Revert #1091 Remove unboxed object code phase 1 + extras.

This should be the last code backout for this. merging this branch
should get us back to the way we were (+ additional code changes for
later changes) as fasr as the unused unboxed code is concerned.

36 files changed, 1544 insertions, 102 deletions

diff --git a/js/src/gc/Marking.cpp b/js/src/gc/Marking.cpp
index a206a8762..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
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/AliasAnalysisShared.cpp b/js/src/jit/AliasAnalysisShared.cpp
index 1a643698f..99c23d2a3 100644
--- a/js/src/jit/AliasAnalysisShared.cpp
+++ b/js/src/jit/AliasAnalysisShared.cpp
@@ -112,6 +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/BaselineInspector.cpp b/js/src/jit/BaselineInspector.cpp
index 9c7b88fb2..c9e09bed7 100644
--- a/js/src/jit/BaselineInspector.cpp
+++ b/js/src/jit/BaselineInspector.cpp
@@ -96,8 +96,13 @@ 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);
 }
 
@@ -165,12 +170,16 @@ GetCacheIRReceiverForUnboxedProperty(ICCacheIR_Monitored* stub, ReceiverGuard* r
 }
 
 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;
@@ -198,7 +207,7 @@ BaselineInspector::maybeInfoForPropertyOp(jsbytecode* pc, ReceiverVector& receiv
             return true;
         }
 
-        if (!AddReceiver(receiver, receivers))
+        if (!AddReceiver(receiver, receivers, convertUnboxedGroups))
             return false;
 
         stub = stub->next();
@@ -691,12 +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);
@@ -708,7 +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) {
@@ -740,19 +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);
@@ -761,7 +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 4a1791798..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);
@@ -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/CodeGenerator.cpp b/js/src/jit/CodeGenerator.cpp
index c3e242991..bb12b09c8 100644
--- a/js/src/jit/CodeGenerator.cpp
+++ b/js/src/jit/CodeGenerator.cpp
@@ -3032,6 +3032,15 @@ GuardReceiver(MacroAssembler& masm, const ReceiverGuard& guard,
 {
     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);
     }
@@ -3069,6 +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) {
@@ -3109,6 +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
@@ -3144,6 +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) {
@@ -3309,6 +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);
@@ -8530,6 +8576,21 @@ 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");
diff --git a/js/src/jit/CodeGenerator.h b/js/src/jit/CodeGenerator.h
index bc8fcccea..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);
@@ -309,6 +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..8bad8ba9c 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;
 }
 
diff --git a/js/src/jit/IonBuilder.cpp b/js/src/jit/IonBuilder.cpp
index a5991cc7b..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;
@@ -6400,7 +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;
@@ -6437,7 +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);
@@ -7734,6 +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;
@@ -8192,7 +8195,8 @@ IonBuilder::maybeMarkEmpty(MDefinition* ins)
 static bool
 ClassHasEffectlessLookup(const Class* clasp)
 {
-    return (clasp == &UnboxedArrayObject::class_) ||
+    return (clasp == &UnboxedPlainObject::class_) ||
+           (clasp == &UnboxedArrayObject::class_) ||
            IsTypedObjectClass(clasp) ||
            (clasp->isNative() && !clasp->getOpsLookupProperty());
 }
@@ -9023,6 +9027,8 @@ IonBuilder::jsop_getelem()
     }
 
     obj = maybeUnboxForPropertyAccess(obj);
+    if (obj->type() == MIRType::Object)
+        obj = convertUnboxedObjects(obj);
 
     bool emitted = false;
 
@@ -10146,7 +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());
@@ -10981,8 +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) {
@@ -11039,6 +11048,8 @@ IonBuilder::getUnboxedOffset(TemporaryTypeSet* types, PropertyName* name, JSValu
             return UINT32_MAX;
         }
 
+        key->watchStateChangeForUnboxedConvertedToNative(constraints());
+
         if (offset == UINT32_MAX) {
             offset = property->offset;
             *punboxedType = property->type;
@@ -11500,6 +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);
@@ -11571,6 +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)
@@ -11936,6 +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)
@@ -12086,14 +12147,45 @@ IonBuilder::loadUnboxedValue(MDefinition* elements, size_t elementsOffset,
     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);
@@ -12117,8 +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;
     }
@@ -12134,7 +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;
     }
@@ -12302,12 +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;
@@ -12330,6 +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;
     }
 
@@ -12571,7 +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();
@@ -12604,6 +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))
@@ -12637,8 +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;
@@ -12653,7 +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;
     }
@@ -12970,6 +13117,40 @@ IonBuilder::storeUnboxedValue(MDefinition* obj, MDefinition* elements, int32_t e
 }
 
 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,
                                    bool barrier, TemporaryTypeSet* objTypes)
@@ -12982,12 +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.
@@ -13007,6 +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;
     }
 
@@ -13705,7 +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;
@@ -14062,6 +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)
 {
@@ -14070,6 +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);
diff --git a/js/src/jit/IonBuilder.h b/js/src/jit/IonBuilder.h
index 1f84b45df..f359c764f 100644
--- a/js/src/jit/IonBuilder.h
+++ b/js/src/jit/IonBuilder.h
@@ -401,6 +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*
@@ -440,6 +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,
@@ -472,6 +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);
@@ -1037,6 +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,
@@ -1054,6 +1061,9 @@ 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,
diff --git a/js/src/jit/Lowering.cpp b/js/src/jit/Lowering.cpp
index c3bd47744..19266bae8 100644
--- a/js/src/jit/Lowering.cpp
+++ b/js/src/jit/Lowering.cpp
@@ -3258,6 +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);
@@ -3775,6 +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 bb06baa29..de66f175b 100644
--- a/js/src/jit/Lowering.h
+++ b/js/src/jit/Lowering.h
@@ -233,6 +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);
@@ -256,6 +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 276b5eba5..f2071dc6a 100644
--- a/js/src/jit/MCallOptimize.cpp
+++ b/js/src/jit/MCallOptimize.cpp
@@ -611,7 +611,7 @@ 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;
@@ -700,7 +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))
@@ -773,7 +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)
@@ -2097,7 +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 403e70c02..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)
 {
@@ -4776,8 +4810,35 @@ 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;
 }
 
@@ -4797,11 +4858,17 @@ MObjectState::MObjectState(JSObject *templateObject, OperandIndexMap* operandInd
     setResultType(MIRType::Object);
     setRecoveredOnBailout();
 
-    MOZ_ASSERT(templateObject->is<NativeObject>());
-
-    NativeObject* nativeObject = &templateObject->as<NativeObject>();
-    numSlots_ = nativeObject->slotSpan();
-    numFixedSlots_ = nativeObject->numFixedSlots();
+    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());
+    }
 
     operandIndex_ = operandIndex;
 }
@@ -4838,21 +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;
 }
@@ -4863,7 +4948,14 @@ MObjectState::New(TempAllocator& alloc, MDefinition* obj)
     JSObject* templateObject = templateObjectOf(obj);
     MOZ_ASSERT(templateObject, "Unexpected object creation.");
 
-    MObjectState* res = new(alloc) MObjectState(templateObject, nullptr);
+    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;
@@ -5770,6 +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)
@@ -5824,6 +5945,8 @@ jit::UnboxedArrayElementType(CompilerConstraintList* constraints, MDefinition* o
             elementType = layout.elementType();
         else
             return JSVAL_TYPE_MAGIC;
+
+        key->watchStateChangeForUnboxedConvertedToNative(constraints);
     }
 
     return elementType;
@@ -6447,6 +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;
 
@@ -6477,6 +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 ebc98a4f8..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
@@ -9749,6 +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,
@@ -11128,6 +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:
@@ -11222,6 +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:
@@ -11330,6 +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,
diff --git a/js/src/jit/MOpcodes.h b/js/src/jit/MOpcodes.h
index f5f59dee6..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)                                              \
@@ -218,6 +220,7 @@ namespace jit {
     _(StoreUnboxedScalar)                                                   \
     _(StoreUnboxedObjectOrNull)                                             \
     _(StoreUnboxedString)                                                   \
+    _(ConvertUnboxedObjectToNative)                                         \
     _(ArrayPopShift)                                                        \
     _(ArrayPush)                                                            \
     _(ArraySlice)                                                           \
diff --git a/js/src/jit/OptimizationTracking.cpp b/js/src/jit/OptimizationTracking.cpp
index b42634d43..308def041 100644
--- a/js/src/jit/OptimizationTracking.cpp
+++ b/js/src/jit/OptimizationTracking.cpp
@@ -15,11 +15,9 @@
 #include "jit/JitcodeMap.h"
 #include "jit/JitSpewer.h"
 #include "js/TrackedOptimizationInfo.h"
-#include "vm/UnboxedObject.h"
 
 #include "vm/ObjectGroup-inl.h"
 #include "vm/TypeInference-inl.h"
-#include "vm/UnboxedObject-inl.h"
 
 using namespace js;
 using namespace js::jit;
diff --git a/js/src/jit/ScalarReplacement.cpp b/js/src/jit/ScalarReplacement.cpp
index 2065c0371..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 +305,12 @@ 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);
@@ -630,6 +656,21 @@ 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)
 {
@@ -659,6 +700,77 @@ ObjectMemoryView::loadOffset(MInstruction* ins, size_t offset)
     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)
 {
diff --git a/js/src/jit/SharedIC.cpp b/js/src/jit/SharedIC.cpp
index 2475dfb22..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"
diff --git a/js/src/jit/shared/LIR-shared.h b/js/src/jit/shared/LIR-shared.h
index 49879eedb..e6aab6ba3 100644
--- a/js/src/jit/shared/LIR-shared.h
+++ b/js/src/jit/shared/LIR-shared.h
@@ -5892,6 +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:
@@ -7414,6 +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 505c0ea03..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)                 \
@@ -284,6 +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 0e2eeebaa..9138a4a92 100644
--- a/js/src/jsapi.h
+++ b/js/src/jsapi.h
@@ -5896,19 +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/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 901a9f505..d4379bd7d 100644
--- a/js/src/jsobj.cpp
+++ b/js/src/jsobj.cpp
@@ -53,7 +53,6 @@
 #include "vm/RegExpStaticsObject.h"
 #include "vm/Shape.h"
 #include "vm/TypedArrayCommon.h"
-#include "vm/UnboxedObject-inl.h"
 
 #include "jsatominlines.h"
 #include "jsboolinlines.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
@@ -1157,27 +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 (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;
+        }
+
+        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
@@ -1199,9 +1220,8 @@ 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>() ||
-			   obj->is<UnboxedArrayObject>());
+    MOZ_ASSERT(obj->is<PlainObject>() || obj->is<UnboxedPlainObject>() ||
+               obj->is<ArrayObject>() || obj->is<UnboxedArrayObject>());
     MOZ_ASSERT(newKind != SingletonObject);
 
     if (obj->is<ArrayObject>() || obj->is<UnboxedArrayObject>()) {
@@ -1320,6 +1340,7 @@ js::XDRObjectLiteral(XDRState<mode>* xdr, MutableHandleObject obj)
     {
         if (mode == XDR_ENCODE) {
             MOZ_ASSERT(obj->is<PlainObject>() ||
+                       obj->is<UnboxedPlainObject>() ||
                        obj->is<ArrayObject>() ||
                        obj->is<UnboxedArrayObject>());
             isArray = (obj->is<ArrayObject>() || obj->is<UnboxedArrayObject>()) ? 1 : 0;
@@ -2315,6 +2336,11 @@ 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);
@@ -2576,6 +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;
@@ -2599,6 +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))
@@ -3611,6 +3645,12 @@ 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>();
diff --git a/js/src/shell/js.cpp b/js/src/shell/js.cpp
index 35a527c5e..8cd821b31 100644
--- a/js/src/shell/js.cpp
+++ b/js/src/shell/js.cpp
@@ -7279,6 +7279,9 @@ SetContextOptions(JSContext* cx, const OptionParser& op)
     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;
diff --git a/js/src/vm/Interpreter-inl.h b/js/src/vm/Interpreter-inl.h
index adefa6e93..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
diff --git a/js/src/vm/Interpreter.cpp b/js/src/vm/Interpreter.cpp
index 834084c4d..274392335 100644
--- a/js/src/vm/Interpreter.cpp
+++ b/js/src/vm/Interpreter.cpp
@@ -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));
 }
@@ -4973,10 +4973,15 @@ 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);
     }
 
     RootedObject obj(cx);
diff --git a/js/src/vm/ReceiverGuard.cpp b/js/src/vm/ReceiverGuard.cpp
index 11c2d0727..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;
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/TypeInference.cpp b/js/src/vm/TypeInference.cpp
index ee5bbef5d..438188a36 100644
--- a/js/src/vm/TypeInference.cpp
+++ b/js/src/vm/TypeInference.cpp
@@ -1999,6 +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)
 {
@@ -3573,6 +3584,7 @@ PreliminaryObjectArrayWithTemplate::maybeAnalyze(ExclusiveContext* cx, ObjectGro
         }
     }
 
+    TryConvertToUnboxedLayout(cx, enter, shape(), group, preliminaryObjects);
     if (group->maybeUnboxedLayout())
         return;
 
@@ -3901,6 +3913,10 @@ 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;
 
diff --git a/js/src/vm/TypeInference.h b/js/src/vm/TypeInference.h
index 537baa21f..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);
 
diff --git a/js/src/vm/UnboxedObject.cpp b/js/src/vm/UnboxedObject.cpp
index 059293a2d..d8c9c774a 100644
--- a/js/src/vm/UnboxedObject.cpp
+++ b/js/src/vm/UnboxedObject.cpp
@@ -1635,12 +1635,227 @@ const Class UnboxedArrayObject::class_ = {
 // 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)
@@ -1666,6 +1881,16 @@ UnboxedArrayObject::fillAfterConvert(ExclusiveContext* cx,
         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)
@@ -1676,6 +1901,181 @@ UnboxedPlainObject::fillAfterConvert(ExclusiveContext* cx,
         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);
diff --git a/js/src/vm/UnboxedObject.h b/js/src/vm/UnboxedObject.h
index 779dd14c7..ecff8be5b 100644
--- a/js/src/vm/UnboxedObject.h
+++ b/js/src/vm/UnboxedObject.h
@@ -317,6 +317,13 @@ class UnboxedPlainObject : public JSObject
     }
 };
 
+// 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
 {
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()