameToId(name_));
+    if (!shape || !shape->hasDefaultGetter() || !shape->hasSlot())
+        return true;
+
+    emitted_ = true;
+
+    EmitReadSlotResult(writer, obj, obj, shape, objId);
+    return true;
+}
+
+bool
 GetPropIRGenerator::tryAttachTypedObject(CacheIRWriter& writer, HandleObject obj, ObjOperandId objId)
 {
     MOZ_ASSERT(!emitted_);
@@ -304,6 +368,13 @@ GetPropIRGenerator::tryAttachObjectLength(CacheIRWriter& writer, HandleObject ob
         return true;
     }
 
+    if (obj->is<UnboxedArrayObject>()) {
+        writer.guardClass(objId, GuardClassKind::UnboxedArray);
+        writer.loadUnboxedArrayLengthResult(objId);
+        emitted_ = true;
+        return true;
+    }
+
     if (obj->is<ArgumentsObject>() && !obj->as<ArgumentsObject>().hasOverriddenLength()) {
         if (obj->is<MappedArgumentsObject>()) {
             writer.guardClass(objId, GuardClassKind::MappedArguments);
diff --git a/js/src/jit/CacheIR.h b/js/src/jit/CacheIR.h
index ae55cfebb..51e55f48b 100644
--- a/js/src/jit/CacheIR.h
+++ b/js/src/jit/CacheIR.h
@@ -87,12 +87,16 @@ class ObjOperandId : public OperandId
     _(GuardClass)                         \
     _(GuardSpecificObject)                \
     _(GuardNoDetachedTypedObjects)        \
+    _(GuardNoUnboxedExpando)              \
+    _(GuardAndLoadUnboxedExpando)         \
     _(LoadObject)                         \
     _(LoadProto)                          \
     _(LoadFixedSlotResult)                \
     _(LoadDynamicSlotResult)              \
+    _(LoadUnboxedPropertyResult)          \
     _(LoadTypedObjectResult)              \
     _(LoadInt32ArrayLengthResult)         \
+    _(LoadUnboxedArrayLengthResult)       \
     _(LoadArgumentsObjectLengthResult)    \
     _(LoadUndefinedResult)
 
@@ -124,6 +128,7 @@ struct StubField {
 enum class GuardClassKind
 {
     Array,
+    UnboxedArray,
     MappedArguments,
     UnmappedArguments,
 };
@@ -271,6 +276,15 @@ class MOZ_RAII CacheIRWriter
     void guardNoDetachedTypedObjects() {
         writeOp(CacheOp::GuardNoDetachedTypedObjects);
     }
+    void guardNoUnboxedExpando(ObjOperandId obj) {
+        writeOpWithOperandId(CacheOp::GuardNoUnboxedExpando, obj);
+    }
+    ObjOperandId guardAndLoadUnboxedExpando(ObjOperandId obj) {
+        ObjOperandId res(nextOperandId_++);
+        writeOpWithOperandId(CacheOp::GuardAndLoadUnboxedExpando, obj);
+        writeOperandId(res);
+        return res;
+    }
 
     ObjOperandId loadObject(JSObject* obj) {
         ObjOperandId res(nextOperandId_++);
@@ -296,6 +310,11 @@ class MOZ_RAII CacheIRWriter
         writeOpWithOperandId(CacheOp::LoadDynamicSlotResult, obj);
         addStubWord(offset, StubField::GCType::NoGCThing);
     }
+    void loadUnboxedPropertyResult(ObjOperandId obj, JSValueType type, size_t offset) {
+        writeOpWithOperandId(CacheOp::LoadUnboxedPropertyResult, obj);
+        buffer_.writeByte(uint32_t(type));
+        addStubWord(offset, StubField::GCType::NoGCThing);
+    }
     void loadTypedObjectResult(ObjOperandId obj, uint32_t offset, TypedThingLayout layout,
                                uint32_t typeDescr) {
         MOZ_ASSERT(uint32_t(layout) <= UINT8_MAX);
@@ -308,6 +327,9 @@ class MOZ_RAII CacheIRWriter
     void loadInt32ArrayLengthResult(ObjOperandId obj) {
         writeOpWithOperandId(CacheOp::LoadInt32ArrayLengthResult, obj);
     }
+    void loadUnboxedArrayLengthResult(ObjOperandId obj) {
+        writeOpWithOperandId(CacheOp::LoadUnboxedArrayLengthResult, obj);
+    }
     void loadArgumentsObjectLengthResult(ObjOperandId obj) {
         writeOpWithOperandId(CacheOp::LoadArgumentsObjectLengthResult, obj);
     }
@@ -389,6 +411,9 @@ class MOZ_RAII GetPropIRGenerator
     PreliminaryObjectAction preliminaryObjectAction_;
 
     MOZ_MUST_USE bool tryAttachNative(CacheIRWriter& writer, HandleObject obj, ObjOperandId objId);
+    MOZ_MUST_USE bool tryAttachUnboxed(CacheIRWriter& writer, HandleObject obj, ObjOperandId objId);
+    MOZ_MUST_USE bool tryAttachUnboxedExpando(CacheIRWriter& writer, HandleObject obj,
+                                              ObjOperandId objId);
     MOZ_MUST_USE bool tryAttachTypedObject(CacheIRWriter& writer, HandleObject obj,
                                            ObjOperandId objId);
     MOZ_MUST_USE bool tryAttachObjectLength(CacheIRWriter& writer, HandleObject obj,
diff --git a/js/src/jit/CodeGenerator.cpp b/js/src/jit/CodeGenerator.cpp
index 2b1c671d1..bb12b09c8 100644
--- a/js/src/jit/CodeGenerator.cpp
+++ b/js/src/jit/CodeGenerator.cpp
@@ -25,7 +25,6 @@
 #include "builtin/Eval.h"
 #include "builtin/TypedObject.h"
 #include "gc/Nursery.h"
-#include "gc/StoreBuffer-inl.h"
 #include "irregexp/NativeRegExpMacroAssembler.h"
 #include "jit/AtomicOperations.h"
 #include "jit/BaselineCompiler.h"
@@ -3029,10 +3028,19 @@ CodeGenerator::visitStoreSlotV(LStoreSlotV* lir)
 
 static void
 GuardReceiver(MacroAssembler& masm, const ReceiverGuard& guard,
-              Register obj, Register scratch, Label* miss)
+              Register obj, Register scratch, Label* miss, bool checkNullExpando)
 {
     if (guard.group) {
         masm.branchTestObjGroup(Assembler::NotEqual, obj, guard.group, miss);
+
+        Address expandoAddress(obj, UnboxedPlainObject::offsetOfExpando());
+        if (guard.shape) {
+            masm.loadPtr(expandoAddress, scratch);
+            masm.branchPtr(Assembler::Equal, scratch, ImmWord(0), miss);
+            masm.branchTestObjShape(Assembler::NotEqual, scratch, guard.shape, miss);
+        } else if (checkNullExpando) {
+            masm.branchPtr(Assembler::NotEqual, expandoAddress, ImmWord(0), miss);
+        }
     } else {
         masm.branchTestObjShape(Assembler::NotEqual, obj, guard.shape, miss);
     }
@@ -3051,11 +3059,13 @@ CodeGenerator::emitGetPropertyPolymorphic(LInstruction* ins, Register obj, Regis
 
         Label next;
         masm.comment("GuardReceiver");
-        GuardReceiver(masm, receiver, obj, scratch, &next);
+        GuardReceiver(masm, receiver, obj, scratch, &next, /* checkNullExpando = */ false);
 
         if (receiver.shape) {
             masm.comment("loadTypedOrValue");
-            Register target = obj;
+            // If this is an unboxed expando access, GuardReceiver loaded the
+            // expando object into scratch.
+            Register target = receiver.group ? scratch : obj;
 
             Shape* shape = mir->shape(i);
             if (shape->slot() < shape->numFixedSlots()) {
@@ -3068,6 +3078,13 @@ CodeGenerator::emitGetPropertyPolymorphic(LInstruction* ins, Register obj, Regis
                 masm.loadPtr(Address(target, NativeObject::offsetOfSlots()), scratch);
                 masm.loadTypedOrValue(Address(scratch, offset), output);
             }
+        } else {
+            masm.comment("loadUnboxedProperty");
+            const UnboxedLayout::Property* property =
+                receiver.group->unboxedLayout().lookup(mir->name());
+            Address propertyAddr(obj, UnboxedPlainObject::offsetOfData() + property->offset);
+
+            masm.loadUnboxedProperty(propertyAddr, property->type, output);
         }
 
         if (i == mir->numReceivers() - 1) {
@@ -3108,6 +3125,8 @@ EmitUnboxedPreBarrier(MacroAssembler &masm, T address, JSValueType type)
         masm.patchableCallPreBarrier(address, MIRType::Object);
     else if (type == JSVAL_TYPE_STRING)
         masm.patchableCallPreBarrier(address, MIRType::String);
+    else
+        MOZ_ASSERT(!UnboxedTypeNeedsPreBarrier(type));
 }
 
 void
@@ -3121,10 +3140,12 @@ CodeGenerator::emitSetPropertyPolymorphic(LInstruction* ins, Register obj, Regis
         ReceiverGuard receiver = mir->receiver(i);
 
         Label next;
-        GuardReceiver(masm, receiver, obj, scratch, &next);
+        GuardReceiver(masm, receiver, obj, scratch, &next, /* checkNullExpando = */ false);
 
         if (receiver.shape) {
-            Register target = obj;
+            // If this is an unboxed expando access, GuardReceiver loaded the
+            // expando object into scratch.
+            Register target = receiver.group ? scratch : obj;
 
             Shape* shape = mir->shape(i);
             if (shape->slot() < shape->numFixedSlots()) {
@@ -3141,6 +3162,13 @@ CodeGenerator::emitSetPropertyPolymorphic(LInstruction* ins, Register obj, Regis
                     emitPreBarrier(addr);
                 masm.storeConstantOrRegister(value, addr);
             }
+        } else {
+            const UnboxedLayout::Property* property =
+                receiver.group->unboxedLayout().lookup(mir->name());
+            Address propertyAddr(obj, UnboxedPlainObject::offsetOfData() + property->offset);
+
+            EmitUnboxedPreBarrier(masm, propertyAddr, property->type);
+            masm.storeUnboxedProperty(propertyAddr, property->type, value, nullptr);
         }
 
         if (i == mir->numReceivers() - 1) {
@@ -3181,7 +3209,9 @@ CodeGenerator::visitSetPropertyPolymorphicT(LSetPropertyPolymorphicT* ins)
 void
 CodeGenerator::visitElements(LElements* lir)
 {
-    Address elements(ToRegister(lir->object()), NativeObject::offsetOfElements());
+    Address elements(ToRegister(lir->object()),
+                     lir->mir()->unboxed() ? UnboxedArrayObject::offsetOfElements()
+                                           : NativeObject::offsetOfElements());
     masm.loadPtr(elements, ToRegister(lir->output()));
 }
 
@@ -3290,7 +3320,7 @@ CodeGenerator::visitGuardReceiverPolymorphic(LGuardReceiverPolymorphic* lir)
         const ReceiverGuard& receiver = mir->receiver(i);
 
         Label next;
-        GuardReceiver(masm, receiver, obj, temp, &next);
+        GuardReceiver(masm, receiver, obj, temp, &next, /* checkNullExpando = */ true);
 
         if (i == mir->numReceivers() - 1) {
             bailoutFrom(&next, lir->snapshot());
@@ -3304,6 +3334,27 @@ CodeGenerator::visitGuardReceiverPolymorphic(LGuardReceiverPolymorphic* lir)
 }
 
 void
+CodeGenerator::visitGuardUnboxedExpando(LGuardUnboxedExpando* lir)
+{
+    Label miss;
+
+    Register obj = ToRegister(lir->object());
+    masm.branchPtr(lir->mir()->requireExpando() ? Assembler::Equal : Assembler::NotEqual,
+                   Address(obj, UnboxedPlainObject::offsetOfExpando()), ImmWord(0), &miss);
+
+    bailoutFrom(&miss, lir->snapshot());
+}
+
+void
+CodeGenerator::visitLoadUnboxedExpando(LLoadUnboxedExpando* lir)
+{
+    Register obj = ToRegister(lir->object());
+    Register result = ToRegister(lir->getDef(0));
+
+    masm.loadPtr(Address(obj, UnboxedPlainObject::offsetOfExpando()), result);
+}
+
+void
 CodeGenerator::visitTypeBarrierV(LTypeBarrierV* lir)
 {
     ValueOperand operand = ToValue(lir, LTypeBarrierV::Input);
@@ -5166,11 +5217,11 @@ static JSObject*
 NewArrayWithGroup(JSContext* cx, uint32_t length, HandleObjectGroup group,
                   bool convertDoubleElements)
 {
-    ArrayObject* res = NewFullyAllocatedArrayTryUseGroup(cx, group, length);
+    JSObject* res = NewFullyAllocatedArrayTryUseGroup(cx, group, length);
     if (!res)
         return nullptr;
     if (convertDoubleElements)
-        res->setShouldConvertDoubleElements();
+        res->as<ArrayObject>().setShouldConvertDoubleElements();
     return res;
 }
 
@@ -5316,7 +5367,7 @@ CodeGenerator::visitNewArrayCopyOnWrite(LNewArrayCopyOnWrite* lir)
     masm.bind(ool->rejoin());
 }
 
-typedef ArrayObject* (*ArrayConstructorOneArgFn)(JSContext*, HandleObjectGroup, int32_t length);
+typedef JSObject* (*ArrayConstructorOneArgFn)(JSContext*, HandleObjectGroup, int32_t length);
 static const VMFunction ArrayConstructorOneArgInfo =
     FunctionInfo<ArrayConstructorOneArgFn>(ArrayConstructorOneArg, "ArrayConstructorOneArg");
 
@@ -5336,11 +5387,21 @@ CodeGenerator::visitNewArrayDynamicLength(LNewArrayDynamicLength* lir)
 
     bool canInline = true;
     size_t inlineLength = 0;
-    if (templateObject->as<ArrayObject>().hasFixedElements()) {
-        size_t numSlots = gc::GetGCKindSlots(templateObject->asTenured().getAllocKind());
-        inlineLength = numSlots - ObjectElements::VALUES_PER_HEADER;
+    if (templateObject->is<ArrayObject>()) {
+        if (templateObject->as<ArrayObject>().hasFixedElements()) {
+            size_t numSlots = gc::GetGCKindSlots(templateObject->asTenured().getAllocKind());
+            inlineLength = numSlots - ObjectElements::VALUES_PER_HEADER;
+        } else {
+            canInline = false;
+        }
     } else {
-        canInline = false;
+        if (templateObject->as<UnboxedArrayObject>().hasInlineElements()) {
+            size_t nbytes =
+                templateObject->tenuredSizeOfThis() - UnboxedArrayObject::offsetOfInlineElements();
+            inlineLength = nbytes / templateObject->as<UnboxedArrayObject>().elementSize();
+        } else {
+            canInline = false;
+        }
     }
 
     if (canInline) {
@@ -7762,7 +7823,7 @@ CodeGenerator::visitSinCos(LSinCos *lir)
     masm.freeStack(sizeof(double) * 2);
 }
 
-typedef ArrayObject* (*StringSplitFn)(JSContext*, HandleObjectGroup, HandleString, HandleString, uint32_t);
+typedef JSObject* (*StringSplitFn)(JSContext*, HandleObjectGroup, HandleString, HandleString, uint32_t);
 static const VMFunction StringSplitInfo =
     FunctionInfo<StringSplitFn>(js::str_split_string, "str_split_string");
 
@@ -7797,6 +7858,49 @@ CodeGenerator::visitSetInitializedLength(LSetInitializedLength* lir)
 }
 
 void
+CodeGenerator::visitUnboxedArrayLength(LUnboxedArrayLength* lir)
+{
+    Register obj = ToRegister(lir->object());
+    Register result = ToRegister(lir->output());
+    masm.load32(Address(obj, UnboxedArrayObject::offsetOfLength()), result);
+}
+
+void
+CodeGenerator::visitUnboxedArrayInitializedLength(LUnboxedArrayInitializedLength* lir)
+{
+    Register obj = ToRegister(lir->object());
+    Register result = ToRegister(lir->output());
+    masm.load32(Address(obj, UnboxedArrayObject::offsetOfCapacityIndexAndInitializedLength()), result);
+    masm.and32(Imm32(UnboxedArrayObject::InitializedLengthMask), result);
+}
+
+void
+CodeGenerator::visitIncrementUnboxedArrayInitializedLength(LIncrementUnboxedArrayInitializedLength* lir)
+{
+    Register obj = ToRegister(lir->object());
+    masm.add32(Imm32(1), Address(obj, UnboxedArrayObject::offsetOfCapacityIndexAndInitializedLength()));
+}
+
+void
+CodeGenerator::visitSetUnboxedArrayInitializedLength(LSetUnboxedArrayInitializedLength* lir)
+{
+    Register obj = ToRegister(lir->object());
+    RegisterOrInt32Constant key = ToRegisterOrInt32Constant(lir->length());
+    Register temp = ToRegister(lir->temp());
+
+    Address initLengthAddr(obj, UnboxedArrayObject::offsetOfCapacityIndexAndInitializedLength());
+    masm.load32(initLengthAddr, temp);
+    masm.and32(Imm32(UnboxedArrayObject::CapacityMask), temp);
+
+    if (key.isRegister())
+        masm.or32(key.reg(), temp);
+    else
+        masm.or32(Imm32(key.constant()), temp);
+
+    masm.store32(temp, initLengthAddr);
+}
+
+void
 CodeGenerator::visitNotO(LNotO* lir)
 {
     MOZ_ASSERT(lir->mir()->operandMightEmulateUndefined(),
@@ -8092,19 +8196,46 @@ CodeGenerator::emitStoreElementHoleT(T* lir)
     OutOfLineStoreElementHole* ool = new(alloc()) OutOfLineStoreElementHole(lir);
     addOutOfLineCode(ool, lir->mir());
 
+    Register obj = ToRegister(lir->object());
     Register elements = ToRegister(lir->elements());
     const LAllocation* index = lir->index();
     RegisterOrInt32Constant key = ToRegisterOrInt32Constant(index);
 
-    Address initLength(elements, ObjectElements::offsetOfInitializedLength());
-    masm.branch32(Assembler::BelowOrEqual, initLength, key, ool->entry());
+    JSValueType unboxedType = lir->mir()->unboxedType();
+    if (unboxedType == JSVAL_TYPE_MAGIC) {
+        Address initLength(elements, ObjectElements::offsetOfInitializedLength());
+        masm.branch32(Assembler::BelowOrEqual, initLength, key, ool->entry());
 
-    if (lir->mir()->needsBarrier())
-        emitPreBarrier(elements, index, 0);
+        if (lir->mir()->needsBarrier())
+            emitPreBarrier(elements, index, 0);
+
+        masm.bind(ool->rejoinStore());
+        emitStoreElementTyped(lir->value(), lir->mir()->value()->type(), lir->mir()->elementType(),
+                              elements, index, 0);
+    } else {
+        Register temp = ToRegister(lir->getTemp(0));
+        Address initLength(obj, UnboxedArrayObject::offsetOfCapacityIndexAndInitializedLength());
+        masm.load32(initLength, temp);
+        masm.and32(Imm32(UnboxedArrayObject::InitializedLengthMask), temp);
+        masm.branch32(Assembler::BelowOrEqual, temp, key, ool->entry());
+
+        ConstantOrRegister v = ToConstantOrRegister(lir->value(), lir->mir()->value()->type());
+
+        if (index->isConstant()) {
+            Address address(elements, ToInt32(index) * UnboxedTypeSize(unboxedType));
+            EmitUnboxedPreBarrier(masm, address, unboxedType);
+
+            masm.bind(ool->rejoinStore());
+            masm.storeUnboxedProperty(address, unboxedType, v, nullptr);
+        } else {
+            BaseIndex address(elements, ToRegister(index),
+                              ScaleFromElemWidth(UnboxedTypeSize(unboxedType)));
+            EmitUnboxedPreBarrier(masm, address, unboxedType);
 
-    masm.bind(ool->rejoinStore());
-    emitStoreElementTyped(lir->value(), lir->mir()->value()->type(), lir->mir()->elementType(),
-                          elements, index, 0);
+            masm.bind(ool->rejoinStore());
+            masm.storeUnboxedProperty(address, unboxedType, v, nullptr);
+        }
+    }
 
     masm.bind(ool->rejoin());
 }
@@ -8124,22 +8255,47 @@ CodeGenerator::emitStoreElementHoleV(T* lir)
     OutOfLineStoreElementHole* ool = new(alloc()) OutOfLineStoreElementHole(lir);
     addOutOfLineCode(ool, lir->mir());
 
+    Register obj = ToRegister(lir->object());
     Register elements = ToRegister(lir->elements());
     const LAllocation* index = lir->index();
     const ValueOperand value = ToValue(lir, T::Value);
     RegisterOrInt32Constant key = ToRegisterOrInt32Constant(index);
 
-    Address initLength(elements, ObjectElements::offsetOfInitializedLength());
-    masm.branch32(Assembler::BelowOrEqual, initLength, key, ool->entry());
+    JSValueType unboxedType = lir->mir()->unboxedType();
+    if (unboxedType == JSVAL_TYPE_MAGIC) {
+        Address initLength(elements, ObjectElements::offsetOfInitializedLength());
+        masm.branch32(Assembler::BelowOrEqual, initLength, key, ool->entry());
 
-    if (lir->mir()->needsBarrier())
-        emitPreBarrier(elements, index, 0);
+        if (lir->mir()->needsBarrier())
+            emitPreBarrier(elements, index, 0);
 
-    masm.bind(ool->rejoinStore());
-    if (index->isConstant())
-        masm.storeValue(value, Address(elements, ToInt32(index) * sizeof(js::Value)));
-    else
-        masm.storeValue(value, BaseIndex(elements, ToRegister(index), TimesEight));
+        masm.bind(ool->rejoinStore());
+        if (index->isConstant())
+            masm.storeValue(value, Address(elements, ToInt32(index) * sizeof(js::Value)));
+        else
+            masm.storeValue(value, BaseIndex(elements, ToRegister(index), TimesEight));
+    } else {
+        Register temp = ToRegister(lir->getTemp(0));
+        Address initLength(obj, UnboxedArrayObject::offsetOfCapacityIndexAndInitializedLength());
+        masm.load32(initLength, temp);
+        masm.and32(Imm32(UnboxedArrayObject::InitializedLengthMask), temp);
+        masm.branch32(Assembler::BelowOrEqual, temp, key, ool->entry());
+
+        if (index->isConstant()) {
+            Address address(elements, ToInt32(index) * UnboxedTypeSize(unboxedType));
+            EmitUnboxedPreBarrier(masm, address, unboxedType);
+
+            masm.bind(ool->rejoinStore());
+            masm.storeUnboxedProperty(address, unboxedType, ConstantOrRegister(value), nullptr);
+        } else {
+            BaseIndex address(elements, ToRegister(index),
+                              ScaleFromElemWidth(UnboxedTypeSize(unboxedType)));
+            EmitUnboxedPreBarrier(masm, address, unboxedType);
+
+            masm.bind(ool->rejoinStore());
+            masm.storeUnboxedProperty(address, unboxedType, ConstantOrRegister(value), nullptr);
+        }
+    }
 
     masm.bind(ool->rejoin());
 }
@@ -8210,10 +8366,11 @@ CodeGenerator::visitFallibleStoreElementV(LFallibleStoreElementV* lir)
     masm.bind(&isFrozen);
 }
 
-typedef bool (*SetDenseElementFn)(JSContext*, HandleNativeObject, int32_t, HandleValue,
-                                  bool strict);
-static const VMFunction SetDenseElementInfo =
-    FunctionInfo<SetDenseElementFn>(jit::SetDenseElement, "SetDenseElement");
+typedef bool (*SetDenseOrUnboxedArrayElementFn)(JSContext*, HandleObject, int32_t,
+                                                HandleValue, bool strict);
+static const VMFunction SetDenseOrUnboxedArrayElementInfo =
+    FunctionInfo<SetDenseOrUnboxedArrayElementFn>(SetDenseOrUnboxedArrayElement,
+                                                  "SetDenseOrUnboxedArrayElement");
 
 void
 CodeGenerator::visitOutOfLineStoreElementHole(OutOfLineStoreElementHole* ool)
@@ -8223,6 +8380,8 @@ CodeGenerator::visitOutOfLineStoreElementHole(OutOfLineStoreElementHole* ool)
     const LAllocation* index;
     MIRType valueType;
     ConstantOrRegister value;
+    JSValueType unboxedType;
+    LDefinition *temp = nullptr;
 
     if (ins->isStoreElementHoleV()) {
         LStoreElementHoleV* store = ins->toStoreElementHoleV();
@@ -8231,6 +8390,8 @@ CodeGenerator::visitOutOfLineStoreElementHole(OutOfLineStoreElementHole* ool)
         index = store->index();
         valueType = store->mir()->value()->type();
         value = TypedOrValueRegister(ToValue(store, LStoreElementHoleV::Value));
+        unboxedType = store->mir()->unboxedType();
+        temp = store->getTemp(0);
     } else if (ins->isFallibleStoreElementV()) {
         LFallibleStoreElementV* store = ins->toFallibleStoreElementV();
         object = ToRegister(store->object());
@@ -8238,6 +8399,8 @@ CodeGenerator::visitOutOfLineStoreElementHole(OutOfLineStoreElementHole* ool)
         index = store->index();
         valueType = store->mir()->value()->type();
         value = TypedOrValueRegister(ToValue(store, LFallibleStoreElementV::Value));
+        unboxedType = store->mir()->unboxedType();
+        temp = store->getTemp(0);
     } else if (ins->isStoreElementHoleT()) {
         LStoreElementHoleT* store = ins->toStoreElementHoleT();
         object = ToRegister(store->object());
@@ -8248,6 +8411,8 @@ CodeGenerator::visitOutOfLineStoreElementHole(OutOfLineStoreElementHole* ool)
             value = ConstantOrRegister(store->value()->toConstant()->toJSValue());
         else
             value = TypedOrValueRegister(valueType, ToAnyRegister(store->value()));
+        unboxedType = store->mir()->unboxedType();
+        temp = store->getTemp(0);
     } else { // ins->isFallibleStoreElementT()
         LFallibleStoreElementT* store = ins->toFallibleStoreElementT();
         object = ToRegister(store->object());
@@ -8258,6 +8423,8 @@ CodeGenerator::visitOutOfLineStoreElementHole(OutOfLineStoreElementHole* ool)
             value = ConstantOrRegister(store->value()->toConstant()->toJSValue());
         else
             value = TypedOrValueRegister(valueType, ToAnyRegister(store->value()));
+        unboxedType = store->mir()->unboxedType();
+        temp = store->getTemp(0);
     }
 
     RegisterOrInt32Constant key = ToRegisterOrInt32Constant(index);
@@ -8268,32 +8435,54 @@ CodeGenerator::visitOutOfLineStoreElementHole(OutOfLineStoreElementHole* ool)
     Label callStub;
 #if defined(JS_CODEGEN_MIPS32) || defined(JS_CODEGEN_MIPS64)
     // Had to reimplement for MIPS because there are no flags.
-    Address initLength(elements, ObjectElements::offsetOfInitializedLength());
-    masm.branch32(Assembler::NotEqual, initLength, key, &callStub);
+    if (unboxedType == JSVAL_TYPE_MAGIC) {
+        Address initLength(elements, ObjectElements::offsetOfInitializedLength());
+        masm.branch32(Assembler::NotEqual, initLength, key, &callStub);
+    } else {
+        Address initLength(object, UnboxedArrayObject::offsetOfCapacityIndexAndInitializedLength());
+        masm.load32(initLength, ToRegister(temp));
+        masm.and32(Imm32(UnboxedArrayObject::InitializedLengthMask), ToRegister(temp));
+        masm.branch32(Assembler::NotEqual, ToRegister(temp), key, &callStub);
+    }
 #else
     masm.j(Assembler::NotEqual, &callStub);
 #endif
 
-    // Check array capacity.
-    masm.branch32(Assembler::BelowOrEqual, Address(elements, ObjectElements::offsetOfCapacity()),
-                  key, &callStub);
+    if (unboxedType == JSVAL_TYPE_MAGIC) {
+        // Check array capacity.
+        masm.branch32(Assembler::BelowOrEqual, Address(elements, ObjectElements::offsetOfCapacity()),
+                      key, &callStub);
 
-    // Update initialized length. The capacity guard above ensures this won't overflow,
-    // due to MAX_DENSE_ELEMENTS_COUNT.
-    masm.inc32(&key);
-    masm.store32(key, Address(elements, ObjectElements::offsetOfInitializedLength()));
+        // Update initialized length. The capacity guard above ensures this won't overflow,
+        // due to MAX_DENSE_ELEMENTS_COUNT.
+        masm.inc32(&key);
+        masm.store32(key, Address(elements, ObjectElements::offsetOfInitializedLength()));
 
-    // Update length if length < initializedLength.
-    Label dontUpdate;
-    masm.branch32(Assembler::AboveOrEqual, Address(elements, ObjectElements::offsetOfLength()),
-                  key, &dontUpdate);
-    masm.store32(key, Address(elements, ObjectElements::offsetOfLength()));
-    masm.bind(&dontUpdate);
+        // Update length if length < initializedLength.
+        Label dontUpdate;
+        masm.branch32(Assembler::AboveOrEqual, Address(elements, ObjectElements::offsetOfLength()),
+                      key, &dontUpdate);
+        masm.store32(key, Address(elements, ObjectElements::offsetOfLength()));
+        masm.bind(&dontUpdate);
 
-    masm.dec32(&key);
+        masm.dec32(&key);
+    } else {
+        // Check array capacity.
+        masm.checkUnboxedArrayCapacity(object, key, ToRegister(temp), &callStub);
+
+        // Update initialized length.
+        masm.add32(Imm32(1), Address(object, UnboxedArrayObject::offsetOfCapacityIndexAndInitializedLength()));
+
+        // Update length if length < initializedLength.
+        Address lengthAddr(object, UnboxedArrayObject::offsetOfLength());
+        Label dontUpdate;
+        masm.branch32(Assembler::Above, lengthAddr, key, &dontUpdate);
+        masm.add32(Imm32(1), lengthAddr);
+        masm.bind(&dontUpdate);
+    }
 
     if ((ins->isStoreElementHoleT() || ins->isFallibleStoreElementT()) &&
-        valueType != MIRType::Double)
+        unboxedType == JSVAL_TYPE_MAGIC && valueType != MIRType::Double)
     {
         // The inline path for StoreElementHoleT and FallibleStoreElementT does not always store
         // the type tag, so we do the store on the OOL path. We use MIRType::None for the element
@@ -8322,7 +8511,7 @@ CodeGenerator::visitOutOfLineStoreElementHole(OutOfLineStoreElementHole* ool)
     else
         pushArg(ToRegister(index));
     pushArg(object);
-    callVM(SetDenseElementInfo, ins);
+    callVM(SetDenseOrUnboxedArrayElementInfo, ins);
 
     restoreLive(ins);
     masm.jump(ool->rejoin());
@@ -8379,6 +8568,29 @@ CodeGenerator::visitStoreUnboxedPointer(LStoreUnboxedPointer* lir)
     }
 }
 
+typedef bool (*ConvertUnboxedObjectToNativeFn)(JSContext*, JSObject*);
+static const VMFunction ConvertUnboxedPlainObjectToNativeInfo =
+    FunctionInfo<ConvertUnboxedObjectToNativeFn>(UnboxedPlainObject::convertToNative,
+                                                 "UnboxedPlainObject::convertToNative");
+static const VMFunction ConvertUnboxedArrayObjectToNativeInfo =
+    FunctionInfo<ConvertUnboxedObjectToNativeFn>(UnboxedArrayObject::convertToNative,
+                                                 "UnboxedArrayObject::convertToNative");
+
+void
+CodeGenerator::visitConvertUnboxedObjectToNative(LConvertUnboxedObjectToNative* lir)
+{
+    Register object = ToRegister(lir->getOperand(0));
+
+    OutOfLineCode* ool = oolCallVM(lir->mir()->group()->unboxedLayoutDontCheckGeneration().isArray()
+                                   ? ConvertUnboxedArrayObjectToNativeInfo
+                                   : ConvertUnboxedPlainObjectToNativeInfo,
+                                   lir, ArgList(object), StoreNothing());
+
+    masm.branchPtr(Assembler::Equal, Address(object, JSObject::offsetOfGroup()),
+                   ImmGCPtr(lir->mir()->group()), ool->entry());
+    masm.bind(ool->rejoin());
+}
+
 typedef bool (*ArrayPopShiftFn)(JSContext*, HandleObject, MutableHandleValue);
 static const VMFunction ArrayPopDenseInfo =
     FunctionInfo<ArrayPopShiftFn>(jit::ArrayPopDense, "ArrayPopDense");
@@ -8403,11 +8615,20 @@ CodeGenerator::emitArrayPopShift(LInstruction* lir, const MArrayPopShift* mir, R
 
     // Load elements and length, and VM call if length != initializedLength.
     RegisterOrInt32Constant key = RegisterOrInt32Constant(lengthTemp);
-    masm.loadPtr(Address(obj, NativeObject::offsetOfElements()), elementsTemp);
-    masm.load32(Address(elementsTemp, ObjectElements::offsetOfLength()), lengthTemp);
+    if (mir->unboxedType() == JSVAL_TYPE_MAGIC) {
+        masm.loadPtr(Address(obj, NativeObject::offsetOfElements()), elementsTemp);
+        masm.load32(Address(elementsTemp, ObjectElements::offsetOfLength()), lengthTemp);
 
-    Address initLength(elementsTemp, ObjectElements::offsetOfInitializedLength());
-    masm.branch32(Assembler::NotEqual, initLength, key, ool->entry());
+        Address initLength(elementsTemp, ObjectElements::offsetOfInitializedLength());
+        masm.branch32(Assembler::NotEqual, initLength, key, ool->entry());
+    } else {
+        masm.loadPtr(Address(obj, UnboxedArrayObject::offsetOfElements()), elementsTemp);
+        masm.load32(Address(obj, UnboxedArrayObject::offsetOfCapacityIndexAndInitializedLength()), lengthTemp);
+        masm.and32(Imm32(UnboxedArrayObject::InitializedLengthMask), lengthTemp);
+
+        Address lengthAddr(obj, UnboxedArrayObject::offsetOfLength());
+        masm.branch32(Assembler::NotEqual, lengthAddr, key, ool->entry());
+    }
 
     // Test for length != 0. On zero length either take a VM call or generate
     // an undefined value, depending on whether the call is known to produce
@@ -8419,10 +8640,13 @@ CodeGenerator::emitArrayPopShift(LInstruction* lir, const MArrayPopShift* mir, R
 
         // According to the spec we need to set the length 0 (which is already 0).
         // This is observable when the array length is made non-writable.
-        // Handle this case in the OOL.
-        Address elementFlags(elementsTemp, ObjectElements::offsetOfFlags());
-        Imm32 bit(ObjectElements::NONWRITABLE_ARRAY_LENGTH);
-        masm.branchTest32(Assembler::NonZero, elementFlags, bit, ool->entry());
+        // Handle this case in the OOL. When freezing an unboxed array it is converted
+        // to an normal array.
+        if (mir->unboxedType() == JSVAL_TYPE_MAGIC) {
+            Address elementFlags(elementsTemp, ObjectElements::offsetOfFlags());
+            Imm32 bit(ObjectElements::NONWRITABLE_ARRAY_LENGTH);
+            masm.branchTest32(Assembler::NonZero, elementFlags, bit, ool->entry());
+        }
 
         masm.moveValue(UndefinedValue(), out.valueReg());
         masm.jump(&done);
@@ -8434,25 +8658,41 @@ CodeGenerator::emitArrayPopShift(LInstruction* lir, const MArrayPopShift* mir, R
     masm.dec32(&key);
 
     if (mir->mode() == MArrayPopShift::Pop) {
-        BaseIndex addr(elementsTemp, lengthTemp, TimesEight);
-        masm.loadElementTypedOrValue(addr, out, mir->needsHoleCheck(), ool->entry());
+        if (mir->unboxedType() == JSVAL_TYPE_MAGIC) {
+            BaseIndex addr(elementsTemp, lengthTemp, TimesEight);
+            masm.loadElementTypedOrValue(addr, out, mir->needsHoleCheck(), ool->entry());
+        } else {
+            size_t elemSize = UnboxedTypeSize(mir->unboxedType());
+            BaseIndex addr(elementsTemp, lengthTemp, ScaleFromElemWidth(elemSize));
+            masm.loadUnboxedProperty(addr, mir->unboxedType(), out);
+        }
     } else {
         MOZ_ASSERT(mir->mode() == MArrayPopShift::Shift);
         Address addr(elementsTemp, 0);
-        masm.loadElementTypedOrValue(addr, out, mir->needsHoleCheck(), ool->entry());
+        if (mir->unboxedType() == JSVAL_TYPE_MAGIC)
+            masm.loadElementTypedOrValue(addr, out, mir->needsHoleCheck(), ool->entry());
+        else
+            masm.loadUnboxedProperty(addr, mir->unboxedType(), out);
     }
 
-    // Handle the failure case when the array length is non-writable in the
-    // OOL path.  (Unlike in the adding-an-element cases, we can't rely on the
-    // capacity <= length invariant for such arrays to avoid an explicit
-    // check.)
-    Address elementFlags(elementsTemp, ObjectElements::offsetOfFlags());
-    Imm32 bit(ObjectElements::NONWRITABLE_ARRAY_LENGTH);
-    masm.branchTest32(Assembler::NonZero, elementFlags, bit, ool->entry());
+    if (mir->unboxedType() == JSVAL_TYPE_MAGIC) {
+        // Handle the failure case when the array length is non-writable in the
+        // OOL path.  (Unlike in the adding-an-element cases, we can't rely on the
+        // capacity <= length invariant for such arrays to avoid an explicit
+        // check.)
+        Address elementFlags(elementsTemp, ObjectElements::offsetOfFlags());
+        Imm32 bit(ObjectElements::NONWRITABLE_ARRAY_LENGTH);
+        masm.branchTest32(Assembler::NonZero, elementFlags, bit, ool->entry());
 
-    // Now adjust length and initializedLength.
-    masm.store32(lengthTemp, Address(elementsTemp, ObjectElements::offsetOfLength()));
-    masm.store32(lengthTemp, Address(elementsTemp, ObjectElements::offsetOfInitializedLength()));
+        // Now adjust length and initializedLength.
+        masm.store32(lengthTemp, Address(elementsTemp, ObjectElements::offsetOfLength()));
+        masm.store32(lengthTemp, Address(elementsTemp, ObjectElements::offsetOfInitializedLength()));
+    } else {
+        // Unboxed arrays always have writable lengths. Adjust length and
+        // initializedLength.
+        masm.store32(lengthTemp, Address(obj, UnboxedArrayObject::offsetOfLength()));
+        masm.add32(Imm32(-1), Address(obj, UnboxedArrayObject::offsetOfCapacityIndexAndInitializedLength()));
+    }
 
     if (mir->mode() == MArrayPopShift::Shift) {
         // Don't save the temp registers.
@@ -8491,7 +8731,7 @@ CodeGenerator::visitArrayPopShiftT(LArrayPopShiftT* lir)
     emitArrayPopShift(lir, lir->mir(), obj, elements, length, out);
 }
 
-typedef bool (*ArrayPushDenseFn)(JSContext*, HandleArrayObject, HandleValue, uint32_t*);
+typedef bool (*ArrayPushDenseFn)(JSContext*, HandleObject, HandleValue, uint32_t*);
 static const VMFunction ArrayPushDenseInfo =
     FunctionInfo<ArrayPushDenseFn>(jit::ArrayPushDense, "ArrayPushDense");
 
@@ -8502,27 +8742,50 @@ CodeGenerator::emitArrayPush(LInstruction* lir, const MArrayPush* mir, Register
     OutOfLineCode* ool = oolCallVM(ArrayPushDenseInfo, lir, ArgList(obj, value), StoreRegisterTo(length));
 
     RegisterOrInt32Constant key = RegisterOrInt32Constant(length);
+    if (mir->unboxedType() == JSVAL_TYPE_MAGIC) {
+        // Load elements and length.
+        masm.loadPtr(Address(obj, NativeObject::offsetOfElements()), elementsTemp);
+        masm.load32(Address(elementsTemp, ObjectElements::offsetOfLength()), length);
 
-    // Load elements and length.
-    masm.loadPtr(Address(obj, NativeObject::offsetOfElements()), elementsTemp);
-    masm.load32(Address(elementsTemp, ObjectElements::offsetOfLength()), length);
+        // Guard length == initializedLength.
+        Address initLength(elementsTemp, ObjectElements::offsetOfInitializedLength());
+        masm.branch32(Assembler::NotEqual, initLength, key, ool->entry());
 
-    // Guard length == initializedLength.
-    Address initLength(elementsTemp, ObjectElements::offsetOfInitializedLength());
-    masm.branch32(Assembler::NotEqual, initLength, key, ool->entry());
+        // Guard length < capacity.
+        Address capacity(elementsTemp, ObjectElements::offsetOfCapacity());
+        masm.branch32(Assembler::BelowOrEqual, capacity, key, ool->entry());
 
-    // Guard length < capacity.
-    Address capacity(elementsTemp, ObjectElements::offsetOfCapacity());
-    masm.branch32(Assembler::BelowOrEqual, capacity, key, ool->entry());
+        // Do the store.
+        masm.storeConstantOrRegister(value, BaseIndex(elementsTemp, length, TimesEight));
+    } else {
+        // Load initialized length.
+        masm.load32(Address(obj, UnboxedArrayObject::offsetOfCapacityIndexAndInitializedLength()), length);
+        masm.and32(Imm32(UnboxedArrayObject::InitializedLengthMask), length);
 
-    // Do the store.
-    masm.storeConstantOrRegister(value, BaseIndex(elementsTemp, length, TimesEight));
+        // Guard length == initializedLength.
+        Address lengthAddr(obj, UnboxedArrayObject::offsetOfLength());
+        masm.branch32(Assembler::NotEqual, lengthAddr, key, ool->entry());
+
+        // Guard length < capacity.
+        masm.checkUnboxedArrayCapacity(obj, key, elementsTemp, ool->entry());
+
+        // Load elements and do the store.
+        masm.loadPtr(Address(obj, UnboxedArrayObject::offsetOfElements()), elementsTemp);
+        size_t elemSize = UnboxedTypeSize(mir->unboxedType());
+        BaseIndex addr(elementsTemp, length, ScaleFromElemWidth(elemSize));
+        masm.storeUnboxedProperty(addr, mir->unboxedType(), value, nullptr);
+    }
 
     masm.inc32(&key);
 
     // Update length and initialized length.
-    masm.store32(length, Address(elementsTemp, ObjectElements::offsetOfLength()));
-    masm.store32(length, Address(elementsTemp, ObjectElements::offsetOfInitializedLength()));
+    if (mir->unboxedType() == JSVAL_TYPE_MAGIC) {
+        masm.store32(length, Address(elementsTemp, ObjectElements::offsetOfLength()));
+        masm.store32(length, Address(elementsTemp, ObjectElements::offsetOfInitializedLength()));
+    } else {
+        masm.store32(length, Address(obj, UnboxedArrayObject::offsetOfLength()));
+        masm.add32(Imm32(1), Address(obj, UnboxedArrayObject::offsetOfCapacityIndexAndInitializedLength()));
+    }
 
     masm.bind(ool->rejoin());
 }
@@ -8671,11 +8934,11 @@ CodeGenerator::visitIteratorStartO(LIteratorStartO* lir)
     masm.loadPtr(Address(niTemp, offsetof(NativeIterator, guard_array)), temp2);
 
     // Compare object with the first receiver guard. The last iterator can only
-    // match for native objects.
+    // match for native objects and unboxed objects.
     {
         Address groupAddr(temp2, offsetof(ReceiverGuard, group));
         Address shapeAddr(temp2, offsetof(ReceiverGuard, shape));
-        Label guardDone, shapeMismatch;
+        Label guardDone, shapeMismatch, noExpando;
         masm.loadObjShape(obj, temp1);
         masm.branchPtr(Assembler::NotEqual, shapeAddr, temp1, &shapeMismatch);
 
@@ -8687,6 +8950,12 @@ CodeGenerator::visitIteratorStartO(LIteratorStartO* lir)
         masm.bind(&shapeMismatch);
         masm.loadObjGroup(obj, temp1);
         masm.branchPtr(Assembler::NotEqual, groupAddr, temp1, ool->entry());
+        masm.loadPtr(Address(obj, UnboxedPlainObject::offsetOfExpando()), temp1);
+        masm.branchTestPtr(Assembler::Zero, temp1, temp1, &noExpando);
+        branchIfNotEmptyObjectElements(temp1, ool->entry());
+        masm.loadObjShape(temp1, temp1);
+        masm.bind(&noExpando);
+        masm.branchPtr(Assembler::NotEqual, shapeAddr, temp1, ool->entry());
         masm.bind(&guardDone);
     }
 
@@ -10343,11 +10612,22 @@ CodeGenerator::visitLoadElementHole(LLoadElementHole* lir)
     else
         masm.branch32(Assembler::BelowOrEqual, initLength, ToRegister(lir->index()), &undefined);
 
-    if (lir->index()->isConstant()) {
-        NativeObject::elementsSizeMustNotOverflow();
-        masm.loadValue(Address(elements, ToInt32(lir->index()) * sizeof(Value)), out);
+    if (mir->unboxedType() != JSVAL_TYPE_MAGIC) {
+        size_t width = UnboxedTypeSize(mir->unboxedType());
+        if (lir->index()->isConstant()) {
+            Address addr(elements, ToInt32(lir->index()) * width);
+            masm.loadUnboxedProperty(addr, mir->unboxedType(), out);
+        } else {
+            BaseIndex addr(elements, ToRegister(lir->index()), ScaleFromElemWidth(width));
+            masm.loadUnboxedProperty(addr, mir->unboxedType(), out);
+        }
     } else {
-        masm.loadValue(BaseObjectElementIndex(elements, ToRegister(lir->index())), out);
+        if (lir->index()->isConstant()) {
+            NativeObject::elementsSizeMustNotOverflow();
+            masm.loadValue(Address(elements, ToInt32(lir->index()) * sizeof(Value)), out);
+        } else {
+            masm.loadValue(BaseObjectElementIndex(elements, ToRegister(lir->index())), out);
+        }
     }
 
     // If a hole check is needed, and the value wasn't a hole, we're done.
@@ -10725,7 +11005,7 @@ CodeGenerator::visitInArray(LInArray* lir)
         }
 
         masm.branch32(Assembler::BelowOrEqual, initLength, Imm32(index), failedInitLength);
-        if (mir->needsHoleCheck()) {
+        if (mir->needsHoleCheck() && mir->unboxedType() == JSVAL_TYPE_MAGIC) {
             NativeObject::elementsSizeMustNotOverflow();
             Address address = Address(elements, index * sizeof(Value));
             masm.branchTestMagic(Assembler::Equal, address, &falseBranch);
@@ -10738,7 +11018,7 @@ CodeGenerator::visitInArray(LInArray* lir)
             failedInitLength = &negativeIntCheck;
 
         masm.branch32(Assembler::BelowOrEqual, initLength, index, failedInitLength);
-        if (mir->needsHoleCheck()) {
+        if (mir->needsHoleCheck() && mir->unboxedType() == JSVAL_TYPE_MAGIC) {
             BaseIndex address = BaseIndex(elements, ToRegister(lir->index()), TimesEight);
             masm.branchTestMagic(Assembler::Equal, address, &falseBranch);
         }
diff --git a/js/src/jit/CodeGenerator.h b/js/src/jit/CodeGenerator.h
index 65acfe274..6a5c7f34f 100644
--- a/js/src/jit/CodeGenerator.h
+++ b/js/src/jit/CodeGenerator.h
@@ -148,6 +148,8 @@ class CodeGenerator final : public CodeGeneratorSpecific
     void visitMaybeCopyElementsForWrite(LMaybeCopyElementsForWrite* lir);
     void visitGuardObjectIdentity(LGuardObjectIdentity* guard);
     void visitGuardReceiverPolymorphic(LGuardReceiverPolymorphic* lir);
+    void visitGuardUnboxedExpando(LGuardUnboxedExpando* lir);
+    void visitLoadUnboxedExpando(LLoadUnboxedExpando* lir);
     void visitTypeBarrierV(LTypeBarrierV* lir);
     void visitTypeBarrierO(LTypeBarrierO* lir);
     void visitMonitorTypes(LMonitorTypes* lir);
@@ -234,6 +236,10 @@ class CodeGenerator final : public CodeGeneratorSpecific
     void visitSubstr(LSubstr* lir);
     void visitInitializedLength(LInitializedLength* lir);
     void visitSetInitializedLength(LSetInitializedLength* lir);
+    void visitUnboxedArrayLength(LUnboxedArrayLength* lir);
+    void visitUnboxedArrayInitializedLength(LUnboxedArrayInitializedLength* lir);
+    void visitIncrementUnboxedArrayInitializedLength(LIncrementUnboxedArrayInitializedLength* lir);
+    void visitSetUnboxedArrayInitializedLength(LSetUnboxedArrayInitializedLength* lir);
     void visitNotO(LNotO* ins);
     void visitNotV(LNotV* ins);
     void visitBoundsCheck(LBoundsCheck* lir);
@@ -305,6 +311,7 @@ class CodeGenerator final : public CodeGeneratorSpecific
     void visitFallibleStoreElementV(LFallibleStoreElementV* lir);
     void visitFallibleStoreElementT(LFallibleStoreElementT* lir);
     void visitStoreUnboxedPointer(LStoreUnboxedPointer* lir);
+    void visitConvertUnboxedObjectToNative(LConvertUnboxedObjectToNative* lir);
     void emitArrayPopShift(LInstruction* lir, const MArrayPopShift* mir, Register obj,
                            Register elementsTemp, Register lengthTemp, TypedOrValueRegister out);
     void visitArrayPopShiftV(LArrayPopShiftV* lir);
diff --git a/js/src/jit/IonAnalysis.cpp b/js/src/jit/IonAnalysis.cpp
index a4724bca4..3c0f2c4b3 100644
--- a/js/src/jit/IonAnalysis.cpp
+++ b/js/src/jit/IonAnalysis.cpp
@@ -3515,6 +3515,8 @@ PassthroughOperand(MDefinition* def)
         return def->toConvertElementsToDoubles()->elements();
     if (def->isMaybeCopyElementsForWrite())
         return def->toMaybeCopyElementsForWrite()->object();
+    if (def->isConvertUnboxedObjectToNative())
+        return def->toConvertUnboxedObjectToNative()->object();
     return nullptr;
 }
 
@@ -4060,7 +4062,7 @@ AnalyzePoppedThis(JSContext* cx, DPAConstraintInfo& constraintInfo, ObjectGroup*
         // Add the property to the object, being careful not to update type information.
         DebugOnly<unsigned> slotSpan = baseobj->slotSpan();
         MOZ_ASSERT(!baseobj->containsPure(id));
-        if (!NativeObject::addDataProperty(cx, baseobj, id, baseobj->slotSpan(), JSPROP_ENUMERATE))
+        if (!baseobj->addDataProperty(cx, id, baseobj->slotSpan(), JSPROP_ENUMERATE))
             return false;
         MOZ_ASSERT(baseobj->slotSpan() != slotSpan);
         MOZ_ASSERT(!baseobj->inDictionaryMode());
diff --git a/js/src/jit/IonBuilder.cpp b/js/src/jit/IonBuilder.cpp
index 1e12f5dbe..f08baf865 100644
--- a/js/src/jit/IonBuilder.cpp
+++ b/js/src/jit/IonBuilder.cpp
@@ -32,6 +32,7 @@
 #include "vm/EnvironmentObject-inl.h"
 #include "vm/NativeObject-inl.h"
 #include "vm/ObjectGroup-inl.h"
+#include "vm/UnboxedObject-inl.h"
 
 using namespace js;
 using namespace js::jit;
@@ -2227,8 +2228,6 @@ IonBuilder::inspectOpcode(JSOp op)
         // update that stale value.
 #endif
       default:
-	    // Any unused opcodes and JSOP_LIMIT will end up here without having
-        // to explicitly specify
         break;
     }
 
@@ -6402,7 +6401,7 @@ IonBuilder::createThisScriptedSingleton(JSFunction* target, MDefinition* callee)
     JSObject* templateObject = inspector->getTemplateObject(pc);
     if (!templateObject)
         return nullptr;
-    if (!templateObject->is<PlainObject>())
+    if (!templateObject->is<PlainObject>() && !templateObject->is<UnboxedPlainObject>())
         return nullptr;
     if (templateObject->staticPrototype() != proto)
         return nullptr;
@@ -6439,7 +6438,7 @@ IonBuilder::createThisScriptedBaseline(MDefinition* callee)
     JSObject* templateObject = inspector->getTemplateObject(pc);
     if (!templateObject)
         return nullptr;
-    if (!templateObject->is<PlainObject>())
+    if (!templateObject->is<PlainObject>() && !templateObject->is<UnboxedPlainObject>())
         return nullptr;
 
     Shape* shape = target->lookupPure(compartment->runtime()->names().prototype);
@@ -7355,6 +7354,12 @@ IonBuilder::newArrayTryTemplateObject(bool* emitted, JSObject* templateObject, u
     if (!templateObject)
         return true;
 
+    if (templateObject->is<UnboxedArrayObject>()) {
+        MOZ_ASSERT(templateObject->as<UnboxedArrayObject>().capacity() >= length);
+        if (!templateObject->as<UnboxedArrayObject>().hasInlineElements())
+            return true;
+    }
+
     MOZ_ASSERT(length <= NativeObject::MAX_DENSE_ELEMENTS_COUNT);
 
     size_t arraySlots =
@@ -7610,6 +7615,7 @@ IonBuilder::jsop_initelem_array()
     // intializer, and that arrays are marked as non-packed when writing holes
     // to them during initialization.
     bool needStub = false;
+    JSValueType unboxedType = JSVAL_TYPE_MAGIC;
     if (shouldAbortOnPreliminaryGroups(obj)) {
         needStub = true;
     } else if (!obj->resultTypeSet() ||
@@ -7620,6 +7626,12 @@ IonBuilder::jsop_initelem_array()
     } else {
         MOZ_ASSERT(obj->resultTypeSet()->getObjectCount() == 1);
         TypeSet::ObjectKey* initializer = obj->resultTypeSet()->getObject(0);
+        if (initializer->clasp() == &UnboxedArrayObject::class_) {
+            if (initializer->group()->unboxedLayout().nativeGroup())
+                needStub = true;
+            else
+                unboxedType = initializer->group()->unboxedLayout().elementType();
+        }
         if (value->type() == MIRType::MagicHole) {
             if (!initializer->hasFlags(constraints(), OBJECT_FLAG_NON_PACKED))
                 needStub = true;
@@ -7639,46 +7651,60 @@ IonBuilder::jsop_initelem_array()
         return resumeAfter(store);
     }
 
-    return initializeArrayElement(obj, index, value, /* addResumePoint = */ true);
+    return initializeArrayElement(obj, index, value, unboxedType, /* addResumePoint = */ true);
 }
 
 bool
 IonBuilder::initializeArrayElement(MDefinition* obj, size_t index, MDefinition* value,
+                                   JSValueType unboxedType,
                                    bool addResumePointAndIncrementInitializedLength)
 {
     MConstant* id = MConstant::New(alloc(), Int32Value(index));
     current->add(id);
 
     // Get the elements vector.
-    MElements* elements = MElements::New(alloc(), obj);
+    MElements* elements = MElements::New(alloc(), obj, unboxedType != JSVAL_TYPE_MAGIC);
     current->add(elements);
 
-    if (NeedsPostBarrier(value))
-        current->add(MPostWriteBarrier::New(alloc(), obj, value));
+    if (unboxedType != JSVAL_TYPE_MAGIC) {
+        // Note: storeUnboxedValue takes care of any post barriers on the value.
+        storeUnboxedValue(obj, elements, 0, id, unboxedType, value, /* preBarrier = */ false);
 
-    if ((obj->isNewArray() && obj->toNewArray()->convertDoubleElements()) ||
-        (obj->isNullarySharedStub() &&
-        obj->resultTypeSet()->convertDoubleElements(constraints()) == TemporaryTypeSet::AlwaysConvertToDoubles))
-    {
-        MInstruction* valueDouble = MToDouble::New(alloc(), value);
-        current->add(valueDouble);
-        value = valueDouble;
-    }
+        if (addResumePointAndIncrementInitializedLength) {
+            MInstruction* increment = MIncrementUnboxedArrayInitializedLength::New(alloc(), obj);
+            current->add(increment);
 
-    // Store the value.
-    MStoreElement* store = MStoreElement::New(alloc(), elements, id, value,
+            if (!resumeAfter(increment))
+                return false;
+        }
+    } else {
+        if (NeedsPostBarrier(value))
+            current->add(MPostWriteBarrier::New(alloc(), obj, value));
+
+        if ((obj->isNewArray() && obj->toNewArray()->convertDoubleElements()) ||
+            (obj->isNullarySharedStub() &&
+            obj->resultTypeSet()->convertDoubleElements(constraints()) == TemporaryTypeSet::AlwaysConvertToDoubles))
+        {
+            MInstruction* valueDouble = MToDouble::New(alloc(), value);
+            current->add(valueDouble);
+            value = valueDouble;
+        }
+
+        // Store the value.
+        MStoreElement* store = MStoreElement::New(alloc(), elements, id, value,
                                                   /* needsHoleCheck = */ false);
-    current->add(store);
+        current->add(store);
 
-    if (addResumePointAndIncrementInitializedLength) {
-        // Update the initialized length. (The template object for this
-        // array has the array's ultimate length, so the length field is
-        // already correct: no updating needed.)
-        MSetInitializedLength* initLength = MSetInitializedLength::New(alloc(), elements, id);
-        current->add(initLength);
+        if (addResumePointAndIncrementInitializedLength) {
+            // Update the initialized length. (The template object for this
+            // array has the array's ultimate length, so the length field is
+            // already correct: no updating needed.)
+            MSetInitializedLength* initLength = MSetInitializedLength::New(alloc(), elements, id);
+            current->add(initLength);
 
-        if (!resumeAfter(initLength))
-            return false;
+            if (!resumeAfter(initLength))
+                return false;
+        }
     }
 
     return true;
@@ -7709,6 +7735,8 @@ IonBuilder::jsop_initprop(PropertyName* name)
             if (templateObject->is<PlainObject>()) {
                 if (!templateObject->as<PlainObject>().containsPure(name))
                     useSlowPath = true;
+            } else {
+                MOZ_ASSERT(templateObject->as<UnboxedPlainObject>().layout().lookup(name));
             }
         } else {
             useSlowPath = true;
@@ -8167,7 +8195,9 @@ IonBuilder::maybeMarkEmpty(MDefinition* ins)
 static bool
 ClassHasEffectlessLookup(const Class* clasp)
 {
-    return IsTypedObjectClass(clasp) ||
+    return (clasp == &UnboxedPlainObject::class_) ||
+           (clasp == &UnboxedArrayObject::class_) ||
+           IsTypedObjectClass(clasp) ||
            (clasp->isNative() && !clasp->getOpsLookupProperty());
 }
 
@@ -8997,6 +9027,8 @@ IonBuilder::jsop_getelem()
     }
 
     obj = maybeUnboxForPropertyAccess(obj);
+    if (obj->type() == MIRType::Object)
+        obj = convertUnboxedObjects(obj);
 
     bool emitted = false;
 
@@ -9441,9 +9473,12 @@ IonBuilder::getElemTryDense(bool* emitted, MDefinition* obj, MDefinition* index)
 {
     MOZ_ASSERT(*emitted == false);
 
-    if (!ElementAccessIsDenseNative(constraints(), obj, index)) {
-        trackOptimizationOutcome(TrackedOutcome::AccessNotDense);
-        return true;
+    JSValueType unboxedType = UnboxedArrayElementType(constraints(), obj, index);
+    if (unboxedType == JSVAL_TYPE_MAGIC) {
+        if (!ElementAccessIsDenseNative(constraints(), obj, index)) {
+            trackOptimizationOutcome(TrackedOutcome::AccessNotDense);
+            return true;
+        }
     }
 
     // Don't generate a fast path if there have been bounds check failures
@@ -9460,7 +9495,7 @@ IonBuilder::getElemTryDense(bool* emitted, MDefinition* obj, MDefinition* index)
         return true;
     }
 
-    if (!jsop_getelem_dense(obj, index))
+    if (!jsop_getelem_dense(obj, index, unboxedType))
         return false;
 
     trackOptimizationSuccess();
@@ -9812,7 +9847,7 @@ IonBuilder::computeHeapType(const TemporaryTypeSet* objTypes, const jsid id)
 }
 
 bool
-IonBuilder::jsop_getelem_dense(MDefinition* obj, MDefinition* index)
+IonBuilder::jsop_getelem_dense(MDefinition* obj, MDefinition* index, JSValueType unboxedType)
 {
     TemporaryTypeSet* types = bytecodeTypes(pc);
 
@@ -9836,7 +9871,7 @@ IonBuilder::jsop_getelem_dense(MDefinition* obj, MDefinition* index)
         !ElementAccessHasExtraIndexedProperty(this, obj);
 
     MIRType knownType = MIRType::Value;
-    if (barrier == BarrierKind::NoBarrier)
+    if (unboxedType == JSVAL_TYPE_MAGIC && barrier == BarrierKind::NoBarrier)
         knownType = GetElemKnownType(needsHoleCheck, types);
 
     // Ensure index is an integer.
@@ -9845,13 +9880,13 @@ IonBuilder::jsop_getelem_dense(MDefinition* obj, MDefinition* index)
     index = idInt32;
 
     // Get the elements vector.
-    MInstruction* elements = MElements::New(alloc(), obj);
+    MInstruction* elements = MElements::New(alloc(), obj, unboxedType != JSVAL_TYPE_MAGIC);
     current->add(elements);
 
     // Note: to help GVN, use the original MElements instruction and not
     // MConvertElementsToDoubles as operand. This is fine because converting
     // elements to double does not change the initialized length.
-    MInstruction* initLength = initializedLength(obj, elements);
+    MInstruction* initLength = initializedLength(obj, elements, unboxedType);
 
     // If we can load the element as a definite double, make sure to check that
     // the array has been converted to homogenous doubles first.
@@ -9867,6 +9902,7 @@ IonBuilder::jsop_getelem_dense(MDefinition* obj, MDefinition* index)
     }
 
     bool loadDouble =
+        unboxedType == JSVAL_TYPE_MAGIC &&
         barrier == BarrierKind::NoBarrier &&
         loopDepth_ &&
         inBounds &&
@@ -9885,13 +9921,18 @@ IonBuilder::jsop_getelem_dense(MDefinition* obj, MDefinition* index)
         // hoisting.
         index = addBoundsCheck(index, initLength);
 
-        load = MLoadElement::New(alloc(), elements, index, needsHoleCheck, loadDouble);
-        current->add(load);
+        if (unboxedType != JSVAL_TYPE_MAGIC) {
+            load = loadUnboxedValue(elements, 0, index, unboxedType, barrier, types);
+        } else {
+            load = MLoadElement::New(alloc(), elements, index, needsHoleCheck, loadDouble);
+            current->add(load);
+        }
     } else {
         // This load may return undefined, so assume that we *can* read holes,
         // or that we can read out-of-bounds accesses. In this case, the bounds
         // check is part of the opcode.
-        load = MLoadElementHole::New(alloc(), elements, index, initLength, needsHoleCheck);
+        load = MLoadElementHole::New(alloc(), elements, index, initLength,
+                                     unboxedType, needsHoleCheck);
         current->add(load);
 
         // If maybeUndefined was true, the typeset must have undefined, and
@@ -9901,7 +9942,8 @@ IonBuilder::jsop_getelem_dense(MDefinition* obj, MDefinition* index)
     }
 
     if (knownType != MIRType::Value) {
-        load->setResultType(knownType);
+        if (unboxedType == JSVAL_TYPE_MAGIC)
+            load->setResultType(knownType);
         load->setResultTypeSet(types);
     }
 
@@ -10110,7 +10152,7 @@ IonBuilder::jsop_setelem()
 
     MDefinition* value = current->pop();
     MDefinition* index = current->pop();
-    MDefinition* object = current->pop();
+    MDefinition* object = convertUnboxedObjects(current->pop());
 
     trackTypeInfo(TrackedTypeSite::Receiver, object->type(), object->resultTypeSet());
     trackTypeInfo(TrackedTypeSite::Index, index->type(), index->resultTypeSet());
@@ -10348,9 +10390,12 @@ IonBuilder::setElemTryDense(bool* emitted, MDefinition* object,
 {
     MOZ_ASSERT(*emitted == false);
 
-    if (!ElementAccessIsDenseNative(constraints(), object, index)) {
-        trackOptimizationOutcome(TrackedOutcome::AccessNotDense);
-        return true;
+    JSValueType unboxedType = UnboxedArrayElementType(constraints(), object, index);
+    if (unboxedType == JSVAL_TYPE_MAGIC) {
+        if (!ElementAccessIsDenseNative(constraints(), object, index)) {
+            trackOptimizationOutcome(TrackedOutcome::AccessNotDense);
+            return true;
+        }
     }
 
     if (PropertyWriteNeedsTypeBarrier(alloc(), constraints(), current,
@@ -10384,7 +10429,7 @@ IonBuilder::setElemTryDense(bool* emitted, MDefinition* object,
     }
 
     // Emit dense setelem variant.
-    if (!jsop_setelem_dense(conversion, object, index, value, writeHole, emitted))
+    if (!jsop_setelem_dense(conversion, object, index, value, unboxedType, writeHole, emitted))
         return false;
 
     if (!*emitted) {
@@ -10474,11 +10519,13 @@ IonBuilder::setElemTryCache(bool* emitted, MDefinition* object,
 bool
 IonBuilder::jsop_setelem_dense(TemporaryTypeSet::DoubleConversion conversion,
                                MDefinition* obj, MDefinition* id, MDefinition* value,
-                               bool writeHole, bool* emitted)
+                               JSValueType unboxedType, bool writeHole, bool* emitted)
 {
     MOZ_ASSERT(*emitted == false);
 
-    MIRType elementType = DenseNativeElementType(constraints(), obj);
+    MIRType elementType = MIRType::None;
+    if (unboxedType == JSVAL_TYPE_MAGIC)
+        elementType = DenseNativeElementType(constraints(), obj);
     bool packed = ElementAccessIsPacked(constraints(), obj);
 
     // Writes which are on holes in the object do not have to bail out if they
@@ -10508,7 +10555,7 @@ IonBuilder::jsop_setelem_dense(TemporaryTypeSet::DoubleConversion conversion,
     obj = addMaybeCopyElementsForWrite(obj, /* checkNative = */ false);
 
     // Get the elements vector.
-    MElements* elements = MElements::New(alloc(), obj);
+    MElements* elements = MElements::New(alloc(), obj, unboxedType != JSVAL_TYPE_MAGIC);
     current->add(elements);
 
     // Ensure the value is a double, if double conversion might be needed.
@@ -10545,7 +10592,7 @@ IonBuilder::jsop_setelem_dense(TemporaryTypeSet::DoubleConversion conversion,
     MInstruction* store;
     MStoreElementCommon* common = nullptr;
     if (writeHole && hasNoExtraIndexedProperty && !mayBeFrozen) {
-        MStoreElementHole* ins = MStoreElementHole::New(alloc(), obj, elements, id, newValue);
+        MStoreElementHole* ins = MStoreElementHole::New(alloc(), obj, elements, id, newValue, unboxedType);
         store = ins;
         common = ins;
 
@@ -10557,23 +10604,27 @@ IonBuilder::jsop_setelem_dense(TemporaryTypeSet::DoubleConversion conversion,
 
         bool strict = IsStrictSetPC(pc);
         MFallibleStoreElement* ins = MFallibleStoreElement::New(alloc(), obj, elements, id,
-                                                                newValue, strict);
+                                                                newValue, unboxedType, strict);
         store = ins;
         common = ins;
 
         current->add(ins);
         current->push(value);
     } else {
-        MInstruction* initLength = initializedLength(obj, elements);
+        MInstruction* initLength = initializedLength(obj, elements, unboxedType);
 
         id = addBoundsCheck(id, initLength);
         bool needsHoleCheck = !packed && !hasNoExtraIndexedProperty;
 
-        MStoreElement* ins = MStoreElement::New(alloc(), elements, id, newValue, needsHoleCheck);
-        store = ins;
-        common = ins;
+        if (unboxedType != JSVAL_TYPE_MAGIC) {
+            store = storeUnboxedValue(obj, elements, 0, id, unboxedType, newValue);
+        } else {
+            MStoreElement* ins = MStoreElement::New(alloc(), elements, id, newValue, needsHoleCheck);
+            store = ins;
+            common = ins;
 
-        current->add(store);
+            current->add(store);
+        }
 
         current->push(value);
     }
@@ -10691,6 +10742,18 @@ IonBuilder::jsop_length_fastPath()
             return true;
         }
 
+        // Compute the length for unboxed array objects.
+        if (UnboxedArrayElementType(constraints(), obj, nullptr) != JSVAL_TYPE_MAGIC &&
+            !objTypes->hasObjectFlags(constraints(), OBJECT_FLAG_LENGTH_OVERFLOW))
+        {
+            current->pop();
+
+            MUnboxedArrayLength* length = MUnboxedArrayLength::New(alloc(), obj);
+            current->add(length);
+            current->push(length);
+            return true;
+        }
+
         // Compute the length for array typed objects.
         TypedObjectPrediction prediction = typedObjectPrediction(obj);
         if (!prediction.isUseless()) {
@@ -10924,8 +10987,11 @@ IonBuilder::getDefiniteSlot(TemporaryTypeSet* types, PropertyName* name, uint32_
         }
 
         // Definite slots will always be fixed slots when they are in the
-        // allowable range for fixed slots.
+        // allowable range for fixed slots, except for objects which were
+        // converted from unboxed objects and have a smaller allocation size.
         size_t nfixed = NativeObject::MAX_FIXED_SLOTS;
+        if (ObjectGroup* group = key->group()->maybeOriginalUnboxedGroup())
+            nfixed = gc::GetGCKindSlots(group->unboxedLayout().getAllocKind());
 
         uint32_t propertySlot = property.maybeTypes()->definiteSlot();
         if (slot == UINT32_MAX) {
@@ -10940,6 +11006,65 @@ IonBuilder::getDefiniteSlot(TemporaryTypeSet* types, PropertyName* name, uint32_
     return slot;
 }
 
+uint32_t
+IonBuilder::getUnboxedOffset(TemporaryTypeSet* types, PropertyName* name, JSValueType* punboxedType)
+{
+    if (!types || types->unknownObject() || !types->objectOrSentinel()) {
+        trackOptimizationOutcome(TrackedOutcome::NoTypeInfo);
+        return UINT32_MAX;
+    }
+
+    uint32_t offset = UINT32_MAX;
+
+    for (size_t i = 0; i < types->getObjectCount(); i++) {
+        TypeSet::ObjectKey* key = types->getObject(i);
+        if (!key)
+            continue;
+
+        if (key->unknownProperties()) {
+            trackOptimizationOutcome(TrackedOutcome::UnknownProperties);
+            return UINT32_MAX;
+        }
+
+        if (key->isSingleton()) {
+            trackOptimizationOutcome(TrackedOutcome::Singleton);
+            return UINT32_MAX;
+        }
+
+        UnboxedLayout* layout = key->group()->maybeUnboxedLayout();
+        if (!layout) {
+            trackOptimizationOutcome(TrackedOutcome::NotUnboxed);
+            return UINT32_MAX;
+        }
+
+        const UnboxedLayout::Property* property = layout->lookup(name);
+        if (!property) {
+            trackOptimizationOutcome(TrackedOutcome::StructNoField);
+            return UINT32_MAX;
+        }
+
+        if (layout->nativeGroup()) {
+            trackOptimizationOutcome(TrackedOutcome::UnboxedConvertedToNative);
+            return UINT32_MAX;
+        }
+
+        key->watchStateChangeForUnboxedConvertedToNative(constraints());
+
+        if (offset == UINT32_MAX) {
+            offset = property->offset;
+            *punboxedType = property->type;
+        } else if (offset != property->offset) {
+            trackOptimizationOutcome(TrackedOutcome::InconsistentFieldOffset);
+            return UINT32_MAX;
+        } else if (*punboxedType != property->type) {
+            trackOptimizationOutcome(TrackedOutcome::InconsistentFieldType);
+            return UINT32_MAX;
+        }
+    }
+
+    return offset;
+}
+
 bool
 IonBuilder::jsop_runonce()
 {
@@ -11386,6 +11511,8 @@ IonBuilder::jsop_getprop(PropertyName* name)
     }
 
     obj = maybeUnboxForPropertyAccess(obj);
+    if (obj->type() == MIRType::Object)
+        obj = convertUnboxedObjects(obj);
 
     BarrierKind barrier = PropertyReadNeedsTypeBarrier(analysisContext, constraints(),
                                                        obj, name, types);
@@ -11457,6 +11584,11 @@ IonBuilder::jsop_getprop(PropertyName* name)
         if (!getPropTryDefiniteSlot(&emitted, obj, name, barrier, types) || emitted)
             return emitted;
 
+        // Try to emit loads from unboxed objects.
+        trackOptimizationAttempt(TrackedStrategy::GetProp_Unboxed);
+        if (!getPropTryUnboxed(&emitted, obj, name, barrier, types) || emitted)
+            return emitted;
+
         // Try to inline a common property getter, or make a call.
         trackOptimizationAttempt(TrackedStrategy::GetProp_CommonGetter);
         if (!getPropTryCommonGetter(&emitted, obj, name, types) || emitted)
@@ -11822,6 +11954,49 @@ IonBuilder::getPropTryComplexPropOfTypedObject(bool* emitted,
                                   fieldPrediction, fieldTypeObj);
 }
 
+MDefinition*
+IonBuilder::convertUnboxedObjects(MDefinition* obj)
+{
+    // If obj might be in any particular unboxed group which should be
+    // converted to a native representation, perform that conversion. This does
+    // not guarantee the object will not have such a group afterwards, if the
+    // object's possible groups are not precisely known.
+    TemporaryTypeSet* types = obj->resultTypeSet();
+    if (!types || types->unknownObject() || !types->objectOrSentinel())
+        return obj;
+
+    BaselineInspector::ObjectGroupVector list(alloc());
+    for (size_t i = 0; i < types->getObjectCount(); i++) {
+        TypeSet::ObjectKey* key = obj->resultTypeSet()->getObject(i);
+        if (!key || !key->isGroup())
+            continue;
+
+        if (UnboxedLayout* layout = key->group()->maybeUnboxedLayout()) {
+            AutoEnterOOMUnsafeRegion oomUnsafe;
+            if (layout->nativeGroup() && !list.append(key->group()))
+                oomUnsafe.crash("IonBuilder::convertUnboxedObjects");
+        }
+    }
+
+    return convertUnboxedObjects(obj, list);
+}
+
+MDefinition*
+IonBuilder::convertUnboxedObjects(MDefinition* obj,
+                                  const BaselineInspector::ObjectGroupVector& list)
+{
+    for (size_t i = 0; i < list.length(); i++) {
+        ObjectGroup* group = list[i];
+        if (TemporaryTypeSet* types = obj->resultTypeSet()) {
+            if (!types->hasType(TypeSet::ObjectType(group)))
+                continue;
+        }
+        obj = MConvertUnboxedObjectToNative::New(alloc(), obj, group);
+        current->add(obj->toInstruction());
+    }
+    return obj;
+}
+
 bool
 IonBuilder::getPropTryDefiniteSlot(bool* emitted, MDefinition* obj, PropertyName* name,
                                    BarrierKind barrier, TemporaryTypeSet* types)
@@ -11906,14 +12081,111 @@ IonBuilder::getPropTryModuleNamespace(bool* emitted, MDefinition* obj, PropertyN
     return true;
 }
 
+MInstruction*
+IonBuilder::loadUnboxedProperty(MDefinition* obj, size_t offset, JSValueType unboxedType,
+                                BarrierKind barrier, TemporaryTypeSet* types)
+{
+    // loadUnboxedValue is designed to load any value as if it were contained in
+    // an array. Thus a property offset is converted to an index, when the
+    // object is reinterpreted as an array of properties of the same size.
+    size_t index = offset / UnboxedTypeSize(unboxedType);
+    MInstruction* indexConstant = MConstant::New(alloc(), Int32Value(index));
+    current->add(indexConstant);
+
+    return loadUnboxedValue(obj, UnboxedPlainObject::offsetOfData(),
+                            indexConstant, unboxedType, barrier, types);
+}
+
+MInstruction*
+IonBuilder::loadUnboxedValue(MDefinition* elements, size_t elementsOffset,
+                             MDefinition* index, JSValueType unboxedType,
+                             BarrierKind barrier, TemporaryTypeSet* types)
+{
+    MInstruction* load;
+    switch (unboxedType) {
+      case JSVAL_TYPE_BOOLEAN:
+        load = MLoadUnboxedScalar::New(alloc(), elements, index, Scalar::Uint8,
+                                       DoesNotRequireMemoryBarrier, elementsOffset);
+        load->setResultType(MIRType::Boolean);
+        break;
+
+      case JSVAL_TYPE_INT32:
+        load = MLoadUnboxedScalar::New(alloc(), elements, index, Scalar::Int32,
+                                       DoesNotRequireMemoryBarrier, elementsOffset);
+        load->setResultType(MIRType::Int32);
+        break;
+
+      case JSVAL_TYPE_DOUBLE:
+        load = MLoadUnboxedScalar::New(alloc(), elements, index, Scalar::Float64,
+                                       DoesNotRequireMemoryBarrier, elementsOffset,
+                                       /* canonicalizeDoubles = */ false);
+        load->setResultType(MIRType::Double);
+        break;
+
+      case JSVAL_TYPE_STRING:
+        load = MLoadUnboxedString::New(alloc(), elements, index, elementsOffset);
+        break;
+
+      case JSVAL_TYPE_OBJECT: {
+        MLoadUnboxedObjectOrNull::NullBehavior nullBehavior;
+        if (types->hasType(TypeSet::NullType()))
+            nullBehavior = MLoadUnboxedObjectOrNull::HandleNull;
+        else if (barrier != BarrierKind::NoBarrier)
+            nullBehavior = MLoadUnboxedObjectOrNull::BailOnNull;
+        else
+            nullBehavior = MLoadUnboxedObjectOrNull::NullNotPossible;
+        load = MLoadUnboxedObjectOrNull::New(alloc(), elements, index, nullBehavior,
+                                             elementsOffset);
+        break;
+      }
+
+      default:
+        MOZ_CRASH();
+    }
+
+    current->add(load);
+    return load;
+}
+
+bool
+IonBuilder::getPropTryUnboxed(bool* emitted, MDefinition* obj, PropertyName* name,
+                              BarrierKind barrier, TemporaryTypeSet* types)
+{
+    MOZ_ASSERT(*emitted == false);
+
+    JSValueType unboxedType;
+    uint32_t offset = getUnboxedOffset(obj->resultTypeSet(), name, &unboxedType);
+    if (offset == UINT32_MAX)
+        return true;
+
+    if (obj->type() != MIRType::Object) {
+        MGuardObject* guard = MGuardObject::New(alloc(), obj);
+        current->add(guard);
+        obj = guard;
+    }
+
+    MInstruction* load = loadUnboxedProperty(obj, offset, unboxedType, barrier, types);
+    current->push(load);
+
+    if (!pushTypeBarrier(load, types, barrier))
+        return false;
+
+    trackOptimizationSuccess();
+    *emitted = true;
+    return true;
+}
+
 MDefinition*
 IonBuilder::addShapeGuardsForGetterSetter(MDefinition* obj, JSObject* holder, Shape* holderShape,
                 const BaselineInspector::ReceiverVector& receivers,
+                const BaselineInspector::ObjectGroupVector& convertUnboxedGroups,
                 bool isOwnProperty)
 {
     MOZ_ASSERT(holder);
     MOZ_ASSERT(holderShape);
 
+    obj = convertUnboxedObjects(obj, convertUnboxedGroups);
+
     if (isOwnProperty) {
         MOZ_ASSERT(receivers.empty());
         return addShapeGuard(obj, holderShape, Bailout_ShapeGuard);
@@ -11937,8 +12209,10 @@ IonBuilder::getPropTryCommonGetter(bool* emitted, MDefinition* obj, PropertyName
     JSObject* foundProto = nullptr;
     bool isOwnProperty = false;
     BaselineInspector::ReceiverVector receivers(alloc());
+    BaselineInspector::ObjectGroupVector convertUnboxedGroups(alloc());
     if (!inspector->commonGetPropFunction(pc, &foundProto, &lastProperty, &commonGetter,
-                                          &globalShape, &isOwnProperty, receivers))
+                                          &globalShape, &isOwnProperty,
+                                          receivers, convertUnboxedGroups))
     {
         return true;
     }
@@ -11954,7 +12228,8 @@ IonBuilder::getPropTryCommonGetter(bool* emitted, MDefinition* obj, PropertyName
         // If type information is bad, we can still optimize the getter if we
         // shape guard.
         obj = addShapeGuardsForGetterSetter(obj, foundProto, lastProperty,
-                                            receivers, isOwnProperty);
+                                            receivers, convertUnboxedGroups,
+                                            isOwnProperty);
         if (!obj)
             return false;
     }
@@ -12122,12 +12397,15 @@ IonBuilder::getPropTryInlineAccess(bool* emitted, MDefinition* obj, PropertyName
     MOZ_ASSERT(*emitted == false);
 
     BaselineInspector::ReceiverVector receivers(alloc());
-    if (!inspector->maybeInfoForPropertyOp(pc, receivers))
+    BaselineInspector::ObjectGroupVector convertUnboxedGroups(alloc());
+    if (!inspector->maybeInfoForPropertyOp(pc, receivers, convertUnboxedGroups))
         return false;
 
     if (!canInlinePropertyOpShapes(receivers))
         return true;
 
+    obj = convertUnboxedObjects(obj, convertUnboxedGroups);
+
     MIRType rvalType = types->getKnownMIRType();
     if (barrier != BarrierKind::NoBarrier || IsNullOrUndefined(rvalType))
         rvalType = MIRType::Value;
@@ -12150,6 +12428,45 @@ IonBuilder::getPropTryInlineAccess(bool* emitted, MDefinition* obj, PropertyName
             return true;
         }
 
+        if (receivers[0].shape) {
+            // Monomorphic load from an unboxed object expando.
+            spew("Inlining monomorphic unboxed expando GETPROP");
+
+            obj = addGroupGuard(obj, receivers[0].group, Bailout_ShapeGuard);
+            obj = addUnboxedExpandoGuard(obj, /* hasExpando = */ true, Bailout_ShapeGuard);
+
+            MInstruction* expando = MLoadUnboxedExpando::New(alloc(), obj);
+            current->add(expando);
+
+            expando = addShapeGuard(expando, receivers[0].shape, Bailout_ShapeGuard);
+
+            Shape* shape = receivers[0].shape->searchLinear(NameToId(name));
+            MOZ_ASSERT(shape);
+
+            if (!loadSlot(expando, shape, rvalType, barrier, types))
+                return false;
+
+            trackOptimizationOutcome(TrackedOutcome::Monomorphic);
+            *emitted = true;
+            return true;
+        }
+
+        // Monomorphic load from an unboxed object.
+        ObjectGroup* group = receivers[0].group;
+        if (obj->resultTypeSet() && !obj->resultTypeSet()->hasType(TypeSet::ObjectType(group)))
+            return true;
+
+        obj = addGroupGuard(obj, group, Bailout_ShapeGuard);
+
+        const UnboxedLayout::Property* property = group->unboxedLayout().lookup(name);
+        MInstruction* load = loadUnboxedProperty(obj, property->offset, property->type, barrier, types);
+        current->push(load);
+
+        if (!pushTypeBarrier(load, types, barrier))
+            return false;
+
+        trackOptimizationOutcome(TrackedOutcome::Monomorphic);
+        *emitted = true;
         return true;
     }
 
@@ -12391,7 +12708,7 @@ bool
 IonBuilder::jsop_setprop(PropertyName* name)
 {
     MDefinition* value = current->pop();
-    MDefinition* obj = current->pop();
+    MDefinition* obj = convertUnboxedObjects(current->pop());
 
     bool emitted = false;
     startTrackingOptimizations();
@@ -12424,6 +12741,13 @@ IonBuilder::jsop_setprop(PropertyName* name)
     bool barrier = PropertyWriteNeedsTypeBarrier(alloc(), constraints(), current, &obj, name, &value,
                                                  /* canModify = */ true);
 
+    if (!forceInlineCaches()) {
+        // Try to emit stores to unboxed objects.
+        trackOptimizationAttempt(TrackedStrategy::SetProp_Unboxed);
+        if (!setPropTryUnboxed(&emitted, obj, name, value, barrier, objTypes) || emitted)
+            return emitted;
+    }
+
     // Add post barrier if needed. The instructions above manage any post
     // barriers they need directly.
     if (NeedsPostBarrier(value))
@@ -12457,8 +12781,10 @@ IonBuilder::setPropTryCommonSetter(bool* emitted, MDefinition* obj,
     JSObject* foundProto = nullptr;
     bool isOwnProperty;
     BaselineInspector::ReceiverVector receivers(alloc());
+    BaselineInspector::ObjectGroupVector convertUnboxedGroups(alloc());
     if (!inspector->commonSetPropFunction(pc, &foundProto, &lastProperty, &commonSetter,
-                                          &isOwnProperty, receivers))
+                                          &isOwnProperty,
+                                          receivers, convertUnboxedGroups))
     {
         trackOptimizationOutcome(TrackedOutcome::NoProtoFound);
         return true;
@@ -12473,7 +12799,8 @@ IonBuilder::setPropTryCommonSetter(bool* emitted, MDefinition* obj,
         // If type information is bad, we can still optimize the setter if we
         // shape guard.
         obj = addShapeGuardsForGetterSetter(obj, foundProto, lastProperty,
-                                            receivers, isOwnProperty);
+                                            receivers, convertUnboxedGroups,
+                                            isOwnProperty);
         if (!obj)
             return false;
     }
@@ -12729,6 +13056,100 @@ IonBuilder::setPropTryDefiniteSlot(bool* emitted, MDefinition* obj,
     return true;
 }
 
+MInstruction*
+IonBuilder::storeUnboxedProperty(MDefinition* obj, size_t offset, JSValueType unboxedType,
+                                 MDefinition* value)
+{
+    size_t scaledOffsetConstant = offset / UnboxedTypeSize(unboxedType);
+    MInstruction* scaledOffset = MConstant::New(alloc(), Int32Value(scaledOffsetConstant));
+    current->add(scaledOffset);
+
+    return storeUnboxedValue(obj, obj, UnboxedPlainObject::offsetOfData(),
+                             scaledOffset, unboxedType, value);
+}
+
+MInstruction*
+IonBuilder::storeUnboxedValue(MDefinition* obj, MDefinition* elements, int32_t elementsOffset,
+                              MDefinition* scaledOffset, JSValueType unboxedType,
+                              MDefinition* value, bool preBarrier /* = true */)
+{
+    MInstruction* store;
+    switch (unboxedType) {
+      case JSVAL_TYPE_BOOLEAN:
+        store = MStoreUnboxedScalar::New(alloc(), elements, scaledOffset, value, Scalar::Uint8,
+                                         MStoreUnboxedScalar::DontTruncateInput,
+                                         DoesNotRequireMemoryBarrier, elementsOffset);
+        break;
+
+      case JSVAL_TYPE_INT32:
+        store = MStoreUnboxedScalar::New(alloc(), elements, scaledOffset, value, Scalar::Int32,
+                                         MStoreUnboxedScalar::DontTruncateInput,
+                                         DoesNotRequireMemoryBarrier, elementsOffset);
+        break;
+
+      case JSVAL_TYPE_DOUBLE:
+        store = MStoreUnboxedScalar::New(alloc(), elements, scaledOffset, value, Scalar::Float64,
+                                         MStoreUnboxedScalar::DontTruncateInput,
+                                         DoesNotRequireMemoryBarrier, elementsOffset);
+        break;
+
+      case JSVAL_TYPE_STRING:
+        store = MStoreUnboxedString::New(alloc(), elements, scaledOffset, value,
+                                         elementsOffset, preBarrier);
+        break;
+
+      case JSVAL_TYPE_OBJECT:
+        MOZ_ASSERT(value->type() == MIRType::Object ||
+                   value->type() == MIRType::Null ||
+                   value->type() == MIRType::Value);
+        MOZ_ASSERT(!value->mightBeType(MIRType::Undefined),
+                   "MToObjectOrNull slow path is invalid for unboxed objects");
+        store = MStoreUnboxedObjectOrNull::New(alloc(), elements, scaledOffset, value, obj,
+                                               elementsOffset, preBarrier);
+        break;
+
+      default:
+        MOZ_CRASH();
+    }
+
+    current->add(store);
+    return store;
+}
+
+bool
+IonBuilder::setPropTryUnboxed(bool* emitted, MDefinition* obj,
+                              PropertyName* name, MDefinition* value,
+                              bool barrier, TemporaryTypeSet* objTypes)
+{
+    MOZ_ASSERT(*emitted == false);
+
+    if (barrier) {
+        trackOptimizationOutcome(TrackedOutcome::NeedsTypeBarrier);
+        return true;
+    }
+
+    JSValueType unboxedType;
+    uint32_t offset = getUnboxedOffset(obj->resultTypeSet(), name, &unboxedType);
+    if (offset == UINT32_MAX)
+        return true;
+
+    if (obj->type() != MIRType::Object) {
+        MGuardObject* guard = MGuardObject::New(alloc(), obj);
+        current->add(guard);
+        obj = guard;
+    }
+
+    MInstruction* store = storeUnboxedProperty(obj, offset, unboxedType, value);
+
+    current->push(value);
+
+    if (!resumeAfter(store))
+        return false;
+
+    *emitted = true;
+    return true;
+}
+
 bool
 IonBuilder::setPropTryInlineAccess(bool* emitted, MDefinition* obj,
                                    PropertyName* name, MDefinition* value,
@@ -12742,12 +13163,15 @@ IonBuilder::setPropTryInlineAccess(bool* emitted, MDefinition* obj,
     }
 
     BaselineInspector::ReceiverVector receivers(alloc());
-    if (!inspector->maybeInfoForPropertyOp(pc, receivers))
+    BaselineInspector::ObjectGroupVector convertUnboxedGroups(alloc());
+    if (!inspector->maybeInfoForPropertyOp(pc, receivers, convertUnboxedGroups))
         return false;
 
     if (!canInlinePropertyOpShapes(receivers))
         return true;
 
+    obj = convertUnboxedObjects(obj, convertUnboxedGroups);
+
     if (receivers.length() == 1) {
         if (!receivers[0].group) {
             // Monomorphic store to a native object.
@@ -12767,6 +13191,46 @@ IonBuilder::setPropTryInlineAccess(bool* emitted, MDefinition* obj,
             return true;
         }
 
+        if (receivers[0].shape) {
+            // Monomorphic store to an unboxed object expando.
+            spew("Inlining monomorphic unboxed expando SETPROP");
+
+            obj = addGroupGuard(obj, receivers[0].group, Bailout_ShapeGuard);
+            obj = addUnboxedExpandoGuard(obj, /* hasExpando = */ true, Bailout_ShapeGuard);
+
+            MInstruction* expando = MLoadUnboxedExpando::New(alloc(), obj);
+            current->add(expando);
+
+            expando = addShapeGuard(expando, receivers[0].shape, Bailout_ShapeGuard);
+
+            Shape* shape = receivers[0].shape->searchLinear(NameToId(name));
+            MOZ_ASSERT(shape);
+
+            bool needsBarrier = objTypes->propertyNeedsBarrier(constraints(), NameToId(name));
+            if (!storeSlot(expando, shape, value, needsBarrier))
+                return false;
+
+            trackOptimizationOutcome(TrackedOutcome::Monomorphic);
+            *emitted = true;
+            return true;
+        }
+
+        // Monomorphic store to an unboxed object.
+        spew("Inlining monomorphic unboxed SETPROP");
+
+        ObjectGroup* group = receivers[0].group;
+        if (!objTypes->hasType(TypeSet::ObjectType(group)))
+            return true;
+
+        obj = addGroupGuard(obj, group, Bailout_ShapeGuard);
+
+        const UnboxedLayout::Property* property = group->unboxedLayout().lookup(name);
+        storeUnboxedProperty(obj, property->offset, property->type, value);
+
+        current->push(value);
+
+        trackOptimizationOutcome(TrackedOutcome::Monomorphic);
+        *emitted = true;
         return true;
     }
 
@@ -13465,7 +13929,7 @@ IonBuilder::jsop_setaliasedvar(EnvironmentCoordinate ec)
 bool
 IonBuilder::jsop_in()
 {
-    MDefinition* obj = current->pop();
+    MDefinition* obj = convertUnboxedObjects(current->pop());
     MDefinition* id = current->pop();
 
     bool emitted = false;
@@ -13492,8 +13956,11 @@ IonBuilder::inTryDense(bool* emitted, MDefinition* obj, MDefinition* id)
     if (shouldAbortOnPreliminaryGroups(obj))
         return true;
 
-    if (!ElementAccessIsDenseNative(constraints(), obj, id))
-        return true;
+    JSValueType unboxedType = UnboxedArrayElementType(constraints(), obj, id);
+    if (unboxedType == JSVAL_TYPE_MAGIC) {
+        if (!ElementAccessIsDenseNative(constraints(), obj, id))
+            return true;
+    }
 
     if (ElementAccessHasExtraIndexedProperty(this, obj))
         return true;
@@ -13508,10 +13975,10 @@ IonBuilder::inTryDense(bool* emitted, MDefinition* obj, MDefinition* id)
     id = idInt32;
 
     // Get the elements vector.
-    MElements* elements = MElements::New(alloc(), obj);
+    MElements* elements = MElements::New(alloc(), obj, unboxedType != JSVAL_TYPE_MAGIC);
     current->add(elements);
 
-    MInstruction* initLength = initializedLength(obj, elements);
+    MInstruction* initLength = initializedLength(obj, elements, unboxedType);
 
     // If there are no holes, speculate the InArray check will not fail.
     if (!needsHoleCheck && !failedBoundsCheck_) {
@@ -13521,7 +13988,8 @@ IonBuilder::inTryDense(bool* emitted, MDefinition* obj, MDefinition* id)
     }
 
     // Check if id < initLength and elem[id] not a hole.
-    MInArray* ins = MInArray::New(alloc(), elements, id, initLength, obj, needsHoleCheck);
+    MInArray* ins = MInArray::New(alloc(), elements, id, initLength, obj, needsHoleCheck,
+                                  unboxedType);
 
     current->add(ins);
     current->push(ins);
@@ -13818,6 +14286,19 @@ IonBuilder::addGroupGuard(MDefinition* obj, ObjectGroup* group, BailoutKind bail
 }
 
 MInstruction*
+IonBuilder::addUnboxedExpandoGuard(MDefinition* obj, bool hasExpando, BailoutKind bailoutKind)
+{
+    MGuardUnboxedExpando* guard = MGuardUnboxedExpando::New(alloc(), obj, hasExpando, bailoutKind);
+    current->add(guard);
+
+    // If a shape guard failed in the past, don't optimize group guards.
+    if (failedShapeGuard_)
+        guard->setNotMovable();
+
+    return guard;
+}
+
+MInstruction*
 IonBuilder::addGuardReceiverPolymorphic(MDefinition* obj,
                                         const BaselineInspector::ReceiverVector& receivers)
 {
@@ -13826,6 +14307,15 @@ IonBuilder::addGuardReceiverPolymorphic(MDefinition* obj,
             // Monomorphic guard on a native object.
             return addShapeGuard(obj, receivers[0].shape, Bailout_ShapeGuard);
         }
+
+        if (!receivers[0].shape) {
+            // Guard on an unboxed object that does not have an expando.
+            obj = addGroupGuard(obj, receivers[0].group, Bailout_ShapeGuard);
+            return addUnboxedExpandoGuard(obj, /* hasExpando = */ false, Bailout_ShapeGuard);
+        }
+
+        // Monomorphic receiver guards are not yet supported when the receiver
+        // is an unboxed object with an expando.
     }
 
     MGuardReceiverPolymorphic* guard = MGuardReceiverPolymorphic::New(alloc(), obj);
@@ -14199,24 +14689,32 @@ IonBuilder::constantInt(int32_t i)
 }
 
 MInstruction*
-IonBuilder::initializedLength(MDefinition* obj, MDefinition* elements)
+IonBuilder::initializedLength(MDefinition* obj, MDefinition* elements, JSValueType unboxedType)
 {
-    MInstruction* res = MInitializedLength::New(alloc(), elements);
+    MInstruction* res;
+    if (unboxedType != JSVAL_TYPE_MAGIC)
+        res = MUnboxedArrayInitializedLength::New(alloc(), obj);
+    else
+        res = MInitializedLength::New(alloc(), elements);
     current->add(res);
     return res;
 }
 
 MInstruction*
-IonBuilder::setInitializedLength(MDefinition* obj, size_t count)
+IonBuilder::setInitializedLength(MDefinition* obj, JSValueType unboxedType, size_t count)
 {
     MOZ_ASSERT(count);
 
-    // MSetInitializedLength takes the index of the last element, rather
-    // than the count itself.
-    MInstruction* elements = MElements::New(alloc(), obj, /* unboxed = */ false);
-    current->add(elements);
-    MInstruction* res = 
-	    MSetInitializedLength::New(alloc(), elements, constant(Int32Value(count - 1)));
+    MInstruction* res;
+    if (unboxedType != JSVAL_TYPE_MAGIC) {
+        res = MSetUnboxedArrayInitializedLength::New(alloc(), obj, constant(Int32Value(count)));
+    } else {
+        // MSetInitializedLength takes the index of the last element, rather
+        // than the count itself.
+        MInstruction* elements = MElements::New(alloc(), obj, /* unboxed = */ false);
+        current->add(elements);
+        res = MSetInitializedLength::New(alloc(), elements, constant(Int32Value(count - 1)));
+    }
     current->add(res);
     return res;
 }
diff --git a/js/src/jit/IonBuilder.h b/js/src/jit/IonBuilder.h
index 6a3b61232..f359c764f 100644
--- a/js/src/jit/IonBuilder.h
+++ b/js/src/jit/IonBuilder.h
@@ -346,8 +346,9 @@ class IonBuilder
 
     MConstant* constant(const Value& v);
     MConstant* constantInt(int32_t i);
-    MInstruction* initializedLength(MDefinition* obj, MDefinition* elements);
-    MInstruction* setInitializedLength(MDefinition* obj, size_t count);
+    MInstruction* initializedLength(MDefinition* obj, MDefinition* elements,
+                                    JSValueType unboxedType);
+    MInstruction* setInitializedLength(MDefinition* obj, JSValueType unboxedType, size_t count);
 
     // Improve the type information at tests
     MOZ_MUST_USE bool improveTypesAtTest(MDefinition* ins, bool trueBranch, MTest* test);
@@ -400,6 +401,7 @@ class IonBuilder
     MInstruction* addBoundsCheck(MDefinition* index, MDefinition* length);
     MInstruction* addShapeGuard(MDefinition* obj, Shape* const shape, BailoutKind bailoutKind);
     MInstruction* addGroupGuard(MDefinition* obj, ObjectGroup* group, BailoutKind bailoutKind);
+    MInstruction* addUnboxedExpandoGuard(MDefinition* obj, bool hasExpando, BailoutKind bailoutKind);
     MInstruction* addSharedTypedArrayGuard(MDefinition* obj);
 
     MInstruction*
@@ -439,6 +441,8 @@ class IonBuilder
                                              BarrierKind barrier, TemporaryTypeSet* types);
     MOZ_MUST_USE bool getPropTryModuleNamespace(bool* emitted, MDefinition* obj, PropertyName* name,
                                                 BarrierKind barrier, TemporaryTypeSet* types);
+    MOZ_MUST_USE bool getPropTryUnboxed(bool* emitted, MDefinition* obj, PropertyName* name,
+                                        BarrierKind barrier, TemporaryTypeSet* types);
     MOZ_MUST_USE bool getPropTryCommonGetter(bool* emitted, MDefinition* obj, PropertyName* name,
                                              TemporaryTypeSet* types);
     MOZ_MUST_USE bool getPropTryInlineAccess(bool* emitted, MDefinition* obj, PropertyName* name,
@@ -471,6 +475,9 @@ class IonBuilder
     MOZ_MUST_USE bool setPropTryDefiniteSlot(bool* emitted, MDefinition* obj,
                                              PropertyName* name, MDefinition* value,
                                              bool barrier, TemporaryTypeSet* objTypes);
+    MOZ_MUST_USE bool setPropTryUnboxed(bool* emitted, MDefinition* obj,
+                                        PropertyName* name, MDefinition* value,
+                                        bool barrier, TemporaryTypeSet* objTypes);
     MOZ_MUST_USE bool setPropTryInlineAccess(bool* emitted, MDefinition* obj,
                                              PropertyName* name, MDefinition* value,
                                              bool barrier, TemporaryTypeSet* objTypes);
@@ -610,6 +617,7 @@ class IonBuilder
                                                         TypedObjectPrediction elemTypeReprs,
                                                         uint32_t elemSize);
     MOZ_MUST_USE bool initializeArrayElement(MDefinition* obj, size_t index, MDefinition* value,
+                                             JSValueType unboxedType,
                                              bool addResumePointAndIncrementInitializedLength);
 
     // jsop_getelem() helpers.
@@ -722,13 +730,15 @@ class IonBuilder
     MOZ_MUST_USE bool jsop_bindname(PropertyName* name);
     MOZ_MUST_USE bool jsop_bindvar();
     MOZ_MUST_USE bool jsop_getelem();
-    MOZ_MUST_USE bool jsop_getelem_dense(MDefinition* obj, MDefinition* index);
+    MOZ_MUST_USE bool jsop_getelem_dense(MDefinition* obj, MDefinition* index,
+                                         JSValueType unboxedType);
     MOZ_MUST_USE bool jsop_getelem_typed(MDefinition* obj, MDefinition* index,
                                          ScalarTypeDescr::Type arrayType);
     MOZ_MUST_USE bool jsop_setelem();
     MOZ_MUST_USE bool jsop_setelem_dense(TemporaryTypeSet::DoubleConversion conversion,
                                          MDefinition* object, MDefinition* index,
-                                         MDefinition* value, bool writeHole, bool* emitted);
+                                         MDefinition* value, JSValueType unboxedType,
+                                         bool writeHole, bool* emitted);
     MOZ_MUST_USE bool jsop_setelem_typed(ScalarTypeDescr::Type arrayType,
                                          MDefinition* object, MDefinition* index,
                                          MDefinition* value);
@@ -1033,6 +1043,7 @@ class IonBuilder
     MDefinition*
     addShapeGuardsForGetterSetter(MDefinition* obj, JSObject* holder, Shape* holderShape,
                                   const BaselineInspector::ReceiverVector& receivers,
+                                  const BaselineInspector::ObjectGroupVector& convertUnboxedGroups,
                                   bool isOwnProperty);
 
     MOZ_MUST_USE bool annotateGetPropertyCache(MDefinition* obj, PropertyName* name,
@@ -1050,6 +1061,22 @@ class IonBuilder
     ResultWithOOM<bool> testNotDefinedProperty(MDefinition* obj, jsid id);
 
     uint32_t getDefiniteSlot(TemporaryTypeSet* types, PropertyName* name, uint32_t* pnfixed);
+    MDefinition* convertUnboxedObjects(MDefinition* obj);
+    MDefinition* convertUnboxedObjects(MDefinition* obj,
+                                       const BaselineInspector::ObjectGroupVector& list);
+    uint32_t getUnboxedOffset(TemporaryTypeSet* types, PropertyName* name,
+                              JSValueType* punboxedType);
+    MInstruction* loadUnboxedProperty(MDefinition* obj, size_t offset, JSValueType unboxedType,
+                                      BarrierKind barrier, TemporaryTypeSet* types);
+    MInstruction* loadUnboxedValue(MDefinition* elements, size_t elementsOffset,
+                                   MDefinition* scaledOffset, JSValueType unboxedType,
+                                   BarrierKind barrier, TemporaryTypeSet* types);
+    MInstruction* storeUnboxedProperty(MDefinition* obj, size_t offset, JSValueType unboxedType,
+                                       MDefinition* value);
+    MInstruction* storeUnboxedValue(MDefinition* obj,
+                                    MDefinition* elements, int32_t elementsOffset,
+                                    MDefinition* scaledOffset, JSValueType unboxedType,
+                                    MDefinition* value, bool preBarrier = true);
     MOZ_MUST_USE bool checkPreliminaryGroups(MDefinition *obj);
     MOZ_MUST_USE bool freezePropTypeSets(TemporaryTypeSet* types,
                                          JSObject* foundProto, PropertyName* name);
diff --git a/js/src/jit/IonCaches.cpp b/js/src/jit/IonCaches.cpp
index fb4291188..c2dc57373 100644
--- a/js/src/jit/IonCaches.cpp
+++ b/js/src/jit/IonCaches.cpp
@@ -31,6 +31,7 @@
 #include "jit/shared/Lowering-shared-inl.h"
 #include "vm/Interpreter-inl.h"
 #include "vm/Shape-inl.h"
+#include "vm/UnboxedObject-inl.h"
 
 using namespace js;
 using namespace js::jit;
@@ -619,7 +620,29 @@ TestMatchingReceiver(MacroAssembler& masm, IonCache::StubAttacher& attacher,
                      Register object, JSObject* obj, Label* failure,
                      bool alwaysCheckGroup = false)
 {
-    if (obj->is<TypedObject>()) {
+    if (obj->is<UnboxedPlainObject>()) {
+        MOZ_ASSERT(failure);
+
+        masm.branchTestObjGroup(Assembler::NotEqual, object, obj->group(), failure);
+        Address expandoAddress(object, UnboxedPlainObject::offsetOfExpando());
+        if (UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando()) {
+            masm.branchPtr(Assembler::Equal, expandoAddress, ImmWord(0), failure);
+            Label success;
+            masm.push(object);
+            masm.loadPtr(expandoAddress, object);
+            masm.branchTestObjShape(Assembler::Equal, object, expando->lastProperty(),
+                                    &success);
+            masm.pop(object);
+            masm.jump(failure);
+            masm.bind(&success);
+            masm.pop(object);
+        } else {
+            masm.branchPtr(Assembler::NotEqual, expandoAddress, ImmWord(0), failure);
+        }
+    } else if (obj->is<UnboxedArrayObject>()) {
+        MOZ_ASSERT(failure);
+        masm.branchTestObjGroup(Assembler::NotEqual, object, obj->group(), failure);
+    } else if (obj->is<TypedObject>()) {
         attacher.branchNextStubOrLabel(masm, Assembler::NotEqual,
                                        Address(object, JSObject::offsetOfGroup()),
                                        ImmGCPtr(obj->group()), failure);
@@ -736,6 +759,7 @@ GenerateReadSlot(JSContext* cx, IonScript* ion, MacroAssembler& masm,
     // jump directly. Otherwise, jump to the end of the stub, so there's a
     // common point to patch.
     bool multipleFailureJumps = (obj != holder)
+                             || obj->is<UnboxedPlainObject>()
                              || (checkTDZ && output.hasValue())
                              || (failures != nullptr && failures->used());
 
@@ -754,6 +778,7 @@ GenerateReadSlot(JSContext* cx, IonScript* ion, MacroAssembler& masm,
     Register scratchReg = Register::FromCode(0); // Quell compiler warning.
 
     if (obj != holder ||
+        obj->is<UnboxedPlainObject>() ||
         !holder->as<NativeObject>().isFixedSlot(shape->slot()))
     {
         if (output.hasValue()) {
@@ -814,6 +839,10 @@ GenerateReadSlot(JSContext* cx, IonScript* ion, MacroAssembler& masm,
 
             holderReg = InvalidReg;
         }
+    } else if (obj->is<UnboxedPlainObject>()) {
+        holder = obj->as<UnboxedPlainObject>().maybeExpando();
+        holderReg = scratchReg;
+        masm.loadPtr(Address(object, UnboxedPlainObject::offsetOfExpando()), holderReg);
     } else {
         holderReg = object;
     }
@@ -841,6 +870,30 @@ GenerateReadSlot(JSContext* cx, IonScript* ion, MacroAssembler& masm,
     attacher.jumpNextStub(masm);
 }
 
+static void
+GenerateReadUnboxed(JSContext* cx, IonScript* ion, MacroAssembler& masm,
+                    IonCache::StubAttacher& attacher, JSObject* obj,
+                    const UnboxedLayout::Property* property,
+                    Register object, TypedOrValueRegister output,
+                    Label* failures = nullptr)
+{
+    // Guard on the group of the object.
+    attacher.branchNextStubOrLabel(masm, Assembler::NotEqual,
+                                   Address(object, JSObject::offsetOfGroup()),
+                                   ImmGCPtr(obj->group()), failures);
+
+    Address address(object, UnboxedPlainObject::offsetOfData() + property->offset);
+
+    masm.loadUnboxedProperty(address, property->type, output);
+
+    attacher.jumpRejoin(masm);
+
+    if (failures) {
+        masm.bind(failures);
+        attacher.jumpNextStub(masm);
+    }
+}
+
 static bool
 EmitGetterCall(JSContext* cx, MacroAssembler& masm,
                IonCache::StubAttacher& attacher, JSObject* obj,
@@ -1135,6 +1188,39 @@ GenerateArrayLength(JSContext* cx, MacroAssembler& masm, IonCache::StubAttacher&
     return true;
 }
 
+static void
+GenerateUnboxedArrayLength(JSContext* cx, MacroAssembler& masm, IonCache::StubAttacher& attacher,
+                           JSObject* array, Register object, TypedOrValueRegister output,
+                           Label* failures)
+{
+    Register outReg;
+    if (output.hasValue()) {
+        outReg = output.valueReg().scratchReg();
+    } else {
+        MOZ_ASSERT(output.type() == MIRType::Int32);
+        outReg = output.typedReg().gpr();
+    }
+    MOZ_ASSERT(object != outReg);
+
+    TestMatchingReceiver(masm, attacher, object, array, failures);
+
+    // Load length.
+    masm.load32(Address(object, UnboxedArrayObject::offsetOfLength()), outReg);
+
+    // Check for a length that fits in an int32.
+    masm.branchTest32(Assembler::Signed, outReg, outReg, failures);
+
+    if (output.hasValue())
+        masm.tagValue(JSVAL_TYPE_INT32, outReg, output.valueReg());
+
+    // Success.
+    attacher.jumpRejoin(masm);
+
+    // Failure.
+    masm.bind(failures);
+    attacher.jumpNextStub(masm);
+}
+
 // In this case, the code for TypedArray and SharedTypedArray is not the same,
 // because the code embeds pointers to the respective class arrays.  Code that
 // caches the stub code must distinguish between the two cases.
@@ -1448,6 +1534,101 @@ GetPropertyIC::tryAttachNative(JSContext* cx, HandleScript outerScript, IonScrip
 }
 
 bool
+GetPropertyIC::tryAttachUnboxed(JSContext* cx, HandleScript outerScript, IonScript* ion,
+                                HandleObject obj, HandleId id, void* returnAddr, bool* emitted)
+{
+    MOZ_ASSERT(canAttachStub());
+    MOZ_ASSERT(!*emitted);
+    MOZ_ASSERT(outerScript->ionScript() == ion);
+
+    if (!obj->is<UnboxedPlainObject>())
+        return true;
+    const UnboxedLayout::Property* property = obj->as<UnboxedPlainObject>().layout().lookup(id);
+    if (!property)
+        return true;
+
+    *emitted = true;
+
+    MacroAssembler masm(cx, ion, outerScript, profilerLeavePc_);
+
+    Label failures;
+    emitIdGuard(masm, id, &failures);
+    Label* maybeFailures = failures.used() ? &failures : nullptr;
+
+    StubAttacher attacher(*this);
+    GenerateReadUnboxed(cx, ion, masm, attacher, obj, property, object(), output(), maybeFailures);
+    return linkAndAttachStub(cx, masm, attacher, ion, "read unboxed",
+                             JS::TrackedOutcome::ICGetPropStub_UnboxedRead);
+}
+
+bool
+GetPropertyIC::tryAttachUnboxedExpando(JSContext* cx, HandleScript outerScript, IonScript* ion,
+                                       HandleObject obj, HandleId id, void* returnAddr, bool* emitted)
+{
+    MOZ_ASSERT(canAttachStub());
+    MOZ_ASSERT(!*emitted);
+    MOZ_ASSERT(outerScript->ionScript() == ion);
+
+    if (!obj->is<UnboxedPlainObject>())
+        return true;
+    Rooted<UnboxedExpandoObject*> expando(cx, obj->as<UnboxedPlainObject>().maybeExpando());
+    if (!expando)
+        return true;
+
+    Shape* shape = expando->lookup(cx, id);
+    if (!shape || !shape->hasDefaultGetter() || !shape->hasSlot())
+        return true;
+
+    *emitted = true;
+
+    MacroAssembler masm(cx, ion, outerScript, profilerLeavePc_);
+
+    Label failures;
+    emitIdGuard(masm, id, &failures);
+    Label* maybeFailures = failures.used() ? &failures : nullptr;
+
+    StubAttacher attacher(*this);
+    GenerateReadSlot(cx, ion, masm, attacher, DontCheckTDZ, obj, obj,
+                     shape, object(), output(), maybeFailures);
+    return linkAndAttachStub(cx, masm, attacher, ion, "read unboxed expando",
+                             JS::TrackedOutcome::ICGetPropStub_UnboxedReadExpando);
+}
+
+bool
+GetPropertyIC::tryAttachUnboxedArrayLength(JSContext* cx, HandleScript outerScript, IonScript* ion,
+                                           HandleObject obj, HandleId id, void* returnAddr,
+                                           bool* emitted)
+{
+    MOZ_ASSERT(canAttachStub());
+    MOZ_ASSERT(!*emitted);
+    MOZ_ASSERT(outerScript->ionScript() == ion);
+
+    if (!obj->is<UnboxedArrayObject>())
+        return true;
+
+    if (!JSID_IS_ATOM(id, cx->names().length))
+        return true;
+
+    if (obj->as<UnboxedArrayObject>().length() > INT32_MAX)
+        return true;
+
+    if (!allowArrayLength(cx))
+        return true;
+
+    *emitted = true;
+
+    MacroAssembler masm(cx, ion, outerScript, profilerLeavePc_);
+
+    Label failures;
+    emitIdGuard(masm, id, &failures);
+
+    StubAttacher attacher(*this);
+    GenerateUnboxedArrayLength(cx, masm, attacher, obj, object(), output(), &failures);
+    return linkAndAttachStub(cx, masm, attacher, ion, "unboxed array length",
+                             JS::TrackedOutcome::ICGetPropStub_UnboxedArrayLength);
+}
+
+bool
 GetPropertyIC::tryAttachTypedArrayLength(JSContext* cx, HandleScript outerScript, IonScript* ion,
                                          HandleObject obj, HandleId id, bool* emitted)
 {
@@ -2016,6 +2197,15 @@ GetPropertyIC::tryAttachStub(JSContext* cx, HandleScript outerScript, IonScript*
         if (!*emitted && !tryAttachNative(cx, outerScript, ion, obj, id, returnAddr, emitted))
             return false;
 
+        if (!*emitted && !tryAttachUnboxed(cx, outerScript, ion, obj, id, returnAddr, emitted))
+            return false;
+
+        if (!*emitted && !tryAttachUnboxedExpando(cx, outerScript, ion, obj, id, returnAddr, emitted))
+            return false;
+
+        if (!*emitted && !tryAttachUnboxedArrayLength(cx, outerScript, ion, obj, id, returnAddr, emitted))
+            return false;
+
         if (!*emitted && !tryAttachTypedArrayLength(cx, outerScript, ion, obj, id, emitted))
             return false;
     }
@@ -2194,6 +2384,12 @@ GenerateSetSlot(JSContext* cx, MacroAssembler& masm, IonCache::StubAttacher& att
 
     NativeObject::slotsSizeMustNotOverflow();
 
+    if (obj->is<UnboxedPlainObject>()) {
+        obj = obj->as<UnboxedPlainObject>().maybeExpando();
+        masm.loadPtr(Address(object, UnboxedPlainObject::offsetOfExpando()), tempReg);
+        object = tempReg;
+    }
+
     if (obj->as<NativeObject>().isFixedSlot(shape->slot())) {
         Address addr(object, NativeObject::getFixedSlotOffset(shape->slot()));
 
@@ -2831,13 +3027,23 @@ GenerateAddSlot(JSContext* cx, MacroAssembler& masm, IonCache::StubAttacher& att
     masm.branchTestObjGroup(Assembler::NotEqual, object, oldGroup, failures);
     if (obj->maybeShape()) {
         masm.branchTestObjShape(Assembler::NotEqual, object, oldShape, failures);
+    } else {
+        MOZ_ASSERT(obj->is<UnboxedPlainObject>());
+
+        Address expandoAddress(object, UnboxedPlainObject::offsetOfExpando());
+        masm.branchPtr(Assembler::Equal, expandoAddress, ImmWord(0), failures);
+
+        masm.loadPtr(expandoAddress, tempReg);
+        masm.branchTestObjShape(Assembler::NotEqual, tempReg, oldShape, failures);
     }
 
     Shape* newShape = obj->maybeShape();
+    if (!newShape)
+        newShape = obj->as<UnboxedPlainObject>().maybeExpando()->lastProperty();
 
     // Guard that the incoming value is in the type set for the property
     // if a type barrier is required.
-    if (newShape && checkTypeset)
+    if (checkTypeset)
         CheckTypeSetForWrite(masm, obj, newShape->propid(), tempReg, value, failures);
 
     // Guard shapes along prototype chain.
@@ -2858,7 +3064,9 @@ GenerateAddSlot(JSContext* cx, MacroAssembler& masm, IonCache::StubAttacher& att
     }
 
     // Call a stub to (re)allocate dynamic slots, if necessary.
-    uint32_t newNumDynamicSlots = obj->as<NativeObject>().numDynamicSlots();
+    uint32_t newNumDynamicSlots = obj->is<UnboxedPlainObject>()
+                                  ? obj->as<UnboxedPlainObject>().maybeExpando()->numDynamicSlots()
+                                  : obj->as<NativeObject>().numDynamicSlots();
     if (NativeObject::dynamicSlotsCount(oldShape) != newNumDynamicSlots) {
         AllocatableRegisterSet regs(RegisterSet::Volatile());
         LiveRegisterSet save(regs.asLiveSet());
@@ -2869,6 +3077,12 @@ GenerateAddSlot(JSContext* cx, MacroAssembler& masm, IonCache::StubAttacher& att
         Register temp1 = regs.takeAnyGeneral();
         Register temp2 = regs.takeAnyGeneral();
 
+        if (obj->is<UnboxedPlainObject>()) {
+            // Pass the expando object to the stub.
+            masm.Push(object);
+            masm.loadPtr(Address(object, UnboxedPlainObject::offsetOfExpando()), object);
+        }
+
         masm.setupUnalignedABICall(temp1);
         masm.loadJSContext(temp1);
         masm.passABIArg(temp1);
@@ -2885,16 +3099,27 @@ GenerateAddSlot(JSContext* cx, MacroAssembler& masm, IonCache::StubAttacher& att
         masm.jump(&allocDone);
 
         masm.bind(&allocFailed);
+        if (obj->is<UnboxedPlainObject>())
+            masm.Pop(object);
         masm.PopRegsInMask(save);
         masm.jump(failures);
 
         masm.bind(&allocDone);
         masm.setFramePushed(framePushedAfterCall);
+        if (obj->is<UnboxedPlainObject>())
+            masm.Pop(object);
         masm.PopRegsInMask(save);
     }
 
     bool popObject = false;
 
+    if (obj->is<UnboxedPlainObject>()) {
+        masm.push(object);
+        popObject = true;
+        obj = obj->as<UnboxedPlainObject>().maybeExpando();
+        masm.loadPtr(Address(object, UnboxedPlainObject::offsetOfExpando()), object);
+    }
+
     // Write the object or expando object's new shape.
     Address shapeAddr(object, ShapedObject::offsetOfShape());
     if (cx->zone()->needsIncrementalBarrier())
@@ -2902,6 +3127,8 @@ GenerateAddSlot(JSContext* cx, MacroAssembler& masm, IonCache::StubAttacher& att
     masm.storePtr(ImmGCPtr(newShape), shapeAddr);
 
     if (oldGroup != obj->group()) {
+        MOZ_ASSERT(!obj->is<UnboxedPlainObject>());
+
         // Changing object's group from a partially to fully initialized group,
         // per the acquired properties analysis. Only change the group if the
         // old group still has a newScript.
@@ -3144,6 +3371,141 @@ CanAttachNativeSetProp(JSContext* cx, HandleObject obj, HandleId id, const Const
     return SetPropertyIC::CanAttachNone;
 }
 
+static void
+GenerateSetUnboxed(JSContext* cx, MacroAssembler& masm, IonCache::StubAttacher& attacher,
+                   JSObject* obj, jsid id, uint32_t unboxedOffset, JSValueType unboxedType,
+                   Register object, Register tempReg, const ConstantOrRegister& value,
+                   bool checkTypeset, Label* failures)
+{
+    // Guard on the type of the object.
+    masm.branchPtr(Assembler::NotEqual,
+                   Address(object, JSObject::offsetOfGroup()),
+                   ImmGCPtr(obj->group()), failures);
+
+    if (checkTypeset)
+        CheckTypeSetForWrite(masm, obj, id, tempReg, value, failures);
+
+    Address address(object, UnboxedPlainObject::offsetOfData() + unboxedOffset);
+
+    if (cx->zone()->needsIncrementalBarrier()) {
+        if (unboxedType == JSVAL_TYPE_OBJECT)
+            masm.callPreBarrier(address, MIRType::Object);
+        else if (unboxedType == JSVAL_TYPE_STRING)
+            masm.callPreBarrier(address, MIRType::String);
+        else
+            MOZ_ASSERT(!UnboxedTypeNeedsPreBarrier(unboxedType));
+    }
+
+    masm.storeUnboxedProperty(address, unboxedType, value, failures);
+
+    attacher.jumpRejoin(masm);
+
+    masm.bind(failures);
+    attacher.jumpNextStub(masm);
+}
+
+static bool
+CanAttachSetUnboxed(JSContext* cx, HandleObject obj, HandleId id, const ConstantOrRegister& val,
+                    bool needsTypeBarrier, bool* checkTypeset,
+                    uint32_t* unboxedOffset, JSValueType* unboxedType)
+{
+    if (!obj->is<UnboxedPlainObject>())
+        return false;
+
+    const UnboxedLayout::Property* property = obj->as<UnboxedPlainObject>().layout().lookup(id);
+    if (property) {
+        *checkTypeset = false;
+        if (needsTypeBarrier && !CanInlineSetPropTypeCheck(obj, id, val, checkTypeset))
+            return false;
+        *unboxedOffset = property->offset;
+        *unboxedType = property->type;
+        return true;
+    }
+
+    return false;
+}
+
+static bool
+CanAttachSetUnboxedExpando(JSContext* cx, HandleObject obj, HandleId id,
+                           const ConstantOrRegister& val,
+                           bool needsTypeBarrier, bool* checkTypeset, Shape** pshape)
+{
+    if (!obj->is<UnboxedPlainObject>())
+        return false;
+
+    Rooted<UnboxedExpandoObject*> expando(cx, obj->as<UnboxedPlainObject>().maybeExpando());
+    if (!expando)
+        return false;
+
+    Shape* shape = expando->lookupPure(id);
+    if (!shape || !shape->hasDefaultSetter() || !shape->hasSlot() || !shape->writable())
+        return false;
+
+    *checkTypeset = false;
+    if (needsTypeBarrier && !CanInlineSetPropTypeCheck(obj, id, val, checkTypeset))
+        return false;
+
+    *pshape = shape;
+    return true;
+}
+
+static bool
+CanAttachAddUnboxedExpando(JSContext* cx, HandleObject obj, HandleShape oldShape,
+                           HandleId id, const ConstantOrRegister& val,
+                           bool needsTypeBarrier, bool* checkTypeset)
+{
+    if (!obj->is<UnboxedPlainObject>())
+        return false;
+
+    Rooted<UnboxedExpandoObject*> expando(cx, obj->as<UnboxedPlainObject>().maybeExpando());
+    if (!expando || expando->inDictionaryMode())
+        return false;
+
+    Shape* newShape = expando->lastProperty();
+    if (newShape->isEmptyShape() || newShape->propid() != id || newShape->previous() != oldShape)
+        return false;
+
+    MOZ_ASSERT(newShape->hasDefaultSetter() && newShape->hasSlot() && newShape->writable());
+
+    if (PrototypeChainShadowsPropertyAdd(cx, obj, id))
+        return false;
+
+    *checkTypeset = false;
+    if (needsTypeBarrier && !CanInlineSetPropTypeCheck(obj, id, val, checkTypeset))
+        return false;
+
+    return true;
+}
+
+bool
+SetPropertyIC::tryAttachUnboxed(JSContext* cx, HandleScript outerScript, IonScript* ion,
+                                HandleObject obj, HandleId id, bool* emitted)
+{
+    MOZ_ASSERT(!*emitted);
+
+    bool checkTypeset = false;
+    uint32_t unboxedOffset;
+    JSValueType unboxedType;
+    if (!CanAttachSetUnboxed(cx, obj, id, value(), needsTypeBarrier(), &checkTypeset,
+                             &unboxedOffset, &unboxedType))
+    {
+        return true;
+    }
+
+    *emitted = true;
+
+    MacroAssembler masm(cx, ion, outerScript, profilerLeavePc_);
+    StubAttacher attacher(*this);
+
+    Label failures;
+    emitIdGuard(masm, id, &failures);
+
+    GenerateSetUnboxed(cx, masm, attacher, obj, id, unboxedOffset, unboxedType,
+                       object(), temp(), value(), checkTypeset, &failures);
+    return linkAndAttachStub(cx, masm, attacher, ion, "set_unboxed",
+                             JS::TrackedOutcome::ICSetPropStub_SetUnboxed);
+}
+
 bool
 SetPropertyIC::tryAttachProxy(JSContext* cx, HandleScript outerScript, IonScript* ion,
                               HandleObject obj, HandleId id, bool* emitted)
@@ -3225,6 +3587,26 @@ SetPropertyIC::tryAttachNative(JSContext* cx, HandleScript outerScript, IonScrip
 }
 
 bool
+SetPropertyIC::tryAttachUnboxedExpando(JSContext* cx, HandleScript outerScript, IonScript* ion,
+                                       HandleObject obj, HandleId id, bool* emitted)
+{
+    MOZ_ASSERT(!*emitted);
+
+    RootedShape shape(cx);
+    bool checkTypeset = false;
+    if (!CanAttachSetUnboxedExpando(cx, obj, id, value(), needsTypeBarrier(),
+                                    &checkTypeset, shape.address()))
+    {
+        return true;
+    }
+
+    if (!attachSetSlot(cx, outerScript, ion, obj, shape, checkTypeset))
+        return false;
+    *emitted = true;
+    return true;
+}
+
+bool
 SetPropertyIC::tryAttachStub(JSContext* cx, HandleScript outerScript, IonScript* ion,
                              HandleObject obj, HandleValue idval, HandleValue value,
                              MutableHandleId id, bool* emitted, bool* tryNativeAddSlot)
@@ -3249,6 +3631,12 @@ SetPropertyIC::tryAttachStub(JSContext* cx, HandleScript outerScript, IonScript*
 
         if (!*emitted && !tryAttachNative(cx, outerScript, ion, obj, id, emitted, tryNativeAddSlot))
             return false;
+
+        if (!*emitted && !tryAttachUnboxed(cx, outerScript, ion, obj, id, emitted))
+            return false;
+
+        if (!*emitted && !tryAttachUnboxedExpando(cx, outerScript, ion, obj, id, emitted))
+            return false;
     }
 
     if (idval.isInt32()) {
@@ -3300,6 +3688,16 @@ SetPropertyIC::tryAttachAddSlot(JSContext* cx, HandleScript outerScript, IonScri
         return true;
     }
 
+    checkTypeset = false;
+    if (CanAttachAddUnboxedExpando(cx, obj, oldShape, id, value(), needsTypeBarrier(),
+                                   &checkTypeset))
+    {
+        if (!attachAddSlot(cx, outerScript, ion, obj, id, oldShape, oldGroup, checkTypeset))
+            return false;
+        *emitted = true;
+        return true;
+    }
+
     return true;
 }
 
@@ -3321,6 +3719,11 @@ SetPropertyIC::update(JSContext* cx, HandleScript outerScript, size_t cacheIndex
             return false;
 
         oldShape = obj->maybeShape();
+        if (obj->is<UnboxedPlainObject>()) {
+            MOZ_ASSERT(!oldShape);
+            if (UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando())
+                oldShape = expando->lastProperty();
+        }
     }
 
     RootedId id(cx);
@@ -3623,7 +4026,7 @@ GetPropertyIC::tryAttachDenseElementHole(JSContext* cx, HandleScript outerScript
 GetPropertyIC::canAttachTypedOrUnboxedArrayElement(JSObject* obj, const Value& idval,
                                                    TypedOrValueRegister output)
 {
-    if (!obj->is<TypedArrayObject>())
+    if (!obj->is<TypedArrayObject>() && !obj->is<UnboxedArrayObject>())
         return false;
 
     MOZ_ASSERT(idval.isInt32() || idval.isString());
@@ -3654,6 +4057,13 @@ GetPropertyIC::canAttachTypedOrUnboxedArrayElement(JSObject* obj, const Value& i
         return output.hasValue() || !output.typedReg().isFloat();
     }
 
+    if (index >= obj->as<UnboxedArrayObject>().initializedLength())
+        return false;
+
+    JSValueType elementType = obj->as<UnboxedArrayObject>().elementType();
+    if (elementType == JSVAL_TYPE_DOUBLE)
+        return output.hasValue();
+
     return output.hasValue() || !output.typedReg().isFloat();
 }
 
@@ -3730,27 +4140,46 @@ GenerateGetTypedOrUnboxedArrayElement(JSContext* cx, MacroAssembler& masm,
 
     Label popObjectAndFail;
 
-    // Guard on the initialized length.
-    Address length(object, TypedArrayObject::lengthOffset());
-    masm.branch32(Assembler::BelowOrEqual, length, indexReg, &failures);
+    if (array->is<TypedArrayObject>()) {
+        // Guard on the initialized length.
+        Address length(object, TypedArrayObject::lengthOffset());
+        masm.branch32(Assembler::BelowOrEqual, length, indexReg, &failures);
 
-    // Save the object register on the stack in case of failure.
-    Register elementReg = object;
-    masm.push(object);
+        // Save the object register on the stack in case of failure.
+        Register elementReg = object;
+        masm.push(object);
 
-    // Load elements vector.
-    masm.loadPtr(Address(object, TypedArrayObject::dataOffset()), elementReg);
+        // Load elements vector.
+        masm.loadPtr(Address(object, TypedArrayObject::dataOffset()), elementReg);
 
-    // Load the value. We use an invalid register because the destination
-    // register is necessary a non double register.
-    Scalar::Type arrayType = array->as<TypedArrayObject>().type();
-    int width = Scalar::byteSize(arrayType);
-    BaseIndex source(elementReg, indexReg, ScaleFromElemWidth(width));
-    if (output.hasValue()) {
-        masm.loadFromTypedArray(arrayType, source, output.valueReg(), allowDoubleResult,
-                                elementReg, &popObjectAndFail);
+        // Load the value. We use an invalid register because the destination
+        // register is necessary a non double register.
+        Scalar::Type arrayType = array->as<TypedArrayObject>().type();
+        int width = Scalar::byteSize(arrayType);
+        BaseIndex source(elementReg, indexReg, ScaleFromElemWidth(width));
+        if (output.hasValue()) {
+            masm.loadFromTypedArray(arrayType, source, output.valueReg(), allowDoubleResult,
+                                    elementReg, &popObjectAndFail);
+        } else {
+            masm.loadFromTypedArray(arrayType, source, output.typedReg(), elementReg, &popObjectAndFail);
+        }
     } else {
-        masm.loadFromTypedArray(arrayType, source, output.typedReg(), elementReg, &popObjectAndFail);
+        // Save the object register on the stack in case of failure.
+        masm.push(object);
+
+        // Guard on the initialized length.
+        masm.load32(Address(object, UnboxedArrayObject::offsetOfCapacityIndexAndInitializedLength()), object);
+        masm.and32(Imm32(UnboxedArrayObject::InitializedLengthMask), object);
+        masm.branch32(Assembler::BelowOrEqual, object, indexReg, &popObjectAndFail);
+
+        // Load elements vector.
+        Register elementReg = object;
+        masm.loadPtr(Address(masm.getStackPointer(), 0), object);
+        masm.loadPtr(Address(object, UnboxedArrayObject::offsetOfElements()), elementReg);
+
+        JSValueType elementType = array->as<UnboxedArrayObject>().elementType();
+        BaseIndex source(elementReg, indexReg, ScaleFromElemWidth(UnboxedTypeSize(elementType)));
+        masm.loadUnboxedProperty(source, elementType, output);
     }
 
     masm.pop(object);
diff --git a/js/src/jit/IonCaches.h b/js/src/jit/IonCaches.h
index b00646538..173e06c6b 100644
--- a/js/src/jit/IonCaches.h
+++ b/js/src/jit/IonCaches.h
@@ -529,6 +529,18 @@ class GetPropertyIC : public IonCache
                                       HandleObject obj, HandleId id, void* returnAddr,
                                       bool* emitted);
 
+    MOZ_MUST_USE bool tryAttachUnboxed(JSContext* cx, HandleScript outerScript, IonScript* ion,
+                                       HandleObject obj, HandleId id, void* returnAddr,
+                                       bool* emitted);
+
+    MOZ_MUST_USE bool tryAttachUnboxedExpando(JSContext* cx, HandleScript outerScript,
+                                              IonScript* ion, HandleObject obj, HandleId id,
+                                              void* returnAddr, bool* emitted);
+
+    MOZ_MUST_USE bool tryAttachUnboxedArrayLength(JSContext* cx, HandleScript outerScript,
+                                                  IonScript* ion, HandleObject obj, HandleId id,
+                                                  void* returnAddr, bool* emitted);
+
     MOZ_MUST_USE bool tryAttachTypedArrayLength(JSContext* cx, HandleScript outerScript,
                                                 IonScript* ion, HandleObject obj, HandleId id,
                                                 bool* emitted);
diff --git a/js/src/jit/JitOptions.cpp b/js/src/jit/JitOptions.cpp
index 3f9d9db88..b9a7c7b27 100644
--- a/js/src/jit/JitOptions.cpp
+++ b/js/src/jit/JitOptions.cpp
@@ -221,6 +221,9 @@ DefaultJitOptions::DefaultJitOptions()
             Warn(forcedRegisterAllocatorEnv, env);
     }
 
+    // Toggles whether unboxed plain objects can be created by the VM.
+    SET_DEFAULT(disableUnboxedObjects, true);
+
     // Test whether Atomics are allowed in asm.js code.
     SET_DEFAULT(asmJSAtomicsEnable, false);
 
diff --git a/js/src/jit/JitOptions.h b/js/src/jit/JitOptions.h
index 719ee14d9..076980b4e 100644
--- a/js/src/jit/JitOptions.h
+++ b/js/src/jit/JitOptions.h
@@ -91,6 +91,9 @@ struct DefaultJitOptions
     mozilla::Maybe<uint32_t> forcedDefaultIonSmallFunctionWarmUpThreshold;
     mozilla::Maybe<IonRegisterAllocator> forcedRegisterAllocator;
 
+    // The options below affect the rest of the VM, and not just the JIT.
+    bool disableUnboxedObjects;
+
     DefaultJitOptions();
     bool isSmallFunction(JSScript* script) const;
     void setEagerCompilation();
diff --git a/js/src/jit/Lowering.cpp b/js/src/jit/Lowering.cpp
index 108450983..19266bae8 100644
--- a/js/src/jit/Lowering.cpp
+++ b/js/src/jit/Lowering.cpp
@@ -2895,6 +2895,32 @@ LIRGenerator::visitSetInitializedLength(MSetInitializedLength* ins)
 }
 
 void
+LIRGenerator::visitUnboxedArrayLength(MUnboxedArrayLength* ins)
+{
+    define(new(alloc()) LUnboxedArrayLength(useRegisterAtStart(ins->object())), ins);
+}
+
+void
+LIRGenerator::visitUnboxedArrayInitializedLength(MUnboxedArrayInitializedLength* ins)
+{
+    define(new(alloc()) LUnboxedArrayInitializedLength(useRegisterAtStart(ins->object())), ins);
+}
+
+void
+LIRGenerator::visitIncrementUnboxedArrayInitializedLength(MIncrementUnboxedArrayInitializedLength* ins)
+{
+    add(new(alloc()) LIncrementUnboxedArrayInitializedLength(useRegister(ins->object())), ins);
+}
+
+void
+LIRGenerator::visitSetUnboxedArrayInitializedLength(MSetUnboxedArrayInitializedLength* ins)
+{
+    add(new(alloc()) LSetUnboxedArrayInitializedLength(useRegister(ins->object()),
+                                                       useRegisterOrConstant(ins->length()),
+                                                       temp()), ins);
+}
+
+void
 LIRGenerator::visitNot(MNot* ins)
 {
     MDefinition* op = ins->input();
@@ -3143,16 +3169,22 @@ LIRGenerator::visitStoreElementHole(MStoreElementHole* ins)
     const LUse elements = useRegister(ins->elements());
     const LAllocation index = useRegisterOrConstant(ins->index());
 
+    // Use a temp register when adding new elements to unboxed arrays.
+    LDefinition tempDef = LDefinition::BogusTemp();
+    if (ins->unboxedType() != JSVAL_TYPE_MAGIC)
+        tempDef = temp();
+
     LInstruction* lir;
     switch (ins->value()->type()) {
       case MIRType::Value:
-        lir = new(alloc()) LStoreElementHoleV(object, elements, index, useBox(ins->value()));
+        lir = new(alloc()) LStoreElementHoleV(object, elements, index, useBox(ins->value()),
+                                              tempDef);
         break;
 
       default:
       {
         const LAllocation value = useRegisterOrNonDoubleConstant(ins->value());
-        lir = new(alloc()) LStoreElementHoleT(object, elements, index, value);
+        lir = new(alloc()) LStoreElementHoleT(object, elements, index, value, tempDef);
         break;
       }
     }
@@ -3171,14 +3203,20 @@ LIRGenerator::visitFallibleStoreElement(MFallibleStoreElement* ins)
     const LUse elements = useRegister(ins->elements());
     const LAllocation index = useRegisterOrConstant(ins->index());
 
+    // Use a temp register when adding new elements to unboxed arrays.
+    LDefinition tempDef = LDefinition::BogusTemp();
+    if (ins->unboxedType() != JSVAL_TYPE_MAGIC)
+        tempDef = temp();
+
     LInstruction* lir;
     switch (ins->value()->type()) {
       case MIRType::Value:
-        lir = new(alloc()) LFallibleStoreElementV(object, elements, index, useBox(ins->value()));
+        lir = new(alloc()) LFallibleStoreElementV(object, elements, index, useBox(ins->value()),
+                                                  tempDef);
         break;
       default:
         const LAllocation value = useRegisterOrNonDoubleConstant(ins->value());
-        lir = new(alloc()) LFallibleStoreElementT(object, elements, index, value);
+        lir = new(alloc()) LFallibleStoreElementT(object, elements, index, value, tempDef);
         break;
     }
 
@@ -3220,6 +3258,14 @@ LIRGenerator::visitStoreUnboxedString(MStoreUnboxedString* ins)
 }
 
 void
+LIRGenerator::visitConvertUnboxedObjectToNative(MConvertUnboxedObjectToNative* ins)
+{
+    LInstruction* check = new(alloc()) LConvertUnboxedObjectToNative(useRegister(ins->object()));
+    add(check, ins);
+    assignSafepoint(check, ins);
+}
+
+void
 LIRGenerator::visitEffectiveAddress(MEffectiveAddress* ins)
 {
     define(new(alloc()) LEffectiveAddress(useRegister(ins->base()), useRegister(ins->index())), ins);
@@ -3737,6 +3783,24 @@ LIRGenerator::visitGuardReceiverPolymorphic(MGuardReceiverPolymorphic* ins)
 }
 
 void
+LIRGenerator::visitGuardUnboxedExpando(MGuardUnboxedExpando* ins)
+{
+    LGuardUnboxedExpando* guard =
+        new(alloc()) LGuardUnboxedExpando(useRegister(ins->object()));
+    assignSnapshot(guard, ins->bailoutKind());
+    add(guard, ins);
+    redefine(ins, ins->object());
+}
+
+void
+LIRGenerator::visitLoadUnboxedExpando(MLoadUnboxedExpando* ins)
+{
+    LLoadUnboxedExpando* lir =
+        new(alloc()) LLoadUnboxedExpando(useRegisterAtStart(ins->object()));
+    define(lir, ins);
+}
+
+void
 LIRGenerator::visitAssertRange(MAssertRange* ins)
 {
     MDefinition* input = ins->input();
diff --git a/js/src/jit/Lowering.h b/js/src/jit/Lowering.h
index 81e6abbbb..de66f175b 100644
--- a/js/src/jit/Lowering.h
+++ b/js/src/jit/Lowering.h
@@ -217,6 +217,10 @@ class LIRGenerator : public LIRGeneratorSpecific
     void visitTypedObjectDescr(MTypedObjectDescr* ins);
     void visitInitializedLength(MInitializedLength* ins);
     void visitSetInitializedLength(MSetInitializedLength* ins);
+    void visitUnboxedArrayLength(MUnboxedArrayLength* ins);
+    void visitUnboxedArrayInitializedLength(MUnboxedArrayInitializedLength* ins);
+    void visitIncrementUnboxedArrayInitializedLength(MIncrementUnboxedArrayInitializedLength* ins);
+    void visitSetUnboxedArrayInitializedLength(MSetUnboxedArrayInitializedLength* ins);
     void visitNot(MNot* ins);
     void visitBoundsCheck(MBoundsCheck* ins);
     void visitBoundsCheckLower(MBoundsCheckLower* ins);
@@ -229,6 +233,7 @@ class LIRGenerator : public LIRGeneratorSpecific
     void visitFallibleStoreElement(MFallibleStoreElement* ins);
     void visitStoreUnboxedObjectOrNull(MStoreUnboxedObjectOrNull* ins);
     void visitStoreUnboxedString(MStoreUnboxedString* ins);
+    void visitConvertUnboxedObjectToNative(MConvertUnboxedObjectToNative* ins);
     void visitEffectiveAddress(MEffectiveAddress* ins);
     void visitArrayPopShift(MArrayPopShift* ins);
     void visitArrayPush(MArrayPush* ins);
@@ -252,6 +257,8 @@ class LIRGenerator : public LIRGeneratorSpecific
     void visitGuardObject(MGuardObject* ins);
     void visitGuardString(MGuardString* ins);
     void visitGuardReceiverPolymorphic(MGuardReceiverPolymorphic* ins);
+    void visitGuardUnboxedExpando(MGuardUnboxedExpando* ins);
+    void visitLoadUnboxedExpando(MLoadUnboxedExpando* ins);
     void visitPolyInlineGuard(MPolyInlineGuard* ins);
     void visitAssertRange(MAssertRange* ins);
     void visitCallGetProperty(MCallGetProperty* ins);
diff --git a/js/src/jit/MCallOptimize.cpp b/js/src/jit/MCallOptimize.cpp
index 236354530..f2071dc6a 100644
--- a/js/src/jit/MCallOptimize.cpp
+++ b/js/src/jit/MCallOptimize.cpp
@@ -30,6 +30,7 @@
 #include "jit/shared/Lowering-shared-inl.h"
 #include "vm/NativeObject-inl.h"
 #include "vm/StringObject-inl.h"
+#include "vm/UnboxedObject-inl.h"
 
 using mozilla::ArrayLength;
 using mozilla::AssertedCast;
@@ -470,6 +471,11 @@ IonBuilder::inlineArray(CallInfo& callInfo)
         return InliningStatus_NotInlined;
     }
 
+    if (templateObject->is<UnboxedArrayObject>()) {
+        if (templateObject->group()->unboxedLayout().nativeGroup())
+            return InliningStatus_NotInlined;
+    }
+
     // Multiple arguments imply array initialization, not just construction.
     if (callInfo.argc() >= 2) {
         initLength = callInfo.argc();
@@ -517,7 +523,7 @@ IonBuilder::inlineArray(CallInfo& callInfo)
         // Make sure initLength matches the template object's length. This is
         // not guaranteed to be the case, for instance if we're inlining the
         // MConstant may come from an outer script.
-        if (initLength != templateObject->as<ArrayObject>().length())
+        if (initLength != GetAnyBoxedOrUnboxedArrayLength(templateObject))
             return InliningStatus_NotInlined;
 
         // Don't inline large allocations.
@@ -532,15 +538,16 @@ IonBuilder::inlineArray(CallInfo& callInfo)
 
     MDefinition* array = current->peek(-1);
     if (callInfo.argc() >= 2) {
+        JSValueType unboxedType = GetBoxedOrUnboxedType(templateObject);
         for (uint32_t i = 0; i < initLength; i++) {
             if (!alloc().ensureBallast())
                 return InliningStatus_Error;
             MDefinition* value = callInfo.getArg(i);
-            if (!initializeArrayElement(array, i, value, /* addResumePoint = */ false))
+            if (!initializeArrayElement(array, i, value, unboxedType, /* addResumePoint = */ false))
                 return InliningStatus_Error;
         }
 
-        MInstruction* setLength = setInitializedLength(array, initLength);
+        MInstruction* setLength = setInitializedLength(array, unboxedType, initLength);
         if (!resumeAfter(setLength))
             return InliningStatus_Error;
     }
@@ -573,7 +580,7 @@ IonBuilder::inlineArrayIsArray(CallInfo& callInfo)
         if (!clasp || clasp->isProxy())
             return InliningStatus_NotInlined;
 
-        isArray = (clasp == &ArrayObject::class_);
+        isArray = (clasp == &ArrayObject::class_ || clasp == &UnboxedArrayObject::class_);
     }
 
     pushConstant(BooleanValue(isArray));
@@ -604,12 +611,12 @@ IonBuilder::inlineArrayPopShift(CallInfo& callInfo, MArrayPopShift::Mode mode)
         OBJECT_FLAG_LENGTH_OVERFLOW |
         OBJECT_FLAG_ITERATED;
 
-    MDefinition* obj = callInfo.thisArg();
+    MDefinition* obj = convertUnboxedObjects(callInfo.thisArg());
     TemporaryTypeSet* thisTypes = obj->resultTypeSet();
     if (!thisTypes)
         return InliningStatus_NotInlined;
     const Class* clasp = thisTypes->getKnownClass(constraints());
-    if (clasp != &ArrayObject::class_)
+    if (clasp != &ArrayObject::class_ && clasp != &UnboxedArrayObject::class_)
         return InliningStatus_NotInlined;
     if (thisTypes->hasObjectFlags(constraints(), unhandledFlags)) {
         trackOptimizationOutcome(TrackedOutcome::ArrayBadFlags);
@@ -622,9 +629,17 @@ IonBuilder::inlineArrayPopShift(CallInfo& callInfo, MArrayPopShift::Mode mode)
         return InliningStatus_NotInlined;
     }
 
+    JSValueType unboxedType = JSVAL_TYPE_MAGIC;
+    if (clasp == &UnboxedArrayObject::class_) {
+        unboxedType = UnboxedArrayElementType(constraints(), obj, nullptr);
+        if (unboxedType == JSVAL_TYPE_MAGIC)
+            return InliningStatus_NotInlined;
+    }
+
     callInfo.setImplicitlyUsedUnchecked();
 
-    obj = addMaybeCopyElementsForWrite(obj, /* checkNative = */ false);
+    if (clasp == &ArrayObject::class_)
+        obj = addMaybeCopyElementsForWrite(obj, /* checkNative = */ false);
 
     TemporaryTypeSet* returnTypes = getInlineReturnTypeSet();
     bool needsHoleCheck = thisTypes->hasObjectFlags(constraints(), OBJECT_FLAG_NON_PACKED);
@@ -635,7 +650,8 @@ IonBuilder::inlineArrayPopShift(CallInfo& callInfo, MArrayPopShift::Mode mode)
     if (barrier != BarrierKind::NoBarrier)
         returnType = MIRType::Value;
 
-    MArrayPopShift* ins = MArrayPopShift::New(alloc(), obj, mode, needsHoleCheck, maybeUndefined);
+    MArrayPopShift* ins = MArrayPopShift::New(alloc(), obj, mode,
+                                              unboxedType, needsHoleCheck, maybeUndefined);
     current->add(ins);
     current->push(ins);
     ins->setResultType(returnType);
@@ -684,7 +700,7 @@ IonBuilder::inlineArrayPush(CallInfo& callInfo)
         return InliningStatus_NotInlined;
     }
 
-    MDefinition* obj = callInfo.thisArg();
+    MDefinition* obj = convertUnboxedObjects(callInfo.thisArg());
     MDefinition* value = callInfo.getArg(0);
     if (PropertyWriteNeedsTypeBarrier(alloc(), constraints(), current,
                                       &obj, nullptr, &value, /* canModify = */ false))
@@ -717,6 +733,13 @@ IonBuilder::inlineArrayPush(CallInfo& callInfo)
         return InliningStatus_NotInlined;
     }
 
+    JSValueType unboxedType = JSVAL_TYPE_MAGIC;
+    if (clasp == &UnboxedArrayObject::class_) {
+        unboxedType = UnboxedArrayElementType(constraints(), obj, nullptr);
+        if (unboxedType == JSVAL_TYPE_MAGIC)
+            return InliningStatus_NotInlined;
+    }
+
     callInfo.setImplicitlyUsedUnchecked();
 
     if (conversion == TemporaryTypeSet::AlwaysConvertToDoubles ||
@@ -727,12 +750,13 @@ IonBuilder::inlineArrayPush(CallInfo& callInfo)
         value = valueDouble;
     }
 
-    obj = addMaybeCopyElementsForWrite(obj, /* checkNative = */ false);
+    if (unboxedType == JSVAL_TYPE_MAGIC)
+        obj = addMaybeCopyElementsForWrite(obj, /* checkNative = */ false);
 
     if (NeedsPostBarrier(value))
         current->add(MPostWriteBarrier::New(alloc(), obj, value));
 
-    MArrayPush* ins = MArrayPush::New(alloc(), obj, value);
+    MArrayPush* ins = MArrayPush::New(alloc(), obj, value, unboxedType);
     current->add(ins);
     current->push(ins);
 
@@ -749,7 +773,7 @@ IonBuilder::inlineArraySlice(CallInfo& callInfo)
         return InliningStatus_NotInlined;
     }
 
-    MDefinition* obj = callInfo.thisArg();
+    MDefinition* obj = convertUnboxedObjects(callInfo.thisArg());
 
     // Ensure |this| and result are objects.
     if (getInlineReturnType() != MIRType::Object)
@@ -773,9 +797,16 @@ IonBuilder::inlineArraySlice(CallInfo& callInfo)
         return InliningStatus_NotInlined;
 
     const Class* clasp = thisTypes->getKnownClass(constraints());
-    if (clasp != &ArrayObject::class_)
+    if (clasp != &ArrayObject::class_ && clasp != &UnboxedArrayObject::class_)
         return InliningStatus_NotInlined;
 
+    JSValueType unboxedType = JSVAL_TYPE_MAGIC;
+    if (clasp == &UnboxedArrayObject::class_) {
+        unboxedType = UnboxedArrayElementType(constraints(), obj, nullptr);
+        if (unboxedType == JSVAL_TYPE_MAGIC)
+            return InliningStatus_NotInlined;
+    }
+
     // Watch out for indexed properties on the object or its prototype.
     if (ElementAccessHasExtraIndexedProperty(this, obj)) {
         trackOptimizationOutcome(TrackedOutcome::ProtoIndexedProps);
@@ -796,8 +827,15 @@ IonBuilder::inlineArraySlice(CallInfo& callInfo)
     if (!templateObj)
         return InliningStatus_NotInlined;
 
-    if (!templateObj->is<ArrayObject>())
-        return InliningStatus_NotInlined;
+    if (unboxedType == JSVAL_TYPE_MAGIC) {
+        if (!templateObj->is<ArrayObject>())
+            return InliningStatus_NotInlined;
+    } else {
+        if (!templateObj->is<UnboxedArrayObject>())
+            return InliningStatus_NotInlined;
+        if (templateObj->as<UnboxedArrayObject>().elementType() != unboxedType)
+            return InliningStatus_NotInlined;
+    }
 
     callInfo.setImplicitlyUsedUnchecked();
 
@@ -816,12 +854,16 @@ IonBuilder::inlineArraySlice(CallInfo& callInfo)
 
         end = MArrayLength::New(alloc(), elements);
         current->add(end->toInstruction());
+    } else {
+        end = MUnboxedArrayLength::New(alloc(), obj);
+        current->add(end->toInstruction());
     }
 
     MArraySlice* ins = MArraySlice::New(alloc(), constraints(),
                                         obj, begin, end,
                                         templateObj,
-                                        templateObj->group()->initialHeap(constraints()));
+                                        templateObj->group()->initialHeap(constraints()),
+                                        unboxedType);
     current->add(ins);
     current->push(ins);
 
@@ -1340,7 +1382,7 @@ IonBuilder::inlineConstantStringSplitString(CallInfo& callInfo)
     // Check if exist a template object in stub.
     JSString* stringStr = nullptr;
     JSString* stringSep = nullptr;
-    ArrayObject* templateObject = nullptr;
+    JSObject* templateObject = nullptr;
     if (!inspector->isOptimizableCallStringSplit(pc, &stringStr, &stringSep, &templateObject))
         return InliningStatus_NotInlined;
 
@@ -1366,13 +1408,13 @@ IonBuilder::inlineConstantStringSplitString(CallInfo& callInfo)
     if (!key.maybeTypes()->hasType(TypeSet::StringType()))
         return InliningStatus_NotInlined;
 
-    uint32_t initLength = templateObject->length();
-    if (templateObject->getDenseInitializedLength() != initLength)
+    uint32_t initLength = GetAnyBoxedOrUnboxedArrayLength(templateObject);
+    if (GetAnyBoxedOrUnboxedInitializedLength(templateObject) != initLength)
         return InliningStatus_NotInlined;
 
     Vector<MConstant*, 0, SystemAllocPolicy> arrayValues;
     for (uint32_t i = 0; i < initLength; i++) {
-        Value str = templateObject->getDenseElement(i);
+        Value str = GetAnyBoxedOrUnboxedDenseElement(templateObject, i);
         MOZ_ASSERT(str.toString()->isAtom());
         MConstant* value = MConstant::New(alloc().fallible(), str, constraints());
         if (!value)
@@ -1403,6 +1445,8 @@ IonBuilder::inlineConstantStringSplitString(CallInfo& callInfo)
         return InliningStatus_Inlined;
     }
 
+    JSValueType unboxedType = GetBoxedOrUnboxedType(templateObject);
+
     // Store all values, no need to initialize the length after each as
     // jsop_initelem_array is doing because we do not expect to bailout
     // because the memory is supposed to be allocated by now.
@@ -1413,11 +1457,11 @@ IonBuilder::inlineConstantStringSplitString(CallInfo& callInfo)
         MConstant* value = arrayValues[i];
         current->add(value);
 
-        if (!initializeArrayElement(array, i, value, /* addResumePoint = */ false))
+        if (!initializeArrayElement(array, i, value, unboxedType, /* addResumePoint = */ false))
             return InliningStatus_Error;
     }
 
-    MInstruction* setLength = setInitializedLength(array, initLength);
+    MInstruction* setLength = setInitializedLength(array, unboxedType, initLength);
     if (!resumeAfter(setLength))
         return InliningStatus_Error;
 
@@ -2053,7 +2097,7 @@ IonBuilder::inlineDefineDataProperty(CallInfo& callInfo)
     if (callInfo.argc() != 3)
         return InliningStatus_NotInlined;
 
-    MDefinition* obj = callInfo.getArg(0);
+    MDefinition* obj = convertUnboxedObjects(callInfo.getArg(0));
     MDefinition* id = callInfo.getArg(1);
     MDefinition* value = callInfo.getArg(2);
 
diff --git a/js/src/jit/MIR.cpp b/js/src/jit/MIR.cpp
index 0cf31adb3..b9e5e8d61 100644
--- a/js/src/jit/MIR.cpp
+++ b/js/src/jit/MIR.cpp
@@ -2630,6 +2630,40 @@ jit::EqualTypes(MIRType type1, TemporaryTypeSet* typeset1,
     return typeset1->equals(typeset2);
 }
 
+// Tests whether input/inputTypes can always be stored to an unboxed
+// object/array property with the given unboxed type.
+bool
+jit::CanStoreUnboxedType(TempAllocator& alloc,
+                         JSValueType unboxedType, MIRType input, TypeSet* inputTypes)
+{
+    TemporaryTypeSet types;
+
+    switch (unboxedType) {
+      case JSVAL_TYPE_BOOLEAN:
+      case JSVAL_TYPE_INT32:
+      case JSVAL_TYPE_DOUBLE:
+      case JSVAL_TYPE_STRING:
+        types.addType(TypeSet::PrimitiveType(unboxedType), alloc.lifoAlloc());
+        break;
+
+      case JSVAL_TYPE_OBJECT:
+        types.addType(TypeSet::AnyObjectType(), alloc.lifoAlloc());
+        types.addType(TypeSet::NullType(), alloc.lifoAlloc());
+        break;
+
+      default:
+        MOZ_CRASH("Bad unboxed type");
+    }
+
+    return TypeSetIncludes(&types, input, inputTypes);
+}
+
+static bool
+CanStoreUnboxedType(TempAllocator& alloc, JSValueType unboxedType, MDefinition* value)
+{
+    return CanStoreUnboxedType(alloc, unboxedType, value->type(), value->resultTypeSet());
+}
+
 bool
 MPhi::specializeType(TempAllocator& alloc)
 {
@@ -4776,31 +4810,67 @@ MBeta::printOpcode(GenericPrinter& out) const
 bool
 MCreateThisWithTemplate::canRecoverOnBailout() const
 {
-    MOZ_ASSERT(templateObject()->is<PlainObject>());
-    MOZ_ASSERT(!templateObject()->as<PlainObject>().denseElementsAreCopyOnWrite());
+    MOZ_ASSERT(templateObject()->is<PlainObject>() || templateObject()->is<UnboxedPlainObject>());
+    MOZ_ASSERT_IF(templateObject()->is<PlainObject>(),
+                  !templateObject()->as<PlainObject>().denseElementsAreCopyOnWrite());
+    return true;
+}
+
+bool
+OperandIndexMap::init(TempAllocator& alloc, JSObject* templateObject)
+{
+    const UnboxedLayout& layout =
+        templateObject->as<UnboxedPlainObject>().layoutDontCheckGeneration();
+
+    const UnboxedLayout::PropertyVector& properties = layout.properties();
+    MOZ_ASSERT(properties.length() < 255);
+
+    // Allocate an array of indexes, where the top of each field correspond to
+    // the index of the operand in the MObjectState instance.
+    if (!map.init(alloc, layout.size()))
+        return false;
+
+    // Reset all indexes to 0, which is an error code.
+    for (size_t i = 0; i < map.length(); i++)
+        map[i] = 0;
+
+    // Map the property offsets to the indexes of MObjectState operands.
+    uint8_t index = 1;
+    for (size_t i = 0; i < properties.length(); i++, index++)
+        map[properties[i].offset] = index;
+
     return true;
 }
 
 MObjectState::MObjectState(MObjectState* state)
   : numSlots_(state->numSlots_),
-    numFixedSlots_(state->numFixedSlots_)
+    numFixedSlots_(state->numFixedSlots_),
+    operandIndex_(state->operandIndex_)
 {
     // This instruction is only used as a summary for bailout paths.
     setResultType(MIRType::Object);
     setRecoveredOnBailout();
 }
 
-MObjectState::MObjectState(JSObject* templateObject)
+MObjectState::MObjectState(JSObject *templateObject, OperandIndexMap* operandIndex)
 {
     // This instruction is only used as a summary for bailout paths.
     setResultType(MIRType::Object);
     setRecoveredOnBailout();
 
-    MOZ_ASSERT(templateObject->is<NativeObject>());
+    if (templateObject->is<NativeObject>()) {
+        NativeObject* nativeObject = &templateObject->as<NativeObject>();
+        numSlots_ = nativeObject->slotSpan();
+        numFixedSlots_ = nativeObject->numFixedSlots();
+    } else {
+        const UnboxedLayout& layout =
+            templateObject->as<UnboxedPlainObject>().layoutDontCheckGeneration();
+        // Same as UnboxedLayout::makeNativeGroup
+        numSlots_ = layout.properties().length();
+        numFixedSlots_ = gc::GetGCKindSlots(layout.getAllocKind());
+    }
 
-    NativeObject* nativeObject = &templateObject->as<NativeObject>();
-    numSlots_ = nativeObject->slotSpan();
-    numFixedSlots_ = nativeObject->numFixedSlots();
+    operandIndex_ = operandIndex;
 }
 
 JSObject*
@@ -4835,21 +4905,39 @@ MObjectState::initFromTemplateObject(TempAllocator& alloc, MDefinition* undefine
     // the template object. This is needed to account values which are baked in
     // the template objects and not visible in IonMonkey, such as the
     // uninitialized-lexical magic value of call objects.
-    NativeObject& nativeObject = templateObject->as<NativeObject>();
-    MOZ_ASSERT(nativeObject.slotSpan() == numSlots());
-
-	MOZ_ASSERT(templateObject->is<NativeObject>());
-    for (size_t i = 0; i < numSlots(); i++) {
-        Value val = nativeObject.getSlot(i);
-        MDefinition *def = undefinedVal;
-        if (!val.isUndefined()) {
-            MConstant* ins = val.isObject() ?
-                MConstant::NewConstraintlessObject(alloc, &val.toObject()) :
-                MConstant::New(alloc, val);
-            block()->insertBefore(this, ins);
-            def = ins;
+    if (templateObject->is<UnboxedPlainObject>()) {
+        UnboxedPlainObject& unboxedObject = templateObject->as<UnboxedPlainObject>();
+        const UnboxedLayout& layout = unboxedObject.layoutDontCheckGeneration();
+        const UnboxedLayout::PropertyVector& properties = layout.properties();
+
+        for (size_t i = 0; i < properties.length(); i++) {
+            Value val = unboxedObject.getValue(properties[i], /* maybeUninitialized = */ true);
+            MDefinition *def = undefinedVal;
+            if (!val.isUndefined()) {
+                MConstant* ins = val.isObject() ?
+                    MConstant::NewConstraintlessObject(alloc, &val.toObject()) :
+                    MConstant::New(alloc, val);
+                block()->insertBefore(this, ins);
+                def = ins;
+            }
+            initSlot(i, def);
+        }
+    } else {
+        NativeObject& nativeObject = templateObject->as<NativeObject>();
+        MOZ_ASSERT(nativeObject.slotSpan() == numSlots());
+
+        for (size_t i = 0; i < numSlots(); i++) {
+            Value val = nativeObject.getSlot(i);
+            MDefinition *def = undefinedVal;
+            if (!val.isUndefined()) {
+                MConstant* ins = val.isObject() ?
+                    MConstant::NewConstraintlessObject(alloc, &val.toObject()) :
+                    MConstant::New(alloc, val);
+                block()->insertBefore(this, ins);
+                def = ins;
+            }
+            initSlot(i, def);
         }
-        initSlot(i, def);
     }
     return true;
 }
@@ -4860,7 +4948,14 @@ MObjectState::New(TempAllocator& alloc, MDefinition* obj)
     JSObject* templateObject = templateObjectOf(obj);
     MOZ_ASSERT(templateObject, "Unexpected object creation.");
 
-    MObjectState* res = new(alloc) MObjectState(templateObject);
+    OperandIndexMap* operandIndex = nullptr;
+    if (templateObject->is<UnboxedPlainObject>()) {
+        operandIndex = new(alloc) OperandIndexMap;
+        if (!operandIndex || !operandIndex->init(alloc, templateObject))
+            return nullptr;
+    }
+
+    MObjectState* res = new(alloc) MObjectState(templateObject, operandIndex);
     if (!res || !res->init(alloc, obj))
         return nullptr;
     return res;
@@ -5767,6 +5862,35 @@ MGetFirstDollarIndex::foldsTo(TempAllocator& alloc)
     return MConstant::New(alloc, Int32Value(index));
 }
 
+MConvertUnboxedObjectToNative*
+MConvertUnboxedObjectToNative::New(TempAllocator& alloc, MDefinition* obj, ObjectGroup* group)
+{
+    MConvertUnboxedObjectToNative* res = new(alloc) MConvertUnboxedObjectToNative(obj, group);
+
+    ObjectGroup* nativeGroup = group->unboxedLayout().nativeGroup();
+
+    // Make a new type set for the result of this instruction which replaces
+    // the input group with the native group we will convert it to.
+    TemporaryTypeSet* types = obj->resultTypeSet();
+    if (types && !types->unknownObject()) {
+        TemporaryTypeSet* newTypes = types->cloneWithoutObjects(alloc.lifoAlloc());
+        if (newTypes) {
+            for (size_t i = 0; i < types->getObjectCount(); i++) {
+                TypeSet::ObjectKey* key = types->getObject(i);
+                if (!key)
+                    continue;
+                if (key->unknownProperties() || !key->isGroup() || key->group() != group)
+                    newTypes->addType(TypeSet::ObjectType(key), alloc.lifoAlloc());
+                else
+                    newTypes->addType(TypeSet::ObjectType(nativeGroup), alloc.lifoAlloc());
+            }
+            res->setResultTypeSet(newTypes);
+        }
+    }
+
+    return res;
+}
+
 bool
 jit::ElementAccessIsDenseNative(CompilerConstraintList* constraints,
                                 MDefinition* obj, MDefinition* id)
@@ -5786,6 +5910,48 @@ jit::ElementAccessIsDenseNative(CompilerConstraintList* constraints,
     return clasp && clasp->isNative() && !IsTypedArrayClass(clasp);
 }
 
+JSValueType
+jit::UnboxedArrayElementType(CompilerConstraintList* constraints, MDefinition* obj,
+                             MDefinition* id)
+{
+    if (obj->mightBeType(MIRType::String))
+        return JSVAL_TYPE_MAGIC;
+
+    if (id && id->type() != MIRType::Int32 && id->type() != MIRType::Double)
+        return JSVAL_TYPE_MAGIC;
+
+    TemporaryTypeSet* types = obj->resultTypeSet();
+    if (!types || types->unknownObject())
+        return JSVAL_TYPE_MAGIC;
+
+    JSValueType elementType = JSVAL_TYPE_MAGIC;
+    for (unsigned i = 0; i < types->getObjectCount(); i++) {
+        TypeSet::ObjectKey* key = types->getObject(i);
+        if (!key)
+            continue;
+
+        if (key->unknownProperties() || !key->isGroup())
+            return JSVAL_TYPE_MAGIC;
+
+        if (key->clasp() != &UnboxedArrayObject::class_)
+            return JSVAL_TYPE_MAGIC;
+
+        const UnboxedLayout &layout = key->group()->unboxedLayout();
+
+        if (layout.nativeGroup())
+            return JSVAL_TYPE_MAGIC;
+
+        if (elementType == layout.elementType() || elementType == JSVAL_TYPE_MAGIC)
+            elementType = layout.elementType();
+        else
+            return JSVAL_TYPE_MAGIC;
+
+        key->watchStateChangeForUnboxedConvertedToNative(constraints);
+    }
+
+    return elementType;
+}
+
 bool
 jit::ElementAccessIsTypedArray(CompilerConstraintList* constraints,
                                MDefinition* obj, MDefinition* id,
@@ -5945,6 +6111,11 @@ ObjectSubsumes(TypeSet::ObjectKey* first, TypeSet::ObjectKey* second)
                firstElements.maybeTypes()->equals(secondElements.maybeTypes());
     }
 
+    if (first->clasp() == &UnboxedArrayObject::class_) {
+        return first->group()->unboxedLayout().elementType() ==
+               second->group()->unboxedLayout().elementType();
+    }
+
     return false;
 }
 
@@ -6399,6 +6570,23 @@ jit::PropertyWriteNeedsTypeBarrier(TempAllocator& alloc, CompilerConstraintList*
         }
     }
 
+    // Perform additional filtering to make sure that any unboxed property
+    // being written can accommodate the value.
+    for (size_t i = 0; i < types->getObjectCount(); i++) {
+        TypeSet::ObjectKey* key = types->getObject(i);
+        if (key && key->isGroup() && key->group()->maybeUnboxedLayout()) {
+            const UnboxedLayout& layout = key->group()->unboxedLayout();
+            if (name) {
+                const UnboxedLayout::Property* property = layout.lookup(name);
+                if (property && !CanStoreUnboxedType(alloc, property->type, *pvalue))
+                    return true;
+            } else {
+                if (layout.isArray() && !CanStoreUnboxedType(alloc, layout.elementType(), *pvalue))
+                    return true;
+            }
+        }
+    }
+
     if (success)
         return false;
 
@@ -6429,6 +6617,17 @@ jit::PropertyWriteNeedsTypeBarrier(TempAllocator& alloc, CompilerConstraintList*
 
     MOZ_ASSERT(excluded);
 
+    // If the excluded object is a group with an unboxed layout, make sure it
+    // does not have a corresponding native group. Objects with the native
+    // group might appear even though they are not in the type set.
+    if (excluded->isGroup()) {
+        if (UnboxedLayout* layout = excluded->group()->maybeUnboxedLayout()) {
+            if (layout->nativeGroup())
+                return true;
+            excluded->watchStateChangeForUnboxedConvertedToNative(constraints);
+        }
+    }
+
     *pobj = AddGroupGuard(alloc, current, *pobj, excluded, /* bailOnEquality = */ true);
     return false;
 }
diff --git a/js/src/jit/MIR.h b/js/src/jit/MIR.h
index 0c1e77f80..af0abc695 100644
--- a/js/src/jit/MIR.h
+++ b/js/src/jit/MIR.h
@@ -30,6 +30,7 @@
 #include "vm/EnvironmentObject.h"
 #include "vm/SharedMem.h"
 #include "vm/TypedArrayCommon.h"
+#include "vm/UnboxedObject.h"
 
 // Undo windows.h damage on Win64
 #undef MemoryBarrier
@@ -375,7 +376,8 @@ class AliasSet {
         Element           = 1 << 1, // A Value member of obj->elements or
                                     // a typed object.
         UnboxedElement    = 1 << 2, // An unboxed scalar or reference member of
-                                    // typed object.
+                                    // a typed array, typed object, or unboxed
+                                    // object.
         DynamicSlot       = 1 << 3, // A Value member of obj->slots.
         FixedSlot         = 1 << 4, // A Value member of obj->fixedSlots().
         DOMProperty       = 1 << 5, // A DOM property
@@ -432,6 +434,9 @@ class AliasSet {
         MOZ_ASSERT(flags && !(flags & Store_));
         return AliasSet(flags | Store_);
     }
+    static uint32_t BoxedOrUnboxedElements(JSValueType type) {
+        return (type == JSVAL_TYPE_MAGIC) ? Element : UnboxedElement;
+    }
 };
 
 typedef Vector<MDefinition*, 6, JitAllocPolicy> MDefinitionVector;
@@ -3758,9 +3763,14 @@ class MObjectState
 {
   private:
     uint32_t numSlots_;
-    uint32_t numFixedSlots_;
+    uint32_t numFixedSlots_;        // valid if isUnboxed() == false.
+    OperandIndexMap* operandIndex_; // valid if isUnboxed() == true.
+
+    bool isUnboxed() const {
+        return operandIndex_ != nullptr;
+    }
 
-    MObjectState(JSObject *templateObject);
+    MObjectState(JSObject *templateObject, OperandIndexMap* operandIndex);
     explicit MObjectState(MObjectState* state);
 
     MOZ_MUST_USE bool init(TempAllocator& alloc, MDefinition* obj);
@@ -3820,6 +3830,18 @@ class MObjectState
         setSlot(slot + numFixedSlots(), def);
     }
 
+    // Interface reserved for unboxed objects.
+    bool hasOffset(uint32_t offset) const {
+        MOZ_ASSERT(isUnboxed());
+        return offset < operandIndex_->map.length() && operandIndex_->map[offset] != 0;
+    }
+    MDefinition* getOffset(uint32_t offset) const {
+        return getOperand(operandIndex_->map[offset]);
+    }
+    void setOffset(uint32_t offset, MDefinition* def) {
+        replaceOperand(operandIndex_->map[offset], def);
+    }
+
     MOZ_MUST_USE bool writeRecoverData(CompactBufferWriter& writer) const override;
     bool canRecoverOnBailout() const override {
         return true;
@@ -8741,6 +8763,102 @@ class MSetInitializedLength
     ALLOW_CLONE(MSetInitializedLength)
 };
 
+// Load the length from an unboxed array.
+class MUnboxedArrayLength
+  : public MUnaryInstruction,
+    public SingleObjectPolicy::Data
+{
+    explicit MUnboxedArrayLength(MDefinition* object)
+      : MUnaryInstruction(object)
+    {
+        setResultType(MIRType::Int32);
+        setMovable();
+    }
+
+  public:
+    INSTRUCTION_HEADER(UnboxedArrayLength)
+    TRIVIAL_NEW_WRAPPERS
+    NAMED_OPERANDS((0, object))
+
+    bool congruentTo(const MDefinition* ins) const override {
+        return congruentIfOperandsEqual(ins);
+    }
+    AliasSet getAliasSet() const override {
+        return AliasSet::Load(AliasSet::ObjectFields);
+    }
+
+    ALLOW_CLONE(MUnboxedArrayLength)
+};
+
+// Load the initialized length from an unboxed array.
+class MUnboxedArrayInitializedLength
+  : public MUnaryInstruction,
+    public SingleObjectPolicy::Data
+{
+    explicit MUnboxedArrayInitializedLength(MDefinition* object)
+      : MUnaryInstruction(object)
+    {
+        setResultType(MIRType::Int32);
+        setMovable();
+    }
+
+  public:
+    INSTRUCTION_HEADER(UnboxedArrayInitializedLength)
+    TRIVIAL_NEW_WRAPPERS
+    NAMED_OPERANDS((0, object))
+
+    bool congruentTo(const MDefinition* ins) const override {
+        return congruentIfOperandsEqual(ins);
+    }
+    AliasSet getAliasSet() const override {
+        return AliasSet::Load(AliasSet::ObjectFields);
+    }
+
+    ALLOW_CLONE(MUnboxedArrayInitializedLength)
+};
+
+// Increment the initialized length of an unboxed array object.
+class MIncrementUnboxedArrayInitializedLength
+  : public MUnaryInstruction,
+    public SingleObjectPolicy::Data
+{
+    explicit MIncrementUnboxedArrayInitializedLength(MDefinition* obj)
+      : MUnaryInstruction(obj)
+    {}
+
+  public:
+    INSTRUCTION_HEADER(IncrementUnboxedArrayInitializedLength)
+    TRIVIAL_NEW_WRAPPERS
+    NAMED_OPERANDS((0, object))
+
+    AliasSet getAliasSet() const override {
+        return AliasSet::Store(AliasSet::ObjectFields);
+    }
+
+    ALLOW_CLONE(MIncrementUnboxedArrayInitializedLength)
+};
+
+// Set the initialized length of an unboxed array object.
+class MSetUnboxedArrayInitializedLength
+  : public MBinaryInstruction,
+    public SingleObjectPolicy::Data
+{
+    explicit MSetUnboxedArrayInitializedLength(MDefinition* obj, MDefinition* length)
+      : MBinaryInstruction(obj, length)
+    {}
+
+  public:
+    INSTRUCTION_HEADER(SetUnboxedArrayInitializedLength)
+    TRIVIAL_NEW_WRAPPERS
+    NAMED_OPERANDS((0, object), (1, length))
+
+    AliasSet getAliasSet() const override {
+        return AliasSet::Store(AliasSet::ObjectFields);
+    }
+
+    ALLOW_CLONE(MSetUnboxedArrayInitializedLength)
+};
+
 // Load the array length from an elements header.
 class MArrayLength
   : public MUnaryInstruction,
@@ -9234,19 +9352,23 @@ class MLoadElement
     ALLOW_CLONE(MLoadElement)
 };
 
-// Load a value from the elements vector of a native object.
+// Load a value from the elements vector for a dense native or unboxed array.
 // If the index is out-of-bounds, or the indexed slot has a hole, undefined is
 // returned instead.
 class MLoadElementHole
   : public MTernaryInstruction,
     public SingleObjectPolicy::Data
 {
+    // Unboxed element type, JSVAL_TYPE_MAGIC for dense native elements.
+    JSValueType unboxedType_;
+
     bool needsNegativeIntCheck_;
     bool needsHoleCheck_;
 
     MLoadElementHole(MDefinition* elements, MDefinition* index, MDefinition* initLength,
-                     bool needsHoleCheck)
+                     JSValueType unboxedType, bool needsHoleCheck)
       : MTernaryInstruction(elements, index, initLength),
+        unboxedType_(unboxedType),
         needsNegativeIntCheck_(true),
         needsHoleCheck_(needsHoleCheck)
     {
@@ -9268,6 +9390,9 @@ class MLoadElementHole
     TRIVIAL_NEW_WRAPPERS
     NAMED_OPERANDS((0, elements), (1, index), (2, initLength))
 
+    JSValueType unboxedType() const {
+        return unboxedType_;
+    }
     bool needsNegativeIntCheck() const {
         return needsNegativeIntCheck_;
     }
@@ -9278,6 +9403,8 @@ class MLoadElementHole
         if (!ins->isLoadElementHole())
             return false;
         const MLoadElementHole* other = ins->toLoadElementHole();
+        if (unboxedType() != other->unboxedType())
+            return false;
         if (needsHoleCheck() != other->needsHoleCheck())
             return false;
         if (needsNegativeIntCheck() != other->needsNegativeIntCheck())
@@ -9285,7 +9412,7 @@ class MLoadElementHole
         return congruentIfOperandsEqual(other);
     }
     AliasSet getAliasSet() const override {
-        return AliasSet::Load(AliasSet::Element);
+        return AliasSet::Load(AliasSet::BoxedOrUnboxedElements(unboxedType()));
     }
     void collectRangeInfoPreTrunc() override;
 
@@ -9465,17 +9592,20 @@ class MStoreElement
     ALLOW_CLONE(MStoreElement)
 };
 
-// Like MStoreElement, but supports indexes >= initialized length. The downside
-// is that we cannot hoist the elements vector and bounds check, since this
-// instruction may update the (initialized) length and reallocate the elements
-// vector.
+// Like MStoreElement, but supports indexes >= initialized length, and can
+// handle unboxed arrays. The downside is that we cannot hoist the elements
+// vector and bounds check, since this instruction may update the (initialized)
+// length and reallocate the elements vector.
 class MStoreElementHole
   : public MAryInstruction<4>,
     public MStoreElementCommon,
     public MixPolicy<SingleObjectPolicy, NoFloatPolicy<3> >::Data
 {
+    JSValueType unboxedType_;
+
     MStoreElementHole(MDefinition* object, MDefinition* elements,
-                      MDefinition* index, MDefinition* value)
+                      MDefinition* index, MDefinition* value, JSValueType unboxedType)
+      : unboxedType_(unboxedType)
     {
         initOperand(0, object);
         initOperand(1, elements);
@@ -9490,6 +9620,10 @@ class MStoreElementHole
     TRIVIAL_NEW_WRAPPERS
     NAMED_OPERANDS((0, object), (1, elements), (2, index), (3, value))
 
+    JSValueType unboxedType() const {
+        return unboxedType_;
+    }
+
     ALLOW_CLONE(MStoreElementHole)
 };
 
@@ -9500,11 +9634,13 @@ class MFallibleStoreElement
     public MStoreElementCommon,
     public MixPolicy<SingleObjectPolicy, NoFloatPolicy<3> >::Data
 {
+    JSValueType unboxedType_;
     bool strict_;
 
     MFallibleStoreElement(MDefinition* object, MDefinition* elements,
                           MDefinition* index, MDefinition* value,
-                          bool strict)
+                          JSValueType unboxedType, bool strict)
+      : unboxedType_(unboxedType)
     {
         initOperand(0, object);
         initOperand(1, elements);
@@ -9520,6 +9656,10 @@ class MFallibleStoreElement
     TRIVIAL_NEW_WRAPPERS
     NAMED_OPERANDS((0, object), (1, elements), (2, index), (3, value))
 
+    JSValueType unboxedType() const {
+        return unboxedType_;
+    }
+
     bool strict() const {
         return strict_;
     }
@@ -9610,6 +9750,59 @@ class MStoreUnboxedString
     ALLOW_CLONE(MStoreUnboxedString)
 };
 
+// Passes through an object, after ensuring it is converted from an unboxed
+// object to a native representation.
+class MConvertUnboxedObjectToNative
+  : public MUnaryInstruction,
+    public SingleObjectPolicy::Data
+{
+    CompilerObjectGroup group_;
+
+    explicit MConvertUnboxedObjectToNative(MDefinition* obj, ObjectGroup* group)
+      : MUnaryInstruction(obj),
+        group_(group)
+    {
+        setGuard();
+        setMovable();
+        setResultType(MIRType::Object);
+    }
+
+  public:
+    INSTRUCTION_HEADER(ConvertUnboxedObjectToNative)
+    NAMED_OPERANDS((0, object))
+
+    static MConvertUnboxedObjectToNative* New(TempAllocator& alloc, MDefinition* obj,
+                                              ObjectGroup* group);
+
+    ObjectGroup* group() const {
+        return group_;
+    }
+    bool congruentTo(const MDefinition* ins) const override {
+        if (!congruentIfOperandsEqual(ins))
+            return false;
+        return ins->toConvertUnboxedObjectToNative()->group() == group();
+    }
+    AliasSet getAliasSet() const override {
+        // This instruction can read and write to all parts of the object, but
+        // is marked as non-effectful so it can be consolidated by LICM and GVN
+        // and avoid inhibiting other optimizations.
+        //
+        // This is valid to do because when unboxed objects might have a native
+        // group they can be converted to, we do not optimize accesses to the
+        // unboxed objects and do not guard on their group or shape (other than
+        // in this opcode).
+        //
+        // Later accesses can assume the object has a native representation
+        // and optimize accordingly. Those accesses cannot be reordered before
+        // this instruction, however. This is prevented by chaining this
+        // instruction with the object itself, in the same way as MBoundsCheck.
+        return AliasSet::None();
+    }
+    bool appendRoots(MRootList& roots) const override {
+        return roots.append(group_);
+    }
+};
+
 // Array.prototype.pop or Array.prototype.shift on a dense array.
 class MArrayPopShift
   : public MUnaryInstruction,
@@ -9623,12 +9816,13 @@ class MArrayPopShift
 
   private:
     Mode mode_;
+    JSValueType unboxedType_;
     bool needsHoleCheck_;
     bool maybeUndefined_;
 
-    MArrayPopShift(MDefinition* object, Mode mode,
+    MArrayPopShift(MDefinition* object, Mode mode, JSValueType unboxedType,
                    bool needsHoleCheck, bool maybeUndefined)
-      : MUnaryInstruction(object), mode_(mode),
+      : MUnaryInstruction(object), mode_(mode), unboxedType_(unboxedType),
         needsHoleCheck_(needsHoleCheck), maybeUndefined_(maybeUndefined)
     { }
 
@@ -9646,8 +9840,12 @@ class MArrayPopShift
     bool mode() const {
         return mode_;
     }
+    JSValueType unboxedType() const {
+        return unboxedType_;
+    }
     AliasSet getAliasSet() const override {
-        return AliasSet::Store(AliasSet::ObjectFields | AliasSet::Element);
+        return AliasSet::Store(AliasSet::ObjectFields |
+                               AliasSet::BoxedOrUnboxedElements(unboxedType()));
     }
 
     ALLOW_CLONE(MArrayPopShift)
@@ -9658,8 +9856,10 @@ class MArrayPush
   : public MBinaryInstruction,
     public MixPolicy<SingleObjectPolicy, NoFloatPolicy<1> >::Data
 {
-    MArrayPush(MDefinition* object, MDefinition* value)
-      : MBinaryInstruction(object, value)
+    JSValueType unboxedType_;
+
+    MArrayPush(MDefinition* object, MDefinition* value, JSValueType unboxedType)
+      : MBinaryInstruction(object, value), unboxedType_(unboxedType)
     {
         setResultType(MIRType::Int32);
     }
@@ -9669,8 +9869,12 @@ class MArrayPush
     TRIVIAL_NEW_WRAPPERS
     NAMED_OPERANDS((0, object), (1, value))
 
+    JSValueType unboxedType() const {
+        return unboxedType_;
+    }
     AliasSet getAliasSet() const override {
-        return AliasSet::Store(AliasSet::ObjectFields | AliasSet::Element);
+        return AliasSet::Store(AliasSet::ObjectFields |
+                               AliasSet::BoxedOrUnboxedElements(unboxedType()));
     }
     void computeRange(TempAllocator& alloc) override;
 
@@ -9684,13 +9888,15 @@ class MArraySlice
 {
     CompilerObject templateObj_;
     gc::InitialHeap initialHeap_;
+    JSValueType unboxedType_;
 
     MArraySlice(CompilerConstraintList* constraints, MDefinition* obj,
                 MDefinition* begin, MDefinition* end,
-                JSObject* templateObj, gc::InitialHeap initialHeap)
+                JSObject* templateObj, gc::InitialHeap initialHeap, JSValueType unboxedType)
       : MTernaryInstruction(obj, begin, end),
         templateObj_(templateObj),
-        initialHeap_(initialHeap)
+        initialHeap_(initialHeap),
+        unboxedType_(unboxedType)
     {
         setResultType(MIRType::Object);
     }
@@ -9708,6 +9914,10 @@ class MArraySlice
         return initialHeap_;
     }
 
+    JSValueType unboxedType() const {
+        return unboxedType_;
+    }
+
     bool possiblyCalls() const override {
         return true;
     }
@@ -10972,6 +11182,11 @@ class MGuardShape
         setMovable();
         setResultType(MIRType::Object);
         setResultTypeSet(obj->resultTypeSet());
+
+        // Disallow guarding on unboxed object shapes. The group is better to
+        // guard on, and guarding on the shape can interact badly with
+        // MConvertUnboxedObjectToNative.
+        MOZ_ASSERT(shape->getObjectClass() != &UnboxedPlainObject::class_);
     }
 
   public:
@@ -11066,6 +11281,11 @@ class MGuardObjectGroup
         setGuard();
         setMovable();
         setResultType(MIRType::Object);
+
+        // Unboxed groups which might be converted to natives can't be guarded
+        // on, due to MConvertUnboxedObjectToNative.
+        MOZ_ASSERT_IF(group->maybeUnboxedLayoutDontCheckGeneration(),
+                      !group->unboxedLayoutDontCheckGeneration().nativeGroup());
     }
 
   public:
@@ -11174,6 +11394,73 @@ class MGuardClass
     ALLOW_CLONE(MGuardClass)
 };
 
+// Guard on the presence or absence of an unboxed object's expando.
+class MGuardUnboxedExpando
+  : public MUnaryInstruction,
+    public SingleObjectPolicy::Data
+{
+    bool requireExpando_;
+    BailoutKind bailoutKind_;
+
+    MGuardUnboxedExpando(MDefinition* obj, bool requireExpando, BailoutKind bailoutKind)
+      : MUnaryInstruction(obj),
+        requireExpando_(requireExpando),
+        bailoutKind_(bailoutKind)
+    {
+        setGuard();
+        setMovable();
+        setResultType(MIRType::Object);
+    }
+
+  public:
+    INSTRUCTION_HEADER(GuardUnboxedExpando)
+    TRIVIAL_NEW_WRAPPERS
+    NAMED_OPERANDS((0, object))
+
+    bool requireExpando() const {
+        return requireExpando_;
+    }
+    BailoutKind bailoutKind() const {
+        return bailoutKind_;
+    }
+    bool congruentTo(const MDefinition* ins) const override {
+        if (!congruentIfOperandsEqual(ins))
+            return false;
+        if (requireExpando() != ins->toGuardUnboxedExpando()->requireExpando())
+            return false;
+        return true;
+    }
+    AliasSet getAliasSet() const override {
+        return AliasSet::Load(AliasSet::ObjectFields);
+    }
+};
+
+// Load an unboxed plain object's expando.
+class MLoadUnboxedExpando
+  : public MUnaryInstruction,
+    public SingleObjectPolicy::Data
+{
+  private:
+    explicit MLoadUnboxedExpando(MDefinition* object)
+      : MUnaryInstruction(object)
+    {
+        setResultType(MIRType::Object);
+        setMovable();
+    }
+
+  public:
+    INSTRUCTION_HEADER(LoadUnboxedExpando)
+    TRIVIAL_NEW_WRAPPERS
+    NAMED_OPERANDS((0, object))
+
+    bool congruentTo(const MDefinition* ins) const override {
+        return congruentIfOperandsEqual(ins);
+    }
+    AliasSet getAliasSet() const override {
+        return AliasSet::Load(AliasSet::ObjectFields);
+    }
+};
+
 // Load from vp[slot] (slots that are not inline in an object).
 class MLoadSlot
   : public MUnaryInstruction,
@@ -12076,13 +12363,15 @@ class MInArray
 {
     bool needsHoleCheck_;
     bool needsNegativeIntCheck_;
+    JSValueType unboxedType_;
 
     MInArray(MDefinition* elements, MDefinition* index,
              MDefinition* initLength, MDefinition* object,
-             bool needsHoleCheck)
+             bool needsHoleCheck, JSValueType unboxedType)
       : MQuaternaryInstruction(elements, index, initLength, object),
         needsHoleCheck_(needsHoleCheck),
-        needsNegativeIntCheck_(true)
+        needsNegativeIntCheck_(true),
+        unboxedType_(unboxedType)
     {
         setResultType(MIRType::Boolean);
         setMovable();
@@ -12102,6 +12391,9 @@ class MInArray
     bool needsNegativeIntCheck() const {
         return needsNegativeIntCheck_;
     }
+    JSValueType unboxedType() const {
+        return unboxedType_;
+    }
     void collectRangeInfoPreTrunc() override;
     AliasSet getAliasSet() const override {
         return AliasSet::Load(AliasSet::Element);
@@ -12114,6 +12406,8 @@ class MInArray
             return false;
         if (needsNegativeIntCheck() != other->needsNegativeIntCheck())
             return false;
+        if (unboxedType() != other->unboxedType())
+            return false;
         return congruentIfOperandsEqual(other);
     }
 };
@@ -14014,6 +14308,8 @@ MDefinition::maybeConstantValue()
 
 bool ElementAccessIsDenseNative(CompilerConstraintList* constraints,
                                 MDefinition* obj, MDefinition* id);
+JSValueType UnboxedArrayElementType(CompilerConstraintList* constraints, MDefinition* obj,
+                                    MDefinition* id);
 bool ElementAccessIsTypedArray(CompilerConstraintList* constraints,
                                MDefinition* obj, MDefinition* id,
                                Scalar::Type* arrayType);
diff --git a/js/src/jit/MOpcodes.h b/js/src/jit/MOpcodes.h
index 589dde077..2f67f8039 100644
--- a/js/src/jit/MOpcodes.h
+++ b/js/src/jit/MOpcodes.h
@@ -188,6 +188,8 @@ namespace jit {
     _(GuardObjectGroup)                                                     \
     _(GuardObjectIdentity)                                                  \
     _(GuardClass)                                                           \
+    _(GuardUnboxedExpando)                                                  \
+    _(LoadUnboxedExpando)                                                   \
     _(ArrayLength)                                                          \
     _(SetArrayLength)                                                       \
     _(GetNextEntryForIterator)                                              \
@@ -199,6 +201,10 @@ namespace jit {
     _(SetTypedObjectOffset)                                                 \
     _(InitializedLength)                                                    \
     _(SetInitializedLength)                                                 \
+    _(UnboxedArrayLength)                                                   \
+    _(UnboxedArrayInitializedLength)                                        \
+    _(IncrementUnboxedArrayInitializedLength)                               \
+    _(SetUnboxedArrayInitializedLength)                                     \
     _(Not)                                                                  \
     _(BoundsCheck)                                                          \
     _(BoundsCheckLower)                                                     \
@@ -214,6 +220,7 @@ namespace jit {
     _(StoreUnboxedScalar)                                                   \
     _(StoreUnboxedObjectOrNull)                                             \
     _(StoreUnboxedString)                                                   \
+    _(ConvertUnboxedObjectToNative)                                         \
     _(ArrayPopShift)                                                        \
     _(ArrayPush)                                                            \
     _(ArraySlice)                                                           \
diff --git a/js/src/jit/MacroAssembler.cpp b/js/src/jit/MacroAssembler.cpp
index a739b9325..f633b9b7b 100644
--- a/js/src/jit/MacroAssembler.cpp
+++ b/js/src/jit/MacroAssembler.cpp
@@ -126,14 +126,20 @@ MacroAssembler::guardTypeSetMightBeIncomplete(TypeSet* types, Register obj, Regi
 {
     // Type set guards might miss when an object's group changes. In this case
     // either its old group's properties will become unknown, or it will change
-    // to a native object. Jump to label if this might have happened for the
-    // input object.
+    // to a native object with an original unboxed group. Jump to label if this
+    // might have happened for the input object.
 
     if (types->unknownObject()) {
         jump(label);
         return;
     }
 
+    loadPtr(Address(obj, JSObject::offsetOfGroup()), scratch);
+    load32(Address(scratch, ObjectGroup::offsetOfFlags()), scratch);
+    and32(Imm32(OBJECT_FLAG_ADDENDUM_MASK), scratch);
+    branch32(Assembler::Equal,
+             scratch, Imm32(ObjectGroup::addendumOriginalUnboxedGroupValue()), label);
+
     for (size_t i = 0; i < types->getObjectCount(); i++) {
         if (JSObject* singleton = types->getSingletonNoBarrier(i)) {
             movePtr(ImmGCPtr(singleton), scratch);
@@ -462,6 +468,268 @@ template void MacroAssembler::loadFromTypedArray(Scalar::Type arrayType, const A
 template void MacroAssembler::loadFromTypedArray(Scalar::Type arrayType, const BaseIndex& src, const ValueOperand& dest,
                                                  bool allowDouble, Register temp, Label* fail);
 
+template <typename T>
+void
+MacroAssembler::loadUnboxedProperty(T address, JSValueType type, TypedOrValueRegister output)
+{
+    switch (type) {
+      case JSVAL_TYPE_INT32: {
+          // Handle loading an int32 into a double reg.
+          if (output.type() == MIRType::Double) {
+              convertInt32ToDouble(address, output.typedReg().fpu());
+              break;
+          }
+          MOZ_FALLTHROUGH;
+      }
+
+      case JSVAL_TYPE_BOOLEAN:
+      case JSVAL_TYPE_STRING: {
+        Register outReg;
+        if (output.hasValue()) {
+            outReg = output.valueReg().scratchReg();
+        } else {
+            MOZ_ASSERT(output.type() == MIRTypeFromValueType(type));
+            outReg = output.typedReg().gpr();
+        }
+
+        switch (type) {
+          case JSVAL_TYPE_BOOLEAN:
+            load8ZeroExtend(address, outReg);
+            break;
+          case JSVAL_TYPE_INT32:
+            load32(address, outReg);
+            break;
+          case JSVAL_TYPE_STRING:
+            loadPtr(address, outReg);
+            break;
+          default:
+            MOZ_CRASH();
+        }
+
+        if (output.hasValue())
+            tagValue(type, outReg, output.valueReg());
+        break;
+      }
+
+      case JSVAL_TYPE_OBJECT:
+        if (output.hasValue()) {
+            Register scratch = output.valueReg().scratchReg();
+            loadPtr(address, scratch);
+
+            Label notNull, done;
+            branchPtr(Assembler::NotEqual, scratch, ImmWord(0), &notNull);
+
+            moveValue(NullValue(), output.valueReg());
+            jump(&done);
+
+            bind(&notNull);
+            tagValue(JSVAL_TYPE_OBJECT, scratch, output.valueReg());
+
+            bind(&done);
+        } else {
+            // Reading null can't be possible here, as otherwise the result
+            // would be a value (either because null has been read before or
+            // because there is a barrier).
+            Register reg = output.typedReg().gpr();
+            loadPtr(address, reg);
+#ifdef DEBUG
+            Label ok;
+            branchTestPtr(Assembler::NonZero, reg, reg, &ok);
+            assumeUnreachable("Null not possible");
+            bind(&ok);
+#endif
+        }
+        break;
+
+      case JSVAL_TYPE_DOUBLE:
+        // Note: doubles in unboxed objects are not accessed through other
+        // views and do not need canonicalization.
+        if (output.hasValue())
+            loadValue(address, output.valueReg());
+        else
+            loadDouble(address, output.typedReg().fpu());
+        break;
+
+      default:
+        MOZ_CRASH();
+    }
+}
+
+template void
+MacroAssembler::loadUnboxedProperty(Address address, JSValueType type,
+                                    TypedOrValueRegister output);
+
+template void
+MacroAssembler::loadUnboxedProperty(BaseIndex address, JSValueType type,
+                                    TypedOrValueRegister output);
+
+static void
+StoreUnboxedFailure(MacroAssembler& masm, Label* failure)
+{
+    // Storing a value to an unboxed property is a fallible operation and
+    // the caller must provide a failure label if a particular unboxed store
+    // might fail. Sometimes, however, a store that cannot succeed (such as
+    // storing a string to an int32 property) will be marked as infallible.
+    // This can only happen if the code involved is unreachable.
+    if (failure)
+        masm.jump(failure);
+    else
+        masm.assumeUnreachable("Incompatible write to unboxed property");
+}
+
+template <typename T>
+void
+MacroAssembler::storeUnboxedProperty(T address, JSValueType type,
+                                     const ConstantOrRegister& value, Label* failure)
+{
+    switch (type) {
+      case JSVAL_TYPE_BOOLEAN:
+        if (value.constant()) {
+            if (value.value().isBoolean())
+                store8(Imm32(value.value().toBoolean()), address);
+            else
+                StoreUnboxedFailure(*this, failure);
+        } else if (value.reg().hasTyped()) {
+            if (value.reg().type() == MIRType::Boolean)
+                store8(value.reg().typedReg().gpr(), address);
+            else
+                StoreUnboxedFailure(*this, failure);
+        } else {
+            if (failure)
+                branchTestBoolean(Assembler::NotEqual, value.reg().valueReg(), failure);
+            storeUnboxedPayload(value.reg().valueReg(), address, /* width = */ 1);
+        }
+        break;
+
+      case JSVAL_TYPE_INT32:
+        if (value.constant()) {
+            if (value.value().isInt32())
+                store32(Imm32(value.value().toInt32()), address);
+            else
+                StoreUnboxedFailure(*this, failure);
+        } else if (value.reg().hasTyped()) {
+            if (value.reg().type() == MIRType::Int32)
+                store32(value.reg().typedReg().gpr(), address);
+            else
+                StoreUnboxedFailure(*this, failure);
+        } else {
+            if (failure)
+                branchTestInt32(Assembler::NotEqual, value.reg().valueReg(), failure);
+            storeUnboxedPayload(value.reg().valueReg(), address, /* width = */ 4);
+        }
+        break;
+
+      case JSVAL_TYPE_DOUBLE:
+        if (value.constant()) {
+            if (value.value().isNumber()) {
+                loadConstantDouble(value.value().toNumber(), ScratchDoubleReg);
+                storeDouble(ScratchDoubleReg, address);
+            } else {
+                StoreUnboxedFailure(*this, failure);
+            }
+        } else if (value.reg().hasTyped()) {
+            if (value.reg().type() == MIRType::Int32) {
+                convertInt32ToDouble(value.reg().typedReg().gpr(), ScratchDoubleReg);
+                storeDouble(ScratchDoubleReg, address);
+            } else if (value.reg().type() == MIRType::Double) {
+                storeDouble(value.reg().typedReg().fpu(), address);
+            } else {
+                StoreUnboxedFailure(*this, failure);
+            }
+        } else {
+            ValueOperand reg = value.reg().valueReg();
+            Label notInt32, end;
+            branchTestInt32(Assembler::NotEqual, reg, &notInt32);
+            int32ValueToDouble(reg, ScratchDoubleReg);
+            storeDouble(ScratchDoubleReg, address);
+            jump(&end);
+            bind(&notInt32);
+            if (failure)
+                branchTestDouble(Assembler::NotEqual, reg, failure);
+            storeValue(reg, address);
+            bind(&end);
+        }
+        break;
+
+      case JSVAL_TYPE_OBJECT:
+        if (value.constant()) {
+            if (value.value().isObjectOrNull())
+                storePtr(ImmGCPtr(value.value().toObjectOrNull()), address);
+            else
+                StoreUnboxedFailure(*this, failure);
+        } else if (value.reg().hasTyped()) {
+            MOZ_ASSERT(value.reg().type() != MIRType::Null);
+            if (value.reg().type() == MIRType::Object)
+                storePtr(value.reg().typedReg().gpr(), address);
+            else
+                StoreUnboxedFailure(*this, failure);
+        } else {
+            if (failure) {
+                Label ok;
+                branchTestNull(Assembler::Equal, value.reg().valueReg(), &ok);
+                branchTestObject(Assembler::NotEqual, value.reg().valueReg(), failure);
+                bind(&ok);
+            }
+            storeUnboxedPayload(value.reg().valueReg(), address, /* width = */ sizeof(uintptr_t));
+        }
+        break;
+
+      case JSVAL_TYPE_STRING:
+        if (value.constant()) {
+            if (value.value().isString())
+                storePtr(ImmGCPtr(value.value().toString()), address);
+            else
+                StoreUnboxedFailure(*this, failure);
+        } else if (value.reg().hasTyped()) {
+            if (value.reg().type() == MIRType::String)
+                storePtr(value.reg().typedReg().gpr(), address);
+            else
+                StoreUnboxedFailure(*this, failure);
+        } else {
+            if (failure)
+                branchTestString(Assembler::NotEqual, value.reg().valueReg(), failure);
+            storeUnboxedPayload(value.reg().valueReg(), address, /* width = */ sizeof(uintptr_t));
+        }
+        break;
+
+      default:
+        MOZ_CRASH();
+    }
+}
+
+template void
+MacroAssembler::storeUnboxedProperty(Address address, JSValueType type,
+                                     const ConstantOrRegister& value, Label* failure);
+
+template void
+MacroAssembler::storeUnboxedProperty(BaseIndex address, JSValueType type,
+                                     const ConstantOrRegister& value, Label* failure);
+
+void
+MacroAssembler::checkUnboxedArrayCapacity(Register obj, const RegisterOrInt32Constant& index,
+                                          Register temp, Label* failure)
+{
+    Address initLengthAddr(obj, UnboxedArrayObject::offsetOfCapacityIndexAndInitializedLength());
+    Address lengthAddr(obj, UnboxedArrayObject::offsetOfLength());
+
+    Label capacityIsIndex, done;
+    load32(initLengthAddr, temp);
+    branchTest32(Assembler::NonZero, temp, Imm32(UnboxedArrayObject::CapacityMask), &capacityIsIndex);
+    branch32(Assembler::BelowOrEqual, lengthAddr, index, failure);
+    jump(&done);
+    bind(&capacityIsIndex);
+
+    // Do a partial shift so that we can get an absolute offset from the base
+    // of CapacityArray to use.
+    JS_STATIC_ASSERT(sizeof(UnboxedArrayObject::CapacityArray[0]) == 4);
+    rshiftPtr(Imm32(UnboxedArrayObject::CapacityShift - 2), temp);
+    and32(Imm32(~0x3), temp);
+
+    addPtr(ImmPtr(&UnboxedArrayObject::CapacityArray), temp);
+    branch32(Assembler::BelowOrEqual, Address(temp, 0), index, failure);
+    bind(&done);
+}
+
 // Inlined version of gc::CheckAllocatorState that checks the bare essentials
 // and bails for anything that cannot be handled with our jit allocators.
 void
@@ -1009,6 +1277,20 @@ MacroAssembler::initGCThing(Register obj, Register temp, JSObject* templateObj,
             nbytes = (nbytes < sizeof(uintptr_t)) ? 0 : nbytes - sizeof(uintptr_t);
             offset += sizeof(uintptr_t);
         }
+    } else if (templateObj->is<UnboxedPlainObject>()) {
+        storePtr(ImmWord(0), Address(obj, UnboxedPlainObject::offsetOfExpando()));
+        if (initContents)
+            initUnboxedObjectContents(obj, &templateObj->as<UnboxedPlainObject>());
+    } else if (templateObj->is<UnboxedArrayObject>()) {
+        MOZ_ASSERT(templateObj->as<UnboxedArrayObject>().hasInlineElements());
+        int elementsOffset = UnboxedArrayObject::offsetOfInlineElements();
+        computeEffectiveAddress(Address(obj, elementsOffset), temp);
+        storePtr(temp, Address(obj, UnboxedArrayObject::offsetOfElements()));
+        store32(Imm32(templateObj->as<UnboxedArrayObject>().length()),
+                Address(obj, UnboxedArrayObject::offsetOfLength()));
+        uint32_t capacityIndex = templateObj->as<UnboxedArrayObject>().capacityIndex();
+        store32(Imm32(capacityIndex << UnboxedArrayObject::CapacityShift),
+                Address(obj, UnboxedArrayObject::offsetOfCapacityIndexAndInitializedLength()));
     } else {
         MOZ_CRASH("Unknown object");
     }
@@ -1030,6 +1312,29 @@ MacroAssembler::initGCThing(Register obj, Register temp, JSObject* templateObj,
 }
 
 void
+MacroAssembler::initUnboxedObjectContents(Register object, UnboxedPlainObject* templateObject)
+{
+    const UnboxedLayout& layout = templateObject->layoutDontCheckGeneration();
+
+    // Initialize reference fields of the object, per UnboxedPlainObject::create.
+    if (const int32_t* list = layout.traceList()) {
+        while (*list != -1) {
+            storePtr(ImmGCPtr(GetJitContext()->runtime->names().empty),
+                     Address(object, UnboxedPlainObject::offsetOfData() + *list));
+            list++;
+        }
+        list++;
+        while (*list != -1) {
+            storePtr(ImmWord(0),
+                     Address(object, UnboxedPlainObject::offsetOfData() + *list));
+            list++;
+        }
+        // Unboxed objects don't have Values to initialize.
+        MOZ_ASSERT(*(list + 1) == -1);
+    }
+}
+
+void
 MacroAssembler::compareStrings(JSOp op, Register left, Register right, Register result,
                                Label* fail)
 {
diff --git a/js/src/jit/MacroAssembler.h b/js/src/jit/MacroAssembler.h
index d5cc95839..b6616321c 100644
--- a/js/src/jit/MacroAssembler.h
+++ b/js/src/jit/MacroAssembler.h
@@ -36,6 +36,7 @@
 #include "vm/ProxyObject.h"
 #include "vm/Shape.h"
 #include "vm/TypedArrayObject.h"
+#include "vm/UnboxedObject.h"
 
 using mozilla::FloatingPoint;
 
@@ -1625,6 +1626,20 @@ class MacroAssembler : public MacroAssemblerSpecific
     void storeToTypedFloatArray(Scalar::Type arrayType, FloatRegister value, const Address& dest,
                                 unsigned numElems = 0);
 
+    // Load a property from an UnboxedPlainObject or UnboxedArrayObject.
+    template <typename T>
+    void loadUnboxedProperty(T address, JSValueType type, TypedOrValueRegister output);
+
+    // Store a property to an UnboxedPlainObject, without triggering barriers.
+    // If failure is null, the value definitely has a type suitable for storing
+    // in the property.
+    template <typename T>
+    void storeUnboxedProperty(T address, JSValueType type,
+                              const ConstantOrRegister& value, Label* failure);
+
+    void checkUnboxedArrayCapacity(Register obj, const RegisterOrInt32Constant& index,
+                                   Register temp, Label* failure);
+
     Register extractString(const Address& address, Register scratch) {
         return extractObject(address, scratch);
     }
@@ -1701,6 +1716,8 @@ class MacroAssembler : public MacroAssemblerSpecific
                              LiveRegisterSet liveRegs, Label* fail,
                              TypedArrayObject* templateObj, TypedArrayLength lengthKind);
 
+    void initUnboxedObjectContents(Register object, UnboxedPlainObject* templateObject);
+
     void newGCString(Register result, Register temp, Label* fail);
     void newGCFatInlineString(Register result, Register temp, Label* fail);
 
diff --git a/js/src/jit/OptimizationTracking.cpp b/js/src/jit/OptimizationTracking.cpp
index 7d72795a0..308def041 100644
--- a/js/src/jit/OptimizationTracking.cpp
+++ b/js/src/jit/OptimizationTracking.cpp
@@ -844,6 +844,8 @@ MaybeConstructorFromType(TypeSet::Type ty)
         return nullptr;
     ObjectGroup* obj = ty.group();
     TypeNewScript* newScript = obj->newScript();
+    if (!newScript && obj->maybeUnboxedLayout())
+        newScript = obj->unboxedLayout().newScript();
     return newScript ? newScript->function() : nullptr;
 }
 
diff --git a/js/src/jit/Recover.cpp b/js/src/jit/Recover.cpp
index 793b631df..6fd71f377 100644
--- a/js/src/jit/Recover.cpp
+++ b/js/src/jit/Recover.cpp
@@ -30,6 +30,7 @@
 
 #include "vm/Interpreter-inl.h"
 #include "vm/NativeObject-inl.h"
+#include "vm/UnboxedObject-inl.h"
 
 using namespace js;
 using namespace js::jit;
@@ -1354,7 +1355,7 @@ RNewArray::recover(JSContext* cx, SnapshotIterator& iter) const
     RootedValue result(cx);
     RootedObjectGroup group(cx, templateObject->group());
 
-    ArrayObject* resultObject = NewFullyAllocatedArrayTryUseGroup(cx, group, count_);
+    JSObject* resultObject = NewFullyAllocatedArrayTryUseGroup(cx, group, count_);
     if (!resultObject)
         return false;
 
@@ -1539,12 +1540,37 @@ RObjectState::recover(JSContext* cx, SnapshotIterator& iter) const
     RootedObject object(cx, &iter.read().toObject());
     RootedValue val(cx);
 
-    RootedNativeObject nativeObject(cx, &object->as<NativeObject>());
-    MOZ_ASSERT(nativeObject->slotSpan() == numSlots());
+    if (object->is<UnboxedPlainObject>()) {
+        const UnboxedLayout& layout = object->as<UnboxedPlainObject>().layout();
 
-    for (size_t i = 0; i < numSlots(); i++) {
-        val = iter.read();
-        nativeObject->setSlot(i, val);
+        RootedId id(cx);
+        RootedValue receiver(cx, ObjectValue(*object));
+        const UnboxedLayout::PropertyVector& properties = layout.properties();
+        for (size_t i = 0; i < properties.length(); i++) {
+            val = iter.read();
+
+            // This is the default placeholder value of MObjectState, when no
+            // properties are defined yet.
+            if (val.isUndefined())
+                continue;
+
+            id = NameToId(properties[i].name);
+            ObjectOpResult result;
+
+            // SetProperty can only fail due to OOM.
+            if (!SetProperty(cx, object, id, val, receiver, result))
+                return false;
+            if (!result)
+                return result.reportError(cx, object, id);
+        }
+    } else {
+        RootedNativeObject nativeObject(cx, &object->as<NativeObject>());
+        MOZ_ASSERT(nativeObject->slotSpan() == numSlots());
+
+        for (size_t i = 0; i < numSlots(); i++) {
+            val = iter.read();
+            nativeObject->setSlot(i, val);
+        }
     }
 
     val.setObject(*object);
diff --git a/js/src/jit/ScalarReplacement.cpp b/js/src/jit/ScalarReplacement.cpp
index 97ba52349..4614b2162 100644
--- a/js/src/jit/ScalarReplacement.cpp
+++ b/js/src/jit/ScalarReplacement.cpp
@@ -13,6 +13,7 @@
 #include "jit/MIR.h"
 #include "jit/MIRGenerator.h"
 #include "jit/MIRGraph.h"
+#include "vm/UnboxedObject.h"
 
 #include "jsobjinlines.h"
 
@@ -182,6 +183,25 @@ IsObjectEscaped(MInstruction* ins, JSObject* objDefault)
             JitSpewDef(JitSpew_Escape, "is escaped by\n", def);
             return true;
 
+          case MDefinition::Op_LoadUnboxedScalar:
+          case MDefinition::Op_StoreUnboxedScalar:
+          case MDefinition::Op_LoadUnboxedObjectOrNull:
+          case MDefinition::Op_StoreUnboxedObjectOrNull:
+          case MDefinition::Op_LoadUnboxedString:
+          case MDefinition::Op_StoreUnboxedString:
+            // Not escaped if it is the first argument.
+            if (def->indexOf(*i) != 0) {
+                JitSpewDef(JitSpew_Escape, "is escaped by\n", def);
+                return true;
+            }
+
+            if (!def->getOperand(1)->isConstant()) {
+                JitSpewDef(JitSpew_Escape, "is addressed with unknown index\n", def);
+                return true;
+            }
+
+            break;
+
           case MDefinition::Op_PostWriteBarrier:
             break;
 
@@ -285,6 +305,16 @@ class ObjectMemoryView : public MDefinitionVisitorDefaultNoop
     void visitGuardShape(MGuardShape* ins);
     void visitFunctionEnvironment(MFunctionEnvironment* ins);
     void visitLambda(MLambda* ins);
+    void visitStoreUnboxedScalar(MStoreUnboxedScalar* ins);
+    void visitLoadUnboxedScalar(MLoadUnboxedScalar* ins);
+    void visitStoreUnboxedObjectOrNull(MStoreUnboxedObjectOrNull* ins);
+    void visitLoadUnboxedObjectOrNull(MLoadUnboxedObjectOrNull* ins);
+    void visitStoreUnboxedString(MStoreUnboxedString* ins);
+    void visitLoadUnboxedString(MLoadUnboxedString* ins);
+
+  private:
+    void storeOffset(MInstruction* ins, size_t offset, MDefinition* value);
+    void loadOffset(MInstruction* ins, size_t offset);
 };
 
 const char* ObjectMemoryView::phaseName = "Scalar Replacement of Object";
@@ -626,6 +656,121 @@ ObjectMemoryView::visitLambda(MLambda* ins)
     ins->setIncompleteObject();
 }
 
+static size_t
+GetOffsetOf(MDefinition* index, size_t width, int32_t baseOffset)
+{
+    int32_t idx = index->toConstant()->toInt32();
+    MOZ_ASSERT(idx >= 0);
+    MOZ_ASSERT(baseOffset >= 0 && size_t(baseOffset) >= UnboxedPlainObject::offsetOfData());
+    return idx * width + baseOffset - UnboxedPlainObject::offsetOfData();
+}
+
+static size_t
+GetOffsetOf(MDefinition* index, Scalar::Type type, int32_t baseOffset)
+{
+    return GetOffsetOf(index, Scalar::byteSize(type), baseOffset);
+}
+
+void
+ObjectMemoryView::storeOffset(MInstruction* ins, size_t offset, MDefinition* value)
+{
+    // Clone the state and update the slot value.
+    MOZ_ASSERT(state_->hasOffset(offset));
+    state_ = BlockState::Copy(alloc_, state_);
+    if (!state_) {
+        oom_ = true;
+        return;
+    }
+
+    state_->setOffset(offset, value);
+    ins->block()->insertBefore(ins, state_);
+
+    // Remove original instruction.
+    ins->block()->discard(ins);
+}
+
+void
+ObjectMemoryView::loadOffset(MInstruction* ins, size_t offset)
+{
+    // Replace load by the slot value.
+    MOZ_ASSERT(state_->hasOffset(offset));
+    ins->replaceAllUsesWith(state_->getOffset(offset));
+
+    // Remove original instruction.
+    ins->block()->discard(ins);
+}
+
+void
+ObjectMemoryView::visitStoreUnboxedScalar(MStoreUnboxedScalar* ins)
+{
+    // Skip stores made on other objects.
+    if (ins->elements() != obj_)
+        return;
+
+    size_t offset = GetOffsetOf(ins->index(), ins->storageType(), ins->offsetAdjustment());
+    storeOffset(ins, offset, ins->value());
+}
+
+void
+ObjectMemoryView::visitLoadUnboxedScalar(MLoadUnboxedScalar* ins)
+{
+    // Skip loads made on other objects.
+    if (ins->elements() != obj_)
+        return;
+
+    // Replace load by the slot value.
+    size_t offset = GetOffsetOf(ins->index(), ins->storageType(), ins->offsetAdjustment());
+    loadOffset(ins, offset);
+}
+
+void
+ObjectMemoryView::visitStoreUnboxedObjectOrNull(MStoreUnboxedObjectOrNull* ins)
+{
+    // Skip stores made on other objects.
+    if (ins->elements() != obj_)
+        return;
+
+    // Clone the state and update the slot value.
+    size_t offset = GetOffsetOf(ins->index(), sizeof(uintptr_t), ins->offsetAdjustment());
+    storeOffset(ins, offset, ins->value());
+}
+
+void
+ObjectMemoryView::visitLoadUnboxedObjectOrNull(MLoadUnboxedObjectOrNull* ins)
+{
+    // Skip loads made on other objects.
+    if (ins->elements() != obj_)
+        return;
+
+    // Replace load by the slot value.
+    size_t offset = GetOffsetOf(ins->index(), sizeof(uintptr_t), ins->offsetAdjustment());
+    loadOffset(ins, offset);
+}
+
+void
+ObjectMemoryView::visitStoreUnboxedString(MStoreUnboxedString* ins)
+{
+    // Skip stores made on other objects.
+    if (ins->elements() != obj_)
+        return;
+
+    // Clone the state and update the slot value.
+    size_t offset = GetOffsetOf(ins->index(), sizeof(uintptr_t), ins->offsetAdjustment());
+    storeOffset(ins, offset, ins->value());
+}
+
+void
+ObjectMemoryView::visitLoadUnboxedString(MLoadUnboxedString* ins)
+{
+    // Skip loads made on other objects.
+    if (ins->elements() != obj_)
+        return;
+
+    // Replace load by the slot value.
+    size_t offset = GetOffsetOf(ins->index(), sizeof(uintptr_t), ins->offsetAdjustment());
+    loadOffset(ins, offset);
+}
+
 static bool
 IndexOf(MDefinition* ins, int32_t* res)
 {
@@ -762,6 +907,11 @@ IsArrayEscaped(MInstruction* ins)
         return true;
     }
 
+    if (obj->is<UnboxedArrayObject>()) {
+        JitSpew(JitSpew_Escape, "Template object is an unboxed plain object.");
+        return true;
+    }
+
     if (length >= 16) {
         JitSpew(JitSpew_Escape, "Array has too many elements");
         return true;
diff --git a/js/src/jit/SharedIC.cpp b/js/src/jit/SharedIC.cpp
index 05a95824f..767cff661 100644
--- a/js/src/jit/SharedIC.cpp
+++ b/js/src/jit/SharedIC.cpp
@@ -27,7 +27,6 @@
 #endif
 #include "jit/VMFunctions.h"
 #include "vm/Interpreter.h"
-#include "vm/NativeObject-inl.h"
 
 #include "jit/MacroAssembler-inl.h"
 #include "vm/Interpreter-inl.h"
@@ -286,6 +285,11 @@ ICStub::trace(JSTracer* trc)
         TraceEdge(trc, &getElemStub->shape(), "baseline-getelem-dense-shape");
         break;
       }
+      case ICStub::GetElem_UnboxedArray: {
+        ICGetElem_UnboxedArray* getElemStub = toGetElem_UnboxedArray();
+        TraceEdge(trc, &getElemStub->group(), "baseline-getelem-unboxed-array-group");
+        break;
+      }
       case ICStub::GetElem_TypedArray: {
         ICGetElem_TypedArray* getElemStub = toGetElem_TypedArray();
         TraceEdge(trc, &getElemStub->shape(), "baseline-getelem-typedarray-shape");
@@ -2244,7 +2248,9 @@ IsCacheableProtoChain(JSObject* obj, JSObject* holder, bool isDOMProxy)
     if (!isDOMProxy && !obj->isNative()) {
         if (obj == holder)
             return false;
-        if (!obj->is<TypedObject>())
+        if (!obj->is<UnboxedPlainObject>() &&
+            !obj->is<UnboxedArrayObject>() &&
+            !obj->is<TypedObject>())
         {
             return false;
         }
@@ -2572,6 +2578,12 @@ CheckHasNoSuchProperty(JSContext* cx, JSObject* obj, PropertyName* name,
         } else if (curObj != obj) {
             // Non-native objects are only handled as the original receiver.
             return false;
+        } else if (curObj->is<UnboxedPlainObject>()) {
+            if (curObj->as<UnboxedPlainObject>().containsUnboxedOrExpandoProperty(cx, NameToId(name)))
+                return false;
+        } else if (curObj->is<UnboxedArrayObject>()) {
+            if (name == cx->names().length)
+                return false;
         } else if (curObj->is<TypedObject>()) {
             if (curObj->as<TypedObject>().typeDescr().hasProperty(cx->names(), NameToId(name)))
                 return false;
@@ -2836,15 +2848,34 @@ GuardReceiverObject(MacroAssembler& masm, ReceiverGuard guard,
 {
     Address groupAddress(ICStubReg, receiverGuardOffset + HeapReceiverGuard::offsetOfGroup());
     Address shapeAddress(ICStubReg, receiverGuardOffset + HeapReceiverGuard::offsetOfShape());
+    Address expandoAddress(object, UnboxedPlainObject::offsetOfExpando());
 
     if (guard.group) {
         masm.loadPtr(groupAddress, scratch);
         masm.branchTestObjGroup(Assembler::NotEqual, object, scratch, failure);
+
+        if (guard.group->clasp() == &UnboxedPlainObject::class_ && !guard.shape) {
+            // Guard the unboxed object has no expando object.
+            masm.branchPtr(Assembler::NotEqual, expandoAddress, ImmWord(0), failure);
+        }
     }
 
     if (guard.shape) {
         masm.loadPtr(shapeAddress, scratch);
-        masm.branchTestObjShape(Assembler::NotEqual, object, scratch, failure);
+        if (guard.group && guard.group->clasp() == &UnboxedPlainObject::class_) {
+            // Guard the unboxed object has a matching expando object.
+            masm.branchPtr(Assembler::Equal, expandoAddress, ImmWord(0), failure);
+            Label done;
+            masm.push(object);
+            masm.loadPtr(expandoAddress, object);
+            masm.branchTestObjShape(Assembler::Equal, object, scratch, &done);
+            masm.pop(object);
+            masm.jump(failure);
+            masm.bind(&done);
+            masm.pop(object);
+        } else {
+            masm.branchTestObjShape(Assembler::NotEqual, object, scratch, failure);
+        }
     }
 }
 
@@ -4228,7 +4259,8 @@ DoNewObject(JSContext* cx, void* payload, ICNewObject_Fallback* stub, MutableHan
                 return false;
 
             if (!stub->invalid() &&
-                !templateObject->as<PlainObject>().hasDynamicSlots())
+                (templateObject->is<UnboxedPlainObject>() ||
+                 !templateObject->as<PlainObject>().hasDynamicSlots()))
             {
                 JitCode* code = GenerateNewObjectWithTemplateCode(cx, templateObject);
                 if (!code)
diff --git a/js/src/jit/VMFunctions.cpp b/js/src/jit/VMFunctions.cpp
index 652c23bf1..10be2836b 100644
--- a/js/src/jit/VMFunctions.cpp
+++ b/js/src/jit/VMFunctions.cpp
@@ -28,7 +28,7 @@
 #include "vm/NativeObject-inl.h"
 #include "vm/StringObject-inl.h"
 #include "vm/TypeInference-inl.h"
-#include "gc/StoreBuffer-inl.h"
+#include "vm/UnboxedObject-inl.h"
 
 using namespace js;
 using namespace js::jit;
@@ -306,7 +306,7 @@ template bool StringsEqual<false>(JSContext* cx, HandleString lhs, HandleString
 bool
 ArrayPopDense(JSContext* cx, HandleObject obj, MutableHandleValue rval)
 {
-    MOZ_ASSERT(obj->is<ArrayObject>());
+    MOZ_ASSERT(obj->is<ArrayObject>() || obj->is<UnboxedArrayObject>());
 
     AutoDetectInvalidation adi(cx, rval);
 
@@ -325,11 +325,12 @@ ArrayPopDense(JSContext* cx, HandleObject obj, MutableHandleValue rval)
 }
 
 bool
-ArrayPushDense(JSContext* cx, HandleArrayObject arr, HandleValue v, uint32_t* length)
+ArrayPushDense(JSContext* cx, HandleObject obj, HandleValue v, uint32_t* length)
 {
-    *length = arr->length();
-    DenseElementResult result = arr->setOrExtendDenseElements(cx, *length, v.address(), 1,
-                                                              ShouldUpdateTypes::DontUpdate);
+    *length = GetAnyBoxedOrUnboxedArrayLength(obj);
+    DenseElementResult result =
+        SetOrExtendAnyBoxedOrUnboxedDenseElements(cx, obj, *length, v.address(), 1,
+                                                  ShouldUpdateTypes::DontUpdate);
     if (result != DenseElementResult::Incomplete) {
         (*length)++;
         return result == DenseElementResult::Success;
@@ -337,7 +338,7 @@ ArrayPushDense(JSContext* cx, HandleArrayObject arr, HandleValue v, uint32_t* le
 
     JS::AutoValueArray<3> argv(cx);
     argv[0].setUndefined();
-    argv[1].setObject(*arr);
+    argv[1].setObject(*obj);
     argv[2].set(v);
     if (!js::array_push(cx, 1, argv.begin()))
         return false;
@@ -349,7 +350,7 @@ ArrayPushDense(JSContext* cx, HandleArrayObject arr, HandleValue v, uint32_t* le
 bool
 ArrayShiftDense(JSContext* cx, HandleObject obj, MutableHandleValue rval)
 {
-    MOZ_ASSERT(obj->is<ArrayObject>());
+    MOZ_ASSERT(obj->is<ArrayObject>() || obj->is<UnboxedArrayObject>());
 
     AutoDetectInvalidation adi(cx, rval);
 
@@ -1130,14 +1131,16 @@ Recompile(JSContext* cx)
 }
 
 bool
-SetDenseElement(JSContext* cx, HandleNativeObject obj, int32_t index, HandleValue value, bool strict)
+SetDenseOrUnboxedArrayElement(JSContext* cx, HandleObject obj, int32_t index,
+                              HandleValue value, bool strict)
 {
     // This function is called from Ion code for StoreElementHole's OOL path.
-    // In this case we know the object is native and that no type changes are
-	// needed.
+    // In this case we know the object is native or an unboxed array and that
+    // no type changes are needed.
 
-    DenseElementResult result = obj->setOrExtendDenseElements(cx, index, value.address(), 1,
-                                                              ShouldUpdateTypes::DontUpdate);
+    DenseElementResult result =
+        SetOrExtendAnyBoxedOrUnboxedDenseElements(cx, obj, index, value.address(), 1,
+                                                  ShouldUpdateTypes::DontUpdate);
     if (result != DenseElementResult::Incomplete)
         return result == DenseElementResult::Success;
 
diff --git a/js/src/jit/VMFunctions.h b/js/src/jit/VMFunctions.h
index 94f741397..7f225c293 100644
--- a/js/src/jit/VMFunctions.h
+++ b/js/src/jit/VMFunctions.h
@@ -622,7 +622,7 @@ template<bool Equal>
 bool StringsEqual(JSContext* cx, HandleString left, HandleString right, bool* res);
 
 MOZ_MUST_USE bool ArrayPopDense(JSContext* cx, HandleObject obj, MutableHandleValue rval);
-MOZ_MUST_USE bool ArrayPushDense(JSContext* cx, HandleArrayObject obj, HandleValue v, uint32_t* length);
+MOZ_MUST_USE bool ArrayPushDense(JSContext* cx, HandleObject obj, HandleValue v, uint32_t* length);
 MOZ_MUST_USE bool ArrayShiftDense(JSContext* cx, HandleObject obj, MutableHandleValue rval);
 JSString* ArrayJoin(JSContext* cx, HandleObject array, HandleString sep);
 
@@ -745,8 +745,8 @@ ForcedRecompile(JSContext* cx);
 JSString* StringReplace(JSContext* cx, HandleString string, HandleString pattern,
                         HandleString repl);
 
-MOZ_MUST_USE bool SetDenseElement(JSContext* cx, HandleNativeObject obj, int32_t index,
-                                  HandleValue value, bool strict);
+MOZ_MUST_USE bool SetDenseOrUnboxedArrayElement(JSContext* cx, HandleObject obj, int32_t index,
+                                                HandleValue value, bool strict);
 
 void AssertValidObjectPtr(JSContext* cx, JSObject* obj);
 void AssertValidObjectOrNullPtr(JSContext* cx, JSObject* obj);
diff --git a/js/src/jit/shared/LIR-shared.h b/js/src/jit/shared/LIR-shared.h
index ff4915d1a..e6aab6ba3 100644
--- a/js/src/jit/shared/LIR-shared.h
+++ b/js/src/jit/shared/LIR-shared.h
@@ -5166,6 +5166,72 @@ class LSetInitializedLength : public LInstructionHelper<0, 2, 0>
     }
 };
 
+class LUnboxedArrayLength : public LInstructionHelper<1, 1, 0>
+{
+  public:
+    LIR_HEADER(UnboxedArrayLength)
+
+    explicit LUnboxedArrayLength(const LAllocation& object) {
+        setOperand(0, object);
+    }
+
+    const LAllocation* object() {
+        return getOperand(0);
+    }
+};
+
+class LUnboxedArrayInitializedLength : public LInstructionHelper<1, 1, 0>
+{
+  public:
+    LIR_HEADER(UnboxedArrayInitializedLength)
+
+    explicit LUnboxedArrayInitializedLength(const LAllocation& object) {
+        setOperand(0, object);
+    }
+
+    const LAllocation* object() {
+        return getOperand(0);
+    }
+};
+
+class LIncrementUnboxedArrayInitializedLength : public LInstructionHelper<0, 1, 0>
+{
+  public:
+    LIR_HEADER(IncrementUnboxedArrayInitializedLength)
+
+    explicit LIncrementUnboxedArrayInitializedLength(const LAllocation& object) {
+        setOperand(0, object);
+    }
+
+    const LAllocation* object() {
+        return getOperand(0);
+    }
+};
+
+class LSetUnboxedArrayInitializedLength : public LInstructionHelper<0, 2, 1>
+{
+  public:
+    LIR_HEADER(SetUnboxedArrayInitializedLength)
+
+    explicit LSetUnboxedArrayInitializedLength(const LAllocation& object,
+                                               const LAllocation& length,
+                                               const LDefinition& temp) {
+        setOperand(0, object);
+        setOperand(1, length);
+        setTemp(0, temp);
+    }
+
+    const LAllocation* object() {
+        return getOperand(0);
+    }
+    const LAllocation* length() {
+        return getOperand(1);
+    }
+    const LDefinition* temp() {
+        return getTemp(0);
+    }
+};
+
 // Load the length from an elements header.
 class LArrayLength : public LInstructionHelper<1, 1, 0>
 {
@@ -5670,17 +5736,19 @@ class LStoreElementT : public LInstructionHelper<0, 3, 0>
 };
 
 // Like LStoreElementV, but supports indexes >= initialized length.
-class LStoreElementHoleV : public LInstructionHelper<0, 3 + BOX_PIECES, 0>
+class LStoreElementHoleV : public LInstructionHelper<0, 3 + BOX_PIECES, 1>
 {
   public:
     LIR_HEADER(StoreElementHoleV)
 
     LStoreElementHoleV(const LAllocation& object, const LAllocation& elements,
-                       const LAllocation& index, const LBoxAllocation& value) {
+                       const LAllocation& index, const LBoxAllocation& value,
+                       const LDefinition& temp) {
         setOperand(0, object);
         setOperand(1, elements);
         setOperand(2, index);
         setBoxOperand(Value, value);
+        setTemp(0, temp);
     }
 
     static const size_t Value = 3;
@@ -5700,17 +5768,19 @@ class LStoreElementHoleV : public LInstructionHelper<0, 3 + BOX_PIECES, 0>
 };
 
 // Like LStoreElementT, but supports indexes >= initialized length.
-class LStoreElementHoleT : public LInstructionHelper<0, 4, 0>
+class LStoreElementHoleT : public LInstructionHelper<0, 4, 1>
 {
   public:
     LIR_HEADER(StoreElementHoleT)
 
     LStoreElementHoleT(const LAllocation& object, const LAllocation& elements,
-                       const LAllocation& index, const LAllocation& value) {
+                       const LAllocation& index, const LAllocation& value,
+                       const LDefinition& temp) {
         setOperand(0, object);
         setOperand(1, elements);
         setOperand(2, index);
         setOperand(3, value);
+        setTemp(0, temp);
     }
 
     const MStoreElementHole* mir() const {
@@ -5731,17 +5801,19 @@ class LStoreElementHoleT : public LInstructionHelper<0, 4, 0>
 };
 
 // Like LStoreElementV, but can just ignore assignment (for eg. frozen objects)
-class LFallibleStoreElementV : public LInstructionHelper<0, 3 + BOX_PIECES, 0>
+class LFallibleStoreElementV : public LInstructionHelper<0, 3 + BOX_PIECES, 1>
 {
   public:
     LIR_HEADER(FallibleStoreElementV)
 
     LFallibleStoreElementV(const LAllocation& object, const LAllocation& elements,
-                           const LAllocation& index, const LBoxAllocation& value) {
+                           const LAllocation& index, const LBoxAllocation& value,
+                           const LDefinition& temp) {
         setOperand(0, object);
         setOperand(1, elements);
         setOperand(2, index);
         setBoxOperand(Value, value);
+        setTemp(0, temp);
     }
 
     static const size_t Value = 3;
@@ -5761,17 +5833,19 @@ class LFallibleStoreElementV : public LInstructionHelper<0, 3 + BOX_PIECES, 0>
 };
 
 // Like LStoreElementT, but can just ignore assignment (for eg. frozen objects)
-class LFallibleStoreElementT : public LInstructionHelper<0, 4, 0>
+class LFallibleStoreElementT : public LInstructionHelper<0, 4, 1>
 {
   public:
     LIR_HEADER(FallibleStoreElementT)
 
     LFallibleStoreElementT(const LAllocation& object, const LAllocation& elements,
-                           const LAllocation& index, const LAllocation& value) {
+                           const LAllocation& index, const LAllocation& value,
+                           const LDefinition& temp) {
         setOperand(0, object);
         setOperand(1, elements);
         setOperand(2, index);
         setOperand(3, value);
+        setTemp(0, temp);
     }
 
     const MFallibleStoreElement* mir() const {
@@ -5818,6 +5892,22 @@ class LStoreUnboxedPointer : public LInstructionHelper<0, 3, 0>
     }
 };
 
+// If necessary, convert an unboxed object in a particular group to its native
+// representation.
+class LConvertUnboxedObjectToNative : public LInstructionHelper<0, 1, 0>
+{
+  public:
+    LIR_HEADER(ConvertUnboxedObjectToNative)
+
+    explicit LConvertUnboxedObjectToNative(const LAllocation& object) {
+        setOperand(0, object);
+    }
+
+    MConvertUnboxedObjectToNative* mir() {
+        return mir_->toConvertUnboxedObjectToNative();
+    }
+};
+
 class LArrayPopShiftV : public LInstructionHelper<BOX_PIECES, 1, 2>
 {
   public:
@@ -7340,6 +7430,38 @@ class LGuardReceiverPolymorphic : public LInstructionHelper<0, 1, 1>
     }
 };
 
+class LGuardUnboxedExpando : public LInstructionHelper<0, 1, 0>
+{
+  public:
+    LIR_HEADER(GuardUnboxedExpando)
+
+    explicit LGuardUnboxedExpando(const LAllocation& in) {
+        setOperand(0, in);
+    }
+    const LAllocation* object() {
+        return getOperand(0);
+    }
+    const MGuardUnboxedExpando* mir() const {
+        return mir_->toGuardUnboxedExpando();
+    }
+};
+
+class LLoadUnboxedExpando : public LInstructionHelper<1, 1, 0>
+{
+  public:
+    LIR_HEADER(LoadUnboxedExpando)
+
+    explicit LLoadUnboxedExpando(const LAllocation& in) {
+        setOperand(0, in);
+    }
+    const LAllocation* object() {
+        return getOperand(0);
+    }
+    const MLoadUnboxedExpando* mir() const {
+        return mir_->toLoadUnboxedExpando();
+    }
+};
+
 // Guard that a value is in a TypeSet.
 class LTypeBarrierV : public LInstructionHelper<0, BOX_PIECES, 1>
 {
diff --git a/js/src/jit/shared/LOpcodes-shared.h b/js/src/jit/shared/LOpcodes-shared.h
index 56b98940a..ea185e1b8 100644
--- a/js/src/jit/shared/LOpcodes-shared.h
+++ b/js/src/jit/shared/LOpcodes-shared.h
@@ -257,6 +257,8 @@
     _(GuardObjectGroup)             \
     _(GuardObjectIdentity)          \
     _(GuardClass)                   \
+    _(GuardUnboxedExpando)          \
+    _(LoadUnboxedExpando)           \
     _(TypeBarrierV)                 \
     _(TypeBarrierO)                 \
     _(MonitorTypes)                 \
@@ -266,6 +268,10 @@
     _(PostWriteElementBarrierV)     \
     _(InitializedLength)            \
     _(SetInitializedLength)         \
+    _(UnboxedArrayLength)           \
+    _(UnboxedArrayInitializedLength) \
+    _(IncrementUnboxedArrayInitializedLength) \
+    _(SetUnboxedArrayInitializedLength) \
     _(BoundsCheck)                  \
     _(BoundsCheckRange)             \
     _(BoundsCheckLower)             \
@@ -280,6 +286,7 @@
     _(StoreElementT)                \
     _(StoreUnboxedScalar)           \
     _(StoreUnboxedPointer)          \
+    _(ConvertUnboxedObjectToNative) \
     _(ArrayPopShiftV)               \
     _(ArrayPopShiftT)               \
     _(ArrayPushV)                   \
diff --git a/js/src/jsapi.cpp b/js/src/jsapi.cpp
index 6483641f3..cb3945152 100644
--- a/js/src/jsapi.cpp
+++ b/js/src/jsapi.cpp
@@ -6588,6 +6588,9 @@ JS_SetGlobalJitCompilerOption(JSContext* cx, JSJitCompilerOption opt, uint32_t v
         }
         jit::JitOptions.jumpThreshold = value;
         break;
+      case JSJITCOMPILER_UNBOXED_OBJECTS:
+        jit::JitOptions.disableUnboxedObjects = !value;
+        break;
       case JSJITCOMPILER_ASMJS_ATOMICS_ENABLE:
         jit::JitOptions.asmJSAtomicsEnable = !!value;
         break;
diff --git a/js/src/jsapi.h b/js/src/jsapi.h
index 7795c5e4c..9138a4a92 100644
--- a/js/src/jsapi.h
+++ b/js/src/jsapi.h
@@ -1096,6 +1096,7 @@ class JS_PUBLIC_API(ContextOptions) {
         wasmAlwaysBaseline_(false),
         throwOnAsmJSValidationFailure_(false),
         nativeRegExp_(true),
+        unboxedArrays_(false),
         asyncStack_(true),
         throwOnDebuggeeWouldRun_(true),
         dumpStackOnDebuggeeWouldRun_(false),
@@ -1172,6 +1173,12 @@ class JS_PUBLIC_API(ContextOptions) {
         return *this;
     }
 
+    bool unboxedArrays() const { return unboxedArrays_; }
+    ContextOptions& setUnboxedArrays(bool flag) {
+        unboxedArrays_ = flag;
+        return *this;
+    }
+
     bool asyncStack() const { return asyncStack_; }
     ContextOptions& setAsyncStack(bool flag) {
         asyncStack_ = flag;
@@ -1234,6 +1241,7 @@ class JS_PUBLIC_API(ContextOptions) {
     bool wasmAlwaysBaseline_ : 1;
     bool throwOnAsmJSValidationFailure_ : 1;
     bool nativeRegExp_ : 1;
+    bool unboxedArrays_ : 1;
     bool asyncStack_ : 1;
     bool throwOnDebuggeeWouldRun_ : 1;
     bool dumpStackOnDebuggeeWouldRun_ : 1;
@@ -5888,19 +5896,20 @@ JS_SetParallelParsingEnabled(JSContext* cx, bool enabled);
 extern JS_PUBLIC_API(void)
 JS_SetOffthreadIonCompilationEnabled(JSContext* cx, bool enabled);
 
-#define JIT_COMPILER_OPTIONS(Register)                                     \
-    Register(BASELINE_WARMUP_TRIGGER, "baseline.warmup.trigger")           \
-    Register(ION_WARMUP_TRIGGER, "ion.warmup.trigger")                     \
-    Register(ION_GVN_ENABLE, "ion.gvn.enable")                             \
-    Register(ION_FORCE_IC, "ion.forceinlineCaches")                        \
-    Register(ION_ENABLE, "ion.enable")                                     \
+#define JIT_COMPILER_OPTIONS(Register)                                      \
+    Register(BASELINE_WARMUP_TRIGGER, "baseline.warmup.trigger")            \
+    Register(ION_WARMUP_TRIGGER, "ion.warmup.trigger")                      \
+    Register(ION_GVN_ENABLE, "ion.gvn.enable")                              \
+    Register(ION_FORCE_IC, "ion.forceinlineCaches")                         \
+    Register(ION_ENABLE, "ion.enable")                                      \
     Register(ION_INTERRUPT_WITHOUT_SIGNAL, "ion.interrupt-without-signals") \
-    Register(ION_CHECK_RANGE_ANALYSIS, "ion.check-range-analysis")         \
-    Register(BASELINE_ENABLE, "baseline.enable")                           \
-    Register(OFFTHREAD_COMPILATION_ENABLE, "offthread-compilation.enable") \
-    Register(JUMP_THRESHOLD, "jump-threshold")                             \
-    Register(ASMJS_ATOMICS_ENABLE, "asmjs.atomics.enable")                 \
-    Register(WASM_TEST_MODE, "wasm.test-mode")                             \
+    Register(ION_CHECK_RANGE_ANALYSIS, "ion.check-range-analysis")          \
+    Register(BASELINE_ENABLE, "baseline.enable")                            \
+    Register(OFFTHREAD_COMPILATION_ENABLE, "offthread-compilation.enable")  \
+    Register(JUMP_THRESHOLD, "jump-threshold")                              \
+    Register(UNBOXED_OBJECTS, "unboxed_objects")                            \
+    Register(ASMJS_ATOMICS_ENABLE, "asmjs.atomics.enable")                  \
+    Register(WASM_TEST_MODE, "wasm.test-mode")                              \
     Register(WASM_FOLD_OFFSETS, "wasm.fold-offsets")
 
 typedef enum JSJitCompilerOption {
diff --git a/js/src/jsarray.cpp b/js/src/jsarray.cpp
index 73243d918..1724a0a66 100644
--- a/js/src/jsarray.cpp
+++ b/js/src/jsarray.cpp
@@ -45,6 +45,7 @@
 #include "vm/Caches-inl.h"
 #include "vm/Interpreter-inl.h"
 #include "vm/NativeObject-inl.h"
+#include "vm/UnboxedObject-inl.h"
 
 using namespace js;
 using namespace js::gc;
@@ -63,7 +64,7 @@ using JS::ToUint32;
 bool
 JS::IsArray(JSContext* cx, HandleObject obj, IsArrayAnswer* answer)
 {
-    if (obj->is<ArrayObject>()) {
+    if (obj->is<ArrayObject>() || obj->is<UnboxedArrayObject>()) {
         *answer = IsArrayAnswer::Array;
         return true;
     }
@@ -99,6 +100,11 @@ js::GetLengthProperty(JSContext* cx, HandleObject obj, uint32_t* lengthp)
         return true;
     }
 
+    if (obj->is<UnboxedArrayObject>()) {
+        *lengthp = obj->as<UnboxedArrayObject>().length();
+        return true;
+    }
+
     if (obj->is<ArgumentsObject>()) {
         ArgumentsObject& argsobj = obj->as<ArgumentsObject>();
         if (!argsobj.hasOverriddenLength()) {
@@ -247,20 +253,18 @@ static bool
 GetElement(JSContext* cx, HandleObject obj, HandleObject receiver,
            uint32_t index, bool* hole, MutableHandleValue vp)
 {
-    if (obj->isNative()) {
-        NativeObject* nobj = &obj->as<NativeObject>();
-        if (index < nobj->getDenseInitializedLength()) {
-            vp.set(nobj->getDenseElement(size_t(index)));
-            if (!vp.isMagic(JS_ELEMENTS_HOLE)) {
-                *hole = false;
-                return true;
-            }
+    AssertGreaterThanZero(index);
+    if (index < GetAnyBoxedOrUnboxedInitializedLength(obj)) {
+        vp.set(GetAnyBoxedOrUnboxedDenseElement(obj, uint32_t(index)));
+        if (!vp.isMagic(JS_ELEMENTS_HOLE)) {
+            *hole = false;
+            return true;
         }
-        if (nobj->is<ArgumentsObject>() && index <= UINT32_MAX) {
-            if (nobj->as<ArgumentsObject>().maybeGetElement(uint32_t(index), vp)) {
-                *hole = false;
-                return true;
-            }
+    }
+    if (obj->is<ArgumentsObject>()) {
+        if (obj->as<ArgumentsObject>().maybeGetElement(uint32_t(index), vp)) {
+            *hole = false;
+            return true;
         }
     }
 
@@ -279,8 +283,8 @@ ElementAdder::append(JSContext* cx, HandleValue v)
 {
     MOZ_ASSERT(index_ < length_);
     if (resObj_) {
-        NativeObject* resObj = &resObj_->as<NativeObject>();
-        DenseElementResult result = resObj->setOrExtendDenseElements(cx, index_, v.address(), 1);
+        DenseElementResult result =
+            SetOrExtendAnyBoxedOrUnboxedDenseElements(cx, resObj_, index_, v.address(), 1);
         if (result == DenseElementResult::Failure)
             return false;
         if (result == DenseElementResult::Incomplete) {
@@ -332,31 +336,37 @@ js::GetElementsWithAdder(JSContext* cx, HandleObject obj, HandleObject receiver,
     return true;
 }
 
-static bool
-GetDenseElements(NativeObject* aobj, uint32_t length, Value* vp)
+template <JSValueType Type>
+DenseElementResult
+GetBoxedOrUnboxedDenseElements(JSObject* aobj, uint32_t length, Value* vp)
 {
     MOZ_ASSERT(!ObjectMayHaveExtraIndexedProperties(aobj));
 
-    if (length > aobj->getDenseInitializedLength())
-        return false;
+    if (length > GetBoxedOrUnboxedInitializedLength<Type>(aobj))
+        return DenseElementResult::Incomplete;
 
     for (size_t i = 0; i < length; i++) {
-        vp[i] = aobj->getDenseElement(i);
+        vp[i] = GetBoxedOrUnboxedDenseElement<Type>(aobj, i);
 
         // No other indexed properties so hole => undefined.
         if (vp[i].isMagic(JS_ELEMENTS_HOLE))
             vp[i] = UndefinedValue();
     }
 
-    return true;
+    return DenseElementResult::Success;
 }
 
+DefineBoxedOrUnboxedFunctor3(GetBoxedOrUnboxedDenseElements,
+                             JSObject*, uint32_t, Value*);
+
 bool
 js::GetElements(JSContext* cx, HandleObject aobj, uint32_t length, Value* vp)
 {
     if (!ObjectMayHaveExtraIndexedProperties(aobj)) {
-        if (GetDenseElements(&aobj->as<NativeObject>(), length, vp))
-            return true;
+        GetBoxedOrUnboxedDenseElementsFunctor functor(aobj, length, vp);
+        DenseElementResult result = CallBoxedOrUnboxedSpecialization(functor, aobj);
+        if (result != DenseElementResult::Incomplete)
+            return result == DenseElementResult::Success;
     }
 
     if (aobj->is<ArgumentsObject>()) {
@@ -388,9 +398,9 @@ SetArrayElement(JSContext* cx, HandleObject obj, double index, HandleValue v)
 {
     MOZ_ASSERT(index >= 0);
 
-    if (obj->is<ArrayObject>() && !obj->isIndexed() && index <= UINT32_MAX) {
-        NativeObject* nobj = &obj->as<NativeObject>();
-        DenseElementResult result = nobj->setOrExtendDenseElements(cx, uint32_t(index), v.address(), 1);
+    if ((obj->is<ArrayObject>() || obj->is<UnboxedArrayObject>()) && !obj->isIndexed() && index <= UINT32_MAX) {
+        DenseElementResult result =
+            SetOrExtendAnyBoxedOrUnboxedDenseElements(cx, obj, uint32_t(index), v.address(), 1);
         if (result != DenseElementResult::Incomplete)
             return result == DenseElementResult::Success;
     }
@@ -510,6 +520,24 @@ struct ReverseIndexComparator
     }
 };
 
+bool
+js::CanonicalizeArrayLengthValue(JSContext* cx, HandleValue v, uint32_t* newLen)
+{
+    double d;
+
+    if (!ToUint32(cx, v, newLen))
+        return false;
+
+    if (!ToNumber(cx, v, &d))
+        return false;
+
+    if (d == *newLen)
+        return true;
+
+    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, JSMSG_BAD_ARRAY_LENGTH);
+    return false;
+}
+
 /* ES6 draft rev 34 (2015 Feb 20) 9.4.2.4 ArraySetLength */
 bool
 js::ArraySetLength(JSContext* cx, Handle<ArrayObject*> arr, HandleId id,
@@ -531,22 +559,12 @@ js::ArraySetLength(JSContext* cx, Handle<ArrayObject*> arr, HandleId id,
     } else {
         // Step 2 is irrelevant in our implementation.
 
-        // Step 3.
-        if (!ToUint32(cx, value, &newLen))
-            return false;
-
-        // Step 4.
-        double d;
-        if (!ToNumber(cx, value, &d))
+        // Steps 3-7.
+        MOZ_ASSERT_IF(attrs & JSPROP_IGNORE_VALUE, value.isUndefined());
+        if (!CanonicalizeArrayLengthValue(cx, value, &newLen))
             return false;
 
-        // Step 5.
-        if (d != newLen) {
-            JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, JSMSG_BAD_ARRAY_LENGTH);
-            return false;
-        }
-
-        // Steps 6-8 are irrelevant in our implementation.
+        // Step 8 is irrelevant in our implementation.
     }
 
     // Steps 9-11.
@@ -805,7 +823,7 @@ array_addProperty(JSContext* cx, HandleObject obj, HandleId id, HandleValue v)
 static inline bool
 ObjectMayHaveExtraIndexedOwnProperties(JSObject* obj)
 {
-    return !obj->isNative() ||
+    return (!obj->isNative() && !obj->is<UnboxedArrayObject>()) ||
            obj->isIndexed() ||
            obj->is<TypedArrayObject>() ||
            ClassMayResolveId(*obj->runtimeFromAnyThread()->commonNames,
@@ -836,7 +854,7 @@ js::ObjectMayHaveExtraIndexedProperties(JSObject* obj)
 
         if (ObjectMayHaveExtraIndexedOwnProperties(obj))
             return true;
-        if (obj->as<NativeObject>().getDenseInitializedLength() != 0)
+        if (GetAnyBoxedOrUnboxedInitializedLength(obj) != 0)
             return true;
     } while (true);
 }
@@ -1046,32 +1064,32 @@ struct StringSeparatorOp
     }
 };
 
-template <typename SeparatorOp>
-static bool
-ArrayJoinDenseKernel(JSContext* cx, SeparatorOp sepOp, HandleNativeObject obj, uint32_t length,
+template <typename SeparatorOp, JSValueType Type>
+static DenseElementResult
+ArrayJoinDenseKernel(JSContext* cx, SeparatorOp sepOp, HandleObject obj, uint32_t length,
                      StringBuffer& sb, uint32_t* numProcessed)
 {
     // This loop handles all elements up to initializedLength. If
     // length > initLength we rely on the second loop to add the
     // other elements.
     MOZ_ASSERT(*numProcessed == 0);
-    uint32_t initLength = Min<uint32_t>(obj->getDenseInitializedLength(),
+    uint32_t initLength = Min<uint32_t>(GetBoxedOrUnboxedInitializedLength<Type>(obj),
                                         length);
     while (*numProcessed < initLength) {
         if (!CheckForInterrupt(cx))
-            return false;
+            return DenseElementResult::Failure;
 
         Value elem = obj->as<NativeObject>().getDenseElement(*numProcessed);
 
         if (elem.isString()) {
             if (!sb.append(elem.toString()))
-                return false;
+                return DenseElementResult::Failure;
         } else if (elem.isNumber()) {
             if (!NumberValueToStringBuffer(cx, elem, sb))
-                return false;
+                return DenseElementResult::Failure;
         } else if (elem.isBoolean()) {
             if (!BooleanToStringBuffer(elem.toBoolean(), sb))
-                return false;
+                return DenseElementResult::Failure;
         } else if (elem.isObject() || elem.isSymbol()) {
             /*
              * Object stringifying could modify the initialized length or make
@@ -1087,13 +1105,33 @@ ArrayJoinDenseKernel(JSContext* cx, SeparatorOp sepOp, HandleNativeObject obj, u
         }
 
         if (++(*numProcessed) != length && !sepOp(cx, sb))
-            return false;
+            return DenseElementResult::Failure;
     }
 
-    return true;
+    return DenseElementResult::Incomplete;
 }
 
 template <typename SeparatorOp>
+struct ArrayJoinDenseKernelFunctor {
+    JSContext* cx;
+    SeparatorOp sepOp;
+    HandleObject obj;
+    uint32_t length;
+    StringBuffer& sb;
+    uint32_t* numProcessed;
+
+    ArrayJoinDenseKernelFunctor(JSContext* cx, SeparatorOp sepOp, HandleObject obj,
+                                uint32_t length, StringBuffer& sb, uint32_t* numProcessed)
+      : cx(cx), sepOp(sepOp), obj(obj), length(length), sb(sb), numProcessed(numProcessed)
+    {}
+
+    template <JSValueType Type>
+    DenseElementResult operator()() {
+        return ArrayJoinDenseKernel<SeparatorOp, Type>(cx, sepOp, obj, length, sb, numProcessed);
+    }
+};
+
+template <typename SeparatorOp>
 static bool
 ArrayJoinKernel(JSContext* cx, SeparatorOp sepOp, HandleObject obj, uint32_t length,
                StringBuffer& sb)
@@ -1101,10 +1139,10 @@ ArrayJoinKernel(JSContext* cx, SeparatorOp sepOp, HandleObject obj, uint32_t len
     uint32_t i = 0;
 
     if (!ObjectMayHaveExtraIndexedProperties(obj)) {
-        if (!ArrayJoinDenseKernel<SeparatorOp>(cx, sepOp, obj.as<NativeObject>(), length, sb, &i))
-        {
+        ArrayJoinDenseKernelFunctor<SeparatorOp> functor(cx, sepOp, obj, length, sb, &i);
+        DenseElementResult result = CallBoxedOrUnboxedSpecialization(functor, obj);
+        if (result == DenseElementResult::Failure)
             return false;
-        }
     }
 
     if (i != length) {
@@ -1175,14 +1213,11 @@ js::array_join(JSContext* cx, unsigned argc, Value* vp)
     // An optimized version of a special case of steps 7-11: when length==1 and
     // the 0th element is a string, ToString() of that element is a no-op and
     // so it can be immediately returned as the result.
-    if (length == 1 && obj->isNative()) {
-        NativeObject* nobj = &obj->as<NativeObject>();
-        if (nobj->getDenseInitializedLength() == 1) {
-            Value elem0 = nobj->getDenseElement(0);
-            if (elem0.isString()) {
-                args.rval().set(elem0);
-                return true;
-            }
+    if (length == 1 && GetAnyBoxedOrUnboxedInitializedLength(obj) == 1) {
+        Value elem0 = GetAnyBoxedOrUnboxedDenseElement(obj, 0);
+        if (elem0.isString()) {
+            args.rval().set(elem0);
+            return true;
         }
     }
 
@@ -1225,7 +1260,7 @@ js::array_join(JSContext* cx, unsigned argc, Value* vp)
     }
 
     // Step 11
-    JSString* str = sb.finishString();
+    JSString *str = sb.finishString();
     if (!str)
         return false;
 
@@ -1254,6 +1289,10 @@ array_toLocaleString(JSContext* cx, unsigned argc, Value* vp)
         args.rval().setString(cx->names().empty);
         return true;
     }
+    if (obj->is<UnboxedArrayObject>() && obj->as<UnboxedArrayObject>().length() == 0) {
+        args.rval().setString(cx->names().empty);
+        return true;
+    }
 
     AutoCycleDetector detector(cx, obj);
     if (!detector.init())
@@ -1290,9 +1329,8 @@ InitArrayElements(JSContext* cx, HandleObject obj, uint32_t start,
         return false;
 
     if (!ObjectMayHaveExtraIndexedProperties(obj)) {
-        NativeObject* nobj = &obj->as<NativeObject>();
-        DenseElementResult result = nobj->setOrExtendDenseElements(cx, uint32_t(start), vector,
-                                                                   count, updateTypes);
+        DenseElementResult result =
+            SetOrExtendAnyBoxedOrUnboxedDenseElements(cx, obj, start, vector, count, updateTypes);
         if (result != DenseElementResult::Incomplete)
             return result == DenseElementResult::Success;
     }
@@ -1326,45 +1364,54 @@ InitArrayElements(JSContext* cx, HandleObject obj, uint32_t start,
     return true;
 }
 
-static DenseElementResult
-ArrayReverseDenseKernel(JSContext* cx, HandleNativeObject obj, uint32_t length)
+template <JSValueType Type>
+DenseElementResult
+ArrayReverseDenseKernel(JSContext* cx, HandleObject obj, uint32_t length)
 {
     /* An empty array or an array with no elements is already reversed. */
-    if (length == 0 || obj->getDenseInitializedLength() == 0)
+    if (length == 0 || GetBoxedOrUnboxedInitializedLength<Type>(obj) == 0)
         return DenseElementResult::Success;
 
-    if (obj->denseElementsAreFrozen())
-        return DenseElementResult::Incomplete;
+    if (Type == JSVAL_TYPE_MAGIC) {
+        if (obj->as<NativeObject>().denseElementsAreFrozen())
+            return DenseElementResult::Incomplete;
 
-    /*
-     * It's actually surprisingly complicated to reverse an array due to the
-     * orthogonality of array length and array capacity while handling
-     * leading and trailing holes correctly.  Reversing seems less likely to
-     * be a common operation than other array mass-mutation methods, so for
-     * now just take a probably-small memory hit (in the absence of too many
-     * holes in the array at its start) and ensure that the capacity is
-     * sufficient to hold all the elements in the array if it were full.
-     */
-    DenseElementResult result = obj->ensureDenseElements(cx, length, 0);
-    if (result != DenseElementResult::Success)
-        return result;
+        /*
+         * It's actually surprisingly complicated to reverse an array due to the
+         * orthogonality of array length and array capacity while handling
+         * leading and trailing holes correctly.  Reversing seems less likely to
+         * be a common operation than other array mass-mutation methods, so for
+         * now just take a probably-small memory hit (in the absence of too many
+         * holes in the array at its start) and ensure that the capacity is
+         * sufficient to hold all the elements in the array if it were full.
+         */
+        DenseElementResult result = obj->as<NativeObject>().ensureDenseElements(cx, length, 0);
+        if (result != DenseElementResult::Success)
+            return result;
 
-    /* Fill out the array's initialized length to its proper length. */
-    obj->ensureDenseInitializedLength(cx, length, 0);
+        /* Fill out the array's initialized length to its proper length. */
+        obj->as<NativeObject>().ensureDenseInitializedLength(cx, length, 0);
+    } else {
+        // Unboxed arrays can only be reversed here if their initialized length
+        // matches their actual length. Otherwise the reversal will place holes
+        // at the beginning of the array, which we don't support.
+        if (length != obj->as<UnboxedArrayObject>().initializedLength())
+            return DenseElementResult::Incomplete;
+    }
 
     RootedValue origlo(cx), orighi(cx);
 
     uint32_t lo = 0, hi = length - 1;
     for (; lo < hi; lo++, hi--) {
-        origlo = obj->getDenseElement(lo);
-        orighi = obj->getDenseElement(hi);
-        obj->setDenseElement(lo, orighi);
+        origlo = GetBoxedOrUnboxedDenseElement<Type>(obj, lo);
+        orighi = GetBoxedOrUnboxedDenseElement<Type>(obj, hi);
+        SetBoxedOrUnboxedDenseElementNoTypeChange<Type>(obj, lo, orighi);
         if (orighi.isMagic(JS_ELEMENTS_HOLE) &&
             !SuppressDeletedProperty(cx, obj, INT_TO_JSID(lo)))
         {
             return DenseElementResult::Failure;
         }
-        obj->setDenseElement(hi, origlo);
+        SetBoxedOrUnboxedDenseElementNoTypeChange<Type>(obj, hi, origlo);
         if (origlo.isMagic(JS_ELEMENTS_HOLE) &&
             !SuppressDeletedProperty(cx, obj, INT_TO_JSID(hi)))
         {
@@ -1375,6 +1422,9 @@ ArrayReverseDenseKernel(JSContext* cx, HandleNativeObject obj, uint32_t length)
     return DenseElementResult::Success;
 }
 
+DefineBoxedOrUnboxedFunctor3(ArrayReverseDenseKernel,
+                             JSContext*, HandleObject, uint32_t);
+
 bool
 js::array_reverse(JSContext* cx, unsigned argc, Value* vp)
 {
@@ -1389,8 +1439,8 @@ js::array_reverse(JSContext* cx, unsigned argc, Value* vp)
         return false;
 
     if (!ObjectMayHaveExtraIndexedProperties(obj)) {
-        DenseElementResult result =
-            ArrayReverseDenseKernel(cx, obj.as<NativeObject>(), uint32_t(len));
+        ArrayReverseDenseKernelFunctor functor(cx, obj, len);
+        DenseElementResult result = CallBoxedOrUnboxedSpecialization(functor, obj);
         if (result != DenseElementResult::Incomplete) {
             /*
              * Per ECMA-262, don't update the length of the array, even if the new
@@ -2033,8 +2083,8 @@ js::array_push(JSContext* cx, unsigned argc, Value* vp)
 
     if (!ObjectMayHaveExtraIndexedProperties(obj)) {
         DenseElementResult result =
-            obj->as<NativeObject>().setOrExtendDenseElements(cx, uint32_t(length),
-                                                             args.array(), args.length());
+            SetOrExtendAnyBoxedOrUnboxedDenseElements(cx, obj, length,
+                                                      args.array(), args.length());
         if (result != DenseElementResult::Incomplete) {
             if (result == DenseElementResult::Failure)
                 return false;
@@ -2042,8 +2092,14 @@ js::array_push(JSContext* cx, unsigned argc, Value* vp)
             uint32_t newlength = length + args.length();
             args.rval().setNumber(newlength);
 
-            // Handle updates to the length of non-arrays here.
-            if (!obj->is<ArrayObject>())
+            // SetOrExtendAnyBoxedOrUnboxedDenseElements takes care of updating the
+            // length for boxed and unboxed arrays. Handle updates to the length of
+            // non-arrays here.
+            bool isArray;
+            if (!IsArray(cx, obj, &isArray))
+                return false;
+
+            if (!isArray)
                 return SetLengthProperty(cx, obj, newlength);
 
             return true;
@@ -2099,47 +2155,42 @@ js::array_pop(JSContext* cx, unsigned argc, Value* vp)
     return SetLengthProperty(cx, obj, index);
 }
 
-void
-js::ArrayShiftMoveElements(NativeObject* obj)
+template <JSValueType Type>
+static inline DenseElementResult
+ShiftMoveBoxedOrUnboxedDenseElements(JSObject* obj)
 {
-    MOZ_ASSERT_IF(obj->is<ArrayObject>(), obj->as<ArrayObject>().lengthIsWritable());
+    MOZ_ASSERT(HasBoxedOrUnboxedDenseElements<Type>(obj));
 
-    size_t initlen = obj->getDenseInitializedLength();
-    
-    if (initlen > 0) {
-      /*
-       * At this point the length and initialized length have already been
-       * decremented and the result fetched, so just shift the array elements
-       * themselves.
-       */
-      obj->moveDenseElementsNoPreBarrier(0, 1, initlen);
+    /*
+     * At this point the length and initialized length have already been
+     * decremented and the result fetched, so just shift the array elements
+     * themselves.
+     */
+    size_t initlen = GetBoxedOrUnboxedInitializedLength<Type>(obj);
+    if (Type == JSVAL_TYPE_MAGIC) {
+        obj->as<NativeObject>().moveDenseElementsNoPreBarrier(0, 1, initlen);
+    } else {
+        uint8_t* data = obj->as<UnboxedArrayObject>().elements();
+        size_t elementSize = UnboxedTypeSize(Type);
+        memmove(data, data + elementSize, initlen * elementSize);
     }
-}
 
-static inline void
-SetInitializedLength(JSContext* cx, NativeObject* obj, size_t initlen)
-{
-    size_t oldInitlen = obj->getDenseInitializedLength();
-    obj->setDenseInitializedLength(initlen);
-    if (initlen < oldInitlen)
-        obj->shrinkElements(cx, initlen);
+    return DenseElementResult::Success;
 }
 
-static DenseElementResult
-MoveDenseElements(JSContext* cx, NativeObject* obj, uint32_t dstStart, uint32_t srcStart,
-                  uint32_t length)
-{
-    if (obj->denseElementsAreFrozen())
-        return DenseElementResult::Incomplete;
+DefineBoxedOrUnboxedFunctor1(ShiftMoveBoxedOrUnboxedDenseElements, JSObject*);
 
-    if (!obj->maybeCopyElementsForWrite(cx))
-        return DenseElementResult::Failure;
-    obj->moveDenseElements(dstStart, srcStart, length);
+void
+js::ArrayShiftMoveElements(JSObject* obj)
+{
+    MOZ_ASSERT_IF(obj->is<ArrayObject>(), obj->as<ArrayObject>().lengthIsWritable());
 
-    return DenseElementResult::Success;
+    ShiftMoveBoxedOrUnboxedDenseElementsFunctor functor(obj);
+    JS_ALWAYS_TRUE(CallBoxedOrUnboxedSpecialization(functor, obj) == DenseElementResult::Success);
 }
 
-static DenseElementResult
+template <JSValueType Type>
+DenseElementResult
 ArrayShiftDenseKernel(JSContext* cx, HandleObject obj, MutableHandleValue rval)
 {
     if (ObjectMayHaveExtraIndexedProperties(obj))
@@ -2152,22 +2203,25 @@ ArrayShiftDenseKernel(JSContext* cx, HandleObject obj, MutableHandleValue rval)
     if (MOZ_UNLIKELY(group->hasAllFlags(OBJECT_FLAG_ITERATED)))
         return DenseElementResult::Incomplete;
 
-    size_t initlen = obj->as<NativeObject>().getDenseInitializedLength();
+    size_t initlen = GetBoxedOrUnboxedInitializedLength<Type>(obj);
     if (initlen == 0)
         return DenseElementResult::Incomplete;
 
-    rval.set(obj->as<NativeObject>().getDenseElement(0));
+    rval.set(GetBoxedOrUnboxedDenseElement<Type>(obj, 0));
     if (rval.isMagic(JS_ELEMENTS_HOLE))
         rval.setUndefined();
 
-    DenseElementResult result = MoveDenseElements(cx, &obj->as<NativeObject>(), 0, 1, initlen - 1);
+    DenseElementResult result = MoveBoxedOrUnboxedDenseElements<Type>(cx, obj, 0, 1, initlen - 1);
     if (result != DenseElementResult::Success)
         return result;
 
-    SetInitializedLength(cx, obj.as<NativeObject>(), initlen - 1);
+    SetBoxedOrUnboxedInitializedLength<Type>(cx, obj, initlen - 1);
     return DenseElementResult::Success;
 }
 
+DefineBoxedOrUnboxedFunctor3(ArrayShiftDenseKernel,
+                             JSContext*, HandleObject, MutableHandleValue);
+
 /* ES5 15.4.4.9 */
 bool
 js::array_shift(JSContext* cx, unsigned argc, Value* vp)
@@ -2199,7 +2253,8 @@ js::array_shift(JSContext* cx, unsigned argc, Value* vp)
     uint32_t newlen = len - 1;
 
     /* Fast paths. */
-    DenseElementResult result = ArrayShiftDenseKernel(cx, obj, args.rval());
+    ArrayShiftDenseKernelFunctor functor(cx, obj, args.rval());
+    DenseElementResult result = CallBoxedOrUnboxedSpecialization(functor, obj);
     if (result != DenseElementResult::Incomplete) {
         if (result == DenseElementResult::Failure)
             return false;
@@ -2253,6 +2308,9 @@ js::array_unshift(JSContext* cx, unsigned argc, Value* vp)
     if (args.length() > 0) {
         /* Slide up the array to make room for all args at the bottom. */
         if (length > 0) {
+            // Only include a fast path for boxed arrays. Unboxed arrays can'nt
+            // be optimized here because unshifting temporarily places holes at
+            // the start of the array.
             bool optimized = false;
             do {
                 if (!obj->is<ArrayObject>())
@@ -2312,10 +2370,10 @@ js::array_unshift(JSContext* cx, unsigned argc, Value* vp)
 }
 
 /*
- * Returns true if this is a dense array whose properties ending at |endIndex|
- * (exclusive) may be accessed (get, set, delete) directly through its
- * contiguous vector of elements without fear of getters, setters, etc. along
- * the prototype chain, or of enumerators requiring notification of
+ * Returns true if this is a dense or unboxed array whose |count| properties
+ * starting from |startingIndex| may be accessed (get, set, delete) directly
+ * through its contiguous vector of elements without fear of getters, setters,
+ * etc. along the prototype chain, or of enumerators requiring notification of
  * modifications.
  */
 static inline bool
@@ -2326,11 +2384,11 @@ CanOptimizeForDenseStorage(HandleObject arr, uint32_t startingIndex, uint32_t co
         return false;
 
     /* There's no optimizing possible if it's not an array. */
-    if (!arr->is<ArrayObject>())
+    if (!arr->is<ArrayObject>() && !arr->is<UnboxedArrayObject>())
         return false;
 
     /* If it's a frozen array, always pick the slow path */
-    if (arr->as<ArrayObject>().denseElementsAreFrozen())
+    if (arr->is<ArrayObject>() && arr->as<ArrayObject>().denseElementsAreFrozen())
         return false;
 
     /*
@@ -2362,23 +2420,7 @@ CanOptimizeForDenseStorage(HandleObject arr, uint32_t startingIndex, uint32_t co
      * is subsumed by the initializedLength comparison.)
      */
     return !ObjectMayHaveExtraIndexedProperties(arr) &&
-           startingIndex + count <= arr->as<NativeObject>().getDenseInitializedLength();
-}
-
-static inline DenseElementResult
-CopyDenseElements(JSContext* cx, NativeObject* dst, NativeObject* src,
-                      uint32_t dstStart, uint32_t srcStart, uint32_t length)
-{
-    MOZ_ASSERT(dst->getDenseInitializedLength() == dstStart);
-    MOZ_ASSERT(src->getDenseInitializedLength() >= srcStart + length);
-    MOZ_ASSERT(dst->getDenseCapacity() >= dstStart + length);
-
-    dst->setDenseInitializedLength(dstStart + length);
-
-    const Value* vp = src->getDenseElements() + srcStart;
-    dst->initDenseElements(dstStart, vp, length);
-
-    return DenseElementResult::Success;
+           startingIndex + count <= GetAnyBoxedOrUnboxedInitializedLength(arr);
 }
 
 static inline bool
@@ -2472,9 +2514,7 @@ array_splice_impl(JSContext* cx, unsigned argc, Value* vp, bool returnValueIsUse
 
                 /* Steps 10-11. */
                 DebugOnly<DenseElementResult> result =
-                    CopyDenseElements(cx, &arr->as<NativeObject>(), 
-                                      &obj->as<NativeObject>(), 0, 
-                                      actualStart, actualDeleteCount);
+                    CopyAnyBoxedOrUnboxedDenseElements(cx, arr, obj, 0, actualStart, actualDeleteCount);
                 MOZ_ASSERT(result.value == DenseElementResult::Success);
 
                 /* Step 12 (implicit). */
@@ -2511,13 +2551,14 @@ array_splice_impl(JSContext* cx, unsigned argc, Value* vp, bool returnValueIsUse
         if (CanOptimizeForDenseStorage(obj, 0, len, cx)) {
             /* Steps 15.a-b. */
             DenseElementResult result =
-                MoveDenseElements(cx, &obj->as<NativeObject>(), targetIndex, sourceIndex, len - sourceIndex);
+                MoveAnyBoxedOrUnboxedDenseElements(cx, obj, targetIndex, sourceIndex,
+                                                   len - sourceIndex);
             MOZ_ASSERT(result != DenseElementResult::Incomplete);
             if (result == DenseElementResult::Failure)
                 return false;
 
             /* Steps 15.c-d. */
-            SetInitializedLength(cx, obj.as<NativeObject>(), finalLength);
+            SetAnyBoxedOrUnboxedInitializedLength(cx, obj, finalLength);
         } else {
             /*
              * This is all very slow if the length is very large. We don't yet
@@ -2597,15 +2638,15 @@ array_splice_impl(JSContext* cx, unsigned argc, Value* vp, bool returnValueIsUse
 
         if (CanOptimizeForDenseStorage(obj, len, itemCount - actualDeleteCount, cx)) {
             DenseElementResult result =
-                MoveDenseElements(cx, &obj->as<NativeObject>(), actualStart + itemCount,
-                                  actualStart + actualDeleteCount,
-                                  len - (actualStart + actualDeleteCount));
+                MoveAnyBoxedOrUnboxedDenseElements(cx, obj, actualStart + itemCount,
+                                                   actualStart + actualDeleteCount,
+                                                   len - (actualStart + actualDeleteCount));
             MOZ_ASSERT(result != DenseElementResult::Incomplete);
             if (result == DenseElementResult::Failure)
                 return false;
 
             /* Steps 16.a-b. */
-            SetInitializedLength(cx, obj.as<NativeObject>(), len + itemCount - actualDeleteCount);
+            SetAnyBoxedOrUnboxedInitializedLength(cx, obj, len + itemCount - actualDeleteCount);
         } else {
             RootedValue fromValue(cx);
             for (double k = len - actualDeleteCount; k > actualStart; k--) {
@@ -2790,7 +2831,7 @@ SliceSlowly(JSContext* cx, HandleObject obj, HandleObject receiver,
 }
 
 static bool
-SliceSparse(JSContext* cx, HandleObject obj, uint32_t begin, uint32_t end, HandleArrayObject result)
+SliceSparse(JSContext* cx, HandleObject obj, uint32_t begin, uint32_t end, HandleObject result)
 {
     MOZ_ASSERT(begin <= end);
 
@@ -2840,28 +2881,26 @@ ArraySliceOrdinary(JSContext* cx, HandleObject obj, uint32_t length, uint32_t be
         begin = end;
 
     if (!ObjectMayHaveExtraIndexedProperties(obj)) {
-        size_t initlen = obj->as<NativeObject>().getDenseInitializedLength();
+        size_t initlen = GetAnyBoxedOrUnboxedInitializedLength(obj);
         size_t count = 0;
         if (initlen > begin)
             count = Min<size_t>(initlen - begin, end - begin);
 
-        RootedArrayObject narr(cx, NewFullyAllocatedArrayTryReuseGroup(cx, obj, count));
+        RootedObject narr(cx, NewFullyAllocatedArrayTryReuseGroup(cx, obj, count));
         if (!narr)
             return false;
-
-        MOZ_ASSERT(count >= narr->as<ArrayObject>().length());
-        narr->as<ArrayObject>().setLength(cx, count);
+        SetAnyBoxedOrUnboxedArrayLength(cx, narr, end - begin);
 
         if (count) {
             DebugOnly<DenseElementResult> result =
-                CopyDenseElements(cx, &narr->as<NativeObject>(), &obj->as<NativeObject>(), 0, begin, count);
+                CopyAnyBoxedOrUnboxedDenseElements(cx, narr, obj, 0, begin, count);
             MOZ_ASSERT(result.value == DenseElementResult::Success);
         }
         arr.set(narr);
         return true;
     }
 
-    RootedArrayObject narr(cx, NewPartlyAllocatedArrayTryReuseGroup(cx, obj, end - begin));
+    RootedObject narr(cx, NewPartlyAllocatedArrayTryReuseGroup(cx, obj, end - begin));
     if (!narr)
         return false;
 
@@ -2978,10 +3017,11 @@ js::array_slice(JSContext* cx, unsigned argc, Value* vp)
     return true;
 }
 
-static bool
-ArraySliceDenseKernel(JSContext* cx, ArrayObject* arr, int32_t beginArg, int32_t endArg, ArrayObject* result)
+template <JSValueType Type>
+DenseElementResult
+ArraySliceDenseKernel(JSContext* cx, JSObject* obj, int32_t beginArg, int32_t endArg, JSObject* result)
 {
-    int32_t length = arr->length();
+    int32_t length = GetAnyBoxedOrUnboxedArrayLength(obj);
 
     uint32_t begin = NormalizeSliceTerm(beginArg, length);
     uint32_t end = NormalizeSliceTerm(endArg, length);
@@ -2989,33 +3029,33 @@ ArraySliceDenseKernel(JSContext* cx, ArrayObject* arr, int32_t beginArg, int32_t
     if (begin > end)
         begin = end;
 
-    size_t initlen = arr->getDenseInitializedLength();
-    size_t count = Min<size_t>(initlen - begin, end - begin);
+    size_t initlen = GetBoxedOrUnboxedInitializedLength<Type>(obj);
     if (initlen > begin) {
+        size_t count = Min<size_t>(initlen - begin, end - begin);
         if (count) {
-            if (!result->ensureElements(cx, count))
-                return false;
-            CopyDenseElements(cx, &result->as<NativeObject>(), &arr->as<NativeObject>(), 0, begin, count);
+            DenseElementResult rv = EnsureBoxedOrUnboxedDenseElements<Type>(cx, result, count);
+            if (rv != DenseElementResult::Success)
+                return rv;
+            CopyBoxedOrUnboxedDenseElements<Type, Type>(cx, result, obj, 0, begin, count);
         }
     }
 
-    MOZ_ASSERT(count >= result->length());
-    result->setLength(cx, count);
-
-    return true;
+    SetAnyBoxedOrUnboxedArrayLength(cx, result, end - begin);
+    return DenseElementResult::Success;
 }
 
+DefineBoxedOrUnboxedFunctor5(ArraySliceDenseKernel,
+                             JSContext*, JSObject*, int32_t, int32_t, JSObject*);
+
 JSObject*
 js::array_slice_dense(JSContext* cx, HandleObject obj, int32_t begin, int32_t end,
                       HandleObject result)
 {
     if (result && IsArraySpecies(cx, obj)) {
-        if (!ArraySliceDenseKernel(cx, &obj->as<ArrayObject>(), begin, end,
-                                   &result->as<ArrayObject>()))
-        {
-            return nullptr;
-        }
-        return result;
+        ArraySliceDenseKernelFunctor functor(cx, obj, begin, end, result);
+        DenseElementResult rv = CallBoxedOrUnboxedSpecialization(functor, result);
+        MOZ_ASSERT(rv != DenseElementResult::Incomplete);
+        return rv == DenseElementResult::Success ? result : nullptr;
     }
 
     // Slower path if the JIT wasn't able to allocate an object inline.
@@ -3046,7 +3086,7 @@ array_isArray(JSContext* cx, unsigned argc, Value* vp)
 static bool
 ArrayFromCallArgs(JSContext* cx, CallArgs& args, HandleObject proto = nullptr)
 {
-    ArrayObject* obj = NewCopiedArrayForCallingAllocationSite(cx, args.array(), args.length(), proto);
+    JSObject* obj = NewCopiedArrayForCallingAllocationSite(cx, args.array(), args.length(), proto);
     if (!obj)
         return false;
 
@@ -3220,7 +3260,7 @@ ArrayConstructorImpl(JSContext* cx, CallArgs& args, bool isConstructor)
         }
     }
 
-    ArrayObject* obj = NewPartlyAllocatedArrayForCallingAllocationSite(cx, length, proto);
+    JSObject* obj = NewPartlyAllocatedArrayForCallingAllocationSite(cx, length, proto);
     if (!obj)
         return false;
 
@@ -3246,7 +3286,7 @@ js::array_construct(JSContext* cx, unsigned argc, Value* vp)
     return ArrayConstructorImpl(cx, args, /* isConstructor = */ false);
 }
 
-ArrayObject*
+JSObject*
 js::ArrayConstructorOneArg(JSContext* cx, HandleObjectGroup group, int32_t lengthInt)
 {
     if (lengthInt < 0) {
@@ -3541,7 +3581,7 @@ js::NewDenseFullyAllocatedArrayWithTemplate(JSContext* cx, uint32_t length, JSOb
     return arr;
 }
 
-ArrayObject*
+JSObject*
 js::NewDenseCopyOnWriteArray(JSContext* cx, HandleArrayObject templateObject, gc::InitialHeap heap)
 {
     MOZ_ASSERT(!gc::IsInsideNursery(templateObject));
@@ -3554,21 +3594,30 @@ js::NewDenseCopyOnWriteArray(JSContext* cx, HandleArrayObject templateObject, gc
     return arr;
 }
 
-// Return a new array with the specified length and allocated capacity (up to
-// maxLength), using the specified group if possible. If the specified group
-// cannot be used, ensure that the created array at least has the given
-// [[Prototype]].
+// Return a new boxed or unboxed array with the specified length and allocated
+// capacity (up to maxLength), using the specified group if possible. If the
+// specified group cannot be used, ensure that the created array at least has
+// the given [[Prototype]].
 template <uint32_t maxLength>
-static inline ArrayObject*
+static inline JSObject*
 NewArrayTryUseGroup(ExclusiveContext* cx, HandleObjectGroup group, size_t length,
                     NewObjectKind newKind = GenericObject)
 {
     MOZ_ASSERT(newKind != SingletonObject);
 
-    if (group->shouldPreTenure())
+    if (group->maybePreliminaryObjects())
+        group->maybePreliminaryObjects()->maybeAnalyze(cx, group);
+
+    if (group->shouldPreTenure() || group->maybePreliminaryObjects())
         newKind = TenuredObject;
 
     RootedObject proto(cx, group->proto().toObject());
+    if (group->maybeUnboxedLayout()) {
+        if (length > UnboxedArrayObject::MaximumCapacity)
+            return NewArray<maxLength>(cx, length, proto, newKind);
+        return UnboxedArrayObject::create(cx, group, length, newKind, maxLength);
+    }
+
     ArrayObject* res = NewArray<maxLength>(cx, length, proto, newKind);
     if (!res)
         return nullptr;
@@ -3580,17 +3629,20 @@ NewArrayTryUseGroup(ExclusiveContext* cx, HandleObjectGroup group, size_t length
     if (res->length() > INT32_MAX)
         res->setLength(cx, res->length());
 
+    if (PreliminaryObjectArray* preliminaryObjects = group->maybePreliminaryObjects())
+        preliminaryObjects->registerNewObject(res);
+
     return res;
 }
 
-ArrayObject*
+JSObject*
 js::NewFullyAllocatedArrayTryUseGroup(ExclusiveContext* cx, HandleObjectGroup group, size_t length,
                                       NewObjectKind newKind)
 {
     return NewArrayTryUseGroup<UINT32_MAX>(cx, group, length, newKind);
 }
 
-ArrayObject*
+JSObject*
 js::NewPartlyAllocatedArrayTryUseGroup(ExclusiveContext* cx, HandleObjectGroup group, size_t length)
 {
     return NewArrayTryUseGroup<ArrayObject::EagerAllocationMaxLength>(cx, group, length);
@@ -3599,13 +3651,16 @@ js::NewPartlyAllocatedArrayTryUseGroup(ExclusiveContext* cx, HandleObjectGroup g
 // Return a new array with the default prototype and specified allocated
 // capacity and length. If possible, try to reuse the group of the input
 // object. The resulting array will either reuse the input object's group or
-// will have unknown property types.
+// will have unknown property types. Additionally, the result will have the
+// same boxed/unboxed elements representation as the input object, unless
+// |length| is larger than the input object's initialized length (in which case
+// UnboxedArrayObject::MaximumCapacity might be exceeded).
 template <uint32_t maxLength>
-static inline ArrayObject*
+static inline JSObject*
 NewArrayTryReuseGroup(JSContext* cx, HandleObject obj, size_t length,
                       NewObjectKind newKind = GenericObject)
 {
-    if (!obj->is<ArrayObject>())
+    if (!obj->is<ArrayObject>() && !obj->is<UnboxedArrayObject>())
         return NewArray<maxLength>(cx, length, nullptr, newKind);
 
     if (obj->staticPrototype() != cx->global()->maybeGetArrayPrototype())
@@ -3618,20 +3673,20 @@ NewArrayTryReuseGroup(JSContext* cx, HandleObject obj, size_t length,
     return NewArrayTryUseGroup<maxLength>(cx, group, length, newKind);
 }
 
-ArrayObject*
+JSObject*
 js::NewFullyAllocatedArrayTryReuseGroup(JSContext* cx, HandleObject obj, size_t length,
                                         NewObjectKind newKind)
 {
     return NewArrayTryReuseGroup<UINT32_MAX>(cx, obj, length, newKind);
 }
 
-ArrayObject*
+JSObject*
 js::NewPartlyAllocatedArrayTryReuseGroup(JSContext* cx, HandleObject obj, size_t length)
 {
     return NewArrayTryReuseGroup<ArrayObject::EagerAllocationMaxLength>(cx, obj, length);
 }
 
-ArrayObject*
+JSObject*
 js::NewFullyAllocatedArrayForCallingAllocationSite(JSContext* cx, size_t length,
                                                    NewObjectKind newKind)
 {
@@ -3641,7 +3696,7 @@ js::NewFullyAllocatedArrayForCallingAllocationSite(JSContext* cx, size_t length,
     return NewArrayTryUseGroup<UINT32_MAX>(cx, group, length, newKind);
 }
 
-ArrayObject*
+JSObject*
 js::NewPartlyAllocatedArrayForCallingAllocationSite(JSContext* cx, size_t length, HandleObject proto)
 {
     RootedObjectGroup group(cx, ObjectGroup::callingAllocationSiteGroup(cx, JSProto_Array, proto));
@@ -3650,23 +3705,68 @@ js::NewPartlyAllocatedArrayForCallingAllocationSite(JSContext* cx, size_t length
     return NewArrayTryUseGroup<ArrayObject::EagerAllocationMaxLength>(cx, group, length);
 }
 
-ArrayObject*
+bool
+js::MaybeAnalyzeBeforeCreatingLargeArray(ExclusiveContext* cx, HandleObjectGroup group,
+                                         const Value* vp, size_t length)
+{
+    static const size_t EagerPreliminaryObjectAnalysisThreshold = 800;
+
+    // Force analysis to see if an unboxed array can be used when making a
+    // sufficiently large array, to avoid excessive analysis and copying later
+    // on. If this is the first array of its group that is being created, first
+    // make a dummy array with the initial elements of the array we are about
+    // to make, so there is some basis for the unboxed array analysis.
+    if (length > EagerPreliminaryObjectAnalysisThreshold) {
+        if (PreliminaryObjectArrayWithTemplate* objects = group->maybePreliminaryObjects()) {
+            if (objects->empty()) {
+                size_t nlength = Min<size_t>(length, 100);
+                JSObject* obj = NewFullyAllocatedArrayTryUseGroup(cx, group, nlength);
+                if (!obj)
+                    return false;
+                DebugOnly<DenseElementResult> result =
+                    SetOrExtendAnyBoxedOrUnboxedDenseElements(cx, obj, 0, vp, nlength,
+                                                              ShouldUpdateTypes::Update);
+                MOZ_ASSERT(result.value == DenseElementResult::Success);
+            }
+            objects->maybeAnalyze(cx, group, /* forceAnalyze = */ true);
+        }
+    }
+    return true;
+}
+
+JSObject*
 js::NewCopiedArrayTryUseGroup(ExclusiveContext* cx, HandleObjectGroup group,
                               const Value* vp, size_t length, NewObjectKind newKind,
                               ShouldUpdateTypes updateTypes)
 {
-    ArrayObject* obj = NewFullyAllocatedArrayTryUseGroup(cx, group, length, newKind);
+    if (!MaybeAnalyzeBeforeCreatingLargeArray(cx, group, vp, length))
+        return nullptr;
+
+    JSObject* obj = NewFullyAllocatedArrayTryUseGroup(cx, group, length, newKind);
     if (!obj)
         return nullptr;
 
-    DenseElementResult result = obj->setOrExtendDenseElements(cx, 0, vp, length, updateTypes);
+    DenseElementResult result =
+        SetOrExtendAnyBoxedOrUnboxedDenseElements(cx, obj, 0, vp, length, updateTypes);
+    if (result == DenseElementResult::Failure)
+        return nullptr;
+    if (result == DenseElementResult::Success)
+        return obj;
+
+    MOZ_ASSERT(obj->is<UnboxedArrayObject>());
+    if (!UnboxedArrayObject::convertToNative(cx->asJSContext(), obj))
+        return nullptr;
+
+    result = SetOrExtendBoxedOrUnboxedDenseElements<JSVAL_TYPE_MAGIC>(cx, obj, 0, vp, length,
+                                                                      updateTypes);
+    MOZ_ASSERT(result != DenseElementResult::Incomplete);
     if (result == DenseElementResult::Failure)
         return nullptr;
-    MOZ_ASSERT(result == DenseElementResult::Success);
+
     return obj;
 }
 
-ArrayObject*
+JSObject*
 js::NewCopiedArrayForCallingAllocationSite(JSContext* cx, const Value* vp, size_t length,
                                            HandleObject proto /* = nullptr */)
 {
diff --git a/js/src/jsarray.h b/js/src/jsarray.h
index d0084731f..0ca580ba7 100644
--- a/js/src/jsarray.h
+++ b/js/src/jsarray.h
@@ -72,37 +72,49 @@ extern ArrayObject*
 NewDenseFullyAllocatedArrayWithTemplate(JSContext* cx, uint32_t length, JSObject* templateObject);
 
 /* Create a dense array with the same copy-on-write elements as another object. */
-extern ArrayObject*
+extern JSObject*
 NewDenseCopyOnWriteArray(JSContext* cx, HandleArrayObject templateObject, gc::InitialHeap heap);
 
-extern ArrayObject*
+// The methods below can create either boxed or unboxed arrays.
+
+extern JSObject*
 NewFullyAllocatedArrayTryUseGroup(ExclusiveContext* cx, HandleObjectGroup group, size_t length,
                                   NewObjectKind newKind = GenericObject);
 
-extern ArrayObject*
+extern JSObject*
 NewPartlyAllocatedArrayTryUseGroup(ExclusiveContext* cx, HandleObjectGroup group, size_t length);
 
-extern ArrayObject*
+extern JSObject*
 NewFullyAllocatedArrayTryReuseGroup(JSContext* cx, HandleObject obj, size_t length,
                                     NewObjectKind newKind = GenericObject);
 
-extern ArrayObject*
+extern JSObject*
 NewPartlyAllocatedArrayTryReuseGroup(JSContext* cx, HandleObject obj, size_t length);
 
-extern ArrayObject*
+extern JSObject*
 NewFullyAllocatedArrayForCallingAllocationSite(JSContext* cx, size_t length,
                                                NewObjectKind newKind = GenericObject);
 
-extern ArrayObject*
+extern JSObject*
 NewPartlyAllocatedArrayForCallingAllocationSite(JSContext* cx, size_t length, HandleObject proto);
 
-extern ArrayObject*
+enum class ShouldUpdateTypes
+{
+    Update,
+    DontUpdate
+};
+
+extern bool
+MaybeAnalyzeBeforeCreatingLargeArray(ExclusiveContext* cx, HandleObjectGroup group,
+                                     const Value* vp, size_t length);
+
+extern JSObject*
 NewCopiedArrayTryUseGroup(ExclusiveContext* cx, HandleObjectGroup group,
                           const Value* vp, size_t length,
                           NewObjectKind newKind = GenericObject,
                           ShouldUpdateTypes updateTypes = ShouldUpdateTypes::Update);
 
-extern ArrayObject*
+extern JSObject*
 NewCopiedArrayForCallingAllocationSite(JSContext* cx, const Value* vp, size_t length,
                                        HandleObject proto = nullptr);
 
@@ -117,6 +129,13 @@ NewValuePair(JSContext* cx, const Value& val1, const Value& val2, MutableHandleV
 extern bool
 WouldDefinePastNonwritableLength(HandleNativeObject obj, uint32_t index);
 
+/*
+ * Canonicalize |vp| to a uint32_t value potentially suitable for use as an
+ * array length.
+ */
+extern bool
+CanonicalizeArrayLengthValue(JSContext* cx, HandleValue v, uint32_t* canonicalized);
+
 extern bool
 GetLengthProperty(JSContext* cx, HandleObject obj, uint32_t* lengthp);
 
@@ -150,7 +169,7 @@ extern bool
 array_join(JSContext* cx, unsigned argc, js::Value* vp);
 
 extern void
-ArrayShiftMoveElements(NativeObject* obj);
+ArrayShiftMoveElements(JSObject* obj);
 
 extern bool
 array_shift(JSContext* cx, unsigned argc, js::Value* vp);
@@ -182,7 +201,7 @@ extern const JSJitInfo array_splice_info;
 extern bool
 NewbornArrayPush(JSContext* cx, HandleObject obj, const Value& v);
 
-extern ArrayObject*
+extern JSObject*
 ArrayConstructorOneArg(JSContext* cx, HandleObjectGroup group, int32_t lengthInt);
 
 #ifdef DEBUG
diff --git a/js/src/jscompartment.h b/js/src/jscompartment.h
index 05ff40b43..d7c6765ff 100644
--- a/js/src/jscompartment.h
+++ b/js/src/jscompartment.h
@@ -528,6 +528,9 @@ struct JSCompartment
     // table manages references from such typed objects to their buffers.
     js::ObjectWeakMap* lazyArrayBuffers;
 
+    // All unboxed layouts in the compartment.
+    mozilla::LinkedList<js::UnboxedLayout> unboxedLayouts;
+
     // WebAssembly state for the compartment.
     js::wasm::Compartment wasm;
 
diff --git a/js/src/jsfriendapi.cpp b/js/src/jsfriendapi.cpp
index bdb3c0a4d..515f62213 100644
--- a/js/src/jsfriendapi.cpp
+++ b/js/src/jsfriendapi.cpp
@@ -268,9 +268,9 @@ js::GetBuiltinClass(JSContext* cx, HandleObject obj, ESClass* cls)
     if (MOZ_UNLIKELY(obj->is<ProxyObject>()))
         return Proxy::getBuiltinClass(cx, obj, cls);
 
-    if (obj->is<PlainObject>())
+    if (obj->is<PlainObject>() || obj->is<UnboxedPlainObject>())
         *cls = ESClass::Object;
-    else if (obj->is<ArrayObject>())
+    else if (obj->is<ArrayObject>() || obj->is<UnboxedArrayObject>())
         *cls = ESClass::Array;
     else if (obj->is<NumberObject>())
         *cls = ESClass::Number;
diff --git a/js/src/jsgc.cpp b/js/src/jsgc.cpp
index 3ad526f74..64573b55a 100644
--- a/js/src/jsgc.cpp
+++ b/js/src/jsgc.cpp
@@ -6176,6 +6176,12 @@ gc::MergeCompartments(JSCompartment* source, JSCompartment* target)
     for (auto group = source->zone()->cellIter<ObjectGroup>(); !group.done(); group.next()) {
         group->setGeneration(target->zone()->types.generation);
         group->compartment_ = target;
+
+        // Remove any unboxed layouts from the list in the off thread
+        // compartment. These do not need to be reinserted in the target
+        // compartment's list, as the list is not required to be complete.
+        if (UnboxedLayout* layout = group->maybeUnboxedLayoutDontCheckGeneration())
+            layout->detachFromCompartment();
     }
 
     // Fixup zone pointers in source's zone to refer to target's zone.
diff --git a/js/src/jsiter.cpp b/js/src/jsiter.cpp
index c58f32382..8b1436b68 100644
--- a/js/src/jsiter.cpp
+++ b/js/src/jsiter.cpp
@@ -158,11 +158,8 @@ SortComparatorIntegerIds(jsid a, jsid b, bool* lessOrEqualp)
 }
 
 static bool
-EnumerateNativeProperties(JSContext* cx,
-                          HandleNativeObject pobj,
-						  unsigned flags,
-						  Maybe<IdSet>& ht,
-                          AutoIdVector* props)
+EnumerateNativeProperties(JSContext* cx, HandleNativeObject pobj, unsigned flags, Maybe<IdSet>& ht,
+                          AutoIdVector* props, Handle<UnboxedPlainObject*> unboxed = nullptr)
 {
     bool enumerateSymbols;
     if (flags & JSITER_SYMBOLSONLY) {
@@ -224,6 +221,16 @@ EnumerateNativeProperties(JSContext* cx,
                 return false;
         }
 
+        if (unboxed) {
+            // If |unboxed| is set then |pobj| is the expando for an unboxed
+            // plain object we are enumerating. Add the unboxed properties
+            // themselves here since they are all property names that were
+            // given to the object before any of the expando's properties.
+            MOZ_ASSERT(pobj->is<UnboxedExpandoObject>());
+            if (!EnumerateExtraProperties(cx, unboxed, flags, ht, props))
+                return false;
+        }
+
         size_t initialLength = props->length();
 
         /* Collect all unique property names from this object's shape. */
@@ -349,12 +356,22 @@ Snapshot(JSContext* cx, HandleObject pobj_, unsigned flags, AutoIdVector* props)
 
     do {
         if (pobj->getOpsEnumerate()) {
-            if (!EnumerateExtraProperties(cx, pobj, flags, ht, props))
-                return false;
-
-            if (pobj->isNative()) {
-                if (!EnumerateNativeProperties(cx, pobj.as<NativeObject>(), flags, ht, props))
+            if (pobj->is<UnboxedPlainObject>() && pobj->as<UnboxedPlainObject>().maybeExpando()) {
+                // Special case unboxed objects with an expando object.
+                RootedNativeObject expando(cx, pobj->as<UnboxedPlainObject>().maybeExpando());
+                if (!EnumerateNativeProperties(cx, expando, flags, ht, props,
+                                               pobj.as<UnboxedPlainObject>()))
+                {
+                    return false;
+                }
+            } else {
+                if (!EnumerateExtraProperties(cx, pobj, flags, ht, props))
                     return false;
+
+                if (pobj->isNative()) {
+                    if (!EnumerateNativeProperties(cx, pobj.as<NativeObject>(), flags, ht, props))
+                        return false;
+                }
             }
         } else if (pobj->isNative()) {
             // Give the object a chance to resolve all lazy properties
@@ -769,6 +786,11 @@ CanCompareIterableObjectToCache(JSObject* obj)
 {
     if (obj->isNative())
         return obj->as<NativeObject>().hasEmptyElements();
+    if (obj->is<UnboxedPlainObject>()) {
+        if (UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando())
+            return expando->hasEmptyElements();
+        return true;
+    }
     return false;
 }
 
diff --git a/js/src/jsobj.cpp b/js/src/jsobj.cpp
index ef1291079..d4379bd7d 100644
--- a/js/src/jsobj.cpp
+++ b/js/src/jsobj.cpp
@@ -42,7 +42,6 @@
 #include "frontend/BytecodeCompiler.h"
 #include "gc/Marking.h"
 #include "gc/Policy.h"
-#include "gc/StoreBuffer-inl.h"
 #include "jit/BaselineJIT.h"
 #include "js/MemoryMetrics.h"
 #include "js/Proxy.h"
@@ -869,6 +868,9 @@ static inline JSObject*
 CreateThisForFunctionWithGroup(JSContext* cx, HandleObjectGroup group,
                                NewObjectKind newKind)
 {
+    if (group->maybeUnboxedLayout() && newKind != SingletonObject)
+        return UnboxedPlainObject::create(cx, group, newKind);
+
     if (TypeNewScript* newScript = group->newScript()) {
         if (newScript->analyzed()) {
             // The definite properties analysis has been performed for this
@@ -1136,18 +1138,19 @@ js::CloneObject(JSContext* cx, HandleObject obj, Handle<js::TaggedProto> proto)
 }
 
 static bool
-GetScriptArrayObjectElements(JSContext* cx, HandleArrayObject arr, MutableHandle<GCVector<Value>> values)
+GetScriptArrayObjectElements(JSContext* cx, HandleObject obj, MutableHandle<GCVector<Value>> values)
 {
-    MOZ_ASSERT(!arr->isSingleton());
-    MOZ_ASSERT(!arr->isIndexed());
+    MOZ_ASSERT(!obj->isSingleton());
+    MOZ_ASSERT(obj->is<ArrayObject>() || obj->is<UnboxedArrayObject>());
+    MOZ_ASSERT(!obj->isIndexed());
 
-    size_t length = arr->length();
+    size_t length = GetAnyBoxedOrUnboxedArrayLength(obj);
     if (!values.appendN(MagicValue(JS_ELEMENTS_HOLE), length))
         return false;
 
-    size_t initlen = arr->getDenseInitializedLength();
+    size_t initlen = GetAnyBoxedOrUnboxedInitializedLength(obj);
     for (size_t i = 0; i < initlen; i++)
-        values[i].set(arr->getDenseElement(i));
+        values[i].set(GetAnyBoxedOrUnboxedDenseElement(obj, i));
 
     return true;
 }
@@ -1156,27 +1159,46 @@ static bool
 GetScriptPlainObjectProperties(JSContext* cx, HandleObject obj,
                                MutableHandle<IdValueVector> properties)
 {
-    MOZ_ASSERT(obj->is<PlainObject>());
-    PlainObject* nobj = &obj->as<PlainObject>();
+    if (obj->is<PlainObject>()) {
+        PlainObject* nobj = &obj->as<PlainObject>();
 
-    if (!properties.appendN(IdValuePair(), nobj->slotSpan()))
-        return false;
+        if (!properties.appendN(IdValuePair(), nobj->slotSpan()))
+            return false;
+
+        for (Shape::Range<NoGC> r(nobj->lastProperty()); !r.empty(); r.popFront()) {
+            Shape& shape = r.front();
+            MOZ_ASSERT(shape.isDataDescriptor());
+            uint32_t slot = shape.slot();
+            properties[slot].get().id = shape.propid();
+            properties[slot].get().value = nobj->getSlot(slot);
+        }
+
+        for (size_t i = 0; i < nobj->getDenseInitializedLength(); i++) {
+            Value v = nobj->getDenseElement(i);
+            if (!v.isMagic(JS_ELEMENTS_HOLE) && !properties.append(IdValuePair(INT_TO_JSID(i), v)))
+                return false;
+        }
 
-    for (Shape::Range<NoGC> r(nobj->lastProperty()); !r.empty(); r.popFront()) {
-        Shape& shape = r.front();
-        MOZ_ASSERT(shape.isDataDescriptor());
-        uint32_t slot = shape.slot();
-        properties[slot].get().id = shape.propid();
-        properties[slot].get().value = nobj->getSlot(slot);
+        return true;
     }
 
-    for (size_t i = 0; i < nobj->getDenseInitializedLength(); i++) {
-        Value v = nobj->getDenseElement(i);
-        if (!v.isMagic(JS_ELEMENTS_HOLE) && !properties.append(IdValuePair(INT_TO_JSID(i), v)))
+    if (obj->is<UnboxedPlainObject>()) {
+        UnboxedPlainObject* nobj = &obj->as<UnboxedPlainObject>();
+
+        const UnboxedLayout& layout = nobj->layout();
+        if (!properties.appendN(IdValuePair(), layout.properties().length()))
             return false;
+
+        for (size_t i = 0; i < layout.properties().length(); i++) {
+            const UnboxedLayout::Property& property = layout.properties()[i];
+            properties[i].get().id = NameToId(property.name);
+            properties[i].get().value = nobj->getValue(property);
+        }
+
+        return true;
     }
 
-    return true;
+    MOZ_CRASH("Bad object kind");
 }
 
 static bool
@@ -1198,13 +1220,13 @@ js::DeepCloneObjectLiteral(JSContext* cx, HandleObject obj, NewObjectKind newKin
     /* NB: Keep this in sync with XDRObjectLiteral. */
     MOZ_ASSERT_IF(obj->isSingleton(),
                   cx->compartment()->behaviors().getSingletonsAsTemplates());
-    MOZ_ASSERT(obj->is<PlainObject>() ||
-               obj->is<ArrayObject>());
+    MOZ_ASSERT(obj->is<PlainObject>() || obj->is<UnboxedPlainObject>() ||
+               obj->is<ArrayObject>() || obj->is<UnboxedArrayObject>());
     MOZ_ASSERT(newKind != SingletonObject);
 
-    if (obj->is<ArrayObject>()) {
+    if (obj->is<ArrayObject>() || obj->is<UnboxedArrayObject>()) {
         Rooted<GCVector<Value>> values(cx, GCVector<Value>(cx));
-        if (!GetScriptArrayObjectElements(cx, obj.as<ArrayObject>(), &values))
+        if (!GetScriptArrayObjectElements(cx, obj, &values))
             return nullptr;
 
         // Deep clone any elements.
@@ -1318,8 +1340,10 @@ js::XDRObjectLiteral(XDRState<mode>* xdr, MutableHandleObject obj)
     {
         if (mode == XDR_ENCODE) {
             MOZ_ASSERT(obj->is<PlainObject>() ||
-                       obj->is<ArrayObject>());
-            isArray = obj->is<ArrayObject>() ? 1 : 0;
+                       obj->is<UnboxedPlainObject>() ||
+                       obj->is<ArrayObject>() ||
+                       obj->is<UnboxedArrayObject>());
+            isArray = (obj->is<ArrayObject>() || obj->is<UnboxedArrayObject>()) ? 1 : 0;
         }
 
         if (!xdr->codeUint32(&isArray))
@@ -1331,11 +1355,8 @@ js::XDRObjectLiteral(XDRState<mode>* xdr, MutableHandleObject obj)
 
     if (isArray) {
         Rooted<GCVector<Value>> values(cx, GCVector<Value>(cx));
-        if (mode == XDR_ENCODE) {
-            RootedArrayObject arr(cx, &obj->as<ArrayObject>());
-            if (!GetScriptArrayObjectElements(cx, arr, &values))
-                return false;
-        }
+        if (mode == XDR_ENCODE && !GetScriptArrayObjectElements(cx, obj, &values))
+            return false;
 
         uint32_t initialized;
         if (mode == XDR_ENCODE)
@@ -2315,6 +2336,16 @@ js::LookupOwnPropertyPure(ExclusiveContext* cx, JSObject* obj, jsid id, Shape**
         // us the resolve hook won't define a property with this id.
         if (ClassMayResolveId(cx->names(), obj->getClass(), id, obj))
             return false;
+    } else if (obj->is<UnboxedPlainObject>()) {
+        if (obj->as<UnboxedPlainObject>().containsUnboxedOrExpandoProperty(cx, id)) {
+            MarkNonNativePropertyFound<NoGC>(propp);
+            return true;
+        }
+    } else if (obj->is<UnboxedArrayObject>()) {
+        if (obj->as<UnboxedArrayObject>().containsProperty(cx, id)) {
+            MarkNonNativePropertyFound<NoGC>(propp);
+            return true;
+        }
     } else if (obj->is<TypedObject>()) {
         if (obj->as<TypedObject>().typeDescr().hasProperty(cx->names(), id)) {
             MarkNonNativePropertyFound<NoGC>(propp);
@@ -2571,6 +2602,11 @@ js::SetPrototype(JSContext* cx, HandleObject obj, HandleObject proto, JS::Object
             break;
     }
 
+    // Convert unboxed objects to their native representations before changing
+    // their prototype/group, as they depend on the group for their layout.
+    if (!MaybeConvertUnboxedObjectToNative(cx, obj))
+        return false;
+
     Rooted<TaggedProto> taggedProto(cx, TaggedProto(proto));
     if (!SetClassAndProto(cx, obj, obj->getClass(), taggedProto))
         return false;
@@ -2594,6 +2630,9 @@ js::PreventExtensions(JSContext* cx, HandleObject obj, ObjectOpResult& result, I
     if (!obj->nonProxyIsExtensible())
         return result.succeed();
 
+    if (!MaybeConvertUnboxedObjectToNative(cx, obj))
+        return false;
+
     // Force lazy properties to be resolved.
     AutoIdVector props(cx);
     if (!js::GetPropertyKeys(cx, obj, JSITER_HIDDEN | JSITER_OWNONLY, &props))
@@ -3606,6 +3645,22 @@ JSObject::allocKindForTenure(const js::Nursery& nursery) const
     if (IsProxy(this))
         return as<ProxyObject>().allocKindForTenure();
 
+    // Unboxed plain objects are sized according to the data they store.
+    if (is<UnboxedPlainObject>()) {
+        size_t nbytes = as<UnboxedPlainObject>().layoutDontCheckGeneration().size();
+        return GetGCObjectKindForBytes(UnboxedPlainObject::offsetOfData() + nbytes);
+    }
+
+    // Unboxed arrays use inline data if their size is small enough.
+    if (is<UnboxedArrayObject>()) {
+        const UnboxedArrayObject* nobj = &as<UnboxedArrayObject>();
+        size_t nbytes = UnboxedArrayObject::offsetOfInlineElements() +
+                        nobj->capacity() * nobj->elementSize();
+        if (nbytes <= JSObject::MAX_BYTE_SIZE)
+            return GetGCObjectKindForBytes(nbytes);
+        return AllocKind::OBJECT0;
+    }
+
     // Inlined typed objects are followed by their data, so make sure we copy
     // it all over to the new object.
     if (is<InlineTypedObject>()) {
diff --git a/js/src/jsobjinlines.h b/js/src/jsobjinlines.h
index 98e740142..a82725ee8 100644
--- a/js/src/jsobjinlines.h
+++ b/js/src/jsobjinlines.h
@@ -32,6 +32,21 @@
 #include "vm/ShapedObject-inl.h"
 #include "vm/TypeInference-inl.h"
 
+namespace js {
+
+// This is needed here for ensureShape() below.
+inline bool
+MaybeConvertUnboxedObjectToNative(ExclusiveContext* cx, JSObject* obj)
+{
+    if (obj->is<UnboxedPlainObject>())
+        return UnboxedPlainObject::convertToNative(cx->asJSContext(), obj);
+    if (obj->is<UnboxedArrayObject>())
+        return UnboxedArrayObject::convertToNative(cx->asJSContext(), obj);
+    return true;
+}
+
+} // namespace js
+
 inline js::Shape*
 JSObject::maybeShape() const
 {
@@ -44,6 +59,8 @@ JSObject::maybeShape() const
 inline js::Shape*
 JSObject::ensureShape(js::ExclusiveContext* cx)
 {
+    if (!js::MaybeConvertUnboxedObjectToNative(cx, this))
+        return nullptr;
     js::Shape* shape = maybeShape();
     MOZ_ASSERT(shape);
     return shape;
diff --git a/js/src/jsstr.cpp b/js/src/jsstr.cpp
index 3964ab84e..5e593846e 100644
--- a/js/src/jsstr.cpp
+++ b/js/src/jsstr.cpp
@@ -2373,7 +2373,7 @@ js::str_replace_string_raw(JSContext* cx, HandleString string, HandleString patt
 }
 
 // ES 2016 draft Mar 25, 2016 21.1.3.17 steps 4, 8, 12-18.
-static ArrayObject*
+static JSObject*
 SplitHelper(JSContext* cx, HandleLinearString str, uint32_t limit, HandleLinearString sep,
             HandleObjectGroup group)
 {
@@ -2470,7 +2470,7 @@ SplitHelper(JSContext* cx, HandleLinearString str, uint32_t limit, HandleLinearS
 }
 
 // Fast-path for splitting a string into a character array via split("").
-static ArrayObject*
+static JSObject*
 CharSplitHelper(JSContext* cx, HandleLinearString str, uint32_t limit, HandleObjectGroup group)
 {
     size_t strLength = str->length();
@@ -2495,7 +2495,7 @@ CharSplitHelper(JSContext* cx, HandleLinearString str, uint32_t limit, HandleObj
 }
 
 // ES 2016 draft Mar 25, 2016 21.1.3.17 steps 4, 8, 12-18.
-ArrayObject*
+JSObject*
 js::str_split_string(JSContext* cx, HandleObjectGroup group, HandleString str, HandleString sep, uint32_t limit)
 
 {
@@ -2906,8 +2906,8 @@ StringObject::assignInitialShape(ExclusiveContext* cx, Handle<StringObject*> obj
 {
     MOZ_ASSERT(obj->empty());
 
-    return NativeObject::addDataProperty(cx, obj, cx->names().length, LENGTH_SLOT,
-                                         JSPROP_PERMANENT | JSPROP_READONLY);
+    return obj->addDataProperty(cx, cx->names().length, LENGTH_SLOT,
+                                JSPROP_PERMANENT | JSPROP_READONLY);
 }
 
 JSObject*
diff --git a/js/src/jsstr.h b/js/src/jsstr.h
index 68175c826..38fbfa85e 100644
--- a/js/src/jsstr.h
+++ b/js/src/jsstr.h
@@ -465,7 +465,7 @@ FileEscapedString(FILE* fp, const char* chars, size_t length, uint32_t quote)
     return res;
 }
 
-ArrayObject*
+JSObject*
 str_split_string(JSContext* cx, HandleObjectGroup group, HandleString str, HandleString sep,
                  uint32_t limit);
 
diff --git a/js/src/moz.build b/js/src/moz.build
index 5287aef00..c0cef9929 100644
--- a/js/src/moz.build
+++ b/js/src/moz.build
@@ -355,6 +355,7 @@ UNIFIED_SOURCES += [
     'vm/UbiNode.cpp',
     'vm/UbiNodeCensus.cpp',
     'vm/UbiNodeShortestPaths.cpp',
+    'vm/UnboxedObject.cpp',
     'vm/Unicode.cpp',
     'vm/Value.cpp',
     'vm/WeakMapPtr.cpp',
diff --git a/js/src/shell/js.cpp b/js/src/shell/js.cpp
index 4b0c858a4..8cd821b31 100644
--- a/js/src/shell/js.cpp
+++ b/js/src/shell/js.cpp
@@ -317,6 +317,7 @@ static bool enableIon = false;
 static bool enableAsmJS = false;
 static bool enableWasm = false;
 static bool enableNativeRegExp = false;
+static bool enableUnboxedArrays = false;
 static bool enableSharedMemory = SHARED_MEMORY_DEFAULT;
 static bool enableWasmAlwaysBaseline = false;
 static bool enableArrayProtoValues = true;
@@ -7262,6 +7263,7 @@ SetContextOptions(JSContext* cx, const OptionParser& op)
     enableAsmJS = !op.getBoolOption("no-asmjs");
     enableWasm = !op.getBoolOption("no-wasm");
     enableNativeRegExp = !op.getBoolOption("no-native-regexp");
+    enableUnboxedArrays = op.getBoolOption("unboxed-arrays");
     enableWasmAlwaysBaseline = op.getBoolOption("wasm-always-baseline");
     enableArrayProtoValues = !op.getBoolOption("no-array-proto-values");
 
@@ -7271,11 +7273,15 @@ SetContextOptions(JSContext* cx, const OptionParser& op)
                              .setWasm(enableWasm)
                              .setWasmAlwaysBaseline(enableWasmAlwaysBaseline)
                              .setNativeRegExp(enableNativeRegExp)
+                             .setUnboxedArrays(enableUnboxedArrays)
                              .setArrayProtoValues(enableArrayProtoValues);
 
     if (op.getBoolOption("wasm-check-bce"))
         jit::JitOptions.wasmAlwaysCheckBounds = true;
 
+    if (op.getBoolOption("no-unboxed-objects"))
+        jit::JitOptions.disableUnboxedObjects = true;
+
     if (const char* str = op.getStringOption("cache-ir-stubs")) {
         if (strcmp(str, "on") == 0)
             jit::JitOptions.disableCacheIR = false;
@@ -7539,6 +7545,7 @@ SetWorkerContextOptions(JSContext* cx)
                              .setWasm(enableWasm)
                              .setWasmAlwaysBaseline(enableWasmAlwaysBaseline)
                              .setNativeRegExp(enableNativeRegExp)
+                             .setUnboxedArrays(enableUnboxedArrays)
                              .setArrayProtoValues(enableArrayProtoValues);
     cx->setOffthreadIonCompilationEnabled(offthreadCompilation);
     cx->profilingScripts = enableCodeCoverage || enableDisassemblyDumps;
@@ -7708,6 +7715,8 @@ main(int argc, char** argv, char** envp)
         || !op.addBoolOption('\0', "no-asmjs", "Disable asm.js compilation")
         || !op.addBoolOption('\0', "no-wasm", "Disable WebAssembly compilation")
         || !op.addBoolOption('\0', "no-native-regexp", "Disable native regexp compilation")
+        || !op.addBoolOption('\0', "no-unboxed-objects", "Disable creating unboxed plain objects")
+        || !op.addBoolOption('\0', "unboxed-arrays", "Allow creating unboxed arrays")
         || !op.addBoolOption('\0', "wasm-always-baseline", "Enable wasm baseline compiler when possible")
         || !op.addBoolOption('\0', "wasm-check-bce", "Always generate wasm bounds check, even redundant ones.")
         || !op.addBoolOption('\0', "no-array-proto-values", "Remove Array.prototype.values")
diff --git a/js/src/vm/ErrorObject.cpp b/js/src/vm/ErrorObject.cpp
index 271132801..d8d29830b 100644
--- a/js/src/vm/ErrorObject.cpp
+++ b/js/src/vm/ErrorObject.cpp
@@ -29,11 +29,11 @@ js::ErrorObject::assignInitialShape(ExclusiveContext* cx, Handle<ErrorObject*> o
 {
     MOZ_ASSERT(obj->empty());
 
-    if (!NativeObject::addDataProperty(cx, obj, cx->names().fileName, FILENAME_SLOT, 0))
+    if (!obj->addDataProperty(cx, cx->names().fileName, FILENAME_SLOT, 0))
         return nullptr;
-    if (!NativeObject::addDataProperty(cx, obj, cx->names().lineNumber, LINENUMBER_SLOT, 0))
+    if (!obj->addDataProperty(cx, cx->names().lineNumber, LINENUMBER_SLOT, 0))
         return nullptr;
-    return NativeObject::addDataProperty(cx, obj, cx->names().columnNumber, COLUMNNUMBER_SLOT, 0);
+    return obj->addDataProperty(cx, cx->names().columnNumber, COLUMNNUMBER_SLOT, 0);
 }
 
 /* static */ bool
@@ -57,7 +57,7 @@ js::ErrorObject::init(JSContext* cx, Handle<ErrorObject*> obj, JSExnType type,
     // |new Error()|.
     RootedShape messageShape(cx);
     if (message) {
-        messageShape = NativeObject::addDataProperty(cx, obj, cx->names().message, MESSAGE_SLOT, 0);
+        messageShape = obj->addDataProperty(cx, cx->names().message, MESSAGE_SLOT, 0);
         if (!messageShape)
             return false;
         MOZ_ASSERT(messageShape->slot() == MESSAGE_SLOT);
diff --git a/js/src/vm/Interpreter-inl.h b/js/src/vm/Interpreter-inl.h
index acfa8f74b..0e81dfef4 100644
--- a/js/src/vm/Interpreter-inl.h
+++ b/js/src/vm/Interpreter-inl.h
@@ -22,6 +22,7 @@
 #include "vm/EnvironmentObject-inl.h"
 #include "vm/Stack-inl.h"
 #include "vm/String-inl.h"
+#include "vm/UnboxedObject-inl.h"
 
 namespace js {
 
@@ -336,10 +337,14 @@ InitGlobalLexicalOperation(JSContext* cx, LexicalEnvironmentObject* lexicalEnvAr
 inline bool
 InitPropertyOperation(JSContext* cx, JSOp op, HandleObject obj, HandleId id, HandleValue rhs)
 {
-  MOZ_ASSERT(obj->is<PlainObject>() || obj->is<JSFunction>());
-  unsigned propAttrs = GetInitDataPropAttrs(op);
-  return NativeDefineProperty(cx, obj.as<NativeObject>(), id, rhs,
-                              nullptr, nullptr, propAttrs);
+    if (obj->is<PlainObject>() || obj->is<JSFunction>()) {
+        unsigned propAttrs = GetInitDataPropAttrs(op);
+        return NativeDefineProperty(cx, obj.as<NativeObject>(), id, rhs, nullptr, nullptr,
+                                    propAttrs);
+    }
+
+    MOZ_ASSERT(obj->as<UnboxedPlainObject>().layout().lookup(id));
+    return PutProperty(cx, obj, id, rhs, false);
 }
 
 inline bool
@@ -593,7 +598,7 @@ InitArrayElemOperation(JSContext* cx, jsbytecode* pc, HandleObject obj, uint32_t
     JSOp op = JSOp(*pc);
     MOZ_ASSERT(op == JSOP_INITELEM_ARRAY || op == JSOP_INITELEM_INC);
 
-    MOZ_ASSERT(obj->is<ArrayObject>());
+    MOZ_ASSERT(obj->is<ArrayObject>() || obj->is<UnboxedArrayObject>());
 
     if (op == JSOP_INITELEM_INC && index == INT32_MAX) {
         JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, JSMSG_SPREAD_TOO_LARGE);
diff --git a/js/src/vm/Interpreter.cpp b/js/src/vm/Interpreter.cpp
index 3cf9b57f6..274392335 100644
--- a/js/src/vm/Interpreter.cpp
+++ b/js/src/vm/Interpreter.cpp
@@ -1939,7 +1939,6 @@ CASE(EnableInterruptsPseudoOpcode)
 /* Various 1-byte no-ops. */
 CASE(JSOP_NOP)
 CASE(JSOP_NOP_DESTRUCTURING)
-CASE(JSOP_UNUSED126)
 CASE(JSOP_UNUSED211)
 CASE(JSOP_TRY_DESTRUCTURING_ITERCLOSE)
 CASE(JSOP_UNUSED221)
@@ -3682,6 +3681,7 @@ CASE(JSOP_NEWINIT)
 END_CASE(JSOP_NEWINIT)
 
 CASE(JSOP_NEWARRAY)
+CASE(JSOP_SPREADCALLARRAY)
 {
     uint32_t length = GET_UINT32(REGS.pc);
     JSObject* obj = NewArrayOperation(cx, script, REGS.pc, length);
@@ -4157,7 +4157,7 @@ CASE(JSOP_INITHOMEOBJECT)
     /* Load the home object */
     ReservedRooted<JSObject*> obj(&rootObject0);
     obj = &REGS.sp[int(-2 - skipOver)].toObject();
-    MOZ_ASSERT(obj->is<PlainObject>() || obj->is<JSFunction>());
+    MOZ_ASSERT(obj->is<PlainObject>() || obj->is<UnboxedPlainObject>() || obj->is<JSFunction>());
 
     func->setExtendedSlot(FunctionExtended::METHOD_HOMEOBJECT_SLOT, ObjectValue(*obj));
 }
@@ -4973,13 +4973,18 @@ js::NewObjectOperation(JSContext* cx, HandleScript script, jsbytecode* pc,
             return nullptr;
         if (group->maybePreliminaryObjects()) {
             group->maybePreliminaryObjects()->maybeAnalyze(cx, group);
+            if (group->maybeUnboxedLayout())
+                group->maybeUnboxedLayout()->setAllocationSite(script, pc);
         }
 
         if (group->shouldPreTenure() || group->maybePreliminaryObjects())
             newKind = TenuredObject;
+
+        if (group->maybeUnboxedLayout())
+            return UnboxedPlainObject::create(cx, group, newKind);
     }
 
-    RootedPlainObject obj(cx);
+    RootedObject obj(cx);
 
     if (*pc == JSOP_NEWOBJECT) {
         RootedPlainObject baseObject(cx, &script->getObject(pc)->as<PlainObject>());
@@ -5016,6 +5021,11 @@ js::NewObjectOperationWithTemplate(JSContext* cx, HandleObject templateObject)
 
     NewObjectKind newKind = templateObject->group()->shouldPreTenure() ? TenuredObject : GenericObject;
 
+    if (templateObject->group()->maybeUnboxedLayout()) {
+        RootedObjectGroup group(cx, templateObject->group());
+        return UnboxedPlainObject::create(cx, group, newKind);
+    }
+
     JSObject* obj = CopyInitializerObject(cx, templateObject.as<PlainObject>(), newKind);
     if (!obj)
         return nullptr;
@@ -5042,6 +5052,9 @@ js::NewArrayOperation(JSContext* cx, HandleScript script, jsbytecode* pc, uint32
 
         if (group->shouldPreTenure() || group->maybePreliminaryObjects())
             newKind = TenuredObject;
+
+        if (group->maybeUnboxedLayout())
+            return UnboxedArrayObject::create(cx, group, length, newKind);
     }
 
     ArrayObject* obj = NewDenseFullyAllocatedArray(cx, length, nullptr, newKind);
@@ -5052,6 +5065,9 @@ js::NewArrayOperation(JSContext* cx, HandleScript script, jsbytecode* pc, uint32
         MOZ_ASSERT(obj->isSingleton());
     } else {
         obj->setGroup(group);
+
+        if (PreliminaryObjectArray* preliminaryObjects = group->maybePreliminaryObjects())
+            preliminaryObjects->registerNewObject(obj);
     }
 
     return obj;
@@ -5064,6 +5080,12 @@ js::NewArrayOperationWithTemplate(JSContext* cx, HandleObject templateObject)
 
     NewObjectKind newKind = templateObject->group()->shouldPreTenure() ? TenuredObject : GenericObject;
 
+    if (templateObject->is<UnboxedArrayObject>()) {
+        uint32_t length = templateObject->as<UnboxedArrayObject>().length();
+        RootedObjectGroup group(cx, templateObject->group());
+        return UnboxedArrayObject::create(cx, group, length, newKind);
+    }
+
     ArrayObject* obj = NewDenseFullyAllocatedArray(cx, templateObject->as<ArrayObject>().length(),
                                                    nullptr, newKind);
     if (!obj)
diff --git a/js/src/vm/JSONParser.cpp b/js/src/vm/JSONParser.cpp
index e50da3bc4..01883bb15 100644
--- a/js/src/vm/JSONParser.cpp
+++ b/js/src/vm/JSONParser.cpp
@@ -606,8 +606,8 @@ JSONParserBase::finishArray(MutableHandleValue vp, ElementVector& elements)
 {
     MOZ_ASSERT(&elements == &stack.back().elements());
 
-    ArrayObject* obj = ObjectGroup::newArrayObject(cx, elements.begin(), elements.length(),
-                                                   GenericObject);
+    JSObject* obj = ObjectGroup::newArrayObject(cx, elements.begin(), elements.length(),
+                                                GenericObject);
     if (!obj)
         return false;
 
diff --git a/js/src/vm/NativeObject-inl.h b/js/src/vm/NativeObject-inl.h
index 030d92c12..205d99aa2 100644
--- a/js/src/vm/NativeObject-inl.h
+++ b/js/src/vm/NativeObject-inl.h
@@ -235,38 +235,6 @@ NativeObject::ensureDenseElements(ExclusiveContext* cx, uint32_t index, uint32_t
     return DenseElementResult::Success;
 }
 
-inline DenseElementResult
-NativeObject::setOrExtendDenseElements(ExclusiveContext* cx, uint32_t start, const Value* vp,
-                                       uint32_t count,
-                                       ShouldUpdateTypes updateTypes)
-{
-    if (denseElementsAreFrozen())
-        return DenseElementResult::Incomplete;
-
-    if (is<ArrayObject>() &&
-        !as<ArrayObject>().lengthIsWritable() &&
-        start + count >= as<ArrayObject>().length())
-    {
-        return DenseElementResult::Incomplete;
-    }
-
-    DenseElementResult result = ensureDenseElements(cx, start, count);
-    if (result != DenseElementResult::Success)
-        return result;
-
-    if (is<ArrayObject>() && start + count >= as<ArrayObject>().length())
-        as<ArrayObject>().setLengthInt32(start + count);
-
-    if (updateTypes == ShouldUpdateTypes::DontUpdate && !shouldConvertDoubleElements()) {
-        copyDenseElements(start, vp, count);
-    } else {
-        for (size_t i = 0; i < count; i++)
-            setDenseElementWithType(cx, start + i, vp[i]);
-    }
-
-    return DenseElementResult::Success;
-}
-
 inline Value
 NativeObject::getDenseOrTypedArrayElement(uint32_t idx)
 {
diff --git a/js/src/vm/NativeObject.cpp b/js/src/vm/NativeObject.cpp
index d801fad06..8b7543d12 100644
--- a/js/src/vm/NativeObject.cpp
+++ b/js/src/vm/NativeObject.cpp
@@ -388,6 +388,33 @@ NativeObject::setLastPropertyMakeNonNative(Shape* shape)
     shape_ = shape;
 }
 
+void
+NativeObject::setLastPropertyMakeNative(ExclusiveContext* cx, Shape* shape)
+{
+    MOZ_ASSERT(getClass()->isNative());
+    MOZ_ASSERT(shape->getObjectClass()->isNative());
+    MOZ_ASSERT(!shape->inDictionary());
+
+    // This method is used to convert unboxed objects into native objects. In
+    // this case, the shape_ field was previously used to store other data and
+    // this should be treated as an initialization.
+    shape_.init(shape);
+
+    slots_ = nullptr;
+    elements_ = emptyObjectElements;
+
+    size_t oldSpan = shape->numFixedSlots();
+    size_t newSpan = shape->slotSpan();
+
+    initializeSlotRange(0, oldSpan);
+
+    // A failure at this point will leave the object as a mutant, and we
+    // can't recover.
+    AutoEnterOOMUnsafeRegion oomUnsafe;
+    if (oldSpan != newSpan && !updateSlotsForSpan(cx, oldSpan, newSpan))
+        oomUnsafe.crash("NativeObject::setLastPropertyMakeNative");
+}
+
 bool
 NativeObject::setSlotSpan(ExclusiveContext* cx, uint32_t span)
 {
@@ -994,22 +1021,23 @@ NativeObject::freeSlot(ExclusiveContext* cx, uint32_t slot)
     setSlot(slot, UndefinedValue());
 }
 
-/* static */ Shape*
-NativeObject::addDataProperty(ExclusiveContext* cx, HandleNativeObject obj,
-                              jsid idArg, uint32_t slot, unsigned attrs)
+Shape*
+NativeObject::addDataProperty(ExclusiveContext* cx, jsid idArg, uint32_t slot, unsigned attrs)
 {
     MOZ_ASSERT(!(attrs & (JSPROP_GETTER | JSPROP_SETTER)));
+    RootedNativeObject self(cx, this);
     RootedId id(cx, idArg);
-    return addProperty(cx, obj, id, nullptr, nullptr, slot, attrs, 0);
+    return addProperty(cx, self, id, nullptr, nullptr, slot, attrs, 0);
 }
 
-/* static */ Shape*
-NativeObject::addDataProperty(ExclusiveContext* cx, HandleNativeObject obj,
-                              HandlePropertyName name, uint32_t slot, unsigned attrs)
+Shape*
+NativeObject::addDataProperty(ExclusiveContext* cx, HandlePropertyName name,
+                              uint32_t slot, unsigned attrs)
 {
     MOZ_ASSERT(!(attrs & (JSPROP_GETTER | JSPROP_SETTER)));
+    RootedNativeObject self(cx, this);
     RootedId id(cx, NameToId(name));
-    return addProperty(cx, obj, id, nullptr, nullptr, slot, attrs, 0);
+    return addProperty(cx, self, id, nullptr, nullptr, slot, attrs, 0);
 }
 
 template <AllowGC allowGC>
diff --git a/js/src/vm/NativeObject.h b/js/src/vm/NativeObject.h
index 9cc6d5436..3a3e50244 100644
--- a/js/src/vm/NativeObject.h
+++ b/js/src/vm/NativeObject.h
@@ -339,19 +339,16 @@ IsObjectValueInCompartment(const Value& v, JSCompartment* comp);
 #endif
 
 // Operations which change an object's dense elements can either succeed, fail,
-// or be unable to complete. The latter is used when the object's elements must
-// become sparse instead. The enum below is used for such operations.
+// or be unable to complete. For native objects, the latter is used when the
+// object's elements must become sparse instead. The enum below is used for
+// such operations, and for similar operations on unboxed arrays and methods
+// that work on both kinds of objects.
 enum class DenseElementResult {
     Failure,
     Success,
     Incomplete
 };
 
-enum class ShouldUpdateTypes {
-    Update,
-    DontUpdate
-};
-
 /*
  * NativeObject specifies the internal implementation of a native object.
  *
@@ -470,6 +467,11 @@ class NativeObject : public ShapedObject
     // that are (temporarily) inconsistent.
     void setLastPropertyMakeNonNative(Shape* shape);
 
+    // As for setLastProperty(), but changes the class associated with the
+    // object to a native one. The object's type has already been changed, and
+    // this brings the shape into sync with it.
+    void setLastPropertyMakeNative(ExclusiveContext* cx, Shape* shape);
+
     // Newly-created TypedArrays that map a SharedArrayBuffer are
     // marked as shared by giving them an ObjectElements that has the
     // ObjectElements::SHARED_MEMORY flag set.
@@ -744,10 +746,10 @@ class NativeObject : public ShapedObject
                               bool allowDictionary = true);
 
     /* Add a data property whose id is not yet in this scope. */
-    static Shape* addDataProperty(ExclusiveContext* cx, HandleNativeObject obj,
-                                  jsid id_, uint32_t slot, unsigned attrs);
-    static Shape* addDataProperty(ExclusiveContext* cx, HandleNativeObject obj,
-                                  HandlePropertyName name, uint32_t slot, unsigned attrs);
+    Shape* addDataProperty(ExclusiveContext* cx,
+                           jsid id_, uint32_t slot, unsigned attrs);
+    Shape* addDataProperty(ExclusiveContext* cx, HandlePropertyName name,
+                           uint32_t slot, unsigned attrs);
 
     /* Add or overwrite a property for id in this scope. */
     static Shape*
@@ -1149,10 +1151,6 @@ class NativeObject : public ShapedObject
         elementsRangeWriteBarrierPost(dstStart, count);
     }
 
-    inline DenseElementResult
-    setOrExtendDenseElements(ExclusiveContext* cx, uint32_t start, const Value* vp, uint32_t count,
-                             ShouldUpdateTypes updateTypes = ShouldUpdateTypes::Update);
-
     bool shouldConvertDoubleElements() {
         return getElementsHeader()->shouldConvertDoubleElements();
     }
diff --git a/js/src/vm/ObjectGroup-inl.h b/js/src/vm/ObjectGroup-inl.h
index d41343be6..9074f4d97 100644
--- a/js/src/vm/ObjectGroup-inl.h
+++ b/js/src/vm/ObjectGroup-inl.h
@@ -108,6 +108,20 @@ ObjectGroup::maybePreliminaryObjects()
     return maybePreliminaryObjectsDontCheckGeneration();
 }
 
+inline UnboxedLayout*
+ObjectGroup::maybeUnboxedLayout()
+{
+    maybeSweep(nullptr);
+    return maybeUnboxedLayoutDontCheckGeneration();
+}
+
+inline UnboxedLayout&
+ObjectGroup::unboxedLayout()
+{
+    maybeSweep(nullptr);
+    return unboxedLayoutDontCheckGeneration();
+}
+
 } // namespace js
 
 #endif /* vm_ObjectGroup_inl_h */
diff --git a/js/src/vm/ObjectGroup.cpp b/js/src/vm/ObjectGroup.cpp
index 91070b3f6..d05a48646 100644
--- a/js/src/vm/ObjectGroup.cpp
+++ b/js/src/vm/ObjectGroup.cpp
@@ -18,10 +18,11 @@
 #include "vm/ArrayObject.h"
 #include "vm/Shape.h"
 #include "vm/TaggedProto.h"
+#include "vm/UnboxedObject.h"
 
 #include "jsobjinlines.h"
 
-#include "vm/NativeObject-inl.h"
+#include "vm/UnboxedObject-inl.h"
 
 using namespace js;
 
@@ -55,6 +56,7 @@ ObjectGroup::finalize(FreeOp* fop)
     if (newScriptDontCheckGeneration())
         newScriptDontCheckGeneration()->clear();
     fop->delete_(newScriptDontCheckGeneration());
+    fop->delete_(maybeUnboxedLayoutDontCheckGeneration());
     if (maybePreliminaryObjectsDontCheckGeneration())
         maybePreliminaryObjectsDontCheckGeneration()->clear();
     fop->delete_(maybePreliminaryObjectsDontCheckGeneration());
@@ -81,6 +83,8 @@ ObjectGroup::sizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf) const
     size_t n = 0;
     if (TypeNewScript* newScript = newScriptDontCheckGeneration())
         n += newScript->sizeOfIncludingThis(mallocSizeOf);
+    if (UnboxedLayout* layout = maybeUnboxedLayoutDontCheckGeneration())
+        n += layout->sizeOfIncludingThis(mallocSizeOf);
     return n;
 }
 
@@ -529,7 +533,8 @@ ObjectGroup::defaultNewGroup(ExclusiveContext* cx, const Class* clasp,
     if (p) {
         ObjectGroup* group = p->group;
         MOZ_ASSERT_IF(clasp, group->clasp() == clasp);
-        MOZ_ASSERT_IF(!clasp, group->clasp() == &PlainObject::class_);
+        MOZ_ASSERT_IF(!clasp, group->clasp() == &PlainObject::class_ ||
+                              group->clasp() == &UnboxedPlainObject::class_);
         MOZ_ASSERT(group->proto() == proto);
         return group;
     }
@@ -772,7 +777,7 @@ GetValueTypeForTable(const Value& v)
     return type;
 }
 
-/* static */ ArrayObject*
+/* static */ JSObject*
 ObjectGroup::newArrayObject(ExclusiveContext* cx,
                             const Value* vp, size_t length,
                             NewObjectKind newKind, NewArrayKind arrayKind)
@@ -836,13 +841,56 @@ ObjectGroup::newArrayObject(ExclusiveContext* cx,
 
         AddTypePropertyId(cx, group, nullptr, JSID_VOID, elementType);
 
+        if (elementType != TypeSet::UnknownType()) {
+            // Keep track of the initial objects we create with this type.
+            // If the initial ones have a consistent shape and property types, we
+            // will try to use an unboxed layout for the group.
+            PreliminaryObjectArrayWithTemplate* preliminaryObjects =
+                cx->new_<PreliminaryObjectArrayWithTemplate>(nullptr);
+            if (!preliminaryObjects)
+                return nullptr;
+            group->setPreliminaryObjects(preliminaryObjects);
+        }
+
         if (!p.add(cx, *table, ObjectGroupCompartment::ArrayObjectKey(elementType), group))
             return nullptr;
     }
 
     // The type of the elements being added will already be reflected in type
-    // information.
+    // information, but make sure when creating an unboxed array that the
+    // common element type is suitable for the unboxed representation.
     ShouldUpdateTypes updateTypes = ShouldUpdateTypes::DontUpdate;
+    if (!MaybeAnalyzeBeforeCreatingLargeArray(cx, group, vp, length))
+        return nullptr;
+    if (group->maybePreliminaryObjects())
+        group->maybePreliminaryObjects()->maybeAnalyze(cx, group);
+    if (group->maybeUnboxedLayout()) {
+        switch (group->unboxedLayout().elementType()) {
+          case JSVAL_TYPE_BOOLEAN:
+            if (elementType != TypeSet::BooleanType())
+                updateTypes = ShouldUpdateTypes::Update;
+            break;
+          case JSVAL_TYPE_INT32:
+            if (elementType != TypeSet::Int32Type())
+                updateTypes = ShouldUpdateTypes::Update;
+            break;
+          case JSVAL_TYPE_DOUBLE:
+            if (elementType != TypeSet::Int32Type() && elementType != TypeSet::DoubleType())
+                updateTypes = ShouldUpdateTypes::Update;
+            break;
+          case JSVAL_TYPE_STRING:
+            if (elementType != TypeSet::StringType())
+                updateTypes = ShouldUpdateTypes::Update;
+            break;
+          case JSVAL_TYPE_OBJECT:
+            if (elementType != TypeSet::NullType() && !elementType.get().isObjectUnchecked())
+                updateTypes = ShouldUpdateTypes::Update;
+            break;
+          default:
+            MOZ_CRASH();
+        }
+    }
+
     return NewCopiedArrayTryUseGroup(cx, group, vp, length, newKind, updateTypes);
 }
 
@@ -852,15 +900,49 @@ GiveObjectGroup(ExclusiveContext* cx, JSObject* source, JSObject* target)
 {
     MOZ_ASSERT(source->group() != target->group());
 
-    if (!target->is<ArrayObject>() || !source->is<ArrayObject>()) {
+    if (!target->is<ArrayObject>() && !target->is<UnboxedArrayObject>())
+        return true;
+
+    if (target->group()->maybePreliminaryObjects()) {
+        bool force = IsInsideNursery(source);
+        target->group()->maybePreliminaryObjects()->maybeAnalyze(cx, target->group(), force);
+    }
+
+    if (target->is<ArrayObject>()) {
+        ObjectGroup* sourceGroup = source->group();
+
+        if (source->is<UnboxedArrayObject>()) {
+            Shape* shape = target->as<ArrayObject>().lastProperty();
+            if (!UnboxedArrayObject::convertToNativeWithGroup(cx, source, target->group(), shape))
+                return false;
+        } else if (source->is<ArrayObject>()) {
+            source->setGroup(target->group());
+        } else {
+            return true;
+        }
+
+        if (sourceGroup->maybePreliminaryObjects())
+            sourceGroup->maybePreliminaryObjects()->unregisterObject(source);
+        if (target->group()->maybePreliminaryObjects())
+            target->group()->maybePreliminaryObjects()->registerNewObject(source);
+
+        for (size_t i = 0; i < source->as<ArrayObject>().getDenseInitializedLength(); i++) {
+            Value v = source->as<ArrayObject>().getDenseElement(i);
+            AddTypePropertyId(cx, source->group(), source, JSID_VOID, v);
+        }
+
         return true;
     }
 
-    source->setGroup(target->group());
+    if (target->is<UnboxedArrayObject>()) {
+        if (!source->is<UnboxedArrayObject>())
+            return true;
+        if (source->as<UnboxedArrayObject>().elementType() != JSVAL_TYPE_INT32)
+            return true;
+        if (target->as<UnboxedArrayObject>().elementType() != JSVAL_TYPE_DOUBLE)
+            return true;
 
-    for (size_t i = 0; i < source->as<ArrayObject>().getDenseInitializedLength(); i++) {
-        Value v = source->as<ArrayObject>().getDenseElement(i);
-        AddTypePropertyId(cx, source->group(), source, JSID_VOID, v);
+        return source->as<UnboxedArrayObject>().convertInt32ToDouble(cx, target->group());
     }
 
     return true;
@@ -969,6 +1051,46 @@ js::CombinePlainObjectPropertyTypes(ExclusiveContext* cx, JSObject* newObj,
                 }
             }
         }
+    } else if (newObj->is<UnboxedPlainObject>()) {
+        const UnboxedLayout& layout = newObj->as<UnboxedPlainObject>().layout();
+        const int32_t* traceList = layout.traceList();
+        if (!traceList)
+            return true;
+
+        uint8_t* newData = newObj->as<UnboxedPlainObject>().data();
+        uint8_t* oldData = oldObj->as<UnboxedPlainObject>().data();
+
+        for (; *traceList != -1; traceList++) {}
+        traceList++;
+        for (; *traceList != -1; traceList++) {
+            JSObject* newInnerObj = *reinterpret_cast<JSObject**>(newData + *traceList);
+            JSObject* oldInnerObj = *reinterpret_cast<JSObject**>(oldData + *traceList);
+
+            if (!newInnerObj || !oldInnerObj || SameGroup(oldInnerObj, newInnerObj))
+                continue;
+
+            if (!GiveObjectGroup(cx, newInnerObj, oldInnerObj))
+                return false;
+
+            if (SameGroup(oldInnerObj, newInnerObj))
+                continue;
+
+            if (!GiveObjectGroup(cx, oldInnerObj, newInnerObj))
+                return false;
+
+            if (SameGroup(oldInnerObj, newInnerObj)) {
+                for (size_t i = 1; i < ncompare; i++) {
+                    if (compare[i].isObject() && SameGroup(&compare[i].toObject(), newObj)) {
+                        uint8_t* otherData = compare[i].toObject().as<UnboxedPlainObject>().data();
+                        JSObject* otherInnerObj = *reinterpret_cast<JSObject**>(otherData + *traceList);
+                        if (otherInnerObj && !SameGroup(otherInnerObj, newInnerObj)) {
+                            if (!GiveObjectGroup(cx, otherInnerObj, newInnerObj))
+                                return false;
+                        }
+                    }
+                }
+            }
+        }
     }
 
     return true;
@@ -1192,6 +1314,12 @@ ObjectGroup::newPlainObject(ExclusiveContext* cx, IdValuePair* properties, size_
 
     RootedObjectGroup group(cx, p->value().group);
 
+    // Watch for existing groups which now use an unboxed layout.
+    if (group->maybeUnboxedLayout()) {
+        MOZ_ASSERT(group->unboxedLayout().properties().length() == nproperties);
+        return UnboxedPlainObject::createWithProperties(cx, group, newKind, properties);
+    }
+
     // Update property types according to the properties we are about to add.
     // Do this before we do anything which can GC, which might move or remove
     // this table entry.
@@ -1378,6 +1506,18 @@ ObjectGroup::allocationSiteGroup(JSContext* cx, JSScript* scriptArg, jsbytecode*
         }
     }
 
+    if (kind == JSProto_Array &&
+        (JSOp(*pc) == JSOP_NEWARRAY || IsCallPC(pc)) &&
+        cx->options().unboxedArrays())
+    {
+        PreliminaryObjectArrayWithTemplate* preliminaryObjects =
+            cx->new_<PreliminaryObjectArrayWithTemplate>(nullptr);
+        if (preliminaryObjects)
+            res->setPreliminaryObjects(preliminaryObjects);
+        else
+            cx->recoverFromOutOfMemory();
+    }
+
     if (!table->add(p, key, res)) {
         ReportOutOfMemory(cx);
         return nullptr;
diff --git a/js/src/vm/ObjectGroup.h b/js/src/vm/ObjectGroup.h
index 0b6eaee51..4e24de9f1 100644
--- a/js/src/vm/ObjectGroup.h
+++ b/js/src/vm/ObjectGroup.h
@@ -20,6 +20,7 @@
 namespace js {
 
 class TypeDescr;
+class UnboxedLayout;
 
 class PreliminaryObjectArrayWithTemplate;
 class TypeNewScript;
@@ -153,6 +154,16 @@ class ObjectGroup : public gc::TenuredCell
         // For some plain objects, the addendum stores a PreliminaryObjectArrayWithTemplate.
         Addendum_PreliminaryObjects,
 
+        // When objects in this group have an unboxed representation, the
+        // addendum stores an UnboxedLayout (which might have a TypeNewScript
+        // as well, if the group is also constructed using 'new').
+        Addendum_UnboxedLayout,
+
+        // If this group is used by objects that have been converted from an
+        // unboxed representation and/or have the same allocation kind as such
+        // objects, the addendum points to that unboxed group.
+        Addendum_OriginalUnboxedGroup,
+
         // When used by typed objects, the addendum stores a TypeDescr.
         Addendum_TypeDescr
     };
@@ -174,6 +185,7 @@ class ObjectGroup : public gc::TenuredCell
         return nullptr;
     }
 
+    TypeNewScript* anyNewScript();
     void detachNewScript(bool writeBarrier, ObjectGroup* replacement);
 
     ObjectGroupFlags flagsDontCheckGeneration() const {
@@ -213,6 +225,34 @@ class ObjectGroup : public gc::TenuredCell
                maybePreliminaryObjectsDontCheckGeneration();
     }
 
+    inline UnboxedLayout* maybeUnboxedLayout();
+    inline UnboxedLayout& unboxedLayout();
+
+    UnboxedLayout* maybeUnboxedLayoutDontCheckGeneration() const {
+        if (addendumKind() == Addendum_UnboxedLayout)
+            return reinterpret_cast<UnboxedLayout*>(addendum_);
+        return nullptr;
+    }
+
+    UnboxedLayout& unboxedLayoutDontCheckGeneration() const {
+        MOZ_ASSERT(addendumKind() == Addendum_UnboxedLayout);
+        return *maybeUnboxedLayoutDontCheckGeneration();
+    }
+
+    void setUnboxedLayout(UnboxedLayout* layout) {
+        setAddendum(Addendum_UnboxedLayout, layout);
+    }
+
+    ObjectGroup* maybeOriginalUnboxedGroup() const {
+        if (addendumKind() == Addendum_OriginalUnboxedGroup)
+            return reinterpret_cast<ObjectGroup*>(addendum_);
+        return nullptr;
+    }
+
+    void setOriginalUnboxedGroup(ObjectGroup* group) {
+        setAddendum(Addendum_OriginalUnboxedGroup, group);
+    }
+
     TypeDescr* maybeTypeDescr() {
         // Note: there is no need to sweep when accessing the type descriptor
         // of an object, as it is strongly held and immutable.
@@ -273,8 +313,9 @@ class ObjectGroup : public gc::TenuredCell
      * that can be read out of that property in actual JS objects. In native
      * objects, property types account for plain data properties (those with a
      * slot and no getter or setter hook) and dense elements. In typed objects
-     * property types account for object and value properties and elements in
-     * the object.
+     * and unboxed objects, property types account for object and value
+     * properties and elements in the object, and expando properties in unboxed
+     * objects.
      *
      * For accesses on these properties, the correspondence is as follows:
      *
@@ -297,9 +338,10 @@ class ObjectGroup : public gc::TenuredCell
      * 2. Array lengths are special cased by the compiler and VM and are not
      *    reflected in property types.
      *
-     * 3. In typed objects, the initial values of properties (null pointers and
-     *    undefined values) are not reflected in the property types. These
-     *    values are always possible when reading the property.
+     * 3. In typed objects (but not unboxed objects), the initial values of
+     *    properties (null pointers and undefined values) are not reflected in
+     *    the property types. These values are always possible when reading the
+     *    property.
      *
      * We establish these by using write barriers on calls to setProperty and
      * defineProperty which are on native properties, and on any jitcode which
@@ -413,6 +455,12 @@ class ObjectGroup : public gc::TenuredCell
         return &flags_;
     }
 
+    // Get the bit pattern stored in an object's addendum when it has an
+    // original unboxed group.
+    static inline int32_t addendumOriginalUnboxedGroupValue() {
+        return Addendum_OriginalUnboxedGroup << OBJECT_FLAG_ADDENDUM_SHIFT;
+    }
+
     inline uint32_t basePropertyCount();
 
   private:
@@ -457,14 +505,14 @@ class ObjectGroup : public gc::TenuredCell
         UnknownIndex  // Make an array with an unknown element type.
     };
 
-    // Create an ArrayObject with the specified elements and a group specialized
-    // for the elements.
-    static ArrayObject* newArrayObject(ExclusiveContext* cx, const Value* vp, size_t length,
-                                       NewObjectKind newKind,
-                                       NewArrayKind arrayKind = NewArrayKind::Normal);
+    // Create an ArrayObject or UnboxedArrayObject with the specified elements
+    // and a group specialized for the elements.
+    static JSObject* newArrayObject(ExclusiveContext* cx, const Value* vp, size_t length,
+                                    NewObjectKind newKind,
+                                    NewArrayKind arrayKind = NewArrayKind::Normal);
 
-    // Create a PlainObject with the specified properties and a group specialized
-    // for those properties.
+    // Create a PlainObject or UnboxedPlainObject with the specified properties
+    // and a group specialized for those properties.
     static JSObject* newPlainObject(ExclusiveContext* cx,
                                     IdValuePair* properties, size_t nproperties,
                                     NewObjectKind newKind);
diff --git a/js/src/vm/Opcodes.h b/js/src/vm/Opcodes.h
index e13cd6221..b26336815 100644
--- a/js/src/vm/Opcodes.h
+++ b/js/src/vm/Opcodes.h
@@ -1281,7 +1281,17 @@
      *   Stack: receiver, obj, propval => obj[propval]
      */ \
     macro(JSOP_GETELEM_SUPER, 125, "getelem-super", NULL, 1,  3,  1, JOF_BYTE |JOF_ELEM|JOF_LEFTASSOC) \
-    macro(JSOP_UNUSED126, 126, "unused126", NULL, 5,  0,  1, JOF_UINT32) \
+    /*
+     * Pushes newly created array for a spread call onto the stack. This has
+     * the same semantics as JSOP_NEWARRAY, but is distinguished to avoid
+     * using unboxed arrays in spread calls, which would make compiling spread
+     * calls in baseline more complex.
+     *   Category: Literals
+     *   Type: Array
+     *   Operands: uint32_t length
+     *   Stack: => obj
+     */ \
+    macro(JSOP_SPREADCALLARRAY, 126, "spreadcallarray", NULL, 5,  0,  1, JOF_UINT32) \
     \
     /*
      * Defines the given function on the current scope.
diff --git a/js/src/vm/ReceiverGuard.cpp b/js/src/vm/ReceiverGuard.cpp
index e95e8a208..97df908c3 100644
--- a/js/src/vm/ReceiverGuard.cpp
+++ b/js/src/vm/ReceiverGuard.cpp
@@ -7,6 +7,7 @@
 #include "vm/ReceiverGuard.h"
 
 #include "builtin/TypedObject.h"
+#include "vm/UnboxedObject.h"
 #include "jsobjinlines.h"
 
 using namespace js;
@@ -15,7 +16,11 @@ ReceiverGuard::ReceiverGuard(JSObject* obj)
   : group(nullptr), shape(nullptr)
 {
     if (obj) {
-        if (obj->is<TypedObject>()) {
+        if (obj->is<UnboxedPlainObject>()) {
+            group = obj->group();
+            if (UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando())
+                shape = expando->lastProperty();
+        } else if (obj->is<UnboxedArrayObject>() || obj->is<TypedObject>()) {
             group = obj->group();
         } else {
             shape = obj->maybeShape();
@@ -28,7 +33,9 @@ ReceiverGuard::ReceiverGuard(ObjectGroup* group, Shape* shape)
 {
     if (group) {
         const Class* clasp = group->clasp();
-        if (IsTypedObjectClass(clasp)) {
+        if (clasp == &UnboxedPlainObject::class_) {
+            // Keep both group and shape.
+        } else if (clasp == &UnboxedArrayObject::class_ || IsTypedObjectClass(clasp)) {
             this->shape = nullptr;
         } else {
             this->group = nullptr;
@@ -39,8 +46,12 @@ ReceiverGuard::ReceiverGuard(ObjectGroup* group, Shape* shape)
 /* static */ int32_t
 HeapReceiverGuard::keyBits(JSObject* obj)
 {
-    if (obj->is<TypedObject>()) {
-        // Only the group needs to be guarded for typed objects.
+    if (obj->is<UnboxedPlainObject>()) {
+        // Both the group and shape need to be guarded for unboxed plain objects.
+        return obj->as<UnboxedPlainObject>().maybeExpando() ? 0 : 1;
+    }
+    if (obj->is<UnboxedArrayObject>() || obj->is<TypedObject>()) {
+        // Only the group needs to be guarded for unboxed arrays and typed objects.
         return 2;
     }
     // Other objects only need the shape to be guarded.
diff --git a/js/src/vm/ReceiverGuard.h b/js/src/vm/ReceiverGuard.h
index c14f0d83b..459cc0012 100644
--- a/js/src/vm/ReceiverGuard.h
+++ b/js/src/vm/ReceiverGuard.h
@@ -28,6 +28,11 @@ namespace js {
 // TypedObject: The structure of a typed object is determined by its group.
 //   All typed objects with the same group have the same class, prototype, and
 //   own properties.
+//
+// UnboxedPlainObject: The structure of an unboxed plain object is determined
+//   by its group and its expando object's shape, if there is one. All unboxed
+//   plain objects with the same group and expando shape have the same
+//   properties except those stored in the expando's dense elements.
 
 class HeapReceiverGuard;
 class RootedReceiverGuard;
diff --git a/js/src/vm/RegExpObject.cpp b/js/src/vm/RegExpObject.cpp
index cd0b54c9d..6223fc10d 100644
--- a/js/src/vm/RegExpObject.cpp
+++ b/js/src/vm/RegExpObject.cpp
@@ -299,8 +299,7 @@ RegExpObject::assignInitialShape(ExclusiveContext* cx, Handle<RegExpObject*> sel
     JS_STATIC_ASSERT(LAST_INDEX_SLOT == 0);
 
     /* The lastIndex property alone is writable but non-configurable. */
-    return NativeObject::addDataProperty(cx, self, cx->names().lastIndex, LAST_INDEX_SLOT,
-                                         JSPROP_PERMANENT);
+    return self->addDataProperty(cx, cx->names().lastIndex, LAST_INDEX_SLOT, JSPROP_PERMANENT);
 }
 
 void
diff --git a/js/src/vm/Stack.cpp b/js/src/vm/Stack.cpp
index 95940eeaf..7eb8f4ab8 100644
--- a/js/src/vm/Stack.cpp
+++ b/js/src/vm/Stack.cpp
@@ -82,7 +82,7 @@ InterpreterFrame::isNonGlobalEvalFrame() const
     return isEvalFrame() && script()->bodyScope()->as<EvalScope>().isNonGlobal();
 }
 
-ArrayObject*
+JSObject*
 InterpreterFrame::createRestParameter(JSContext* cx)
 {
     MOZ_ASSERT(script()->hasRest());
diff --git a/js/src/vm/Stack.h b/js/src/vm/Stack.h
index fe04a00f2..d08544213 100644
--- a/js/src/vm/Stack.h
+++ b/js/src/vm/Stack.h
@@ -523,7 +523,7 @@ class InterpreterFrame
     ArgumentsObject& argsObj() const;
     void initArgsObj(ArgumentsObject& argsobj);
 
-    ArrayObject* createRestParameter(JSContext* cx);
+    JSObject* createRestParameter(JSContext* cx);
 
     /*
      * Environment chain
diff --git a/js/src/vm/TypeInference-inl.h b/js/src/vm/TypeInference-inl.h
index 2af252cea..da47fa898 100644
--- a/js/src/vm/TypeInference-inl.h
+++ b/js/src/vm/TypeInference-inl.h
@@ -23,6 +23,7 @@
 #include "vm/SharedArrayObject.h"
 #include "vm/StringObject.h"
 #include "vm/TypedArrayObject.h"
+#include "vm/UnboxedObject.h"
 
 #include "jscntxtinlines.h"
 
@@ -284,6 +285,10 @@ TypeIdString(jsid id)
  */
 struct AutoEnterAnalysis
 {
+    // For use when initializing an UnboxedLayout.  The UniquePtr's destructor
+    // must run when GC is not suppressed.
+    UniquePtr<UnboxedLayout> unboxedLayoutToCleanUp;
+
     // Prevent GC activity in the middle of analysis.
     gc::AutoSuppressGC suppressGC;
 
diff --git a/js/src/vm/TypeInference.cpp b/js/src/vm/TypeInference.cpp
index 2b1fa0e3b..438188a36 100644
--- a/js/src/vm/TypeInference.cpp
+++ b/js/src/vm/TypeInference.cpp
@@ -35,6 +35,7 @@
 #include "vm/Opcodes.h"
 #include "vm/Shape.h"
 #include "vm/Time.h"
+#include "vm/UnboxedObject.h"
 
 #include "jsatominlines.h"
 #include "jsscriptinlines.h"
@@ -296,6 +297,9 @@ js::ObjectGroupHasProperty(JSContext* cx, ObjectGroup* group, jsid id, const Val
                         return true;
                 }
             }
+            JSObject* obj = &value.toObject();
+            if (!obj->hasLazyGroup() && obj->group()->maybeOriginalUnboxedGroup())
+                return true;
         }
 
         if (!types->hasType(type)) {
@@ -1944,6 +1948,33 @@ class ConstraintDataFreezeObjectForTypedArrayData
     }
 };
 
+// Constraint which triggers recompilation if an unboxed object in some group
+// is converted to a native object.
+class ConstraintDataFreezeObjectForUnboxedConvertedToNative
+{
+  public:
+    ConstraintDataFreezeObjectForUnboxedConvertedToNative()
+    {}
+
+    const char* kind() { return "freezeObjectForUnboxedConvertedToNative"; }
+
+    bool invalidateOnNewType(TypeSet::Type type) { return false; }
+    bool invalidateOnNewPropertyState(TypeSet* property) { return false; }
+    bool invalidateOnNewObjectState(ObjectGroup* group) {
+        return group->unboxedLayout().nativeGroup() != nullptr;
+    }
+
+    bool constraintHolds(JSContext* cx,
+                         const HeapTypeSetKey& property, TemporaryTypeSet* expected)
+    {
+        return !invalidateOnNewObjectState(property.object()->maybeGroup());
+    }
+
+    bool shouldSweep() { return false; }
+
+    JSCompartment* maybeCompartment() { return nullptr; }
+};
+
 } /* anonymous namespace */
 
 void
@@ -1968,6 +1999,17 @@ TypeSet::ObjectKey::watchStateChangeForTypedArrayData(CompilerConstraintList* co
                                     ConstraintDataFreezeObjectForTypedArrayData(tarray)));
 }
 
+void
+TypeSet::ObjectKey::watchStateChangeForUnboxedConvertedToNative(CompilerConstraintList* constraints)
+{
+    HeapTypeSetKey objectProperty = property(JSID_EMPTY);
+    LifoAlloc* alloc = constraints->alloc();
+
+    typedef CompilerConstraintInstance<ConstraintDataFreezeObjectForUnboxedConvertedToNative> T;
+    constraints->add(alloc->new_<T>(alloc, objectProperty,
+                                    ConstraintDataFreezeObjectForUnboxedConvertedToNative()));
+}
+
 static void
 ObjectStateChange(ExclusiveContext* cxArg, ObjectGroup* group, bool markingUnknown)
 {
@@ -2478,6 +2520,8 @@ TemporaryTypeSet::propertyNeedsBarrier(CompilerConstraintList* constraints, jsid
 bool
 js::ClassCanHaveExtraProperties(const Class* clasp)
 {
+    if (clasp == &UnboxedPlainObject::class_ || clasp == &UnboxedArrayObject::class_)
+        return false;
     return clasp->getResolve()
         || clasp->getOpsLookupProperty()
         || clasp->getOpsGetProperty()
@@ -2768,6 +2812,15 @@ js::AddTypePropertyId(ExclusiveContext* cx, ObjectGroup* group, JSObject* obj, j
     // from acquiring the fully initialized group.
     if (group->newScript() && group->newScript()->initializedGroup())
         AddTypePropertyId(cx, group->newScript()->initializedGroup(), nullptr, id, type);
+
+    // Maintain equivalent type information for unboxed object groups and their
+    // corresponding native group. Since type sets might contain the unboxed
+    // group but not the native group, this ensures optimizations based on the
+    // unboxed group are valid for the native group.
+    if (group->maybeUnboxedLayout() && group->maybeUnboxedLayout()->nativeGroup())
+        AddTypePropertyId(cx, group->maybeUnboxedLayout()->nativeGroup(), nullptr, id, type);
+    if (ObjectGroup* unboxedGroup = group->maybeOriginalUnboxedGroup())
+        AddTypePropertyId(cx, unboxedGroup, nullptr, id, type);
 }
 
 void
@@ -2839,6 +2892,12 @@ ObjectGroup::setFlags(ExclusiveContext* cx, ObjectGroupFlags flags)
     // acquired properties analysis.
     if (newScript() && newScript()->initializedGroup())
         newScript()->initializedGroup()->setFlags(cx, flags);
+
+    // Propagate flag changes between unboxed and corresponding native groups.
+    if (maybeUnboxedLayout() && maybeUnboxedLayout()->nativeGroup())
+        maybeUnboxedLayout()->nativeGroup()->setFlags(cx, flags);
+    if (ObjectGroup* unboxedGroup = maybeOriginalUnboxedGroup())
+        unboxedGroup->setFlags(cx, flags);
 }
 
 void
@@ -2871,6 +2930,23 @@ ObjectGroup::markUnknown(ExclusiveContext* cx)
             prop->types.setNonDataProperty(cx);
         }
     }
+
+    if (ObjectGroup* unboxedGroup = maybeOriginalUnboxedGroup())
+        MarkObjectGroupUnknownProperties(cx, unboxedGroup);
+    if (maybeUnboxedLayout() && maybeUnboxedLayout()->nativeGroup())
+        MarkObjectGroupUnknownProperties(cx, maybeUnboxedLayout()->nativeGroup());
+    if (ObjectGroup* unboxedGroup = maybeOriginalUnboxedGroup())
+        MarkObjectGroupUnknownProperties(cx, unboxedGroup);
+}
+
+TypeNewScript*
+ObjectGroup::anyNewScript()
+{
+    if (newScript())
+        return newScript();
+    if (maybeUnboxedLayout())
+        return unboxedLayout().newScript();
+    return nullptr;
 }
 
 void
@@ -2880,7 +2956,7 @@ ObjectGroup::detachNewScript(bool writeBarrier, ObjectGroup* replacement)
     // analyzed, remove it from the newObjectGroups table so that it will not be
     // produced by calling 'new' on the associated function anymore.
     // The TypeNewScript is not actually destroyed.
-    TypeNewScript* newScript = this->newScript();
+    TypeNewScript* newScript = anyNewScript();
     MOZ_ASSERT(newScript);
 
     if (newScript->analyzed()) {
@@ -2899,7 +2975,10 @@ ObjectGroup::detachNewScript(bool writeBarrier, ObjectGroup* replacement)
         MOZ_ASSERT(!replacement);
     }
 
-    setAddendum(Addendum_None, nullptr, writeBarrier);
+    if (this->newScript())
+        setAddendum(Addendum_None, nullptr, writeBarrier);
+    else
+        unboxedLayout().setNewScript(nullptr, writeBarrier);
 }
 
 void
@@ -2910,7 +2989,7 @@ ObjectGroup::maybeClearNewScriptOnOOM()
     if (!isMarked())
         return;
 
-    TypeNewScript* newScript = this->newScript();
+    TypeNewScript* newScript = anyNewScript();
     if (!newScript)
         return;
 
@@ -2925,7 +3004,7 @@ ObjectGroup::maybeClearNewScriptOnOOM()
 void
 ObjectGroup::clearNewScript(ExclusiveContext* cx, ObjectGroup* replacement /* = nullptr*/)
 {
-    TypeNewScript* newScript = this->newScript();
+    TypeNewScript* newScript = anyNewScript();
     if (!newScript)
         return;
 
@@ -3332,7 +3411,7 @@ JSFunction::setTypeForScriptedFunction(ExclusiveContext* cx, HandleFunction fun,
 /////////////////////////////////////////////////////////////////////
 
 void
-PreliminaryObjectArray::registerNewObject(PlainObject* res)
+PreliminaryObjectArray::registerNewObject(JSObject* res)
 {
     // The preliminary object pointers are weak, and won't be swept properly
     // during nursery collections, so the preliminary objects need to be
@@ -3350,7 +3429,7 @@ PreliminaryObjectArray::registerNewObject(PlainObject* res)
 }
 
 void
-PreliminaryObjectArray::unregisterObject(PlainObject* obj)
+PreliminaryObjectArray::unregisterObject(JSObject* obj)
 {
     for (size_t i = 0; i < COUNT; i++) {
         if (objects[i] == obj) {
@@ -3390,6 +3469,22 @@ PreliminaryObjectArray::sweep()
     for (size_t i = 0; i < COUNT; i++) {
         JSObject** ptr = &objects[i];
         if (*ptr && IsAboutToBeFinalizedUnbarriered(ptr)) {
+            // Before we clear this reference, change the object's group to the
+            // Object.prototype group. This is done to ensure JSObject::finalize
+            // sees a NativeObject Class even if we change the current group's
+            // Class to one of the unboxed object classes in the meantime. If
+            // the compartment's global is dead, we don't do anything as the
+            // group's Class is not going to change in that case.
+            JSObject* obj = *ptr;
+            GlobalObject* global = obj->compartment()->unsafeUnbarrieredMaybeGlobal();
+            if (global && !obj->isSingleton()) {
+                JSObject* objectProto = GetBuiltinPrototypePure(global, JSProto_Object);
+                obj->setGroup(objectProto->groupRaw());
+                MOZ_ASSERT(obj->is<NativeObject>());
+                MOZ_ASSERT(obj->getClass() == objectProto->getClass());
+                MOZ_ASSERT(!obj->getClass()->hasFinalize());
+            }
+
             *ptr = nullptr;
         }
     }
@@ -3489,11 +3584,17 @@ PreliminaryObjectArrayWithTemplate::maybeAnalyze(ExclusiveContext* cx, ObjectGro
         }
     }
 
-    // Since the preliminary objects still reflect the template object's
-    // properties, and all objects in the future will be created with those
-    // properties, the properties can be marked as definitive for objects in
-    // the group.
-    group->addDefiniteProperties(cx, shape());
+    TryConvertToUnboxedLayout(cx, enter, shape(), group, preliminaryObjects);
+    if (group->maybeUnboxedLayout())
+        return;
+
+    if (shape()) {
+        // We weren't able to use an unboxed layout, but since the preliminary
+        // objects still reflect the template object's properties, and all
+        // objects in the future will be created with those properties, the
+        // properties can be marked as definite for objects in the group.
+        group->addDefiniteProperties(cx, shape());
+    }
 }
 
 /////////////////////////////////////////////////////////////////////
@@ -3507,6 +3608,7 @@ TypeNewScript::make(JSContext* cx, ObjectGroup* group, JSFunction* fun)
 {
     MOZ_ASSERT(cx->zone()->types.activeAnalysis);
     MOZ_ASSERT(!group->newScript());
+    MOZ_ASSERT(!group->maybeUnboxedLayout());
 
     // rollbackPartiallyInitializedObjects expects function_ to be
     // canonicalized.
@@ -3811,9 +3913,34 @@ TypeNewScript::maybeAnalyze(JSContext* cx, ObjectGroup* group, bool* regenerate,
         PodCopy(initializerList, initializerVector.begin(), initializerVector.length());
     }
 
+    // Try to use an unboxed representation for the group.
+    if (!TryConvertToUnboxedLayout(cx, enter, templateObject()->lastProperty(), group, preliminaryObjects))
+        return false;
+
     js_delete(preliminaryObjects);
     preliminaryObjects = nullptr;
 
+    if (group->maybeUnboxedLayout()) {
+        // An unboxed layout was constructed for the group, and this has already
+        // been hooked into it.
+        MOZ_ASSERT(group->unboxedLayout().newScript() == this);
+        destroyNewScript.group = nullptr;
+
+        // Clear out the template object, which is not used for TypeNewScripts
+        // with an unboxed layout. Currently it is a mutant object with a
+        // non-native group and native shape, so make it safe for GC by changing
+        // its group to the default for its prototype.
+        AutoEnterOOMUnsafeRegion oomUnsafe;
+        ObjectGroup* plainGroup = ObjectGroup::defaultNewGroup(cx, &PlainObject::class_,
+                                                               group->proto());
+        if (!plainGroup)
+            oomUnsafe.crash("TypeNewScript::maybeAnalyze");
+        templateObject_->setGroup(plainGroup);
+        templateObject_ = nullptr;
+
+        return true;
+    }
+
     if (prefixShape->slotSpan() == templateObject()->slotSpan()) {
         // The definite properties analysis found exactly the properties that
         // are held in common by the preliminary objects. No further analysis
@@ -3927,6 +4054,12 @@ TypeNewScript::rollbackPartiallyInitializedObjects(JSContext* cx, ObjectGroup* g
             continue;
         }
 
+        if (thisv.toObject().is<UnboxedPlainObject>()) {
+            AutoEnterOOMUnsafeRegion oomUnsafe;
+            if (!UnboxedPlainObject::convertToNative(cx, &thisv.toObject()))
+                oomUnsafe.crash("rollbackPartiallyInitializedObjects");
+        }
+
         // Found a matching frame.
         RootedPlainObject obj(cx, &thisv.toObject().as<PlainObject>());
 
@@ -4120,6 +4253,12 @@ ConstraintTypeSet::sweep(Zone* zone, AutoClearTypeInferenceStateOnOOM& oom)
                 // Object sets containing objects with unknown properties might
                 // not be complete. Mark the type set as unknown, which it will
                 // be treated as during Ion compilation.
+                //
+                // Note that we don't have to do this when the type set might
+                // be missing the native group corresponding to an unboxed
+                // object group. In this case, the native group points to the
+                // unboxed object group via its addendum, so as long as objects
+                // with either group exist, neither group will be finalized.
                 flags |= TYPE_FLAG_ANYOBJECT;
                 clearObjects();
                 objectCount = 0;
@@ -4203,6 +4342,21 @@ ObjectGroup::sweep(AutoClearTypeInferenceStateOnOOM* oom)
     Maybe<AutoClearTypeInferenceStateOnOOM> fallbackOOM;
     EnsureHasAutoClearTypeInferenceStateOnOOM(oom, zone(), fallbackOOM);
 
+    if (maybeUnboxedLayout()) {
+        // Remove unboxed layouts that are about to be finalized from the
+        // compartment wide list while we are still on the main thread.
+        ObjectGroup* group = this;
+        if (IsAboutToBeFinalizedUnbarriered(&group))
+            unboxedLayout().detachFromCompartment();
+
+        if (unboxedLayout().newScript())
+            unboxedLayout().newScript()->sweep();
+
+        // Discard constructor code to avoid holding onto ExecutablePools.
+        if (zone()->isGCCompacting())
+            unboxedLayout().setConstructorCode(nullptr);
+    }
+
     if (maybePreliminaryObjects())
         maybePreliminaryObjects()->sweep();
 
diff --git a/js/src/vm/TypeInference.h b/js/src/vm/TypeInference.h
index fd021fc96..9ec1bf927 100644
--- a/js/src/vm/TypeInference.h
+++ b/js/src/vm/TypeInference.h
@@ -262,6 +262,7 @@ class TypeSet
         bool hasStableClassAndProto(CompilerConstraintList* constraints);
         void watchStateChangeForInlinedCall(CompilerConstraintList* constraints);
         void watchStateChangeForTypedArrayData(CompilerConstraintList* constraints);
+        void watchStateChangeForUnboxedConvertedToNative(CompilerConstraintList* constraints);
         HeapTypeSetKey property(jsid id);
         void ensureTrackedProperty(JSContext* cx, jsid id);
 
@@ -871,8 +872,8 @@ class PreliminaryObjectArray
   public:
     PreliminaryObjectArray() = default;
 
-    void registerNewObject(PlainObject* res);
-    void unregisterObject(PlainObject* obj);
+    void registerNewObject(JSObject* res);
+    void unregisterObject(JSObject* obj);
 
     JSObject* get(size_t i) const {
         MOZ_ASSERT(i < COUNT);
diff --git a/js/src/vm/UnboxedObject-inl.h b/js/src/vm/UnboxedObject-inl.h
new file mode 100644
index 000000000..93ad7bf28
--- /dev/null
+++ b/js/src/vm/UnboxedObject-inl.h
@@ -0,0 +1,840 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ * vim: set ts=8 sts=4 et sw=4 tw=99:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef vm_UnboxedObject_inl_h
+#define vm_UnboxedObject_inl_h
+
+#include "vm/UnboxedObject.h"
+
+#include "gc/StoreBuffer-inl.h"
+#include "vm/ArrayObject-inl.h"
+#include "vm/NativeObject-inl.h"
+
+namespace js {
+
+static inline Value
+GetUnboxedValue(uint8_t* p, JSValueType type, bool maybeUninitialized)
+{
+    switch (type) {
+      case JSVAL_TYPE_BOOLEAN:
+        return BooleanValue(*p != 0);
+
+      case JSVAL_TYPE_INT32:
+        return Int32Value(*reinterpret_cast<int32_t*>(p));
+
+      case JSVAL_TYPE_DOUBLE: {
+        // During unboxed plain object creation, non-GC thing properties are
+        // left uninitialized. This is normally fine, since the properties will
+        // be filled in shortly, but if they are read before that happens we
+        // need to make sure that doubles are canonical.
+        double d = *reinterpret_cast<double*>(p);
+        if (maybeUninitialized)
+            return DoubleValue(JS::CanonicalizeNaN(d));
+        return DoubleValue(d);
+      }
+
+      case JSVAL_TYPE_STRING:
+        return StringValue(*reinterpret_cast<JSString**>(p));
+
+      case JSVAL_TYPE_OBJECT:
+        return ObjectOrNullValue(*reinterpret_cast<JSObject**>(p));
+
+      default:
+        MOZ_CRASH("Invalid type for unboxed value");
+    }
+}
+
+static inline void
+SetUnboxedValueNoTypeChange(JSObject* unboxedObject,
+                            uint8_t* p, JSValueType type, const Value& v,
+                            bool preBarrier)
+{
+    switch (type) {
+      case JSVAL_TYPE_BOOLEAN:
+        *p = v.toBoolean();
+        return;
+
+      case JSVAL_TYPE_INT32:
+        *reinterpret_cast<int32_t*>(p) = v.toInt32();
+        return;
+
+      case JSVAL_TYPE_DOUBLE:
+        *reinterpret_cast<double*>(p) = v.toNumber();
+        return;
+
+      case JSVAL_TYPE_STRING: {
+        MOZ_ASSERT(!IsInsideNursery(v.toString()));
+        JSString** np = reinterpret_cast<JSString**>(p);
+        if (preBarrier)
+            JSString::writeBarrierPre(*np);
+        *np = v.toString();
+        return;
+      }
+
+      case JSVAL_TYPE_OBJECT: {
+        JSObject** np = reinterpret_cast<JSObject**>(p);
+
+        // Manually trigger post barriers on the whole object. If we treat
+        // the pointer as a HeapPtrObject we will get confused later if the
+        // object is converted to its native representation.
+        JSObject* obj = v.toObjectOrNull();
+        if (IsInsideNursery(obj) && !IsInsideNursery(unboxedObject)) {
+            JSRuntime* rt = unboxedObject->runtimeFromMainThread();
+            rt->gc.storeBuffer.putWholeCell(unboxedObject);
+        }
+
+        if (preBarrier)
+            JSObject::writeBarrierPre(*np);
+        *np = obj;
+        return;
+      }
+
+      default:
+        MOZ_CRASH("Invalid type for unboxed value");
+    }
+}
+
+static inline bool
+SetUnboxedValue(ExclusiveContext* cx, JSObject* unboxedObject, jsid id,
+                uint8_t* p, JSValueType type, const Value& v, bool preBarrier)
+{
+    switch (type) {
+      case JSVAL_TYPE_BOOLEAN:
+        if (v.isBoolean()) {
+            *p = v.toBoolean();
+            return true;
+        }
+        return false;
+
+      case JSVAL_TYPE_INT32:
+        if (v.isInt32()) {
+            *reinterpret_cast<int32_t*>(p) = v.toInt32();
+            return true;
+        }
+        return false;
+
+      case JSVAL_TYPE_DOUBLE:
+        if (v.isNumber()) {
+            *reinterpret_cast<double*>(p) = v.toNumber();
+            return true;
+        }
+        return false;
+
+      case JSVAL_TYPE_STRING:
+        if (v.isString()) {
+            MOZ_ASSERT(!IsInsideNursery(v.toString()));
+            JSString** np = reinterpret_cast<JSString**>(p);
+            if (preBarrier)
+                JSString::writeBarrierPre(*np);
+            *np = v.toString();
+            return true;
+        }
+        return false;
+
+      case JSVAL_TYPE_OBJECT:
+        if (v.isObjectOrNull()) {
+            JSObject** np = reinterpret_cast<JSObject**>(p);
+
+            // Update property types when writing object properties. Types for
+            // other properties were captured when the unboxed layout was
+            // created.
+            AddTypePropertyId(cx, unboxedObject, id, v);
+
+            // As above, trigger post barriers on the whole object.
+            JSObject* obj = v.toObjectOrNull();
+            if (IsInsideNursery(v.toObjectOrNull()) && !IsInsideNursery(unboxedObject)) {
+                JSRuntime* rt = unboxedObject->runtimeFromMainThread();
+                rt->gc.storeBuffer.putWholeCell(unboxedObject);
+            }
+
+            if (preBarrier)
+                JSObject::writeBarrierPre(*np);
+            *np = obj;
+            return true;
+        }
+        return false;
+
+      default:
+        MOZ_CRASH("Invalid type for unboxed value");
+    }
+}
+
+/////////////////////////////////////////////////////////////////////
+// UnboxedPlainObject
+/////////////////////////////////////////////////////////////////////
+
+inline const UnboxedLayout&
+UnboxedPlainObject::layout() const
+{
+    return group()->unboxedLayout();
+}
+
+/////////////////////////////////////////////////////////////////////
+// UnboxedArrayObject
+/////////////////////////////////////////////////////////////////////
+
+inline const UnboxedLayout&
+UnboxedArrayObject::layout() const
+{
+    return group()->unboxedLayout();
+}
+
+inline void
+UnboxedArrayObject::setLength(ExclusiveContext* cx, uint32_t length)
+{
+    if (length > INT32_MAX) {
+        // Track objects with overflowing lengths in type information.
+        MarkObjectGroupFlags(cx, this, OBJECT_FLAG_LENGTH_OVERFLOW);
+    }
+
+    length_ = length;
+}
+
+inline void
+UnboxedArrayObject::setInitializedLength(uint32_t initlen)
+{
+    if (initlen < initializedLength()) {
+        switch (elementType()) {
+          case JSVAL_TYPE_STRING:
+            for (size_t i = initlen; i < initializedLength(); i++)
+                triggerPreBarrier<JSVAL_TYPE_STRING>(i);
+            break;
+          case JSVAL_TYPE_OBJECT:
+            for (size_t i = initlen; i < initializedLength(); i++)
+                triggerPreBarrier<JSVAL_TYPE_OBJECT>(i);
+            break;
+          default:
+            MOZ_ASSERT(!UnboxedTypeNeedsPreBarrier(elementType()));
+        }
+    }
+    setInitializedLengthNoBarrier(initlen);
+}
+
+template <JSValueType Type>
+inline bool
+UnboxedArrayObject::setElementSpecific(ExclusiveContext* cx, size_t index, const Value& v)
+{
+    MOZ_ASSERT(index < initializedLength());
+    MOZ_ASSERT(Type == elementType());
+    uint8_t* p = elements() + index * UnboxedTypeSize(Type);
+    return SetUnboxedValue(cx, this, JSID_VOID, p, elementType(), v, /* preBarrier = */ true);
+}
+
+template <JSValueType Type>
+inline void
+UnboxedArrayObject::setElementNoTypeChangeSpecific(size_t index, const Value& v)
+{
+    MOZ_ASSERT(index < initializedLength());
+    MOZ_ASSERT(Type == elementType());
+    uint8_t* p = elements() + index * UnboxedTypeSize(Type);
+    return SetUnboxedValueNoTypeChange(this, p, elementType(), v, /* preBarrier = */ true);
+}
+
+template <JSValueType Type>
+inline bool
+UnboxedArrayObject::initElementSpecific(ExclusiveContext* cx, size_t index, const Value& v)
+{
+    MOZ_ASSERT(index < initializedLength());
+    MOZ_ASSERT(Type == elementType());
+    uint8_t* p = elements() + index * UnboxedTypeSize(Type);
+    return SetUnboxedValue(cx, this, JSID_VOID, p, elementType(), v, /* preBarrier = */ false);
+}
+
+template <JSValueType Type>
+inline void
+UnboxedArrayObject::initElementNoTypeChangeSpecific(size_t index, const Value& v)
+{
+    MOZ_ASSERT(index < initializedLength());
+    MOZ_ASSERT(Type == elementType());
+    uint8_t* p = elements() + index * UnboxedTypeSize(Type);
+    return SetUnboxedValueNoTypeChange(this, p, elementType(), v, /* preBarrier = */ false);
+}
+
+template <JSValueType Type>
+inline Value
+UnboxedArrayObject::getElementSpecific(size_t index)
+{
+    MOZ_ASSERT(index < initializedLength());
+    MOZ_ASSERT(Type == elementType());
+    uint8_t* p = elements() + index * UnboxedTypeSize(Type);
+    return GetUnboxedValue(p, Type, /* maybeUninitialized = */ false);
+}
+
+template <JSValueType Type>
+inline void
+UnboxedArrayObject::triggerPreBarrier(size_t index)
+{
+    MOZ_ASSERT(UnboxedTypeNeedsPreBarrier(Type));
+
+    uint8_t* p = elements() + index * UnboxedTypeSize(Type);
+
+    switch (Type) {
+      case JSVAL_TYPE_STRING: {
+        JSString** np = reinterpret_cast<JSString**>(p);
+        JSString::writeBarrierPre(*np);
+        break;
+      }
+
+      case JSVAL_TYPE_OBJECT: {
+        JSObject** np = reinterpret_cast<JSObject**>(p);
+        JSObject::writeBarrierPre(*np);
+        break;
+      }
+
+      default:
+        MOZ_CRASH("Bad type");
+    }
+}
+
+/////////////////////////////////////////////////////////////////////
+// Combined methods for NativeObject and UnboxedArrayObject accesses.
+/////////////////////////////////////////////////////////////////////
+
+static inline bool
+HasAnyBoxedOrUnboxedDenseElements(JSObject* obj)
+{
+    return obj->isNative() || obj->is<UnboxedArrayObject>();
+}
+
+static inline size_t
+GetAnyBoxedOrUnboxedInitializedLength(JSObject* obj)
+{
+    if (obj->isNative())
+        return obj->as<NativeObject>().getDenseInitializedLength();
+    if (obj->is<UnboxedArrayObject>())
+        return obj->as<UnboxedArrayObject>().initializedLength();
+    return 0;
+}
+
+static inline size_t
+GetAnyBoxedOrUnboxedCapacity(JSObject* obj)
+{
+    if (obj->isNative())
+        return obj->as<NativeObject>().getDenseCapacity();
+    if (obj->is<UnboxedArrayObject>())
+        return obj->as<UnboxedArrayObject>().capacity();
+    return 0;
+}
+
+static inline Value
+GetAnyBoxedOrUnboxedDenseElement(JSObject* obj, size_t index)
+{
+    if (obj->isNative())
+        return obj->as<NativeObject>().getDenseElement(index);
+    return obj->as<UnboxedArrayObject>().getElement(index);
+}
+
+static inline size_t
+GetAnyBoxedOrUnboxedArrayLength(JSObject* obj)
+{
+    if (obj->is<ArrayObject>())
+        return obj->as<ArrayObject>().length();
+    return obj->as<UnboxedArrayObject>().length();
+}
+
+static inline void
+SetAnyBoxedOrUnboxedArrayLength(JSContext* cx, JSObject* obj, size_t length)
+{
+    if (obj->is<ArrayObject>()) {
+        MOZ_ASSERT(length >= obj->as<ArrayObject>().length());
+        obj->as<ArrayObject>().setLength(cx, length);
+    } else {
+        MOZ_ASSERT(length >= obj->as<UnboxedArrayObject>().length());
+        obj->as<UnboxedArrayObject>().setLength(cx, length);
+    }
+}
+
+static inline bool
+SetAnyBoxedOrUnboxedDenseElement(JSContext* cx, JSObject* obj, size_t index, const Value& value)
+{
+    if (obj->isNative()) {
+        obj->as<NativeObject>().setDenseElementWithType(cx, index, value);
+        return true;
+    }
+    return obj->as<UnboxedArrayObject>().setElement(cx, index, value);
+}
+
+static inline bool
+InitAnyBoxedOrUnboxedDenseElement(JSContext* cx, JSObject* obj, size_t index, const Value& value)
+{
+    if (obj->isNative()) {
+        obj->as<NativeObject>().initDenseElementWithType(cx, index, value);
+        return true;
+    }
+    return obj->as<UnboxedArrayObject>().initElement(cx, index, value);
+}
+
+/////////////////////////////////////////////////////////////////////
+// Template methods for NativeObject and UnboxedArrayObject accesses.
+/////////////////////////////////////////////////////////////////////
+
+static inline JSValueType
+GetBoxedOrUnboxedType(JSObject* obj)
+{
+    if (obj->isNative())
+        return JSVAL_TYPE_MAGIC;
+    return obj->as<UnboxedArrayObject>().elementType();
+}
+
+template <JSValueType Type>
+static inline bool
+HasBoxedOrUnboxedDenseElements(JSObject* obj)
+{
+    if (Type == JSVAL_TYPE_MAGIC)
+        return obj->isNative();
+    return obj->is<UnboxedArrayObject>() && obj->as<UnboxedArrayObject>().elementType() == Type;
+}
+
+template <JSValueType Type>
+static inline size_t
+GetBoxedOrUnboxedInitializedLength(JSObject* obj)
+{
+    if (Type == JSVAL_TYPE_MAGIC)
+        return obj->as<NativeObject>().getDenseInitializedLength();
+    return obj->as<UnboxedArrayObject>().initializedLength();
+}
+
+template <JSValueType Type>
+static inline DenseElementResult
+SetBoxedOrUnboxedInitializedLength(JSContext* cx, JSObject* obj, size_t initlen)
+{
+    size_t oldInitlen = GetBoxedOrUnboxedInitializedLength<Type>(obj);
+    if (Type == JSVAL_TYPE_MAGIC) {
+        obj->as<NativeObject>().setDenseInitializedLength(initlen);
+        if (initlen < oldInitlen)
+            obj->as<NativeObject>().shrinkElements(cx, initlen);
+    } else {
+        obj->as<UnboxedArrayObject>().setInitializedLength(initlen);
+        if (initlen < oldInitlen)
+            obj->as<UnboxedArrayObject>().shrinkElements(cx, initlen);
+    }
+    return DenseElementResult::Success;
+}
+
+template <JSValueType Type>
+static inline size_t
+GetBoxedOrUnboxedCapacity(JSObject* obj)
+{
+    if (Type == JSVAL_TYPE_MAGIC)
+        return obj->as<NativeObject>().getDenseCapacity();
+    return obj->as<UnboxedArrayObject>().capacity();
+}
+
+template <JSValueType Type>
+static inline Value
+GetBoxedOrUnboxedDenseElement(JSObject* obj, size_t index)
+{
+    if (Type == JSVAL_TYPE_MAGIC)
+        return obj->as<NativeObject>().getDenseElement(index);
+    return obj->as<UnboxedArrayObject>().getElementSpecific<Type>(index);
+}
+
+template <JSValueType Type>
+static inline void
+SetBoxedOrUnboxedDenseElementNoTypeChange(JSObject* obj, size_t index, const Value& value)
+{
+    if (Type == JSVAL_TYPE_MAGIC)
+        obj->as<NativeObject>().setDenseElement(index, value);
+    else
+        obj->as<UnboxedArrayObject>().setElementNoTypeChangeSpecific<Type>(index, value);
+}
+
+template <JSValueType Type>
+static inline bool
+SetBoxedOrUnboxedDenseElement(JSContext* cx, JSObject* obj, size_t index, const Value& value)
+{
+    if (Type == JSVAL_TYPE_MAGIC) {
+        obj->as<NativeObject>().setDenseElementWithType(cx, index, value);
+        return true;
+    }
+    return obj->as<UnboxedArrayObject>().setElementSpecific<Type>(cx, index, value);
+}
+
+template <JSValueType Type>
+static inline DenseElementResult
+EnsureBoxedOrUnboxedDenseElements(JSContext* cx, JSObject* obj, size_t count)
+{
+    if (Type == JSVAL_TYPE_MAGIC) {
+        if (!obj->as<ArrayObject>().ensureElements(cx, count))
+            return DenseElementResult::Failure;
+    } else {
+        if (obj->as<UnboxedArrayObject>().capacity() < count) {
+            if (!obj->as<UnboxedArrayObject>().growElements(cx, count))
+                return DenseElementResult::Failure;
+        }
+    }
+    return DenseElementResult::Success;
+}
+
+template <JSValueType Type>
+static inline DenseElementResult
+SetOrExtendBoxedOrUnboxedDenseElements(ExclusiveContext* cx, JSObject* obj,
+                                       uint32_t start, const Value* vp, uint32_t count,
+                                       ShouldUpdateTypes updateTypes = ShouldUpdateTypes::Update)
+{
+    if (Type == JSVAL_TYPE_MAGIC) {
+        NativeObject* nobj = &obj->as<NativeObject>();
+
+        if (nobj->denseElementsAreFrozen())
+            return DenseElementResult::Incomplete;
+
+        if (obj->is<ArrayObject>() &&
+            !obj->as<ArrayObject>().lengthIsWritable() &&
+            start + count >= obj->as<ArrayObject>().length())
+        {
+            return DenseElementResult::Incomplete;
+        }
+
+        DenseElementResult result = nobj->ensureDenseElements(cx, start, count);
+        if (result != DenseElementResult::Success)
+            return result;
+
+        if (obj->is<ArrayObject>() && start + count >= obj->as<ArrayObject>().length())
+            obj->as<ArrayObject>().setLengthInt32(start + count);
+
+        if (updateTypes == ShouldUpdateTypes::DontUpdate && !nobj->shouldConvertDoubleElements()) {
+            nobj->copyDenseElements(start, vp, count);
+        } else {
+            for (size_t i = 0; i < count; i++)
+                nobj->setDenseElementWithType(cx, start + i, vp[i]);
+        }
+
+        return DenseElementResult::Success;
+    }
+
+    UnboxedArrayObject* nobj = &obj->as<UnboxedArrayObject>();
+
+    if (start > nobj->initializedLength())
+        return DenseElementResult::Incomplete;
+
+    if (start + count >= UnboxedArrayObject::MaximumCapacity)
+        return DenseElementResult::Incomplete;
+
+    if (start + count > nobj->capacity() && !nobj->growElements(cx, start + count))
+        return DenseElementResult::Failure;
+
+    size_t oldInitlen = nobj->initializedLength();
+
+    // Overwrite any existing elements covered by the new range. If we fail
+    // after this point due to some incompatible type being written to the
+    // object's elements, afterwards the contents will be different from when
+    // we started. The caller must retry the operation using a generic path,
+    // which will overwrite the already-modified elements as well as the ones
+    // that were left alone.
+    size_t i = 0;
+    if (updateTypes == ShouldUpdateTypes::DontUpdate) {
+        for (size_t j = start; i < count && j < oldInitlen; i++, j++)
+            nobj->setElementNoTypeChangeSpecific<Type>(j, vp[i]);
+    } else {
+        for (size_t j = start; i < count && j < oldInitlen; i++, j++) {
+            if (!nobj->setElementSpecific<Type>(cx, j, vp[i]))
+                return DenseElementResult::Incomplete;
+        }
+    }
+
+    if (i != count) {
+        obj->as<UnboxedArrayObject>().setInitializedLength(start + count);
+        if (updateTypes == ShouldUpdateTypes::DontUpdate) {
+            for (; i < count; i++)
+                nobj->initElementNoTypeChangeSpecific<Type>(start + i, vp[i]);
+        } else {
+            for (; i < count; i++) {
+                if (!nobj->initElementSpecific<Type>(cx, start + i, vp[i])) {
+                    nobj->setInitializedLengthNoBarrier(oldInitlen);
+                    return DenseElementResult::Incomplete;
+                }
+            }
+        }
+    }
+
+    if (start + count >= nobj->length())
+        nobj->setLength(cx, start + count);
+
+    return DenseElementResult::Success;
+}
+
+template <JSValueType Type>
+static inline DenseElementResult
+MoveBoxedOrUnboxedDenseElements(JSContext* cx, JSObject* obj, uint32_t dstStart, uint32_t srcStart,
+                                uint32_t length)
+{
+    MOZ_ASSERT(HasBoxedOrUnboxedDenseElements<Type>(obj));
+
+    if (Type == JSVAL_TYPE_MAGIC) {
+        if (obj->as<NativeObject>().denseElementsAreFrozen())
+            return DenseElementResult::Incomplete;
+
+        if (!obj->as<NativeObject>().maybeCopyElementsForWrite(cx))
+            return DenseElementResult::Failure;
+        obj->as<NativeObject>().moveDenseElements(dstStart, srcStart, length);
+    } else {
+        uint8_t* data = obj->as<UnboxedArrayObject>().elements();
+        size_t elementSize = UnboxedTypeSize(Type);
+
+        if (UnboxedTypeNeedsPreBarrier(Type) &&
+            JS::shadow::Zone::asShadowZone(obj->zone())->needsIncrementalBarrier())
+        {
+            // Trigger pre barriers on any elements we are overwriting. See
+            // NativeObject::moveDenseElements. No post barrier is needed as
+            // only whole cell post barriers are used with unboxed objects.
+            for (size_t i = 0; i < length; i++)
+                obj->as<UnboxedArrayObject>().triggerPreBarrier<Type>(dstStart + i);
+        }
+
+        memmove(data + dstStart * elementSize,
+                data + srcStart * elementSize,
+                length * elementSize);
+    }
+
+    return DenseElementResult::Success;
+}
+
+template <JSValueType DstType, JSValueType SrcType>
+static inline DenseElementResult
+CopyBoxedOrUnboxedDenseElements(JSContext* cx, JSObject* dst, JSObject* src,
+                                uint32_t dstStart, uint32_t srcStart, uint32_t length)
+{
+    MOZ_ASSERT(HasBoxedOrUnboxedDenseElements<SrcType>(src));
+    MOZ_ASSERT(HasBoxedOrUnboxedDenseElements<DstType>(dst));
+    MOZ_ASSERT(GetBoxedOrUnboxedInitializedLength<DstType>(dst) == dstStart);
+    MOZ_ASSERT(GetBoxedOrUnboxedInitializedLength<SrcType>(src) >= srcStart + length);
+    MOZ_ASSERT(GetBoxedOrUnboxedCapacity<DstType>(dst) >= dstStart + length);
+
+    SetBoxedOrUnboxedInitializedLength<DstType>(cx, dst, dstStart + length);
+
+    if (DstType == JSVAL_TYPE_MAGIC) {
+        if (SrcType == JSVAL_TYPE_MAGIC) {
+            const Value* vp = src->as<NativeObject>().getDenseElements() + srcStart;
+            dst->as<NativeObject>().initDenseElements(dstStart, vp, length);
+        } else {
+            for (size_t i = 0; i < length; i++) {
+                Value v = GetBoxedOrUnboxedDenseElement<SrcType>(src, srcStart + i);
+                dst->as<NativeObject>().initDenseElement(dstStart + i, v);
+            }
+        }
+    } else if (DstType == SrcType) {
+        uint8_t* dstData = dst->as<UnboxedArrayObject>().elements();
+        uint8_t* srcData = src->as<UnboxedArrayObject>().elements();
+        size_t elementSize = UnboxedTypeSize(DstType);
+
+        memcpy(dstData + dstStart * elementSize,
+               srcData + srcStart * elementSize,
+               length * elementSize);
+
+        // Add a store buffer entry if we might have copied a nursery pointer to dst.
+        if (UnboxedTypeNeedsPostBarrier(DstType) && !IsInsideNursery(dst))
+            dst->runtimeFromMainThread()->gc.storeBuffer.putWholeCell(dst);
+    } else if (DstType == JSVAL_TYPE_DOUBLE && SrcType == JSVAL_TYPE_INT32) {
+        uint8_t* dstData = dst->as<UnboxedArrayObject>().elements();
+        uint8_t* srcData = src->as<UnboxedArrayObject>().elements();
+
+        for (size_t i = 0; i < length; i++) {
+            int32_t v = *reinterpret_cast<int32_t*>(srcData + (srcStart + i) * sizeof(int32_t));
+            *reinterpret_cast<double*>(dstData + (dstStart + i) * sizeof(double)) = v;
+        }
+    } else {
+        for (size_t i = 0; i < length; i++) {
+            Value v = GetBoxedOrUnboxedDenseElement<SrcType>(src, srcStart + i);
+            dst->as<UnboxedArrayObject>().initElementNoTypeChangeSpecific<DstType>(dstStart + i, v);
+        }
+    }
+
+    return DenseElementResult::Success;
+}
+
+/////////////////////////////////////////////////////////////////////
+// Dispatch to specialized methods based on the type of an object.
+/////////////////////////////////////////////////////////////////////
+
+// Goop to fix MSVC. See DispatchTraceKindTyped in TraceKind.h.
+// The clang-cl front end defines _MSC_VER, but still requires the explicit
+// template declaration, so we must test for __clang__ here as well.
+#if defined(_MSC_VER) && !defined(__clang__)
+# define DEPENDENT_TEMPLATE_HINT
+#else
+# define DEPENDENT_TEMPLATE_HINT template
+#endif
+
+// Function to dispatch a method specialized to whatever boxed or unboxed dense
+// elements which an input object has.
+template <typename F>
+DenseElementResult
+CallBoxedOrUnboxedSpecialization(F f, JSObject* obj)
+{
+    if (!HasAnyBoxedOrUnboxedDenseElements(obj))
+        return DenseElementResult::Incomplete;
+    switch (GetBoxedOrUnboxedType(obj)) {
+      case JSVAL_TYPE_MAGIC:
+        return f. DEPENDENT_TEMPLATE_HINT operator()<JSVAL_TYPE_MAGIC>();
+      case JSVAL_TYPE_BOOLEAN:
+        return f. DEPENDENT_TEMPLATE_HINT operator()<JSVAL_TYPE_BOOLEAN>();
+      case JSVAL_TYPE_INT32:
+        return f. DEPENDENT_TEMPLATE_HINT operator()<JSVAL_TYPE_INT32>();
+      case JSVAL_TYPE_DOUBLE:
+        return f. DEPENDENT_TEMPLATE_HINT operator()<JSVAL_TYPE_DOUBLE>();
+      case JSVAL_TYPE_STRING:
+        return f. DEPENDENT_TEMPLATE_HINT operator()<JSVAL_TYPE_STRING>();
+      case JSVAL_TYPE_OBJECT:
+        return f. DEPENDENT_TEMPLATE_HINT operator()<JSVAL_TYPE_OBJECT>();
+      default:
+        MOZ_CRASH();
+    }
+}
+
+// As above, except the specialization can reflect the unboxed type of two objects.
+template <typename F>
+DenseElementResult
+CallBoxedOrUnboxedSpecialization(F f, JSObject* obj1, JSObject* obj2)
+{
+    if (!HasAnyBoxedOrUnboxedDenseElements(obj1) || !HasAnyBoxedOrUnboxedDenseElements(obj2))
+        return DenseElementResult::Incomplete;
+
+#define SPECIALIZE_OBJ2(TYPE)                                                     \
+    switch (GetBoxedOrUnboxedType(obj2)) {                                        \
+      case JSVAL_TYPE_MAGIC:                                                      \
+        return f. DEPENDENT_TEMPLATE_HINT operator()<TYPE, JSVAL_TYPE_MAGIC>();   \
+      case JSVAL_TYPE_BOOLEAN:                                                    \
+        return f. DEPENDENT_TEMPLATE_HINT operator()<TYPE, JSVAL_TYPE_BOOLEAN>(); \
+      case JSVAL_TYPE_INT32:                                                      \
+        return f. DEPENDENT_TEMPLATE_HINT operator()<TYPE, JSVAL_TYPE_INT32>();   \
+      case JSVAL_TYPE_DOUBLE:                                                     \
+        return f. DEPENDENT_TEMPLATE_HINT operator()<TYPE, JSVAL_TYPE_DOUBLE>();  \
+      case JSVAL_TYPE_STRING:                                                     \
+        return f. DEPENDENT_TEMPLATE_HINT operator()<TYPE, JSVAL_TYPE_STRING>();  \
+      case JSVAL_TYPE_OBJECT:                                                     \
+        return f. DEPENDENT_TEMPLATE_HINT operator()<TYPE, JSVAL_TYPE_OBJECT>();  \
+      default:                                                                    \
+        MOZ_CRASH();                                                              \
+    }
+
+    switch (GetBoxedOrUnboxedType(obj1)) {
+      case JSVAL_TYPE_MAGIC:
+        SPECIALIZE_OBJ2(JSVAL_TYPE_MAGIC)
+      case JSVAL_TYPE_BOOLEAN:
+        SPECIALIZE_OBJ2(JSVAL_TYPE_BOOLEAN)
+      case JSVAL_TYPE_INT32:
+        SPECIALIZE_OBJ2(JSVAL_TYPE_INT32)
+      case JSVAL_TYPE_DOUBLE:
+        SPECIALIZE_OBJ2(JSVAL_TYPE_DOUBLE)
+      case JSVAL_TYPE_STRING:
+        SPECIALIZE_OBJ2(JSVAL_TYPE_STRING)
+      case JSVAL_TYPE_OBJECT:
+        SPECIALIZE_OBJ2(JSVAL_TYPE_OBJECT)
+      default:
+        MOZ_CRASH();
+    }
+
+#undef SPECIALIZE_OBJ2
+}
+
+#undef DEPENDENT_TEMPLATE_HINT
+
+#define DefineBoxedOrUnboxedFunctor1(Signature, A)                      \
+struct Signature ## Functor {                                           \
+    A a;                                                                \
+    explicit Signature ## Functor(A a)                                  \
+      : a(a)                                                            \
+    {}                                                                  \
+    template <JSValueType Type>                                         \
+    DenseElementResult operator()() {                                   \
+        return Signature<Type>(a);                                      \
+    }                                                                   \
+}
+
+#define DefineBoxedOrUnboxedFunctor3(Signature, A, B, C)                \
+struct Signature ## Functor {                                           \
+    A a; B b; C c;                                                      \
+    Signature ## Functor(A a, B b, C c)                                 \
+      : a(a), b(b), c(c)                                                \
+    {}                                                                  \
+    template <JSValueType Type>                                         \
+    DenseElementResult operator()() {                                   \
+        return Signature<Type>(a, b, c);                                \
+    }                                                                   \
+}
+
+#define DefineBoxedOrUnboxedFunctor4(Signature, A, B, C, D)             \
+struct Signature ## Functor {                                           \
+    A a; B b; C c; D d;                                                 \
+    Signature ## Functor(A a, B b, C c, D d)                            \
+      : a(a), b(b), c(c), d(d)                                          \
+    {}                                                                  \
+    template <JSValueType Type>                                         \
+    DenseElementResult operator()() {                                   \
+        return Signature<Type>(a, b, c, d);                             \
+    }                                                                   \
+}
+
+#define DefineBoxedOrUnboxedFunctorPair4(Signature, A, B, C, D)         \
+struct Signature ## Functor {                                           \
+    A a; B b; C c; D d;                                                 \
+    Signature ## Functor(A a, B b, C c, D d)                            \
+      : a(a), b(b), c(c), d(d)                                          \
+    {}                                                                  \
+    template <JSValueType TypeOne, JSValueType TypeTwo>                 \
+    DenseElementResult operator()() {                                   \
+        return Signature<TypeOne, TypeTwo>(a, b, c, d);                 \
+    }                                                                   \
+}
+
+#define DefineBoxedOrUnboxedFunctor5(Signature, A, B, C, D, E)          \
+struct Signature ## Functor {                                           \
+    A a; B b; C c; D d; E e;                                            \
+    Signature ## Functor(A a, B b, C c, D d, E e)                       \
+      : a(a), b(b), c(c), d(d), e(e)                                    \
+    {}                                                                  \
+    template <JSValueType Type>                                         \
+    DenseElementResult operator()() {                                   \
+        return Signature<Type>(a, b, c, d, e);                          \
+    }                                                                   \
+}
+
+#define DefineBoxedOrUnboxedFunctor6(Signature, A, B, C, D, E, F)       \
+struct Signature ## Functor {                                           \
+    A a; B b; C c; D d; E e; F f;                                       \
+    Signature ## Functor(A a, B b, C c, D d, E e, F f)                  \
+      : a(a), b(b), c(c), d(d), e(e), f(f)                              \
+    {}                                                                  \
+    template <JSValueType Type>                                         \
+    DenseElementResult operator()() {                                   \
+        return Signature<Type>(a, b, c, d, e, f);                       \
+    }                                                                   \
+}
+
+#define DefineBoxedOrUnboxedFunctorPair6(Signature, A, B, C, D, E, F)   \
+struct Signature ## Functor {                                           \
+    A a; B b; C c; D d; E e; F f;                                       \
+    Signature ## Functor(A a, B b, C c, D d, E e, F f)                  \
+      : a(a), b(b), c(c), d(d), e(e), f(f)                              \
+    {}                                                                  \
+    template <JSValueType TypeOne, JSValueType TypeTwo>                 \
+    DenseElementResult operator()() {                                   \
+        return Signature<TypeOne, TypeTwo>(a, b, c, d, e, f);           \
+    }                                                                   \
+}
+
+DenseElementResult
+SetOrExtendAnyBoxedOrUnboxedDenseElements(ExclusiveContext* cx, JSObject* obj,
+                                          uint32_t start, const Value* vp, uint32_t count,
+                                          ShouldUpdateTypes updateTypes = ShouldUpdateTypes::Update);
+
+DenseElementResult
+MoveAnyBoxedOrUnboxedDenseElements(JSContext* cx, JSObject* obj,
+                                   uint32_t dstStart, uint32_t srcStart, uint32_t length);
+
+DenseElementResult
+CopyAnyBoxedOrUnboxedDenseElements(JSContext* cx, JSObject* dst, JSObject* src,
+                                   uint32_t dstStart, uint32_t srcStart, uint32_t length);
+
+void
+SetAnyBoxedOrUnboxedInitializedLength(JSContext* cx, JSObject* obj, size_t initlen);
+
+DenseElementResult
+EnsureAnyBoxedOrUnboxedDenseElements(JSContext* cx, JSObject* obj, size_t count);
+
+} // namespace js
+
+#endif // vm_UnboxedObject_inl_h
diff --git a/js/src/vm/UnboxedObject.cpp b/js/src/vm/UnboxedObject.cpp
new file mode 100644
index 000000000..d8c9c774a
--- /dev/null
+++ b/js/src/vm/UnboxedObject.cpp
@@ -0,0 +1,2132 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ * vim: set ts=8 sts=4 et sw=4 tw=99:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "vm/UnboxedObject-inl.h"
+
+#include "jit/BaselineIC.h"
+#include "jit/ExecutableAllocator.h"
+#include "jit/JitCommon.h"
+#include "jit/Linker.h"
+
+#include "jsobjinlines.h"
+
+#include "gc/Nursery-inl.h"
+#include "jit/MacroAssembler-inl.h"
+#include "vm/Shape-inl.h"
+
+using mozilla::ArrayLength;
+using mozilla::DebugOnly;
+using mozilla::PodCopy;
+
+using namespace js;
+
+/////////////////////////////////////////////////////////////////////
+// UnboxedLayout
+/////////////////////////////////////////////////////////////////////
+
+void
+UnboxedLayout::trace(JSTracer* trc)
+{
+    for (size_t i = 0; i < properties_.length(); i++)
+        TraceManuallyBarrieredEdge(trc, &properties_[i].name, "unboxed_layout_name");
+
+    if (newScript())
+        newScript()->trace(trc);
+
+    TraceNullableEdge(trc, &nativeGroup_, "unboxed_layout_nativeGroup");
+    TraceNullableEdge(trc, &nativeShape_, "unboxed_layout_nativeShape");
+    TraceNullableEdge(trc, &allocationScript_, "unboxed_layout_allocationScript");
+    TraceNullableEdge(trc, &replacementGroup_, "unboxed_layout_replacementGroup");
+    TraceNullableEdge(trc, &constructorCode_, "unboxed_layout_constructorCode");
+}
+
+size_t
+UnboxedLayout::sizeOfIncludingThis(mozilla::MallocSizeOf mallocSizeOf)
+{
+    return mallocSizeOf(this)
+         + properties_.sizeOfExcludingThis(mallocSizeOf)
+         + (newScript() ? newScript()->sizeOfIncludingThis(mallocSizeOf) : 0)
+         + mallocSizeOf(traceList());
+}
+
+void
+UnboxedLayout::setNewScript(TypeNewScript* newScript, bool writeBarrier /* = true */)
+{
+    if (newScript_ && writeBarrier)
+        TypeNewScript::writeBarrierPre(newScript_);
+    newScript_ = newScript;
+}
+
+// Constructor code returns a 0x1 value to indicate the constructor code should
+// be cleared.
+static const uintptr_t CLEAR_CONSTRUCTOR_CODE_TOKEN = 0x1;
+
+/* static */ bool
+UnboxedLayout::makeConstructorCode(JSContext* cx, HandleObjectGroup group)
+{
+    gc::AutoSuppressGC suppress(cx);
+
+    using namespace jit;
+
+    if (!cx->compartment()->ensureJitCompartmentExists(cx))
+        return false;
+
+    UnboxedLayout& layout = group->unboxedLayout();
+    MOZ_ASSERT(!layout.constructorCode());
+
+    UnboxedPlainObject* templateObject = UnboxedPlainObject::create(cx, group, TenuredObject);
+    if (!templateObject)
+        return false;
+
+    JitContext jitContext(cx, nullptr);
+
+    MacroAssembler masm;
+
+    Register propertiesReg, newKindReg;
+#ifdef JS_CODEGEN_X86
+    propertiesReg = eax;
+    newKindReg = ecx;
+    masm.loadPtr(Address(masm.getStackPointer(), sizeof(void*)), propertiesReg);
+    masm.loadPtr(Address(masm.getStackPointer(), 2 * sizeof(void*)), newKindReg);
+#else
+    propertiesReg = IntArgReg0;
+    newKindReg = IntArgReg1;
+#endif
+
+#ifdef JS_CODEGEN_ARM64
+    // ARM64 communicates stack address via sp, but uses a pseudo-sp for addressing.
+    masm.initStackPtr();
+#endif
+
+    MOZ_ASSERT(propertiesReg.volatile_());
+    MOZ_ASSERT(newKindReg.volatile_());
+
+    AllocatableGeneralRegisterSet regs(GeneralRegisterSet::All());
+    regs.take(propertiesReg);
+    regs.take(newKindReg);
+    Register object = regs.takeAny(), scratch1 = regs.takeAny(), scratch2 = regs.takeAny();
+
+    LiveGeneralRegisterSet savedNonVolatileRegisters = SavedNonVolatileRegisters(regs);
+    masm.PushRegsInMask(savedNonVolatileRegisters);
+
+    // The scratch double register might be used by MacroAssembler methods.
+    if (ScratchDoubleReg.volatile_())
+        masm.push(ScratchDoubleReg);
+
+    Label failure, tenuredObject, allocated;
+    masm.branch32(Assembler::NotEqual, newKindReg, Imm32(GenericObject), &tenuredObject);
+    masm.branchTest32(Assembler::NonZero, AbsoluteAddress(group->addressOfFlags()),
+                      Imm32(OBJECT_FLAG_PRE_TENURE), &tenuredObject);
+
+    // Allocate an object in the nursery
+    masm.createGCObject(object, scratch1, templateObject, gc::DefaultHeap, &failure,
+                        /* initFixedSlots = */ false);
+
+    masm.jump(&allocated);
+    masm.bind(&tenuredObject);
+
+    // Allocate an object in the tenured heap.
+    masm.createGCObject(object, scratch1, templateObject, gc::TenuredHeap, &failure,
+                        /* initFixedSlots = */ false);
+
+    // If any of the properties being stored are in the nursery, add a store
+    // buffer entry for the new object.
+    Label postBarrier;
+    for (size_t i = 0; i < layout.properties().length(); i++) {
+        const UnboxedLayout::Property& property = layout.properties()[i];
+        if (property.type == JSVAL_TYPE_OBJECT) {
+            Address valueAddress(propertiesReg, i * sizeof(IdValuePair) + offsetof(IdValuePair, value));
+            Label notObject;
+            masm.branchTestObject(Assembler::NotEqual, valueAddress, &notObject);
+            Register valueObject = masm.extractObject(valueAddress, scratch1);
+            masm.branchPtrInNurseryChunk(Assembler::Equal, valueObject, scratch2, &postBarrier);
+            masm.bind(&notObject);
+        }
+    }
+
+    masm.jump(&allocated);
+    masm.bind(&postBarrier);
+
+    LiveGeneralRegisterSet liveVolatileRegisters;
+    liveVolatileRegisters.add(propertiesReg);
+    if (object.volatile_())
+        liveVolatileRegisters.add(object);
+    masm.PushRegsInMask(liveVolatileRegisters);
+
+    masm.mov(ImmPtr(cx->runtime()), scratch1);
+    masm.setupUnalignedABICall(scratch2);
+    masm.passABIArg(scratch1);
+    masm.passABIArg(object);
+    masm.callWithABI(JS_FUNC_TO_DATA_PTR(void*, PostWriteBarrier));
+
+    masm.PopRegsInMask(liveVolatileRegisters);
+
+    masm.bind(&allocated);
+
+    ValueOperand valueOperand;
+#ifdef JS_NUNBOX32
+    valueOperand = ValueOperand(scratch1, scratch2);
+#else
+    valueOperand = ValueOperand(scratch1);
+#endif
+
+    Label failureStoreOther, failureStoreObject;
+
+    for (size_t i = 0; i < layout.properties().length(); i++) {
+        const UnboxedLayout::Property& property = layout.properties()[i];
+        Address valueAddress(propertiesReg, i * sizeof(IdValuePair) + offsetof(IdValuePair, value));
+        Address targetAddress(object, UnboxedPlainObject::offsetOfData() + property.offset);
+
+        masm.loadValue(valueAddress, valueOperand);
+
+        if (property.type == JSVAL_TYPE_OBJECT) {
+            HeapTypeSet* types = group->maybeGetProperty(IdToTypeId(NameToId(property.name)));
+
+            Label notObject;
+            masm.branchTestObject(Assembler::NotEqual, valueOperand,
+                                  types->mightBeMIRType(MIRType::Null) ? &notObject : &failureStoreObject);
+
+            Register payloadReg = masm.extractObject(valueOperand, scratch1);
+
+            if (!types->hasType(TypeSet::AnyObjectType())) {
+                Register scratch = (payloadReg == scratch1) ? scratch2 : scratch1;
+                masm.guardObjectType(payloadReg, types, scratch, &failureStoreObject);
+            }
+
+            masm.storeUnboxedProperty(targetAddress, JSVAL_TYPE_OBJECT,
+                                      TypedOrValueRegister(MIRType::Object,
+                                                           AnyRegister(payloadReg)), nullptr);
+
+            if (notObject.used()) {
+                Label done;
+                masm.jump(&done);
+                masm.bind(&notObject);
+                masm.branchTestNull(Assembler::NotEqual, valueOperand, &failureStoreOther);
+                masm.storeUnboxedProperty(targetAddress, JSVAL_TYPE_OBJECT, NullValue(), nullptr);
+                masm.bind(&done);
+            }
+        } else {
+            masm.storeUnboxedProperty(targetAddress, property.type,
+                                      ConstantOrRegister(valueOperand), &failureStoreOther);
+        }
+    }
+
+    Label done;
+    masm.bind(&done);
+
+    if (object != ReturnReg)
+        masm.movePtr(object, ReturnReg);
+
+    // Restore non-volatile registers which were saved on entry.
+    if (ScratchDoubleReg.volatile_())
+        masm.pop(ScratchDoubleReg);
+    masm.PopRegsInMask(savedNonVolatileRegisters);
+
+    masm.abiret();
+
+    masm.bind(&failureStoreOther);
+
+    // There was a failure while storing a value which cannot be stored at all
+    // in the unboxed object. Initialize the object so it is safe for GC and
+    // return null.
+    masm.initUnboxedObjectContents(object, templateObject);
+
+    masm.bind(&failure);
+
+    masm.movePtr(ImmWord(0), object);
+    masm.jump(&done);
+
+    masm.bind(&failureStoreObject);
+
+    // There was a failure while storing a value to an object slot of the
+    // unboxed object. If the value is storable, the failure occurred due to
+    // incomplete type information in the object, so return a token to trigger
+    // regeneration of the jitcode after a new object is created in the VM.
+    {
+        Label isObject;
+        masm.branchTestObject(Assembler::Equal, valueOperand, &isObject);
+        masm.branchTestNull(Assembler::NotEqual, valueOperand, &failureStoreOther);
+        masm.bind(&isObject);
+    }
+
+    // Initialize the object so it is safe for GC.
+    masm.initUnboxedObjectContents(object, templateObject);
+
+    masm.movePtr(ImmWord(CLEAR_CONSTRUCTOR_CODE_TOKEN), object);
+    masm.jump(&done);
+
+    Linker linker(masm);
+    AutoFlushICache afc("UnboxedObject");
+    JitCode* code = linker.newCode<NoGC>(cx, OTHER_CODE);
+    if (!code)
+        return false;
+
+    layout.setConstructorCode(code);
+    return true;
+}
+
+void
+UnboxedLayout::detachFromCompartment()
+{
+    if (isInList())
+        remove();
+}
+
+/////////////////////////////////////////////////////////////////////
+// UnboxedPlainObject
+/////////////////////////////////////////////////////////////////////
+
+bool
+UnboxedPlainObject::setValue(ExclusiveContext* cx, const UnboxedLayout::Property& property,
+                             const Value& v)
+{
+    uint8_t* p = &data_[property.offset];
+    return SetUnboxedValue(cx, this, NameToId(property.name), p, property.type, v,
+                           /* preBarrier = */ true);
+}
+
+Value
+UnboxedPlainObject::getValue(const UnboxedLayout::Property& property,
+                             bool maybeUninitialized /* = false */)
+{
+    uint8_t* p = &data_[property.offset];
+    return GetUnboxedValue(p, property.type, maybeUninitialized);
+}
+
+void
+UnboxedPlainObject::trace(JSTracer* trc, JSObject* obj)
+{
+    if (obj->as<UnboxedPlainObject>().expando_) {
+        TraceManuallyBarrieredEdge(trc,
+            reinterpret_cast<NativeObject**>(&obj->as<UnboxedPlainObject>().expando_),
+            "unboxed_expando");
+    }
+
+    const UnboxedLayout& layout = obj->as<UnboxedPlainObject>().layoutDontCheckGeneration();
+    const int32_t* list = layout.traceList();
+    if (!list)
+        return;
+
+    uint8_t* data = obj->as<UnboxedPlainObject>().data();
+    while (*list != -1) {
+        GCPtrString* heap = reinterpret_cast<GCPtrString*>(data + *list);
+        TraceEdge(trc, heap, "unboxed_string");
+        list++;
+    }
+    list++;
+    while (*list != -1) {
+        GCPtrObject* heap = reinterpret_cast<GCPtrObject*>(data + *list);
+        TraceNullableEdge(trc, heap, "unboxed_object");
+        list++;
+    }
+
+    // Unboxed objects don't have Values to trace.
+    MOZ_ASSERT(*(list + 1) == -1);
+}
+
+/* static */ UnboxedExpandoObject*
+UnboxedPlainObject::ensureExpando(JSContext* cx, Handle<UnboxedPlainObject*> obj)
+{
+    if (obj->expando_)
+        return obj->expando_;
+
+    UnboxedExpandoObject* expando =
+        NewObjectWithGivenProto<UnboxedExpandoObject>(cx, nullptr, gc::AllocKind::OBJECT4);
+    if (!expando)
+        return nullptr;
+
+    // Don't track property types for expando objects. This allows Baseline
+    // and Ion AddSlot ICs to guard on the unboxed group without guarding on
+    // the expando group.
+    MarkObjectGroupUnknownProperties(cx, expando->group());
+
+    // If the expando is tenured then the original object must also be tenured.
+    // Otherwise barriers triggered on the original object for writes to the
+    // expando (as can happen in the JIT) won't see the tenured->nursery edge.
+    // See WholeCellEdges::mark.
+    MOZ_ASSERT_IF(!IsInsideNursery(expando), !IsInsideNursery(obj));
+
+    // As with setValue(), we need to manually trigger post barriers on the
+    // whole object. If we treat the field as a GCPtrObject and later
+    // convert the object to its native representation, we will end up with a
+    // corrupted store buffer entry.
+    if (IsInsideNursery(expando) && !IsInsideNursery(obj))
+        cx->runtime()->gc.storeBuffer.putWholeCell(obj);
+
+    obj->expando_ = expando;
+    return expando;
+}
+
+bool
+UnboxedPlainObject::containsUnboxedOrExpandoProperty(ExclusiveContext* cx, jsid id) const
+{
+    if (layout().lookup(id))
+        return true;
+
+    if (maybeExpando() && maybeExpando()->containsShapeOrElement(cx, id))
+        return true;
+
+    return false;
+}
+
+static bool
+PropagatePropertyTypes(JSContext* cx, jsid id, ObjectGroup* oldGroup, ObjectGroup* newGroup)
+{
+    HeapTypeSet* typeProperty = oldGroup->maybeGetProperty(id);
+    TypeSet::TypeList types;
+    if (!typeProperty->enumerateTypes(&types)) {
+        ReportOutOfMemory(cx);
+        return false;
+    }
+    for (size_t j = 0; j < types.length(); j++)
+        AddTypePropertyId(cx, newGroup, nullptr, id, types[j]);
+    return true;
+}
+
+static PlainObject*
+MakeReplacementTemplateObject(JSContext* cx, HandleObjectGroup group, const UnboxedLayout &layout)
+{
+    PlainObject* obj = NewObjectWithGroup<PlainObject>(cx, group, layout.getAllocKind(),
+                                                       TenuredObject);
+    if (!obj)
+        return nullptr;
+
+    for (size_t i = 0; i < layout.properties().length(); i++) {
+        const UnboxedLayout::Property& property = layout.properties()[i];
+        if (!obj->addDataProperty(cx, NameToId(property.name), i, JSPROP_ENUMERATE))
+            return nullptr;
+        MOZ_ASSERT(obj->slotSpan() == i + 1);
+        MOZ_ASSERT(!obj->inDictionaryMode());
+    }
+
+    return obj;
+}
+
+/* static */ bool
+UnboxedLayout::makeNativeGroup(JSContext* cx, ObjectGroup* group)
+{
+    AutoEnterAnalysis enter(cx);
+
+    UnboxedLayout& layout = group->unboxedLayout();
+    Rooted<TaggedProto> proto(cx, group->proto());
+
+    MOZ_ASSERT(!layout.nativeGroup());
+
+    RootedObjectGroup replacementGroup(cx);
+
+    const Class* clasp = layout.isArray() ? &ArrayObject::class_ : &PlainObject::class_;
+
+    // Immediately clear any new script on the group. This is done by replacing
+    // the existing new script with one for a replacement default new group.
+    // This is done so that the size of the replacment group's objects is the
+    // same as that for the unboxed group, so that we do not see polymorphic
+    // slot accesses later on for sites that see converted objects from this
+    // group and objects that were allocated using the replacement new group.
+    if (layout.newScript()) {
+        MOZ_ASSERT(!layout.isArray());
+
+        replacementGroup = ObjectGroupCompartment::makeGroup(cx, &PlainObject::class_, proto);
+        if (!replacementGroup)
+            return false;
+
+        PlainObject* templateObject = MakeReplacementTemplateObject(cx, replacementGroup, layout);
+        if (!templateObject)
+            return false;
+
+        TypeNewScript* replacementNewScript =
+            TypeNewScript::makeNativeVersion(cx, layout.newScript(), templateObject);
+        if (!replacementNewScript)
+            return false;
+
+        replacementGroup->setNewScript(replacementNewScript);
+        gc::TraceTypeNewScript(replacementGroup);
+
+        group->clearNewScript(cx, replacementGroup);
+    }
+
+    // Similarly, if this group is keyed to an allocation site, replace its
+    // entry with a new group that has no unboxed layout.
+    if (layout.allocationScript()) {
+        RootedScript script(cx, layout.allocationScript());
+        jsbytecode* pc = layout.allocationPc();
+
+        replacementGroup = ObjectGroupCompartment::makeGroup(cx, clasp, proto);
+        if (!replacementGroup)
+            return false;
+
+        PlainObject* templateObject = &script->getObject(pc)->as<PlainObject>();
+        replacementGroup->addDefiniteProperties(cx, templateObject->lastProperty());
+
+        JSProtoKey key = layout.isArray() ? JSProto_Array : JSProto_Object;
+        cx->compartment()->objectGroups.replaceAllocationSiteGroup(script, pc, key,
+                                                                   replacementGroup);
+
+        // Clear any baseline information at this opcode which might use the old group.
+        if (script->hasBaselineScript()) {
+            jit::ICEntry& entry = script->baselineScript()->icEntryFromPCOffset(script->pcToOffset(pc));
+            jit::ICFallbackStub* fallback = entry.fallbackStub();
+            for (jit::ICStubIterator iter = fallback->beginChain(); !iter.atEnd(); iter++)
+                iter.unlink(cx);
+            if (fallback->isNewObject_Fallback())
+                fallback->toNewObject_Fallback()->setTemplateObject(nullptr);
+            else if (fallback->isNewArray_Fallback())
+                fallback->toNewArray_Fallback()->setTemplateGroup(replacementGroup);
+        }
+    }
+
+    size_t nfixed = layout.isArray() ? 0 : gc::GetGCKindSlots(layout.getAllocKind());
+
+    if (layout.isArray()) {
+        // The length shape to use for arrays is cached via a modified initial
+        // shape for array objects. Create an array now to make sure this entry
+        // is instantiated.
+        if (!NewDenseEmptyArray(cx))
+            return false;
+    }
+
+    RootedShape shape(cx, EmptyShape::getInitialShape(cx, clasp, proto, nfixed, 0));
+    if (!shape)
+        return false;
+
+    MOZ_ASSERT_IF(layout.isArray(), !shape->isEmptyShape() && shape->slotSpan() == 0);
+
+    // Add shapes for each property, if this is for a plain object.
+    for (size_t i = 0; i < layout.properties().length(); i++) {
+        const UnboxedLayout::Property& property = layout.properties()[i];
+
+        Rooted<StackShape> child(cx, StackShape(shape->base()->unowned(), NameToId(property.name),
+                                                i, JSPROP_ENUMERATE, 0));
+        shape = cx->zone()->propertyTree.getChild(cx, shape, child);
+        if (!shape)
+            return false;
+    }
+
+    ObjectGroup* nativeGroup =
+        ObjectGroupCompartment::makeGroup(cx, clasp, proto,
+                                          group->flags() & OBJECT_FLAG_DYNAMIC_MASK);
+    if (!nativeGroup)
+        return false;
+
+    // No sense propagating if we don't know what we started with.
+    if (!group->unknownProperties()) {
+        // Propagate all property types from the old group to the new group.
+        if (layout.isArray()) {
+            if (!PropagatePropertyTypes(cx, JSID_VOID, group, nativeGroup))
+                return false;
+        } else {
+            for (size_t i = 0; i < layout.properties().length(); i++) {
+                const UnboxedLayout::Property& property = layout.properties()[i];
+                jsid id = NameToId(property.name);
+                if (!PropagatePropertyTypes(cx, id, group, nativeGroup))
+                    return false;
+
+                // If we are OOM we may not be able to propagate properties.
+                if (nativeGroup->unknownProperties())
+                    break;
+
+                HeapTypeSet* nativeProperty = nativeGroup->maybeGetProperty(id);
+                if (nativeProperty && nativeProperty->canSetDefinite(i))
+                    nativeProperty->setDefinite(i);
+            }
+        }
+    } else {
+        // If we skip, though, the new group had better agree.
+        MOZ_ASSERT(nativeGroup->unknownProperties());
+    }
+
+    layout.nativeGroup_ = nativeGroup;
+    layout.nativeShape_ = shape;
+    layout.replacementGroup_ = replacementGroup;
+
+    nativeGroup->setOriginalUnboxedGroup(group);
+
+    group->markStateChange(cx);
+
+    return true;
+}
+
+/* static */ bool
+UnboxedPlainObject::convertToNative(JSContext* cx, JSObject* obj)
+{
+    const UnboxedLayout& layout = obj->as<UnboxedPlainObject>().layout();
+    UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando();
+
+    if (!layout.nativeGroup()) {
+        if (!UnboxedLayout::makeNativeGroup(cx, obj->group()))
+            return false;
+
+        // makeNativeGroup can reentrantly invoke this method.
+        if (obj->is<PlainObject>())
+            return true;
+    }
+
+    AutoValueVector values(cx);
+    for (size_t i = 0; i < layout.properties().length(); i++) {
+        // We might be reading properties off the object which have not been
+        // initialized yet. Make sure any double values we read here are
+        // canonicalized.
+        if (!values.append(obj->as<UnboxedPlainObject>().getValue(layout.properties()[i], true)))
+            return false;
+    }
+
+    // We are eliminating the expando edge with the conversion, so trigger a
+    // pre barrier.
+    JSObject::writeBarrierPre(expando);
+
+    // Additionally trigger a post barrier on the expando itself. Whole cell
+    // store buffer entries can be added on the original unboxed object for
+    // writes to the expando (see WholeCellEdges::trace), so after conversion
+    // we need to make sure the expando itself will still be traced.
+    if (expando && !IsInsideNursery(expando))
+        cx->runtime()->gc.storeBuffer.putWholeCell(expando);
+
+    obj->setGroup(layout.nativeGroup());
+    obj->as<PlainObject>().setLastPropertyMakeNative(cx, layout.nativeShape());
+
+    for (size_t i = 0; i < values.length(); i++)
+        obj->as<PlainObject>().initSlotUnchecked(i, values[i]);
+
+    if (expando) {
+        // Add properties from the expando object to the object, in order.
+        // Suppress GC here, so that callers don't need to worry about this
+        // method collecting. The stuff below can only fail due to OOM, in
+        // which case the object will not have been completely filled back in.
+        gc::AutoSuppressGC suppress(cx);
+
+        Vector<jsid> ids(cx);
+        for (Shape::Range<NoGC> r(expando->lastProperty()); !r.empty(); r.popFront()) {
+            if (!ids.append(r.front().propid()))
+                return false;
+        }
+        for (size_t i = 0; i < expando->getDenseInitializedLength(); i++) {
+            if (!expando->getDenseElement(i).isMagic(JS_ELEMENTS_HOLE)) {
+                if (!ids.append(INT_TO_JSID(i)))
+                    return false;
+            }
+        }
+        ::Reverse(ids.begin(), ids.end());
+
+        RootedPlainObject nobj(cx, &obj->as<PlainObject>());
+        Rooted<UnboxedExpandoObject*> nexpando(cx, expando);
+        RootedId id(cx);
+        Rooted<PropertyDescriptor> desc(cx);
+        for (size_t i = 0; i < ids.length(); i++) {
+            id = ids[i];
+            if (!GetOwnPropertyDescriptor(cx, nexpando, id, &desc))
+                return false;
+            ObjectOpResult result;
+            if (!DefineProperty(cx, nobj, id, desc, result))
+                return false;
+            MOZ_ASSERT(result.ok());
+        }
+    }
+
+    return true;
+}
+
+/* static */
+UnboxedPlainObject*
+UnboxedPlainObject::create(ExclusiveContext* cx, HandleObjectGroup group, NewObjectKind newKind)
+{
+    AutoSetNewObjectMetadata metadata(cx);
+
+    MOZ_ASSERT(group->clasp() == &class_);
+    gc::AllocKind allocKind = group->unboxedLayout().getAllocKind();
+
+    UnboxedPlainObject* res =
+        NewObjectWithGroup<UnboxedPlainObject>(cx, group, allocKind, newKind);
+    if (!res)
+        return nullptr;
+
+    // Overwrite the dummy shape which was written to the object's expando field.
+    res->initExpando();
+
+    // Initialize reference fields of the object. All fields in the object will
+    // be overwritten shortly, but references need to be safe for the GC.
+    const int32_t* list = res->layout().traceList();
+    if (list) {
+        uint8_t* data = res->data();
+        while (*list != -1) {
+            GCPtrString* heap = reinterpret_cast<GCPtrString*>(data + *list);
+            heap->init(cx->names().empty);
+            list++;
+        }
+        list++;
+        while (*list != -1) {
+            GCPtrObject* heap = reinterpret_cast<GCPtrObject*>(data + *list);
+            heap->init(nullptr);
+            list++;
+        }
+        // Unboxed objects don't have Values to initialize.
+        MOZ_ASSERT(*(list + 1) == -1);
+    }
+
+    return res;
+}
+
+/* static */ JSObject*
+UnboxedPlainObject::createWithProperties(ExclusiveContext* cx, HandleObjectGroup group,
+                                         NewObjectKind newKind, IdValuePair* properties)
+{
+    MOZ_ASSERT(newKind == GenericObject || newKind == TenuredObject);
+
+    UnboxedLayout& layout = group->unboxedLayout();
+
+    if (layout.constructorCode()) {
+        MOZ_ASSERT(cx->isJSContext());
+
+        typedef JSObject* (*ConstructorCodeSignature)(IdValuePair*, NewObjectKind);
+        ConstructorCodeSignature function =
+            reinterpret_cast<ConstructorCodeSignature>(layout.constructorCode()->raw());
+
+        JSObject* obj;
+        {
+            JS::AutoSuppressGCAnalysis nogc;
+            obj = reinterpret_cast<JSObject*>(CALL_GENERATED_2(function, properties, newKind));
+        }
+        if (obj > reinterpret_cast<JSObject*>(CLEAR_CONSTRUCTOR_CODE_TOKEN))
+            return obj;
+
+        if (obj == reinterpret_cast<JSObject*>(CLEAR_CONSTRUCTOR_CODE_TOKEN))
+            layout.setConstructorCode(nullptr);
+    }
+
+    UnboxedPlainObject* obj = UnboxedPlainObject::create(cx, group, newKind);
+    if (!obj)
+        return nullptr;
+
+    for (size_t i = 0; i < layout.properties().length(); i++) {
+        if (!obj->setValue(cx, layout.properties()[i], properties[i].value))
+            return NewPlainObjectWithProperties(cx, properties, layout.properties().length(), newKind);
+    }
+
+#ifndef JS_CODEGEN_NONE
+    if (cx->isJSContext() &&
+        !group->unknownProperties() &&
+        !layout.constructorCode() &&
+        cx->asJSContext()->runtime()->jitSupportsFloatingPoint &&
+        jit::CanLikelyAllocateMoreExecutableMemory())
+    {
+        if (!UnboxedLayout::makeConstructorCode(cx->asJSContext(), group))
+            return nullptr;
+    }
+#endif
+
+    return obj;
+}
+
+/* static */ bool
+UnboxedPlainObject::obj_lookupProperty(JSContext* cx, HandleObject obj,
+                                       HandleId id, MutableHandleObject objp,
+                                       MutableHandleShape propp)
+{
+    if (obj->as<UnboxedPlainObject>().containsUnboxedOrExpandoProperty(cx, id)) {
+        MarkNonNativePropertyFound<CanGC>(propp);
+        objp.set(obj);
+        return true;
+    }
+
+    RootedObject proto(cx, obj->staticPrototype());
+    if (!proto) {
+        objp.set(nullptr);
+        propp.set(nullptr);
+        return true;
+    }
+
+    return LookupProperty(cx, proto, id, objp, propp);
+}
+
+/* static */ bool
+UnboxedPlainObject::obj_defineProperty(JSContext* cx, HandleObject obj, HandleId id,
+                                       Handle<PropertyDescriptor> desc,
+                                       ObjectOpResult& result)
+{
+    const UnboxedLayout& layout = obj->as<UnboxedPlainObject>().layout();
+
+    if (const UnboxedLayout::Property* property = layout.lookup(id)) {
+        if (!desc.getter() && !desc.setter() && desc.attributes() == JSPROP_ENUMERATE) {
+            // This define is equivalent to setting an existing property.
+            if (obj->as<UnboxedPlainObject>().setValue(cx, *property, desc.value()))
+                return result.succeed();
+        }
+
+        // Trying to incompatibly redefine an existing property requires the
+        // object to be converted to a native object.
+        if (!convertToNative(cx, obj))
+            return false;
+
+        return DefineProperty(cx, obj, id, desc, result);
+    }
+
+    // Define the property on the expando object.
+    Rooted<UnboxedExpandoObject*> expando(cx, ensureExpando(cx, obj.as<UnboxedPlainObject>()));
+    if (!expando)
+        return false;
+
+    // Update property types on the unboxed object as well.
+    AddTypePropertyId(cx, obj, id, desc.value());
+
+    return DefineProperty(cx, expando, id, desc, result);
+}
+
+/* static */ bool
+UnboxedPlainObject::obj_hasProperty(JSContext* cx, HandleObject obj, HandleId id, bool* foundp)
+{
+    if (obj->as<UnboxedPlainObject>().containsUnboxedOrExpandoProperty(cx, id)) {
+        *foundp = true;
+        return true;
+    }
+
+    RootedObject proto(cx, obj->staticPrototype());
+    if (!proto) {
+        *foundp = false;
+        return true;
+    }
+
+    return HasProperty(cx, proto, id, foundp);
+}
+
+/* static */ bool
+UnboxedPlainObject::obj_getProperty(JSContext* cx, HandleObject obj, HandleValue receiver,
+                                    HandleId id, MutableHandleValue vp)
+{
+    const UnboxedLayout& layout = obj->as<UnboxedPlainObject>().layout();
+
+    if (const UnboxedLayout::Property* property = layout.lookup(id)) {
+        vp.set(obj->as<UnboxedPlainObject>().getValue(*property));
+        return true;
+    }
+
+    if (UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando()) {
+        if (expando->containsShapeOrElement(cx, id)) {
+            RootedObject nexpando(cx, expando);
+            return GetProperty(cx, nexpando, receiver, id, vp);
+        }
+    }
+
+    RootedObject proto(cx, obj->staticPrototype());
+    if (!proto) {
+        vp.setUndefined();
+        return true;
+    }
+
+    return GetProperty(cx, proto, receiver, id, vp);
+}
+
+/* static */ bool
+UnboxedPlainObject::obj_setProperty(JSContext* cx, HandleObject obj, HandleId id, HandleValue v,
+                                    HandleValue receiver, ObjectOpResult& result)
+{
+    const UnboxedLayout& layout = obj->as<UnboxedPlainObject>().layout();
+
+    if (const UnboxedLayout::Property* property = layout.lookup(id)) {
+        if (receiver.isObject() && obj == &receiver.toObject()) {
+            if (obj->as<UnboxedPlainObject>().setValue(cx, *property, v))
+                return result.succeed();
+
+            if (!convertToNative(cx, obj))
+                return false;
+            return SetProperty(cx, obj, id, v, receiver, result);
+        }
+
+        return SetPropertyByDefining(cx, id, v, receiver, result);
+    }
+
+    if (UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando()) {
+        if (expando->containsShapeOrElement(cx, id)) {
+            // Update property types on the unboxed object as well.
+            AddTypePropertyId(cx, obj, id, v);
+
+            RootedObject nexpando(cx, expando);
+            return SetProperty(cx, nexpando, id, v, receiver, result);
+        }
+    }
+
+    return SetPropertyOnProto(cx, obj, id, v, receiver, result);
+}
+
+/* static */ bool
+UnboxedPlainObject::obj_getOwnPropertyDescriptor(JSContext* cx, HandleObject obj, HandleId id,
+                                                 MutableHandle<PropertyDescriptor> desc)
+{
+    const UnboxedLayout& layout = obj->as<UnboxedPlainObject>().layout();
+
+    if (const UnboxedLayout::Property* property = layout.lookup(id)) {
+        desc.value().set(obj->as<UnboxedPlainObject>().getValue(*property));
+        desc.setAttributes(JSPROP_ENUMERATE);
+        desc.object().set(obj);
+        return true;
+    }
+
+    if (UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando()) {
+        if (expando->containsShapeOrElement(cx, id)) {
+            RootedObject nexpando(cx, expando);
+            if (!GetOwnPropertyDescriptor(cx, nexpando, id, desc))
+                return false;
+            if (desc.object() == nexpando)
+                desc.object().set(obj);
+            return true;
+        }
+    }
+
+    desc.object().set(nullptr);
+    return true;
+}
+
+/* static */ bool
+UnboxedPlainObject::obj_deleteProperty(JSContext* cx, HandleObject obj, HandleId id,
+                                       ObjectOpResult& result)
+{
+    if (!convertToNative(cx, obj))
+        return false;
+    return DeleteProperty(cx, obj, id, result);
+}
+
+/* static */ bool
+UnboxedPlainObject::obj_enumerate(JSContext* cx, HandleObject obj, AutoIdVector& properties,
+                                  bool enumerableOnly)
+{
+    // Ignore expando properties here, they are special-cased by the property
+    // enumeration code.
+
+    const UnboxedLayout::PropertyVector& unboxed = obj->as<UnboxedPlainObject>().layout().properties();
+    for (size_t i = 0; i < unboxed.length(); i++) {
+        if (!properties.append(NameToId(unboxed[i].name)))
+            return false;
+    }
+
+    return true;
+}
+
+const Class UnboxedExpandoObject::class_ = {
+    "UnboxedExpandoObject",
+    0
+};
+
+static const ClassOps UnboxedPlainObjectClassOps = {
+    nullptr,        /* addProperty */
+    nullptr,        /* delProperty */
+    nullptr,        /* getProperty */
+    nullptr,        /* setProperty */
+    nullptr,        /* enumerate   */
+    nullptr,        /* resolve     */
+    nullptr,        /* mayResolve  */
+    nullptr,        /* finalize    */
+    nullptr,        /* call        */
+    nullptr,        /* hasInstance */
+    nullptr,        /* construct   */
+    UnboxedPlainObject::trace,
+};
+
+static const ObjectOps UnboxedPlainObjectObjectOps = {
+    UnboxedPlainObject::obj_lookupProperty,
+    UnboxedPlainObject::obj_defineProperty,
+    UnboxedPlainObject::obj_hasProperty,
+    UnboxedPlainObject::obj_getProperty,
+    UnboxedPlainObject::obj_setProperty,
+    UnboxedPlainObject::obj_getOwnPropertyDescriptor,
+    UnboxedPlainObject::obj_deleteProperty,
+    nullptr,   /* getElements */
+    UnboxedPlainObject::obj_enumerate,
+    nullptr    /* funToString */
+};
+
+const Class UnboxedPlainObject::class_ = {
+    js_Object_str,
+    Class::NON_NATIVE |
+    JSCLASS_HAS_CACHED_PROTO(JSProto_Object) |
+    JSCLASS_DELAY_METADATA_BUILDER,
+    &UnboxedPlainObjectClassOps,
+    JS_NULL_CLASS_SPEC,
+    JS_NULL_CLASS_EXT,
+    &UnboxedPlainObjectObjectOps
+};
+
+/////////////////////////////////////////////////////////////////////
+// UnboxedArrayObject
+/////////////////////////////////////////////////////////////////////
+
+template <JSValueType Type>
+DenseElementResult
+AppendUnboxedDenseElements(UnboxedArrayObject* obj, uint32_t initlen,
+                           MutableHandle<GCVector<Value>> values)
+{
+    for (size_t i = 0; i < initlen; i++)
+        values.infallibleAppend(obj->getElementSpecific<Type>(i));
+    return DenseElementResult::Success;
+}
+
+DefineBoxedOrUnboxedFunctor3(AppendUnboxedDenseElements,
+                             UnboxedArrayObject*, uint32_t, MutableHandle<GCVector<Value>>);
+
+/* static */ bool
+UnboxedArrayObject::convertToNativeWithGroup(ExclusiveContext* cx, JSObject* obj,
+                                             ObjectGroup* group, Shape* shape)
+{
+    size_t length = obj->as<UnboxedArrayObject>().length();
+    size_t initlen = obj->as<UnboxedArrayObject>().initializedLength();
+
+    Rooted<GCVector<Value>> values(cx, GCVector<Value>(cx));
+    if (!values.reserve(initlen))
+        return false;
+
+    AppendUnboxedDenseElementsFunctor functor(&obj->as<UnboxedArrayObject>(), initlen, &values);
+    DebugOnly<DenseElementResult> result = CallBoxedOrUnboxedSpecialization(functor, obj);
+    MOZ_ASSERT(result.value == DenseElementResult::Success);
+
+    obj->setGroup(group);
+
+    ArrayObject* aobj = &obj->as<ArrayObject>();
+    aobj->setLastPropertyMakeNative(cx, shape);
+
+    // Make sure there is at least one element, so that this array does not
+    // use emptyObjectElements / emptyObjectElementsShared.
+    if (!aobj->ensureElements(cx, Max<size_t>(initlen, 1)))
+        return false;
+
+    MOZ_ASSERT(!aobj->getDenseInitializedLength());
+    aobj->setDenseInitializedLength(initlen);
+    aobj->initDenseElements(0, values.begin(), initlen);
+    aobj->setLengthInt32(length);
+
+    return true;
+}
+
+/* static */ bool
+UnboxedArrayObject::convertToNative(JSContext* cx, JSObject* obj)
+{
+    const UnboxedLayout& layout = obj->as<UnboxedArrayObject>().layout();
+
+    if (!layout.nativeGroup()) {
+        if (!UnboxedLayout::makeNativeGroup(cx, obj->group()))
+            return false;
+    }
+
+    return convertToNativeWithGroup(cx, obj, layout.nativeGroup(), layout.nativeShape());
+}
+
+bool
+UnboxedArrayObject::convertInt32ToDouble(ExclusiveContext* cx, ObjectGroup* group)
+{
+    MOZ_ASSERT(elementType() == JSVAL_TYPE_INT32);
+    MOZ_ASSERT(group->unboxedLayout().elementType() == JSVAL_TYPE_DOUBLE);
+
+    Vector<int32_t> values(cx);
+    if (!values.reserve(initializedLength()))
+        return false;
+    for (size_t i = 0; i < initializedLength(); i++)
+        values.infallibleAppend(getElementSpecific<JSVAL_TYPE_INT32>(i).toInt32());
+
+    uint8_t* newElements;
+    if (hasInlineElements()) {
+        newElements = AllocateObjectBuffer<uint8_t>(cx, this, capacity() * sizeof(double));
+    } else {
+        newElements = ReallocateObjectBuffer<uint8_t>(cx, this, elements(),
+                                                      capacity() * sizeof(int32_t),
+                                                      capacity() * sizeof(double));
+    }
+    if (!newElements)
+        return false;
+
+    setGroup(group);
+    elements_ = newElements;
+
+    for (size_t i = 0; i < initializedLength(); i++)
+        setElementNoTypeChangeSpecific<JSVAL_TYPE_DOUBLE>(i, DoubleValue(values[i]));
+
+    return true;
+}
+
+/* static */ UnboxedArrayObject*
+UnboxedArrayObject::create(ExclusiveContext* cx, HandleObjectGroup group, uint32_t length,
+                           NewObjectKind newKind, uint32_t maxLength)
+{
+    MOZ_ASSERT(length <= MaximumCapacity);
+
+    MOZ_ASSERT(group->clasp() == &class_);
+    uint32_t elementSize = UnboxedTypeSize(group->unboxedLayout().elementType());
+    uint32_t capacity = Min(length, maxLength);
+    uint32_t nbytes = offsetOfInlineElements() + elementSize * capacity;
+
+    UnboxedArrayObject* res;
+    if (nbytes <= JSObject::MAX_BYTE_SIZE) {
+        gc::AllocKind allocKind = gc::GetGCObjectKindForBytes(nbytes);
+
+        // If there was no provided length information, pick an allocation kind
+        // to accommodate small arrays (as is done for normal native arrays).
+        if (capacity == 0)
+            allocKind = gc::AllocKind::OBJECT8;
+
+        res = NewObjectWithGroup<UnboxedArrayObject>(cx, group, allocKind, newKind);
+        if (!res)
+            return nullptr;
+        res->setInitializedLengthNoBarrier(0);
+        res->setInlineElements();
+
+        size_t actualCapacity = (GetGCKindBytes(allocKind) - offsetOfInlineElements()) / elementSize;
+        MOZ_ASSERT(actualCapacity >= capacity);
+        res->setCapacityIndex(exactCapacityIndex(actualCapacity));
+    } else {
+        res = NewObjectWithGroup<UnboxedArrayObject>(cx, group, gc::AllocKind::OBJECT0, newKind);
+        if (!res)
+            return nullptr;
+        res->setInitializedLengthNoBarrier(0);
+
+        uint32_t capacityIndex = (capacity == length)
+                                 ? CapacityMatchesLengthIndex
+                                 : chooseCapacityIndex(capacity, length);
+        uint32_t actualCapacity = computeCapacity(capacityIndex, length);
+
+        res->elements_ = AllocateObjectBuffer<uint8_t>(cx, res, actualCapacity * elementSize);
+        if (!res->elements_) {
+            // Make the object safe for GC.
+            res->setInlineElements();
+            return nullptr;
+        }
+
+        res->setCapacityIndex(capacityIndex);
+    }
+
+    res->setLength(cx, length);
+    return res;
+}
+
+bool
+UnboxedArrayObject::setElement(ExclusiveContext* cx, size_t index, const Value& v)
+{
+    MOZ_ASSERT(index < initializedLength());
+    uint8_t* p = elements() + index * elementSize();
+    return SetUnboxedValue(cx, this, JSID_VOID, p, elementType(), v, /* preBarrier = */ true);
+}
+
+bool
+UnboxedArrayObject::initElement(ExclusiveContext* cx, size_t index, const Value& v)
+{
+    MOZ_ASSERT(index < initializedLength());
+    uint8_t* p = elements() + index * elementSize();
+    return SetUnboxedValue(cx, this, JSID_VOID, p, elementType(), v, /* preBarrier = */ false);
+}
+
+void
+UnboxedArrayObject::initElementNoTypeChange(size_t index, const Value& v)
+{
+    MOZ_ASSERT(index < initializedLength());
+    uint8_t* p = elements() + index * elementSize();
+    if (UnboxedTypeNeedsPreBarrier(elementType()))
+        *reinterpret_cast<void**>(p) = nullptr;
+    SetUnboxedValueNoTypeChange(this, p, elementType(), v, /* preBarrier = */ false);
+}
+
+Value
+UnboxedArrayObject::getElement(size_t index)
+{
+    MOZ_ASSERT(index < initializedLength());
+    uint8_t* p = elements() + index * elementSize();
+    return GetUnboxedValue(p, elementType(), /* maybeUninitialized = */ false);
+}
+
+/* static */ void
+UnboxedArrayObject::trace(JSTracer* trc, JSObject* obj)
+{
+    JSValueType type = obj->as<UnboxedArrayObject>().elementType();
+    if (!UnboxedTypeNeedsPreBarrier(type))
+        return;
+
+    MOZ_ASSERT(obj->as<UnboxedArrayObject>().elementSize() == sizeof(uintptr_t));
+    size_t initlen = obj->as<UnboxedArrayObject>().initializedLength();
+    void** elements = reinterpret_cast<void**>(obj->as<UnboxedArrayObject>().elements());
+
+    switch (type) {
+      case JSVAL_TYPE_OBJECT:
+        for (size_t i = 0; i < initlen; i++) {
+            GCPtrObject* heap = reinterpret_cast<GCPtrObject*>(elements + i);
+            TraceNullableEdge(trc, heap, "unboxed_object");
+        }
+        break;
+
+      case JSVAL_TYPE_STRING:
+        for (size_t i = 0; i < initlen; i++) {
+            GCPtrString* heap = reinterpret_cast<GCPtrString*>(elements + i);
+            TraceEdge(trc, heap, "unboxed_string");
+        }
+        break;
+
+      default:
+        MOZ_CRASH();
+    }
+}
+
+/* static */ void
+UnboxedArrayObject::objectMoved(JSObject* obj, const JSObject* old)
+{
+    UnboxedArrayObject& dst = obj->as<UnboxedArrayObject>();
+    const UnboxedArrayObject& src = old->as<UnboxedArrayObject>();
+
+    // Fix up possible inline data pointer.
+    if (src.hasInlineElements())
+        dst.setInlineElements();
+}
+
+/* static */ void
+UnboxedArrayObject::finalize(FreeOp* fop, JSObject* obj)
+{
+    MOZ_ASSERT(!IsInsideNursery(obj));
+    if (!obj->as<UnboxedArrayObject>().hasInlineElements())
+        js_free(obj->as<UnboxedArrayObject>().elements());
+}
+
+/* static */ size_t
+UnboxedArrayObject::objectMovedDuringMinorGC(JSTracer* trc, JSObject* dst, JSObject* src,
+                                             gc::AllocKind allocKind)
+{
+    UnboxedArrayObject* ndst = &dst->as<UnboxedArrayObject>();
+    UnboxedArrayObject* nsrc = &src->as<UnboxedArrayObject>();
+    MOZ_ASSERT(ndst->elements() == nsrc->elements());
+
+    Nursery& nursery = trc->runtime()->gc.nursery;
+
+    if (!nursery.isInside(nsrc->elements())) {
+        nursery.removeMallocedBuffer(nsrc->elements());
+        return 0;
+    }
+
+    // Determine if we can use inline data for the target array. If this is
+    // possible, the nursery will have picked an allocation size that is large
+    // enough.
+    size_t nbytes = nsrc->capacity() * nsrc->elementSize();
+    if (offsetOfInlineElements() + nbytes <= GetGCKindBytes(allocKind)) {
+        ndst->setInlineElements();
+    } else {
+        MOZ_ASSERT(allocKind == gc::AllocKind::OBJECT0);
+
+        AutoEnterOOMUnsafeRegion oomUnsafe;
+        uint8_t* data = nsrc->zone()->pod_malloc<uint8_t>(nbytes);
+        if (!data)
+            oomUnsafe.crash("Failed to allocate unboxed array elements while tenuring.");
+        ndst->elements_ = data;
+    }
+
+    PodCopy(ndst->elements(), nsrc->elements(), nsrc->initializedLength() * nsrc->elementSize());
+
+    // Set a forwarding pointer for the element buffers in case they were
+    // preserved on the stack by Ion.
+    bool direct = nsrc->capacity() * nsrc->elementSize() >= sizeof(uintptr_t);
+    nursery.maybeSetForwardingPointer(trc, nsrc->elements(), ndst->elements(), direct);
+
+    return ndst->hasInlineElements() ? 0 : nbytes;
+}
+
+// Possible capacities for unboxed arrays. Some of these capacities might seem
+// a little weird, but were chosen to allow the inline data of objects of each
+// size to be fully utilized for arrays of the various types on both 32 bit and
+// 64 bit platforms.
+//
+// To find the possible inline capacities, the following script was used:
+//
+// var fixedSlotCapacities = [0, 2, 4, 8, 12, 16];
+// var dataSizes = [1, 4, 8];
+// var header32 = 4 * 2 + 4 * 2;
+// var header64 = 8 * 2 + 4 * 2;
+//
+// for (var i = 0; i < fixedSlotCapacities.length; i++) {
+//    var nfixed = fixedSlotCapacities[i];
+//    var size32 = 4 * 4 + 8 * nfixed - header32;
+//    var size64 = 8 * 4 + 8 * nfixed - header64;
+//    for (var j = 0; j < dataSizes.length; j++) {
+//        print(size32 / dataSizes[j]);
+//        print(size64 / dataSizes[j]);
+//    }
+// }
+//
+/* static */ const uint32_t
+UnboxedArrayObject::CapacityArray[] = {
+    UINT32_MAX, // For CapacityMatchesLengthIndex.
+    0, 1, 2, 3, 4, 5, 6, 8, 9, 10, 12, 13, 16, 17, 18, 24, 26, 32, 34, 40, 64, 72, 96, 104, 128, 136,
+    256, 512, 1024, 2048, 4096, 8192, 16384, 32768, 65536, 131072, 262144, 524288,
+    1048576, 2097152, 3145728, 4194304, 5242880, 6291456, 7340032, 8388608, 9437184, 11534336,
+    13631488, 15728640, 17825792, 20971520, 24117248, 27262976, 31457280, 35651584, 40894464,
+    46137344, 52428800, 59768832, MaximumCapacity
+};
+
+static const uint32_t
+Pow2CapacityIndexes[] = {
+    2,  // 1
+    3,  // 2
+    5,  // 4
+    8,  // 8
+    13, // 16
+    18, // 32
+    21, // 64
+    25, // 128
+    27, // 256
+    28, // 512
+    29, // 1024
+    30, // 2048
+    31, // 4096
+    32, // 8192
+    33, // 16384
+    34, // 32768
+    35, // 65536
+    36, // 131072
+    37, // 262144
+    38, // 524288
+    39  // 1048576
+};
+
+static const uint32_t MebiCapacityIndex = 39;
+
+/* static */ uint32_t
+UnboxedArrayObject::chooseCapacityIndex(uint32_t capacity, uint32_t length)
+{
+    // Note: the structure and behavior of this method follow along with
+    // NativeObject::goodAllocated. Changes to the allocation strategy in one
+    // should generally be matched by the other.
+
+    // Make sure we have enough space to store all possible values for the capacity index.
+    // This ought to be a static_assert, but MSVC doesn't like that.
+    MOZ_ASSERT(mozilla::ArrayLength(CapacityArray) - 1 <= (CapacityMask >> CapacityShift));
+
+    // The caller should have ensured the capacity is possible for an unboxed array.
+    MOZ_ASSERT(capacity <= MaximumCapacity);
+
+    static const uint32_t Mebi = 1024 * 1024;
+
+    if (capacity <= Mebi) {
+        capacity = mozilla::RoundUpPow2(capacity);
+
+        // When the required capacity is close to the array length, then round
+        // up to the array length itself, as for NativeObject.
+        if (length >= capacity && capacity > (length / 3) * 2)
+            return CapacityMatchesLengthIndex;
+
+        if (capacity < MinimumDynamicCapacity)
+            capacity = MinimumDynamicCapacity;
+
+        uint32_t bit = mozilla::FloorLog2Size(capacity);
+        MOZ_ASSERT(capacity == uint32_t(1 << bit));
+        MOZ_ASSERT(bit <= 20);
+        MOZ_ASSERT(mozilla::ArrayLength(Pow2CapacityIndexes) == 21);
+
+        uint32_t index = Pow2CapacityIndexes[bit];
+        MOZ_ASSERT(CapacityArray[index] == capacity);
+
+        return index;
+    }
+
+    MOZ_ASSERT(CapacityArray[MebiCapacityIndex] == Mebi);
+
+    for (uint32_t i = MebiCapacityIndex + 1;; i++) {
+        if (CapacityArray[i] >= capacity)
+            return i;
+    }
+
+    MOZ_CRASH("Invalid capacity");
+}
+
+/* static */ uint32_t
+UnboxedArrayObject::exactCapacityIndex(uint32_t capacity)
+{
+    for (size_t i = CapacityMatchesLengthIndex + 1; i < ArrayLength(CapacityArray); i++) {
+        if (CapacityArray[i] == capacity)
+            return i;
+    }
+    MOZ_CRASH();
+}
+
+bool
+UnboxedArrayObject::growElements(ExclusiveContext* cx, size_t cap)
+{
+    // The caller should have checked if this capacity is possible for an
+    // unboxed array, so the only way this call can fail is from OOM.
+    MOZ_ASSERT(cap <= MaximumCapacity);
+
+    uint32_t oldCapacity = capacity();
+    uint32_t newCapacityIndex = chooseCapacityIndex(cap, length());
+    uint32_t newCapacity = computeCapacity(newCapacityIndex, length());
+
+    MOZ_ASSERT(oldCapacity < cap);
+    MOZ_ASSERT(cap <= newCapacity);
+
+    // The allocation size computation below cannot have integer overflows.
+    JS_STATIC_ASSERT(MaximumCapacity < UINT32_MAX / sizeof(double));
+
+    uint8_t* newElements;
+    if (hasInlineElements()) {
+        newElements = AllocateObjectBuffer<uint8_t>(cx, this, newCapacity * elementSize());
+        if (!newElements)
+            return false;
+        js_memcpy(newElements, elements(), initializedLength() * elementSize());
+    } else {
+        newElements = ReallocateObjectBuffer<uint8_t>(cx, this, elements(),
+                                                      oldCapacity * elementSize(),
+                                                      newCapacity * elementSize());
+        if (!newElements)
+            return false;
+    }
+
+    elements_ = newElements;
+    setCapacityIndex(newCapacityIndex);
+
+    return true;
+}
+
+void
+UnboxedArrayObject::shrinkElements(ExclusiveContext* cx, size_t cap)
+{
+    if (hasInlineElements())
+        return;
+
+    uint32_t oldCapacity = capacity();
+    uint32_t newCapacityIndex = chooseCapacityIndex(cap, 0);
+    uint32_t newCapacity = computeCapacity(newCapacityIndex, 0);
+
+    MOZ_ASSERT(cap < oldCapacity);
+    MOZ_ASSERT(cap <= newCapacity);
+
+    if (newCapacity >= oldCapacity)
+        return;
+
+    uint8_t* newElements = ReallocateObjectBuffer<uint8_t>(cx, this, elements(),
+                                                           oldCapacity * elementSize(),
+                                                           newCapacity * elementSize());
+    if (!newElements)
+        return;
+
+    elements_ = newElements;
+    setCapacityIndex(newCapacityIndex);
+}
+
+bool
+UnboxedArrayObject::containsProperty(ExclusiveContext* cx, jsid id)
+{
+    if (JSID_IS_INT(id) && uint32_t(JSID_TO_INT(id)) < initializedLength())
+        return true;
+    if (JSID_IS_ATOM(id) && JSID_TO_ATOM(id) == cx->names().length)
+        return true;
+    return false;
+}
+
+/* static */ bool
+UnboxedArrayObject::obj_lookupProperty(JSContext* cx, HandleObject obj,
+                                       HandleId id, MutableHandleObject objp,
+                                       MutableHandleShape propp)
+{
+    if (obj->as<UnboxedArrayObject>().containsProperty(cx, id)) {
+        MarkNonNativePropertyFound<CanGC>(propp);
+        objp.set(obj);
+        return true;
+    }
+
+    RootedObject proto(cx, obj->staticPrototype());
+    if (!proto) {
+        objp.set(nullptr);
+        propp.set(nullptr);
+        return true;
+    }
+
+    return LookupProperty(cx, proto, id, objp, propp);
+}
+
+/* static */ bool
+UnboxedArrayObject::obj_defineProperty(JSContext* cx, HandleObject obj, HandleId id,
+                                       Handle<PropertyDescriptor> desc,
+                                       ObjectOpResult& result)
+{
+    if (JSID_IS_INT(id) && !desc.getter() && !desc.setter() && desc.attributes() == JSPROP_ENUMERATE) {
+        UnboxedArrayObject* nobj = &obj->as<UnboxedArrayObject>();
+
+        uint32_t index = JSID_TO_INT(id);
+        if (index < nobj->initializedLength()) {
+            if (nobj->setElement(cx, index, desc.value()))
+                return result.succeed();
+        } else if (index == nobj->initializedLength() && index < MaximumCapacity) {
+            if (nobj->initializedLength() == nobj->capacity()) {
+                if (!nobj->growElements(cx, index + 1))
+                    return false;
+            }
+            nobj->setInitializedLength(index + 1);
+            if (nobj->initElement(cx, index, desc.value())) {
+                if (nobj->length() <= index)
+                    nobj->setLengthInt32(index + 1);
+                return result.succeed();
+            }
+            nobj->setInitializedLengthNoBarrier(index);
+        }
+    }
+
+    if (!convertToNative(cx, obj))
+        return false;
+
+    return DefineProperty(cx, obj, id, desc, result);
+}
+
+/* static */ bool
+UnboxedArrayObject::obj_hasProperty(JSContext* cx, HandleObject obj, HandleId id, bool* foundp)
+{
+    if (obj->as<UnboxedArrayObject>().containsProperty(cx, id)) {
+        *foundp = true;
+        return true;
+    }
+
+    RootedObject proto(cx, obj->staticPrototype());
+    if (!proto) {
+        *foundp = false;
+        return true;
+    }
+
+    return HasProperty(cx, proto, id, foundp);
+}
+
+/* static */ bool
+UnboxedArrayObject::obj_getProperty(JSContext* cx, HandleObject obj, HandleValue receiver,
+                                    HandleId id, MutableHandleValue vp)
+{
+    if (obj->as<UnboxedArrayObject>().containsProperty(cx, id)) {
+        if (JSID_IS_INT(id))
+            vp.set(obj->as<UnboxedArrayObject>().getElement(JSID_TO_INT(id)));
+        else
+            vp.set(Int32Value(obj->as<UnboxedArrayObject>().length()));
+        return true;
+    }
+
+    RootedObject proto(cx, obj->staticPrototype());
+    if (!proto) {
+        vp.setUndefined();
+        return true;
+    }
+
+    return GetProperty(cx, proto, receiver, id, vp);
+}
+
+/* static */ bool
+UnboxedArrayObject::obj_setProperty(JSContext* cx, HandleObject obj, HandleId id, HandleValue v,
+                                    HandleValue receiver, ObjectOpResult& result)
+{
+    if (obj->as<UnboxedArrayObject>().containsProperty(cx, id)) {
+        if (receiver.isObject() && obj == &receiver.toObject()) {
+            if (JSID_IS_INT(id)) {
+                if (obj->as<UnboxedArrayObject>().setElement(cx, JSID_TO_INT(id), v))
+                    return result.succeed();
+            } else {
+                uint32_t len;
+                if (!CanonicalizeArrayLengthValue(cx, v, &len))
+                    return false;
+                UnboxedArrayObject* nobj = &obj->as<UnboxedArrayObject>();
+                if (len < nobj->initializedLength()) {
+                    nobj->setInitializedLength(len);
+                    nobj->shrinkElements(cx, len);
+                }
+                nobj->setLength(cx, len);
+                return result.succeed();
+            }
+
+            if (!convertToNative(cx, obj))
+                return false;
+            return SetProperty(cx, obj, id, v, receiver, result);
+        }
+
+        return SetPropertyByDefining(cx, id, v, receiver, result);
+    }
+
+    return SetPropertyOnProto(cx, obj, id, v, receiver, result);
+}
+
+/* static */ bool
+UnboxedArrayObject::obj_getOwnPropertyDescriptor(JSContext* cx, HandleObject obj, HandleId id,
+                                                 MutableHandle<PropertyDescriptor> desc)
+{
+    if (obj->as<UnboxedArrayObject>().containsProperty(cx, id)) {
+        if (JSID_IS_INT(id)) {
+            desc.value().set(obj->as<UnboxedArrayObject>().getElement(JSID_TO_INT(id)));
+            desc.setAttributes(JSPROP_ENUMERATE);
+        } else {
+            desc.value().set(Int32Value(obj->as<UnboxedArrayObject>().length()));
+            desc.setAttributes(JSPROP_PERMANENT);
+        }
+        desc.object().set(obj);
+        return true;
+    }
+
+    desc.object().set(nullptr);
+    return true;
+}
+
+/* static */ bool
+UnboxedArrayObject::obj_deleteProperty(JSContext* cx, HandleObject obj, HandleId id,
+                                       ObjectOpResult& result)
+{
+    if (obj->as<UnboxedArrayObject>().containsProperty(cx, id)) {
+        size_t initlen = obj->as<UnboxedArrayObject>().initializedLength();
+        if (JSID_IS_INT(id) && JSID_TO_INT(id) == int32_t(initlen - 1)) {
+            obj->as<UnboxedArrayObject>().setInitializedLength(initlen - 1);
+            obj->as<UnboxedArrayObject>().shrinkElements(cx, initlen - 1);
+            return result.succeed();
+        }
+    }
+
+    if (!convertToNative(cx, obj))
+        return false;
+    return DeleteProperty(cx, obj, id, result);
+}
+
+/* static */ bool
+UnboxedArrayObject::obj_enumerate(JSContext* cx, HandleObject obj, AutoIdVector& properties,
+                                  bool enumerableOnly)
+{
+    for (size_t i = 0; i < obj->as<UnboxedArrayObject>().initializedLength(); i++) {
+        if (!properties.append(INT_TO_JSID(i)))
+            return false;
+    }
+
+    if (!enumerableOnly && !properties.append(NameToId(cx->names().length)))
+        return false;
+
+    return true;
+}
+
+static const ClassOps UnboxedArrayObjectClassOps = {
+    nullptr,        /* addProperty */
+    nullptr,        /* delProperty */
+    nullptr,        /* getProperty */
+    nullptr,        /* setProperty */
+    nullptr,        /* enumerate   */
+    nullptr,        /* resolve     */
+    nullptr,        /* mayResolve  */
+    UnboxedArrayObject::finalize,
+    nullptr,        /* call        */
+    nullptr,        /* hasInstance */
+    nullptr,        /* construct   */
+    UnboxedArrayObject::trace,
+};
+
+static const ClassExtension UnboxedArrayObjectClassExtension = {
+    nullptr,    /* weakmapKeyDelegateOp */
+    UnboxedArrayObject::objectMoved
+};
+
+static const ObjectOps UnboxedArrayObjectObjectOps = {
+    UnboxedArrayObject::obj_lookupProperty,
+    UnboxedArrayObject::obj_defineProperty,
+    UnboxedArrayObject::obj_hasProperty,
+    UnboxedArrayObject::obj_getProperty,
+    UnboxedArrayObject::obj_setProperty,
+    UnboxedArrayObject::obj_getOwnPropertyDescriptor,
+    UnboxedArrayObject::obj_deleteProperty,
+    nullptr,   /* getElements */
+    UnboxedArrayObject::obj_enumerate,
+    nullptr    /* funToString */
+};
+
+const Class UnboxedArrayObject::class_ = {
+    "Array",
+    Class::NON_NATIVE |
+    JSCLASS_SKIP_NURSERY_FINALIZE |
+    JSCLASS_BACKGROUND_FINALIZE,
+    &UnboxedArrayObjectClassOps,
+    JS_NULL_CLASS_SPEC,
+    &UnboxedArrayObjectClassExtension,
+    &UnboxedArrayObjectObjectOps
+};
+
+/////////////////////////////////////////////////////////////////////
+// API
+/////////////////////////////////////////////////////////////////////
+
+static bool
+UnboxedTypeIncludes(JSValueType supertype, JSValueType subtype)
+{
+    if (supertype == JSVAL_TYPE_DOUBLE && subtype == JSVAL_TYPE_INT32)
+        return true;
+    if (supertype == JSVAL_TYPE_OBJECT && subtype == JSVAL_TYPE_NULL)
+        return true;
+    return false;
+}
+
+static bool
+CombineUnboxedTypes(const Value& value, JSValueType* existing)
+{
+    JSValueType type = value.isDouble() ? JSVAL_TYPE_DOUBLE : value.extractNonDoubleType();
+
+    if (*existing == JSVAL_TYPE_MAGIC || *existing == type || UnboxedTypeIncludes(type, *existing)) {
+        *existing = type;
+        return true;
+    }
+    if (UnboxedTypeIncludes(*existing, type))
+        return true;
+    return false;
+}
+
+// Return whether the property names and types in layout are a subset of the
+// specified vector.
+static bool
+PropertiesAreSuperset(const UnboxedLayout::PropertyVector& properties, UnboxedLayout* layout)
+{
+    for (size_t i = 0; i < layout->properties().length(); i++) {
+        const UnboxedLayout::Property& layoutProperty = layout->properties()[i];
+        bool found = false;
+        for (size_t j = 0; j < properties.length(); j++) {
+            if (layoutProperty.name == properties[j].name) {
+                found = (layoutProperty.type == properties[j].type);
+                break;
+            }
+        }
+        if (!found)
+            return false;
+    }
+    return true;
+}
+
+static bool
+CombinePlainObjectProperties(PlainObject* obj, Shape* templateShape,
+                             UnboxedLayout::PropertyVector& properties)
+{
+    // All preliminary objects must have been created with enough space to
+    // fill in their unboxed data inline. This is ensured either by using
+    // the largest allocation kind (which limits the maximum size of an
+    // unboxed object), or by using an allocation kind that covers all
+    // properties in the template, as the space used by unboxed properties
+    // is less than or equal to that used by boxed properties.
+    MOZ_ASSERT(gc::GetGCKindSlots(obj->asTenured().getAllocKind()) >=
+               Min(NativeObject::MAX_FIXED_SLOTS, templateShape->slotSpan()));
+
+    if (obj->lastProperty() != templateShape || obj->hasDynamicElements()) {
+        // Only use an unboxed representation if all created objects match
+        // the template shape exactly.
+        return false;
+    }
+
+    for (size_t i = 0; i < templateShape->slotSpan(); i++) {
+        Value val = obj->getSlot(i);
+
+        JSValueType& existing = properties[i].type;
+        if (!CombineUnboxedTypes(val, &existing))
+            return false;
+    }
+
+    return true;
+}
+
+static bool
+CombineArrayObjectElements(ExclusiveContext* cx, ArrayObject* obj, JSValueType* elementType)
+{
+    if (obj->inDictionaryMode() ||
+        obj->lastProperty()->propid() != AtomToId(cx->names().length) ||
+        !obj->lastProperty()->previous()->isEmptyShape())
+    {
+        // Only use an unboxed representation if the object has no properties.
+        return false;
+    }
+
+    for (size_t i = 0; i < obj->getDenseInitializedLength(); i++) {
+        Value val = obj->getDenseElement(i);
+
+        // For now, unboxed arrays cannot have holes.
+        if (val.isMagic(JS_ELEMENTS_HOLE))
+            return false;
+
+        if (!CombineUnboxedTypes(val, elementType))
+            return false;
+    }
+
+    return true;
+}
+
+static size_t
+ComputePlainObjectLayout(ExclusiveContext* cx, Shape* templateShape,
+                         UnboxedLayout::PropertyVector& properties)
+{
+    // Fill in the names for all the object's properties.
+    for (Shape::Range<NoGC> r(templateShape); !r.empty(); r.popFront()) {
+        size_t slot = r.front().slot();
+        MOZ_ASSERT(!properties[slot].name);
+        properties[slot].name = JSID_TO_ATOM(r.front().propid())->asPropertyName();
+    }
+
+    // Fill in all the unboxed object's property offsets.
+    uint32_t offset = 0;
+
+    // Search for an existing unboxed layout which is a subset of this one.
+    // If there are multiple such layouts, use the largest one. If we're able
+    // to find such a layout, use the same property offsets for the shared
+    // properties, which will allow us to generate better code if the objects
+    // have a subtype/supertype relation and are accessed at common sites.
+    UnboxedLayout* bestExisting = nullptr;
+    for (UnboxedLayout* existing : cx->compartment()->unboxedLayouts) {
+        if (PropertiesAreSuperset(properties, existing)) {
+            if (!bestExisting ||
+                existing->properties().length() > bestExisting->properties().length())
+            {
+                bestExisting = existing;
+            }
+        }
+    }
+    if (bestExisting) {
+        for (size_t i = 0; i < bestExisting->properties().length(); i++) {
+            const UnboxedLayout::Property& existingProperty = bestExisting->properties()[i];
+            for (size_t j = 0; j < templateShape->slotSpan(); j++) {
+                if (existingProperty.name == properties[j].name) {
+                    MOZ_ASSERT(existingProperty.type == properties[j].type);
+                    properties[j].offset = existingProperty.offset;
+                }
+            }
+        }
+        offset = bestExisting->size();
+    }
+
+    // Order remaining properties from the largest down for the best space
+    // utilization.
+    static const size_t typeSizes[] = { 8, 4, 1 };
+
+    for (size_t i = 0; i < ArrayLength(typeSizes); i++) {
+        size_t size = typeSizes[i];
+        for (size_t j = 0; j < templateShape->slotSpan(); j++) {
+            if (properties[j].offset != UINT32_MAX)
+                continue;
+            JSValueType type = properties[j].type;
+            if (UnboxedTypeSize(type) == size) {
+                offset = JS_ROUNDUP(offset, size);
+                properties[j].offset = offset;
+                offset += size;
+            }
+        }
+    }
+
+    // The final offset is the amount of data needed by the object.
+    return offset;
+}
+
+static bool
+SetLayoutTraceList(ExclusiveContext* cx, UnboxedLayout* layout)
+{
+    // Figure out the offsets of any objects or string properties.
+    Vector<int32_t, 8, SystemAllocPolicy> objectOffsets, stringOffsets;
+    for (size_t i = 0; i < layout->properties().length(); i++) {
+        const UnboxedLayout::Property& property = layout->properties()[i];
+        MOZ_ASSERT(property.offset != UINT32_MAX);
+        if (property.type == JSVAL_TYPE_OBJECT) {
+            if (!objectOffsets.append(property.offset))
+                return false;
+        } else if (property.type == JSVAL_TYPE_STRING) {
+            if (!stringOffsets.append(property.offset))
+                return false;
+        }
+    }
+
+    // Construct the layout's trace list.
+    if (!objectOffsets.empty() || !stringOffsets.empty()) {
+        Vector<int32_t, 8, SystemAllocPolicy> entries;
+        if (!entries.appendAll(stringOffsets) ||
+            !entries.append(-1) ||
+            !entries.appendAll(objectOffsets) ||
+            !entries.append(-1) ||
+            !entries.append(-1))
+        {
+            return false;
+        }
+        int32_t* traceList = cx->zone()->pod_malloc<int32_t>(entries.length());
+        if (!traceList)
+            return false;
+        PodCopy(traceList, entries.begin(), entries.length());
+        layout->setTraceList(traceList);
+    }
+
+    return true;
+}
+
+static inline Value
+NextValue(Handle<GCVector<Value>> values, size_t* valueCursor)
+{
+    return values[(*valueCursor)++];
+}
+
+static bool
+GetValuesFromPreliminaryArrayObject(ArrayObject* obj, MutableHandle<GCVector<Value>> values)
+{
+    if (!values.append(Int32Value(obj->length())))
+        return false;
+    if (!values.append(Int32Value(obj->getDenseInitializedLength())))
+        return false;
+    for (size_t i = 0; i < obj->getDenseInitializedLength(); i++) {
+        if (!values.append(obj->getDenseElement(i)))
+            return false;
+    }
+    return true;
+}
+
+void
+UnboxedArrayObject::fillAfterConvert(ExclusiveContext* cx,
+                                     Handle<GCVector<Value>> values, size_t* valueCursor)
+{
+    MOZ_ASSERT(CapacityArray[1] == 0);
+    setCapacityIndex(1);
+    setInitializedLengthNoBarrier(0);
+    setInlineElements();
+
+    setLength(cx, NextValue(values, valueCursor).toInt32());
+
+    int32_t initlen = NextValue(values, valueCursor).toInt32();
+    if (!initlen)
+        return;
+
+    AutoEnterOOMUnsafeRegion oomUnsafe;
+    if (!growElements(cx, initlen))
+        oomUnsafe.crash("UnboxedArrayObject::fillAfterConvert");
+
+    setInitializedLength(initlen);
+
+    for (size_t i = 0; i < size_t(initlen); i++)
+        JS_ALWAYS_TRUE(initElement(cx, i, NextValue(values, valueCursor)));
+}
+
+static bool
+GetValuesFromPreliminaryPlainObject(PlainObject* obj, MutableHandle<GCVector<Value>> values)
+{
+    for (size_t i = 0; i < obj->slotSpan(); i++) {
+        if (!values.append(obj->getSlot(i)))
+            return false;
+    }
+    return true;
+}
+
+void
+UnboxedPlainObject::fillAfterConvert(ExclusiveContext* cx,
+                                     Handle<GCVector<Value>> values, size_t* valueCursor)
+{
+    initExpando();
+    memset(data(), 0, layout().size());
+    for (size_t i = 0; i < layout().properties().length(); i++)
+        JS_ALWAYS_TRUE(setValue(cx, layout().properties()[i], NextValue(values, valueCursor)));
+}
+
+bool
+js::TryConvertToUnboxedLayout(ExclusiveContext* cx, AutoEnterAnalysis& enter, Shape* templateShape,
+                              ObjectGroup* group, PreliminaryObjectArray* objects)
+{
+    bool isArray = !templateShape;
+
+    // Unboxed arrays are nightly only for now. The getenv() call will be
+    // removed when they are on by default. See bug 1153266.
+    if (isArray) {
+#ifdef NIGHTLY_BUILD
+        if (!getenv("JS_OPTION_USE_UNBOXED_ARRAYS")) {
+            if (!cx->options().unboxedArrays())
+                return true;
+        }
+#else
+        return true;
+#endif
+    } else {
+        if (jit::JitOptions.disableUnboxedObjects)
+            return true;
+    }
+
+    MOZ_ASSERT_IF(templateShape, !templateShape->getObjectFlags());
+
+    if (group->runtimeFromAnyThread()->isSelfHostingGlobal(cx->global()))
+        return true;
+
+    if (!isArray && templateShape->slotSpan() == 0)
+        return true;
+
+    UnboxedLayout::PropertyVector properties;
+    if (!isArray) {
+        if (!properties.appendN(UnboxedLayout::Property(), templateShape->slotSpan()))
+            return false;
+    }
+    JSValueType elementType = JSVAL_TYPE_MAGIC;
+
+    size_t objectCount = 0;
+    for (size_t i = 0; i < PreliminaryObjectArray::COUNT; i++) {
+        JSObject* obj = objects->get(i);
+        if (!obj)
+            continue;
+
+        if (obj->isSingleton() || obj->group() != group)
+            return true;
+
+        objectCount++;
+
+        if (isArray) {
+            if (!CombineArrayObjectElements(cx, &obj->as<ArrayObject>(), &elementType))
+                return true;
+        } else {
+            if (!CombinePlainObjectProperties(&obj->as<PlainObject>(), templateShape, properties))
+                return true;
+        }
+    }
+
+    size_t layoutSize = 0;
+    if (isArray) {
+        // Don't use an unboxed representation if we couldn't determine an
+        // element type for the objects.
+        if (UnboxedTypeSize(elementType) == 0)
+            return true;
+    } else {
+        if (objectCount <= 1) {
+            // If only one of the objects has been created, it is more likely
+            // to have new properties added later. This heuristic is not used
+            // for array objects, where we might want an unboxed representation
+            // even if there is only one large array.
+            return true;
+        }
+
+        for (size_t i = 0; i < templateShape->slotSpan(); i++) {
+            // We can't use an unboxed representation if e.g. all the objects have
+            // a null value for one of the properties, as we can't decide what type
+            // it is supposed to have.
+            if (UnboxedTypeSize(properties[i].type) == 0)
+                return true;
+        }
+
+        // Make sure that all properties on the template shape are property
+        // names, and not indexes.
+        for (Shape::Range<NoGC> r(templateShape); !r.empty(); r.popFront()) {
+            jsid id = r.front().propid();
+            uint32_t dummy;
+            if (!JSID_IS_ATOM(id) || JSID_TO_ATOM(id)->isIndex(&dummy))
+                return true;
+        }
+
+        layoutSize = ComputePlainObjectLayout(cx, templateShape, properties);
+
+        // The entire object must be allocatable inline.
+        if (UnboxedPlainObject::offsetOfData() + layoutSize > JSObject::MAX_BYTE_SIZE)
+            return true;
+    }
+
+    UniquePtr<UnboxedLayout>& layout = enter.unboxedLayoutToCleanUp;
+    MOZ_ASSERT(!layout);
+    layout = group->zone()->make_unique<UnboxedLayout>();
+    if (!layout)
+        return false;
+
+    if (isArray) {
+        layout->initArray(elementType);
+    } else {
+        if (!layout->initProperties(properties, layoutSize))
+            return false;
+
+        // The unboxedLayouts list only tracks layouts for plain objects.
+        cx->compartment()->unboxedLayouts.insertFront(layout.get());
+
+        if (!SetLayoutTraceList(cx, layout.get()))
+            return false;
+    }
+
+    // We've determined that all the preliminary objects can use the new layout
+    // just constructed, so convert the existing group to use the unboxed class,
+    // and update the preliminary objects to use the new layout. Do the
+    // fallible stuff first before modifying any objects.
+
+    // Get an empty shape which we can use for the preliminary objects.
+    const Class* clasp = isArray ? &UnboxedArrayObject::class_ : &UnboxedPlainObject::class_;
+    Shape* newShape = EmptyShape::getInitialShape(cx, clasp, group->proto(), 0);
+    if (!newShape) {
+        cx->recoverFromOutOfMemory();
+        return false;
+    }
+
+    // Accumulate a list of all the values in each preliminary object, and
+    // update their shapes.
+    Rooted<GCVector<Value>> values(cx, GCVector<Value>(cx));
+    for (size_t i = 0; i < PreliminaryObjectArray::COUNT; i++) {
+        JSObject* obj = objects->get(i);
+        if (!obj)
+            continue;
+
+        bool ok;
+        if (isArray)
+            ok = GetValuesFromPreliminaryArrayObject(&obj->as<ArrayObject>(), &values);
+        else
+            ok = GetValuesFromPreliminaryPlainObject(&obj->as<PlainObject>(), &values);
+
+        if (!ok) {
+            cx->recoverFromOutOfMemory();
+            return false;
+        }
+    }
+
+    if (TypeNewScript* newScript = group->newScript())
+        layout->setNewScript(newScript);
+
+    for (size_t i = 0; i < PreliminaryObjectArray::COUNT; i++) {
+        if (JSObject* obj = objects->get(i))
+            obj->as<NativeObject>().setLastPropertyMakeNonNative(newShape);
+    }
+
+    group->setClasp(clasp);
+    group->setUnboxedLayout(layout.release());
+
+    size_t valueCursor = 0;
+    for (size_t i = 0; i < PreliminaryObjectArray::COUNT; i++) {
+        JSObject* obj = objects->get(i);
+        if (!obj)
+            continue;
+
+        if (isArray)
+            obj->as<UnboxedArrayObject>().fillAfterConvert(cx, values, &valueCursor);
+        else
+            obj->as<UnboxedPlainObject>().fillAfterConvert(cx, values, &valueCursor);
+    }
+
+    MOZ_ASSERT(valueCursor == values.length());
+    return true;
+}
+
+DefineBoxedOrUnboxedFunctor6(SetOrExtendBoxedOrUnboxedDenseElements,
+                             ExclusiveContext*, JSObject*, uint32_t, const Value*, uint32_t,
+                             ShouldUpdateTypes);
+
+DenseElementResult
+js::SetOrExtendAnyBoxedOrUnboxedDenseElements(ExclusiveContext* cx, JSObject* obj,
+                                              uint32_t start, const Value* vp, uint32_t count,
+                                              ShouldUpdateTypes updateTypes)
+{
+    SetOrExtendBoxedOrUnboxedDenseElementsFunctor functor(cx, obj, start, vp, count, updateTypes);
+    return CallBoxedOrUnboxedSpecialization(functor, obj);
+};
+
+DefineBoxedOrUnboxedFunctor5(MoveBoxedOrUnboxedDenseElements,
+                             JSContext*, JSObject*, uint32_t, uint32_t, uint32_t);
+
+DenseElementResult
+js::MoveAnyBoxedOrUnboxedDenseElements(JSContext* cx, JSObject* obj,
+                                       uint32_t dstStart, uint32_t srcStart, uint32_t length)
+{
+    MoveBoxedOrUnboxedDenseElementsFunctor functor(cx, obj, dstStart, srcStart, length);
+    return CallBoxedOrUnboxedSpecialization(functor, obj);
+}
+
+DefineBoxedOrUnboxedFunctorPair6(CopyBoxedOrUnboxedDenseElements,
+                                 JSContext*, JSObject*, JSObject*, uint32_t, uint32_t, uint32_t);
+
+DenseElementResult
+js::CopyAnyBoxedOrUnboxedDenseElements(JSContext* cx, JSObject* dst, JSObject* src,
+                                       uint32_t dstStart, uint32_t srcStart, uint32_t length)
+{
+    CopyBoxedOrUnboxedDenseElementsFunctor functor(cx, dst, src, dstStart, srcStart, length);
+    return CallBoxedOrUnboxedSpecialization(functor, dst, src);
+}
+
+DefineBoxedOrUnboxedFunctor3(SetBoxedOrUnboxedInitializedLength,
+                             JSContext*, JSObject*, size_t);
+
+void
+js::SetAnyBoxedOrUnboxedInitializedLength(JSContext* cx, JSObject* obj, size_t initlen)
+{
+    SetBoxedOrUnboxedInitializedLengthFunctor functor(cx, obj, initlen);
+    JS_ALWAYS_TRUE(CallBoxedOrUnboxedSpecialization(functor, obj) == DenseElementResult::Success);
+}
+
+DefineBoxedOrUnboxedFunctor3(EnsureBoxedOrUnboxedDenseElements,
+                             JSContext*, JSObject*, size_t);
+
+DenseElementResult
+js::EnsureAnyBoxedOrUnboxedDenseElements(JSContext* cx, JSObject* obj, size_t initlen)
+{
+    EnsureBoxedOrUnboxedDenseElementsFunctor functor(cx, obj, initlen);
+    return CallBoxedOrUnboxedSpecialization(functor, obj);
+}
diff --git a/js/src/vm/UnboxedObject.h b/js/src/vm/UnboxedObject.h
new file mode 100644
index 000000000..ecff8be5b
--- /dev/null
+++ b/js/src/vm/UnboxedObject.h
@@ -0,0 +1,531 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ * vim: set ts=8 sts=4 et sw=4 tw=99:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef vm_UnboxedObject_h
+#define vm_UnboxedObject_h
+
+#include "jsgc.h"
+#include "jsobj.h"
+
+#include "vm/Runtime.h"
+#include "vm/TypeInference.h"
+
+namespace js {
+
+// Memory required for an unboxed value of a given type. Returns zero for types
+// which can't be used for unboxed objects.
+static inline size_t
+UnboxedTypeSize(JSValueType type)
+{
+    switch (type) {
+      case JSVAL_TYPE_BOOLEAN: return 1;
+      case JSVAL_TYPE_INT32:   return 4;
+      case JSVAL_TYPE_DOUBLE:  return 8;
+      case JSVAL_TYPE_STRING:  return sizeof(void*);
+      case JSVAL_TYPE_OBJECT:  return sizeof(void*);
+      default:                 return 0;
+    }
+}
+
+static inline bool
+UnboxedTypeNeedsPreBarrier(JSValueType type)
+{
+    return type == JSVAL_TYPE_STRING || type == JSVAL_TYPE_OBJECT;
+}
+
+static inline bool
+UnboxedTypeNeedsPostBarrier(JSValueType type)
+{
+    return type == JSVAL_TYPE_OBJECT;
+}
+
+// Class tracking information specific to unboxed objects.
+class UnboxedLayout : public mozilla::LinkedListElement<UnboxedLayout>
+{
+  public:
+    struct Property {
+        PropertyName* name;
+        uint32_t offset;
+        JSValueType type;
+
+        Property()
+          : name(nullptr), offset(UINT32_MAX), type(JSVAL_TYPE_MAGIC)
+        {}
+    };
+
+    typedef Vector<Property, 0, SystemAllocPolicy> PropertyVector;
+
+  private:
+    // If objects in this group have ever been converted to native objects,
+    // these store the corresponding native group and initial shape for such
+    // objects. Type information for this object is reflected in nativeGroup.
+    GCPtrObjectGroup nativeGroup_;
+    GCPtrShape nativeShape_;
+
+    // Any script/pc which the associated group is created for.
+    GCPtrScript allocationScript_;
+    jsbytecode* allocationPc_;
+
+    // If nativeGroup is set and this object originally had a TypeNewScript or
+    // was keyed to an allocation site, this points to the group which replaced
+    // this one. This link is only needed to keep the replacement group from
+    // being GC'ed. If it were GC'ed and a new one regenerated later, that new
+    // group might have a different allocation kind from this group.
+    GCPtrObjectGroup replacementGroup_;
+
+    // The following members are only used for unboxed plain objects.
+
+    // All properties on objects with this layout, in enumeration order.
+    PropertyVector properties_;
+
+    // Byte size of the data for objects with this layout.
+    size_t size_;
+
+    // Any 'new' script information associated with this layout.
+    TypeNewScript* newScript_;
+
+    // List for use in tracing objects with this layout. This has the same
+    // structure as the trace list on a TypeDescr.
+    int32_t* traceList_;
+
+    // If this layout has been used to construct script or JSON constant
+    // objects, this code might be filled in to more quickly fill in objects
+    // from an array of values.
+    GCPtrJitCode constructorCode_;
+
+    // The following members are only used for unboxed arrays.
+
+    // The type of array elements.
+    JSValueType elementType_;
+
+  public:
+    UnboxedLayout()
+      : nativeGroup_(nullptr), nativeShape_(nullptr),
+        allocationScript_(nullptr), allocationPc_(nullptr), replacementGroup_(nullptr),
+        size_(0), newScript_(nullptr), traceList_(nullptr), constructorCode_(nullptr),
+        elementType_(JSVAL_TYPE_MAGIC)
+    {}
+
+    bool initProperties(const PropertyVector& properties, size_t size) {
+        size_ = size;
+        return properties_.appendAll(properties);
+    }
+
+    void initArray(JSValueType elementType) {
+        elementType_ = elementType;
+    }
+
+    ~UnboxedLayout() {
+        if (newScript_)
+            newScript_->clear();
+        js_delete(newScript_);
+        js_free(traceList_);
+
+        nativeGroup_.init(nullptr);
+        nativeShape_.init(nullptr);
+        replacementGroup_.init(nullptr);
+        constructorCode_.init(nullptr);
+    }
+
+    bool isArray() const {
+        return elementType_ != JSVAL_TYPE_MAGIC;
+    }
+
+    void detachFromCompartment();
+
+    const PropertyVector& properties() const {
+        return properties_;
+    }
+
+    TypeNewScript* newScript() const {
+        return newScript_;
+    }
+
+    void setNewScript(TypeNewScript* newScript, bool writeBarrier = true);
+
+    JSScript* allocationScript() const {
+        return allocationScript_;
+    }
+
+    jsbytecode* allocationPc() const {
+        return allocationPc_;
+    }
+
+    void setAllocationSite(JSScript* script, jsbytecode* pc) {
+        allocationScript_ = script;
+        allocationPc_ = pc;
+    }
+
+    const int32_t* traceList() const {
+        return traceList_;
+    }
+
+    void setTraceList(int32_t* traceList) {
+        traceList_ = traceList;
+    }
+
+    const Property* lookup(JSAtom* atom) const {
+        for (size_t i = 0; i < properties_.length(); i++) {
+            if (properties_[i].name == atom)
+                return &properties_[i];
+        }
+        return nullptr;
+    }
+
+    const Property* lookup(jsid id) const {
+        if (JSID_IS_STRING(id))
+            return lookup(JSID_TO_ATOM(id));
+        return nullptr;
+    }
+
+    size_t size() const {
+        return size_;
+    }
+
+    ObjectGroup* nativeGroup() const {
+        return nativeGroup_;
+    }
+
+    Shape* nativeShape() const {
+        return nativeShape_;
+    }
+
+    jit::JitCode* constructorCode() const {
+        return constructorCode_;
+    }
+
+    void setConstructorCode(jit::JitCode* code) {
+        constructorCode_ = code;
+    }
+
+    JSValueType elementType() const {
+        return elementType_;
+    }
+
+    inline gc::AllocKind getAllocKind() const;
+
+    void trace(JSTracer* trc);
+
+    size_t sizeOfIncludingThis(mozilla::MallocSizeOf mallocSizeOf);
+
+    static bool makeNativeGroup(JSContext* cx, ObjectGroup* group);
+    static bool makeConstructorCode(JSContext* cx, HandleObjectGroup group);
+};
+
+// Class for expando objects holding extra properties given to an unboxed plain
+// object. These objects behave identically to normal native plain objects, and
+// have a separate Class to distinguish them for memory usage reporting.
+class UnboxedExpandoObject : public NativeObject
+{
+  public:
+    static const Class class_;
+};
+
+// Class for a plain object using an unboxed representation. The physical
+// layout of these objects is identical to that of an InlineTypedObject, though
+// these objects use an UnboxedLayout instead of a TypeDescr to keep track of
+// how their properties are stored.
+class UnboxedPlainObject : public JSObject
+{
+    // Optional object which stores extra properties on this object. This is
+    // not automatically barriered to avoid problems if the object is converted
+    // to a native. See ensureExpando().
+    UnboxedExpandoObject* expando_;
+
+    // Start of the inline data, which immediately follows the group and extra properties.
+    uint8_t data_[1];
+
+  public:
+    static const Class class_;
+
+    static bool obj_lookupProperty(JSContext* cx, HandleObject obj,
+                                   HandleId id, MutableHandleObject objp,
+                                   MutableHandleShape propp);
+
+    static bool obj_defineProperty(JSContext* cx, HandleObject obj, HandleId id,
+                                   Handle<PropertyDescriptor> desc,
+                                   ObjectOpResult& result);
+
+    static bool obj_hasProperty(JSContext* cx, HandleObject obj, HandleId id, bool* foundp);
+
+    static bool obj_getProperty(JSContext* cx, HandleObject obj, HandleValue receiver,
+                                HandleId id, MutableHandleValue vp);
+
+    static bool obj_setProperty(JSContext* cx, HandleObject obj, HandleId id, HandleValue v,
+                                HandleValue receiver, ObjectOpResult& result);
+
+    static bool obj_getOwnPropertyDescriptor(JSContext* cx, HandleObject obj, HandleId id,
+                                             MutableHandle<PropertyDescriptor> desc);
+
+    static bool obj_deleteProperty(JSContext* cx, HandleObject obj, HandleId id,
+                                   ObjectOpResult& result);
+
+    static bool obj_enumerate(JSContext* cx, HandleObject obj, AutoIdVector& properties,
+                              bool enumerableOnly);
+    static bool obj_watch(JSContext* cx, HandleObject obj, HandleId id, HandleObject callable);
+
+    inline const UnboxedLayout& layout() const;
+
+    const UnboxedLayout& layoutDontCheckGeneration() const {
+        return group()->unboxedLayoutDontCheckGeneration();
+    }
+
+    uint8_t* data() {
+        return &data_[0];
+    }
+
+    UnboxedExpandoObject* maybeExpando() const {
+        return expando_;
+    }
+
+    void initExpando() {
+        expando_ = nullptr;
+    }
+
+    // For use during GC.
+    JSObject** addressOfExpando() {
+        return reinterpret_cast<JSObject**>(&expando_);
+    }
+
+    bool containsUnboxedOrExpandoProperty(ExclusiveContext* cx, jsid id) const;
+
+    static UnboxedExpandoObject* ensureExpando(JSContext* cx, Handle<UnboxedPlainObject*> obj);
+
+    bool setValue(ExclusiveContext* cx, const UnboxedLayout::Property& property, const Value& v);
+    Value getValue(const UnboxedLayout::Property& property, bool maybeUninitialized = false);
+
+    static bool convertToNative(JSContext* cx, JSObject* obj);
+    static UnboxedPlainObject* create(ExclusiveContext* cx, HandleObjectGroup group,
+                                      NewObjectKind newKind);
+    static JSObject* createWithProperties(ExclusiveContext* cx, HandleObjectGroup group,
+                                          NewObjectKind newKind, IdValuePair* properties);
+
+    void fillAfterConvert(ExclusiveContext* cx,
+                          Handle<GCVector<Value>> values, size_t* valueCursor);
+
+    static void trace(JSTracer* trc, JSObject* object);
+
+    static size_t offsetOfExpando() {
+        return offsetof(UnboxedPlainObject, expando_);
+    }
+
+    static size_t offsetOfData() {
+        return offsetof(UnboxedPlainObject, data_[0]);
+    }
+};
+
+// Try to construct an UnboxedLayout for each of the preliminary objects,
+// provided they all match the template shape. If successful, converts the
+// preliminary objects and their group to the new unboxed representation.
+bool
+TryConvertToUnboxedLayout(ExclusiveContext* cx, AutoEnterAnalysis& enter, Shape* templateShape,
+                          ObjectGroup* group, PreliminaryObjectArray* objects);
+
+inline gc::AllocKind
+UnboxedLayout::getAllocKind() const
+{
+    MOZ_ASSERT(size());
+    return gc::GetGCObjectKindForBytes(UnboxedPlainObject::offsetOfData() + size());
+}
+
+// Class for an array object using an unboxed representation.
+class UnboxedArrayObject : public JSObject
+{
+    // Elements pointer for the object.
+    uint8_t* elements_;
+
+    // The nominal array length. This always fits in an int32_t.
+    uint32_t length_;
+
+    // Value indicating the allocated capacity and initialized length of the
+    // array. The top CapacityBits bits are an index into CapacityArray, which
+    // indicates the elements capacity. The low InitializedLengthBits store the
+    // initialized length of the array.
+    uint32_t capacityIndexAndInitializedLength_;
+
+    // If the elements are inline, they will point here.
+    uint8_t inlineElements_[1];
+
+  public:
+    static const uint32_t CapacityBits = 6;
+    static const uint32_t CapacityShift = 26;
+
+    static const uint32_t CapacityMask = uint32_t(-1) << CapacityShift;
+    static const uint32_t InitializedLengthMask = (1 << CapacityShift) - 1;
+
+    static const uint32_t MaximumCapacity = InitializedLengthMask;
+    static const uint32_t MinimumDynamicCapacity = 8;
+
+    static const uint32_t CapacityArray[];
+
+    // Capacity index which indicates the array's length is also its capacity.
+    static const uint32_t CapacityMatchesLengthIndex = 0;
+
+  private:
+    static inline uint32_t computeCapacity(uint32_t index, uint32_t length) {
+        if (index == CapacityMatchesLengthIndex)
+            return length;
+        return CapacityArray[index];
+    }
+
+    static uint32_t chooseCapacityIndex(uint32_t capacity, uint32_t length);
+    static uint32_t exactCapacityIndex(uint32_t capacity);
+
+  public:
+    static const Class class_;
+
+    static bool obj_lookupProperty(JSContext* cx, HandleObject obj,
+                                   HandleId id, MutableHandleObject objp,
+                                   MutableHandleShape propp);
+
+    static bool obj_defineProperty(JSContext* cx, HandleObject obj, HandleId id,
+                                   Handle<PropertyDescriptor> desc,
+                                   ObjectOpResult& result);
+
+    static bool obj_hasProperty(JSContext* cx, HandleObject obj, HandleId id, bool* foundp);
+
+    static bool obj_getProperty(JSContext* cx, HandleObject obj, HandleValue receiver,
+                                HandleId id, MutableHandleValue vp);
+
+    static bool obj_setProperty(JSContext* cx, HandleObject obj, HandleId id, HandleValue v,
+                                HandleValue receiver, ObjectOpResult& result);
+
+    static bool obj_getOwnPropertyDescriptor(JSContext* cx, HandleObject obj, HandleId id,
+                                             MutableHandle<PropertyDescriptor> desc);
+
+    static bool obj_deleteProperty(JSContext* cx, HandleObject obj, HandleId id,
+                                   ObjectOpResult& result);
+
+    static bool obj_enumerate(JSContext* cx, HandleObject obj, AutoIdVector& properties,
+                              bool enumerableOnly);
+    static bool obj_watch(JSContext* cx, HandleObject obj, HandleId id, HandleObject callable);
+
+    inline const UnboxedLayout& layout() const;
+
+    const UnboxedLayout& layoutDontCheckGeneration() const {
+        return group()->unboxedLayoutDontCheckGeneration();
+    }
+
+    JSValueType elementType() const {
+        return layoutDontCheckGeneration().elementType();
+    }
+
+    uint32_t elementSize() const {
+        return UnboxedTypeSize(elementType());
+    }
+
+    static bool convertToNative(JSContext* cx, JSObject* obj);
+    static UnboxedArrayObject* create(ExclusiveContext* cx, HandleObjectGroup group,
+                                      uint32_t length, NewObjectKind newKind,
+                                      uint32_t maxLength = MaximumCapacity);
+
+    static bool convertToNativeWithGroup(ExclusiveContext* cx, JSObject* obj,
+                                         ObjectGroup* group, Shape* shape);
+    bool convertInt32ToDouble(ExclusiveContext* cx, ObjectGroup* group);
+
+    void fillAfterConvert(ExclusiveContext* cx,
+                          Handle<GCVector<Value>> values, size_t* valueCursor);
+
+    static void trace(JSTracer* trc, JSObject* object);
+    static void objectMoved(JSObject* obj, const JSObject* old);
+    static void finalize(FreeOp* fop, JSObject* obj);
+
+    static size_t objectMovedDuringMinorGC(JSTracer* trc, JSObject* dst, JSObject* src,
+                                           gc::AllocKind allocKind);
+
+    uint8_t* elements() {
+        return elements_;
+    }
+
+    bool hasInlineElements() const {
+        return elements_ == &inlineElements_[0];
+    }
+
+    uint32_t length() const {
+        return length_;
+    }
+
+    uint32_t initializedLength() const {
+        return capacityIndexAndInitializedLength_ & InitializedLengthMask;
+    }
+
+    uint32_t capacityIndex() const {
+        return (capacityIndexAndInitializedLength_ & CapacityMask) >> CapacityShift;
+    }
+
+    uint32_t capacity() const {
+        return computeCapacity(capacityIndex(), length());
+    }
+
+    bool containsProperty(ExclusiveContext* cx, jsid id);
+
+    bool setElement(ExclusiveContext* cx, size_t index, const Value& v);
+    bool initElement(ExclusiveContext* cx, size_t index, const Value& v);
+    void initElementNoTypeChange(size_t index, const Value& v);
+    Value getElement(size_t index);
+
+    template <JSValueType Type> inline bool setElementSpecific(ExclusiveContext* cx, size_t index,
+                                                               const Value& v);
+    template <JSValueType Type> inline void setElementNoTypeChangeSpecific(size_t index, const Value& v);
+    template <JSValueType Type> inline bool initElementSpecific(ExclusiveContext* cx, size_t index,
+                                                                const Value& v);
+    template <JSValueType Type> inline void initElementNoTypeChangeSpecific(size_t index, const Value& v);
+    template <JSValueType Type> inline Value getElementSpecific(size_t index);
+    template <JSValueType Type> inline void triggerPreBarrier(size_t index);
+
+    bool growElements(ExclusiveContext* cx, size_t cap);
+    void shrinkElements(ExclusiveContext* cx, size_t cap);
+
+    static uint32_t offsetOfElements() {
+        return offsetof(UnboxedArrayObject, elements_);
+    }
+    static uint32_t offsetOfLength() {
+        return offsetof(UnboxedArrayObject, length_);
+    }
+    static uint32_t offsetOfCapacityIndexAndInitializedLength() {
+        return offsetof(UnboxedArrayObject, capacityIndexAndInitializedLength_);
+    }
+    static uint32_t offsetOfInlineElements() {
+        return offsetof(UnboxedArrayObject, inlineElements_);
+    }
+
+    void setLengthInt32(uint32_t length) {
+        MOZ_ASSERT(length <= INT32_MAX);
+        length_ = length;
+    }
+
+    inline void setLength(ExclusiveContext* cx, uint32_t len);
+    inline void setInitializedLength(uint32_t initlen);
+
+    inline void setInitializedLengthNoBarrier(uint32_t initlen) {
+        MOZ_ASSERT(initlen <= InitializedLengthMask);
+        capacityIndexAndInitializedLength_ =
+            (capacityIndexAndInitializedLength_ & CapacityMask) | initlen;
+    }
+
+  private:
+    void setInlineElements() {
+        elements_ = &inlineElements_[0];
+    }
+
+    void setCapacityIndex(uint32_t index) {
+        MOZ_ASSERT(index <= (CapacityMask >> CapacityShift));
+        capacityIndexAndInitializedLength_ =
+            (index << CapacityShift) | initializedLength();
+    }
+};
+
+} // namespace js
+
+namespace JS {
+
+template <>
+struct DeletePolicy<js::UnboxedLayout> : public js::GCManagedDeletePolicy<js::UnboxedLayout>
+{};
+
+} /* namespace JS */
+
+#endif /* vm_UnboxedObject_h */
diff --git a/js/xpconnect/src/XPCJSContext.cpp b/js/xpconnect/src/XPCJSContext.cpp
index 0243d80e3..bde949a96 100644
--- a/js/xpconnect/src/XPCJSContext.cpp
+++ b/js/xpconnect/src/XPCJSContext.cpp
@@ -1427,6 +1427,8 @@ ReloadPrefsCallback(const char* pref, void* data)
 
     bool extraWarnings = Preferences::GetBool(JS_OPTIONS_DOT_STR "strict");
 
+    bool unboxedObjects = Preferences::GetBool(JS_OPTIONS_DOT_STR "unboxed_objects");
+    
     sSharedMemoryEnabled = Preferences::GetBool(JS_OPTIONS_DOT_STR "shared_memory");
 
 #ifdef DEBUG
@@ -1455,6 +1457,8 @@ ReloadPrefsCallback(const char* pref, void* data)
                                   useBaselineEager ? 0 : -1);
     JS_SetGlobalJitCompilerOption(cx, JSJITCOMPILER_ION_WARMUP_TRIGGER,
                                   useIonEager ? 0 : -1);
+    JS_SetGlobalJitCompilerOption(cx, JSJITCOMPILER_UNBOXED_OBJECTS,
+                                  unboxedObjects);
 }
 
 XPCJSContext::~XPCJSContext()