Merge pull request #1461 from MoonchildProductions/ubbo

Back out unboxed array/object removals

35 files changed, 3611 insertions, 390 deletions

diff --git a/js/src/jit/AliasAnalysisShared.cpp b/js/src/jit/AliasAnalysisShared.cpp
index 400626b33..99c23d2a3 100644
--- a/js/src/jit/AliasAnalysisShared.cpp
+++ b/js/src/jit/AliasAnalysisShared.cpp
@@ -91,6 +91,10 @@ GetObject(const MDefinition* ins)
       case MDefinition::Op_Elements:
       case MDefinition::Op_MaybeCopyElementsForWrite:
       case MDefinition::Op_MaybeToDoubleElement:
+      case MDefinition::Op_UnboxedArrayLength:
+      case MDefinition::Op_UnboxedArrayInitializedLength:
+      case MDefinition::Op_IncrementUnboxedArrayInitializedLength:
+      case MDefinition::Op_SetUnboxedArrayInitializedLength:
       case MDefinition::Op_TypedArrayLength:
       case MDefinition::Op_SetTypedObjectOffset:
       case MDefinition::Op_SetDisjointTypedElements:
@@ -108,6 +112,8 @@ GetObject(const MDefinition* ins)
       case MDefinition::Op_GuardObjectGroup:
       case MDefinition::Op_GuardObjectIdentity:
       case MDefinition::Op_GuardClass:
+      case MDefinition::Op_GuardUnboxedExpando:
+      case MDefinition::Op_LoadUnboxedExpando:
       case MDefinition::Op_LoadSlot:
       case MDefinition::Op_StoreSlot:
       case MDefinition::Op_InArray:
diff --git a/js/src/jit/BaselineCacheIR.cpp b/js/src/jit/BaselineCacheIR.cpp
index 67c80473b..bf96932d1 100644
--- a/js/src/jit/BaselineCacheIR.cpp
+++ b/js/src/jit/BaselineCacheIR.cpp
@@ -16,7 +16,7 @@ using namespace js;
 using namespace js::jit;
 
 // OperandLocation represents the location of an OperandId. The operand is
-// either in a register or on the stack.
+// either in a register or on the stack, and is either boxed or unboxed.
 class OperandLocation
 {
   public:
@@ -787,6 +787,9 @@ BaselineCacheIRCompiler::emitGuardClass()
       case GuardClassKind::Array:
         clasp = &ArrayObject::class_;
         break;
+      case GuardClassKind::UnboxedArray:
+        clasp = &UnboxedArrayObject::class_;
+        break;
       case GuardClassKind::MappedArguments:
         clasp = &MappedArgumentsObject::class_;
         break;
@@ -815,6 +818,36 @@ BaselineCacheIRCompiler::emitGuardSpecificObject()
 }
 
 bool
+BaselineCacheIRCompiler::emitGuardNoUnboxedExpando()
+{
+    Register obj = allocator.useRegister(masm, reader.objOperandId());
+
+    FailurePath* failure;
+    if (!addFailurePath(&failure))
+        return false;
+
+    Address expandoAddr(obj, UnboxedPlainObject::offsetOfExpando());
+    masm.branchPtr(Assembler::NotEqual, expandoAddr, ImmWord(0), failure->label());
+    return true;
+}
+
+bool
+BaselineCacheIRCompiler::emitGuardAndLoadUnboxedExpando()
+{
+    Register obj = allocator.useRegister(masm, reader.objOperandId());
+    Register output = allocator.defineRegister(masm, reader.objOperandId());
+
+    FailurePath* failure;
+    if (!addFailurePath(&failure))
+        return false;
+
+    Address expandoAddr(obj, UnboxedPlainObject::offsetOfExpando());
+    masm.loadPtr(expandoAddr, output);
+    masm.branchTestPtr(Assembler::Zero, output, output, failure->label());
+    return true;
+}
+
+bool
 BaselineCacheIRCompiler::emitLoadFixedSlotResult()
 {
     Register obj = allocator.useRegister(masm, reader.objOperandId());
@@ -841,6 +874,26 @@ BaselineCacheIRCompiler::emitLoadDynamicSlotResult()
 }
 
 bool
+BaselineCacheIRCompiler::emitLoadUnboxedPropertyResult()
+{
+    Register obj = allocator.useRegister(masm, reader.objOperandId());
+    AutoScratchRegister scratch(allocator, masm);
+
+    JSValueType fieldType = reader.valueType();
+
+    Address fieldOffset(stubAddress(reader.stubOffset()));
+    masm.load32(fieldOffset, scratch);
+    masm.loadUnboxedProperty(BaseIndex(obj, scratch, TimesOne), fieldType, R0);
+
+    if (fieldType == JSVAL_TYPE_OBJECT)
+        emitEnterTypeMonitorIC();
+    else
+        emitReturnFromIC();
+
+    return true;
+}
+
+bool
 BaselineCacheIRCompiler::emitGuardNoDetachedTypedObjects()
 {
     FailurePath* failure;
@@ -951,6 +1004,19 @@ BaselineCacheIRCompiler::emitLoadInt32ArrayLengthResult()
 }
 
 bool
+BaselineCacheIRCompiler::emitLoadUnboxedArrayLengthResult()
+{
+    Register obj = allocator.useRegister(masm, reader.objOperandId());
+    masm.load32(Address(obj, UnboxedArrayObject::offsetOfLength()), R0.scratchReg());
+    masm.tagValue(JSVAL_TYPE_INT32, R0.scratchReg(), R0);
+
+    // The int32 type was monitored when attaching the stub, so we can
+    // just return.
+    emitReturnFromIC();
+    return true;
+}
+
+bool
 BaselineCacheIRCompiler::emitLoadArgumentsObjectLengthResult()
 {
     Register obj = allocator.useRegister(masm, reader.objOperandId());
diff --git a/js/src/jit/BaselineCompiler.cpp b/js/src/jit/BaselineCompiler.cpp
index d254b9826..ae5a2e666 100644
--- a/js/src/jit/BaselineCompiler.cpp
+++ b/js/src/jit/BaselineCompiler.cpp
@@ -2050,7 +2050,13 @@ BaselineCompiler::emit_JSOP_NEWARRAY()
     return true;
 }
 
-typedef ArrayObject* (*NewArrayCopyOnWriteFn)(JSContext*, HandleArrayObject, gc::InitialHeap);
+bool
+BaselineCompiler::emit_JSOP_SPREADCALLARRAY()
+{
+    return emit_JSOP_NEWARRAY();
+}
+
+typedef JSObject* (*NewArrayCopyOnWriteFn)(JSContext*, HandleArrayObject, gc::InitialHeap);
 const VMFunction jit::NewArrayCopyOnWriteInfo =
     FunctionInfo<NewArrayCopyOnWriteFn>(js::NewDenseCopyOnWriteArray, "NewDenseCopyOnWriteArray");
 
@@ -4181,14 +4187,14 @@ BaselineCompiler::emit_JSOP_REST()
 {
     frame.syncStack(0);
 
-    ArrayObject* templateObject =
+    JSObject* templateObject =
         ObjectGroup::newArrayObject(cx, nullptr, 0, TenuredObject,
                                     ObjectGroup::NewArrayKind::UnknownIndex);
     if (!templateObject)
         return false;
 
     // Call IC.
-    ICRest_Fallback::Compiler compiler(cx, templateObject);
+    ICRest_Fallback::Compiler compiler(cx, &templateObject->as<ArrayObject>());
     if (!emitOpIC(compiler.getStub(&stubSpace_)))
         return false;
 
diff --git a/js/src/jit/BaselineCompiler.h b/js/src/jit/BaselineCompiler.h
index a200f7ab9..7b1af092a 100644
--- a/js/src/jit/BaselineCompiler.h
+++ b/js/src/jit/BaselineCompiler.h
@@ -100,6 +100,7 @@ namespace jit {
     _(JSOP_BITNOT)             \
     _(JSOP_NEG)                \
     _(JSOP_NEWARRAY)           \
+    _(JSOP_SPREADCALLARRAY)    \
     _(JSOP_NEWARRAY_COPYONWRITE) \
     _(JSOP_INITELEM_ARRAY)     \
     _(JSOP_NEWOBJECT)          \
diff --git a/js/src/jit/BaselineIC.cpp b/js/src/jit/BaselineIC.cpp
index e65f10aac..9c8cd9835 100644
--- a/js/src/jit/BaselineIC.cpp
+++ b/js/src/jit/BaselineIC.cpp
@@ -44,8 +44,8 @@
 #include "jit/shared/Lowering-shared-inl.h"
 #include "vm/EnvironmentObject-inl.h"
 #include "vm/Interpreter-inl.h"
-#include "vm/NativeObject-inl.h"
 #include "vm/StringObject-inl.h"
+#include "vm/UnboxedObject-inl.h"
 
 using mozilla::DebugOnly;
 
@@ -741,6 +741,11 @@ LastPropertyForSetProp(JSObject* obj)
     if (obj->isNative())
         return obj->as<NativeObject>().lastProperty();
 
+    if (obj->is<UnboxedPlainObject>()) {
+        UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando();
+        return expando ? expando->lastProperty() : nullptr;
+    }
+
     return nullptr;
 }
 
@@ -1157,6 +1162,56 @@ TryAttachNativeOrUnboxedGetValueElemStub(JSContext* cx, HandleScript script, jsb
 
     ICStub* monitorStub = stub->fallbackMonitorStub()->firstMonitorStub();
 
+    if (obj->is<UnboxedPlainObject>() && holder == obj) {
+        const UnboxedLayout::Property* property = obj->as<UnboxedPlainObject>().layout().lookup(id);
+
+        // Once unboxed objects support symbol-keys, we need to change the following accordingly
+        MOZ_ASSERT_IF(!keyVal.isString(), !property);
+
+        if (property) {
+            if (!cx->runtime()->jitSupportsFloatingPoint)
+                return true;
+
+            RootedPropertyName name(cx, JSID_TO_ATOM(id)->asPropertyName());
+            ICGetElemNativeCompiler<PropertyName*> compiler(cx, ICStub::GetElem_UnboxedPropertyName,
+                                                            monitorStub, obj, holder,
+                                                            name,
+                                                            ICGetElemNativeStub::UnboxedProperty,
+                                                            needsAtomize, property->offset +
+                                                            UnboxedPlainObject::offsetOfData(),
+                                                            property->type);
+            ICStub* newStub = compiler.getStub(compiler.getStubSpace(script));
+            if (!newStub)
+                return false;
+
+            stub->addNewStub(newStub);
+            *attached = true;
+            return true;
+        }
+
+        Shape* shape = obj->as<UnboxedPlainObject>().maybeExpando()->lookup(cx, id);
+        if (!shape->hasDefaultGetter() || !shape->hasSlot())
+            return true;
+
+        bool isFixedSlot;
+        uint32_t offset;
+        GetFixedOrDynamicSlotOffset(shape, &isFixedSlot, &offset);
+
+        ICGetElemNativeStub::AccessType acctype =
+            isFixedSlot ? ICGetElemNativeStub::FixedSlot
+                        : ICGetElemNativeStub::DynamicSlot;
+        ICGetElemNativeCompiler<T> compiler(cx, getGetElemStubKind<T>(ICStub::GetElem_NativeSlotName),
+                                            monitorStub, obj, holder, key,
+                                            acctype, needsAtomize, offset);
+        ICStub* newStub = compiler.getStub(compiler.getStubSpace(script));
+        if (!newStub)
+            return false;
+
+        stub->addNewStub(newStub);
+        *attached = true;
+        return true;
+    }
+
     if (!holder->isNative())
         return true;
 
@@ -1320,7 +1375,7 @@ IsNativeDenseElementAccess(HandleObject obj, HandleValue key)
 static bool
 IsNativeOrUnboxedDenseElementAccess(HandleObject obj, HandleValue key)
 {
-    if (!obj->isNative())
+    if (!obj->isNative() && !obj->is<UnboxedArrayObject>())
         return false;
     if (key.isInt32() && key.toInt32() >= 0 && !obj->is<TypedArrayObject>())
         return true;
@@ -1404,7 +1459,7 @@ TryAttachGetElemStub(JSContext* cx, JSScript* script, jsbytecode* pc, ICGetElem_
     }
 
     // Check for NativeObject[id] and UnboxedPlainObject[id] shape-optimizable accesses.
-    if (obj->isNative()) {
+    if (obj->isNative() || obj->is<UnboxedPlainObject>()) {
         RootedScript rootedScript(cx, script);
         if (rhs.isString()) {
             if (!TryAttachNativeOrUnboxedGetValueElemStub<PropertyName*>(cx, rootedScript, pc, stub,
@@ -1424,6 +1479,20 @@ TryAttachGetElemStub(JSContext* cx, JSScript* script, jsbytecode* pc, ICGetElem_
         script = rootedScript;
     }
 
+    // Check for UnboxedArray[int] accesses.
+    if (obj->is<UnboxedArrayObject>() && rhs.isInt32() && rhs.toInt32() >= 0) {
+        JitSpew(JitSpew_BaselineIC, "  Generating GetElem(UnboxedArray[Int32]) stub");
+        ICGetElem_UnboxedArray::Compiler compiler(cx, stub->fallbackMonitorStub()->firstMonitorStub(),
+                                                  obj->group());
+        ICStub* unboxedStub = compiler.getStub(compiler.getStubSpace(script));
+        if (!unboxedStub)
+            return false;
+
+        stub->addNewStub(unboxedStub);
+        *attached = true;
+        return true;
+    }
+
     // Check for TypedArray[int] => Number and TypedObject[int] => Number accesses.
     if ((obj->is<TypedArrayObject>() || IsPrimitiveArrayTypedObject(obj)) &&
         rhs.isNumber() &&
@@ -1816,6 +1885,14 @@ ICGetElemNativeCompiler<T>::generateStubCode(MacroAssembler& masm)
     Register holderReg;
     if (obj_ == holder_) {
         holderReg = objReg;
+
+        if (obj_->is<UnboxedPlainObject>() && acctype_ != ICGetElemNativeStub::UnboxedProperty) {
+            // The property will be loaded off the unboxed expando.
+            masm.push(R1.scratchReg());
+            popR1 = true;
+            holderReg = R1.scratchReg();
+            masm.loadPtr(Address(objReg, UnboxedPlainObject::offsetOfExpando()), holderReg);
+        }
     } else {
         // Shape guard holder.
         if (regs.empty()) {
@@ -1866,6 +1943,13 @@ ICGetElemNativeCompiler<T>::generateStubCode(MacroAssembler& masm)
         if (popR1)
             masm.addToStackPtr(ImmWord(sizeof(size_t)));
 
+    } else if (acctype_ == ICGetElemNativeStub::UnboxedProperty) {
+        masm.load32(Address(ICStubReg, ICGetElemNativeSlotStub<T>::offsetOfOffset()),
+                    scratchReg);
+        masm.loadUnboxedProperty(BaseIndex(objReg, scratchReg, TimesOne), unboxedType_,
+                                 TypedOrValueRegister(R0));
+        if (popR1)
+            masm.addToStackPtr(ImmWord(sizeof(size_t)));
     } else {
         MOZ_ASSERT(acctype_ == ICGetElemNativeStub::NativeGetter ||
                    acctype_ == ICGetElemNativeStub::ScriptedGetter);
@@ -2015,6 +2099,56 @@ ICGetElem_Dense::Compiler::generateStubCode(MacroAssembler& masm)
 }
 
 //
+// GetElem_UnboxedArray
+//
+
+bool
+ICGetElem_UnboxedArray::Compiler::generateStubCode(MacroAssembler& masm)
+{
+    MOZ_ASSERT(engine_ == Engine::Baseline);
+
+    Label failure;
+    masm.branchTestObject(Assembler::NotEqual, R0, &failure);
+    masm.branchTestInt32(Assembler::NotEqual, R1, &failure);
+
+    AllocatableGeneralRegisterSet regs(availableGeneralRegs(2));
+    Register scratchReg = regs.takeAny();
+
+    // Unbox R0 and group guard.
+    Register obj = masm.extractObject(R0, ExtractTemp0);
+    masm.loadPtr(Address(ICStubReg, ICGetElem_UnboxedArray::offsetOfGroup()), scratchReg);
+    masm.branchTestObjGroup(Assembler::NotEqual, obj, scratchReg, &failure);
+
+    // Unbox key.
+    Register key = masm.extractInt32(R1, ExtractTemp1);
+
+    // Bounds check.
+    masm.load32(Address(obj, UnboxedArrayObject::offsetOfCapacityIndexAndInitializedLength()),
+                scratchReg);
+    masm.and32(Imm32(UnboxedArrayObject::InitializedLengthMask), scratchReg);
+    masm.branch32(Assembler::BelowOrEqual, scratchReg, key, &failure);
+
+    // Load obj->elements.
+    masm.loadPtr(Address(obj, UnboxedArrayObject::offsetOfElements()), scratchReg);
+
+    // Load value.
+    size_t width = UnboxedTypeSize(elementType_);
+    BaseIndex addr(scratchReg, key, ScaleFromElemWidth(width));
+    masm.loadUnboxedProperty(addr, elementType_, R0);
+
+    // Only monitor the result if its type might change.
+    if (elementType_ == JSVAL_TYPE_OBJECT)
+        EmitEnterTypeMonitorIC(masm);
+    else
+        EmitReturnFromIC(masm);
+
+    // Failure case - jump to next stub
+    masm.bind(&failure);
+    EmitStubGuardFailure(masm);
+    return true;
+}
+
+//
 // GetElem_TypedArray
 //
 
@@ -2318,8 +2452,8 @@ CanOptimizeDenseOrUnboxedArraySetElem(JSObject* obj, uint32_t index,
                                       Shape* oldShape, uint32_t oldCapacity, uint32_t oldInitLength,
                                       bool* isAddingCaseOut, size_t* protoDepthOut)
 {
-    uint32_t initLength = obj->as<NativeObject>().getDenseInitializedLength();
-    uint32_t capacity = obj->as<NativeObject>().getDenseCapacity();
+    uint32_t initLength = GetAnyBoxedOrUnboxedInitializedLength(obj);
+    uint32_t capacity = GetAnyBoxedOrUnboxedCapacity(obj);
 
     *isAddingCaseOut = false;
     *protoDepthOut = 0;
@@ -2328,6 +2462,10 @@ CanOptimizeDenseOrUnboxedArraySetElem(JSObject* obj, uint32_t index,
     if (initLength < oldInitLength || capacity < oldCapacity)
         return false;
 
+    // Unboxed arrays need to be able to emit floating point code.
+    if (obj->is<UnboxedArrayObject>() && !obj->runtimeFromMainThread()->jitSupportsFloatingPoint)
+        return false;
+
     Shape* shape = obj->maybeShape();
 
     // Cannot optimize if the shape changed.
@@ -2409,8 +2547,8 @@ DoSetElemFallback(JSContext* cx, BaselineFrame* frame, ICSetElem_Fallback* stub_
     uint32_t oldCapacity = 0;
     uint32_t oldInitLength = 0;
     if (index.isInt32() && index.toInt32() >= 0) {
-        oldCapacity = obj->as<NativeObject>().getDenseCapacity();
-        oldInitLength = obj->as<NativeObject>().getDenseInitializedLength();
+        oldCapacity = GetAnyBoxedOrUnboxedCapacity(obj);
+        oldInitLength = GetAnyBoxedOrUnboxedInitializedLength(obj);
     }
 
     if (op == JSOP_INITELEM || op == JSOP_INITHIDDENELEM) {
@@ -2618,6 +2756,18 @@ BaselineScript::noteArrayWriteHole(uint32_t pcOffset)
 // SetElem_DenseOrUnboxedArray
 //
 
+template <typename T>
+void
+EmitUnboxedPreBarrierForBaseline(MacroAssembler &masm, T address, JSValueType type)
+{
+    if (type == JSVAL_TYPE_OBJECT)
+        EmitPreBarrier(masm, address, MIRType::Object);
+    else if (type == JSVAL_TYPE_STRING)
+        EmitPreBarrier(masm, address, MIRType::String);
+    else
+        MOZ_ASSERT(!UnboxedTypeNeedsPreBarrier(type));
+}
+
 bool
 ICSetElem_DenseOrUnboxedArray::Compiler::generateStubCode(MacroAssembler& masm)
 {
@@ -2736,6 +2886,29 @@ ICSetElem_DenseOrUnboxedArray::Compiler::generateStubCode(MacroAssembler& masm)
         masm.loadValue(valueAddr, tmpVal);
         EmitPreBarrier(masm, element, MIRType::Value);
         masm.storeValue(tmpVal, element);
+    } else {
+        // Set element on an unboxed array.
+
+        // Bounds check.
+        Address initLength(obj, UnboxedArrayObject::offsetOfCapacityIndexAndInitializedLength());
+        masm.load32(initLength, scratchReg);
+        masm.and32(Imm32(UnboxedArrayObject::InitializedLengthMask), scratchReg);
+        masm.branch32(Assembler::BelowOrEqual, scratchReg, key, &failure);
+
+        // Load obj->elements.
+        masm.loadPtr(Address(obj, UnboxedArrayObject::offsetOfElements()), scratchReg);
+
+        // Compute the address being written to.
+        BaseIndex address(scratchReg, key, ScaleFromElemWidth(UnboxedTypeSize(unboxedType_)));
+
+        EmitUnboxedPreBarrierForBaseline(masm, address, unboxedType_);
+
+        Address valueAddr(masm.getStackPointer(), ICStackValueOffset + sizeof(Value));
+        masm.Push(R0);
+        masm.loadValue(valueAddr, R0);
+        masm.storeUnboxedProperty(address, unboxedType_,
+                                  ConstantOrRegister(TypedOrValueRegister(R0)), &failurePopR0);
+        masm.Pop(R0);
     }
 
     EmitReturnFromIC(masm);
@@ -2929,6 +3102,40 @@ ICSetElemDenseOrUnboxedArrayAddCompiler::generateStubCode(MacroAssembler& masm)
         BaseIndex element(scratchReg, key, TimesEight);
         masm.loadValue(valueAddr, tmpVal);
         masm.storeValue(tmpVal, element);
+    } else {
+        // Adding element to an unboxed array.
+
+        // Bounds check (key == initLength)
+        Address initLengthAddr(obj, UnboxedArrayObject::offsetOfCapacityIndexAndInitializedLength());
+        masm.load32(initLengthAddr, scratchReg);
+        masm.and32(Imm32(UnboxedArrayObject::InitializedLengthMask), scratchReg);
+        masm.branch32(Assembler::NotEqual, scratchReg, key, &failure);
+
+        // Capacity check.
+        masm.checkUnboxedArrayCapacity(obj, RegisterOrInt32Constant(key), scratchReg, &failure);
+
+        // Load obj->elements.
+        masm.loadPtr(Address(obj, UnboxedArrayObject::offsetOfElements()), scratchReg);
+
+        // Write the value first, since this can fail. No need for pre-barrier
+        // since we're not overwriting an old value.
+        masm.Push(R0);
+        Address valueAddr(masm.getStackPointer(), ICStackValueOffset + sizeof(Value));
+        masm.loadValue(valueAddr, R0);
+        BaseIndex address(scratchReg, key, ScaleFromElemWidth(UnboxedTypeSize(unboxedType_)));
+        masm.storeUnboxedProperty(address, unboxedType_,
+                                  ConstantOrRegister(TypedOrValueRegister(R0)), &failurePopR0);
+        masm.Pop(R0);
+
+        // Increment initialized length.
+        masm.add32(Imm32(1), initLengthAddr);
+
+        // If length is now <= key, increment length.
+        Address lengthAddr(obj, UnboxedArrayObject::offsetOfLength());
+        Label skipIncrementLength;
+        masm.branch32(Assembler::Above, lengthAddr, key, &skipIncrementLength);
+        masm.add32(Imm32(1), lengthAddr);
+        masm.bind(&skipIncrementLength);
     }
 
     EmitReturnFromIC(masm);
@@ -4061,7 +4268,18 @@ TryAttachSetValuePropStub(JSContext* cx, HandleScript script, jsbytecode* pc, IC
         return true;
 
     if (!obj->isNative()) {
-        return true;
+        if (obj->is<UnboxedPlainObject>()) {
+            UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando();
+            if (expando) {
+                shape = expando->lookup(cx, name);
+                if (!shape)
+                    return true;
+            } else {
+                return true;
+            }
+        } else {
+            return true;
+        }
     }
 
     size_t chainDepth;
@@ -4209,6 +4427,40 @@ TryAttachSetAccessorPropStub(JSContext* cx, HandleScript script, jsbytecode* pc,
 }
 
 static bool
+TryAttachUnboxedSetPropStub(JSContext* cx, HandleScript script,
+                            ICSetProp_Fallback* stub, HandleId id,
+                            HandleObject obj, HandleValue rhs, bool* attached)
+{
+    MOZ_ASSERT(!*attached);
+
+    if (!cx->runtime()->jitSupportsFloatingPoint)
+        return true;
+
+    if (!obj->is<UnboxedPlainObject>())
+        return true;
+
+    const UnboxedLayout::Property* property = obj->as<UnboxedPlainObject>().layout().lookup(id);
+    if (!property)
+        return true;
+
+    ICSetProp_Unboxed::Compiler compiler(cx, obj->group(),
+                                         property->offset + UnboxedPlainObject::offsetOfData(),
+                                         property->type);
+    ICUpdatedStub* newStub = compiler.getStub(compiler.getStubSpace(script));
+    if (!newStub)
+        return false;
+    if (compiler.needsUpdateStubs() && !newStub->addUpdateStubForValue(cx, script, obj, id, rhs))
+        return false;
+
+    stub->addNewStub(newStub);
+
+    StripPreliminaryObjectStubs(cx, stub);
+
+    *attached = true;
+    return true;
+}
+
+static bool
 TryAttachTypedObjectSetPropStub(JSContext* cx, HandleScript script,
                                 ICSetProp_Fallback* stub, HandleId id,
                                 HandleObject obj, HandleValue rhs, bool* attached)
@@ -4291,6 +4543,12 @@ DoSetPropFallback(JSContext* cx, BaselineFrame* frame, ICSetProp_Fallback* stub_
         return false;
     RootedReceiverGuard oldGuard(cx, ReceiverGuard(obj));
 
+    if (obj->is<UnboxedPlainObject>()) {
+        MOZ_ASSERT(!oldShape);
+        if (UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando())
+            oldShape = expando->lastProperty();
+    }
+
     bool attached = false;
     // There are some reasons we can fail to attach a stub that are temporary.
     // We want to avoid calling noteUnoptimizableAccess() if the reason we
@@ -4363,6 +4621,15 @@ DoSetPropFallback(JSContext* cx, BaselineFrame* frame, ICSetProp_Fallback* stub_
 
     if (!attached &&
         lhs.isObject() &&
+        !TryAttachUnboxedSetPropStub(cx, script, stub, id, obj, rhs, &attached))
+    {
+        return false;
+    }
+    if (attached)
+        return true;
+
+    if (!attached &&
+        lhs.isObject() &&
         !TryAttachTypedObjectSetPropStub(cx, script, stub, id, obj, rhs, &attached))
     {
         return false;
@@ -4445,7 +4712,20 @@ GuardGroupAndShapeMaybeUnboxedExpando(MacroAssembler& masm, JSObject* obj,
 
     // Guard against shape or expando shape.
     masm.loadPtr(Address(ICStubReg, offsetOfShape), scratch);
-    masm.branchTestObjShape(Assembler::NotEqual, object, scratch, failure);
+    if (obj->is<UnboxedPlainObject>()) {
+        Address expandoAddress(object, UnboxedPlainObject::offsetOfExpando());
+        masm.branchPtr(Assembler::Equal, expandoAddress, ImmWord(0), failure);
+        Label done;
+        masm.push(object);
+        masm.loadPtr(expandoAddress, object);
+        masm.branchTestObjShape(Assembler::Equal, object, scratch, &done);
+        masm.pop(object);
+        masm.jump(failure);
+        masm.bind(&done);
+        masm.pop(object);
+    } else {
+        masm.branchTestObjShape(Assembler::NotEqual, object, scratch, failure);
+    }
 }
 
 bool
@@ -4484,7 +4764,13 @@ ICSetProp_Native::Compiler::generateStubCode(MacroAssembler& masm)
     regs.takeUnchecked(objReg);
 
     Register holderReg;
-    if (isFixedSlot_) {
+    if (obj_->is<UnboxedPlainObject>()) {
+        // We are loading off the expando object, so use that for the holder.
+        holderReg = regs.takeAny();
+        masm.loadPtr(Address(objReg, UnboxedPlainObject::offsetOfExpando()), holderReg);
+        if (!isFixedSlot_)
+            masm.loadPtr(Address(holderReg, NativeObject::offsetOfSlots()), holderReg);
+    } else if (isFixedSlot_) {
         holderReg = objReg;
     } else {
         holderReg = regs.takeAny();
@@ -4621,17 +4907,31 @@ ICSetPropNativeAddCompiler::generateStubCode(MacroAssembler& masm)
     regs.add(R0);
     regs.takeUnchecked(objReg);
 
-    // Write the object's new shape.
-    Address shapeAddr(objReg, ShapedObject::offsetOfShape());
-    EmitPreBarrier(masm, shapeAddr, MIRType::Shape);
-    masm.loadPtr(Address(ICStubReg, ICSetProp_NativeAdd::offsetOfNewShape()), scratch);
-    masm.storePtr(scratch, shapeAddr);
+    if (obj_->is<UnboxedPlainObject>()) {
+        holderReg = regs.takeAny();
+        masm.loadPtr(Address(objReg, UnboxedPlainObject::offsetOfExpando()), holderReg);
+
+        // Write the expando object's new shape.
+        Address shapeAddr(holderReg, ShapedObject::offsetOfShape());
+        EmitPreBarrier(masm, shapeAddr, MIRType::Shape);
+        masm.loadPtr(Address(ICStubReg, ICSetProp_NativeAdd::offsetOfNewShape()), scratch);
+        masm.storePtr(scratch, shapeAddr);
 
-    if (isFixedSlot_) {
-        holderReg = objReg;
+        if (!isFixedSlot_)
+            masm.loadPtr(Address(holderReg, NativeObject::offsetOfSlots()), holderReg);
     } else {
-        holderReg = regs.takeAny();
-        masm.loadPtr(Address(objReg, NativeObject::offsetOfSlots()), holderReg);
+        // Write the object's new shape.
+        Address shapeAddr(objReg, ShapedObject::offsetOfShape());
+        EmitPreBarrier(masm, shapeAddr, MIRType::Shape);
+        masm.loadPtr(Address(ICStubReg, ICSetProp_NativeAdd::offsetOfNewShape()), scratch);
+        masm.storePtr(scratch, shapeAddr);
+
+        if (isFixedSlot_) {
+            holderReg = objReg;
+        } else {
+            holderReg = regs.takeAny();
+            masm.loadPtr(Address(objReg, NativeObject::offsetOfSlots()), holderReg);
+        }
     }
 
     // Perform the store.  No write barrier required since this is a new
@@ -4663,6 +4963,70 @@ ICSetPropNativeAddCompiler::generateStubCode(MacroAssembler& masm)
 }
 
 bool
+ICSetProp_Unboxed::Compiler::generateStubCode(MacroAssembler& masm)
+{
+    MOZ_ASSERT(engine_ == Engine::Baseline);
+
+    Label failure;
+
+    // Guard input is an object.
+    masm.branchTestObject(Assembler::NotEqual, R0, &failure);
+
+    AllocatableGeneralRegisterSet regs(availableGeneralRegs(2));
+    Register scratch = regs.takeAny();
+
+    // Unbox and group guard.
+    Register object = masm.extractObject(R0, ExtractTemp0);
+    masm.loadPtr(Address(ICStubReg, ICSetProp_Unboxed::offsetOfGroup()), scratch);
+    masm.branchPtr(Assembler::NotEqual, Address(object, JSObject::offsetOfGroup()), scratch,
+                   &failure);
+
+    if (needsUpdateStubs()) {
+        // Stow both R0 and R1 (object and value).
+        EmitStowICValues(masm, 2);
+
+        // Move RHS into R0 for TypeUpdate check.
+        masm.moveValue(R1, R0);
+
+        // Call the type update stub.
+        if (!callTypeUpdateIC(masm, sizeof(Value)))
+            return false;
+
+        // Unstow R0 and R1 (object and key)
+        EmitUnstowICValues(masm, 2);
+
+        // The TypeUpdate IC may have smashed object. Rederive it.
+        masm.unboxObject(R0, object);
+
+        // Trigger post barriers here on the values being written. Fields which
+        // objects can be written to also need update stubs.
+        LiveGeneralRegisterSet saveRegs;
+        saveRegs.add(R0);
+        saveRegs.add(R1);
+        saveRegs.addUnchecked(object);
+        saveRegs.add(ICStubReg);
+        emitPostWriteBarrierSlot(masm, object, R1, scratch, saveRegs);
+    }
+
+    // Compute the address being written to.
+    masm.load32(Address(ICStubReg, ICSetProp_Unboxed::offsetOfFieldOffset()), scratch);
+    BaseIndex address(object, scratch, TimesOne);
+
+    EmitUnboxedPreBarrierForBaseline(masm, address, fieldType_);
+    masm.storeUnboxedProperty(address, fieldType_,
+                              ConstantOrRegister(TypedOrValueRegister(R1)), &failure);
+
+    // The RHS has to be in R0.
+    masm.moveValue(R1, R0);
+
+    EmitReturnFromIC(masm);
+
+    masm.bind(&failure);
+    EmitStubGuardFailure(masm);
+    return true;
+}
+
+bool
 ICSetProp_TypedObject::Compiler::generateStubCode(MacroAssembler& masm)
 {
     MOZ_ASSERT(engine_ == Engine::Baseline);
@@ -5135,6 +5499,13 @@ GetTemplateObjectForSimd(JSContext* cx, JSFunction* target, MutableHandleObject
     return true;
 }
 
+static void
+EnsureArrayGroupAnalyzed(JSContext* cx, JSObject* obj)
+{
+    if (PreliminaryObjectArrayWithTemplate* objects = obj->group()->maybePreliminaryObjects())
+        objects->maybeAnalyze(cx, obj->group(), /* forceAnalyze = */ true);
+}
+
 static bool
 GetTemplateObjectForNative(JSContext* cx, HandleFunction target, const CallArgs& args,
                            MutableHandleObject res, bool* skipAttach)
@@ -5166,7 +5537,10 @@ GetTemplateObjectForNative(JSContext* cx, HandleFunction target, const CallArgs&
             // With this and other array templates, analyze the group so that
             // we don't end up with a template whose structure might change later.
             res.set(NewFullyAllocatedArrayForCallingAllocationSite(cx, count, TenuredObject));
-            return !!res;
+            if (!res)
+                return false;
+            EnsureArrayGroupAnalyzed(cx, res);
+            return true;
         }
     }
 
@@ -5191,7 +5565,10 @@ GetTemplateObjectForNative(JSContext* cx, HandleFunction target, const CallArgs&
                     return true;
                 }
                 res.set(NewFullyAllocatedArrayTryReuseGroup(cx, obj, 0, TenuredObject));
-                return !!res;
+                if (!res)
+                    return false;
+                EnsureArrayGroupAnalyzed(cx, res);
+                return true;
             }
         }
     }
@@ -5208,7 +5585,10 @@ GetTemplateObjectForNative(JSContext* cx, HandleFunction target, const CallArgs&
         }
 
         res.set(NewFullyAllocatedArrayForCallingAllocationSite(cx, 0, TenuredObject));
-        return !!res;
+        if (!res)
+            return false;
+        EnsureArrayGroupAnalyzed(cx, res);
+        return true;
     }
 
     if (native == StringConstructor) {
@@ -5421,7 +5801,7 @@ TryAttachCallStub(JSContext* cx, ICCall_Fallback* stub, HandleScript script, jsb
             if (!thisObject)
                 return false;
 
-            if (thisObject->is<PlainObject>())
+            if (thisObject->is<PlainObject>() || thisObject->is<UnboxedPlainObject>())
                 templateObject = thisObject;
         }
 
@@ -5521,24 +5901,15 @@ TryAttachCallStub(JSContext* cx, ICCall_Fallback* stub, HandleScript script, jsb
 }
 
 static bool
-CopyArray(JSContext* cx, HandleArrayObject arr, MutableHandleValue result)
+CopyArray(JSContext* cx, HandleObject obj, MutableHandleValue result)
 {
-    uint32_t length = arr->length();
-    ArrayObject* nobj = NewFullyAllocatedArrayTryReuseGroup(cx, arr, length, TenuredObject);
+    uint32_t length = GetAnyBoxedOrUnboxedArrayLength(obj);
+    JSObject* nobj = NewFullyAllocatedArrayTryReuseGroup(cx, obj, length, TenuredObject);
     if (!nobj)
         return false;
-    
-    MOZ_ASSERT(arr->isNative());
-    MOZ_ASSERT(nobj->isNative());
-    MOZ_ASSERT(nobj->as<NativeObject>().getDenseInitializedLength() == 0);
-    MOZ_ASSERT(arr->as<NativeObject>().getDenseInitializedLength() >= length);
-    MOZ_ASSERT(nobj->as<NativeObject>().getDenseCapacity() >= length);
-
-    nobj->as<NativeObject>().setDenseInitializedLength(length);
-
-    const Value* vp = arr->as<NativeObject>().getDenseElements();
-    nobj->as<NativeObject>().initDenseElements(0, vp, length);
-    
+    EnsureArrayGroupAnalyzed(cx, nobj);
+    CopyAnyBoxedOrUnboxedDenseElements(cx, nobj, obj, 0, 0, length);
+
     result.setObject(*nobj);
     return true;
 }
@@ -5569,22 +5940,26 @@ TryAttachStringSplit(JSContext* cx, ICCall_Fallback* stub, HandleScript script,
     RootedValue arr(cx);
 
     // Copy the array before storing in stub.
-    if (!CopyArray(cx, obj.as<ArrayObject>(), &arr))
+    if (!CopyArray(cx, obj, &arr))
         return false;
 
     // Atomize all elements of the array.
-    RootedArrayObject arrObj(cx, &arr.toObject().as<ArrayObject>());
-    uint32_t initLength = arrObj->length();
+    RootedObject arrObj(cx, &arr.toObject());
+    uint32_t initLength = GetAnyBoxedOrUnboxedArrayLength(arrObj);
     for (uint32_t i = 0; i < initLength; i++) {
-        JSAtom* str = js::AtomizeString(cx, arrObj->getDenseElement(i).toString());
+        JSAtom* str = js::AtomizeString(cx, GetAnyBoxedOrUnboxedDenseElement(arrObj, i).toString());
         if (!str)
             return false;
 
-        arrObj->setDenseElementWithType(cx, i, StringValue(str));
+        if (!SetAnyBoxedOrUnboxedDenseElement(cx, arrObj, i, StringValue(str))) {
+            // The value could not be stored to an unboxed dense element.
+            return true;
+        }
     }
 
     ICCall_StringSplit::Compiler compiler(cx, stub->fallbackMonitorStub()->firstMonitorStub(),
-                                          script->pcToOffset(pc), str, sep, arrObj);
+                                          script->pcToOffset(pc), str, sep,
+                                          arr);
     ICStub* newStub = compiler.getStub(compiler.getStubSpace(script));
     if (!newStub)
         return false;
@@ -6472,7 +6847,7 @@ ICCallScriptedCompiler::generateStubCode(MacroAssembler& masm)
     return true;
 }
 
-typedef bool (*CopyArrayFn)(JSContext*, HandleArrayObject, MutableHandleValue);
+typedef bool (*CopyArrayFn)(JSContext*, HandleObject, MutableHandleValue);
 static const VMFunction CopyArrayInfo = FunctionInfo<CopyArrayFn>(CopyArray, "CopyArray");
 
 bool
@@ -7949,6 +8324,19 @@ ICGetElem_Dense::Clone(JSContext* cx, ICStubSpace* space, ICStub* firstMonitorSt
     return New<ICGetElem_Dense>(cx, space, other.jitCode(), firstMonitorStub, other.shape_);
 }
 
+ICGetElem_UnboxedArray::ICGetElem_UnboxedArray(JitCode* stubCode, ICStub* firstMonitorStub,
+                                               ObjectGroup *group)
+  : ICMonitoredStub(GetElem_UnboxedArray, stubCode, firstMonitorStub),
+    group_(group)
+{ }
+
+/* static */ ICGetElem_UnboxedArray*
+ICGetElem_UnboxedArray::Clone(JSContext* cx, ICStubSpace* space, ICStub* firstMonitorStub,
+                              ICGetElem_UnboxedArray& other)
+{
+    return New<ICGetElem_UnboxedArray>(cx, space, other.jitCode(), firstMonitorStub, other.group_);
+}
+
 ICGetElem_TypedArray::ICGetElem_TypedArray(JitCode* stubCode, Shape* shape, Scalar::Type type)
   : ICStub(GetElem_TypedArray, stubCode),
     shape_(shape)
@@ -8324,8 +8712,8 @@ static bool DoRestFallback(JSContext* cx, BaselineFrame* frame, ICRest_Fallback*
     unsigned numRest = numActuals > numFormals ? numActuals - numFormals : 0;
     Value* rest = frame->argv() + numFormals;
 
-    ArrayObject* obj = ObjectGroup::newArrayObject(cx, rest, numRest, GenericObject,
-                                                   ObjectGroup::NewArrayKind::UnknownIndex);
+    JSObject* obj = ObjectGroup::newArrayObject(cx, rest, numRest, GenericObject,
+                                                ObjectGroup::NewArrayKind::UnknownIndex);
     if (!obj)
         return false;
     res.setObject(*obj);
diff --git a/js/src/jit/BaselineIC.h b/js/src/jit/BaselineIC.h
index e1ad12559..5600f816a 100644
--- a/js/src/jit/BaselineIC.h
+++ b/js/src/jit/BaselineIC.h
@@ -22,6 +22,7 @@
 #include "jit/SharedICRegisters.h"
 #include "js/GCVector.h"
 #include "vm/ArrayObject.h"
+#include "vm/UnboxedObject.h"
 
 namespace js {
 namespace jit {
@@ -891,6 +892,54 @@ class ICGetElem_Dense : public ICMonitoredStub
     };
 };
 
+class ICGetElem_UnboxedArray : public ICMonitoredStub
+{
+    friend class ICStubSpace;
+
+    GCPtrObjectGroup group_;
+
+    ICGetElem_UnboxedArray(JitCode* stubCode, ICStub* firstMonitorStub, ObjectGroup* group);
+
+  public:
+    static ICGetElem_UnboxedArray* Clone(JSContext* cx, ICStubSpace* space,
+                                         ICStub* firstMonitorStub, ICGetElem_UnboxedArray& other);
+
+    static size_t offsetOfGroup() {
+        return offsetof(ICGetElem_UnboxedArray, group_);
+    }
+
+    GCPtrObjectGroup& group() {
+        return group_;
+    }
+
+    class Compiler : public ICStubCompiler {
+      ICStub* firstMonitorStub_;
+      RootedObjectGroup group_;
+      JSValueType elementType_;
+
+      protected:
+        MOZ_MUST_USE bool generateStubCode(MacroAssembler& masm);
+
+        virtual int32_t getKey() const {
+            return static_cast<int32_t>(engine_) |
+                  (static_cast<int32_t>(kind) << 1) |
+                  (static_cast<int32_t>(elementType_) << 17);
+        }
+
+      public:
+        Compiler(JSContext* cx, ICStub* firstMonitorStub, ObjectGroup* group)
+          : ICStubCompiler(cx, ICStub::GetElem_UnboxedArray, Engine::Baseline),
+            firstMonitorStub_(firstMonitorStub),
+            group_(cx, group),
+            elementType_(group->unboxedLayoutDontCheckGeneration().elementType())
+        {}
+
+        ICStub* getStub(ICStubSpace* space) {
+            return newStub<ICGetElem_UnboxedArray>(space, getStubCode(), firstMonitorStub_, group_);
+        }
+    };
+};
+
 // Accesses scalar elements of a typed array or typed object.
 class ICGetElem_TypedArray : public ICStub
 {
@@ -1066,7 +1115,9 @@ class ICSetElem_DenseOrUnboxedArray : public ICUpdatedStub
           : ICStubCompiler(cx, ICStub::SetElem_DenseOrUnboxedArray, Engine::Baseline),
             shape_(cx, shape),
             group_(cx, group),
-            unboxedType_(JSVAL_TYPE_MAGIC)
+            unboxedType_(shape
+                         ? JSVAL_TYPE_MAGIC
+                         : group->unboxedLayoutDontCheckGeneration().elementType())
         {}
 
         ICUpdatedStub* getStub(ICStubSpace* space) {
@@ -1174,7 +1225,9 @@ class ICSetElemDenseOrUnboxedArrayAddCompiler : public ICStubCompiler {
         : ICStubCompiler(cx, ICStub::SetElem_DenseOrUnboxedArrayAdd, Engine::Baseline),
           obj_(cx, obj),
           protoChainDepth_(protoChainDepth),
-          unboxedType_(JSVAL_TYPE_MAGIC)
+          unboxedType_(obj->is<UnboxedArrayObject>()
+                       ? obj->as<UnboxedArrayObject>().elementType()
+                       : JSVAL_TYPE_MAGIC)
     {}
 
     template <size_t ProtoChainDepth>
@@ -1822,7 +1875,8 @@ class ICSetProp_Native : public ICUpdatedStub
         virtual int32_t getKey() const {
             return static_cast<int32_t>(engine_) |
                   (static_cast<int32_t>(kind) << 1) |
-                  (static_cast<int32_t>(isFixedSlot_) << 17);
+                  (static_cast<int32_t>(isFixedSlot_) << 17) |
+                  (static_cast<int32_t>(obj_->is<UnboxedPlainObject>()) << 18);
         }
 
         MOZ_MUST_USE bool generateStubCode(MacroAssembler& masm);
@@ -1927,6 +1981,7 @@ class ICSetPropNativeAddCompiler : public ICStubCompiler
         return static_cast<int32_t>(engine_) |
               (static_cast<int32_t>(kind) << 1) |
               (static_cast<int32_t>(isFixedSlot_) << 17) |
+              (static_cast<int32_t>(obj_->is<UnboxedPlainObject>()) << 18) |
               (static_cast<int32_t>(protoChainDepth_) << 19);
     }
 
@@ -1951,7 +2006,10 @@ class ICSetPropNativeAddCompiler : public ICStubCompiler
             newGroup = nullptr;
 
         RootedShape newShape(cx);
-        newShape = obj_->as<NativeObject>().lastProperty();
+        if (obj_->isNative())
+            newShape = obj_->as<NativeObject>().lastProperty();
+        else
+            newShape = obj_->as<UnboxedPlainObject>().maybeExpando()->lastProperty();
 
         return newStub<ICSetProp_NativeAddImpl<ProtoChainDepth>>(
             space, getStubCode(), oldGroup_, shapes, newShape, newGroup, offset_);
@@ -2816,10 +2874,10 @@ class ICCall_StringSplit : public ICMonitoredStub
     uint32_t pcOffset_;
     GCPtrString expectedStr_;
     GCPtrString expectedSep_;
-    GCPtrArrayObject templateObject_;
+    GCPtrObject templateObject_;
 
     ICCall_StringSplit(JitCode* stubCode, ICStub* firstMonitorStub, uint32_t pcOffset, JSString* str,
-                       JSString* sep, ArrayObject* templateObject)
+                       JSString* sep, JSObject* templateObject)
       : ICMonitoredStub(ICStub::Call_StringSplit, stubCode, firstMonitorStub),
         pcOffset_(pcOffset), expectedStr_(str), expectedSep_(sep),
         templateObject_(templateObject)
@@ -2846,7 +2904,7 @@ class ICCall_StringSplit : public ICMonitoredStub
         return expectedSep_;
     }
 
-    GCPtrArrayObject& templateObject() {
+    GCPtrObject& templateObject() {
         return templateObject_;
     }
 
@@ -2856,7 +2914,7 @@ class ICCall_StringSplit : public ICMonitoredStub
         uint32_t pcOffset_;
         RootedString expectedStr_;
         RootedString expectedSep_;
-        RootedArrayObject templateObject_;
+        RootedObject templateObject_;
 
         MOZ_MUST_USE bool generateStubCode(MacroAssembler& masm);
 
@@ -2867,13 +2925,13 @@ class ICCall_StringSplit : public ICMonitoredStub
 
       public:
         Compiler(JSContext* cx, ICStub* firstMonitorStub, uint32_t pcOffset, HandleString str,
-                 HandleString sep, HandleArrayObject templateObject)
+                 HandleString sep, HandleValue templateObject)
           : ICCallStubCompiler(cx, ICStub::Call_StringSplit),
             firstMonitorStub_(firstMonitorStub),
             pcOffset_(pcOffset),
             expectedStr_(cx, str),
             expectedSep_(cx, sep),
-            templateObject_(cx, templateObject)
+            templateObject_(cx, &templateObject.toObject())
         { }
 
         ICStub* getStub(ICStubSpace* space) {
diff --git a/js/src/jit/BaselineInspector.cpp b/js/src/jit/BaselineInspector.cpp
index 3b852debf..c9e09bed7 100644
--- a/js/src/jit/BaselineInspector.cpp
+++ b/js/src/jit/BaselineInspector.cpp
@@ -96,19 +96,32 @@ VectorAppendNoDuplicate(S& list, T value)
 
 static bool
 AddReceiver(const ReceiverGuard& receiver,
-            BaselineInspector::ReceiverVector& receivers)
+            BaselineInspector::ReceiverVector& receivers,
+            BaselineInspector::ObjectGroupVector& convertUnboxedGroups)
 {
+    if (receiver.group && receiver.group->maybeUnboxedLayout()) {
+        if (receiver.group->unboxedLayout().nativeGroup())
+            return VectorAppendNoDuplicate(convertUnboxedGroups, receiver.group);
+    }
     return VectorAppendNoDuplicate(receivers, receiver);
 }
 
 static bool
 GetCacheIRReceiverForNativeReadSlot(ICCacheIR_Monitored* stub, ReceiverGuard* receiver)
 {
-    // We match:
+    // We match either:
     //
     //   GuardIsObject 0
     //   GuardShape 0
     //   LoadFixedSlotResult 0 or LoadDynamicSlotResult 0
+    //
+    // or
+    //
+    //   GuardIsObject 0
+    //   GuardGroup 0
+    //   1: GuardAndLoadUnboxedExpando 0
+    //   GuardShape 1
+    //   LoadFixedSlotResult 1 or LoadDynamicSlotResult 1
 
     *receiver = ReceiverGuard();
     CacheIRReader reader(stub->stubInfo());
@@ -117,6 +130,14 @@ GetCacheIRReceiverForNativeReadSlot(ICCacheIR_Monitored* stub, ReceiverGuard* re
     if (!reader.matchOp(CacheOp::GuardIsObject, objId))
         return false;
 
+    if (reader.matchOp(CacheOp::GuardGroup, objId)) {
+        receiver->group = stub->stubInfo()->getStubField<ObjectGroup*>(stub, reader.stubOffset());
+
+        if (!reader.matchOp(CacheOp::GuardAndLoadUnboxedExpando, objId))
+            return false;
+        objId = reader.objOperandId();
+    }
+
     if (reader.matchOp(CacheOp::GuardShape, objId)) {
         receiver->shape = stub->stubInfo()->getStubField<Shape*>(stub, reader.stubOffset());
         return reader.matchOpEither(CacheOp::LoadFixedSlotResult, CacheOp::LoadDynamicSlotResult);
@@ -125,13 +146,40 @@ GetCacheIRReceiverForNativeReadSlot(ICCacheIR_Monitored* stub, ReceiverGuard* re
     return false;
 }
 
+static bool
+GetCacheIRReceiverForUnboxedProperty(ICCacheIR_Monitored* stub, ReceiverGuard* receiver)
+{
+    // We match:
+    //
+    //   GuardIsObject 0
+    //   GuardGroup 0
+    //   LoadUnboxedPropertyResult 0 ..
+
+    *receiver = ReceiverGuard();
+    CacheIRReader reader(stub->stubInfo());
+
+    ObjOperandId objId = ObjOperandId(0);
+    if (!reader.matchOp(CacheOp::GuardIsObject, objId))
+        return false;
+
+    if (!reader.matchOp(CacheOp::GuardGroup, objId))
+        return false;
+    receiver->group = stub->stubInfo()->getStubField<ObjectGroup*>(stub, reader.stubOffset());
+
+    return reader.matchOp(CacheOp::LoadUnboxedPropertyResult, objId);
+}
+
 bool
-BaselineInspector::maybeInfoForPropertyOp(jsbytecode* pc, ReceiverVector& receivers)
+BaselineInspector::maybeInfoForPropertyOp(jsbytecode* pc, ReceiverVector& receivers,
+                                          ObjectGroupVector& convertUnboxedGroups)
 {
     // Return a list of the receivers seen by the baseline IC for the current
     // op. Empty lists indicate no receivers are known, or there was an
-    // uncacheable access.
+    // uncacheable access. convertUnboxedGroups is used for unboxed object
+    // groups which have been seen, but have had instances converted to native
+    // objects and should be eagerly converted by Ion.
     MOZ_ASSERT(receivers.empty());
+    MOZ_ASSERT(convertUnboxedGroups.empty());
 
     if (!hasBaselineScript())
         return true;
@@ -143,7 +191,8 @@ BaselineInspector::maybeInfoForPropertyOp(jsbytecode* pc, ReceiverVector& receiv
     while (stub->next()) {
         ReceiverGuard receiver;
         if (stub->isCacheIR_Monitored()) {
-            if (!GetCacheIRReceiverForNativeReadSlot(stub->toCacheIR_Monitored(), &receiver))
+            if (!GetCacheIRReceiverForNativeReadSlot(stub->toCacheIR_Monitored(), &receiver) &&
+                !GetCacheIRReceiverForUnboxedProperty(stub->toCacheIR_Monitored(), &receiver))
             {
                 receivers.clear();
                 return true;
@@ -151,12 +200,14 @@ BaselineInspector::maybeInfoForPropertyOp(jsbytecode* pc, ReceiverVector& receiv
         } else if (stub->isSetProp_Native()) {
             receiver = ReceiverGuard(stub->toSetProp_Native()->group(),
                                      stub->toSetProp_Native()->shape());
+        } else if (stub->isSetProp_Unboxed()) {
+            receiver = ReceiverGuard(stub->toSetProp_Unboxed()->group(), nullptr);
         } else {
             receivers.clear();
             return true;
         }
 
-        if (!AddReceiver(receiver, receivers))
+        if (!AddReceiver(receiver, receivers, convertUnboxedGroups))
             return false;
 
         stub = stub->next();
@@ -538,7 +589,7 @@ BaselineInspector::getTemplateObjectForNative(jsbytecode* pc, Native native)
 
 bool
 BaselineInspector::isOptimizableCallStringSplit(jsbytecode* pc, JSString** strOut, JSString** sepOut,
-                                                ArrayObject** objOut)
+                                                JSObject** objOut)
 {
     if (!hasBaselineScript())
         return false;
@@ -649,12 +700,14 @@ bool
 BaselineInspector::commonGetPropFunction(jsbytecode* pc, JSObject** holder, Shape** holderShape,
                                          JSFunction** commonGetter, Shape** globalShape,
                                          bool* isOwnProperty,
-                                         ReceiverVector& receivers)
+                                         ReceiverVector& receivers,
+                                         ObjectGroupVector& convertUnboxedGroups)
 {
     if (!hasBaselineScript())
         return false;
 
     MOZ_ASSERT(receivers.empty());
+    MOZ_ASSERT(convertUnboxedGroups.empty());
 
     *holder = nullptr;
     const ICEntry& entry = icEntryFromPC(pc);
@@ -666,7 +719,7 @@ BaselineInspector::commonGetPropFunction(jsbytecode* pc, JSObject** holder, Shap
         {
             ICGetPropCallGetter* nstub = static_cast<ICGetPropCallGetter*>(stub);
             bool isOwn = nstub->isOwnGetter();
-            if (!isOwn && !AddReceiver(nstub->receiverGuard(), receivers))
+            if (!isOwn && !AddReceiver(nstub->receiverGuard(), receivers, convertUnboxedGroups))
                 return false;
 
             if (!*holder) {
@@ -698,19 +751,21 @@ BaselineInspector::commonGetPropFunction(jsbytecode* pc, JSObject** holder, Shap
     if (!*holder)
         return false;
 
-    MOZ_ASSERT(*isOwnProperty == (receivers.empty()));
+    MOZ_ASSERT(*isOwnProperty == (receivers.empty() && convertUnboxedGroups.empty()));
     return true;
 }
 
 bool
 BaselineInspector::commonSetPropFunction(jsbytecode* pc, JSObject** holder, Shape** holderShape,
                                          JSFunction** commonSetter, bool* isOwnProperty,
-                                         ReceiverVector& receivers)
+                                         ReceiverVector& receivers,
+                                         ObjectGroupVector& convertUnboxedGroups)
 {
     if (!hasBaselineScript())
         return false;
 
     MOZ_ASSERT(receivers.empty());
+    MOZ_ASSERT(convertUnboxedGroups.empty());
 
     *holder = nullptr;
     const ICEntry& entry = icEntryFromPC(pc);
@@ -719,7 +774,7 @@ BaselineInspector::commonSetPropFunction(jsbytecode* pc, JSObject** holder, Shap
         if (stub->isSetProp_CallScripted() || stub->isSetProp_CallNative()) {
             ICSetPropCallSetter* nstub = static_cast<ICSetPropCallSetter*>(stub);
             bool isOwn = nstub->isOwnSetter();
-            if (!isOwn && !AddReceiver(nstub->receiverGuard(), receivers))
+            if (!isOwn && !AddReceiver(nstub->receiverGuard(), receivers, convertUnboxedGroups))
                 return false;
 
             if (!*holder) {
diff --git a/js/src/jit/BaselineInspector.h b/js/src/jit/BaselineInspector.h
index 1ed4b5547..961df18aa 100644
--- a/js/src/jit/BaselineInspector.h
+++ b/js/src/jit/BaselineInspector.h
@@ -95,7 +95,8 @@ class BaselineInspector
   public:
     typedef Vector<ReceiverGuard, 4, JitAllocPolicy> ReceiverVector;
     typedef Vector<ObjectGroup*, 4, JitAllocPolicy> ObjectGroupVector;
-    MOZ_MUST_USE bool maybeInfoForPropertyOp(jsbytecode* pc, ReceiverVector& receivers);
+    MOZ_MUST_USE bool maybeInfoForPropertyOp(jsbytecode* pc, ReceiverVector& receivers,
+                                             ObjectGroupVector& convertUnboxedGroups);
 
     SetElemICInspector setElemICInspector(jsbytecode* pc) {
         return makeICInspector<SetElemICInspector>(pc, ICStub::SetElem_Fallback);
@@ -113,7 +114,7 @@ class BaselineInspector
     bool hasSeenNonStringIterMore(jsbytecode* pc);
 
     MOZ_MUST_USE bool isOptimizableCallStringSplit(jsbytecode* pc, JSString** strOut,
-                                                   JSString** sepOut, ArrayObject** objOut);
+                                                   JSString** sepOut, JSObject** objOut);
     JSObject* getTemplateObject(jsbytecode* pc);
     JSObject* getTemplateObjectForNative(jsbytecode* pc, Native native);
     JSObject* getTemplateObjectForClassHook(jsbytecode* pc, const Class* clasp);
@@ -130,10 +131,12 @@ class BaselineInspector
 
     MOZ_MUST_USE bool commonGetPropFunction(jsbytecode* pc, JSObject** holder, Shape** holderShape,
                                             JSFunction** commonGetter, Shape** globalShape,
-                                            bool* isOwnProperty, ReceiverVector& receivers);
+                                            bool* isOwnProperty, ReceiverVector& receivers,
+                                            ObjectGroupVector& convertUnboxedGroups);
     MOZ_MUST_USE bool commonSetPropFunction(jsbytecode* pc, JSObject** holder, Shape** holderShape,
                                             JSFunction** commonSetter, bool* isOwnProperty,
-                                            ReceiverVector& receivers);
+                                            ReceiverVector& receivers,
+                                            ObjectGroupVector& convertUnboxedGroups);
 
     MOZ_MUST_USE bool instanceOfData(jsbytecode* pc, Shape** shape, uint32_t* slot,
                                      JSObject** prototypeObject);
diff --git a/js/src/jit/CacheIR.cpp b/js/src/jit/CacheIR.cpp
index d184ea40c..f1061af70 100644
--- a/js/src/jit/CacheIR.cpp
+++ b/js/src/jit/CacheIR.cpp
@@ -10,7 +10,8 @@
 #include "jit/IonCaches.h"
 
 #include "jsobjinlines.h"
-#include "vm/NativeObject-inl.h"
+
+#include "vm/UnboxedObject-inl.h"
 
 using namespace js;
 using namespace js::jit;
@@ -59,6 +60,10 @@ GetPropIRGenerator::tryAttachStub(Maybe<CacheIRWriter>& writer)
             return false;
         if (!emitted_ && !tryAttachNative(*writer, obj, objId))
             return false;
+        if (!emitted_ && !tryAttachUnboxed(*writer, obj, objId))
+            return false;
+        if (!emitted_ && !tryAttachUnboxedExpando(*writer, obj, objId))
+            return false;
         if (!emitted_ && !tryAttachTypedObject(*writer, obj, objId))
             return false;
         if (!emitted_ && !tryAttachModuleNamespace(*writer, obj, objId))
@@ -158,9 +163,19 @@ GeneratePrototypeGuards(CacheIRWriter& writer, JSObject* obj, JSObject* holder,
 }
 
 static void
-TestMatchingReceiver(CacheIRWriter& writer, JSObject* obj, Shape* shape, ObjOperandId objId)
+TestMatchingReceiver(CacheIRWriter& writer, JSObject* obj, Shape* shape, ObjOperandId objId,
+                     Maybe<ObjOperandId>* expandoId)
 {
-    if (obj->is<TypedObject>()) {
+    if (obj->is<UnboxedPlainObject>()) {
+        writer.guardGroup(objId, obj->group());
+
+        if (UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando()) {
+            expandoId->emplace(writer.guardAndLoadUnboxedExpando(objId));
+            writer.guardShape(expandoId->ref(), expando->lastProperty());
+        } else {
+            writer.guardNoUnboxedExpando(objId);
+        }
+    } else if (obj->is<UnboxedArrayObject>() || obj->is<TypedObject>()) {
         writer.guardGroup(objId, obj->group());
     } else {
         Shape* shape = obj->maybeShape();
@@ -173,7 +188,8 @@ static void
 EmitReadSlotResult(CacheIRWriter& writer, JSObject* obj, JSObject* holder,
                    Shape* shape, ObjOperandId objId)
 {
-    TestMatchingReceiver(writer, obj, shape, objId);
+    Maybe<ObjOperandId> expandoId;
+    TestMatchingReceiver(writer, obj, shape, objId, &expandoId);
 
     ObjOperandId holderId;
     if (obj != holder) {
@@ -196,6 +212,9 @@ EmitReadSlotResult(CacheIRWriter& writer, JSObject* obj, JSObject* holder,
                 lastObjId = protoId;
             }
         }
+    } else if (obj->is<UnboxedPlainObject>()) {
+        holder = obj->as<UnboxedPlainObject>().maybeExpando();
+        holderId = *expandoId;
     } else {
         holderId = objId;
     }
@@ -247,6 +266,51 @@ GetPropIRGenerator::tryAttachNative(CacheIRWriter& writer, HandleObject obj, Obj
 }
 
 bool
+GetPropIRGenerator::tryAttachUnboxed(CacheIRWriter& writer, HandleObject obj, ObjOperandId objId)
+{
+    MOZ_ASSERT(!emitted_);
+
+    if (!obj->is<UnboxedPlainObject>())
+        return true;
+
+    const UnboxedLayout::Property* property = obj->as<UnboxedPlainObject>().layout().lookup(name_);
+    if (!property)
+        return true;
+
+    if (!cx_->runtime()->jitSupportsFloatingPoint)
+        return true;
+
+    writer.guardGroup(objId, obj->group());
+    writer.loadUnboxedPropertyResult(objId, property->type,
+                                     UnboxedPlainObject::offsetOfData() + property->offset);
+    emitted_ = true;
+    preliminaryObjectAction_ = PreliminaryObjectAction::Unlink;
+    return true;
+}
+
+bool
+GetPropIRGenerator::tryAttachUnboxedExpando(CacheIRWriter& writer, HandleObject obj, ObjOperandId objId)
+{
+    MOZ_ASSERT(!emitted_);
+
+    if (!obj->is<UnboxedPlainObject>())
+        return true;
+
+    UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando();
+    if (!expando)
+        return true;
+
+    Shape* shape = expando->lookup(cx_, NameToId(name_));
+    if (!shape || !shape->hasDefaultGetter() || !shape->hasSlot())
+        return true;
+
+    emitted_ = true;
+
+    EmitReadSlotResult(writer, obj, obj, shape, objId);
+    return true;
+}
+
+bool
 GetPropIRGenerator::tryAttachTypedObject(CacheIRWriter& writer, HandleObject obj, ObjOperandId objId)
 {
     MOZ_ASSERT(!emitted_);
@@ -304,6 +368,13 @@ GetPropIRGenerator::tryAttachObjectLength(CacheIRWriter& writer, HandleObject ob
         return true;
     }
 
+    if (obj->is<UnboxedArrayObject>()) {
+        writer.guardClass(objId, GuardClassKind::UnboxedArray);
+        writer.loadUnboxedArrayLengthResult(objId);
+        emitted_ = true;
+        return true;
+    }
+
     if (obj->is<ArgumentsObject>() && !obj->as<ArgumentsObject>().hasOverriddenLength()) {
         if (obj->is<MappedArgumentsObject>()) {
             writer.guardClass(objId, GuardClassKind::MappedArguments);
diff --git a/js/src/jit/CacheIR.h b/js/src/jit/CacheIR.h
index ae55cfebb..51e55f48b 100644
--- a/js/src/jit/CacheIR.h
+++ b/js/src/jit/CacheIR.h
@@ -87,12 +87,16 @@ class ObjOperandId : public OperandId
     _(GuardClass)                         \
     _(GuardSpecificObject)                \
     _(GuardNoDetachedTypedObjects)        \
+    _(GuardNoUnboxedExpando)              \
+    _(GuardAndLoadUnboxedExpando)         \
     _(LoadObject)                         \
     _(LoadProto)                          \
     _(LoadFixedSlotResult)                \
     _(LoadDynamicSlotResult)              \
+    _(LoadUnboxedPropertyResult)          \
     _(LoadTypedObjectResult)              \
     _(LoadInt32ArrayLengthResult)         \
+    _(LoadUnboxedArrayLengthResult)       \
     _(LoadArgumentsObjectLengthResult)    \
     _(LoadUndefinedResult)
 
@@ -124,6 +128,7 @@ struct StubField {
 enum class GuardClassKind
 {
     Array,
+    UnboxedArray,
     MappedArguments,
     UnmappedArguments,
 };
@@ -271,6 +276,15 @@ class MOZ_RAII CacheIRWriter
     void guardNoDetachedTypedObjects() {
         writeOp(CacheOp::GuardNoDetachedTypedObjects);
     }
+    void guardNoUnboxedExpando(ObjOperandId obj) {
+        writeOpWithOperandId(CacheOp::GuardNoUnboxedExpando, obj);
+    }
+    ObjOperandId guardAndLoadUnboxedExpando(ObjOperandId obj) {
+        ObjOperandId res(nextOperandId_++);
+        writeOpWithOperandId(CacheOp::GuardAndLoadUnboxedExpando, obj);
+        writeOperandId(res);
+        return res;
+    }
 
     ObjOperandId loadObject(JSObject* obj) {
         ObjOperandId res(nextOperandId_++);
@@ -296,6 +310,11 @@ class MOZ_RAII CacheIRWriter
         writeOpWithOperandId(CacheOp::LoadDynamicSlotResult, obj);
         addStubWord(offset, StubField::GCType::NoGCThing);
     }
+    void loadUnboxedPropertyResult(ObjOperandId obj, JSValueType type, size_t offset) {
+        writeOpWithOperandId(CacheOp::LoadUnboxedPropertyResult, obj);
+        buffer_.writeByte(uint32_t(type));
+        addStubWord(offset, StubField::GCType::NoGCThing);
+    }
     void loadTypedObjectResult(ObjOperandId obj, uint32_t offset, TypedThingLayout layout,
                                uint32_t typeDescr) {
         MOZ_ASSERT(uint32_t(layout) <= UINT8_MAX);
@@ -308,6 +327,9 @@ class MOZ_RAII CacheIRWriter
     void loadInt32ArrayLengthResult(ObjOperandId obj) {
         writeOpWithOperandId(CacheOp::LoadInt32ArrayLengthResult, obj);
     }
+    void loadUnboxedArrayLengthResult(ObjOperandId obj) {
+        writeOpWithOperandId(CacheOp::LoadUnboxedArrayLengthResult, obj);
+    }
     void loadArgumentsObjectLengthResult(ObjOperandId obj) {
         writeOpWithOperandId(CacheOp::LoadArgumentsObjectLengthResult, obj);
     }
@@ -389,6 +411,9 @@ class MOZ_RAII GetPropIRGenerator
     PreliminaryObjectAction preliminaryObjectAction_;
 
     MOZ_MUST_USE bool tryAttachNative(CacheIRWriter& writer, HandleObject obj, ObjOperandId objId);
+    MOZ_MUST_USE bool tryAttachUnboxed(CacheIRWriter& writer, HandleObject obj, ObjOperandId objId);
+    MOZ_MUST_USE bool tryAttachUnboxedExpando(CacheIRWriter& writer, HandleObject obj,
+                                              ObjOperandId objId);
     MOZ_MUST_USE bool tryAttachTypedObject(CacheIRWriter& writer, HandleObject obj,
                                            ObjOperandId objId);
     MOZ_MUST_USE bool tryAttachObjectLength(CacheIRWriter& writer, HandleObject obj,
diff --git a/js/src/jit/CodeGenerator.cpp b/js/src/jit/CodeGenerator.cpp
index 2b1c671d1..bb12b09c8 100644
--- a/js/src/jit/CodeGenerator.cpp
+++ b/js/src/jit/CodeGenerator.cpp
@@ -25,7 +25,6 @@
 #include "builtin/Eval.h"
 #include "builtin/TypedObject.h"
 #include "gc/Nursery.h"
-#include "gc/StoreBuffer-inl.h"
 #include "irregexp/NativeRegExpMacroAssembler.h"
 #include "jit/AtomicOperations.h"
 #include "jit/BaselineCompiler.h"
@@ -3029,10 +3028,19 @@ CodeGenerator::visitStoreSlotV(LStoreSlotV* lir)
 
 static void
 GuardReceiver(MacroAssembler& masm, const ReceiverGuard& guard,
-              Register obj, Register scratch, Label* miss)
+              Register obj, Register scratch, Label* miss, bool checkNullExpando)
 {
     if (guard.group) {
         masm.branchTestObjGroup(Assembler::NotEqual, obj, guard.group, miss);
+
+        Address expandoAddress(obj, UnboxedPlainObject::offsetOfExpando());
+        if (guard.shape) {
+            masm.loadPtr(expandoAddress, scratch);
+            masm.branchPtr(Assembler::Equal, scratch, ImmWord(0), miss);
+            masm.branchTestObjShape(Assembler::NotEqual, scratch, guard.shape, miss);
+        } else if (checkNullExpando) {
+            masm.branchPtr(Assembler::NotEqual, expandoAddress, ImmWord(0), miss);
+        }
     } else {
         masm.branchTestObjShape(Assembler::NotEqual, obj, guard.shape, miss);
     }
@@ -3051,11 +3059,13 @@ CodeGenerator::emitGetPropertyPolymorphic(LInstruction* ins, Register obj, Regis
 
         Label next;
         masm.comment("GuardReceiver");
-        GuardReceiver(masm, receiver, obj, scratch, &next);
+        GuardReceiver(masm, receiver, obj, scratch, &next, /* checkNullExpando = */ false);
 
         if (receiver.shape) {
             masm.comment("loadTypedOrValue");
-            Register target = obj;
+            // If this is an unboxed expando access, GuardReceiver loaded the
+            // expando object into scratch.
+            Register target = receiver.group ? scratch : obj;
 
             Shape* shape = mir->shape(i);
             if (shape->slot() < shape->numFixedSlots()) {
@@ -3068,6 +3078,13 @@ CodeGenerator::emitGetPropertyPolymorphic(LInstruction* ins, Register obj, Regis
                 masm.loadPtr(Address(target, NativeObject::offsetOfSlots()), scratch);
                 masm.loadTypedOrValue(Address(scratch, offset), output);
             }
+        } else {
+            masm.comment("loadUnboxedProperty");
+            const UnboxedLayout::Property* property =
+                receiver.group->unboxedLayout().lookup(mir->name());
+            Address propertyAddr(obj, UnboxedPlainObject::offsetOfData() + property->offset);
+
+            masm.loadUnboxedProperty(propertyAddr, property->type, output);
         }
 
         if (i == mir->numReceivers() - 1) {
@@ -3108,6 +3125,8 @@ EmitUnboxedPreBarrier(MacroAssembler &masm, T address, JSValueType type)
         masm.patchableCallPreBarrier(address, MIRType::Object);
     else if (type == JSVAL_TYPE_STRING)
         masm.patchableCallPreBarrier(address, MIRType::String);
+    else
+        MOZ_ASSERT(!UnboxedTypeNeedsPreBarrier(type));
 }
 
 void
@@ -3121,10 +3140,12 @@ CodeGenerator::emitSetPropertyPolymorphic(LInstruction* ins, Register obj, Regis
         ReceiverGuard receiver = mir->receiver(i);
 
         Label next;
-        GuardReceiver(masm, receiver, obj, scratch, &next);
+        GuardReceiver(masm, receiver, obj, scratch, &next, /* checkNullExpando = */ false);
 
         if (receiver.shape) {
-            Register target = obj;
+            // If this is an unboxed expando access, GuardReceiver loaded the
+            // expando object into scratch.
+            Register target = receiver.group ? scratch : obj;
 
             Shape* shape = mir->shape(i);
             if (shape->slot() < shape->numFixedSlots()) {
@@ -3141,6 +3162,13 @@ CodeGenerator::emitSetPropertyPolymorphic(LInstruction* ins, Register obj, Regis
                     emitPreBarrier(addr);
                 masm.storeConstantOrRegister(value, addr);
             }
+        } else {
+            const UnboxedLayout::Property* property =
+                receiver.group->unboxedLayout().lookup(mir->name());
+            Address propertyAddr(obj, UnboxedPlainObject::offsetOfData() + property->offset);
+
+            EmitUnboxedPreBarrier(masm, propertyAddr, property->type);
+            masm.storeUnboxedProperty(propertyAddr, property->type, value, nullptr);
         }
 
         if (i == mir->numReceivers() - 1) {
@@ -3181,7 +3209,9 @@ CodeGenerator::visitSetPropertyPolymorphicT(LSetPropertyPolymorphicT* ins)
 void
 CodeGenerator::visitElements(LElements* lir)
 {
-    Address elements(ToRegister(lir->object()), NativeObject::offsetOfElements());
+    Address elements(ToRegister(lir->object()),
+                     lir->mir()->unboxed() ? UnboxedArrayObject::offsetOfElements()
+                                           : NativeObject::offsetOfElements());
     masm.loadPtr(elements, ToRegister(lir->output()));
 }
 
@@ -3290,7 +3320,7 @@ CodeGenerator::visitGuardReceiverPolymorphic(LGuardReceiverPolymorphic* lir)
         const ReceiverGuard& receiver = mir->receiver(i);
 
         Label next;
-        GuardReceiver(masm, receiver, obj, temp, &next);
+        GuardReceiver(masm, receiver, obj, temp, &next, /* checkNullExpando = */ true);
 
         if (i == mir->numReceivers() - 1) {
             bailoutFrom(&next, lir->snapshot());
@@ -3304,6 +3334,27 @@ CodeGenerator::visitGuardReceiverPolymorphic(LGuardReceiverPolymorphic* lir)
 }
 
 void
+CodeGenerator::visitGuardUnboxedExpando(LGuardUnboxedExpando* lir)
+{
+    Label miss;
+
+    Register obj = ToRegister(lir->object());
+    masm.branchPtr(lir->mir()->requireExpando() ? Assembler::Equal : Assembler::NotEqual,
+                   Address(obj, UnboxedPlainObject::offsetOfExpando()), ImmWord(0), &miss);
+
+    bailoutFrom(&miss, lir->snapshot());
+}
+
+void
+CodeGenerator::visitLoadUnboxedExpando(LLoadUnboxedExpando* lir)
+{
+    Register obj = ToRegister(lir->object());
+    Register result = ToRegister(lir->getDef(0));
+
+    masm.loadPtr(Address(obj, UnboxedPlainObject::offsetOfExpando()), result);
+}
+
+void
 CodeGenerator::visitTypeBarrierV(LTypeBarrierV* lir)
 {
     ValueOperand operand = ToValue(lir, LTypeBarrierV::Input);
@@ -5166,11 +5217,11 @@ static JSObject*
 NewArrayWithGroup(JSContext* cx, uint32_t length, HandleObjectGroup group,
                   bool convertDoubleElements)
 {
-    ArrayObject* res = NewFullyAllocatedArrayTryUseGroup(cx, group, length);
+    JSObject* res = NewFullyAllocatedArrayTryUseGroup(cx, group, length);
     if (!res)
         return nullptr;
     if (convertDoubleElements)
-        res->setShouldConvertDoubleElements();
+        res->as<ArrayObject>().setShouldConvertDoubleElements();
     return res;
 }
 
@@ -5316,7 +5367,7 @@ CodeGenerator::visitNewArrayCopyOnWrite(LNewArrayCopyOnWrite* lir)
     masm.bind(ool->rejoin());
 }
 
-typedef ArrayObject* (*ArrayConstructorOneArgFn)(JSContext*, HandleObjectGroup, int32_t length);
+typedef JSObject* (*ArrayConstructorOneArgFn)(JSContext*, HandleObjectGroup, int32_t length);
 static const VMFunction ArrayConstructorOneArgInfo =
     FunctionInfo<ArrayConstructorOneArgFn>(ArrayConstructorOneArg, "ArrayConstructorOneArg");
 
@@ -5336,11 +5387,21 @@ CodeGenerator::visitNewArrayDynamicLength(LNewArrayDynamicLength* lir)
 
     bool canInline = true;
     size_t inlineLength = 0;
-    if (templateObject->as<ArrayObject>().hasFixedElements()) {
-        size_t numSlots = gc::GetGCKindSlots(templateObject->asTenured().getAllocKind());
-        inlineLength = numSlots - ObjectElements::VALUES_PER_HEADER;
+    if (templateObject->is<ArrayObject>()) {
+        if (templateObject->as<ArrayObject>().hasFixedElements()) {
+            size_t numSlots = gc::GetGCKindSlots(templateObject->asTenured().getAllocKind());
+            inlineLength = numSlots - ObjectElements::VALUES_PER_HEADER;
+        } else {
+            canInline = false;
+        }
     } else {
-        canInline = false;
+        if (templateObject->as<UnboxedArrayObject>().hasInlineElements()) {
+            size_t nbytes =
+                templateObject->tenuredSizeOfThis() - UnboxedArrayObject::offsetOfInlineElements();
+            inlineLength = nbytes / templateObject->as<UnboxedArrayObject>().elementSize();
+        } else {
+            canInline = false;
+        }
     }
 
     if (canInline) {
@@ -7762,7 +7823,7 @@ CodeGenerator::visitSinCos(LSinCos *lir)
     masm.freeStack(sizeof(double) * 2);
 }
 
-typedef ArrayObject* (*StringSplitFn)(JSContext*, HandleObjectGroup, HandleString, HandleString, uint32_t);
+typedef JSObject* (*StringSplitFn)(JSContext*, HandleObjectGroup, HandleString, HandleString, uint32_t);
 static const VMFunction StringSplitInfo =
     FunctionInfo<StringSplitFn>(js::str_split_string, "str_split_string");
 
@@ -7797,6 +7858,49 @@ CodeGenerator::visitSetInitializedLength(LSetInitializedLength* lir)
 }
 
 void
+CodeGenerator::visitUnboxedArrayLength(LUnboxedArrayLength* lir)
+{
+    Register obj = ToRegister(lir->object());
+    Register result = ToRegister(lir->output());
+    masm.load32(Address(obj, UnboxedArrayObject::offsetOfLength()), result);
+}
+
+void
+CodeGenerator::visitUnboxedArrayInitializedLength(LUnboxedArrayInitializedLength* lir)
+{
+    Register obj = ToRegister(lir->object());
+    Register result = ToRegister(lir->output());
+    masm.load32(Address(obj, UnboxedArrayObject::offsetOfCapacityIndexAndInitializedLength()), result);
+    masm.and32(Imm32(UnboxedArrayObject::InitializedLengthMask), result);
+}
+
+void
+CodeGenerator::visitIncrementUnboxedArrayInitializedLength(LIncrementUnboxedArrayInitializedLength* lir)
+{
+    Register obj = ToRegister(lir->object());
+    masm.add32(Imm32(1), Address(obj, UnboxedArrayObject::offsetOfCapacityIndexAndInitializedLength()));
+}
+
+void
+CodeGenerator::visitSetUnboxedArrayInitializedLength(LSetUnboxedArrayInitializedLength* lir)
+{
+    Register obj = ToRegister(lir->object());
+    RegisterOrInt32Constant key = ToRegisterOrInt32Constant(lir->length());
+    Register temp = ToRegister(lir->temp());
+
+    Address initLengthAddr(obj, UnboxedArrayObject::offsetOfCapacityIndexAndInitializedLength());
+    masm.load32(initLengthAddr, temp);
+    masm.and32(Imm32(UnboxedArrayObject::CapacityMask), temp);
+
+    if (key.isRegister())
+        masm.or32(key.reg(), temp);
+    else
+        masm.or32(Imm32(key.constant()), temp);
+
+    masm.store32(temp, initLengthAddr);
+}
+
+void
 CodeGenerator::visitNotO(LNotO* lir)
 {
     MOZ_ASSERT(lir->mir()->operandMightEmulateUndefined(),
@@ -8092,19 +8196,46 @@ CodeGenerator::emitStoreElementHoleT(T* lir)
     OutOfLineStoreElementHole* ool = new(alloc()) OutOfLineStoreElementHole(lir);
     addOutOfLineCode(ool, lir->mir());
 
+    Register obj = ToRegister(lir->object());
     Register elements = ToRegister(lir->elements());
     const LAllocation* index = lir->index();
     RegisterOrInt32Constant key = ToRegisterOrInt32Constant(index);
 
-    Address initLength(elements, ObjectElements::offsetOfInitializedLength());
-    masm.branch32(Assembler::BelowOrEqual, initLength, key, ool->entry());
+    JSValueType unboxedType = lir->mir()->unboxedType();
+    if (unboxedType == JSVAL_TYPE_MAGIC) {
+        Address initLength(elements, ObjectElements::offsetOfInitializedLength());
+        masm.branch32(Assembler::BelowOrEqual, initLength, key, ool->entry());
 
-    if (lir->mir()->needsBarrier())
-        emitPreBarrier(elements, index, 0);
+        if (lir->mir()->needsBarrier())
+            emitPreBarrier(elements, index, 0);
+
+        masm.bind(ool->rejoinStore());
+        emitStoreElementTyped(lir->value(), lir->mir()->value()->type(), lir->mir()->elementType(),
+                              elements, index, 0);
+    } else {
+        Register temp = ToRegister(lir->getTemp(0));
+        Address initLength(obj, UnboxedArrayObject::offsetOfCapacityIndexAndInitializedLength());
+        masm.load32(initLength, temp);
+        masm.and32(Imm32(UnboxedArrayObject::InitializedLengthMask), temp);
+        masm.branch32(Assembler::BelowOrEqual, temp, key, ool->entry());
+
+        ConstantOrRegister v = ToConstantOrRegister(lir->value(), lir->mir()->value()->type());
+
+        if (index->isConstant()) {
+            Address address(elements, ToInt32(index) * UnboxedTypeSize(unboxedType));
+            EmitUnboxedPreBarrier(masm, address, unboxedType);
+
+            masm.bind(ool->rejoinStore());
+            masm.storeUnboxedProperty(address, unboxedType, v, nullptr);
+        } else {
+            BaseIndex address(elements, ToRegister(index),
+                              ScaleFromElemWidth(UnboxedTypeSize(unboxedType)));
+            EmitUnboxedPreBarrier(masm, address, unboxedType);
 
-    masm.bind(ool->rejoinStore());
-    emitStoreElementTyped(lir->value(), lir->mir()->value()->type(), lir->mir()->elementType(),
-                          elements, index, 0);
+            masm.bind(ool->rejoinStore());
+            masm.storeUnboxedProperty(address, unboxedType, v, nullptr);
+        }
+    }
 
     masm.bind(ool->rejoin());
 }
@@ -8124,22 +8255,47 @@ CodeGenerator::emitStoreElementHoleV(T* lir)
     OutOfLineStoreElementHole* ool = new(alloc()) OutOfLineStoreElementHole(lir);
     addOutOfLineCode(ool, lir->mir());
 
+    Register obj = ToRegister(lir->object());
     Register elements = ToRegister(lir->elements());
     const LAllocation* index = lir->index();
     const ValueOperand value = ToValue(lir, T::Value);
     RegisterOrInt32Constant key = ToRegisterOrInt32Constant(index);
 
-    Address initLength(elements, ObjectElements::offsetOfInitializedLength());
-    masm.branch32(Assembler::BelowOrEqual, initLength, key, ool->entry());
+    JSValueType unboxedType = lir->mir()->unboxedType();
+    if (unboxedType == JSVAL_TYPE_MAGIC) {
+        Address initLength(elements, ObjectElements::offsetOfInitializedLength());
+        masm.branch32(Assembler::BelowOrEqual, initLength, key, ool->entry());
 
-    if (lir->mir()->needsBarrier())
-        emitPreBarrier(elements, index, 0);
+        if (lir->mir()->needsBarrier())
+            emitPreBarrier(elements, index, 0);
 
-    masm.bind(ool->rejoinStore());
-    if (index->isConstant())
-        masm.storeValue(value, Address(elements, ToInt32(index) * sizeof(js::Value)));
-    else
-        masm.storeValue(value, BaseIndex(elements, ToRegister(index), TimesEight));
+        masm.bind(ool->rejoinStore());
+        if (index->isConstant())
+            masm.storeValue(value, Address(elements, ToInt32(index) * sizeof(js::Value)));
+        else
+            masm.storeValue(value, BaseIndex(elements, ToRegister(index), TimesEight));
+    } else {
+        Register temp = ToRegister(lir->getTemp(0));
+        Address initLength(obj, UnboxedArrayObject::offsetOfCapacityIndexAndInitializedLength());
+        masm.load32(initLength, temp);
+        masm.and32(Imm32(UnboxedArrayObject::InitializedLengthMask), temp);
+        masm.branch32(Assembler::BelowOrEqual, temp, key, ool->entry());
+
+        if (index->isConstant()) {
+            Address address(elements, ToInt32(index) * UnboxedTypeSize(unboxedType));
+            EmitUnboxedPreBarrier(masm, address, unboxedType);
+
+            masm.bind(ool->rejoinStore());
+            masm.storeUnboxedProperty(address, unboxedType, ConstantOrRegister(value), nullptr);
+        } else {
+            BaseIndex address(elements, ToRegister(index),
+                              ScaleFromElemWidth(UnboxedTypeSize(unboxedType)));
+            EmitUnboxedPreBarrier(masm, address, unboxedType);
+
+            masm.bind(ool->rejoinStore());
+            masm.storeUnboxedProperty(address, unboxedType, ConstantOrRegister(value), nullptr);
+        }
+    }
 
     masm.bind(ool->rejoin());
 }
@@ -8210,10 +8366,11 @@ CodeGenerator::visitFallibleStoreElementV(LFallibleStoreElementV* lir)
     masm.bind(&isFrozen);
 }
 
-typedef bool (*SetDenseElementFn)(JSContext*, HandleNativeObject, int32_t, HandleValue,
-                                  bool strict);
-static const VMFunction SetDenseElementInfo =
-    FunctionInfo<SetDenseElementFn>(jit::SetDenseElement, "SetDenseElement");
+typedef bool (*SetDenseOrUnboxedArrayElementFn)(JSContext*, HandleObject, int32_t,
+                                                HandleValue, bool strict);
+static const VMFunction SetDenseOrUnboxedArrayElementInfo =
+    FunctionInfo<SetDenseOrUnboxedArrayElementFn>(SetDenseOrUnboxedArrayElement,
+                                                  "SetDenseOrUnboxedArrayElement");
 
 void
 CodeGenerator::visitOutOfLineStoreElementHole(OutOfLineStoreElementHole* ool)
@@ -8223,6 +8380,8 @@ CodeGenerator::visitOutOfLineStoreElementHole(OutOfLineStoreElementHole* ool)
     const LAllocation* index;
     MIRType valueType;
     ConstantOrRegister value;
+    JSValueType unboxedType;
+    LDefinition *temp = nullptr;
 
     if (ins->isStoreElementHoleV()) {
         LStoreElementHoleV* store = ins->toStoreElementHoleV();
@@ -8231,6 +8390,8 @@ CodeGenerator::visitOutOfLineStoreElementHole(OutOfLineStoreElementHole* ool)
         index = store->index();
         valueType = store->mir()->value()->type();
         value = TypedOrValueRegister(ToValue(store, LStoreElementHoleV::Value));
+        unboxedType = store->mir()->unboxedType();
+        temp = store->getTemp(0);
     } else if (ins->isFallibleStoreElementV()) {
         LFallibleStoreElementV* store = ins->toFallibleStoreElementV();
         object = ToRegister(store->object());
@@ -8238,6 +8399,8 @@ CodeGenerator::visitOutOfLineStoreElementHole(OutOfLineStoreElementHole* ool)
         index = store->index();
         valueType = store->mir()->value()->type();
         value = TypedOrValueRegister(ToValue(store, LFallibleStoreElementV::Value));
+        unboxedType = store->mir()->unboxedType();
+        temp = store->getTemp(0);
     } else if (ins->isStoreElementHoleT()) {
         LStoreElementHoleT* store = ins->toStoreElementHoleT();
         object = ToRegister(store->object());
@@ -8248,6 +8411,8 @@ CodeGenerator::visitOutOfLineStoreElementHole(OutOfLineStoreElementHole* ool)
             value = ConstantOrRegister(store->value()->toConstant()->toJSValue());
         else
             value = TypedOrValueRegister(valueType, ToAnyRegister(store->value()));
+        unboxedType = store->mir()->unboxedType();
+        temp = store->getTemp(0);
     } else { // ins->isFallibleStoreElementT()
         LFallibleStoreElementT* store = ins->toFallibleStoreElementT();
         object = ToRegister(store->object());
@@ -8258,6 +8423,8 @@ CodeGenerator::visitOutOfLineStoreElementHole(OutOfLineStoreElementHole* ool)
             value = ConstantOrRegister(store->value()->toConstant()->toJSValue());
         else
             value = TypedOrValueRegister(valueType, ToAnyRegister(store->value()));
+        unboxedType = store->mir()->unboxedType();
+        temp = store->getTemp(0);
     }
 
     RegisterOrInt32Constant key = ToRegisterOrInt32Constant(index);
@@ -8268,32 +8435,54 @@ CodeGenerator::visitOutOfLineStoreElementHole(OutOfLineStoreElementHole* ool)
     Label callStub;
 #if defined(JS_CODEGEN_MIPS32) || defined(JS_CODEGEN_MIPS64)
     // Had to reimplement for MIPS because there are no flags.
-    Address initLength(elements, ObjectElements::offsetOfInitializedLength());
-    masm.branch32(Assembler::NotEqual, initLength, key, &callStub);
+    if (unboxedType == JSVAL_TYPE_MAGIC) {
+        Address initLength(elements, ObjectElements::offsetOfInitializedLength());
+        masm.branch32(Assembler::NotEqual, initLength, key, &callStub);
+    } else {
+        Address initLength(object, UnboxedArrayObject::offsetOfCapacityIndexAndInitializedLength());
+        masm.load32(initLength, ToRegister(temp));
+        masm.and32(Imm32(UnboxedArrayObject::InitializedLengthMask), ToRegister(temp));
+        masm.branch32(Assembler::NotEqual, ToRegister(temp), key, &callStub);
+    }
 #else
     masm.j(Assembler::NotEqual, &callStub);
 #endif
 
-    // Check array capacity.
-    masm.branch32(Assembler::BelowOrEqual, Address(elements, ObjectElements::offsetOfCapacity()),
-                  key, &callStub);
+    if (unboxedType == JSVAL_TYPE_MAGIC) {
+        // Check array capacity.
+        masm.branch32(Assembler::BelowOrEqual, Address(elements, ObjectElements::offsetOfCapacity()),
+                      key, &callStub);
 
-    // Update initialized length. The capacity guard above ensures this won't overflow,
-    // due to MAX_DENSE_ELEMENTS_COUNT.
-    masm.inc32(&key);
-    masm.store32(key, Address(elements, ObjectElements::offsetOfInitializedLength()));
+        // Update initialized length. The capacity guard above ensures this won't overflow,
+        // due to MAX_DENSE_ELEMENTS_COUNT.
+        masm.inc32(&key);
+        masm.store32(key, Address(elements, ObjectElements::offsetOfInitializedLength()));
 
-    // Update length if length < initializedLength.
-    Label dontUpdate;
-    masm.branch32(Assembler::AboveOrEqual, Address(elements, ObjectElements::offsetOfLength()),
-                  key, &dontUpdate);
-    masm.store32(key, Address(elements, ObjectElements::offsetOfLength()));
-    masm.bind(&dontUpdate);
+        // Update length if length < initializedLength.
+        Label dontUpdate;
+        masm.branch32(Assembler::AboveOrEqual, Address(elements, ObjectElements::offsetOfLength()),
+                      key, &dontUpdate);
+        masm.store32(key, Address(elements, ObjectElements::offsetOfLength()));
+        masm.bind(&dontUpdate);
 
-    masm.dec32(&key);
+        masm.dec32(&key);
+    } else {
+        // Check array capacity.
+        masm.checkUnboxedArrayCapacity(object, key, ToRegister(temp), &callStub);
+
+        // Update initialized length.
+        masm.add32(Imm32(1), Address(object, UnboxedArrayObject::offsetOfCapacityIndexAndInitializedLength()));
+
+        // Update length if length < initializedLength.
+        Address lengthAddr(object, UnboxedArrayObject::offsetOfLength());
+        Label dontUpdate;
+        masm.branch32(Assembler::Above, lengthAddr, key, &dontUpdate);
+        masm.add32(Imm32(1), lengthAddr);
+        masm.bind(&dontUpdate);
+    }
 
     if ((ins->isStoreElementHoleT() || ins->isFallibleStoreElementT()) &&
-        valueType != MIRType::Double)
+        unboxedType == JSVAL_TYPE_MAGIC && valueType != MIRType::Double)
     {
         // The inline path for StoreElementHoleT and FallibleStoreElementT does not always store
         // the type tag, so we do the store on the OOL path. We use MIRType::None for the element
@@ -8322,7 +8511,7 @@ CodeGenerator::visitOutOfLineStoreElementHole(OutOfLineStoreElementHole* ool)
     else
         pushArg(ToRegister(index));
     pushArg(object);
-    callVM(SetDenseElementInfo, ins);
+    callVM(SetDenseOrUnboxedArrayElementInfo, ins);
 
     restoreLive(ins);
     masm.jump(ool->rejoin());
@@ -8379,6 +8568,29 @@ CodeGenerator::visitStoreUnboxedPointer(LStoreUnboxedPointer* lir)
     }
 }
 
+typedef bool (*ConvertUnboxedObjectToNativeFn)(JSContext*, JSObject*);
+static const VMFunction ConvertUnboxedPlainObjectToNativeInfo =
+    FunctionInfo<ConvertUnboxedObjectToNativeFn>(UnboxedPlainObject::convertToNative,
+                                                 "UnboxedPlainObject::convertToNative");
+static const VMFunction ConvertUnboxedArrayObjectToNativeInfo =
+    FunctionInfo<ConvertUnboxedObjectToNativeFn>(UnboxedArrayObject::convertToNative,
+                                                 "UnboxedArrayObject::convertToNative");
+
+void
+CodeGenerator::visitConvertUnboxedObjectToNative(LConvertUnboxedObjectToNative* lir)
+{
+    Register object = ToRegister(lir->getOperand(0));
+
+    OutOfLineCode* ool = oolCallVM(lir->mir()->group()->unboxedLayoutDontCheckGeneration().isArray()
+                                   ? ConvertUnboxedArrayObjectToNativeInfo
+                                   : ConvertUnboxedPlainObjectToNativeInfo,
+                                   lir, ArgList(object), StoreNothing());
+
+    masm.branchPtr(Assembler::Equal, Address(object, JSObject::offsetOfGroup()),
+                   ImmGCPtr(lir->mir()->group()), ool->entry());
+    masm.bind(ool->rejoin());
+}
+
 typedef bool (*ArrayPopShiftFn)(JSContext*, HandleObject, MutableHandleValue);
 static const VMFunction ArrayPopDenseInfo =
     FunctionInfo<ArrayPopShiftFn>(jit::ArrayPopDense, "ArrayPopDense");
@@ -8403,11 +8615,20 @@ CodeGenerator::emitArrayPopShift(LInstruction* lir, const MArrayPopShift* mir, R
 
     // Load elements and length, and VM call if length != initializedLength.
     RegisterOrInt32Constant key = RegisterOrInt32Constant(lengthTemp);
-    masm.loadPtr(Address(obj, NativeObject::offsetOfElements()), elementsTemp);
-    masm.load32(Address(elementsTemp, ObjectElements::offsetOfLength()), lengthTemp);
+    if (mir->unboxedType() == JSVAL_TYPE_MAGIC) {
+        masm.loadPtr(Address(obj, NativeObject::offsetOfElements()), elementsTemp);
+        masm.load32(Address(elementsTemp, ObjectElements::offsetOfLength()), lengthTemp);
 
-    Address initLength(elementsTemp, ObjectElements::offsetOfInitializedLength());
-    masm.branch32(Assembler::NotEqual, initLength, key, ool->entry());
+        Address initLength(elementsTemp, ObjectElements::offsetOfInitializedLength());
+        masm.branch32(Assembler::NotEqual, initLength, key, ool->entry());
+    } else {
+        masm.loadPtr(Address(obj, UnboxedArrayObject::offsetOfElements()), elementsTemp);
+        masm.load32(Address(obj, UnboxedArrayObject::offsetOfCapacityIndexAndInitializedLength()), lengthTemp);
+        masm.and32(Imm32(UnboxedArrayObject::InitializedLengthMask), lengthTemp);
+
+        Address lengthAddr(obj, UnboxedArrayObject::offsetOfLength());
+        masm.branch32(Assembler::NotEqual, lengthAddr, key, ool->entry());
+    }
 
     // Test for length != 0. On zero length either take a VM call or generate
     // an undefined value, depending on whether the call is known to produce
@@ -8419,10 +8640,13 @@ CodeGenerator::emitArrayPopShift(LInstruction* lir, const MArrayPopShift* mir, R
 
         // According to the spec we need to set the length 0 (which is already 0).
         // This is observable when the array length is made non-writable.
-        // Handle this case in the OOL.
-        Address elementFlags(elementsTemp, ObjectElements::offsetOfFlags());
-        Imm32 bit(ObjectElements::NONWRITABLE_ARRAY_LENGTH);
-        masm.branchTest32(Assembler::NonZero, elementFlags, bit, ool->entry());
+        // Handle this case in the OOL. When freezing an unboxed array it is converted
+        // to an normal array.
+        if (mir->unboxedType() == JSVAL_TYPE_MAGIC) {
+            Address elementFlags(elementsTemp, ObjectElements::offsetOfFlags());
+            Imm32 bit(ObjectElements::NONWRITABLE_ARRAY_LENGTH);
+            masm.branchTest32(Assembler::NonZero, elementFlags, bit, ool->entry());
+        }
 
         masm.moveValue(UndefinedValue(), out.valueReg());
         masm.jump(&done);
@@ -8434,25 +8658,41 @@ CodeGenerator::emitArrayPopShift(LInstruction* lir, const MArrayPopShift* mir, R
     masm.dec32(&key);
 
     if (mir->mode() == MArrayPopShift::Pop) {
-        BaseIndex addr(elementsTemp, lengthTemp, TimesEight);
-        masm.loadElementTypedOrValue(addr, out, mir->needsHoleCheck(), ool->entry());
+        if (mir->unboxedType() == JSVAL_TYPE_MAGIC) {
+            BaseIndex addr(elementsTemp, lengthTemp, TimesEight);
+            masm.loadElementTypedOrValue(addr, out, mir->needsHoleCheck(), ool->entry());
+        } else {
+            size_t elemSize = UnboxedTypeSize(mir->unboxedType());
+            BaseIndex addr(elementsTemp, lengthTemp, ScaleFromElemWidth(elemSize));
+            masm.loadUnboxedProperty(addr, mir->unboxedType(), out);
+        }
     } else {
         MOZ_ASSERT(mir->mode() == MArrayPopShift::Shift);
         Address addr(elementsTemp, 0);
-        masm.loadElementTypedOrValue(addr, out, mir->needsHoleCheck(), ool->entry());
+        if (mir->unboxedType() == JSVAL_TYPE_MAGIC)
+            masm.loadElementTypedOrValue(addr, out, mir->needsHoleCheck(), ool->entry());
+        else
+            masm.loadUnboxedProperty(addr, mir->unboxedType(), out);
     }
 
-    // Handle the failure case when the array length is non-writable in the
-    // OOL path.  (Unlike in the adding-an-element cases, we can't rely on the
-    // capacity <= length invariant for such arrays to avoid an explicit
-    // check.)
-    Address elementFlags(elementsTemp, ObjectElements::offsetOfFlags());
-    Imm32 bit(ObjectElements::NONWRITABLE_ARRAY_LENGTH);
-    masm.branchTest32(Assembler::NonZero, elementFlags, bit, ool->entry());
+    if (mir->unboxedType() == JSVAL_TYPE_MAGIC) {
+        // Handle the failure case when the array length is non-writable in the
+        // OOL path.  (Unlike in the adding-an-element cases, we can't rely on the
+        // capacity <= length invariant for such arrays to avoid an explicit
+        // check.)
+        Address elementFlags(elementsTemp, ObjectElements::offsetOfFlags());
+        Imm32 bit(ObjectElements::NONWRITABLE_ARRAY_LENGTH);
+        masm.branchTest32(Assembler::NonZero, elementFlags, bit, ool->entry());
 
-    // Now adjust length and initializedLength.
-    masm.store32(lengthTemp, Address(elementsTemp, ObjectElements::offsetOfLength()));
-    masm.store32(lengthTemp, Address(elementsTemp, ObjectElements::offsetOfInitializedLength()));
+        // Now adjust length and initializedLength.
+        masm.store32(lengthTemp, Address(elementsTemp, ObjectElements::offsetOfLength()));
+        masm.store32(lengthTemp, Address(elementsTemp, ObjectElements::offsetOfInitializedLength()));
+    } else {
+        // Unboxed arrays always have writable lengths. Adjust length and
+        // initializedLength.
+        masm.store32(lengthTemp, Address(obj, UnboxedArrayObject::offsetOfLength()));
+        masm.add32(Imm32(-1), Address(obj, UnboxedArrayObject::offsetOfCapacityIndexAndInitializedLength()));
+    }
 
     if (mir->mode() == MArrayPopShift::Shift) {
         // Don't save the temp registers.
@@ -8491,7 +8731,7 @@ CodeGenerator::visitArrayPopShiftT(LArrayPopShiftT* lir)
     emitArrayPopShift(lir, lir->mir(), obj, elements, length, out);
 }
 
-typedef bool (*ArrayPushDenseFn)(JSContext*, HandleArrayObject, HandleValue, uint32_t*);
+typedef bool (*ArrayPushDenseFn)(JSContext*, HandleObject, HandleValue, uint32_t*);
 static const VMFunction ArrayPushDenseInfo =
     FunctionInfo<ArrayPushDenseFn>(jit::ArrayPushDense, "ArrayPushDense");
 
@@ -8502,27 +8742,50 @@ CodeGenerator::emitArrayPush(LInstruction* lir, const MArrayPush* mir, Register
     OutOfLineCode* ool = oolCallVM(ArrayPushDenseInfo, lir, ArgList(obj, value), StoreRegisterTo(length));
 
     RegisterOrInt32Constant key = RegisterOrInt32Constant(length);
+    if (mir->unboxedType() == JSVAL_TYPE_MAGIC) {
+        // Load elements and length.
+        masm.loadPtr(Address(obj, NativeObject::offsetOfElements()), elementsTemp);
+        masm.load32(Address(elementsTemp, ObjectElements::offsetOfLength()), length);
 
-    // Load elements and length.
-    masm.loadPtr(Address(obj, NativeObject::offsetOfElements()), elementsTemp);
-    masm.load32(Address(elementsTemp, ObjectElements::offsetOfLength()), length);
+        // Guard length == initializedLength.
+        Address initLength(elementsTemp, ObjectElements::offsetOfInitializedLength());
+        masm.branch32(Assembler::NotEqual, initLength, key, ool->entry());
 
-    // Guard length == initializedLength.
-    Address initLength(elementsTemp, ObjectElements::offsetOfInitializedLength());
-    masm.branch32(Assembler::NotEqual, initLength, key, ool->entry());
+        // Guard length < capacity.
+        Address capacity(elementsTemp, ObjectElements::offsetOfCapacity());
+        masm.branch32(Assembler::BelowOrEqual, capacity, key, ool->entry());
 
-    // Guard length < capacity.
-    Address capacity(elementsTemp, ObjectElements::offsetOfCapacity());
-    masm.branch32(Assembler::BelowOrEqual, capacity, key, ool->entry());
+        // Do the store.
+        masm.storeConstantOrRegister(value, BaseIndex(elementsTemp, length, TimesEight));
+    } else {
+        // Load initialized length.
+        masm.load32(Address(obj, UnboxedArrayObject::offsetOfCapacityIndexAndInitializedLength()), length);
+        masm.and32(Imm32(UnboxedArrayObject::InitializedLengthMask), length);
 
-    // Do the store.
-    masm.storeConstantOrRegister(value, BaseIndex(elementsTemp, length, TimesEight));
+        // Guard length == initializedLength.
+        Address lengthAddr(obj, UnboxedArrayObject::offsetOfLength());
+        masm.branch32(Assembler::NotEqual, lengthAddr, key, ool->entry());
+
+        // Guard length < capacity.
+        masm.checkUnboxedArrayCapacity(obj, key, elementsTemp, ool->entry());
+
+        // Load elements and do the store.
+        masm.loadPtr(Address(obj, UnboxedArrayObject::offsetOfElements()), elementsTemp);
+        size_t elemSize = UnboxedTypeSize(mir->unboxedType());
+        BaseIndex addr(elementsTemp, length, ScaleFromElemWidth(elemSize));
+        masm.storeUnboxedProperty(addr, mir->unboxedType(), value, nullptr);
+    }
 
     masm.inc32(&key);
 
     // Update length and initialized length.
-    masm.store32(length, Address(elementsTemp, ObjectElements::offsetOfLength()));
-    masm.store32(length, Address(elementsTemp, ObjectElements::offsetOfInitializedLength()));
+    if (mir->unboxedType() == JSVAL_TYPE_MAGIC) {
+        masm.store32(length, Address(elementsTemp, ObjectElements::offsetOfLength()));
+        masm.store32(length, Address(elementsTemp, ObjectElements::offsetOfInitializedLength()));
+    } else {
+        masm.store32(length, Address(obj, UnboxedArrayObject::offsetOfLength()));
+        masm.add32(Imm32(1), Address(obj, UnboxedArrayObject::offsetOfCapacityIndexAndInitializedLength()));
+    }
 
     masm.bind(ool->rejoin());
 }
@@ -8671,11 +8934,11 @@ CodeGenerator::visitIteratorStartO(LIteratorStartO* lir)
     masm.loadPtr(Address(niTemp, offsetof(NativeIterator, guard_array)), temp2);
 
     // Compare object with the first receiver guard. The last iterator can only
-    // match for native objects.
+    // match for native objects and unboxed objects.
     {
         Address groupAddr(temp2, offsetof(ReceiverGuard, group));
         Address shapeAddr(temp2, offsetof(ReceiverGuard, shape));
-        Label guardDone, shapeMismatch;
+        Label guardDone, shapeMismatch, noExpando;
         masm.loadObjShape(obj, temp1);
         masm.branchPtr(Assembler::NotEqual, shapeAddr, temp1, &shapeMismatch);
 
@@ -8687,6 +8950,12 @@ CodeGenerator::visitIteratorStartO(LIteratorStartO* lir)
         masm.bind(&shapeMismatch);
         masm.loadObjGroup(obj, temp1);
         masm.branchPtr(Assembler::NotEqual, groupAddr, temp1, ool->entry());
+        masm.loadPtr(Address(obj, UnboxedPlainObject::offsetOfExpando()), temp1);
+        masm.branchTestPtr(Assembler::Zero, temp1, temp1, &noExpando);
+        branchIfNotEmptyObjectElements(temp1, ool->entry());
+        masm.loadObjShape(temp1, temp1);
+        masm.bind(&noExpando);
+        masm.branchPtr(Assembler::NotEqual, shapeAddr, temp1, ool->entry());
         masm.bind(&guardDone);
     }
 
@@ -10343,11 +10612,22 @@ CodeGenerator::visitLoadElementHole(LLoadElementHole* lir)
     else
         masm.branch32(Assembler::BelowOrEqual, initLength, ToRegister(lir->index()), &undefined);
 
-    if (lir->index()->isConstant()) {
-        NativeObject::elementsSizeMustNotOverflow();
-        masm.loadValue(Address(elements, ToInt32(lir->index()) * sizeof(Value)), out);
+    if (mir->unboxedType() != JSVAL_TYPE_MAGIC) {
+        size_t width = UnboxedTypeSize(mir->unboxedType());
+        if (lir->index()->isConstant()) {
+            Address addr(elements, ToInt32(lir->index()) * width);
+            masm.loadUnboxedProperty(addr, mir->unboxedType(), out);
+        } else {
+            BaseIndex addr(elements, ToRegister(lir->index()), ScaleFromElemWidth(width));
+            masm.loadUnboxedProperty(addr, mir->unboxedType(), out);
+        }
     } else {
-        masm.loadValue(BaseObjectElementIndex(elements, ToRegister(lir->index())), out);
+        if (lir->index()->isConstant()) {
+            NativeObject::elementsSizeMustNotOverflow();
+            masm.loadValue(Address(elements, ToInt32(lir->index()) * sizeof(Value)), out);
+        } else {
+            masm.loadValue(BaseObjectElementIndex(elements, ToRegister(lir->index())), out);
+        }
     }
 
     // If a hole check is needed, and the value wasn't a hole, we're done.
@@ -10725,7 +11005,7 @@ CodeGenerator::visitInArray(LInArray* lir)
         }
 
         masm.branch32(Assembler::BelowOrEqual, initLength, Imm32(index), failedInitLength);
-        if (mir->needsHoleCheck()) {
+        if (mir->needsHoleCheck() && mir->unboxedType() == JSVAL_TYPE_MAGIC) {
             NativeObject::elementsSizeMustNotOverflow();
             Address address = Address(elements, index * sizeof(Value));
             masm.branchTestMagic(Assembler::Equal, address, &falseBranch);
@@ -10738,7 +11018,7 @@ CodeGenerator::visitInArray(LInArray* lir)
             failedInitLength = &negativeIntCheck;
 
         masm.branch32(Assembler::BelowOrEqual, initLength, index, failedInitLength);
-        if (mir->needsHoleCheck()) {
+        if (mir->needsHoleCheck() && mir->unboxedType() == JSVAL_TYPE_MAGIC) {
             BaseIndex address = BaseIndex(elements, ToRegister(lir->index()), TimesEight);
             masm.branchTestMagic(Assembler::Equal, address, &falseBranch);
         }
diff --git a/js/src/jit/CodeGenerator.h b/js/src/jit/CodeGenerator.h
index 65acfe274..6a5c7f34f 100644
--- a/js/src/jit/CodeGenerator.h
+++ b/js/src/jit/CodeGenerator.h
@@ -148,6 +148,8 @@ class CodeGenerator final : public CodeGeneratorSpecific
     void visitMaybeCopyElementsForWrite(LMaybeCopyElementsForWrite* lir);
     void visitGuardObjectIdentity(LGuardObjectIdentity* guard);
     void visitGuardReceiverPolymorphic(LGuardReceiverPolymorphic* lir);
+    void visitGuardUnboxedExpando(LGuardUnboxedExpando* lir);
+    void visitLoadUnboxedExpando(LLoadUnboxedExpando* lir);
     void visitTypeBarrierV(LTypeBarrierV* lir);
     void visitTypeBarrierO(LTypeBarrierO* lir);
     void visitMonitorTypes(LMonitorTypes* lir);
@@ -234,6 +236,10 @@ class CodeGenerator final : public CodeGeneratorSpecific
     void visitSubstr(LSubstr* lir);
     void visitInitializedLength(LInitializedLength* lir);
     void visitSetInitializedLength(LSetInitializedLength* lir);
+    void visitUnboxedArrayLength(LUnboxedArrayLength* lir);
+    void visitUnboxedArrayInitializedLength(LUnboxedArrayInitializedLength* lir);
+    void visitIncrementUnboxedArrayInitializedLength(LIncrementUnboxedArrayInitializedLength* lir);
+    void visitSetUnboxedArrayInitializedLength(LSetUnboxedArrayInitializedLength* lir);
     void visitNotO(LNotO* ins);
     void visitNotV(LNotV* ins);
     void visitBoundsCheck(LBoundsCheck* lir);
@@ -305,6 +311,7 @@ class CodeGenerator final : public CodeGeneratorSpecific
     void visitFallibleStoreElementV(LFallibleStoreElementV* lir);
     void visitFallibleStoreElementT(LFallibleStoreElementT* lir);
     void visitStoreUnboxedPointer(LStoreUnboxedPointer* lir);
+    void visitConvertUnboxedObjectToNative(LConvertUnboxedObjectToNative* lir);
     void emitArrayPopShift(LInstruction* lir, const MArrayPopShift* mir, Register obj,
                            Register elementsTemp, Register lengthTemp, TypedOrValueRegister out);
     void visitArrayPopShiftV(LArrayPopShiftV* lir);
diff --git a/js/src/jit/IonAnalysis.cpp b/js/src/jit/IonAnalysis.cpp
index a4724bca4..3c0f2c4b3 100644
--- a/js/src/jit/IonAnalysis.cpp
+++ b/js/src/jit/IonAnalysis.cpp
@@ -3515,6 +3515,8 @@ PassthroughOperand(MDefinition* def)
         return def->toConvertElementsToDoubles()->elements();
     if (def->isMaybeCopyElementsForWrite())
         return def->toMaybeCopyElementsForWrite()->object();
+    if (def->isConvertUnboxedObjectToNative())
+        return def->toConvertUnboxedObjectToNative()->object();
     return nullptr;
 }
 
@@ -4060,7 +4062,7 @@ AnalyzePoppedThis(JSContext* cx, DPAConstraintInfo& constraintInfo, ObjectGroup*
         // Add the property to the object, being careful not to update type information.
         DebugOnly<unsigned> slotSpan = baseobj->slotSpan();
         MOZ_ASSERT(!baseobj->containsPure(id));
-        if (!NativeObject::addDataProperty(cx, baseobj, id, baseobj->slotSpan(), JSPROP_ENUMERATE))
+        if (!baseobj->addDataProperty(cx, id, baseobj->slotSpan(), JSPROP_ENUMERATE))
             return false;
         MOZ_ASSERT(baseobj->slotSpan() != slotSpan);
         MOZ_ASSERT(!baseobj->inDictionaryMode());
diff --git a/js/src/jit/IonBuilder.cpp b/js/src/jit/IonBuilder.cpp
index 1e12f5dbe..f08baf865 100644
--- a/js/src/jit/IonBuilder.cpp
+++ b/js/src/jit/IonBuilder.cpp
@@ -32,6 +32,7 @@
 #include "vm/EnvironmentObject-inl.h"
 #include "vm/NativeObject-inl.h"
 #include "vm/ObjectGroup-inl.h"
+#include "vm/UnboxedObject-inl.h"
 
 using namespace js;
 using namespace js::jit;
@@ -2227,8 +2228,6 @@ IonBuilder::inspectOpcode(JSOp op)
         // update that stale value.
 #endif
       default:
-	    // Any unused opcodes and JSOP_LIMIT will end up here without having
-        // to explicitly specify
         break;
     }
 
@@ -6402,7 +6401,7 @@ IonBuilder::createThisScriptedSingleton(JSFunction* target, MDefinition* callee)
     JSObject* templateObject = inspector->getTemplateObject(pc);
     if (!templateObject)
         return nullptr;
-    if (!templateObject->is<PlainObject>())
+    if (!templateObject->is<PlainObject>() && !templateObject->is<UnboxedPlainObject>())
         return nullptr;
     if (templateObject->staticPrototype() != proto)
         return nullptr;
@@ -6439,7 +6438,7 @@ IonBuilder::createThisScriptedBaseline(MDefinition* callee)
     JSObject* templateObject = inspector->getTemplateObject(pc);
     if (!templateObject)
         return nullptr;
-    if (!templateObject->is<PlainObject>())
+    if (!templateObject->is<PlainObject>() && !templateObject->is<UnboxedPlainObject>())
         return nullptr;
 
     Shape* shape = target->lookupPure(compartment->runtime()->names().prototype);
@@ -7355,6 +7354,12 @@ IonBuilder::newArrayTryTemplateObject(bool* emitted, JSObject* templateObject, u
     if (!templateObject)
         return true;
 
+    if (templateObject->is<UnboxedArrayObject>()) {
+        MOZ_ASSERT(templateObject->as<UnboxedArrayObject>().capacity() >= length);
+        if (!templateObject->as<UnboxedArrayObject>().hasInlineElements())
+            return true;
+    }
+
     MOZ_ASSERT(length <= NativeObject::MAX_DENSE_ELEMENTS_COUNT);
 
     size_t arraySlots =
@@ -7610,6 +7615,7 @@ IonBuilder::jsop_initelem_array()
     // intializer, and that arrays are marked as non-packed when writing holes
     // to them during initialization.
     bool needStub = false;
+    JSValueType unboxedType = JSVAL_TYPE_MAGIC;
     if (shouldAbortOnPreliminaryGroups(obj)) {
         needStub = true;
     } else if (!obj->resultTypeSet() ||
@@ -7620,6 +7626,12 @@ IonBuilder::jsop_initelem_array()
     } else {
         MOZ_ASSERT(obj->resultTypeSet()->getObjectCount() == 1);
         TypeSet::ObjectKey* initializer = obj->resultTypeSet()->getObject(0);
+        if (initializer->clasp() == &UnboxedArrayObject::class_) {
+            if (initializer->group()->unboxedLayout().nativeGroup())
+                needStub = true;
+            else
+                unboxedType = initializer->group()->unboxedLayout().elementType();
+        }
         if (value->type() == MIRType::MagicHole) {
             if (!initializer->hasFlags(constraints(), OBJECT_FLAG_NON_PACKED))
                 needStub = true;
@@ -7639,46 +7651,60 @@ IonBuilder::jsop_initelem_array()
         return resumeAfter(store);
     }
 
-    return initializeArrayElement(obj, index, value, /* addResumePoint = */ true);
+    return initializeArrayElement(obj, index, value, unboxedType, /* addResumePoint = */ true);
 }
 
 bool
 IonBuilder::initializeArrayElement(MDefinition* obj, size_t index, MDefinition* value,
+                                   JSValueType unboxedType,
                                    bool addResumePointAndIncrementInitializedLength)
 {
     MConstant* id = MConstant::New(alloc(), Int32Value(index));
     current->add(id);
 
     // Get the elements vector.
-    MElements* elements = MElements::New(alloc(), obj);
+    MElements* elements = MElements::New(alloc(), obj, unboxedType != JSVAL_TYPE_MAGIC);
     current->add(elements);
 
-    if (NeedsPostBarrier(value))
-        current->add(MPostWriteBarrier::New(alloc(), obj, value));
+    if (unboxedType != JSVAL_TYPE_MAGIC) {
+        // Note: storeUnboxedValue takes care of any post barriers on the value.
+        storeUnboxedValue(obj, elements, 0, id, unboxedType, value, /* preBarrier = */ false);
 
-    if ((obj->isNewArray() && obj->toNewArray()->convertDoubleElements()) ||
-        (obj->isNullarySharedStub() &&
-        obj->resultTypeSet()->convertDoubleElements(constraints()) == TemporaryTypeSet::AlwaysConvertToDoubles))
-    {
-        MInstruction* valueDouble = MToDouble::New(alloc(), value);
-        current->add(valueDouble);
-        value = valueDouble;
-    }
+        if (addResumePointAndIncrementInitializedLength) {
+            MInstruction* increment = MIncrementUnboxedArrayInitializedLength::New(alloc(), obj);
+            current->add(increment);
 
-    // Store the value.
-    MStoreElement* store = MStoreElement::New(alloc(), elements, id, value,
+            if (!resumeAfter(increment))
+                return false;
+        }
+    } else {
+        if (NeedsPostBarrier(value))
+            current->add(MPostWriteBarrier::New(alloc(), obj, value));
+
+        if ((obj->isNewArray() && obj->toNewArray()->convertDoubleElements()) ||
+            (obj->isNullarySharedStub() &&
+            obj->resultTypeSet()->convertDoubleElements(constraints()) == TemporaryTypeSet::AlwaysConvertToDoubles))
+        {
+            MInstruction* valueDouble = MToDouble::New(alloc(), value);
+            current->add(valueDouble);
+            value = valueDouble;
+        }
+
+        // Store the value.
+        MStoreElement* store = MStoreElement::New(alloc(), elements, id, value,
                                                   /* needsHoleCheck = */ false);
-    current->add(store);
+        current->add(store);
 
-    if (addResumePointAndIncrementInitializedLength) {
-        // Update the initialized length. (The template object for this
-        // array has the array's ultimate length, so the length field is
-        // already correct: no updating needed.)
-        MSetInitializedLength* initLength = MSetInitializedLength::New(alloc(), elements, id);
-        current->add(initLength);
+        if (addResumePointAndIncrementInitializedLength) {
+            // Update the initialized length. (The template object for this
+            // array has the array's ultimate length, so the length field is
+            // already correct: no updating needed.)
+            MSetInitializedLength* initLength = MSetInitializedLength::New(alloc(), elements, id);
+            current->add(initLength);
 
-        if (!resumeAfter(initLength))
-            return false;
+            if (!resumeAfter(initLength))
+                return false;
+        }
     }
 
     return true;
@@ -7709,6 +7735,8 @@ IonBuilder::jsop_initprop(PropertyName* name)
             if (templateObject->is<PlainObject>()) {
                 if (!templateObject->as<PlainObject>().containsPure(name))
                     useSlowPath = true;
+            } else {
+                MOZ_ASSERT(templateObject->as<UnboxedPlainObject>().layout().lookup(name));
             }
         } else {
             useSlowPath = true;
@@ -8167,7 +8195,9 @@ IonBuilder::maybeMarkEmpty(MDefinition* ins)
 static bool
 ClassHasEffectlessLookup(const Class* clasp)
 {
-    return IsTypedObjectClass(clasp) ||
+    return (clasp == &UnboxedPlainObject::class_) ||
+           (clasp == &UnboxedArrayObject::class_) ||
+           IsTypedObjectClass(clasp) ||
            (clasp->isNative() && !clasp->getOpsLookupProperty());
 }
 
@@ -8997,6 +9027,8 @@ IonBuilder::jsop_getelem()
     }
 
     obj = maybeUnboxForPropertyAccess(obj);
+    if (obj->type() == MIRType::Object)
+        obj = convertUnboxedObjects(obj);
 
     bool emitted = false;
 
@@ -9441,9 +9473,12 @@ IonBuilder::getElemTryDense(bool* emitted, MDefinition* obj, MDefinition* index)
 {
     MOZ_ASSERT(*emitted == false);
 
-    if (!ElementAccessIsDenseNative(constraints(), obj, index)) {
-        trackOptimizationOutcome(TrackedOutcome::AccessNotDense);
-        return true;
+    JSValueType unboxedType = UnboxedArrayElementType(constraints(), obj, index);
+    if (unboxedType == JSVAL_TYPE_MAGIC) {
+        if (!ElementAccessIsDenseNative(constraints(), obj, index)) {
+            trackOptimizationOutcome(TrackedOutcome::AccessNotDense);
+            return true;
+        }
     }
 
     // Don't generate a fast path if there have been bounds check failures
@@ -9460,7 +9495,7 @@ IonBuilder::getElemTryDense(bool* emitted, MDefinition* obj, MDefinition* index)
         return true;
     }
 
-    if (!jsop_getelem_dense(obj, index))
+    if (!jsop_getelem_dense(obj, index, unboxedType))
         return false;
 
     trackOptimizationSuccess();
@@ -9812,7 +9847,7 @@ IonBuilder::computeHeapType(const TemporaryTypeSet* objTypes, const jsid id)
 }
 
 bool
-IonBuilder::jsop_getelem_dense(MDefinition* obj, MDefinition* index)
+IonBuilder::jsop_getelem_dense(MDefinition* obj, MDefinition* index, JSValueType unboxedType)
 {
     TemporaryTypeSet* types = bytecodeTypes(pc);
 
@@ -9836,7 +9871,7 @@ IonBuilder::jsop_getelem_dense(MDefinition* obj, MDefinition* index)
         !ElementAccessHasExtraIndexedProperty(this, obj);
 
     MIRType knownType = MIRType::Value;
-    if (barrier == BarrierKind::NoBarrier)
+    if (unboxedType == JSVAL_TYPE_MAGIC && barrier == BarrierKind::NoBarrier)
         knownType = GetElemKnownType(needsHoleCheck, types);
 
     // Ensure index is an integer.
@@ -9845,13 +9880,13 @@ IonBuilder::jsop_getelem_dense(MDefinition* obj, MDefinition* index)
     index = idInt32;
 
     // Get the elements vector.
-    MInstruction* elements = MElements::New(alloc(), obj);
+    MInstruction* elements = MElements::New(alloc(), obj, unboxedType != JSVAL_TYPE_MAGIC);
     current->add(elements);
 
     // Note: to help GVN, use the original MElements instruction and not
     // MConvertElementsToDoubles as operand. This is fine because converting
     // elements to double does not change the initialized length.
-    MInstruction* initLength = initializedLength(obj, elements);
+    MInstruction* initLength = initializedLength(obj, elements, unboxedType);
 
     // If we can load the element as a definite double, make sure to check that
     // the array has been converted to homogenous doubles first.
@@ -9867,6 +9902,7 @@ IonBuilder::jsop_getelem_dense(MDefinition* obj, MDefinition* index)
     }
 
     bool loadDouble =
+        unboxedType == JSVAL_TYPE_MAGIC &&
         barrier == BarrierKind::NoBarrier &&
         loopDepth_ &&
         inBounds &&
@@ -9885,13 +9921,18 @@ IonBuilder::jsop_getelem_dense(MDefinition* obj, MDefinition* index)
         // hoisting.
         index = addBoundsCheck(index, initLength);
 
-        load = MLoadElement::New(alloc(), elements, index, needsHoleCheck, loadDouble);
-        current->add(load);
+        if (unboxedType != JSVAL_TYPE_MAGIC) {
+            load = loadUnboxedValue(elements, 0, index, unboxedType, barrier, types);
+        } else {
+            load = MLoadElement::New(alloc(), elements, index, needsHoleCheck, loadDouble);
+            current->add(load);
+        }
     } else {
         // This load may return undefined, so assume that we *can* read holes,
         // or that we can read out-of-bounds accesses. In this case, the bounds
         // check is part of the opcode.
-        load = MLoadElementHole::New(alloc(), elements, index, initLength, needsHoleCheck);
+        load = MLoadElementHole::New(alloc(), elements, index, initLength,
+                                     unboxedType, needsHoleCheck);
         current->add(load);
 
         // If maybeUndefined was true, the typeset must have undefined, and
@@ -9901,7 +9942,8 @@ IonBuilder::jsop_getelem_dense(MDefinition* obj, MDefinition* index)
     }
 
     if (knownType != MIRType::Value) {
-        load->setResultType(knownType);
+        if (unboxedType == JSVAL_TYPE_MAGIC)
+            load->setResultType(knownType);
         load->setResultTypeSet(types);
     }
 
@@ -10110,7 +10152,7 @@ IonBuilder::jsop_setelem()
 
     MDefinition* value = current->pop();
     MDefinition* index = current->pop();
-    MDefinition* object = current->pop();
+    MDefinition* object = convertUnboxedObjects(current->pop());
 
     trackTypeInfo(TrackedTypeSite::Receiver, object->type(), object->resultTypeSet());
     trackTypeInfo(TrackedTypeSite::Index, index->type(), index->resultTypeSet());
@@ -10348,9 +10390,12 @@ IonBuilder::setElemTryDense(bool* emitted, MDefinition* object,
 {
     MOZ_ASSERT(*emitted == false);
 
-    if (!ElementAccessIsDenseNative(constraints(), object, index)) {
-        trackOptimizationOutcome(TrackedOutcome::AccessNotDense);
-        return true;
+    JSValueType unboxedType = UnboxedArrayElementType(constraints(), object, index);
+    if (unboxedType == JSVAL_TYPE_MAGIC) {
+        if (!ElementAccessIsDenseNative(constraints(), object, index)) {
+            trackOptimizationOutcome(TrackedOutcome::AccessNotDense);
+            return true;
+        }
     }
 
     if (PropertyWriteNeedsTypeBarrier(alloc(), constraints(), current,
@@ -10384,7 +10429,7 @@ IonBuilder::setElemTryDense(bool* emitted, MDefinition* object,
     }
 
     // Emit dense setelem variant.
-    if (!jsop_setelem_dense(conversion, object, index, value, writeHole, emitted))
+    if (!jsop_setelem_dense(conversion, object, index, value, unboxedType, writeHole, emitted))
         return false;
 
     if (!*emitted) {
@@ -10474,11 +10519,13 @@ IonBuilder::setElemTryCache(bool* emitted, MDefinition* object,
 bool
 IonBuilder::jsop_setelem_dense(TemporaryTypeSet::DoubleConversion conversion,
                                MDefinition* obj, MDefinition* id, MDefinition* value,
-                               bool writeHole, bool* emitted)
+                               JSValueType unboxedType, bool writeHole, bool* emitted)
 {
     MOZ_ASSERT(*emitted == false);
 
-    MIRType elementType = DenseNativeElementType(constraints(), obj);
+    MIRType elementType = MIRType::None;
+    if (unboxedType == JSVAL_TYPE_MAGIC)
+        elementType = DenseNativeElementType(constraints(), obj);
     bool packed = ElementAccessIsPacked(constraints(), obj);
 
     // Writes which are on holes in the object do not have to bail out if they
@@ -10508,7 +10555,7 @@ IonBuilder::jsop_setelem_dense(TemporaryTypeSet::DoubleConversion conversion,
     obj = addMaybeCopyElementsForWrite(obj, /* checkNative = */ false);
 
     // Get the elements vector.
-    MElements* elements = MElements::New(alloc(), obj);
+    MElements* elements = MElements::New(alloc(), obj, unboxedType != JSVAL_TYPE_MAGIC);
     current->add(elements);
 
     // Ensure the value is a double, if double conversion might be needed.
@@ -10545,7 +10592,7 @@ IonBuilder::jsop_setelem_dense(TemporaryTypeSet::DoubleConversion conversion,
     MInstruction* store;
     MStoreElementCommon* common = nullptr;
     if (writeHole && hasNoExtraIndexedProperty && !mayBeFrozen) {
-        MStoreElementHole* ins = MStoreElementHole::New(alloc(), obj, elements, id, newValue);
+        MStoreElementHole* ins = MStoreElementHole::New(alloc(), obj, elements, id, newValue, unboxedType);
         store = ins;
         common = ins;
 
@@ -10557,23 +10604,27 @@ IonBuilder::jsop_setelem_dense(TemporaryTypeSet::DoubleConversion conversion,
 
         bool strict = IsStrictSetPC(pc);
         MFallibleStoreElement* ins = MFallibleStoreElement::New(alloc(), obj, elements, id,
-                                                                newValue, strict);
+                                                                newValue, unboxedType, strict);
         store = ins;
         common = ins;
 
         current->add(ins);
         current->push(value);
     } else {
-        MInstruction* initLength = initializedLength(obj, elements);
+        MInstruction* initLength = initializedLength(obj, elements, unboxedType);
 
         id = addBoundsCheck(id, initLength);
         bool needsHoleCheck = !packed && !hasNoExtraIndexedProperty;
 
-        MStoreElement* ins = MStoreElement::New(alloc(), elements, id, newValue, needsHoleCheck);
-        store = ins;
-        common = ins;
+        if (unboxedType != JSVAL_TYPE_MAGIC) {
+            store = storeUnboxedValue(obj, elements, 0, id, unboxedType, newValue);
+        } else {
+            MStoreElement* ins = MStoreElement::New(alloc(), elements, id, newValue, needsHoleCheck);
+            store = ins;
+            common = ins;
 
-        current->add(store);
+            current->add(store);
+        }
 
         current->push(value);
     }
@@ -10691,6 +10742,18 @@ IonBuilder::jsop_length_fastPath()
             return true;
         }
 
+        // Compute the length for unboxed array objects.
+        if (UnboxedArrayElementType(constraints(), obj, nullptr) != JSVAL_TYPE_MAGIC &&
+            !objTypes->hasObjectFlags(constraints(), OBJECT_FLAG_LENGTH_OVERFLOW))
+        {
+            current->pop();
+
+            MUnboxedArrayLength* length = MUnboxedArrayLength::New(alloc(), obj);
+            current->add(length);
+            current->push(length);
+            return true;
+        }
+
         // Compute the length for array typed objects.
         TypedObjectPrediction prediction = typedObjectPrediction(obj);
         if (!prediction.isUseless()) {
@@ -10924,8 +10987,11 @@ IonBuilder::getDefiniteSlot(TemporaryTypeSet* types, PropertyName* name, uint32_
         }
 
         // Definite slots will always be fixed slots when they are in the
-        // allowable range for fixed slots.
+        // allowable range for fixed slots, except for objects which were
+        // converted from unboxed objects and have a smaller allocation size.
         size_t nfixed = NativeObject::MAX_FIXED_SLOTS;
+        if (ObjectGroup* group = key->group()->maybeOriginalUnboxedGroup())
+            nfixed = gc::GetGCKindSlots(group->unboxedLayout().getAllocKind());
 
         uint32_t propertySlot = property.maybeTypes()->definiteSlot();
         if (slot == UINT32_MAX) {
@@ -10940,6 +11006,65 @@ IonBuilder::getDefiniteSlot(TemporaryTypeSet* types, PropertyName* name, uint32_
     return slot;
 }
 
+uint32_t
+IonBuilder::getUnboxedOffset(TemporaryTypeSet* types, PropertyName* name, JSValueType* punboxedType)
+{
+    if (!types || types->unknownObject() || !types->objectOrSentinel()) {
+        trackOptimizationOutcome(TrackedOutcome::NoTypeInfo);
+        return UINT32_MAX;
+    }
+
+    uint32_t offset = UINT32_MAX;
+
+    for (size_t i = 0; i < types->getObjectCount(); i++) {
+        TypeSet::ObjectKey* key = types->getObject(i);
+        if (!key)
+            continue;
+
+        if (key->unknownProperties()) {
+            trackOptimizationOutcome(TrackedOutcome::UnknownProperties);
+            return UINT32_MAX;
+        }
+
+        if (key->isSingleton()) {
+            trackOptimizationOutcome(TrackedOutcome::Singleton);
+            return UINT32_MAX;
+        }
+
+        UnboxedLayout* layout = key->group()->maybeUnboxedLayout();
+        if (!layout) {
+            trackOptimizationOutcome(TrackedOutcome::NotUnboxed);
+            return UINT32_MAX;
+        }
+
+        const UnboxedLayout::Property* property = layout->lookup(name);
+        if (!property) {
+            trackOptimizationOutcome(TrackedOutcome::StructNoField);
+            return UINT32_MAX;
+        }
+
+        if (layout->nativeGroup()) {
+            trackOptimizationOutcome(TrackedOutcome::UnboxedConvertedToNative);
+            return UINT32_MAX;
+        }
+
+        key->watchStateChangeForUnboxedConvertedToNative(constraints());
+
+        if (offset == UINT32_MAX) {
+            offset = property->offset;
+            *punboxedType = property->type;
+        } else if (offset != property->offset) {
+            trackOptimizationOutcome(TrackedOutcome::InconsistentFieldOffset);
+            return UINT32_MAX;
+        } else if (*punboxedType != property->type) {
+            trackOptimizationOutcome(TrackedOutcome::InconsistentFieldType);
+            return UINT32_MAX;
+        }
+    }
+
+    return offset;
+}
+
 bool
 IonBuilder::jsop_runonce()
 {
@@ -11386,6 +11511,8 @@ IonBuilder::jsop_getprop(PropertyName* name)
     }
 
     obj = maybeUnboxForPropertyAccess(obj);
+    if (obj->type() == MIRType::Object)
+        obj = convertUnboxedObjects(obj);
 
     BarrierKind barrier = PropertyReadNeedsTypeBarrier(analysisContext, constraints(),
                                                        obj, name, types);
@@ -11457,6 +11584,11 @@ IonBuilder::jsop_getprop(PropertyName* name)
         if (!getPropTryDefiniteSlot(&emitted, obj, name, barrier, types) || emitted)
             return emitted;
 
+        // Try to emit loads from unboxed objects.
+        trackOptimizationAttempt(TrackedStrategy::GetProp_Unboxed);
+        if (!getPropTryUnboxed(&emitted, obj, name, barrier, types) || emitted)
+            return emitted;
+
         // Try to inline a common property getter, or make a call.
         trackOptimizationAttempt(TrackedStrategy::GetProp_CommonGetter);
         if (!getPropTryCommonGetter(&emitted, obj, name, types) || emitted)
@@ -11822,6 +11954,49 @@ IonBuilder::getPropTryComplexPropOfTypedObject(bool* emitted,
                                   fieldPrediction, fieldTypeObj);
 }
 
+MDefinition*
+IonBuilder::convertUnboxedObjects(MDefinition* obj)
+{
+    // If obj might be in any particular unboxed group which should be
+    // converted to a native representation, perform that conversion. This does
+    // not guarantee the object will not have such a group afterwards, if the
+    // object's possible groups are not precisely known.
+    TemporaryTypeSet* types = obj->resultTypeSet();
+    if (!types || types->unknownObject() || !types->objectOrSentinel())
+        return obj;
+
+    BaselineInspector::ObjectGroupVector list(alloc());
+    for (size_t i = 0; i < types->getObjectCount(); i++) {
+        TypeSet::ObjectKey* key = obj->resultTypeSet()->getObject(i);
+        if (!key || !key->isGroup())
+            continue;
+
+        if (UnboxedLayout* layout = key->group()->maybeUnboxedLayout()) {
+            AutoEnterOOMUnsafeRegion oomUnsafe;
+            if (layout->nativeGroup() && !list.append(key->group()))
+                oomUnsafe.crash("IonBuilder::convertUnboxedObjects");
+        }
+    }
+
+    return convertUnboxedObjects(obj, list);
+}
+
+MDefinition*
+IonBuilder::convertUnboxedObjects(MDefinition* obj,
+                                  const BaselineInspector::ObjectGroupVector& list)
+{
+    for (size_t i = 0; i < list.length(); i++) {
+        ObjectGroup* group = list[i];
+        if (TemporaryTypeSet* types = obj->resultTypeSet()) {
+            if (!types->hasType(TypeSet::ObjectType(group)))
+                continue;
+        }
+        obj = MConvertUnboxedObjectToNative::New(alloc(), obj, group);
+        current->add(obj->toInstruction());
+    }
+    return obj;
+}
+
 bool
 IonBuilder::getPropTryDefiniteSlot(bool* emitted, MDefinition* obj, PropertyName* name,
                                    BarrierKind barrier, TemporaryTypeSet* types)
@@ -11906,14 +12081,111 @@ IonBuilder::getPropTryModuleNamespace(bool* emitted, MDefinition* obj, PropertyN
     return true;
 }
 
+MInstruction*
+IonBuilder::loadUnboxedProperty(MDefinition* obj, size_t offset, JSValueType unboxedType,
+                                BarrierKind barrier, TemporaryTypeSet* types)
+{
+    // loadUnboxedValue is designed to load any value as if it were contained in
+    // an array. Thus a property offset is converted to an index, when the
+    // object is reinterpreted as an array of properties of the same size.
+    size_t index = offset / UnboxedTypeSize(unboxedType);
+    MInstruction* indexConstant = MConstant::New(alloc(), Int32Value(index));
+    current->add(indexConstant);
+
+    return loadUnboxedValue(obj, UnboxedPlainObject::offsetOfData(),
+                            indexConstant, unboxedType, barrier, types);
+}
+
+MInstruction*
+IonBuilder::loadUnboxedValue(MDefinition* elements, size_t elementsOffset,
+                             MDefinition* index, JSValueType unboxedType,
+                             BarrierKind barrier, TemporaryTypeSet* types)
+{
+    MInstruction* load;
+    switch (unboxedType) {
+      case JSVAL_TYPE_BOOLEAN:
+        load = MLoadUnboxedScalar::New(alloc(), elements, index, Scalar::Uint8,
+                                       DoesNotRequireMemoryBarrier, elementsOffset);
+        load->setResultType(MIRType::Boolean);
+        break;
+
+      case JSVAL_TYPE_INT32:
+        load = MLoadUnboxedScalar::New(alloc(), elements, index, Scalar::Int32,
+                                       DoesNotRequireMemoryBarrier, elementsOffset);
+        load->setResultType(MIRType::Int32);
+        break;
+
+      case JSVAL_TYPE_DOUBLE:
+        load = MLoadUnboxedScalar::New(alloc(), elements, index, Scalar::Float64,
+                                       DoesNotRequireMemoryBarrier, elementsOffset,
+                                       /* canonicalizeDoubles = */ false);
+        load->setResultType(MIRType::Double);
+        break;
+
+      case JSVAL_TYPE_STRING:
+        load = MLoadUnboxedString::New(alloc(), elements, index, elementsOffset);
+        break;
+
+      case JSVAL_TYPE_OBJECT: {
+        MLoadUnboxedObjectOrNull::NullBehavior nullBehavior;
+        if (types->hasType(TypeSet::NullType()))
+            nullBehavior = MLoadUnboxedObjectOrNull::HandleNull;
+        else if (barrier != BarrierKind::NoBarrier)
+            nullBehavior = MLoadUnboxedObjectOrNull::BailOnNull;
+        else
+            nullBehavior = MLoadUnboxedObjectOrNull::NullNotPossible;
+        load = MLoadUnboxedObjectOrNull::New(alloc(), elements, index, nullBehavior,
+                                             elementsOffset);
+        break;
+      }
+
+      default:
+        MOZ_CRASH();
+    }
+
+    current->add(load);
+    return load;
+}
+
+bool
+IonBuilder::getPropTryUnboxed(bool* emitted, MDefinition* obj, PropertyName* name,
+                              BarrierKind barrier, TemporaryTypeSet* types)
+{
+    MOZ_ASSERT(*emitted == false);
+
+    JSValueType unboxedType;
+    uint32_t offset = getUnboxedOffset(obj->resultTypeSet(), name, &unboxedType);
+    if (offset == UINT32_MAX)
+        return true;
+
+    if (obj->type() != MIRType::Object) {
+        MGuardObject* guard = MGuardObject::New(alloc(), obj);
+        current->add(guard);
+        obj = guard;
+    }
+
+    MInstruction* load = loadUnboxedProperty(obj, offset, unboxedType, barrier, types);
+    current->push(load);
+
+    if (!pushTypeBarrier(load, types, barrier))
+        return false;
+
+    trackOptimizationSuccess();
+    *emitted = true;
+    return true;
+}
+
 MDefinition*
 IonBuilder::addShapeGuardsForGetterSetter(MDefinition* obj, JSObject* holder, Shape* holderShape,
                 const BaselineInspector::ReceiverVector& receivers,
+                const BaselineInspector::ObjectGroupVector& convertUnboxedGroups,
                 bool isOwnProperty)
 {
     MOZ_ASSERT(holder);
     MOZ_ASSERT(holderShape);
 
+    obj = convertUnboxedObjects(obj, convertUnboxedGroups);
+
     if (isOwnProperty) {
         MOZ_ASSERT(receivers.empty());
         return addShapeGuard(obj, holderShape, Bailout_ShapeGuard);
@@ -11937,8 +12209,10 @@ IonBuilder::getPropTryCommonGetter(bool* emitted, MDefinition* obj, PropertyName
     JSObject* foundProto = nullptr;
     bool isOwnProperty = false;
     BaselineInspector::ReceiverVector receivers(alloc());
+    BaselineInspector::ObjectGroupVector convertUnboxedGroups(alloc());
     if (!inspector->commonGetPropFunction(pc, &foundProto, &lastProperty, &commonGetter,
-                                          &globalShape, &isOwnProperty, receivers))
+                                          &globalShape, &isOwnProperty,
+                                          receivers, convertUnboxedGroups))
     {
         return true;
     }
@@ -11954,7 +12228,8 @@ IonBuilder::getPropTryCommonGetter(bool* emitted, MDefinition* obj, PropertyName
         // If type information is bad, we can still optimize the getter if we
         // shape guard.
         obj = addShapeGuardsForGetterSetter(obj, foundProto, lastProperty,
-                                            receivers, isOwnProperty);
+                                            receivers, convertUnboxedGroups,
+                                            isOwnProperty);
         if (!obj)
             return false;
     }
@@ -12122,12 +12397,15 @@ IonBuilder::getPropTryInlineAccess(bool* emitted, MDefinition* obj, PropertyName
     MOZ_ASSERT(*emitted == false);
 
     BaselineInspector::ReceiverVector receivers(alloc());
-    if (!inspector->maybeInfoForPropertyOp(pc, receivers))
+    BaselineInspector::ObjectGroupVector convertUnboxedGroups(alloc());
+    if (!inspector->maybeInfoForPropertyOp(pc, receivers, convertUnboxedGroups))
         return false;
 
     if (!canInlinePropertyOpShapes(receivers))
         return true;
 
+    obj = convertUnboxedObjects(obj, convertUnboxedGroups);
+
     MIRType rvalType = types->getKnownMIRType();
     if (barrier != BarrierKind::NoBarrier || IsNullOrUndefined(rvalType))
         rvalType = MIRType::Value;
@@ -12150,6 +12428,45 @@ IonBuilder::getPropTryInlineAccess(bool* emitted, MDefinition* obj, PropertyName
             return true;
         }
 
+        if (receivers[0].shape) {
+            // Monomorphic load from an unboxed object expando.
+            spew("Inlining monomorphic unboxed expando GETPROP");
+
+            obj = addGroupGuard(obj, receivers[0].group, Bailout_ShapeGuard);
+            obj = addUnboxedExpandoGuard(obj, /* hasExpando = */ true, Bailout_ShapeGuard);
+
+            MInstruction* expando = MLoadUnboxedExpando::New(alloc(), obj);
+            current->add(expando);
+
+            expando = addShapeGuard(expando, receivers[0].shape, Bailout_ShapeGuard);
+
+            Shape* shape = receivers[0].shape->searchLinear(NameToId(name));
+            MOZ_ASSERT(shape);
+
+            if (!loadSlot(expando, shape, rvalType, barrier, types))
+                return false;
+
+            trackOptimizationOutcome(TrackedOutcome::Monomorphic);
+            *emitted = true;
+            return true;
+        }
+
+        // Monomorphic load from an unboxed object.
+        ObjectGroup* group = receivers[0].group;
+        if (obj->resultTypeSet() && !obj->resultTypeSet()->hasType(TypeSet::ObjectType(group)))
+            return true;
+
+        obj = addGroupGuard(obj, group, Bailout_ShapeGuard);
+
+        const UnboxedLayout::Property* property = group->unboxedLayout().lookup(name);
+        MInstruction* load = loadUnboxedProperty(obj, property->offset, property->type, barrier, types);
+        current->push(load);
+
+        if (!pushTypeBarrier(load, types, barrier))
+            return false;
+
+        trackOptimizationOutcome(TrackedOutcome::Monomorphic);
+        *emitted = true;
         return true;
     }
 
@@ -12391,7 +12708,7 @@ bool
 IonBuilder::jsop_setprop(PropertyName* name)
 {
     MDefinition* value = current->pop();
-    MDefinition* obj = current->pop();
+    MDefinition* obj = convertUnboxedObjects(current->pop());
 
     bool emitted = false;
     startTrackingOptimizations();
@@ -12424,6 +12741,13 @@ IonBuilder::jsop_setprop(PropertyName* name)
     bool barrier = PropertyWriteNeedsTypeBarrier(alloc(), constraints(), current, &obj, name, &value,
                                                  /* canModify = */ true);
 
+    if (!forceInlineCaches()) {
+        // Try to emit stores to unboxed objects.
+        trackOptimizationAttempt(TrackedStrategy::SetProp_Unboxed);
+        if (!setPropTryUnboxed(&emitted, obj, name, value, barrier, objTypes) || emitted)
+            return emitted;
+    }
+
     // Add post barrier if needed. The instructions above manage any post
     // barriers they need directly.
     if (NeedsPostBarrier(value))
@@ -12457,8 +12781,10 @@ IonBuilder::setPropTryCommonSetter(bool* emitted, MDefinition* obj,
     JSObject* foundProto = nullptr;
     bool isOwnProperty;
     BaselineInspector::ReceiverVector receivers(alloc());
+    BaselineInspector::ObjectGroupVector convertUnboxedGroups(alloc());
     if (!inspector->commonSetPropFunction(pc, &foundProto, &lastProperty, &commonSetter,
-                                          &isOwnProperty, receivers))
+                                          &isOwnProperty,
+                                          receivers, convertUnboxedGroups))
     {
         trackOptimizationOutcome(TrackedOutcome::NoProtoFound);
         return true;
@@ -12473,7 +12799,8 @@ IonBuilder::setPropTryCommonSetter(bool* emitted, MDefinition* obj,
         // If type information is bad, we can still optimize the setter if we
         // shape guard.
         obj = addShapeGuardsForGetterSetter(obj, foundProto, lastProperty,
-                                            receivers, isOwnProperty);
+                                            receivers, convertUnboxedGroups,
+                                            isOwnProperty);
         if (!obj)
             return false;
     }
@@ -12729,6 +13056,100 @@ IonBuilder::setPropTryDefiniteSlot(bool* emitted, MDefinition* obj,
     return true;
 }
 
+MInstruction*
+IonBuilder::storeUnboxedProperty(MDefinition* obj, size_t offset, JSValueType unboxedType,
+                                 MDefinition* value)
+{
+    size_t scaledOffsetConstant = offset / UnboxedTypeSize(unboxedType);
+    MInstruction* scaledOffset = MConstant::New(alloc(), Int32Value(scaledOffsetConstant));
+    current->add(scaledOffset);
+
+    return storeUnboxedValue(obj, obj, UnboxedPlainObject::offsetOfData(),
+                             scaledOffset, unboxedType, value);
+}
+
+MInstruction*
+IonBuilder::storeUnboxedValue(MDefinition* obj, MDefinition* elements, int32_t elementsOffset,
+                              MDefinition* scaledOffset, JSValueType unboxedType,
+                              MDefinition* value, bool preBarrier /* = true */)
+{
+    MInstruction* store;
+    switch (unboxedType) {
+      case JSVAL_TYPE_BOOLEAN:
+        store = MStoreUnboxedScalar::New(alloc(), elements, scaledOffset, value, Scalar::Uint8,
+                                         MStoreUnboxedScalar::DontTruncateInput,
+                                         DoesNotRequireMemoryBarrier, elementsOffset);
+        break;
+
+      case JSVAL_TYPE_INT32:
+        store = MStoreUnboxedScalar::New(alloc(), elements, scaledOffset, value, Scalar::Int32,
+                                         MStoreUnboxedScalar::DontTruncateInput,
+                                         DoesNotRequireMemoryBarrier, elementsOffset);
+        break;
+
+      case JSVAL_TYPE_DOUBLE:
+        store = MStoreUnboxedScalar::New(alloc(), elements, scaledOffset, value, Scalar::Float64,
+                                         MStoreUnboxedScalar::DontTruncateInput,
+                                         DoesNotRequireMemoryBarrier, elementsOffset);
+        break;
+
+      case JSVAL_TYPE_STRING:
+        store = MStoreUnboxedString::New(alloc(), elements, scaledOffset, value,
+                                         elementsOffset, preBarrier);
+        break;
+
+      case JSVAL_TYPE_OBJECT:
+        MOZ_ASSERT(value->type() == MIRType::Object ||
+                   value->type() == MIRType::Null ||
+                   value->type() == MIRType::Value);
+        MOZ_ASSERT(!value->mightBeType(MIRType::Undefined),
+                   "MToObjectOrNull slow path is invalid for unboxed objects");
+        store = MStoreUnboxedObjectOrNull::New(alloc(), elements, scaledOffset, value, obj,
+                                               elementsOffset, preBarrier);
+        break;
+
+      default:
+        MOZ_CRASH();
+    }
+
+    current->add(store);
+    return store;
+}
+
+bool
+IonBuilder::setPropTryUnboxed(bool* emitted, MDefinition* obj,
+                              PropertyName* name, MDefinition* value,
+                              bool barrier, TemporaryTypeSet* objTypes)
+{
+    MOZ_ASSERT(*emitted == false);
+
+    if (barrier) {
+        trackOptimizationOutcome(TrackedOutcome::NeedsTypeBarrier);
+        return true;
+    }
+
+    JSValueType unboxedType;
+    uint32_t offset = getUnboxedOffset(obj->resultTypeSet(), name, &unboxedType);
+    if (offset == UINT32_MAX)
+        return true;
+
+    if (obj->type() != MIRType::Object) {
+        MGuardObject* guard = MGuardObject::New(alloc(), obj);
+        current->add(guard);
+        obj = guard;
+    }
+
+    MInstruction* store = storeUnboxedProperty(obj, offset, unboxedType, value);
+
+    current->push(value);
+
+    if (!resumeAfter(store))
+        return false;
+
+    *emitted = true;
+    return true;
+}
+
 bool
 IonBuilder::setPropTryInlineAccess(bool* emitted, MDefinition* obj,
                                    PropertyName* name, MDefinition* value,
@@ -12742,12 +13163,15 @@ IonBuilder::setPropTryInlineAccess(bool* emitted, MDefinition* obj,
     }
 
     BaselineInspector::ReceiverVector receivers(alloc());
-    if (!inspector->maybeInfoForPropertyOp(pc, receivers))
+    BaselineInspector::ObjectGroupVector convertUnboxedGroups(alloc());
+    if (!inspector->maybeInfoForPropertyOp(pc, receivers, convertUnboxedGroups))
         return false;
 
     if (!canInlinePropertyOpShapes(receivers))
         return true;
 
+    obj = convertUnboxedObjects(obj, convertUnboxedGroups);
+
     if (receivers.length() == 1) {
         if (!receivers[0].group) {
             // Monomorphic store to a native object.
@@ -12767,6 +13191,46 @@ IonBuilder::setPropTryInlineAccess(bool* emitted, MDefinition* obj,
             return true;
         }
 
+        if (receivers[0].shape) {
+            // Monomorphic store to an unboxed object expando.
+            spew("Inlining monomorphic unboxed expando SETPROP");
+
+            obj = addGroupGuard(obj, receivers[0].group, Bailout_ShapeGuard);
+            obj = addUnboxedExpandoGuard(obj, /* hasExpando = */ true, Bailout_ShapeGuard);
+
+            MInstruction* expando = MLoadUnboxedExpando::New(alloc(), obj);
+            current->add(expando);
+
+            expando = addShapeGuard(expando, receivers[0].shape, Bailout_ShapeGuard);
+
+            Shape* shape = receivers[0].shape->searchLinear(NameToId(name));
+            MOZ_ASSERT(shape);
+
+            bool needsBarrier = objTypes->propertyNeedsBarrier(constraints(), NameToId(name));
+            if (!storeSlot(expando, shape, value, needsBarrier))
+                return false;
+
+            trackOptimizationOutcome(TrackedOutcome::Monomorphic);
+            *emitted = true;
+            return true;
+        }
+
+        // Monomorphic store to an unboxed object.
+        spew("Inlining monomorphic unboxed SETPROP");
+
+        ObjectGroup* group = receivers[0].group;
+        if (!objTypes->hasType(TypeSet::ObjectType(group)))
+            return true;
+
+        obj = addGroupGuard(obj, group, Bailout_ShapeGuard);
+
+        const UnboxedLayout::Property* property = group->unboxedLayout().lookup(name);
+        storeUnboxedProperty(obj, property->offset, property->type, value);
+
+        current->push(value);
+
+        trackOptimizationOutcome(TrackedOutcome::Monomorphic);
+        *emitted = true;
         return true;
     }
 
@@ -13465,7 +13929,7 @@ IonBuilder::jsop_setaliasedvar(EnvironmentCoordinate ec)
 bool
 IonBuilder::jsop_in()
 {
-    MDefinition* obj = current->pop();
+    MDefinition* obj = convertUnboxedObjects(current->pop());
     MDefinition* id = current->pop();
 
     bool emitted = false;
@@ -13492,8 +13956,11 @@ IonBuilder::inTryDense(bool* emitted, MDefinition* obj, MDefinition* id)
     if (shouldAbortOnPreliminaryGroups(obj))
         return true;
 
-    if (!ElementAccessIsDenseNative(constraints(), obj, id))
-        return true;
+    JSValueType unboxedType = UnboxedArrayElementType(constraints(), obj, id);
+    if (unboxedType == JSVAL_TYPE_MAGIC) {
+        if (!ElementAccessIsDenseNative(constraints(), obj, id))
+            return true;
+    }
 
     if (ElementAccessHasExtraIndexedProperty(this, obj))
         return true;
@@ -13508,10 +13975,10 @@ IonBuilder::inTryDense(bool* emitted, MDefinition* obj, MDefinition* id)
     id = idInt32;
 
     // Get the elements vector.
-    MElements* elements = MElements::New(alloc(), obj);
+    MElements* elements = MElements::New(alloc(), obj, unboxedType != JSVAL_TYPE_MAGIC);
     current->add(elements);
 
-    MInstruction* initLength = initializedLength(obj, elements);
+    MInstruction* initLength = initializedLength(obj, elements, unboxedType);
 
     // If there are no holes, speculate the InArray check will not fail.
     if (!needsHoleCheck && !failedBoundsCheck_) {
@@ -13521,7 +13988,8 @@ IonBuilder::inTryDense(bool* emitted, MDefinition* obj, MDefinition* id)
     }
 
     // Check if id < initLength and elem[id] not a hole.
-    MInArray* ins = MInArray::New(alloc(), elements, id, initLength, obj, needsHoleCheck);
+    MInArray* ins = MInArray::New(alloc(), elements, id, initLength, obj, needsHoleCheck,
+                                  unboxedType);
 
     current->add(ins);
     current->push(ins);
@@ -13818,6 +14286,19 @@ IonBuilder::addGroupGuard(MDefinition* obj, ObjectGroup* group, BailoutKind bail
 }
 
 MInstruction*
+IonBuilder::addUnboxedExpandoGuard(MDefinition* obj, bool hasExpando, BailoutKind bailoutKind)
+{
+    MGuardUnboxedExpando* guard = MGuardUnboxedExpando::New(alloc(), obj, hasExpando, bailoutKind);
+    current->add(guard);
+
+    // If a shape guard failed in the past, don't optimize group guards.
+    if (failedShapeGuard_)
+        guard->setNotMovable();
+
+    return guard;
+}
+
+MInstruction*
 IonBuilder::addGuardReceiverPolymorphic(MDefinition* obj,
                                         const BaselineInspector::ReceiverVector& receivers)
 {
@@ -13826,6 +14307,15 @@ IonBuilder::addGuardReceiverPolymorphic(MDefinition* obj,
             // Monomorphic guard on a native object.
             return addShapeGuard(obj, receivers[0].shape, Bailout_ShapeGuard);
         }
+
+        if (!receivers[0].shape) {
+            // Guard on an unboxed object that does not have an expando.
+            obj = addGroupGuard(obj, receivers[0].group, Bailout_ShapeGuard);
+            return addUnboxedExpandoGuard(obj, /* hasExpando = */ false, Bailout_ShapeGuard);
+        }
+
+        // Monomorphic receiver guards are not yet supported when the receiver
+        // is an unboxed object with an expando.
     }
 
     MGuardReceiverPolymorphic* guard = MGuardReceiverPolymorphic::New(alloc(), obj);
@@ -14199,24 +14689,32 @@ IonBuilder::constantInt(int32_t i)
 }
 
 MInstruction*
-IonBuilder::initializedLength(MDefinition* obj, MDefinition* elements)
+IonBuilder::initializedLength(MDefinition* obj, MDefinition* elements, JSValueType unboxedType)
 {
-    MInstruction* res = MInitializedLength::New(alloc(), elements);
+    MInstruction* res;
+    if (unboxedType != JSVAL_TYPE_MAGIC)
+        res = MUnboxedArrayInitializedLength::New(alloc(), obj);
+    else
+        res = MInitializedLength::New(alloc(), elements);
     current->add(res);
     return res;
 }
 
 MInstruction*
-IonBuilder::setInitializedLength(MDefinition* obj, size_t count)
+IonBuilder::setInitializedLength(MDefinition* obj, JSValueType unboxedType, size_t count)
 {
     MOZ_ASSERT(count);
 
-    // MSetInitializedLength takes the index of the last element, rather
-    // than the count itself.
-    MInstruction* elements = MElements::New(alloc(), obj, /* unboxed = */ false);
-    current->add(elements);
-    MInstruction* res = 
-	    MSetInitializedLength::New(alloc(), elements, constant(Int32Value(count - 1)));
+    MInstruction* res;
+    if (unboxedType != JSVAL_TYPE_MAGIC) {
+        res = MSetUnboxedArrayInitializedLength::New(alloc(), obj, constant(Int32Value(count)));
+    } else {
+        // MSetInitializedLength takes the index of the last element, rather
+        // than the count itself.
+        MInstruction* elements = MElements::New(alloc(), obj, /* unboxed = */ false);
+        current->add(elements);
+        res = MSetInitializedLength::New(alloc(), elements, constant(Int32Value(count - 1)));
+    }
     current->add(res);
     return res;
 }
diff --git a/js/src/jit/IonBuilder.h b/js/src/jit/IonBuilder.h
index 6a3b61232..f359c764f 100644
--- a/js/src/jit/IonBuilder.h
+++ b/js/src/jit/IonBuilder.h
@@ -346,8 +346,9 @@ class IonBuilder
 
     MConstant* constant(const Value& v);
     MConstant* constantInt(int32_t i);
-    MInstruction* initializedLength(MDefinition* obj, MDefinition* elements);
-    MInstruction* setInitializedLength(MDefinition* obj, size_t count);
+    MInstruction* initializedLength(MDefinition* obj, MDefinition* elements,
+                                    JSValueType unboxedType);
+    MInstruction* setInitializedLength(MDefinition* obj, JSValueType unboxedType, size_t count);
 
     // Improve the type information at tests
     MOZ_MUST_USE bool improveTypesAtTest(MDefinition* ins, bool trueBranch, MTest* test);
@@ -400,6 +401,7 @@ class IonBuilder
     MInstruction* addBoundsCheck(MDefinition* index, MDefinition* length);
     MInstruction* addShapeGuard(MDefinition* obj, Shape* const shape, BailoutKind bailoutKind);
     MInstruction* addGroupGuard(MDefinition* obj, ObjectGroup* group, BailoutKind bailoutKind);
+    MInstruction* addUnboxedExpandoGuard(MDefinition* obj, bool hasExpando, BailoutKind bailoutKind);
     MInstruction* addSharedTypedArrayGuard(MDefinition* obj);
 
     MInstruction*
@@ -439,6 +441,8 @@ class IonBuilder
                                              BarrierKind barrier, TemporaryTypeSet* types);
     MOZ_MUST_USE bool getPropTryModuleNamespace(bool* emitted, MDefinition* obj, PropertyName* name,
                                                 BarrierKind barrier, TemporaryTypeSet* types);
+    MOZ_MUST_USE bool getPropTryUnboxed(bool* emitted, MDefinition* obj, PropertyName* name,
+                                        BarrierKind barrier, TemporaryTypeSet* types);
     MOZ_MUST_USE bool getPropTryCommonGetter(bool* emitted, MDefinition* obj, PropertyName* name,
                                              TemporaryTypeSet* types);
     MOZ_MUST_USE bool getPropTryInlineAccess(bool* emitted, MDefinition* obj, PropertyName* name,
@@ -471,6 +475,9 @@ class IonBuilder
     MOZ_MUST_USE bool setPropTryDefiniteSlot(bool* emitted, MDefinition* obj,
                                              PropertyName* name, MDefinition* value,
                                              bool barrier, TemporaryTypeSet* objTypes);
+    MOZ_MUST_USE bool setPropTryUnboxed(bool* emitted, MDefinition* obj,
+                                        PropertyName* name, MDefinition* value,
+                                        bool barrier, TemporaryTypeSet* objTypes);
     MOZ_MUST_USE bool setPropTryInlineAccess(bool* emitted, MDefinition* obj,
                                              PropertyName* name, MDefinition* value,
                                              bool barrier, TemporaryTypeSet* objTypes);
@@ -610,6 +617,7 @@ class IonBuilder
                                                         TypedObjectPrediction elemTypeReprs,
                                                         uint32_t elemSize);
     MOZ_MUST_USE bool initializeArrayElement(MDefinition* obj, size_t index, MDefinition* value,
+                                             JSValueType unboxedType,
                                              bool addResumePointAndIncrementInitializedLength);
 
     // jsop_getelem() helpers.
@@ -722,13 +730,15 @@ class IonBuilder
     MOZ_MUST_USE bool jsop_bindname(PropertyName* name);
     MOZ_MUST_USE bool jsop_bindvar();
     MOZ_MUST_USE bool jsop_getelem();
-    MOZ_MUST_USE bool jsop_getelem_dense(MDefinition* obj, MDefinition* index);
+    MOZ_MUST_USE bool jsop_getelem_dense(MDefinition* obj, MDefinition* index,
+                                         JSValueType unboxedType);
     MOZ_MUST_USE bool jsop_getelem_typed(MDefinition* obj, MDefinition* index,
                                          ScalarTypeDescr::Type arrayType);
     MOZ_MUST_USE bool jsop_setelem();
     MOZ_MUST_USE bool jsop_setelem_dense(TemporaryTypeSet::DoubleConversion conversion,
                                          MDefinition* object, MDefinition* index,
-                                         MDefinition* value, bool writeHole, bool* emitted);
+                                         MDefinition* value, JSValueType unboxedType,
+                                         bool writeHole, bool* emitted);
     MOZ_MUST_USE bool jsop_setelem_typed(ScalarTypeDescr::Type arrayType,
                                          MDefinition* object, MDefinition* index,
                                          MDefinition* value);
@@ -1033,6 +1043,7 @@ class IonBuilder
     MDefinition*
     addShapeGuardsForGetterSetter(MDefinition* obj, JSObject* holder, Shape* holderShape,
                                   const BaselineInspector::ReceiverVector& receivers,
+                                  const BaselineInspector::ObjectGroupVector& convertUnboxedGroups,
                                   bool isOwnProperty);
 
     MOZ_MUST_USE bool annotateGetPropertyCache(MDefinition* obj, PropertyName* name,
@@ -1050,6 +1061,22 @@ class IonBuilder
     ResultWithOOM<bool> testNotDefinedProperty(MDefinition* obj, jsid id);
 
     uint32_t getDefiniteSlot(TemporaryTypeSet* types, PropertyName* name, uint32_t* pnfixed);
+    MDefinition* convertUnboxedObjects(MDefinition* obj);
+    MDefinition* convertUnboxedObjects(MDefinition* obj,
+                                       const BaselineInspector::ObjectGroupVector& list);
+    uint32_t getUnboxedOffset(TemporaryTypeSet* types, PropertyName* name,
+                              JSValueType* punboxedType);
+    MInstruction* loadUnboxedProperty(MDefinition* obj, size_t offset, JSValueType unboxedType,
+                                      BarrierKind barrier, TemporaryTypeSet* types);
+    MInstruction* loadUnboxedValue(MDefinition* elements, size_t elementsOffset,
+                                   MDefinition* scaledOffset, JSValueType unboxedType,
+                                   BarrierKind barrier, TemporaryTypeSet* types);
+    MInstruction* storeUnboxedProperty(MDefinition* obj, size_t offset, JSValueType unboxedType,
+                                       MDefinition* value);
+    MInstruction* storeUnboxedValue(MDefinition* obj,
+                                    MDefinition* elements, int32_t elementsOffset,
+                                    MDefinition* scaledOffset, JSValueType unboxedType,
+                                    MDefinition* value, bool preBarrier = true);
     MOZ_MUST_USE bool checkPreliminaryGroups(MDefinition *obj);
     MOZ_MUST_USE bool freezePropTypeSets(TemporaryTypeSet* types,
                                          JSObject* foundProto, PropertyName* name);
diff --git a/js/src/jit/IonCaches.cpp b/js/src/jit/IonCaches.cpp
index fb4291188..c2dc57373 100644
--- a/js/src/jit/IonCaches.cpp
+++ b/js/src/jit/IonCaches.cpp
@@ -31,6 +31,7 @@
 #include "jit/shared/Lowering-shared-inl.h"
 #include "vm/Interpreter-inl.h"
 #include "vm/Shape-inl.h"
+#include "vm/UnboxedObject-inl.h"
 
 using namespace js;
 using namespace js::jit;
@@ -619,7 +620,29 @@ TestMatchingReceiver(MacroAssembler& masm, IonCache::StubAttacher& attacher,
                      Register object, JSObject* obj, Label* failure,
                      bool alwaysCheckGroup = false)
 {
-    if (obj->is<TypedObject>()) {
+    if (obj->is<UnboxedPlainObject>()) {
+        MOZ_ASSERT(failure);
+
+        masm.branchTestObjGroup(Assembler::NotEqual, object, obj->group(), failure);
+        Address expandoAddress(object, UnboxedPlainObject::offsetOfExpando());
+        if (UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando()) {
+            masm.branchPtr(Assembler::Equal, expandoAddress, ImmWord(0), failure);
+            Label success;
+            masm.push(object);
+            masm.loadPtr(expandoAddress, object);
+            masm.branchTestObjShape(Assembler::Equal, object, expando->lastProperty(),
+                                    &success);
+            masm.pop(object);
+            masm.jump(failure);
+            masm.bind(&success);
+            masm.pop(object);
+        } else {
+            masm.branchPtr(Assembler::NotEqual, expandoAddress, ImmWord(0), failure);
+        }
+    } else if (obj->is<UnboxedArrayObject>()) {
+        MOZ_ASSERT(failure);
+        masm.branchTestObjGroup(Assembler::NotEqual, object, obj->group(), failure);
+    } else if (obj->is<TypedObject>()) {
         attacher.branchNextStubOrLabel(masm, Assembler::NotEqual,
                                        Address(object, JSObject::offsetOfGroup()),
                                        ImmGCPtr(obj->group()), failure);
@@ -736,6 +759,7 @@ GenerateReadSlot(JSContext* cx, IonScript* ion, MacroAssembler& masm,
     // jump directly. Otherwise, jump to the end of the stub, so there's a
     // common point to patch.
     bool multipleFailureJumps = (obj != holder)
+                             || obj->is<UnboxedPlainObject>()
                              || (checkTDZ && output.hasValue())
                              || (failures != nullptr && failures->used());
 
@@ -754,6 +778,7 @@ GenerateReadSlot(JSContext* cx, IonScript* ion, MacroAssembler& masm,
     Register scratchReg = Register::FromCode(0); // Quell compiler warning.
 
     if (obj != holder ||
+        obj->is<UnboxedPlainObject>() ||
         !holder->as<NativeObject>().isFixedSlot(shape->slot()))
     {
         if (output.hasValue()) {
@@ -814,6 +839,10 @@ GenerateReadSlot(JSContext* cx, IonScript* ion, MacroAssembler& masm,
 
             holderReg = InvalidReg;
         }
+    } else if (obj->is<UnboxedPlainObject>()) {
+        holder = obj->as<UnboxedPlainObject>().maybeExpando();
+        holderReg = scratchReg;
+        masm.loadPtr(Address(object, UnboxedPlainObject::offsetOfExpando()), holderReg);
     } else {
         holderReg = object;
     }
@@ -841,6 +870,30 @@ GenerateReadSlot(JSContext* cx, IonScript* ion, MacroAssembler& masm,
     attacher.jumpNextStub(masm);
 }
 
+static void
+GenerateReadUnboxed(JSContext* cx, IonScript* ion, MacroAssembler& masm,
+                    IonCache::StubAttacher& attacher, JSObject* obj,
+                    const UnboxedLayout::Property* property,
+                    Register object, TypedOrValueRegister output,
+                    Label* failures = nullptr)
+{
+    // Guard on the group of the object.
+    attacher.branchNextStubOrLabel(masm, Assembler::NotEqual,
+                                   Address(object, JSObject::offsetOfGroup()),
+                                   ImmGCPtr(obj->group()), failures);
+
+    Address address(object, UnboxedPlainObject::offsetOfData() + property->offset);
+
+    masm.loadUnboxedProperty(address, property->type, output);
+
+    attacher.jumpRejoin(masm);
+
+    if (failures) {
+        masm.bind(failures);
+        attacher.jumpNextStub(masm);
+    }
+}
+
 static bool
 EmitGetterCall(JSContext* cx, MacroAssembler& masm,
                IonCache::StubAttacher& attacher, JSObject* obj,
@@ -1135,6 +1188,39 @@ GenerateArrayLength(JSContext* cx, MacroAssembler& masm, IonCache::StubAttacher&
     return true;
 }
 
+static void
+GenerateUnboxedArrayLength(JSContext* cx, MacroAssembler& masm, IonCache::StubAttacher& attacher,
+                           JSObject* array, Register object, TypedOrValueRegister output,
+                           Label* failures)
+{
+    Register outReg;
+    if (output.hasValue()) {
+        outReg = output.valueReg().scratchReg();
+    } else {
+        MOZ_ASSERT(output.type() == MIRType::Int32);
+        outReg = output.typedReg().gpr();
+    }
+    MOZ_ASSERT(object != outReg);
+
+    TestMatchingReceiver(masm, attacher, object, array, failures);
+
+    // Load length.
+    masm.load32(Address(object, UnboxedArrayObject::offsetOfLength()), outReg);
+
+    // Check for a length that fits in an int32.
+    masm.branchTest32(Assembler::Signed, outReg, outReg, failures);
+
+    if (output.hasValue())
+        masm.tagValue(JSVAL_TYPE_INT32, outReg, output.valueReg());
+
+    // Success.
+    attacher.jumpRejoin(masm);
+
+    // Failure.
+    masm.bind(failures);
+    attacher.jumpNextStub(masm);
+}
+
 // In this case, the code for TypedArray and SharedTypedArray is not the same,
 // because the code embeds pointers to the respective class arrays.  Code that
 // caches the stub code must distinguish between the two cases.
@@ -1448,6 +1534,101 @@ GetPropertyIC::tryAttachNative(JSContext* cx, HandleScript outerScript, IonScrip
 }
 
 bool
+GetPropertyIC::tryAttachUnboxed(JSContext* cx, HandleScript outerScript, IonScript* ion,
+                                HandleObject obj, HandleId id, void* returnAddr, bool* emitted)
+{
+    MOZ_ASSERT(canAttachStub());
+    MOZ_ASSERT(!*emitted);
+    MOZ_ASSERT(outerScript->ionScript() == ion);
+
+    if (!obj->is<UnboxedPlainObject>())
+        return true;
+    const UnboxedLayout::Property* property = obj->as<UnboxedPlainObject>().layout().lookup(id);
+    if (!property)
+        return true;
+
+    *emitted = true;
+
+    MacroAssembler masm(cx, ion, outerScript, profilerLeavePc_);
+
+    Label failures;
+    emitIdGuard(masm, id, &failures);
+    Label* maybeFailures = failures.used() ? &failures : nullptr;
+
+    StubAttacher attacher(*this);
+    GenerateReadUnboxed(cx, ion, masm, attacher, obj, property, object(), output(), maybeFailures);
+    return linkAndAttachStub(cx, masm, attacher, ion, "read unboxed",
+                             JS::TrackedOutcome::ICGetPropStub_UnboxedRead);
+}
+
+bool
+GetPropertyIC::tryAttachUnboxedExpando(JSContext* cx, HandleScript outerScript, IonScript* ion,
+                                       HandleObject obj, HandleId id, void* returnAddr, bool* emitted)
+{
+    MOZ_ASSERT(canAttachStub());
+    MOZ_ASSERT(!*emitted);
+    MOZ_ASSERT(outerScript->ionScript() == ion);
+
+    if (!obj->is<UnboxedPlainObject>())
+        return true;
+    Rooted<UnboxedExpandoObject*> expando(cx, obj->as<UnboxedPlainObject>().maybeExpando());
+    if (!expando)
+        return true;
+
+    Shape* shape = expando->lookup(cx, id);
+    if (!shape || !shape->hasDefaultGetter() || !shape->hasSlot())
+        return true;
+
+    *emitted = true;
+
+    MacroAssembler masm(cx, ion, outerScript, profilerLeavePc_);
+
+    Label failures;
+    emitIdGuard(masm, id, &failures);
+    Label* maybeFailures = failures.used() ? &failures : nullptr;
+
+    StubAttacher attacher(*this);
+    GenerateReadSlot(cx, ion, masm, attacher, DontCheckTDZ, obj, obj,
+                     shape, object(), output(), maybeFailures);
+    return linkAndAttachStub(cx, masm, attacher, ion, "read unboxed expando",
+                             JS::TrackedOutcome::ICGetPropStub_UnboxedReadExpando);
+}
+
+bool
+GetPropertyIC::tryAttachUnboxedArrayLength(JSContext* cx, HandleScript outerScript, IonScript* ion,
+                                           HandleObject obj, HandleId id, void* returnAddr,
+                                           bool* emitted)
+{
+    MOZ_ASSERT(canAttachStub());
+    MOZ_ASSERT(!*emitted);
+    MOZ_ASSERT(outerScript->ionScript() == ion);
+
+    if (!obj->is<UnboxedArrayObject>())
+        return true;
+
+    if (!JSID_IS_ATOM(id, cx->names().length))
+        return true;
+
+    if (obj->as<UnboxedArrayObject>().length() > INT32_MAX)
+        return true;
+
+    if (!allowArrayLength(cx))
+        return true;
+
+    *emitted = true;
+
+    MacroAssembler masm(cx, ion, outerScript, profilerLeavePc_);
+
+    Label failures;
+    emitIdGuard(masm, id, &failures);
+
+    StubAttacher attacher(*this);
+    GenerateUnboxedArrayLength(cx, masm, attacher, obj, object(), output(), &failures);
+    return linkAndAttachStub(cx, masm, attacher, ion, "unboxed array length",
+                             JS::TrackedOutcome::ICGetPropStub_UnboxedArrayLength);
+}
+
+bool
 GetPropertyIC::tryAttachTypedArrayLength(JSContext* cx, HandleScript outerScript, IonScript* ion,
                                          HandleObject obj, HandleId id, bool* emitted)
 {
@@ -2016,6 +2197,15 @@ GetPropertyIC::tryAttachStub(JSContext* cx, HandleScript outerScript, IonScript*
         if (!*emitted && !tryAttachNative(cx, outerScript, ion, obj, id, returnAddr, emitted))
             return false;
 
+        if (!*emitted && !tryAttachUnboxed(cx, outerScript, ion, obj, id, returnAddr, emitted))
+            return false;
+
+        if (!*emitted && !tryAttachUnboxedExpando(cx, outerScript, ion, obj, id, returnAddr, emitted))
+            return false;
+
+        if (!*emitted && !tryAttachUnboxedArrayLength(cx, outerScript, ion, obj, id, returnAddr, emitted))
+            return false;
+
         if (!*emitted && !tryAttachTypedArrayLength(cx, outerScript, ion, obj, id, emitted))
             return false;
     }
@@ -2194,6 +2384,12 @@ GenerateSetSlot(JSContext* cx, MacroAssembler& masm, IonCache::StubAttacher& att
 
     NativeObject::slotsSizeMustNotOverflow();
 
+    if (obj->is<UnboxedPlainObject>()) {
+        obj = obj->as<UnboxedPlainObject>().maybeExpando();
+        masm.loadPtr(Address(object, UnboxedPlainObject::offsetOfExpando()), tempReg);
+        object = tempReg;
+    }
+
     if (obj->as<NativeObject>().isFixedSlot(shape->slot())) {
         Address addr(object, NativeObject::getFixedSlotOffset(shape->slot()));
 
@@ -2831,13 +3027,23 @@ GenerateAddSlot(JSContext* cx, MacroAssembler& masm, IonCache::StubAttacher& att
     masm.branchTestObjGroup(Assembler::NotEqual, object, oldGroup, failures);
     if (obj->maybeShape()) {
         masm.branchTestObjShape(Assembler::NotEqual, object, oldShape, failures);
+    } else {
+        MOZ_ASSERT(obj->is<UnboxedPlainObject>());
+
+        Address expandoAddress(object, UnboxedPlainObject::offsetOfExpando());
+        masm.branchPtr(Assembler::Equal, expandoAddress, ImmWord(0), failures);
+
+        masm.loadPtr(expandoAddress, tempReg);
+        masm.branchTestObjShape(Assembler::NotEqual, tempReg, oldShape, failures);
     }
 
     Shape* newShape = obj->maybeShape();
+    if (!newShape)
+        newShape = obj->as<UnboxedPlainObject>().maybeExpando()->lastProperty();
 
     // Guard that the incoming value is in the type set for the property
     // if a type barrier is required.
-    if (newShape && checkTypeset)
+    if (checkTypeset)
         CheckTypeSetForWrite(masm, obj, newShape->propid(), tempReg, value, failures);
 
     // Guard shapes along prototype chain.
@@ -2858,7 +3064,9 @@ GenerateAddSlot(JSContext* cx, MacroAssembler& masm, IonCache::StubAttacher& att
     }
 
     // Call a stub to (re)allocate dynamic slots, if necessary.
-    uint32_t newNumDynamicSlots = obj->as<NativeObject>().numDynamicSlots();
+    uint32_t newNumDynamicSlots = obj->is<UnboxedPlainObject>()
+                                  ? obj->as<UnboxedPlainObject>().maybeExpando()->numDynamicSlots()
+                                  : obj->as<NativeObject>().numDynamicSlots();
     if (NativeObject::dynamicSlotsCount(oldShape) != newNumDynamicSlots) {
         AllocatableRegisterSet regs(RegisterSet::Volatile());
         LiveRegisterSet save(regs.asLiveSet());
@@ -2869,6 +3077,12 @@ GenerateAddSlot(JSContext* cx, MacroAssembler& masm, IonCache::StubAttacher& att
         Register temp1 = regs.takeAnyGeneral();
         Register temp2 = regs.takeAnyGeneral();
 
+        if (obj->is<UnboxedPlainObject>()) {
+            // Pass the expando object to the stub.
+            masm.Push(object);
+            masm.loadPtr(Address(object, UnboxedPlainObject::offsetOfExpando()), object);
+        }
+
         masm.setupUnalignedABICall(temp1);
         masm.loadJSContext(temp1);
         masm.passABIArg(temp1);
@@ -2885,16 +3099,27 @@ GenerateAddSlot(JSContext* cx, MacroAssembler& masm, IonCache::StubAttacher& att
         masm.jump(&allocDone);
 
         masm.bind(&allocFailed);
+        if (obj->is<UnboxedPlainObject>())
+            masm.Pop(object);
         masm.PopRegsInMask(save);
         masm.jump(failures);
 
         masm.bind(&allocDone);
         masm.setFramePushed(framePushedAfterCall);
+        if (obj->is<UnboxedPlainObject>())
+            masm.Pop(object);
         masm.PopRegsInMask(save);
     }
 
     bool popObject = false;
 
+    if (obj->is<UnboxedPlainObject>()) {
+        masm.push(object);
+        popObject = true;
+        obj = obj->as<UnboxedPlainObject>().maybeExpando();
+        masm.loadPtr(Address(object, UnboxedPlainObject::offsetOfExpando()), object);
+    }
+
     // Write the object or expando object's new shape.
     Address shapeAddr(object, ShapedObject::offsetOfShape());
     if (cx->zone()->needsIncrementalBarrier())
@@ -2902,6 +3127,8 @@ GenerateAddSlot(JSContext* cx, MacroAssembler& masm, IonCache::StubAttacher& att
     masm.storePtr(ImmGCPtr(newShape), shapeAddr);
 
     if (oldGroup != obj->group()) {
+        MOZ_ASSERT(!obj->is<UnboxedPlainObject>());
+
         // Changing object's group from a partially to fully initialized group,
         // per the acquired properties analysis. Only change the group if the
         // old group still has a newScript.
@@ -3144,6 +3371,141 @@ CanAttachNativeSetProp(JSContext* cx, HandleObject obj, HandleId id, const Const
     return SetPropertyIC::CanAttachNone;
 }
 
+static void
+GenerateSetUnboxed(JSContext* cx, MacroAssembler& masm, IonCache::StubAttacher& attacher,
+                   JSObject* obj, jsid id, uint32_t unboxedOffset, JSValueType unboxedType,
+                   Register object, Register tempReg, const ConstantOrRegister& value,
+                   bool checkTypeset, Label* failures)
+{
+    // Guard on the type of the object.
+    masm.branchPtr(Assembler::NotEqual,
+                   Address(object, JSObject::offsetOfGroup()),
+                   ImmGCPtr(obj->group()), failures);
+
+    if (checkTypeset)
+        CheckTypeSetForWrite(masm, obj, id, tempReg, value, failures);
+
+    Address address(object, UnboxedPlainObject::offsetOfData() + unboxedOffset);
+
+    if (cx->zone()->needsIncrementalBarrier()) {
+        if (unboxedType == JSVAL_TYPE_OBJECT)
+            masm.callPreBarrier(address, MIRType::Object);
+        else if (unboxedType == JSVAL_TYPE_STRING)
+            masm.callPreBarrier(address, MIRType::String);
+        else
+            MOZ_ASSERT(!UnboxedTypeNeedsPreBarrier(unboxedType));
+    }
+
+    masm.storeUnboxedProperty(address, unboxedType, value, failures);
+
+    attacher.jumpRejoin(masm);
+
+    masm.bind(failures);
+    attacher.jumpNextStub(masm);
+}
+
+static bool
+CanAttachSetUnboxed(JSContext* cx, HandleObject obj, HandleId id, const ConstantOrRegister& val,
+                    bool needsTypeBarrier, bool* checkTypeset,
+                    uint32_t* unboxedOffset, JSValueType* unboxedType)
+{
+    if (!obj->is<UnboxedPlainObject>())
+        return false;
+
+    const UnboxedLayout::Property* property = obj->as<UnboxedPlainObject>().layout().lookup(id);
+    if (property) {
+        *checkTypeset = false;
+        if (needsTypeBarrier && !CanInlineSetPropTypeCheck(obj, id, val, checkTypeset))
+            return false;
+        *unboxedOffset = property->offset;
+        *unboxedType = property->type;
+        return true;
+    }
+
+    return false;
+}
+
+static bool
+CanAttachSetUnboxedExpando(JSContext* cx, HandleObject obj, HandleId id,
+                           const ConstantOrRegister& val,
+                           bool needsTypeBarrier, bool* checkTypeset, Shape** pshape)
+{
+    if (!obj->is<UnboxedPlainObject>())
+        return false;
+
+    Rooted<UnboxedExpandoObject*> expando(cx, obj->as<UnboxedPlainObject>().maybeExpando());
+    if (!expando)
+        return false;
+
+    Shape* shape = expando->lookupPure(id);
+    if (!shape || !shape->hasDefaultSetter() || !shape->hasSlot() || !shape->writable())
+        return false;
+
+    *checkTypeset = false;
+    if (needsTypeBarrier && !CanInlineSetPropTypeCheck(obj, id, val, checkTypeset))
+        return false;
+
+    *pshape = shape;
+    return true;
+}
+
+static bool
+CanAttachAddUnboxedExpando(JSContext* cx, HandleObject obj, HandleShape oldShape,
+                           HandleId id, const ConstantOrRegister& val,
+                           bool needsTypeBarrier, bool* checkTypeset)
+{
+    if (!obj->is<UnboxedPlainObject>())
+        return false;
+
+    Rooted<UnboxedExpandoObject*> expando(cx, obj->as<UnboxedPlainObject>().maybeExpando());
+    if (!expando || expando->inDictionaryMode())
+        return false;
+
+    Shape* newShape = expando->lastProperty();
+    if (newShape->isEmptyShape() || newShape->propid() != id || newShape->previous() != oldShape)
+        return false;
+
+    MOZ_ASSERT(newShape->hasDefaultSetter() && newShape->hasSlot() && newShape->writable());
+
+    if (PrototypeChainShadowsPropertyAdd(cx, obj, id))
+        return false;
+
+    *checkTypeset = false;
+    if (needsTypeBarrier && !CanInlineSetPropTypeCheck(obj, id, val, checkTypeset))
+        return false;
+
+    return true;
+}
+
+bool
+SetPropertyIC::tryAttachUnboxed(JSContext* cx, HandleScript outerScript, IonScript* ion,
+                                HandleObject obj, HandleId id, bool* emitted)
+{
+    MOZ_ASSERT(!*emitted);
+
+    bool checkTypeset = false;
+    uint32_t unboxedOffset;
+    JSValueType unboxedType;
+    if (!CanAttachSetUnboxed(cx, obj, id, value(), needsTypeBarrier(), &checkTypeset,
+                             &unboxedOffset, &unboxedType))
+    {
+        return true;
+    }
+
+    *emitted = true;
+
+    MacroAssembler masm(cx, ion, outerScript, profilerLeavePc_);
+    StubAttacher attacher(*this);
+
+    Label failures;
+    emitIdGuard(masm, id, &failures);
+
+    GenerateSetUnboxed(cx, masm, attacher, obj, id, unboxedOffset, unboxedType,
+                       object(), temp(), value(), checkTypeset, &failures);
+    return linkAndAttachStub(cx, masm, attacher, ion, "set_unboxed",
+                             JS::TrackedOutcome::ICSetPropStub_SetUnboxed);
+}
+
 bool
 SetPropertyIC::tryAttachProxy(JSContext* cx, HandleScript outerScript, IonScript* ion,
                               HandleObject obj, HandleId id, bool* emitted)
@@ -3225,6 +3587,26 @@ SetPropertyIC::tryAttachNative(JSContext* cx, HandleScript outerScript, IonScrip
 }
 
 bool
+SetPropertyIC::tryAttachUnboxedExpando(JSContext* cx, HandleScript outerScript, IonScript* ion,
+                                       HandleObject obj, HandleId id, bool* emitted)
+{
+    MOZ_ASSERT(!*emitted);
+
+    RootedShape shape(cx);
+    bool checkTypeset = false;
+    if (!CanAttachSetUnboxedExpando(cx, obj, id, value(), needsTypeBarrier(),
+                                    &checkTypeset, shape.address()))
+    {
+        return true;
+    }
+
+    if (!attachSetSlot(cx, outerScript, ion, obj, shape, checkTypeset))
+        return false;
+    *emitted = true;
+    return true;
+}
+
+bool
 SetPropertyIC::tryAttachStub(JSContext* cx, HandleScript outerScript, IonScript* ion,
                              HandleObject obj, HandleValue idval, HandleValue value,
                              MutableHandleId id, bool* emitted, bool* tryNativeAddSlot)
@@ -3249,6 +3631,12 @@ SetPropertyIC::tryAttachStub(JSContext* cx, HandleScript outerScript, IonScript*
 
         if (!*emitted && !tryAttachNative(cx, outerScript, ion, obj, id, emitted, tryNativeAddSlot))
             return false;
+
+        if (!*emitted && !tryAttachUnboxed(cx, outerScript, ion, obj, id, emitted))
+            return false;
+
+        if (!*emitted && !tryAttachUnboxedExpando(cx, outerScript, ion, obj, id, emitted))
+            return false;
     }
 
     if (idval.isInt32()) {
@@ -3300,6 +3688,16 @@ SetPropertyIC::tryAttachAddSlot(JSContext* cx, HandleScript outerScript, IonScri
         return true;
     }
 
+    checkTypeset = false;
+    if (CanAttachAddUnboxedExpando(cx, obj, oldShape, id, value(), needsTypeBarrier(),
+                                   &checkTypeset))
+    {
+        if (!attachAddSlot(cx, outerScript, ion, obj, id, oldShape, oldGroup, checkTypeset))
+            return false;
+        *emitted = true;
+        return true;
+    }
+
     return true;
 }
 
@@ -3321,6 +3719,11 @@ SetPropertyIC::update(JSContext* cx, HandleScript outerScript, size_t cacheIndex
             return false;
 
         oldShape = obj->maybeShape();
+        if (obj->is<UnboxedPlainObject>()) {
+            MOZ_ASSERT(!oldShape);
+            if (UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando())
+                oldShape = expando->lastProperty();
+        }
     }
 
     RootedId id(cx);
@@ -3623,7 +4026,7 @@ GetPropertyIC::tryAttachDenseElementHole(JSContext* cx, HandleScript outerScript
 GetPropertyIC::canAttachTypedOrUnboxedArrayElement(JSObject* obj, const Value& idval,
                                                    TypedOrValueRegister output)
 {
-    if (!obj->is<TypedArrayObject>())
+    if (!obj->is<TypedArrayObject>() && !obj->is<UnboxedArrayObject>())
         return false;
 
     MOZ_ASSERT(idval.isInt32() || idval.isString());
@@ -3654,6 +4057,13 @@ GetPropertyIC::canAttachTypedOrUnboxedArrayElement(JSObject* obj, const Value& i
         return output.hasValue() || !output.typedReg().isFloat();
     }
 
+    if (index >= obj->as<UnboxedArrayObject>().initializedLength())
+        return false;
+
+    JSValueType elementType = obj->as<UnboxedArrayObject>().elementType();
+    if (elementType == JSVAL_TYPE_DOUBLE)
+        return output.hasValue();
+
     return output.hasValue() || !output.typedReg().isFloat();
 }
 
@@ -3730,27 +4140,46 @@ GenerateGetTypedOrUnboxedArrayElement(JSContext* cx, MacroAssembler& masm,
 
     Label popObjectAndFail;
 
-    // Guard on the initialized length.
-    Address length(object, TypedArrayObject::lengthOffset());
-    masm.branch32(Assembler::BelowOrEqual, length, indexReg, &failures);
+    if (array->is<TypedArrayObject>()) {
+        // Guard on the initialized length.
+        Address length(object, TypedArrayObject::lengthOffset());
+        masm.branch32(Assembler::BelowOrEqual, length, indexReg, &failures);
 
-    // Save the object register on the stack in case of failure.
-    Register elementReg = object;
-    masm.push(object);
+        // Save the object register on the stack in case of failure.
+        Register elementReg = object;
+        masm.push(object);
 
-    // Load elements vector.
-    masm.loadPtr(Address(object, TypedArrayObject::dataOffset()), elementReg);
+        // Load elements vector.
+        masm.loadPtr(Address(object, TypedArrayObject::dataOffset()), elementReg);
 
-    // Load the value. We use an invalid register because the destination
-    // register is necessary a non double register.
-    Scalar::Type arrayType = array->as<TypedArrayObject>().type();
-    int width = Scalar::byteSize(arrayType);
-    BaseIndex source(elementReg, indexReg, ScaleFromElemWidth(width));
-    if (output.hasValue()) {
-        masm.loadFromTypedArray(arrayType, source, output.valueReg(), allowDoubleResult,
-                                elementReg, &popObjectAndFail);
+        // Load the value. We use an invalid register because the destination
+        // register is necessary a non double register.
+        Scalar::Type arrayType = array->as<TypedArrayObject>().type();
+        int width = Scalar::byteSize(arrayType);
+        BaseIndex source(elementReg, indexReg, ScaleFromElemWidth(width));
+        if (output.hasValue()) {
+            masm.loadFromTypedArray(arrayType, source, output.valueReg(), allowDoubleResult,
+                                    elementReg, &popObjectAndFail);
+        } else {
+            masm.loadFromTypedArray(arrayType, source, output.typedReg(), elementReg, &popObjectAndFail);
+        }
     } else {
-        masm.loadFromTypedArray(arrayType, source, output.typedReg(), elementReg, &popObjectAndFail);
+        // Save the object register on the stack in case of failure.
+        masm.push(object);
+
+        // Guard on the initialized length.
+        masm.load32(Address(object, UnboxedArrayObject::offsetOfCapacityIndexAndInitializedLength()), object);
+        masm.and32(Imm32(UnboxedArrayObject::InitializedLengthMask), object);
+        masm.branch32(Assembler::BelowOrEqual, object, indexReg, &popObjectAndFail);
+
+        // Load elements vector.
+        Register elementReg = object;
+        masm.loadPtr(Address(masm.getStackPointer(), 0), object);
+        masm.loadPtr(Address(object, UnboxedArrayObject::offsetOfElements()), elementReg);
+
+        JSValueType elementType = array->as<UnboxedArrayObject>().elementType();
+        BaseIndex source(elementReg, indexReg, ScaleFromElemWidth(UnboxedTypeSize(elementType)));
+        masm.loadUnboxedProperty(source, elementType, output);
     }
 
     masm.pop(object);
diff --git a/js/src/jit/IonCaches.h b/js/src/jit/IonCaches.h
index b00646538..173e06c6b 100644
--- a/js/src/jit/IonCaches.h
+++ b/js/src/jit/IonCaches.h
@@ -529,6 +529,18 @@ class GetPropertyIC : public IonCache
                                       HandleObject obj, HandleId id, void* returnAddr,
                                       bool* emitted);
 
+    MOZ_MUST_USE bool tryAttachUnboxed(JSContext* cx, HandleScript outerScript, IonScript* ion,
+                                       HandleObject obj, HandleId id, void* returnAddr,
+                                       bool* emitted);
+
+    MOZ_MUST_USE bool tryAttachUnboxedExpando(JSContext* cx, HandleScript outerScript,
+                                              IonScript* ion, HandleObject obj, HandleId id,
+                                              void* returnAddr, bool* emitted);
+
+    MOZ_MUST_USE bool tryAttachUnboxedArrayLength(JSContext* cx, HandleScript outerScript,
+                                                  IonScript* ion, HandleObject obj, HandleId id,
+                                                  void* returnAddr, bool* emitted);
+
     MOZ_MUST_USE bool tryAttachTypedArrayLength(JSContext* cx, HandleScript outerScript,
                                                 IonScript* ion, HandleObject obj, HandleId id,
                                                 bool* emitted);
diff --git a/js/src/jit/JitOptions.cpp b/js/src/jit/JitOptions.cpp
index 3f9d9db88..b9a7c7b27 100644
--- a/js/src/jit/JitOptions.cpp
+++ b/js/src/jit/JitOptions.cpp
@@ -221,6 +221,9 @@ DefaultJitOptions::DefaultJitOptions()
             Warn(forcedRegisterAllocatorEnv, env);
     }
 
+    // Toggles whether unboxed plain objects can be created by the VM.
+    SET_DEFAULT(disableUnboxedObjects, true);
+
     // Test whether Atomics are allowed in asm.js code.
     SET_DEFAULT(asmJSAtomicsEnable, false);
 
diff --git a/js/src/jit/JitOptions.h b/js/src/jit/JitOptions.h
index 719ee14d9..076980b4e 100644
--- a/js/src/jit/JitOptions.h
+++ b/js/src/jit/JitOptions.h
@@ -91,6 +91,9 @@ struct DefaultJitOptions
     mozilla::Maybe<uint32_t> forcedDefaultIonSmallFunctionWarmUpThreshold;
     mozilla::Maybe<IonRegisterAllocator> forcedRegisterAllocator;
 
+    // The options below affect the rest of the VM, and not just the JIT.
+    bool disableUnboxedObjects;
+
     DefaultJitOptions();
     bool isSmallFunction(JSScript* script) const;
     void setEagerCompilation();
diff --git a/js/src/jit/Lowering.cpp b/js/src/jit/Lowering.cpp
index 108450983..19266bae8 100644
--- a/js/src/jit/Lowering.cpp
+++ b/js/src/jit/Lowering.cpp
@@ -2895,6 +2895,32 @@ LIRGenerator::visitSetInitializedLength(MSetInitializedLength* ins)
 }
 
 void
+LIRGenerator::visitUnboxedArrayLength(MUnboxedArrayLength* ins)
+{
+    define(new(alloc()) LUnboxedArrayLength(useRegisterAtStart(ins->object())), ins);
+}
+
+void
+LIRGenerator::visitUnboxedArrayInitializedLength(MUnboxedArrayInitializedLength* ins)
+{
+    define(new(alloc()) LUnboxedArrayInitializedLength(useRegisterAtStart(ins->object())), ins);
+}
+
+void
+LIRGenerator::visitIncrementUnboxedArrayInitializedLength(MIncrementUnboxedArrayInitializedLength* ins)
+{
+    add(new(alloc()) LIncrementUnboxedArrayInitializedLength(useRegister(ins->object())), ins);
+}
+
+void
+LIRGenerator::visitSetUnboxedArrayInitializedLength(MSetUnboxedArrayInitializedLength* ins)
+{
+    add(new(alloc()) LSetUnboxedArrayInitializedLength(useRegister(ins->object()),
+                                                       useRegisterOrConstant(ins->length()),
+                                                       temp()), ins);
+}
+
+void
 LIRGenerator::visitNot(MNot* ins)
 {
     MDefinition* op = ins->input();
@@ -3143,16 +3169,22 @@ LIRGenerator::visitStoreElementHole(MStoreElementHole* ins)
     const LUse elements = useRegister(ins->elements());
     const LAllocation index = useRegisterOrConstant(ins->index());
 
+    // Use a temp register when adding new elements to unboxed arrays.
+    LDefinition tempDef = LDefinition::BogusTemp();
+    if (ins->unboxedType() != JSVAL_TYPE_MAGIC)
+        tempDef = temp();
+
     LInstruction* lir;
     switch (ins->value()->type()) {
       case MIRType::Value:
-        lir = new(alloc()) LStoreElementHoleV(object, elements, index, useBox(ins->value()));
+        lir = new(alloc()) LStoreElementHoleV(object, elements, index, useBox(ins->value()),
+                                              tempDef);
         break;
 
       default:
       {
         const LAllocation value = useRegisterOrNonDoubleConstant(ins->value());
-        lir = new(alloc()) LStoreElementHoleT(object, elements, index, value);
+        lir = new(alloc()) LStoreElementHoleT(object, elements, index, value, tempDef);
         break;
       }
     }
@@ -3171,14 +3203,20 @@ LIRGenerator::visitFallibleStoreElement(MFallibleStoreElement* ins)
     const LUse elements = useRegister(ins->elements());
     const LAllocation index = useRegisterOrConstant(ins->index());
 
+    // Use a temp register when adding new elements to unboxed arrays.
+    LDefinition tempDef = LDefinition::BogusTemp();
+    if (ins->unboxedType() != JSVAL_TYPE_MAGIC)
+        tempDef = temp();
+
     LInstruction* lir;
     switch (ins->value()->type()) {
       case MIRType::Value:
-        lir = new(alloc()) LFallibleStoreElementV(object, elements, index, useBox(ins->value()));
+        lir = new(alloc()) LFallibleStoreElementV(object, elements, index, useBox(ins->value()),
+                                                  tempDef);
         break;
       default:
         const LAllocation value = useRegisterOrNonDoubleConstant(ins->value());
-        lir = new(alloc()) LFallibleStoreElementT(object, elements, index, value);
+        lir = new(alloc()) LFallibleStoreElementT(object, elements, index, value, tempDef);
         break;
     }
 
@@ -3220,6 +3258,14 @@ LIRGenerator::visitStoreUnboxedString(MStoreUnboxedString* ins)
 }
 
 void
+LIRGenerator::visitConvertUnboxedObjectToNative(MConvertUnboxedObjectToNative* ins)
+{
+    LInstruction* check = new(alloc()) LConvertUnboxedObjectToNative(useRegister(ins->object()));
+    add(check, ins);
+    assignSafepoint(check, ins);
+}
+
+void
 LIRGenerator::visitEffectiveAddress(MEffectiveAddress* ins)
 {
     define(new(alloc()) LEffectiveAddress(useRegister(ins->base()), useRegister(ins->index())), ins);
@@ -3737,6 +3783,24 @@ LIRGenerator::visitGuardReceiverPolymorphic(MGuardReceiverPolymorphic* ins)
 }
 
 void
+LIRGenerator::visitGuardUnboxedExpando(MGuardUnboxedExpando* ins)
+{
+    LGuardUnboxedExpando* guard =
+        new(alloc()) LGuardUnboxedExpando(useRegister(ins->object()));
+    assignSnapshot(guard, ins->bailoutKind());
+    add(guard, ins);
+    redefine(ins, ins->object());
+}
+
+void
+LIRGenerator::visitLoadUnboxedExpando(MLoadUnboxedExpando* ins)
+{
+    LLoadUnboxedExpando* lir =
+        new(alloc()) LLoadUnboxedExpando(useRegisterAtStart(ins->object()));
+    define(lir, ins);
+}
+
+void
 LIRGenerator::visitAssertRange(MAssertRange* ins)
 {
     MDefinition* input = ins->input();
diff --git a/js/src/jit/Lowering.h b/js/src/jit/Lowering.h
index 81e6abbbb..de66f175b 100644
--- a/js/src/jit/Lowering.h
+++ b/js/src/jit/Lowering.h
@@ -217,6 +217,10 @@ class LIRGenerator : public LIRGeneratorSpecific
     void visitTypedObjectDescr(MTypedObjectDescr* ins);
     void visitInitializedLength(MInitializedLength* ins);
     void visitSetInitializedLength(MSetInitializedLength* ins);
+    void visitUnboxedArrayLength(MUnboxedArrayLength* ins);
+    void visitUnboxedArrayInitializedLength(MUnboxedArrayInitializedLength* ins);
+    void visitIncrementUnboxedArrayInitializedLength(MIncrementUnboxedArrayInitializedLength* ins);
+    void visitSetUnboxedArrayInitializedLength(MSetUnboxedArrayInitializedLength* ins);
     void visitNot(MNot* ins);
     void visitBoundsCheck(MBoundsCheck* ins);
     void visitBoundsCheckLower(MBoundsCheckLower* ins);
@@ -229,6 +233,7 @@ class LIRGenerator : public LIRGeneratorSpecific
     void visitFallibleStoreElement(MFallibleStoreElement* ins);
     void visitStoreUnboxedObjectOrNull(MStoreUnboxedObjectOrNull* ins);
     void visitStoreUnboxedString(MStoreUnboxedString* ins);
+    void visitConvertUnboxedObjectToNative(MConvertUnboxedObjectToNative* ins);
     void visitEffectiveAddress(MEffectiveAddress* ins);
     void visitArrayPopShift(MArrayPopShift* ins);
     void visitArrayPush(MArrayPush* ins);
@@ -252,6 +257,8 @@ class LIRGenerator : public LIRGeneratorSpecific
     void visitGuardObject(MGuardObject* ins);
     void visitGuardString(MGuardString* ins);
     void visitGuardReceiverPolymorphic(MGuardReceiverPolymorphic* ins);
+    void visitGuardUnboxedExpando(MGuardUnboxedExpando* ins);
+    void visitLoadUnboxedExpando(MLoadUnboxedExpando* ins);
     void visitPolyInlineGuard(MPolyInlineGuard* ins);
     void visitAssertRange(MAssertRange* ins);
     void visitCallGetProperty(MCallGetProperty* ins);
diff --git a/js/src/jit/MCallOptimize.cpp b/js/src/jit/MCallOptimize.cpp
index 236354530..f2071dc6a 100644
--- a/js/src/jit/MCallOptimize.cpp
+++ b/js/src/jit/MCallOptimize.cpp
@@ -30,6 +30,7 @@
 #include "jit/shared/Lowering-shared-inl.h"
 #include "vm/NativeObject-inl.h"
 #include "vm/StringObject-inl.h"
+#include "vm/UnboxedObject-inl.h"
 
 using mozilla::ArrayLength;
 using mozilla::AssertedCast;
@@ -470,6 +471,11 @@ IonBuilder::inlineArray(CallInfo& callInfo)
         return InliningStatus_NotInlined;
     }
 
+    if (templateObject->is<UnboxedArrayObject>()) {
+        if (templateObject->group()->unboxedLayout().nativeGroup())
+            return InliningStatus_NotInlined;
+    }
+
     // Multiple arguments imply array initialization, not just construction.
     if (callInfo.argc() >= 2) {
         initLength = callInfo.argc();
@@ -517,7 +523,7 @@ IonBuilder::inlineArray(CallInfo& callInfo)
         // Make sure initLength matches the template object's length. This is
         // not guaranteed to be the case, for instance if we're inlining the
         // MConstant may come from an outer script.
-        if (initLength != templateObject->as<ArrayObject>().length())
+        if (initLength != GetAnyBoxedOrUnboxedArrayLength(templateObject))
             return InliningStatus_NotInlined;
 
         // Don't inline large allocations.
@@ -532,15 +538,16 @@ IonBuilder::inlineArray(CallInfo& callInfo)
 
     MDefinition* array = current->peek(-1);
     if (callInfo.argc() >= 2) {
+        JSValueType unboxedType = GetBoxedOrUnboxedType(templateObject);
         for (uint32_t i = 0; i < initLength; i++) {
             if (!alloc().ensureBallast())
                 return InliningStatus_Error;
             MDefinition* value = callInfo.getArg(i);
-            if (!initializeArrayElement(array, i, value, /* addResumePoint = */ false))
+            if (!initializeArrayElement(array, i, value, unboxedType, /* addResumePoint = */ false))
                 return InliningStatus_Error;
         }
 
-        MInstruction* setLength = setInitializedLength(array, initLength);
+        MInstruction* setLength = setInitializedLength(array, unboxedType, initLength);
         if (!resumeAfter(setLength))
             return InliningStatus_Error;
     }
@@ -573,7 +580,7 @@ IonBuilder::inlineArrayIsArray(CallInfo& callInfo)
         if (!clasp || clasp->isProxy())
             return InliningStatus_NotInlined;
 
-        isArray = (clasp == &ArrayObject::class_);
+        isArray = (clasp == &ArrayObject::class_ || clasp == &UnboxedArrayObject::class_);
     }
 
     pushConstant(BooleanValue(isArray));
@@ -604,12 +611,12 @@ IonBuilder::inlineArrayPopShift(CallInfo& callInfo, MArrayPopShift::Mode mode)
         OBJECT_FLAG_LENGTH_OVERFLOW |
         OBJECT_FLAG_ITERATED;
 
-    MDefinition* obj = callInfo.thisArg();
+    MDefinition* obj = convertUnboxedObjects(callInfo.thisArg());
     TemporaryTypeSet* thisTypes = obj->resultTypeSet();
     if (!thisTypes)
         return InliningStatus_NotInlined;
     const Class* clasp = thisTypes->getKnownClass(constraints());
-    if (clasp != &ArrayObject::class_)
+    if (clasp != &ArrayObject::class_ && clasp != &UnboxedArrayObject::class_)
         return InliningStatus_NotInlined;
     if (thisTypes->hasObjectFlags(constraints(), unhandledFlags)) {
         trackOptimizationOutcome(TrackedOutcome::ArrayBadFlags);
@@ -622,9 +629,17 @@ IonBuilder::inlineArrayPopShift(CallInfo& callInfo, MArrayPopShift::Mode mode)
         return InliningStatus_NotInlined;
     }
 
+    JSValueType unboxedType = JSVAL_TYPE_MAGIC;
+    if (clasp == &UnboxedArrayObject::class_) {
+        unboxedType = UnboxedArrayElementType(constraints(), obj, nullptr);
+        if (unboxedType == JSVAL_TYPE_MAGIC)
+            return InliningStatus_NotInlined;
+    }
+
     callInfo.setImplicitlyUsedUnchecked();
 
-    obj = addMaybeCopyElementsForWrite(obj, /* checkNative = */ false);
+    if (clasp == &ArrayObject::class_)
+        obj = addMaybeCopyElementsForWrite(obj, /* checkNative = */ false);
 
     TemporaryTypeSet* returnTypes = getInlineReturnTypeSet();
     bool needsHoleCheck = thisTypes->hasObjectFlags(constraints(), OBJECT_FLAG_NON_PACKED);
@@ -635,7 +650,8 @@ IonBuilder::inlineArrayPopShift(CallInfo& callInfo, MArrayPopShift::Mode mode)
     if (barrier != BarrierKind::NoBarrier)
         returnType = MIRType::Value;
 
-    MArrayPopShift* ins = MArrayPopShift::New(alloc(), obj, mode, needsHoleCheck, maybeUndefined);
+    MArrayPopShift* ins = MArrayPopShift::New(alloc(), obj, mode,
+                                              unboxedType, needsHoleCheck, maybeUndefined);
     current->add(ins);
     current->push(ins);
     ins->setResultType(returnType);
@@ -684,7 +700,7 @@ IonBuilder::inlineArrayPush(CallInfo& callInfo)
         return InliningStatus_NotInlined;
     }
 
-    MDefinition* obj = callInfo.thisArg();
+    MDefinition* obj = convertUnboxedObjects(callInfo.thisArg());
     MDefinition* value = callInfo.getArg(0);
     if (PropertyWriteNeedsTypeBarrier(alloc(), constraints(), current,
                                       &obj, nullptr, &value, /* canModify = */ false))
@@ -717,6 +733,13 @@ IonBuilder::inlineArrayPush(CallInfo& callInfo)
         return InliningStatus_NotInlined;
     }
 
+    JSValueType unboxedType = JSVAL_TYPE_MAGIC;
+    if (clasp == &UnboxedArrayObject::class_) {
+        unboxedType = UnboxedArrayElementType(constraints(), obj, nullptr);
+        if (unboxedType == JSVAL_TYPE_MAGIC)
+            return InliningStatus_NotInlined;
+    }
+
     callInfo.setImplicitlyUsedUnchecked();
 
     if (conversion == TemporaryTypeSet::AlwaysConvertToDoubles ||
@@ -727,12 +750,13 @@ IonBuilder::inlineArrayPush(CallInfo& callInfo)
         value = valueDouble;
     }
 
-    obj = addMaybeCopyElementsForWrite(obj, /* checkNative = */ false);
+    if (unboxedType == JSVAL_TYPE_MAGIC)
+        obj = addMaybeCopyElementsForWrite(obj, /* checkNative = */ false);
 
     if (NeedsPostBarrier(value))
         current->add(MPostWriteBarrier::New(alloc(), obj, value));
 
-    MArrayPush* ins = MArrayPush::New(alloc(), obj, value);
+    MArrayPush* ins = MArrayPush::New(alloc(), obj, value, unboxedType);
     current->add(ins);
     current->push(ins);
 
@@ -749,7 +773,7 @@ IonBuilder::inlineArraySlice(CallInfo& callInfo)
         return InliningStatus_NotInlined;
     }
 
-    MDefinition* obj = callInfo.thisArg();
+    MDefinition* obj = convertUnboxedObjects(callInfo.thisArg());
 
     // Ensure |this| and result are objects.
     if (getInlineReturnType() != MIRType::Object)
@@ -773,9 +797,16 @@ IonBuilder::inlineArraySlice(CallInfo& callInfo)
         return InliningStatus_NotInlined;
 
     const Class* clasp = thisTypes->getKnownClass(constraints());
-    if (clasp != &ArrayObject::class_)
+    if (clasp != &ArrayObject::class_ && clasp != &UnboxedArrayObject::class_)
         return InliningStatus_NotInlined;
 
+    JSValueType unboxedType = JSVAL_TYPE_MAGIC;
+    if (clasp == &UnboxedArrayObject::class_) {
+        unboxedType = UnboxedArrayElementType(constraints(), obj, nullptr);
+        if (unboxedType == JSVAL_TYPE_MAGIC)
+            return InliningStatus_NotInlined;
+    }
+
     // Watch out for indexed properties on the object or its prototype.
     if (ElementAccessHasExtraIndexedProperty(this, obj)) {
         trackOptimizationOutcome(TrackedOutcome::ProtoIndexedProps);
@@ -796,8 +827,15 @@ IonBuilder::inlineArraySlice(CallInfo& callInfo)
     if (!templateObj)
         return InliningStatus_NotInlined;
 
-    if (!templateObj->is<ArrayObject>())
-        return InliningStatus_NotInlined;
+    if (unboxedType == JSVAL_TYPE_MAGIC) {
+        if (!templateObj->is<ArrayObject>())
+            return InliningStatus_NotInlined;
+    } else {
+        if (!templateObj->is<UnboxedArrayObject>())
+            return InliningStatus_NotInlined;
+        if (templateObj->as<UnboxedArrayObject>().elementType() != unboxedType)
+            return InliningStatus_NotInlined;
+    }
 
     callInfo.setImplicitlyUsedUnchecked();
 
@@ -816,12 +854,16 @@ IonBuilder::inlineArraySlice(CallInfo& callInfo)
 
         end = MArrayLength::New(alloc(), elements);
         current->add(end->toInstruction());
+    } else {
+        end = MUnboxedArrayLength::New(alloc(), obj);
+        current->add(end->toInstruction());
     }
 
     MArraySlice* ins = MArraySlice::New(alloc(), constraints(),
                                         obj, begin, end,
                                         templateObj,
-                                        templateObj->group()->initialHeap(constraints()));
+                                        templateObj->group()->initialHeap(constraints()),
+                                        unboxedType);
     current->add(ins);
     current->push(ins);
 
@@ -1340,7 +1382,7 @@ IonBuilder::inlineConstantStringSplitString(CallInfo& callInfo)
     // Check if exist a template object in stub.
     JSString* stringStr = nullptr;
     JSString* stringSep = nullptr;
-    ArrayObject* templateObject = nullptr;
+    JSObject* templateObject = nullptr;
     if (!inspector->isOptimizableCallStringSplit(pc, &stringStr, &stringSep, &templateObject))
         return InliningStatus_NotInlined;
 
@@ -1366,13 +1408,13 @@ IonBuilder::inlineConstantStringSplitString(CallInfo& callInfo)
     if (!key.maybeTypes()->hasType(TypeSet::StringType()))
         return InliningStatus_NotInlined;
 
-    uint32_t initLength = templateObject->length();
-    if (templateObject->getDenseInitializedLength() != initLength)
+    uint32_t initLength = GetAnyBoxedOrUnboxedArrayLength(templateObject);
+    if (GetAnyBoxedOrUnboxedInitializedLength(templateObject) != initLength)
         return InliningStatus_NotInlined;
 
     Vector<MConstant*, 0, SystemAllocPolicy> arrayValues;
     for (uint32_t i = 0; i < initLength; i++) {
-        Value str = templateObject->getDenseElement(i);
+        Value str = GetAnyBoxedOrUnboxedDenseElement(templateObject, i);
         MOZ_ASSERT(str.toString()->isAtom());
         MConstant* value = MConstant::New(alloc().fallible(), str, constraints());
         if (!value)
@@ -1403,6 +1445,8 @@ IonBuilder::inlineConstantStringSplitString(CallInfo& callInfo)
         return InliningStatus_Inlined;
     }
 
+    JSValueType unboxedType = GetBoxedOrUnboxedType(templateObject);
+
     // Store all values, no need to initialize the length after each as
     // jsop_initelem_array is doing because we do not expect to bailout
     // because the memory is supposed to be allocated by now.
@@ -1413,11 +1457,11 @@ IonBuilder::inlineConstantStringSplitString(CallInfo& callInfo)
         MConstant* value = arrayValues[i];
         current->add(value);
 
-        if (!initializeArrayElement(array, i, value, /* addResumePoint = */ false))
+        if (!initializeArrayElement(array, i, value, unboxedType, /* addResumePoint = */ false))
             return InliningStatus_Error;
     }
 
-    MInstruction* setLength = setInitializedLength(array, initLength);
+    MInstruction* setLength = setInitializedLength(array, unboxedType, initLength);
     if (!resumeAfter(setLength))
         return InliningStatus_Error;
 
@@ -2053,7 +2097,7 @@ IonBuilder::inlineDefineDataProperty(CallInfo& callInfo)
     if (callInfo.argc() != 3)
         return InliningStatus_NotInlined;
 
-    MDefinition* obj = callInfo.getArg(0);
+    MDefinition* obj = convertUnboxedObjects(callInfo.getArg(0));
     MDefinition* id = callInfo.getArg(1);
     MDefinition* value = callInfo.getArg(2);
 
diff --git a/js/src/jit/MIR.cpp b/js/src/jit/MIR.cpp
index 0cf31adb3..b9e5e8d61 100644
--- a/js/src/jit/MIR.cpp
+++ b/js/src/jit/MIR.cpp
@@ -2630,6 +2630,40 @@ jit::EqualTypes(MIRType type1, TemporaryTypeSet* typeset1,
     return typeset1->equals(typeset2);
 }
 
+// Tests whether input/inputTypes can always be stored to an unboxed
+// object/array property with the given unboxed type.
+bool
+jit::CanStoreUnboxedType(TempAllocator& alloc,
+                         JSValueType unboxedType, MIRType input, TypeSet* inputTypes)
+{
+    TemporaryTypeSet types;
+
+    switch (unboxedType) {
+      case JSVAL_TYPE_BOOLEAN:
+      case JSVAL_TYPE_INT32:
+      case JSVAL_TYPE_DOUBLE:
+      case JSVAL_TYPE_STRING:
+        types.addType(TypeSet::PrimitiveType(unboxedType), alloc.lifoAlloc());
+        break;
+
+      case JSVAL_TYPE_OBJECT:
+        types.addType(TypeSet::AnyObjectType(), alloc.lifoAlloc());
+        types.addType(TypeSet::NullType(), alloc.lifoAlloc());
+        break;
+
+      default:
+        MOZ_CRASH("Bad unboxed type");
+    }
+
+    return TypeSetIncludes(&types, input, inputTypes);
+}
+
+static bool
+CanStoreUnboxedType(TempAllocator& alloc, JSValueType unboxedType, MDefinition* value)
+{
+    return CanStoreUnboxedType(alloc, unboxedType, value->type(), value->resultTypeSet());
+}
+
 bool
 MPhi::specializeType(TempAllocator& alloc)
 {
@@ -4776,31 +4810,67 @@ MBeta::printOpcode(GenericPrinter& out) const
 bool
 MCreateThisWithTemplate::canRecoverOnBailout() const
 {
-    MOZ_ASSERT(templateObject()->is<PlainObject>());
-    MOZ_ASSERT(!templateObject()->as<PlainObject>().denseElementsAreCopyOnWrite());
+    MOZ_ASSERT(templateObject()->is<PlainObject>() || templateObject()->is<UnboxedPlainObject>());
+    MOZ_ASSERT_IF(templateObject()->is<PlainObject>(),
+                  !templateObject()->as<PlainObject>().denseElementsAreCopyOnWrite());
+    return true;
+}
+
+bool
+OperandIndexMap::init(TempAllocator& alloc, JSObject* templateObject)
+{
+    const UnboxedLayout& layout =
+        templateObject->as<UnboxedPlainObject>().layoutDontCheckGeneration();
+
+    const UnboxedLayout::PropertyVector& properties = layout.properties();
+    MOZ_ASSERT(properties.length() < 255);
+
+    // Allocate an array of indexes, where the top of each field correspond to
+    // the index of the operand in the MObjectState instance.
+    if (!map.init(alloc, layout.size()))
+        return false;
+
+    // Reset all indexes to 0, which is an error code.
+    for (size_t i = 0; i < map.length(); i++)
+        map[i] = 0;
+
+    // Map the property offsets to the indexes of MObjectState operands.
+    uint8_t index = 1;
+    for (size_t i = 0; i < properties.length(); i++, index++)
+        map[properties[i].offset] = index;
+
     return true;
 }
 
 MObjectState::MObjectState(MObjectState* state)
   : numSlots_(state->numSlots_),
-    numFixedSlots_(state->numFixedSlots_)
+    numFixedSlots_(state->numFixedSlots_),
+    operandIndex_(state->operandIndex_)
 {
     // This instruction is only used as a summary for bailout paths.
     setResultType(MIRType::Object);
     setRecoveredOnBailout();
 }
 
-MObjectState::MObjectState(JSObject* templateObject)
+MObjectState::MObjectState(JSObject *templateObject, OperandIndexMap* operandIndex)
 {
     // This instruction is only used as a summary for bailout paths.
     setResultType(MIRType::Object);
     setRecoveredOnBailout();
 
-    MOZ_ASSERT(templateObject->is<NativeObject>());
+    if (templateObject->is<NativeObject>()) {
+        NativeObject* nativeObject = &templateObject->as<NativeObject>();
+        numSlots_ = nativeObject->slotSpan();
+        numFixedSlots_ = nativeObject->numFixedSlots();
+    } else {
+        const UnboxedLayout& layout =
+            templateObject->as<UnboxedPlainObject>().layoutDontCheckGeneration();
+        // Same as UnboxedLayout::makeNativeGroup
+        numSlots_ = layout.properties().length();
+        numFixedSlots_ = gc::GetGCKindSlots(layout.getAllocKind());
+    }
 
-    NativeObject* nativeObject = &templateObject->as<NativeObject>();
-    numSlots_ = nativeObject->slotSpan();
-    numFixedSlots_ = nativeObject->numFixedSlots();
+    operandIndex_ = operandIndex;
 }
 
 JSObject*
@@ -4835,21 +4905,39 @@ MObjectState::initFromTemplateObject(TempAllocator& alloc, MDefinition* undefine
     // the template object. This is needed to account values which are baked in
     // the template objects and not visible in IonMonkey, such as the
     // uninitialized-lexical magic value of call objects.
-    NativeObject& nativeObject = templateObject->as<NativeObject>();
-    MOZ_ASSERT(nativeObject.slotSpan() == numSlots());
-
-	MOZ_ASSERT(templateObject->is<NativeObject>());
-    for (size_t i = 0; i < numSlots(); i++) {
-        Value val = nativeObject.getSlot(i);
-        MDefinition *def = undefinedVal;
-        if (!val.isUndefined()) {
-            MConstant* ins = val.isObject() ?
-                MConstant::NewConstraintlessObject(alloc, &val.toObject()) :
-                MConstant::New(alloc, val);
-            block()->insertBefore(this, ins);
-            def = ins;
+    if (templateObject->is<UnboxedPlainObject>()) {
+        UnboxedPlainObject& unboxedObject = templateObject->as<UnboxedPlainObject>();
+        const UnboxedLayout& layout = unboxedObject.layoutDontCheckGeneration();
+        const UnboxedLayout::PropertyVector& properties = layout.properties();
+
+        for (size_t i = 0; i < properties.length(); i++) {
+            Value val = unboxedObject.getValue(properties[i], /* maybeUninitialized = */ true);
+            MDefinition *def = undefinedVal;
+            if (!val.isUndefined()) {
+                MConstant* ins = val.isObject() ?
+                    MConstant::NewConstraintlessObject(alloc, &val.toObject()) :
+                    MConstant::New(alloc, val);
+                block()->insertBefore(this, ins);
+                def = ins;
+            }
+            initSlot(i, def);
+        }
+    } else {
+        NativeObject& nativeObject = templateObject->as<NativeObject>();
+        MOZ_ASSERT(nativeObject.slotSpan() == numSlots());
+
+        for (size_t i = 0; i < numSlots(); i++) {
+            Value val = nativeObject.getSlot(i);
+            MDefinition *def = undefinedVal;
+            if (!val.isUndefined()) {
+                MConstant* ins = val.isObject() ?
+                    MConstant::NewConstraintlessObject(alloc, &val.toObject()) :
+                    MConstant::New(alloc, val);
+                block()->insertBefore(this, ins);
+                def = ins;
+            }
+            initSlot(i, def);
         }
-        initSlot(i, def);
     }
     return true;
 }
@@ -4860,7 +4948,14 @@ MObjectState::New(TempAllocator& alloc, MDefinition* obj)
     JSObject* templateObject = templateObjectOf(obj);
     MOZ_ASSERT(templateObject, "Unexpected object creation.");
 
-    MObjectState* res = new(alloc) MObjectState(templateObject);
+    OperandIndexMap* operandIndex = nullptr;
+    if (templateObject->is<UnboxedPlainObject>()) {
+        operandIndex = new(alloc) OperandIndexMap;
+        if (!operandIndex || !operandIndex->init(alloc, templateObject))
+            return nullptr;
+    }
+
+    MObjectState* res = new(alloc) MObjectState(templateObject, operandIndex);
     if (!res || !res->init(alloc, obj))
         return nullptr;
     return res;
@@ -5767,6 +5862,35 @@ MGetFirstDollarIndex::foldsTo(TempAllocator& alloc)
     return MConstant::New(alloc, Int32Value(index));
 }
 
+MConvertUnboxedObjectToNative*
+MConvertUnboxedObjectToNative::New(TempAllocator& alloc, MDefinition* obj, ObjectGroup* group)
+{
+    MConvertUnboxedObjectToNative* res = new(alloc) MConvertUnboxedObjectToNative(obj, group);
+
+    ObjectGroup* nativeGroup = group->unboxedLayout().nativeGroup();
+
+    // Make a new type set for the result of this instruction which replaces
+    // the input group with the native group we will convert it to.
+    TemporaryTypeSet* types = obj->resultTypeSet();
+    if (types && !types->unknownObject()) {
+        TemporaryTypeSet* newTypes = types->cloneWithoutObjects(alloc.lifoAlloc());
+        if (newTypes) {
+            for (size_t i = 0; i < types->getObjectCount(); i++) {
+                TypeSet::ObjectKey* key = types->getObject(i);
+                if (!key)
+                    continue;
+                if (key->unknownProperties() || !key->isGroup() || key->group() != group)
+                    newTypes->addType(TypeSet::ObjectType(key), alloc.lifoAlloc());
+                else
+                    newTypes->addType(TypeSet::ObjectType(nativeGroup), alloc.lifoAlloc());
+            }
+            res->setResultTypeSet(newTypes);
+        }
+    }
+
+    return res;
+}
+
 bool
 jit::ElementAccessIsDenseNative(CompilerConstraintList* constraints,
                                 MDefinition* obj, MDefinition* id)
@@ -5786,6 +5910,48 @@ jit::ElementAccessIsDenseNative(CompilerConstraintList* constraints,
     return clasp && clasp->isNative() && !IsTypedArrayClass(clasp);
 }
 
+JSValueType
+jit::UnboxedArrayElementType(CompilerConstraintList* constraints, MDefinition* obj,
+                             MDefinition* id)
+{
+    if (obj->mightBeType(MIRType::String))
+        return JSVAL_TYPE_MAGIC;
+
+    if (id && id->type() != MIRType::Int32 && id->type() != MIRType::Double)
+        return JSVAL_TYPE_MAGIC;
+
+    TemporaryTypeSet* types = obj->resultTypeSet();
+    if (!types || types->unknownObject())
+        return JSVAL_TYPE_MAGIC;
+
+    JSValueType elementType = JSVAL_TYPE_MAGIC;
+    for (unsigned i = 0; i < types->getObjectCount(); i++) {
+        TypeSet::ObjectKey* key = types->getObject(i);
+        if (!key)
+            continue;
+
+        if (key->unknownProperties() || !key->isGroup())
+            return JSVAL_TYPE_MAGIC;
+
+        if (key->clasp() != &UnboxedArrayObject::class_)
+            return JSVAL_TYPE_MAGIC;
+
+        const UnboxedLayout &layout = key->group()->unboxedLayout();
+
+        if (layout.nativeGroup())
+            return JSVAL_TYPE_MAGIC;
+
+        if (elementType == layout.elementType() || elementType == JSVAL_TYPE_MAGIC)
+            elementType = layout.elementType();
+        else
+            return JSVAL_TYPE_MAGIC;
+
+        key->watchStateChangeForUnboxedConvertedToNative(constraints);
+    }
+
+    return elementType;
+}
+
 bool
 jit::ElementAccessIsTypedArray(CompilerConstraintList* constraints,
                                MDefinition* obj, MDefinition* id,
@@ -5945,6 +6111,11 @@ ObjectSubsumes(TypeSet::ObjectKey* first, TypeSet::ObjectKey* second)
                firstElements.maybeTypes()->equals(secondElements.maybeTypes());
     }
 
+    if (first->clasp() == &UnboxedArrayObject::class_) {
+        return first->group()->unboxedLayout().elementType() ==
+               second->group()->unboxedLayout().elementType();
+    }
+
     return false;
 }
 
@@ -6399,6 +6570,23 @@ jit::PropertyWriteNeedsTypeBarrier(TempAllocator& alloc, CompilerConstraintList*
         }
     }
 
+    // Perform additional filtering to make sure that any unboxed property
+    // being written can accommodate the value.
+    for (size_t i = 0; i < types->getObjectCount(); i++) {
+        TypeSet::ObjectKey* key = types->getObject(i);
+        if (key && key->isGroup() && key->group()->maybeUnboxedLayout()) {
+            const UnboxedLayout& layout = key->group()->unboxedLayout();
+            if (name) {
+                const UnboxedLayout::Property* property = layout.lookup(name);
+                if (property && !CanStoreUnboxedType(alloc, property->type, *pvalue))
+                    return true;
+            } else {
+                if (layout.isArray() && !CanStoreUnboxedType(alloc, layout.elementType(), *pvalue))
+                    return true;
+            }
+        }
+    }
+
     if (success)
         return false;
 
@@ -6429,6 +6617,17 @@ jit::PropertyWriteNeedsTypeBarrier(TempAllocator& alloc, CompilerConstraintList*
 
     MOZ_ASSERT(excluded);
 
+    // If the excluded object is a group with an unboxed layout, make sure it
+    // does not have a corresponding native group. Objects with the native
+    // group might appear even though they are not in the type set.
+    if (excluded->isGroup()) {
+        if (UnboxedLayout* layout = excluded->group()->maybeUnboxedLayout()) {
+            if (layout->nativeGroup())
+                return true;
+            excluded->watchStateChangeForUnboxedConvertedToNative(constraints);
+        }
+    }
+
     *pobj = AddGroupGuard(alloc, current, *pobj, excluded, /* bailOnEquality = */ true);
     return false;
 }
diff --git a/js/src/jit/MIR.h b/js/src/jit/MIR.h
index 0c1e77f80..af0abc695 100644
--- a/js/src/jit/MIR.h
+++ b/js/src/jit/MIR.h
@@ -30,6 +30,7 @@
 #include "vm/EnvironmentObject.h"
 #include "vm/SharedMem.h"
 #include "vm/TypedArrayCommon.h"
+#include "vm/UnboxedObject.h"
 
 // Undo windows.h damage on Win64
 #undef MemoryBarrier
@@ -375,7 +376,8 @@ class AliasSet {
         Element           = 1 << 1, // A Value member of obj->elements or
                                     // a typed object.
         UnboxedElement    = 1 << 2, // An unboxed scalar or reference member of
-                                    // typed object.
+                                    // a typed array, typed object, or unboxed
+                                    // object.
         DynamicSlot       = 1 << 3, // A Value member of obj->slots.
         FixedSlot         = 1 << 4, // A Value member of obj->fixedSlots().
         DOMProperty       = 1 << 5, // A DOM property
@@ -432,6 +434,9 @@ class AliasSet {
         MOZ_ASSERT(flags && !(flags & Store_));
         return AliasSet(flags | Store_);
     }
+    static uint32_t BoxedOrUnboxedElements(JSValueType type) {
+        return (type == JSVAL_TYPE_MAGIC) ? Element : UnboxedElement;
+    }
 };
 
 typedef Vector<MDefinition*, 6, JitAllocPolicy> MDefinitionVector;
@@ -3758,9 +3763,14 @@ class MObjectState
 {
   private:
     uint32_t numSlots_;
-    uint32_t numFixedSlots_;
+    uint32_t numFixedSlots_;        // valid if isUnboxed() == false.
+    OperandIndexMap* operandIndex_; // valid if isUnboxed() == true.
+
+    bool isUnboxed() const {
+        return operandIndex_ != nullptr;
+    }
 
-    MObjectState(JSObject *templateObject);
+    MObjectState(JSObject *templateObject, OperandIndexMap* operandIndex);
     explicit MObjectState(MObjectState* state);
 
     MOZ_MUST_USE bool init(TempAllocator& alloc, MDefinition* obj);
@@ -3820,6 +3830,18 @@ class MObjectState
         setSlot(slot + numFixedSlots(), def);
     }
 
+    // Interface reserved for unboxed objects.
+    bool hasOffset(uint32_t offset) const {
+        MOZ_ASSERT(isUnboxed());
+        return offset < operandIndex_->map.length() && operandIndex_->map[offset] != 0;
+    }
+    MDefinition* getOffset(uint32_t offset) const {
+        return getOperand(operandIndex_->map[offset]);
+    }
+    void setOffset(uint32_t offset, MDefinition* def) {
+        replaceOperand(operandIndex_->map[offset], def);
+    }
+
     MOZ_MUST_USE bool writeRecoverData(CompactBufferWriter& writer) const override;
     bool canRecoverOnBailout() const override {
         return true;
@@ -8741,6 +8763,102 @@ class MSetInitializedLength
     ALLOW_CLONE(MSetInitializedLength)
 };
 
+// Load the length from an unboxed array.
+class MUnboxedArrayLength
+  : public MUnaryInstruction,
+    public SingleObjectPolicy::Data
+{
+    explicit MUnboxedArrayLength(MDefinition* object)
+      : MUnaryInstruction(object)
+    {
+        setResultType(MIRType::Int32);
+        setMovable();
+    }
+
+  public:
+    INSTRUCTION_HEADER(UnboxedArrayLength)
+    TRIVIAL_NEW_WRAPPERS
+    NAMED_OPERANDS((0, object))
+
+    bool congruentTo(const MDefinition* ins) const override {
+        return congruentIfOperandsEqual(ins);
+    }
+    AliasSet getAliasSet() const override {
+        return AliasSet::Load(AliasSet::ObjectFields);
+    }
+
+    ALLOW_CLONE(MUnboxedArrayLength)
+};
+
+// Load the initialized length from an unboxed array.
+class MUnboxedArrayInitializedLength
+  : public MUnaryInstruction,
+    public SingleObjectPolicy::Data
+{
+    explicit MUnboxedArrayInitializedLength(MDefinition* object)
+      : MUnaryInstruction(object)
+    {
+        setResultType(MIRType::Int32);
+        setMovable();
+    }
+
+  public:
+    INSTRUCTION_HEADER(UnboxedArrayInitializedLength)
+    TRIVIAL_NEW_WRAPPERS
+    NAMED_OPERANDS((0, object))
+
+    bool congruentTo(const MDefinition* ins) const override {
+        return congruentIfOperandsEqual(ins);
+    }
+    AliasSet getAliasSet() const override {
+        return AliasSet::Load(AliasSet::ObjectFields);
+    }
+
+    ALLOW_CLONE(MUnboxedArrayInitializedLength)
+};
+
+// Increment the initialized length of an unboxed array object.
+class MIncrementUnboxedArrayInitializedLength
+  : public MUnaryInstruction,
+    public SingleObjectPolicy::Data
+{
+    explicit MIncrementUnboxedArrayInitializedLength(MDefinition* obj)
+      : MUnaryInstruction(obj)
+    {}
+
+  public:
+    INSTRUCTION_HEADER(IncrementUnboxedArrayInitializedLength)
+    TRIVIAL_NEW_WRAPPERS
+    NAMED_OPERANDS((0, object))
+
+    AliasSet getAliasSet() const override {
+        return AliasSet::Store(AliasSet::ObjectFields);
+    }
+
+    ALLOW_CLONE(MIncrementUnboxedArrayInitializedLength)
+};
+
+// Set the initialized length of an unboxed array object.
+class MSetUnboxedArrayInitializedLength
+  : public MBinaryInstruction,
+    public SingleObjectPolicy::Data
+{
+    explicit MSetUnboxedArrayInitializedLength(MDefinition* obj, MDefinition* length)
+      : MBinaryInstruction(obj, length)
+    {}
+
+  public:
+    INSTRUCTION_HEADER(SetUnboxedArrayInitializedLength)
+    TRIVIAL_NEW_WRAPPERS
+    NAMED_OPERANDS((0, object), (1, length))
+
+    AliasSet getAliasSet() const override {
+        return AliasSet::Store(AliasSet::ObjectFields);
+    }
+
+    ALLOW_CLONE(MSetUnboxedArrayInitializedLength)
+};
+
 // Load the array length from an elements header.
 class MArrayLength
   : public MUnaryInstruction,
@@ -9234,19 +9352,23 @@ class MLoadElement
     ALLOW_CLONE(MLoadElement)
 };
 
-// Load a value from the elements vector of a native object.
+// Load a value from the elements vector for a dense native or unboxed array.
 // If the index is out-of-bounds, or the indexed slot has a hole, undefined is
 // returned instead.
 class MLoadElementHole
   : public MTernaryInstruction,
     public SingleObjectPolicy::Data
 {
+    // Unboxed element type, JSVAL_TYPE_MAGIC for dense native elements.
+    JSValueType unboxedType_;
+
     bool needsNegativeIntCheck_;
     bool needsHoleCheck_;
 
     MLoadElementHole(MDefinition* elements, MDefinition* index, MDefinition* initLength,
-                     bool needsHoleCheck)
+                     JSValueType unboxedType, bool needsHoleCheck)
       : MTernaryInstruction(elements, index, initLength),
+        unboxedType_(unboxedType),
         needsNegativeIntCheck_(true),
         needsHoleCheck_(needsHoleCheck)
     {
@@ -9268,6 +9390,9 @@ class MLoadElementHole
     TRIVIAL_NEW_WRAPPERS
     NAMED_OPERANDS((0, elements), (1, index), (2, initLength))
 
+    JSValueType unboxedType() const {
+        return unboxedType_;
+    }
     bool needsNegativeIntCheck() const {
         return needsNegativeIntCheck_;
     }
@@ -9278,6 +9403,8 @@ class MLoadElementHole
         if (!ins->isLoadElementHole())
             return false;
         const MLoadElementHole* other = ins->toLoadElementHole();
+        if (unboxedType() != other->unboxedType())
+            return false;
         if (needsHoleCheck() != other->needsHoleCheck())
             return false;
         if (needsNegativeIntCheck() != other->needsNegativeIntCheck())
@@ -9285,7 +9412,7 @@ class MLoadElementHole
         return congruentIfOperandsEqual(other);
     }
     AliasSet getAliasSet() const override {
-        return AliasSet::Load(AliasSet::Element);
+        return AliasSet::Load(AliasSet::BoxedOrUnboxedElements(unboxedType()));
     }
     void collectRangeInfoPreTrunc() override;
 
@@ -9465,17 +9592,20 @@ class MStoreElement
     ALLOW_CLONE(MStoreElement)
 };
 
-// Like MStoreElement, but supports indexes >= initialized length. The downside
-// is that we cannot hoist the elements vector and bounds check, since this
-// instruction may update the (initialized) length and reallocate the elements
-// vector.
+// Like MStoreElement, but supports indexes >= initialized length, and can
+// handle unboxed arrays. The downside is that we cannot hoist the elements
+// vector and bounds check, since this instruction may update the (initialized)
+// length and reallocate the elements vector.
 class MStoreElementHole
   : public MAryInstruction<4>,
     public MStoreElementCommon,
     public MixPolicy<SingleObjectPolicy, NoFloatPolicy<3> >::Data
 {
+    JSValueType unboxedType_;
+
     MStoreElementHole(MDefinition* object, MDefinition* elements,
-                      MDefinition* index, MDefinition* value)
+                      MDefinition* index, MDefinition* value, JSValueType unboxedType)
+      : unboxedType_(unboxedType)
     {
         initOperand(0, object);
         initOperand(1, elements);
@@ -9490,6 +9620,10 @@ class MStoreElementHole
     TRIVIAL_NEW_WRAPPERS
     NAMED_OPERANDS((0, object), (1, elements), (2, index), (3, value))
 
+    JSValueType unboxedType() const {
+        return unboxedType_;
+    }
+
     ALLOW_CLONE(MStoreElementHole)
 };
 
@@ -9500,11 +9634,13 @@ class MFallibleStoreElement
     public MStoreElementCommon,
     public MixPolicy<SingleObjectPolicy, NoFloatPolicy<3> >::Data
 {
+    JSValueType unboxedType_;
     bool strict_;
 
     MFallibleStoreElement(MDefinition* object, MDefinition* elements,
                           MDefinition* index, MDefinition* value,
-                          bool strict)
+                          JSValueType unboxedType, bool strict)
+      : unboxedType_(unboxedType)
     {
         initOperand(0, object);
         initOperand(1, elements);
@@ -9520,6 +9656,10 @@ class MFallibleStoreElement
     TRIVIAL_NEW_WRAPPERS
     NAMED_OPERANDS((0, object), (1, elements), (2, index), (3, value))
 
+    JSValueType unboxedType() const {
+        return unboxedType_;
+    }
+
     bool strict() const {
         return strict_;
     }
@@ -9610,6 +9750,59 @@ class MStoreUnboxedString
     ALLOW_CLONE(MStoreUnboxedString)
 };
 
+// Passes through an object, after ensuring it is converted from an unboxed
+// object to a native representation.
+class MConvertUnboxedObjectToNative
+  : public MUnaryInstruction,
+    public SingleObjectPolicy::Data
+{
+    CompilerObjectGroup group_;
+
+    explicit MConvertUnboxedObjectToNative(MDefinition* obj, ObjectGroup* group)
+      : MUnaryInstruction(obj),
+        group_(group)
+    {
+        setGuard();
+        setMovable();
+        setResultType(MIRType::Object);
+    }
+
+  public:
+    INSTRUCTION_HEADER(ConvertUnboxedObjectToNative)
+    NAMED_OPERANDS((0, object))
+
+    static MConvertUnboxedObjectToNative* New(TempAllocator& alloc, MDefinition* obj,
+                                              ObjectGroup* group);
+
+    ObjectGroup* group() const {
+        return group_;
+    }
+    bool congruentTo(const MDefinition* ins) const override {
+        if (!congruentIfOperandsEqual(ins))
+            return false;
+        return ins->toConvertUnboxedObjectToNative()->group() == group();
+    }
+    AliasSet getAliasSet() const override {
+        // This instruction can read and write to all parts of the object, but
+        // is marked as non-effectful so it can be consolidated by LICM and GVN
+        // and avoid inhibiting other optimizations.
+        //
+        // This is valid to do because when unboxed objects might have a native
+        // group they can be converted to, we do not optimize accesses to the
+        // unboxed objects and do not guard on their group or shape (other than
+        // in this opcode).
+        //
+        // Later accesses can assume the object has a native representation
+        // and optimize accordingly. Those accesses cannot be reordered before
+        // this instruction, however. This is prevented by chaining this
+        // instruction with the object itself, in the same way as MBoundsCheck.
+        return AliasSet::None();
+    }
+    bool appendRoots(MRootList& roots) const override {
+        return roots.append(group_);
+    }
+};
+
 // Array.prototype.pop or Array.prototype.shift on a dense array.
 class MArrayPopShift
   : public MUnaryInstruction,
@@ -9623,12 +9816,13 @@ class MArrayPopShift
 
   private:
     Mode mode_;
+    JSValueType unboxedType_;
     bool needsHoleCheck_;
     bool maybeUndefined_;
 
-    MArrayPopShift(MDefinition* object, Mode mode,
+    MArrayPopShift(MDefinition* object, Mode mode, JSValueType unboxedType,
                    bool needsHoleCheck, bool maybeUndefined)
-      : MUnaryInstruction(object), mode_(mode),
+      : MUnaryInstruction(object), mode_(mode), unboxedType_(unboxedType),
         needsHoleCheck_(needsHoleCheck), maybeUndefined_(maybeUndefined)
     { }
 
@@ -9646,8 +9840,12 @@ class MArrayPopShift
     bool mode() const {
         return mode_;
     }
+    JSValueType unboxedType() const {
+        return unboxedType_;
+    }
     AliasSet getAliasSet() const override {
-        return AliasSet::Store(AliasSet::ObjectFields | AliasSet::Element);
+        return AliasSet::Store(AliasSet::ObjectFields |
+                               AliasSet::BoxedOrUnboxedElements(unboxedType()));
     }
 
     ALLOW_CLONE(MArrayPopShift)
@@ -9658,8 +9856,10 @@ class MArrayPush
   : public MBinaryInstruction,
     public MixPolicy<SingleObjectPolicy, NoFloatPolicy<1> >::Data
 {
-    MArrayPush(MDefinition* object, MDefinition* value)
-      : MBinaryInstruction(object, value)
+    JSValueType unboxedType_;
+
+    MArrayPush(MDefinition* object, MDefinition* value, JSValueType unboxedType)
+      : MBinaryInstruction(object, value), unboxedType_(unboxedType)
     {
         setResultType(MIRType::Int32);
     }
@@ -9669,8 +9869,12 @@ class MArrayPush
     TRIVIAL_NEW_WRAPPERS
     NAMED_OPERANDS((0, object), (1, value))
 
+    JSValueType unboxedType() const {
+        return unboxedType_;
+    }
     AliasSet getAliasSet() const override {
-        return AliasSet::Store(AliasSet::ObjectFields | AliasSet::Element);
+        return AliasSet::Store(AliasSet::ObjectFields |
+                               AliasSet::BoxedOrUnboxedElements(unboxedType()));
     }
     void computeRange(TempAllocator& alloc) override;
 
@@ -9684,13 +9888,15 @@ class MArraySlice
 {
     CompilerObject templateObj_;
     gc::InitialHeap initialHeap_;
+    JSValueType unboxedType_;
 
     MArraySlice(CompilerConstraintList* constraints, MDefinition* obj,
                 MDefinition* begin, MDefinition* end,
-                JSObject* templateObj, gc::InitialHeap initialHeap)
+                JSObject* templateObj, gc::InitialHeap initialHeap, JSValueType unboxedType)
       : MTernaryInstruction(obj, begin, end),
         templateObj_(templateObj),
-        initialHeap_(initialHeap)
+        initialHeap_(initialHeap),
+        unboxedType_(unboxedType)
     {
         setResultType(MIRType::Object);
     }
@@ -9708,6 +9914,10 @@ class MArraySlice
         return initialHeap_;
     }
 
+    JSValueType unboxedType() const {
+        return unboxedType_;
+    }
+
     bool possiblyCalls() const override {
         return true;
     }
@@ -10972,6 +11182,11 @@ class MGuardShape
         setMovable();
         setResultType(MIRType::Object);
         setResultTypeSet(obj->resultTypeSet());
+
+        // Disallow guarding on unboxed object shapes. The group is better to
+        // guard on, and guarding on the shape can interact badly with
+        // MConvertUnboxedObjectToNative.
+        MOZ_ASSERT(shape->getObjectClass() != &UnboxedPlainObject::class_);
     }
 
   public:
@@ -11066,6 +11281,11 @@ class MGuardObjectGroup
         setGuard();
         setMovable();
         setResultType(MIRType::Object);
+
+        // Unboxed groups which might be converted to natives can't be guarded
+        // on, due to MConvertUnboxedObjectToNative.
+        MOZ_ASSERT_IF(group->maybeUnboxedLayoutDontCheckGeneration(),
+                      !group->unboxedLayoutDontCheckGeneration().nativeGroup());
     }
 
   public:
@@ -11174,6 +11394,73 @@ class MGuardClass
     ALLOW_CLONE(MGuardClass)
 };
 
+// Guard on the presence or absence of an unboxed object's expando.
+class MGuardUnboxedExpando
+  : public MUnaryInstruction,
+    public SingleObjectPolicy::Data
+{
+    bool requireExpando_;
+    BailoutKind bailoutKind_;
+
+    MGuardUnboxedExpando(MDefinition* obj, bool requireExpando, BailoutKind bailoutKind)
+      : MUnaryInstruction(obj),
+        requireExpando_(requireExpando),
+        bailoutKind_(bailoutKind)
+    {
+        setGuard();
+        setMovable();
+        setResultType(MIRType::Object);
+    }
+
+  public:
+    INSTRUCTION_HEADER(GuardUnboxedExpando)
+    TRIVIAL_NEW_WRAPPERS
+    NAMED_OPERANDS((0, object))
+
+    bool requireExpando() const {
+        return requireExpando_;
+    }
+    BailoutKind bailoutKind() const {
+        return bailoutKind_;
+    }
+    bool congruentTo(const MDefinition* ins) const override {
+        if (!congruentIfOperandsEqual(ins))
+            return false;
+        if (requireExpando() != ins->toGuardUnboxedExpando()->requireExpando())
+            return false;
+        return true;
+    }
+    AliasSet getAliasSet() const override {
+        return AliasSet::Load(AliasSet::ObjectFields);
+    }
+};
+
+// Load an unboxed plain object's expando.
+class MLoadUnboxedExpando
+  : public MUnaryInstruction,
+    public SingleObjectPolicy::Data
+{
+  private:
+    explicit MLoadUnboxedExpando(MDefinition* object)
+      : MUnaryInstruction(object)
+    {
+        setResultType(MIRType::Object);
+        setMovable();
+    }
+
+  public:
+    INSTRUCTION_HEADER(LoadUnboxedExpando)
+    TRIVIAL_NEW_WRAPPERS
+    NAMED_OPERANDS((0, object))
+
+    bool congruentTo(const MDefinition* ins) const override {
+        return congruentIfOperandsEqual(ins);
+    }
+    AliasSet getAliasSet() const override {
+        return AliasSet::Load(AliasSet::ObjectFields);
+    }
+};
+
 // Load from vp[slot] (slots that are not inline in an object).
 class MLoadSlot
   : public MUnaryInstruction,
@@ -12076,13 +12363,15 @@ class MInArray
 {
     bool needsHoleCheck_;
     bool needsNegativeIntCheck_;
+    JSValueType unboxedType_;
 
     MInArray(MDefinition* elements, MDefinition* index,
              MDefinition* initLength, MDefinition* object,
-             bool needsHoleCheck)
+             bool needsHoleCheck, JSValueType unboxedType)
       : MQuaternaryInstruction(elements, index, initLength, object),
         needsHoleCheck_(needsHoleCheck),
-        needsNegativeIntCheck_(true)
+        needsNegativeIntCheck_(true),
+        unboxedType_(unboxedType)
     {
         setResultType(MIRType::Boolean);
         setMovable();
@@ -12102,6 +12391,9 @@ class MInArray
     bool needsNegativeIntCheck() const {
         return needsNegativeIntCheck_;
     }
+    JSValueType unboxedType() const {
+        return unboxedType_;
+    }
     void collectRangeInfoPreTrunc() override;
     AliasSet getAliasSet() const override {
         return AliasSet::Load(AliasSet::Element);
@@ -12114,6 +12406,8 @@ class MInArray
             return false;
         if (needsNegativeIntCheck() != other->needsNegativeIntCheck())
             return false;
+        if (unboxedType() != other->unboxedType())
+            return false;
         return congruentIfOperandsEqual(other);
     }
 };
@@ -14014,6 +14308,8 @@ MDefinition::maybeConstantValue()
 
 bool ElementAccessIsDenseNative(CompilerConstraintList* constraints,
                                 MDefinition* obj, MDefinition* id);
+JSValueType UnboxedArrayElementType(CompilerConstraintList* constraints, MDefinition* obj,
+                                    MDefinition* id);
 bool ElementAccessIsTypedArray(CompilerConstraintList* constraints,
                                MDefinition* obj, MDefinition* id,
                                Scalar::Type* arrayType);
diff --git a/js/src/jit/MOpcodes.h b/js/src/jit/MOpcodes.h
index 589dde077..2f67f8039 100644
--- a/js/src/jit/MOpcodes.h
+++ b/js/src/jit/MOpcodes.h
@@ -188,6 +188,8 @@ namespace jit {
     _(GuardObjectGroup)                                                     \
     _(GuardObjectIdentity)                                                  \
     _(GuardClass)                                                           \
+    _(GuardUnboxedExpando)                                                  \
+    _(LoadUnboxedExpando)                                                   \
     _(ArrayLength)                                                          \
     _(SetArrayLength)                                                       \
     _(GetNextEntryForIterator)                                              \
@@ -199,6 +201,10 @@ namespace jit {
     _(SetTypedObjectOffset)                                                 \
     _(InitializedLength)                                                    \
     _(SetInitializedLength)                                                 \
+    _(UnboxedArrayLength)                                                   \
+    _(UnboxedArrayInitializedLength)                                        \
+    _(IncrementUnboxedArrayInitializedLength)                               \
+    _(SetUnboxedArrayInitializedLength)                                     \
     _(Not)                                                                  \
     _(BoundsCheck)                                                          \
     _(BoundsCheckLower)                                                     \
@@ -214,6 +220,7 @@ namespace jit {
     _(StoreUnboxedScalar)                                                   \
     _(StoreUnboxedObjectOrNull)                                             \
     _(StoreUnboxedString)                                                   \
+    _(ConvertUnboxedObjectToNative)                                         \
     _(ArrayPopShift)                                                        \
     _(ArrayPush)                                                            \
     _(ArraySlice)                                                           \
diff --git a/js/src/jit/MacroAssembler.cpp b/js/src/jit/MacroAssembler.cpp
index a739b9325..f633b9b7b 100644
--- a/js/src/jit/MacroAssembler.cpp
+++ b/js/src/jit/MacroAssembler.cpp
@@ -126,14 +126,20 @@ MacroAssembler::guardTypeSetMightBeIncomplete(TypeSet* types, Register obj, Regi
 {
     // Type set guards might miss when an object's group changes. In this case
     // either its old group's properties will become unknown, or it will change
-    // to a native object. Jump to label if this might have happened for the
-    // input object.
+    // to a native object with an original unboxed group. Jump to label if this
+    // might have happened for the input object.
 
     if (types->unknownObject()) {
         jump(label);
         return;
     }
 
+    loadPtr(Address(obj, JSObject::offsetOfGroup()), scratch);
+    load32(Address(scratch, ObjectGroup::offsetOfFlags()), scratch);
+    and32(Imm32(OBJECT_FLAG_ADDENDUM_MASK), scratch);
+    branch32(Assembler::Equal,
+             scratch, Imm32(ObjectGroup::addendumOriginalUnboxedGroupValue()), label);
+
     for (size_t i = 0; i < types->getObjectCount(); i++) {
         if (JSObject* singleton = types->getSingletonNoBarrier(i)) {
             movePtr(ImmGCPtr(singleton), scratch);
@@ -462,6 +468,268 @@ template void MacroAssembler::loadFromTypedArray(Scalar::Type arrayType, const A
 template void MacroAssembler::loadFromTypedArray(Scalar::Type arrayType, const BaseIndex& src, const ValueOperand& dest,
                                                  bool allowDouble, Register temp, Label* fail);
 
+template <typename T>
+void
+MacroAssembler::loadUnboxedProperty(T address, JSValueType type, TypedOrValueRegister output)
+{
+    switch (type) {
+      case JSVAL_TYPE_INT32: {
+          // Handle loading an int32 into a double reg.
+          if (output.type() == MIRType::Double) {
+              convertInt32ToDouble(address, output.typedReg().fpu());
+              break;
+          }
+          MOZ_FALLTHROUGH;
+      }
+
+      case JSVAL_TYPE_BOOLEAN:
+      case JSVAL_TYPE_STRING: {
+        Register outReg;
+        if (output.hasValue()) {
+            outReg = output.valueReg().scratchReg();
+        } else {
+            MOZ_ASSERT(output.type() == MIRTypeFromValueType(type));
+            outReg = output.typedReg().gpr();
+        }
+
+        switch (type) {
+          case JSVAL_TYPE_BOOLEAN:
+            load8ZeroExtend(address, outReg);
+            break;
+          case JSVAL_TYPE_INT32:
+            load32(address, outReg);
+            break;
+          case JSVAL_TYPE_STRING:
+            loadPtr(address, outReg);
+            break;
+          default:
+            MOZ_CRASH();
+        }
+
+        if (output.hasValue())
+            tagValue(type, outReg, output.valueReg());
+        break;
+      }
+
+      case JSVAL_TYPE_OBJECT:
+        if (output.hasValue()) {
+            Register scratch = output.valueReg().scratchReg();
+            loadPtr(address, scratch);
+
+            Label notNull, done;
+            branchPtr(Assembler::NotEqual, scratch, ImmWord(0), &notNull);
+
+            moveValue(NullValue(), output.valueReg());
+            jump(&done);
+
+            bind(&notNull);
+            tagValue(JSVAL_TYPE_OBJECT, scratch, output.valueReg());
+
+            bind(&done);
+        } else {
+            // Reading null can't be possible here, as otherwise the result
+            // would be a value (either because null has been read before or
+            // because there is a barrier).
+            Register reg = output.typedReg().gpr();
+            loadPtr(address, reg);
+#ifdef DEBUG
+            Label ok;
+            branchTestPtr(Assembler::NonZero, reg, reg, &ok);
+            assumeUnreachable("Null not possible");
+            bind(&ok);
+#endif
+        }
+        break;
+
+      case JSVAL_TYPE_DOUBLE:
+        // Note: doubles in unboxed objects are not accessed through other
+        // views and do not need canonicalization.
+        if (output.hasValue())
+            loadValue(address, output.valueReg());
+        else
+            loadDouble(address, output.typedReg().fpu());
+        break;
+
+      default:
+        MOZ_CRASH();
+    }
+}
+
+template void
+MacroAssembler::loadUnboxedProperty(Address address, JSValueType type,
+                                    TypedOrValueRegister output);
+
+template void
+MacroAssembler::loadUnboxedProperty(BaseIndex address, JSValueType type,
+                                    TypedOrValueRegister output);
+
+static void
+StoreUnboxedFailure(MacroAssembler& masm, Label* failure)
+{
+    // Storing a value to an unboxed property is a fallible operation and
+    // the caller must provide a failure label if a particular unboxed store
+    // might fail. Sometimes, however, a store that cannot succeed (such as
+    // storing a string to an int32 property) will be marked as infallible.
+    // This can only happen if the code involved is unreachable.
+    if (failure)
+        masm.jump(failure);
+    else
+        masm.assumeUnreachable("Incompatible write to unboxed property");
+}
+
+template <typename T>
+void
+MacroAssembler::storeUnboxedProperty(T address, JSValueType type,
+                                     const ConstantOrRegister& value, Label* failure)
+{
+    switch (type) {
+      case JSVAL_TYPE_BOOLEAN:
+        if (value.constant()) {
+            if (value.value().isBoolean())
+                store8(Imm32(value.value().toBoolean()), address);
+            else
+                StoreUnboxedFailure(*this, failure);
+        } else if (value.reg().hasTyped()) {
+            if (value.reg().type() == MIRType::Boolean)
+                store8(value.reg().typedReg().gpr(), address);
+            else
+                StoreUnboxedFailure(*this, failure);
+        } else {
+            if (failure)
+                branchTestBoolean(Assembler::NotEqual, value.reg().valueReg(), failure);
+            storeUnboxedPayload(value.reg().valueReg(), address, /* width = */ 1);
+        }
+        break;
+
+      case JSVAL_TYPE_INT32:
+        if (value.constant()) {
+            if (value.value().isInt32())
+                store32(Imm32(value.value().toInt32()), address);
+            else
+                StoreUnboxedFailure(*this, failure);
+        } else if (value.reg().hasTyped()) {
+            if (value.reg().type() == MIRType::Int32)
+                store32(value.reg().typedReg().gpr(), address);
+            else
+                StoreUnboxedFailure(*this, failure);
+        } else {
+            if (failure)
+                branchTestInt32(Assembler::NotEqual, value.reg().valueReg(), failure);
+            storeUnboxedPayload(value.reg().valueReg(), address, /* width = */ 4);
+        }
+        break;
+
+      case JSVAL_TYPE_DOUBLE:
+        if (value.constant()) {
+            if (value.value().isNumber()) {
+                loadConstantDouble(value.value().toNumber(), ScratchDoubleReg);
+                storeDouble(ScratchDoubleReg, address);
+            } else {
+                StoreUnboxedFailure(*this, failure);
+            }
+        } else if (value.reg().hasTyped()) {
+            if (value.reg().type() == MIRType::Int32) {
+                convertInt32ToDouble(value.reg().typedReg().gpr(), ScratchDoubleReg);
+                storeDouble(ScratchDoubleReg, address);
+            } else if (value.reg().type() == MIRType::Double) {
+                storeDouble(value.reg().typedReg().fpu(), address);
+            } else {
+                StoreUnboxedFailure(*this, failure);
+            }
+        } else {
+            ValueOperand reg = value.reg().valueReg();
+            Label notInt32, end;
+            branchTestInt32(Assembler::NotEqual, reg, &notInt32);
+            int32ValueToDouble(reg, ScratchDoubleReg);
+            storeDouble(ScratchDoubleReg, address);
+            jump(&end);
+            bind(&notInt32);
+            if (failure)
+                branchTestDouble(Assembler::NotEqual, reg, failure);
+            storeValue(reg, address);
+            bind(&end);
+        }
+        break;
+
+      case JSVAL_TYPE_OBJECT:
+        if (value.constant()) {
+            if (value.value().isObjectOrNull())
+                storePtr(ImmGCPtr(value.value().toObjectOrNull()), address);
+            else
+                StoreUnboxedFailure(*this, failure);
+        } else if (value.reg().hasTyped()) {
+            MOZ_ASSERT(value.reg().type() != MIRType::Null);
+            if (value.reg().type() == MIRType::Object)
+                storePtr(value.reg().typedReg().gpr(), address);
+            else
+                StoreUnboxedFailure(*this, failure);
+        } else {
+            if (failure) {
+                Label ok;
+                branchTestNull(Assembler::Equal, value.reg().valueReg(), &ok);
+                branchTestObject(Assembler::NotEqual, value.reg().valueReg(), failure);
+                bind(&ok);
+            }
+            storeUnboxedPayload(value.reg().valueReg(), address, /* width = */ sizeof(uintptr_t));
+        }
+        break;
+
+      case JSVAL_TYPE_STRING:
+        if (value.constant()) {
+            if (value.value().isString())
+                storePtr(ImmGCPtr(value.value().toString()), address);
+            else
+                StoreUnboxedFailure(*this, failure);
+        } else if (value.reg().hasTyped()) {
+            if (value.reg().type() == MIRType::String)
+                storePtr(value.reg().typedReg().gpr(), address);
+            else
+                StoreUnboxedFailure(*this, failure);
+        } else {
+            if (failure)
+                branchTestString(Assembler::NotEqual, value.reg().valueReg(), failure);
+            storeUnboxedPayload(value.reg().valueReg(), address, /* width = */ sizeof(uintptr_t));
+        }
+        break;
+
+      default:
+        MOZ_CRASH();
+    }
+}
+
+template void
+MacroAssembler::storeUnboxedProperty(Address address, JSValueType type,
+                                     const ConstantOrRegister& value, Label* failure);
+
+template void
+MacroAssembler::storeUnboxedProperty(BaseIndex address, JSValueType type,
+                                     const ConstantOrRegister& value, Label* failure);
+
+void
+MacroAssembler::checkUnboxedArrayCapacity(Register obj, const RegisterOrInt32Constant& index,
+                                          Register temp, Label* failure)
+{
+    Address initLengthAddr(obj, UnboxedArrayObject::offsetOfCapacityIndexAndInitializedLength());
+    Address lengthAddr(obj, UnboxedArrayObject::offsetOfLength());
+
+    Label capacityIsIndex, done;
+    load32(initLengthAddr, temp);
+    branchTest32(Assembler::NonZero, temp, Imm32(UnboxedArrayObject::CapacityMask), &capacityIsIndex);
+    branch32(Assembler::BelowOrEqual, lengthAddr, index, failure);
+    jump(&done);
+    bind(&capacityIsIndex);
+
+    // Do a partial shift so that we can get an absolute offset from the base
+    // of CapacityArray to use.
+    JS_STATIC_ASSERT(sizeof(UnboxedArrayObject::CapacityArray[0]) == 4);
+    rshiftPtr(Imm32(UnboxedArrayObject::CapacityShift - 2), temp);
+    and32(Imm32(~0x3), temp);
+
+    addPtr(ImmPtr(&UnboxedArrayObject::CapacityArray), temp);
+    branch32(Assembler::BelowOrEqual, Address(temp, 0), index, failure);
+    bind(&done);
+}
+
 // Inlined version of gc::CheckAllocatorState that checks the bare essentials
 // and bails for anything that cannot be handled with our jit allocators.
 void
@@ -1009,6 +1277,20 @@ MacroAssembler::initGCThing(Register obj, Register temp, JSObject* templateObj,
             nbytes = (nbytes < sizeof(uintptr_t)) ? 0 : nbytes - sizeof(uintptr_t);
             offset += sizeof(uintptr_t);
         }
+    } else if (templateObj->is<UnboxedPlainObject>()) {
+        storePtr(ImmWord(0), Address(obj, UnboxedPlainObject::offsetOfExpando()));
+        if (initContents)
+            initUnboxedObjectContents(obj, &templateObj->as<UnboxedPlainObject>());
+    } else if (templateObj->is<UnboxedArrayObject>()) {
+        MOZ_ASSERT(templateObj->as<UnboxedArrayObject>().hasInlineElements());
+        int elementsOffset = UnboxedArrayObject::offsetOfInlineElements();
+        computeEffectiveAddress(Address(obj, elementsOffset), temp);
+        storePtr(temp, Address(obj, UnboxedArrayObject::offsetOfElements()));
+        store32(Imm32(templateObj->as<UnboxedArrayObject>().length()),
+                Address(obj, UnboxedArrayObject::offsetOfLength()));
+        uint32_t capacityIndex = templateObj->as<UnboxedArrayObject>().capacityIndex();
+        store32(Imm32(capacityIndex << UnboxedArrayObject::CapacityShift),
+                Address(obj, UnboxedArrayObject::offsetOfCapacityIndexAndInitializedLength()));
     } else {
         MOZ_CRASH("Unknown object");
     }
@@ -1030,6 +1312,29 @@ MacroAssembler::initGCThing(Register obj, Register temp, JSObject* templateObj,
 }
 
 void
+MacroAssembler::initUnboxedObjectContents(Register object, UnboxedPlainObject* templateObject)
+{
+    const UnboxedLayout& layout = templateObject->layoutDontCheckGeneration();
+
+    // Initialize reference fields of the object, per UnboxedPlainObject::create.
+    if (const int32_t* list = layout.traceList()) {
+        while (*list != -1) {
+            storePtr(ImmGCPtr(GetJitContext()->runtime->names().empty),
+                     Address(object, UnboxedPlainObject::offsetOfData() + *list));
+            list++;
+        }
+        list++;
+        while (*list != -1) {
+            storePtr(ImmWord(0),
+                     Address(object, UnboxedPlainObject::offsetOfData() + *list));
+            list++;
+        }
+        // Unboxed objects don't have Values to initialize.
+        MOZ_ASSERT(*(list + 1) == -1);
+    }
+}
+
+void
 MacroAssembler::compareStrings(JSOp op, Register left, Register right, Register result,
                                Label* fail)
 {
diff --git a/js/src/jit/MacroAssembler.h b/js/src/jit/MacroAssembler.h
index d5cc95839..b6616321c 100644
--- a/js/src/jit/MacroAssembler.h
+++ b/js/src/jit/MacroAssembler.h
@@ -36,6 +36,7 @@
 #include "vm/ProxyObject.h"
 #include "vm/Shape.h"
 #include "vm/TypedArrayObject.h"
+#include "vm/UnboxedObject.h"
 
 using mozilla::FloatingPoint;
 
@@ -1625,6 +1626,20 @@ class MacroAssembler : public MacroAssemblerSpecific
     void storeToTypedFloatArray(Scalar::Type arrayType, FloatRegister value, const Address& dest,
                                 unsigned numElems = 0);
 
+    // Load a property from an UnboxedPlainObject or UnboxedArrayObject.
+    template <typename T>
+    void loadUnboxedProperty(T address, JSValueType type, TypedOrValueRegister output);
+
+    // Store a property to an UnboxedPlainObject, without triggering barriers.
+    // If failure is null, the value definitely has a type suitable for storing
+    // in the property.
+    template <typename T>
+    void storeUnboxedProperty(T address, JSValueType type,
+                              const ConstantOrRegister& value, Label* failure);
+
+    void checkUnboxedArrayCapacity(Register obj, const RegisterOrInt32Constant& index,
+                                   Register temp, Label* failure);
+
     Register extractString(const Address& address, Register scratch) {
         return extractObject(address, scratch);
     }
@@ -1701,6 +1716,8 @@ class MacroAssembler : public MacroAssemblerSpecific
                              LiveRegisterSet liveRegs, Label* fail,
                              TypedArrayObject* templateObj, TypedArrayLength lengthKind);
 
+    void initUnboxedObjectContents(Register object, UnboxedPlainObject* templateObject);
+
     void newGCString(Register result, Register temp, Label* fail);
     void newGCFatInlineString(Register result, Register temp, Label* fail);
 
diff --git a/js/src/jit/OptimizationTracking.cpp b/js/src/jit/OptimizationTracking.cpp
index 7d72795a0..308def041 100644
--- a/js/src/jit/OptimizationTracking.cpp
+++ b/js/src/jit/OptimizationTracking.cpp
@@ -844,6 +844,8 @@ MaybeConstructorFromType(TypeSet::Type ty)
         return nullptr;
     ObjectGroup* obj = ty.group();
     TypeNewScript* newScript = obj->newScript();
+    if (!newScript && obj->maybeUnboxedLayout())
+        newScript = obj->unboxedLayout().newScript();
     return newScript ? newScript->function() : nullptr;
 }
 
diff --git a/js/src/jit/Recover.cpp b/js/src/jit/Recover.cpp
index 793b631df..6fd71f377 100644
--- a/js/src/jit/Recover.cpp
+++ b/js/src/jit/Recover.cpp
@@ -30,6 +30,7 @@
 
 #include "vm/Interpreter-inl.h"
 #include "vm/NativeObject-inl.h"
+#include "vm/UnboxedObject-inl.h"
 
 using namespace js;
 using namespace js::jit;
@@ -1354,7 +1355,7 @@ RNewArray::recover(JSContext* cx, SnapshotIterator& iter) const
     RootedValue result(cx);
     RootedObjectGroup group(cx, templateObject->group());
 
-    ArrayObject* resultObject = NewFullyAllocatedArrayTryUseGroup(cx, group, count_);
+    JSObject* resultObject = NewFullyAllocatedArrayTryUseGroup(cx, group, count_);
     if (!resultObject)
         return false;
 
@@ -1539,12 +1540,37 @@ RObjectState::recover(JSContext* cx, SnapshotIterator& iter) const
     RootedObject object(cx, &iter.read().toObject());
     RootedValue val(cx);
 
-    RootedNativeObject nativeObject(cx, &object->as<NativeObject>());
-    MOZ_ASSERT(nativeObject->slotSpan() == numSlots());
+    if (object->is<UnboxedPlainObject>()) {
+        const UnboxedLayout& layout = object->as<UnboxedPlainObject>().layout();
 
-    for (size_t i = 0; i < numSlots(); i++) {
-        val = iter.read();
-        nativeObject->setSlot(i, val);
+        RootedId id(cx);
+        RootedValue receiver(cx, ObjectValue(*object));
+        const UnboxedLayout::PropertyVector& properties = layout.properties();
+        for (size_t i = 0; i < properties.length(); i++) {
+            val = iter.read();
+
+            // This is the default placeholder value of MObjectState, when no
+            // properties are defined yet.
+            if (val.isUndefined())
+                continue;
+
+            id = NameToId(properties[i].name);
+            ObjectOpResult result;
+
+            // SetProperty can only fail due to OOM.
+            if (!SetProperty(cx, object, id, val, receiver, result))
+                return false;
+            if (!result)
+                return result.reportError(cx, object, id);
+        }
+    } else {
+        RootedNativeObject nativeObject(cx, &object->as<NativeObject>());
+        MOZ_ASSERT(nativeObject->slotSpan() == numSlots());
+
+        for (size_t i = 0; i < numSlots(); i++) {
+            val = iter.read();
+            nativeObject->setSlot(i, val);
+        }
     }
 
     val.setObject(*object);
diff --git a/js/src/jit/ScalarReplacement.cpp b/js/src/jit/ScalarReplacement.cpp
index 97ba52349..4614b2162 100644
--- a/js/src/jit/ScalarReplacement.cpp
+++ b/js/src/jit/ScalarReplacement.cpp
@@ -13,6 +13,7 @@
 #include "jit/MIR.h"
 #include "jit/MIRGenerator.h"
 #include "jit/MIRGraph.h"
+#include "vm/UnboxedObject.h"
 
 #include "jsobjinlines.h"
 
@@ -182,6 +183,25 @@ IsObjectEscaped(MInstruction* ins, JSObject* objDefault)
             JitSpewDef(JitSpew_Escape, "is escaped by\n", def);
             return true;
 
+          case MDefinition::Op_LoadUnboxedScalar:
+          case MDefinition::Op_StoreUnboxedScalar:
+          case MDefinition::Op_LoadUnboxedObjectOrNull:
+          case MDefinition::Op_StoreUnboxedObjectOrNull:
+          case MDefinition::Op_LoadUnboxedString:
+          case MDefinition::Op_StoreUnboxedString:
+            // Not escaped if it is the first argument.
+            if (def->indexOf(*i) != 0) {
+                JitSpewDef(JitSpew_Escape, "is escaped by\n", def);
+                return true;
+            }
+
+            if (!def->getOperand(1)->isConstant()) {
+                JitSpewDef(JitSpew_Escape, "is addressed with unknown index\n", def);
+                return true;
+            }
+
+            break;
+
           case MDefinition::Op_PostWriteBarrier:
             break;
 
@@ -285,6 +305,16 @@ class ObjectMemoryView : public MDefinitionVisitorDefaultNoop
     void visitGuardShape(MGuardShape* ins);
     void visitFunctionEnvironment(MFunctionEnvironment* ins);
     void visitLambda(MLambda* ins);
+    void visitStoreUnboxedScalar(MStoreUnboxedScalar* ins);
+    void visitLoadUnboxedScalar(MLoadUnboxedScalar* ins);
+    void visitStoreUnboxedObjectOrNull(MStoreUnboxedObjectOrNull* ins);
+    void visitLoadUnboxedObjectOrNull(MLoadUnboxedObjectOrNull* ins);
+    void visitStoreUnboxedString(MStoreUnboxedString* ins);
+    void visitLoadUnboxedString(MLoadUnboxedString* ins);
+
+  private:
+    void storeOffset(MInstruction* ins, size_t offset, MDefinition* value);
+    void loadOffset(MInstruction* ins, size_t offset);
 };
 
 const char* ObjectMemoryView::phaseName = "Scalar Replacement of Object";
@@ -626,6 +656,121 @@ ObjectMemoryView::visitLambda(MLambda* ins)
     ins->setIncompleteObject();
 }
 
+static size_t
+GetOffsetOf(MDefinition* index, size_t width, int32_t baseOffset)
+{
+    int32_t idx = index->toConstant()->toInt32();
+    MOZ_ASSERT(idx >= 0);
+    MOZ_ASSERT(baseOffset >= 0 && size_t(baseOffset) >= UnboxedPlainObject::offsetOfData());
+    return idx * width + baseOffset - UnboxedPlainObject::offsetOfData();
+}
+
+static size_t
+GetOffsetOf(MDefinition* index, Scalar::Type type, int32_t baseOffset)
+{
+    return GetOffsetOf(index, Scalar::byteSize(type), baseOffset);
+}
+
+void
+ObjectMemoryView::storeOffset(MInstruction* ins, size_t offset, MDefinition* value)
+{
+    // Clone the state and update the slot value.
+    MOZ_ASSERT(state_->hasOffset(offset));
+    state_ = BlockState::Copy(alloc_, state_);
+    if (!state_) {
+        oom_ = true;
+        return;
+    }
+
+    state_->setOffset(offset, value);
+    ins->block()->insertBefore(ins, state_);
+
+    // Remove original instruction.
+    ins->block()->discard(ins);
+}
+
+void
+ObjectMemoryView::loadOffset(MInstruction* ins, size_t offset)
+{
+    // Replace load by the slot value.
+    MOZ_ASSERT(state_->hasOffset(offset));
+    ins->replaceAllUsesWith(state_->getOffset(offset));
+
+    // Remove original instruction.
+    ins->block()->discard(ins);
+}
+
+void
+ObjectMemoryView::visitStoreUnboxedScalar(MStoreUnboxedScalar* ins)
+{
+    // Skip stores made on other objects.
+    if (ins->elements() != obj_)
+        return;
+
+    size_t offset = GetOffsetOf(ins->index(), ins->storageType(), ins->offsetAdjustment());
+    storeOffset(ins, offset, ins->value());
+}
+
+void
+ObjectMemoryView::visitLoadUnboxedScalar(MLoadUnboxedScalar* ins)
+{
+    // Skip loads made on other objects.
+    if (ins->elements() != obj_)
+        return;
+
+    // Replace load by the slot value.
+    size_t offset = GetOffsetOf(ins->index(), ins->storageType(), ins->offsetAdjustment());
+    loadOffset(ins, offset);
+}
+
+void
+ObjectMemoryView::visitStoreUnboxedObjectOrNull(MStoreUnboxedObjectOrNull* ins)
+{
+    // Skip stores made on other objects.
+    if (ins->elements() != obj_)
+        return;
+
+    // Clone the state and update the slot value.
+    size_t offset = GetOffsetOf(ins->index(), sizeof(uintptr_t), ins->offsetAdjustment());
+    storeOffset(ins, offset, ins->value());
+}
+
+void
+ObjectMemoryView::visitLoadUnboxedObjectOrNull(MLoadUnboxedObjectOrNull* ins)
+{
+    // Skip loads made on other objects.
+    if (ins->elements() != obj_)
+        return;
+
+    // Replace load by the slot value.
+    size_t offset = GetOffsetOf(ins->index(), sizeof(uintptr_t), ins->offsetAdjustment());
+    loadOffset(ins, offset);
+}
+
+void
+ObjectMemoryView::visitStoreUnboxedString(MStoreUnboxedString* ins)
+{
+    // Skip stores made on other objects.
+    if (ins->elements() != obj_)
+        return;
+
+    // Clone the state and update the slot value.
+    size_t offset = GetOffsetOf(ins->index(), sizeof(uintptr_t), ins->offsetAdjustment());
+    storeOffset(ins, offset, ins->value());
+}
+
+void
+ObjectMemoryView::visitLoadUnboxedString(MLoadUnboxedString* ins)
+{
+    // Skip loads made on other objects.
+    if (ins->elements() != obj_)
+        return;
+
+    // Replace load by the slot value.
+    size_t offset = GetOffsetOf(ins->index(), sizeof(uintptr_t), ins->offsetAdjustment());
+    loadOffset(ins, offset);
+}
+
 static bool
 IndexOf(MDefinition* ins, int32_t* res)
 {
@@ -762,6 +907,11 @@ IsArrayEscaped(MInstruction* ins)
         return true;
     }
 
+    if (obj->is<UnboxedArrayObject>()) {
+        JitSpew(JitSpew_Escape, "Template object is an unboxed plain object.");
+        return true;
+    }
+
     if (length >= 16) {
         JitSpew(JitSpew_Escape, "Array has too many elements");
         return true;
diff --git a/js/src/jit/SharedIC.cpp b/js/src/jit/SharedIC.cpp
index 05a95824f..767cff661 100644
--- a/js/src/jit/SharedIC.cpp
+++ b/js/src/jit/SharedIC.cpp
@@ -27,7 +27,6 @@
 #endif
 #include "jit/VMFunctions.h"
 #include "vm/Interpreter.h"
-#include "vm/NativeObject-inl.h"
 
 #include "jit/MacroAssembler-inl.h"
 #include "vm/Interpreter-inl.h"
@@ -286,6 +285,11 @@ ICStub::trace(JSTracer* trc)
         TraceEdge(trc, &getElemStub->shape(), "baseline-getelem-dense-shape");
         break;
       }
+      case ICStub::GetElem_UnboxedArray: {
+        ICGetElem_UnboxedArray* getElemStub = toGetElem_UnboxedArray();
+        TraceEdge(trc, &getElemStub->group(), "baseline-getelem-unboxed-array-group");
+        break;
+      }
       case ICStub::GetElem_TypedArray: {
         ICGetElem_TypedArray* getElemStub = toGetElem_TypedArray();
         TraceEdge(trc, &getElemStub->shape(), "baseline-getelem-typedarray-shape");
@@ -2244,7 +2248,9 @@ IsCacheableProtoChain(JSObject* obj, JSObject* holder, bool isDOMProxy)
     if (!isDOMProxy && !obj->isNative()) {
         if (obj == holder)
             return false;
-        if (!obj->is<TypedObject>())
+        if (!obj->is<UnboxedPlainObject>() &&
+            !obj->is<UnboxedArrayObject>() &&
+            !obj->is<TypedObject>())
         {
             return false;
         }
@@ -2572,6 +2578,12 @@ CheckHasNoSuchProperty(JSContext* cx, JSObject* obj, PropertyName* name,
         } else if (curObj != obj) {
             // Non-native objects are only handled as the original receiver.
             return false;
+        } else if (curObj->is<UnboxedPlainObject>()) {
+            if (curObj->as<UnboxedPlainObject>().containsUnboxedOrExpandoProperty(cx, NameToId(name)))
+                return false;
+        } else if (curObj->is<UnboxedArrayObject>()) {
+            if (name == cx->names().length)
+                return false;
         } else if (curObj->is<TypedObject>()) {
             if (curObj->as<TypedObject>().typeDescr().hasProperty(cx->names(), NameToId(name)))
                 return false;
@@ -2836,15 +2848,34 @@ GuardReceiverObject(MacroAssembler& masm, ReceiverGuard guard,
 {
     Address groupAddress(ICStubReg, receiverGuardOffset + HeapReceiverGuard::offsetOfGroup());
     Address shapeAddress(ICStubReg, receiverGuardOffset + HeapReceiverGuard::offsetOfShape());
+    Address expandoAddress(object, UnboxedPlainObject::offsetOfExpando());
 
     if (guard.group) {
         masm.loadPtr(groupAddress, scratch);
         masm.branchTestObjGroup(Assembler::NotEqual, object, scratch, failure);
+
+        if (guard.group->clasp() == &UnboxedPlainObject::class_ && !guard.shape) {
+            // Guard the unboxed object has no expando object.
+            masm.branchPtr(Assembler::NotEqual, expandoAddress, ImmWord(0), failure);
+        }
     }
 
     if (guard.shape) {
         masm.loadPtr(shapeAddress, scratch);
-        masm.branchTestObjShape(Assembler::NotEqual, object, scratch, failure);
+        if (guard.group && guard.group->clasp() == &UnboxedPlainObject::class_) {
+            // Guard the unboxed object has a matching expando object.
+            masm.branchPtr(Assembler::Equal, expandoAddress, ImmWord(0), failure);
+            Label done;
+            masm.push(object);
+            masm.loadPtr(expandoAddress, object);
+            masm.branchTestObjShape(Assembler::Equal, object, scratch, &done);
+            masm.pop(object);
+            masm.jump(failure);
+            masm.bind(&done);
+            masm.pop(object);
+        } else {
+            masm.branchTestObjShape(Assembler::NotEqual, object, scratch, failure);
+        }
     }
 }
 
@@ -4228,7 +4259,8 @@ DoNewObject(JSContext* cx, void* payload, ICNewObject_Fallback* stub, MutableHan
                 return false;
 
             if (!stub->invalid() &&
-                !templateObject->as<PlainObject>().hasDynamicSlots())
+                (templateObject->is<UnboxedPlainObject>() ||
+                 !templateObject->as<PlainObject>().hasDynamicSlots()))
             {
                 JitCode* code = GenerateNewObjectWithTemplateCode(cx, templateObject);
                 if (!code)
diff --git a/js/src/jit/VMFunctions.cpp b/js/src/jit/VMFunctions.cpp
index 652c23bf1..10be2836b 100644
--- a/js/src/jit/VMFunctions.cpp
+++ b/js/src/jit/VMFunctions.cpp
@@ -28,7 +28,7 @@
 #include "vm/NativeObject-inl.h"
 #include "vm/StringObject-inl.h"
 #include "vm/TypeInference-inl.h"
-#include "gc/StoreBuffer-inl.h"
+#include "vm/UnboxedObject-inl.h"
 
 using namespace js;
 using namespace js::jit;
@@ -306,7 +306,7 @@ template bool StringsEqual<false>(JSContext* cx, HandleString lhs, HandleString
 bool
 ArrayPopDense(JSContext* cx, HandleObject obj, MutableHandleValue rval)
 {
-    MOZ_ASSERT(obj->is<ArrayObject>());
+    MOZ_ASSERT(obj->is<ArrayObject>() || obj->is<UnboxedArrayObject>());
 
     AutoDetectInvalidation adi(cx, rval);
 
@@ -325,11 +325,12 @@ ArrayPopDense(JSContext* cx, HandleObject obj, MutableHandleValue rval)
 }
 
 bool
-ArrayPushDense(JSContext* cx, HandleArrayObject arr, HandleValue v, uint32_t* length)
+ArrayPushDense(JSContext* cx, HandleObject obj, HandleValue v, uint32_t* length)
 {
-    *length = arr->length();
-    DenseElementResult result = arr->setOrExtendDenseElements(cx, *length, v.address(), 1,
-                                                              ShouldUpdateTypes::DontUpdate);
+    *length = GetAnyBoxedOrUnboxedArrayLength(obj);
+    DenseElementResult result =
+        SetOrExtendAnyBoxedOrUnboxedDenseElements(cx, obj, *length, v.address(), 1,
+                                                  ShouldUpdateTypes::DontUpdate);
     if (result != DenseElementResult::Incomplete) {
         (*length)++;
         return result == DenseElementResult::Success;
@@ -337,7 +338,7 @@ ArrayPushDense(JSContext* cx, HandleArrayObject arr, HandleValue v, uint32_t* le
 
     JS::AutoValueArray<3> argv(cx);
     argv[0].setUndefined();
-    argv[1].setObject(*arr);
+    argv[1].setObject(*obj);
     argv[2].set(v);
     if (!js::array_push(cx, 1, argv.begin()))
         return false;
@@ -349,7 +350,7 @@ ArrayPushDense(JSContext* cx, HandleArrayObject arr, HandleValue v, uint32_t* le
 bool
 ArrayShiftDense(JSContext* cx, HandleObject obj, MutableHandleValue rval)
 {
-    MOZ_ASSERT(obj->is<ArrayObject>());
+    MOZ_ASSERT(obj->is<ArrayObject>() || obj->is<UnboxedArrayObject>());
 
     AutoDetectInvalidation adi(cx, rval);
 
@@ -1130,14 +1131,16 @@ Recompile(JSContext* cx)
 }
 
 bool
-SetDenseElement(JSContext* cx, HandleNativeObject obj, int32_t index, HandleValue value, bool strict)
+SetDenseOrUnboxedArrayElement(JSContext* cx, HandleObject obj, int32_t index,
+                              HandleValue value, bool strict)
 {
     // This function is called from Ion code for StoreElementHole's OOL path.
-    // In this case we know the object is native and that no type changes are
-	// needed.
+    // In this case we know the object is native or an unboxed array and that
+    // no type changes are needed.
 
-    DenseElementResult result = obj->setOrExtendDenseElements(cx, index, value.address(), 1,
-                                                              ShouldUpdateTypes::DontUpdate);
+    DenseElementResult result =
+        SetOrExtendAnyBoxedOrUnboxedDenseElements(cx, obj, index, value.address(), 1,
+                                                  ShouldUpdateTypes::DontUpdate);
     if (result != DenseElementResult::Incomplete)
         return result == DenseElementResult::Success;
 
diff --git a/js/src/jit/VMFunctions.h b/js/src/jit/VMFunctions.h
index 94f741397..7f225c293 100644
--- a/js/src/jit/VMFunctions.h
+++ b/js/src/jit/VMFunctions.h
@@ -622,7 +622,7 @@ template<bool Equal>
 bool StringsEqual(JSContext* cx, HandleString left, HandleString right, bool* res);
 
 MOZ_MUST_USE bool ArrayPopDense(JSContext* cx, HandleObject obj, MutableHandleValue rval);
-MOZ_MUST_USE bool ArrayPushDense(JSContext* cx, HandleArrayObject obj, HandleValue v, uint32_t* length);
+MOZ_MUST_USE bool ArrayPushDense(JSContext* cx, HandleObject obj, HandleValue v, uint32_t* length);
 MOZ_MUST_USE bool ArrayShiftDense(JSContext* cx, HandleObject obj, MutableHandleValue rval);
 JSString* ArrayJoin(JSContext* cx, HandleObject array, HandleString sep);
 
@@ -745,8 +745,8 @@ ForcedRecompile(JSContext* cx);
 JSString* StringReplace(JSContext* cx, HandleString string, HandleString pattern,
                         HandleString repl);
 
-MOZ_MUST_USE bool SetDenseElement(JSContext* cx, HandleNativeObject obj, int32_t index,
-                                  HandleValue value, bool strict);
+MOZ_MUST_USE bool SetDenseOrUnboxedArrayElement(JSContext* cx, HandleObject obj, int32_t index,
+                                                HandleValue value, bool strict);
 
 void AssertValidObjectPtr(JSContext* cx, JSObject* obj);
 void AssertValidObjectOrNullPtr(JSContext* cx, JSObject* obj);
diff --git a/js/src/jit/shared/LIR-shared.h b/js/src/jit/shared/LIR-shared.h
index ff4915d1a..e6aab6ba3 100644
--- a/js/src/jit/shared/LIR-shared.h
+++ b/js/src/jit/shared/LIR-shared.h
@@ -5166,6 +5166,72 @@ class LSetInitializedLength : public LInstructionHelper<0, 2, 0>
     }
 };
 
+class LUnboxedArrayLength : public LInstructionHelper<1, 1, 0>
+{
+  public:
+    LIR_HEADER(UnboxedArrayLength)
+
+    explicit LUnboxedArrayLength(const LAllocation& object) {
+        setOperand(0, object);
+    }
+
+    const LAllocation* object() {
+        return getOperand(0);
+    }
+};
+
+class LUnboxedArrayInitializedLength : public LInstructionHelper<1, 1, 0>
+{
+  public:
+    LIR_HEADER(UnboxedArrayInitializedLength)
+
+    explicit LUnboxedArrayInitializedLength(const LAllocation& object) {
+        setOperand(0, object);
+    }
+
+    const LAllocation* object() {
+        return getOperand(0);
+    }
+};
+
+class LIncrementUnboxedArrayInitializedLength : public LInstructionHelper<0, 1, 0>
+{
+  public:
+    LIR_HEADER(IncrementUnboxedArrayInitializedLength)
+
+    explicit LIncrementUnboxedArrayInitializedLength(const LAllocation& object) {
+        setOperand(0, object);
+    }
+
+    const LAllocation* object() {
+        return getOperand(0);
+    }
+};
+
+class LSetUnboxedArrayInitializedLength : public LInstructionHelper<0, 2, 1>
+{
+  public:
+    LIR_HEADER(SetUnboxedArrayInitializedLength)
+
+    explicit LSetUnboxedArrayInitializedLength(const LAllocation& object,
+                                               const LAllocation& length,
+                                               const LDefinition& temp) {
+        setOperand(0, object);
+        setOperand(1, length);
+        setTemp(0, temp);
+    }
+
+    const LAllocation* object() {
+        return getOperand(0);
+    }
+    const LAllocation* length() {
+        return getOperand(1);
+    }
+    const LDefinition* temp() {
+        return getTemp(0);
+    }
+};
+
 // Load the length from an elements header.
 class LArrayLength : public LInstructionHelper<1, 1, 0>
 {
@@ -5670,17 +5736,19 @@ class LStoreElementT : public LInstructionHelper<0, 3, 0>
 };
 
 // Like LStoreElementV, but supports indexes >= initialized length.
-class LStoreElementHoleV : public LInstructionHelper<0, 3 + BOX_PIECES, 0>
+class LStoreElementHoleV : public LInstructionHelper<0, 3 + BOX_PIECES, 1>
 {
   public:
     LIR_HEADER(StoreElementHoleV)
 
     LStoreElementHoleV(const LAllocation& object, const LAllocation& elements,
-                       const LAllocation& index, const LBoxAllocation& value) {
+                       const LAllocation& index, const LBoxAllocation& value,
+                       const LDefinition& temp) {
         setOperand(0, object);
         setOperand(1, elements);
         setOperand(2, index);
         setBoxOperand(Value, value);
+        setTemp(0, temp);
     }
 
     static const size_t Value = 3;
@@ -5700,17 +5768,19 @@ class LStoreElementHoleV : public LInstructionHelper<0, 3 + BOX_PIECES, 0>
 };
 
 // Like LStoreElementT, but supports indexes >= initialized length.
-class LStoreElementHoleT : public LInstructionHelper<0, 4, 0>
+class LStoreElementHoleT : public LInstructionHelper<0, 4, 1>
 {
   public:
     LIR_HEADER(StoreElementHoleT)
 
     LStoreElementHoleT(const LAllocation& object, const LAllocation& elements,
-                       const LAllocation& index, const LAllocation& value) {
+                       const LAllocation& index, const LAllocation& value,
+                       const LDefinition& temp) {
         setOperand(0, object);
         setOperand(1, elements);
         setOperand(2, index);
         setOperand(3, value);
+        setTemp(0, temp);
     }
 
     const MStoreElementHole* mir() const {
@@ -5731,17 +5801,19 @@ class LStoreElementHoleT : public LInstructionHelper<0, 4, 0>
 };
 
 // Like LStoreElementV, but can just ignore assignment (for eg. frozen objects)
-class LFallibleStoreElementV : public LInstructionHelper<0, 3 + BOX_PIECES, 0>
+class LFallibleStoreElementV : public LInstructionHelper<0, 3 + BOX_PIECES, 1>
 {
   public:
     LIR_HEADER(FallibleStoreElementV)
 
     LFallibleStoreElementV(const LAllocation& object, const LAllocation& elements,
-                           const LAllocation& index, const LBoxAllocation& value) {
+                           const LAllocation& index, const LBoxAllocation& value,
+                           const LDefinition& temp) {
         setOperand(0, object);
         setOperand(1, elements);
         setOperand(2, index);
         setBoxOperand(Value, value);
+        setTemp(0, temp);
     }
 
     static const size_t Value = 3;
@@ -5761,17 +5833,19 @@ class LFallibleStoreElementV : public LInstructionHelper<0, 3 + BOX_PIECES, 0>
 };
 
 // Like LStoreElementT, but can just ignore assignment (for eg. frozen objects)
-class LFallibleStoreElementT : public LInstructionHelper<0, 4, 0>
+class LFallibleStoreElementT : public LInstructionHelper<0, 4, 1>
 {
   public:
     LIR_HEADER(FallibleStoreElementT)
 
     LFallibleStoreElementT(const LAllocation& object, const LAllocation& elements,
-                           const LAllocation& index, const LAllocation& value) {
+                           const LAllocation& index, const LAllocation& value,
+                           const LDefinition& temp) {
         setOperand(0, object);
         setOperand(1, elements);
         setOperand(2, index);
         setOperand(3, value);
+        setTemp(0, temp);
     }
 
     const MFallibleStoreElement* mir() const {
@@ -5818,6 +5892,22 @@ class LStoreUnboxedPointer : public LInstructionHelper<0, 3, 0>
     }
 };
 
+// If necessary, convert an unboxed object in a particular group to its native
+// representation.
+class LConvertUnboxedObjectToNative : public LInstructionHelper<0, 1, 0>
+{
+  public:
+    LIR_HEADER(ConvertUnboxedObjectToNative)
+
+    explicit LConvertUnboxedObjectToNative(const LAllocation& object) {
+        setOperand(0, object);
+    }
+
+    MConvertUnboxedObjectToNative* mir() {
+        return mir_->toConvertUnboxedObjectToNative();
+    }
+};
+
 class LArrayPopShiftV : public LInstructionHelper<BOX_PIECES, 1, 2>
 {
   public:
@@ -7340,6 +7430,38 @@ class LGuardReceiverPolymorphic : public LInstructionHelper<0, 1, 1>
     }
 };
 
+class LGuardUnboxedExpando : public LInstructionHelper<0, 1, 0>
+{
+  public:
+    LIR_HEADER(GuardUnboxedExpando)
+
+    explicit LGuardUnboxedExpando(const LAllocation& in) {
+        setOperand(0, in);
+    }
+    const LAllocation* object() {
+        return getOperand(0);
+    }
+    const MGuardUnboxedExpando* mir() const {
+        return mir_->toGuardUnboxedExpando();
+    }
+};
+
+class LLoadUnboxedExpando : public LInstructionHelper<1, 1, 0>
+{
+  public:
+    LIR_HEADER(LoadUnboxedExpando)
+
+    explicit LLoadUnboxedExpando(const LAllocation& in) {
+        setOperand(0, in);
+    }
+    const LAllocation* object() {
+        return getOperand(0);
+    }
+    const MLoadUnboxedExpando* mir() const {
+        return mir_->toLoadUnboxedExpando();
+    }
+};
+
 // Guard that a value is in a TypeSet.
 class LTypeBarrierV : public LInstructionHelper<0, BOX_PIECES, 1>
 {
diff --git a/js/src/jit/shared/LOpcodes-shared.h b/js/src/jit/shared/LOpcodes-shared.h
index 56b98940a..ea185e1b8 100644
--- a/js/src/jit/shared/LOpcodes-shared.h
+++ b/js/src/jit/shared/LOpcodes-shared.h
@@ -257,6 +257,8 @@
     _(GuardObjectGroup)             \
     _(GuardObjectIdentity)          \
     _(GuardClass)                   \
+    _(GuardUnboxedExpando)          \
+    _(LoadUnboxedExpando)           \
     _(TypeBarrierV)                 \
     _(TypeBarrierO)                 \
     _(MonitorTypes)                 \
@@ -266,6 +268,10 @@
     _(PostWriteElementBarrierV)     \
     _(InitializedLength)            \
     _(SetInitializedLength)         \
+    _(UnboxedArrayLength)           \
+    _(UnboxedArrayInitializedLength) \
+    _(IncrementUnboxedArrayInitializedLength) \
+    _(SetUnboxedArrayInitializedLength) \
     _(BoundsCheck)                  \
     _(BoundsCheckRange)             \
     _(BoundsCheckLower)             \
@@ -280,6 +286,7 @@
     _(StoreElementT)                \
     _(StoreUnboxedScalar)           \
     _(StoreUnboxedPointer)          \
+    _(ConvertUnboxedObjectToNative) \
     _(ArrayPopShiftV)               \
     _(ArrayPopShiftT)               \
     _(ArrayPushV)                   \