Revert #1137 - Remove unboxed arrays

- accounting for removal of watch()/unwatch()
- updated for intermediate code changes.

59 files changed, 3543 insertions, 630 deletions

diff --git a/js/src/builtin/ModuleObject.cpp b/js/src/builtin/ModuleObject.cpp
index 575bab0b0c..b275cb9681 100644
--- a/js/src/builtin/ModuleObject.cpp
+++ b/js/src/builtin/ModuleObject.cpp
@@ -927,7 +927,7 @@ ModuleObject::evaluate(JSContext* cx, HandleModuleObject self, MutableHandleValu
 ModuleObject::createNamespace(JSContext* cx, HandleModuleObject self, HandleObject exports)
 {
     MOZ_ASSERT(!self->namespace_());
-    MOZ_ASSERT(exports->is<ArrayObject>());
+    MOZ_ASSERT(exports->is<ArrayObject>() || exports->is<UnboxedArrayObject>());
 
     RootedModuleNamespaceObject ns(cx, ModuleNamespaceObject::create(cx, self));
     if (!ns)
diff --git a/js/src/frontend/BytecodeEmitter.cpp b/js/src/frontend/BytecodeEmitter.cpp
index f4574b2489..8cd06feac7 100644
--- a/js/src/frontend/BytecodeEmitter.cpp
+++ b/js/src/frontend/BytecodeEmitter.cpp
@@ -6495,8 +6495,8 @@ ParseNode::getConstantValue(ExclusiveContext* cx, AllowConstantObjects allowObje
         }
         MOZ_ASSERT(idx == count);
 
-        ArrayObject* obj = ObjectGroup::newArrayObject(cx, values.begin(), values.length(),
-                                                       newKind, arrayKind);
+        JSObject* obj = ObjectGroup::newArrayObject(cx, values.begin(), values.length(),
+                                                    newKind, arrayKind);
         if (!obj)
             return false;
 
@@ -9623,7 +9623,7 @@ BytecodeEmitter::emitCallOrNew(ParseNode* pn, ValueUsage valueUsage /* = ValueUs
                 return false;
         }
 
-        if (!emitArray(args, argc))
+        if (!emitArray(args, argc, JSOP_SPREADCALLARRAY))
             return false;
 
         if (optCodeEmitted) {
@@ -10138,11 +10138,11 @@ BytecodeEmitter::emitArrayLiteral(ParseNode* pn)
         }
     }
 
-    return emitArray(pn->pn_head, pn->pn_count);
+    return emitArray(pn->pn_head, pn->pn_count, JSOP_NEWARRAY);
 }
 
 bool
-BytecodeEmitter::emitArray(ParseNode* pn, uint32_t count)
+BytecodeEmitter::emitArray(ParseNode* pn, uint32_t count, JSOp op)
 {
 
     /*
@@ -10153,6 +10153,7 @@ BytecodeEmitter::emitArray(ParseNode* pn, uint32_t count)
      * to avoid dup'ing and popping the array as each element is added, as
      * JSOP_SETELEM/JSOP_SETPROP would do.
      */
+    MOZ_ASSERT(op == JSOP_NEWARRAY || op == JSOP_SPREADCALLARRAY);
 
     uint32_t nspread = 0;
     for (ParseNode* elt = pn; elt; elt = elt->pn_next) {
@@ -10173,7 +10174,7 @@ BytecodeEmitter::emitArray(ParseNode* pn, uint32_t count)
 
     // For arrays with spread, this is a very pessimistic allocation, the
     // minimum possible final size.
-    if (!emitUint32Operand(JSOP_NEWARRAY, count - nspread))         // ARRAY
+    if (!emitUint32Operand(op, count - nspread))                    // ARRAY
         return false;
 
     ParseNode* pn2 = pn;
@@ -11314,8 +11315,8 @@ BytecodeEmitter::setSrcNoteOffset(unsigned index, unsigned which, ptrdiff_t offs
         /* Maybe this offset was already set to a four-byte value. */
         if (!(*sn & SN_4BYTE_OFFSET_FLAG)) {
             /* Insert three dummy bytes that will be overwritten shortly. */
-            if (MOZ_UNLIKELY(notes.length() + 3 > MaxSrcNotesLength)) {

-                ReportAllocationOverflow(cx);

+            if (MOZ_UNLIKELY(notes.length() + 3 > MaxSrcNotesLength)) {
+                ReportAllocationOverflow(cx);
                 return false;
             }
             jssrcnote dummy = 0;
diff --git a/js/src/frontend/BytecodeEmitter.h b/js/src/frontend/BytecodeEmitter.h
index 8ad409c11e..77f77599c3 100644
--- a/js/src/frontend/BytecodeEmitter.h
+++ b/js/src/frontend/BytecodeEmitter.h
@@ -543,7 +543,7 @@ struct MOZ_STACK_CLASS BytecodeEmitter
     MOZ_MUST_USE bool emitAtomOp(ParseNode* pn, JSOp op);
 
     MOZ_MUST_USE bool emitArrayLiteral(ParseNode* pn);
-    MOZ_MUST_USE bool emitArray(ParseNode* pn, uint32_t count);
+    MOZ_MUST_USE bool emitArray(ParseNode* pn, uint32_t count, JSOp op);
     MOZ_MUST_USE bool emitArrayComp(ParseNode* pn);
 
     MOZ_MUST_USE bool emitInternedScopeOp(uint32_t index, JSOp op);
diff --git a/js/src/gc/Marking.cpp b/js/src/gc/Marking.cpp
index 262fc8cbcc..a206a87626 100644
--- a/js/src/gc/Marking.cpp
+++ b/js/src/gc/Marking.cpp
@@ -2504,6 +2504,8 @@ js::TenuringTracer::moveObjectToTenured(JSObject* dst, JSObject* src, AllocKind
         InlineTypedObject::objectMovedDuringMinorGC(this, dst, src);
     } else if (src->is<TypedArrayObject>()) {
         tenuredSize += TypedArrayObject::objectMovedDuringMinorGC(this, dst, src, dstKind);
+    } else if (src->is<UnboxedArrayObject>()) {
+        tenuredSize += UnboxedArrayObject::objectMovedDuringMinorGC(this, dst, src, dstKind);
     } else if (src->is<ArgumentsObject>()) {
         tenuredSize += ArgumentsObject::objectMovedDuringMinorGC(this, dst, src);
     } else if (src->is<ProxyObject>()) {
diff --git a/js/src/jit/AliasAnalysisShared.cpp b/js/src/jit/AliasAnalysisShared.cpp
index 400626b336..1a643698f9 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:
diff --git a/js/src/jit/BaselineCacheIR.cpp b/js/src/jit/BaselineCacheIR.cpp
index 7fb5868115..bf96932d1e 100644
--- a/js/src/jit/BaselineCacheIR.cpp
+++ b/js/src/jit/BaselineCacheIR.cpp
@@ -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;
@@ -1001,6 +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 d254b9826f..ae5a2e666f 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 a200f7ab9f..7b1af092ae 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 1b98325b79..9c8cd98350 100644
--- a/js/src/jit/BaselineIC.cpp
+++ b/js/src/jit/BaselineIC.cpp
@@ -1375,7 +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;
@@ -1479,6 +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() &&
@@ -2085,6 +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
 //
 
@@ -2388,8 +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;
@@ -2398,6 +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.
@@ -2479,8 +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) {
@@ -2818,6 +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);
@@ -3011,6 +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);
@@ -5374,6 +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)
@@ -5405,7 +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;
         }
     }
 
@@ -5430,7 +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;
             }
         }
     }
@@ -5447,7 +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) {
@@ -5760,24 +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;
 }
@@ -5808,22 +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;
@@ -6711,7 +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
@@ -8188,6 +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)
@@ -8563,8 +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 a1291a3bbf..5600f816ab 100644
--- a/js/src/jit/BaselineIC.h
+++ b/js/src/jit/BaselineIC.h
@@ -892,6 +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
 {
@@ -1067,7 +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) {
@@ -1175,7 +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>
@@ -2822,10 +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)
@@ -2852,7 +2904,7 @@ class ICCall_StringSplit : public ICMonitoredStub
         return expectedSep_;
     }
 
-    GCPtrArrayObject& templateObject() {
+    GCPtrObject& templateObject() {
         return templateObject_;
     }
 
@@ -2862,7 +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);
 
@@ -2873,13 +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 bcb527516c..9c7b88fb22 100644
--- a/js/src/jit/BaselineInspector.cpp
+++ b/js/src/jit/BaselineInspector.cpp
@@ -580,7 +580,7 @@ BaselineInspector::getTemplateObjectForNative(jsbytecode* pc, Native native)
 
 bool
 BaselineInspector::isOptimizableCallStringSplit(jsbytecode* pc, JSString** strOut, JSString** sepOut,
-                                                ArrayObject** objOut)
+                                                JSObject** objOut)
 {
     if (!hasBaselineScript())
         return false;
diff --git a/js/src/jit/BaselineInspector.h b/js/src/jit/BaselineInspector.h
index 1ed4b55475..4a17917982 100644
--- a/js/src/jit/BaselineInspector.h
+++ b/js/src/jit/BaselineInspector.h
@@ -113,7 +113,7 @@ 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);
diff --git a/js/src/jit/CacheIR.cpp b/js/src/jit/CacheIR.cpp
index 6822a70afe..f1061af701 100644
--- a/js/src/jit/CacheIR.cpp
+++ b/js/src/jit/CacheIR.cpp
@@ -175,7 +175,7 @@ TestMatchingReceiver(CacheIRWriter& writer, JSObject* obj, Shape* shape, ObjOper
         } else {
             writer.guardNoUnboxedExpando(objId);
         }
-    } else if (obj->is<TypedObject>()) {
+    } else if (obj->is<UnboxedArrayObject>() || obj->is<TypedObject>()) {
         writer.guardGroup(objId, obj->group());
     } else {
         Shape* shape = obj->maybeShape();
@@ -368,6 +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 4fd8575f01..51e55f48b8 100644
--- a/js/src/jit/CacheIR.h
+++ b/js/src/jit/CacheIR.h
@@ -96,6 +96,7 @@ class ObjOperandId : public OperandId
     _(LoadUnboxedPropertyResult)          \
     _(LoadTypedObjectResult)              \
     _(LoadInt32ArrayLengthResult)         \
+    _(LoadUnboxedArrayLengthResult)       \
     _(LoadArgumentsObjectLengthResult)    \
     _(LoadUndefinedResult)
 
@@ -127,6 +128,7 @@ struct StubField {
 enum class GuardClassKind
 {
     Array,
+    UnboxedArray,
     MappedArguments,
     UnmappedArguments,
 };
@@ -325,6 +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);
     }
diff --git a/js/src/jit/CodeGenerator.cpp b/js/src/jit/CodeGenerator.cpp
index 901e9ea939..c3e2429913 100644
--- a/js/src/jit/CodeGenerator.cpp
+++ b/js/src/jit/CodeGenerator.cpp
@@ -3184,7 +3184,9 @@ 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()));
 }
 
@@ -5169,11 +5171,11 @@ 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;
 }
 
@@ -5319,7 +5321,7 @@ 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");
 
@@ -5339,11 +5341,21 @@ 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) {
@@ -7765,7 +7777,7 @@ 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");
 
@@ -7800,6 +7812,49 @@ 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(),
@@ -8095,19 +8150,46 @@ 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());
 
-    masm.bind(ool->rejoinStore());
-    emitStoreElementTyped(lir->value(), lir->mir()->value()->type(), lir->mir()->elementType(),
-                          elements, index, 0);
+        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());
+            masm.storeUnboxedProperty(address, unboxedType, v, nullptr);
+        }
+    }
 
     masm.bind(ool->rejoin());
 }
@@ -8127,22 +8209,47 @@ 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());
 }
@@ -8213,10 +8320,11 @@ 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)
@@ -8226,6 +8334,8 @@ CodeGenerator::visitOutOfLineStoreElementHole(OutOfLineStoreElementHole* ool)
     const LAllocation* index;
     MIRType valueType;
     ConstantOrRegister value;
+    JSValueType unboxedType;
+    LDefinition *temp = nullptr;
 
     if (ins->isStoreElementHoleV()) {
         LStoreElementHoleV* store = ins->toStoreElementHoleV();
@@ -8234,6 +8344,8 @@ 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());
@@ -8241,6 +8353,8 @@ 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());
@@ -8251,6 +8365,8 @@ 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());
@@ -8261,6 +8377,8 @@ 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);
@@ -8271,32 +8389,54 @@ 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
@@ -8325,7 +8465,7 @@ CodeGenerator::visitOutOfLineStoreElementHole(OutOfLineStoreElementHole* ool)
     else
         pushArg(ToRegister(index));
     pushArg(object);
-    callVM(SetDenseElementInfo, ins);
+    callVM(SetDenseOrUnboxedArrayElementInfo, ins);
 
     restoreLive(ins);
     masm.jump(ool->rejoin());
@@ -8386,6 +8526,9 @@ 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");
 
 typedef bool (*ArrayPopShiftFn)(JSContext*, HandleObject, MutableHandleValue);
 static const VMFunction ArrayPopDenseInfo =
@@ -8411,11 +8554,20 @@ 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
@@ -8427,10 +8579,13 @@ 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);
@@ -8442,25 +8597,41 @@ 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.
@@ -8499,7 +8670,7 @@ 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");
 
@@ -8510,27 +8681,50 @@ 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);
+
+        // Guard length == initializedLength.
+        Address initLength(elementsTemp, ObjectElements::offsetOfInitializedLength());
+        masm.branch32(Assembler::NotEqual, initLength, key, ool->entry());
 
-    // Load elements and length.
-    masm.loadPtr(Address(obj, NativeObject::offsetOfElements()), elementsTemp);
-    masm.load32(Address(elementsTemp, ObjectElements::offsetOfLength()), length);
+        // Guard length < capacity.
+        Address capacity(elementsTemp, ObjectElements::offsetOfCapacity());
+        masm.branch32(Assembler::BelowOrEqual, capacity, key, ool->entry());
 
-    // Guard length == initializedLength.
-    Address initLength(elementsTemp, ObjectElements::offsetOfInitializedLength());
-    masm.branch32(Assembler::NotEqual, initLength, 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);
 
-    // Guard length < capacity.
-    Address capacity(elementsTemp, ObjectElements::offsetOfCapacity());
-    masm.branch32(Assembler::BelowOrEqual, capacity, key, ool->entry());
+        // Guard length == initializedLength.
+        Address lengthAddr(obj, UnboxedArrayObject::offsetOfLength());
+        masm.branch32(Assembler::NotEqual, lengthAddr, key, ool->entry());
 
-    // Do the store.
-    masm.storeConstantOrRegister(value, BaseIndex(elementsTemp, length, TimesEight));
+        // 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());
 }
@@ -10357,11 +10551,22 @@ 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.
@@ -10739,7 +10944,7 @@ 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);
@@ -10752,7 +10957,7 @@ 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 65acfe2740..bc8fcccea2 100644
--- a/js/src/jit/CodeGenerator.h
+++ b/js/src/jit/CodeGenerator.h
@@ -234,6 +234,10 @@ 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);
diff --git a/js/src/jit/IonBuilder.cpp b/js/src/jit/IonBuilder.cpp
index f00167d929..a5991cc7bd 100644
--- a/js/src/jit/IonBuilder.cpp
+++ b/js/src/jit/IonBuilder.cpp
@@ -2227,8 +2227,6 @@ 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;
     }
 
@@ -7355,6 +7353,12 @@ 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 +7614,7 @@ 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 +7625,12 @@ 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 +7650,60 @@ 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;
@@ -8167,7 +8192,8 @@ IonBuilder::maybeMarkEmpty(MDefinition* ins)
 static bool
 ClassHasEffectlessLookup(const Class* clasp)
 {
-    return IsTypedObjectClass(clasp) ||
+    return (clasp == &UnboxedArrayObject::class_) ||
+           IsTypedObjectClass(clasp) ||
            (clasp->isNative() && !clasp->getOpsLookupProperty());
 }
 
@@ -9441,9 +9467,12 @@ 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 +9489,7 @@ 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 +9841,7 @@ 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 +9865,7 @@ 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 +9874,13 @@ 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 +9896,7 @@ IonBuilder::jsop_getelem_dense(MDefinition* obj, MDefinition* index)
     }
 
     bool loadDouble =
+        unboxedType == JSVAL_TYPE_MAGIC &&
         barrier == BarrierKind::NoBarrier &&
         loopDepth_ &&
         inBounds &&
@@ -9885,13 +9915,18 @@ 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 +9936,8 @@ 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);
     }
 
@@ -10348,9 +10384,12 @@ 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 +10423,7 @@ 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 +10513,13 @@ 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 +10549,7 @@ 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 +10586,7 @@ 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 +10598,27 @@ 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 +10736,18 @@ 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()) {
@@ -13675,8 +13732,11 @@ 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;
@@ -13691,10 +13751,10 @@ 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_) {
@@ -13704,7 +13764,8 @@ 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);
@@ -14382,24 +14443,32 @@ 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 78af0e412e..1f84b45df9 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);
@@ -610,6 +611,7 @@ 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 +724,15 @@ 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);
diff --git a/js/src/jit/IonCaches.cpp b/js/src/jit/IonCaches.cpp
index f5e4659c1a..c2dc573738 100644
--- a/js/src/jit/IonCaches.cpp
+++ b/js/src/jit/IonCaches.cpp
@@ -639,6 +639,9 @@ TestMatchingReceiver(MacroAssembler& masm, IonCache::StubAttacher& attacher,
         } 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()),
@@ -1185,6 +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.
@@ -1559,6 +1595,40 @@ GetPropertyIC::tryAttachUnboxedExpando(JSContext* cx, HandleScript outerScript,
 }
 
 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)
 {
@@ -2133,6 +2203,9 @@ GetPropertyIC::tryAttachStub(JSContext* cx, HandleScript outerScript, IonScript*
         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;
     }
@@ -3953,7 +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());
@@ -3984,6 +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();
 }
 
@@ -4060,27 +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/Lowering.cpp b/js/src/jit/Lowering.cpp
index 1084509833..c3bd477441 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;
     }
 
diff --git a/js/src/jit/Lowering.h b/js/src/jit/Lowering.h
index 81e6abbbbd..bb06baa294 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);
diff --git a/js/src/jit/MCallOptimize.cpp b/js/src/jit/MCallOptimize.cpp
index a1c336391b..276b5eba50 100644
--- a/js/src/jit/MCallOptimize.cpp
+++ b/js/src/jit/MCallOptimize.cpp
@@ -471,6 +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();
@@ -518,7 +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.
@@ -533,15 +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;
     }
@@ -574,7 +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));
@@ -610,7 +616,7 @@ IonBuilder::inlineArrayPopShift(CallInfo& callInfo, MArrayPopShift::Mode mode)
     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);
@@ -623,9 +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);
@@ -636,7 +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);
@@ -718,6 +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 ||
@@ -728,12 +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);
 
@@ -774,9 +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);
@@ -797,8 +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();
 
@@ -817,12 +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);
 
@@ -1341,7 +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;
 
@@ -1367,13 +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)
@@ -1404,6 +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.
@@ -1414,11 +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;
 
diff --git a/js/src/jit/MIR.cpp b/js/src/jit/MIR.cpp
index 1e4ee170f1..403e70c024 100644
--- a/js/src/jit/MIR.cpp
+++ b/js/src/jit/MIR.cpp
@@ -5789,6 +5789,46 @@ 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;
+    }
+
+    return elementType;
+}
+
 bool
 jit::ElementAccessIsTypedArray(CompilerConstraintList* constraints,
                                MDefinition* obj, MDefinition* id,
@@ -5948,6 +5988,11 @@ 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;
 }
 
diff --git a/js/src/jit/MIR.h b/js/src/jit/MIR.h
index cafdbab714..ebc98a4f80 100644
--- a/js/src/jit/MIR.h
+++ b/js/src/jit/MIR.h
@@ -375,7 +375,8 @@ 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 or unboxed 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 +433,9 @@ 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;
@@ -8758,6 +8762,102 @@ 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,
@@ -9251,19 +9351,23 @@ 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)
     {
@@ -9285,6 +9389,9 @@ class MLoadElementHole
     TRIVIAL_NEW_WRAPPERS
     NAMED_OPERANDS((0, elements), (1, index), (2, initLength))
 
+    JSValueType unboxedType() const {
+        return unboxedType_;
+    }
     bool needsNegativeIntCheck() const {
         return needsNegativeIntCheck_;
     }
@@ -9295,6 +9402,8 @@ 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())
@@ -9302,7 +9411,7 @@ class MLoadElementHole
         return congruentIfOperandsEqual(other);
     }
     AliasSet getAliasSet() const override {
-        return AliasSet::Load(AliasSet::Element);
+        return AliasSet::Load(AliasSet::BoxedOrUnboxedElements(unboxedType()));
     }
     void collectRangeInfoPreTrunc() override;
 
@@ -9482,17 +9591,20 @@ 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);
@@ -9507,6 +9619,10 @@ class MStoreElementHole
     TRIVIAL_NEW_WRAPPERS
     NAMED_OPERANDS((0, object), (1, elements), (2, index), (3, value))
 
+    JSValueType unboxedType() const {
+        return unboxedType_;
+    }
+
     ALLOW_CLONE(MStoreElementHole)
 };
 
@@ -9517,11 +9633,13 @@ 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);
@@ -9537,6 +9655,10 @@ 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_;
     }
@@ -9640,12 +9762,13 @@ 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)
     { }
 
@@ -9663,8 +9786,12 @@ 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)
@@ -9675,8 +9802,10 @@ 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);
     }
@@ -9686,8 +9815,12 @@ 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;
 
@@ -9701,13 +9834,15 @@ 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);
     }
@@ -9725,6 +9860,10 @@ class MArraySlice
         return initialHeap_;
     }
 
+    JSValueType unboxedType() const {
+        return unboxedType_;
+    }
+
     bool possiblyCalls() const override {
         return true;
     }
@@ -12093,13 +12232,15 @@ 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();
@@ -12119,6 +12260,9 @@ class MInArray
     bool needsNegativeIntCheck() const {
         return needsNegativeIntCheck_;
     }
+    JSValueType unboxedType() const {
+        return unboxedType_;
+    }
     void collectRangeInfoPreTrunc() override;
     AliasSet getAliasSet() const override {
         return AliasSet::Load(AliasSet::Element);
@@ -12131,6 +12275,8 @@ class MInArray
             return false;
         if (needsNegativeIntCheck() != other->needsNegativeIntCheck())
             return false;
+        if (unboxedType() != other->unboxedType())
+            return false;
         return congruentIfOperandsEqual(other);
     }
 };
@@ -14031,6 +14177,8 @@ 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 589dde0774..f5f59dee6e 100644
--- a/js/src/jit/MOpcodes.h
+++ b/js/src/jit/MOpcodes.h
@@ -199,6 +199,10 @@ namespace jit {
     _(SetTypedObjectOffset)                                                 \
     _(InitializedLength)                                                    \
     _(SetInitializedLength)                                                 \
+    _(UnboxedArrayLength)                                                   \
+    _(UnboxedArrayInitializedLength)                                        \
+    _(IncrementUnboxedArrayInitializedLength)                               \
+    _(SetUnboxedArrayInitializedLength)                                     \
     _(Not)                                                                  \
     _(BoundsCheck)                                                          \
     _(BoundsCheckLower)                                                     \
diff --git a/js/src/jit/MacroAssembler.cpp b/js/src/jit/MacroAssembler.cpp
index e50f687227..f633b9b7b9 100644
--- a/js/src/jit/MacroAssembler.cpp
+++ b/js/src/jit/MacroAssembler.cpp
@@ -705,6 +705,31 @@ 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
@@ -1256,6 +1281,16 @@ MacroAssembler::initGCThing(Register obj, Register temp, JSObject* templateObj,
         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");
     }
diff --git a/js/src/jit/MacroAssembler.h b/js/src/jit/MacroAssembler.h
index 6ee989463d..b6616321c3 100644
--- a/js/src/jit/MacroAssembler.h
+++ b/js/src/jit/MacroAssembler.h
@@ -1626,7 +1626,7 @@ class MacroAssembler : public MacroAssemblerSpecific
     void storeToTypedFloatArray(Scalar::Type arrayType, FloatRegister value, const Address& dest,
                                 unsigned numElems = 0);
 
-    // Load a property from an UnboxedPlainObject.
+    // Load a property from an UnboxedPlainObject or UnboxedArrayObject.
     template <typename T>
     void loadUnboxedProperty(T address, JSValueType type, TypedOrValueRegister output);
 
@@ -1637,6 +1637,9 @@ class MacroAssembler : public MacroAssemblerSpecific
     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);
     }
diff --git a/js/src/jit/Recover.cpp b/js/src/jit/Recover.cpp
index 8fe6ee3fb5..6fd71f3774 100644
--- a/js/src/jit/Recover.cpp
+++ b/js/src/jit/Recover.cpp
@@ -1355,7 +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;
 
diff --git a/js/src/jit/ScalarReplacement.cpp b/js/src/jit/ScalarReplacement.cpp
index be9ceee2ee..2065c0371b 100644
--- a/js/src/jit/ScalarReplacement.cpp
+++ b/js/src/jit/ScalarReplacement.cpp
@@ -795,6 +795,11 @@ 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 3139574629..2475dfb228 100644
--- a/js/src/jit/SharedIC.cpp
+++ b/js/src/jit/SharedIC.cpp
@@ -286,6 +286,11 @@ 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");
@@ -2245,6 +2250,7 @@ IsCacheableProtoChain(JSObject* obj, JSObject* holder, bool isDOMProxy)
         if (obj == holder)
             return false;
         if (!obj->is<UnboxedPlainObject>() &&
+            !obj->is<UnboxedArrayObject>() &&
             !obj->is<TypedObject>())
         {
             return false;
@@ -2576,6 +2582,9 @@ CheckHasNoSuchProperty(JSContext* cx, JSObject* obj, PropertyName* name,
         } 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;
diff --git a/js/src/jit/VMFunctions.cpp b/js/src/jit/VMFunctions.cpp
index 1cb731de85..10be2836b4 100644
--- a/js/src/jit/VMFunctions.cpp
+++ b/js/src/jit/VMFunctions.cpp
@@ -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 94f7413971..7f225c2933 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 ff4915d1a7..49879eedb4 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 {
diff --git a/js/src/jit/shared/LOpcodes-shared.h b/js/src/jit/shared/LOpcodes-shared.h
index 56b98940af..505c0ea035 100644
--- a/js/src/jit/shared/LOpcodes-shared.h
+++ b/js/src/jit/shared/LOpcodes-shared.h
@@ -266,6 +266,10 @@
     _(PostWriteElementBarrierV)     \
     _(InitializedLength)            \
     _(SetInitializedLength)         \
+    _(UnboxedArrayLength)           \
+    _(UnboxedArrayInitializedLength) \
+    _(IncrementUnboxedArrayInitializedLength) \
+    _(SetUnboxedArrayInitializedLength) \
     _(BoundsCheck)                  \
     _(BoundsCheckRange)             \
     _(BoundsCheckLower)             \
diff --git a/js/src/jsapi.h b/js/src/jsapi.h
index 7795c5e4ca..0e2eeebaa3 100644
--- a/js/src/jsapi.h
+++ b/js/src/jsapi.h
@@ -1096,6 +1096,7 @@ class JS_PUBLIC_API(ContextOptions) {
         wasmAlwaysBaseline_(false),
         throwOnAsmJSValidationFailure_(false),
         nativeRegExp_(true),
+        unboxedArrays_(false),
         asyncStack_(true),
         throwOnDebuggeeWouldRun_(true),
         dumpStackOnDebuggeeWouldRun_(false),
@@ -1172,6 +1173,12 @@ class JS_PUBLIC_API(ContextOptions) {
         return *this;
     }
 
+    bool unboxedArrays() const { return unboxedArrays_; }
+    ContextOptions& setUnboxedArrays(bool flag) {
+        unboxedArrays_ = flag;
+        return *this;
+    }
+
     bool asyncStack() const { return asyncStack_; }
     ContextOptions& setAsyncStack(bool flag) {
         asyncStack_ = flag;
@@ -1234,6 +1241,7 @@ class JS_PUBLIC_API(ContextOptions) {
     bool wasmAlwaysBaseline_ : 1;
     bool throwOnAsmJSValidationFailure_ : 1;
     bool nativeRegExp_ : 1;
+    bool unboxedArrays_ : 1;
     bool asyncStack_ : 1;
     bool throwOnDebuggeeWouldRun_ : 1;
     bool dumpStackOnDebuggeeWouldRun_ : 1;
diff --git a/js/src/jsarray.cpp b/js/src/jsarray.cpp
index 4ee967d4c8..1724a0a667 100644
--- a/js/src/jsarray.cpp
+++ b/js/src/jsarray.cpp
@@ -64,7 +64,7 @@ using JS::ToUint32;
 bool
 JS::IsArray(JSContext* cx, HandleObject obj, IsArrayAnswer* answer)
 {
-    if (obj->is<ArrayObject>()) {
+    if (obj->is<ArrayObject>() || obj->is<UnboxedArrayObject>()) {
         *answer = IsArrayAnswer::Array;
         return true;
     }
@@ -100,6 +100,11 @@ js::GetLengthProperty(JSContext* cx, HandleObject obj, uint32_t* lengthp)
         return true;
     }
 
+    if (obj->is<UnboxedArrayObject>()) {
+        *lengthp = obj->as<UnboxedArrayObject>().length();
+        return true;
+    }
+
     if (obj->is<ArgumentsObject>()) {
         ArgumentsObject& argsobj = obj->as<ArgumentsObject>();
         if (!argsobj.hasOverriddenLength()) {
@@ -248,20 +253,18 @@ static bool
 GetElement(JSContext* cx, HandleObject obj, HandleObject receiver,
            uint32_t index, bool* hole, MutableHandleValue vp)
 {
-    if (obj->isNative()) {
-        NativeObject* nobj = &obj->as<NativeObject>();
-        if (index < nobj->getDenseInitializedLength()) {
-            vp.set(nobj->getDenseElement(size_t(index)));
-            if (!vp.isMagic(JS_ELEMENTS_HOLE)) {
-                *hole = false;
-                return true;
-            }
+    AssertGreaterThanZero(index);
+    if (index < GetAnyBoxedOrUnboxedInitializedLength(obj)) {
+        vp.set(GetAnyBoxedOrUnboxedDenseElement(obj, uint32_t(index)));
+        if (!vp.isMagic(JS_ELEMENTS_HOLE)) {
+            *hole = false;
+            return true;
         }
-        if (nobj->is<ArgumentsObject>() && index <= UINT32_MAX) {
-            if (nobj->as<ArgumentsObject>().maybeGetElement(uint32_t(index), vp)) {
-                *hole = false;
-                return true;
-            }
+    }
+    if (obj->is<ArgumentsObject>()) {
+        if (obj->as<ArgumentsObject>().maybeGetElement(uint32_t(index), vp)) {
+            *hole = false;
+            return true;
         }
     }
 
@@ -280,8 +283,8 @@ ElementAdder::append(JSContext* cx, HandleValue v)
 {
     MOZ_ASSERT(index_ < length_);
     if (resObj_) {
-        NativeObject* resObj = &resObj_->as<NativeObject>();
-        DenseElementResult result = resObj->setOrExtendDenseElements(cx, index_, v.address(), 1);
+        DenseElementResult result =
+            SetOrExtendAnyBoxedOrUnboxedDenseElements(cx, resObj_, index_, v.address(), 1);
         if (result == DenseElementResult::Failure)
             return false;
         if (result == DenseElementResult::Incomplete) {
@@ -333,31 +336,37 @@ js::GetElementsWithAdder(JSContext* cx, HandleObject obj, HandleObject receiver,
     return true;
 }
 
-static bool
-GetDenseElements(NativeObject* aobj, uint32_t length, Value* vp)
+template <JSValueType Type>
+DenseElementResult
+GetBoxedOrUnboxedDenseElements(JSObject* aobj, uint32_t length, Value* vp)
 {
     MOZ_ASSERT(!ObjectMayHaveExtraIndexedProperties(aobj));
 
-    if (length > aobj->getDenseInitializedLength())
-        return false;
+    if (length > GetBoxedOrUnboxedInitializedLength<Type>(aobj))
+        return DenseElementResult::Incomplete;
 
     for (size_t i = 0; i < length; i++) {
-        vp[i] = aobj->getDenseElement(i);
+        vp[i] = GetBoxedOrUnboxedDenseElement<Type>(aobj, i);
 
         // No other indexed properties so hole => undefined.
         if (vp[i].isMagic(JS_ELEMENTS_HOLE))
             vp[i] = UndefinedValue();
     }
 
-    return true;
+    return DenseElementResult::Success;
 }
 
+DefineBoxedOrUnboxedFunctor3(GetBoxedOrUnboxedDenseElements,
+                             JSObject*, uint32_t, Value*);
+
 bool
 js::GetElements(JSContext* cx, HandleObject aobj, uint32_t length, Value* vp)
 {
     if (!ObjectMayHaveExtraIndexedProperties(aobj)) {
-        if (GetDenseElements(&aobj->as<NativeObject>(), length, vp))
-            return true;
+        GetBoxedOrUnboxedDenseElementsFunctor functor(aobj, length, vp);
+        DenseElementResult result = CallBoxedOrUnboxedSpecialization(functor, aobj);
+        if (result != DenseElementResult::Incomplete)
+            return result == DenseElementResult::Success;
     }
 
     if (aobj->is<ArgumentsObject>()) {
@@ -389,9 +398,9 @@ SetArrayElement(JSContext* cx, HandleObject obj, double index, HandleValue v)
 {
     MOZ_ASSERT(index >= 0);
 
-    if (obj->is<ArrayObject>() && !obj->isIndexed() && index <= UINT32_MAX) {
-        NativeObject* nobj = &obj->as<NativeObject>();
-        DenseElementResult result = nobj->setOrExtendDenseElements(cx, uint32_t(index), v.address(), 1);
+    if ((obj->is<ArrayObject>() || obj->is<UnboxedArrayObject>()) && !obj->isIndexed() && index <= UINT32_MAX) {
+        DenseElementResult result =
+            SetOrExtendAnyBoxedOrUnboxedDenseElements(cx, obj, uint32_t(index), v.address(), 1);
         if (result != DenseElementResult::Incomplete)
             return result == DenseElementResult::Success;
     }
@@ -511,6 +520,24 @@ struct ReverseIndexComparator
     }
 };
 
+bool
+js::CanonicalizeArrayLengthValue(JSContext* cx, HandleValue v, uint32_t* newLen)
+{
+    double d;
+
+    if (!ToUint32(cx, v, newLen))
+        return false;
+
+    if (!ToNumber(cx, v, &d))
+        return false;
+
+    if (d == *newLen)
+        return true;
+
+    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, JSMSG_BAD_ARRAY_LENGTH);
+    return false;
+}
+
 /* ES6 draft rev 34 (2015 Feb 20) 9.4.2.4 ArraySetLength */
 bool
 js::ArraySetLength(JSContext* cx, Handle<ArrayObject*> arr, HandleId id,
@@ -532,22 +559,12 @@ js::ArraySetLength(JSContext* cx, Handle<ArrayObject*> arr, HandleId id,
     } else {
         // Step 2 is irrelevant in our implementation.
 
-        // Step 3.
-        if (!ToUint32(cx, value, &newLen))
-            return false;
-
-        // Step 4.
-        double d;
-        if (!ToNumber(cx, value, &d))
+        // Steps 3-7.
+        MOZ_ASSERT_IF(attrs & JSPROP_IGNORE_VALUE, value.isUndefined());
+        if (!CanonicalizeArrayLengthValue(cx, value, &newLen))
             return false;
 
-        // Step 5.
-        if (d != newLen) {
-            JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, JSMSG_BAD_ARRAY_LENGTH);
-            return false;
-        }
-
-        // Steps 6-8 are irrelevant in our implementation.
+        // Step 8 is irrelevant in our implementation.
     }
 
     // Steps 9-11.
@@ -806,7 +823,7 @@ array_addProperty(JSContext* cx, HandleObject obj, HandleId id, HandleValue v)
 static inline bool
 ObjectMayHaveExtraIndexedOwnProperties(JSObject* obj)
 {
-    return !obj->isNative() ||
+    return (!obj->isNative() && !obj->is<UnboxedArrayObject>()) ||
            obj->isIndexed() ||
            obj->is<TypedArrayObject>() ||
            ClassMayResolveId(*obj->runtimeFromAnyThread()->commonNames,
@@ -837,7 +854,7 @@ js::ObjectMayHaveExtraIndexedProperties(JSObject* obj)
 
         if (ObjectMayHaveExtraIndexedOwnProperties(obj))
             return true;
-        if (obj->as<NativeObject>().getDenseInitializedLength() != 0)
+        if (GetAnyBoxedOrUnboxedInitializedLength(obj) != 0)
             return true;
     } while (true);
 }
@@ -1047,32 +1064,32 @@ struct StringSeparatorOp
     }
 };
 
-template <typename SeparatorOp>
-static bool
-ArrayJoinDenseKernel(JSContext* cx, SeparatorOp sepOp, HandleNativeObject obj, uint32_t length,
+template <typename SeparatorOp, JSValueType Type>
+static DenseElementResult
+ArrayJoinDenseKernel(JSContext* cx, SeparatorOp sepOp, HandleObject obj, uint32_t length,
                      StringBuffer& sb, uint32_t* numProcessed)
 {
     // This loop handles all elements up to initializedLength. If
     // length > initLength we rely on the second loop to add the
     // other elements.
     MOZ_ASSERT(*numProcessed == 0);
-    uint32_t initLength = Min<uint32_t>(obj->getDenseInitializedLength(),
+    uint32_t initLength = Min<uint32_t>(GetBoxedOrUnboxedInitializedLength<Type>(obj),
                                         length);
     while (*numProcessed < initLength) {
         if (!CheckForInterrupt(cx))
-            return false;
+            return DenseElementResult::Failure;
 
         Value elem = obj->as<NativeObject>().getDenseElement(*numProcessed);
 
         if (elem.isString()) {
             if (!sb.append(elem.toString()))
-                return false;
+                return DenseElementResult::Failure;
         } else if (elem.isNumber()) {
             if (!NumberValueToStringBuffer(cx, elem, sb))
-                return false;
+                return DenseElementResult::Failure;
         } else if (elem.isBoolean()) {
             if (!BooleanToStringBuffer(elem.toBoolean(), sb))
-                return false;
+                return DenseElementResult::Failure;
         } else if (elem.isObject() || elem.isSymbol()) {
             /*
              * Object stringifying could modify the initialized length or make
@@ -1088,13 +1105,33 @@ ArrayJoinDenseKernel(JSContext* cx, SeparatorOp sepOp, HandleNativeObject obj, u
         }
 
         if (++(*numProcessed) != length && !sepOp(cx, sb))
-            return false;
+            return DenseElementResult::Failure;
     }
 
-    return true;
+    return DenseElementResult::Incomplete;
 }
 
 template <typename SeparatorOp>
+struct ArrayJoinDenseKernelFunctor {
+    JSContext* cx;
+    SeparatorOp sepOp;
+    HandleObject obj;
+    uint32_t length;
+    StringBuffer& sb;
+    uint32_t* numProcessed;
+
+    ArrayJoinDenseKernelFunctor(JSContext* cx, SeparatorOp sepOp, HandleObject obj,
+                                uint32_t length, StringBuffer& sb, uint32_t* numProcessed)
+      : cx(cx), sepOp(sepOp), obj(obj), length(length), sb(sb), numProcessed(numProcessed)
+    {}
+
+    template <JSValueType Type>
+    DenseElementResult operator()() {
+        return ArrayJoinDenseKernel<SeparatorOp, Type>(cx, sepOp, obj, length, sb, numProcessed);
+    }
+};
+
+template <typename SeparatorOp>
 static bool
 ArrayJoinKernel(JSContext* cx, SeparatorOp sepOp, HandleObject obj, uint32_t length,
                StringBuffer& sb)
@@ -1102,10 +1139,10 @@ ArrayJoinKernel(JSContext* cx, SeparatorOp sepOp, HandleObject obj, uint32_t len
     uint32_t i = 0;
 
     if (!ObjectMayHaveExtraIndexedProperties(obj)) {
-        if (!ArrayJoinDenseKernel<SeparatorOp>(cx, sepOp, obj.as<NativeObject>(), length, sb, &i))
-        {
+        ArrayJoinDenseKernelFunctor<SeparatorOp> functor(cx, sepOp, obj, length, sb, &i);
+        DenseElementResult result = CallBoxedOrUnboxedSpecialization(functor, obj);
+        if (result == DenseElementResult::Failure)
             return false;
-        }
     }
 
     if (i != length) {
@@ -1176,14 +1213,11 @@ js::array_join(JSContext* cx, unsigned argc, Value* vp)
     // An optimized version of a special case of steps 7-11: when length==1 and
     // the 0th element is a string, ToString() of that element is a no-op and
     // so it can be immediately returned as the result.
-    if (length == 1 && obj->isNative()) {
-        NativeObject* nobj = &obj->as<NativeObject>();
-        if (nobj->getDenseInitializedLength() == 1) {
-            Value elem0 = nobj->getDenseElement(0);
-            if (elem0.isString()) {
-                args.rval().set(elem0);
-                return true;
-            }
+    if (length == 1 && GetAnyBoxedOrUnboxedInitializedLength(obj) == 1) {
+        Value elem0 = GetAnyBoxedOrUnboxedDenseElement(obj, 0);
+        if (elem0.isString()) {
+            args.rval().set(elem0);
+            return true;
         }
     }
 
@@ -1226,7 +1260,7 @@ js::array_join(JSContext* cx, unsigned argc, Value* vp)
     }
 
     // Step 11
-    JSString* str = sb.finishString();
+    JSString *str = sb.finishString();
     if (!str)
         return false;
 
@@ -1255,6 +1289,10 @@ array_toLocaleString(JSContext* cx, unsigned argc, Value* vp)
         args.rval().setString(cx->names().empty);
         return true;
     }
+    if (obj->is<UnboxedArrayObject>() && obj->as<UnboxedArrayObject>().length() == 0) {
+        args.rval().setString(cx->names().empty);
+        return true;
+    }
 
     AutoCycleDetector detector(cx, obj);
     if (!detector.init())
@@ -1291,9 +1329,8 @@ InitArrayElements(JSContext* cx, HandleObject obj, uint32_t start,
         return false;
 
     if (!ObjectMayHaveExtraIndexedProperties(obj)) {
-        NativeObject* nobj = &obj->as<NativeObject>();
-        DenseElementResult result = nobj->setOrExtendDenseElements(cx, uint32_t(start), vector,
-                                                                   count, updateTypes);
+        DenseElementResult result =
+            SetOrExtendAnyBoxedOrUnboxedDenseElements(cx, obj, start, vector, count, updateTypes);
         if (result != DenseElementResult::Incomplete)
             return result == DenseElementResult::Success;
     }
@@ -1327,45 +1364,54 @@ InitArrayElements(JSContext* cx, HandleObject obj, uint32_t start,
     return true;
 }
 
-static DenseElementResult
-ArrayReverseDenseKernel(JSContext* cx, HandleNativeObject obj, uint32_t length)
+template <JSValueType Type>
+DenseElementResult
+ArrayReverseDenseKernel(JSContext* cx, HandleObject obj, uint32_t length)
 {
     /* An empty array or an array with no elements is already reversed. */
-    if (length == 0 || obj->getDenseInitializedLength() == 0)
+    if (length == 0 || GetBoxedOrUnboxedInitializedLength<Type>(obj) == 0)
         return DenseElementResult::Success;
 
-    if (obj->denseElementsAreFrozen())
-        return DenseElementResult::Incomplete;
+    if (Type == JSVAL_TYPE_MAGIC) {
+        if (obj->as<NativeObject>().denseElementsAreFrozen())
+            return DenseElementResult::Incomplete;
 
-    /*
-     * It's actually surprisingly complicated to reverse an array due to the
-     * orthogonality of array length and array capacity while handling
-     * leading and trailing holes correctly.  Reversing seems less likely to
-     * be a common operation than other array mass-mutation methods, so for
-     * now just take a probably-small memory hit (in the absence of too many
-     * holes in the array at its start) and ensure that the capacity is
-     * sufficient to hold all the elements in the array if it were full.
-     */
-    DenseElementResult result = obj->ensureDenseElements(cx, length, 0);
-    if (result != DenseElementResult::Success)
-        return result;
+        /*
+         * It's actually surprisingly complicated to reverse an array due to the
+         * orthogonality of array length and array capacity while handling
+         * leading and trailing holes correctly.  Reversing seems less likely to
+         * be a common operation than other array mass-mutation methods, so for
+         * now just take a probably-small memory hit (in the absence of too many
+         * holes in the array at its start) and ensure that the capacity is
+         * sufficient to hold all the elements in the array if it were full.
+         */
+        DenseElementResult result = obj->as<NativeObject>().ensureDenseElements(cx, length, 0);
+        if (result != DenseElementResult::Success)
+            return result;
 
-    /* Fill out the array's initialized length to its proper length. */
-    obj->ensureDenseInitializedLength(cx, length, 0);
+        /* Fill out the array's initialized length to its proper length. */
+        obj->as<NativeObject>().ensureDenseInitializedLength(cx, length, 0);
+    } else {
+        // Unboxed arrays can only be reversed here if their initialized length
+        // matches their actual length. Otherwise the reversal will place holes
+        // at the beginning of the array, which we don't support.
+        if (length != obj->as<UnboxedArrayObject>().initializedLength())
+            return DenseElementResult::Incomplete;
+    }
 
     RootedValue origlo(cx), orighi(cx);
 
     uint32_t lo = 0, hi = length - 1;
     for (; lo < hi; lo++, hi--) {
-        origlo = obj->getDenseElement(lo);
-        orighi = obj->getDenseElement(hi);
-        obj->setDenseElement(lo, orighi);
+        origlo = GetBoxedOrUnboxedDenseElement<Type>(obj, lo);
+        orighi = GetBoxedOrUnboxedDenseElement<Type>(obj, hi);
+        SetBoxedOrUnboxedDenseElementNoTypeChange<Type>(obj, lo, orighi);
         if (orighi.isMagic(JS_ELEMENTS_HOLE) &&
             !SuppressDeletedProperty(cx, obj, INT_TO_JSID(lo)))
         {
             return DenseElementResult::Failure;
         }
-        obj->setDenseElement(hi, origlo);
+        SetBoxedOrUnboxedDenseElementNoTypeChange<Type>(obj, hi, origlo);
         if (origlo.isMagic(JS_ELEMENTS_HOLE) &&
             !SuppressDeletedProperty(cx, obj, INT_TO_JSID(hi)))
         {
@@ -1376,6 +1422,9 @@ ArrayReverseDenseKernel(JSContext* cx, HandleNativeObject obj, uint32_t length)
     return DenseElementResult::Success;
 }
 
+DefineBoxedOrUnboxedFunctor3(ArrayReverseDenseKernel,
+                             JSContext*, HandleObject, uint32_t);
+
 bool
 js::array_reverse(JSContext* cx, unsigned argc, Value* vp)
 {
@@ -1390,8 +1439,8 @@ js::array_reverse(JSContext* cx, unsigned argc, Value* vp)
         return false;
 
     if (!ObjectMayHaveExtraIndexedProperties(obj)) {
-        DenseElementResult result =
-            ArrayReverseDenseKernel(cx, obj.as<NativeObject>(), uint32_t(len));
+        ArrayReverseDenseKernelFunctor functor(cx, obj, len);
+        DenseElementResult result = CallBoxedOrUnboxedSpecialization(functor, obj);
         if (result != DenseElementResult::Incomplete) {
             /*
              * Per ECMA-262, don't update the length of the array, even if the new
@@ -2034,8 +2083,8 @@ js::array_push(JSContext* cx, unsigned argc, Value* vp)
 
     if (!ObjectMayHaveExtraIndexedProperties(obj)) {
         DenseElementResult result =
-            obj->as<NativeObject>().setOrExtendDenseElements(cx, uint32_t(length),
-                                                             args.array(), args.length());
+            SetOrExtendAnyBoxedOrUnboxedDenseElements(cx, obj, length,
+                                                      args.array(), args.length());
         if (result != DenseElementResult::Incomplete) {
             if (result == DenseElementResult::Failure)
                 return false;
@@ -2043,8 +2092,14 @@ js::array_push(JSContext* cx, unsigned argc, Value* vp)
             uint32_t newlength = length + args.length();
             args.rval().setNumber(newlength);
 
-            // Handle updates to the length of non-arrays here.
-            if (!obj->is<ArrayObject>())
+            // SetOrExtendAnyBoxedOrUnboxedDenseElements takes care of updating the
+            // length for boxed and unboxed arrays. Handle updates to the length of
+            // non-arrays here.
+            bool isArray;
+            if (!IsArray(cx, obj, &isArray))
+                return false;
+
+            if (!isArray)
                 return SetLengthProperty(cx, obj, newlength);
 
             return true;
@@ -2100,46 +2155,42 @@ js::array_pop(JSContext* cx, unsigned argc, Value* vp)
     return SetLengthProperty(cx, obj, index);
 }
 
-void
-js::ArrayShiftMoveElements(NativeObject* obj)
+template <JSValueType Type>
+static inline DenseElementResult
+ShiftMoveBoxedOrUnboxedDenseElements(JSObject* obj)
 {
-    MOZ_ASSERT_IF(obj->is<ArrayObject>(), obj->as<ArrayObject>().lengthIsWritable());
-
-    size_t initlen = obj->getDenseInitializedLength();
-    MOZ_ASSERT(initlen > 0);
+    MOZ_ASSERT(HasBoxedOrUnboxedDenseElements<Type>(obj));
 
     /*
      * At this point the length and initialized length have already been
      * decremented and the result fetched, so just shift the array elements
      * themselves.
      */
-    obj->moveDenseElementsNoPreBarrier(0, 1, initlen);
-}
+    size_t initlen = GetBoxedOrUnboxedInitializedLength<Type>(obj);
+    if (Type == JSVAL_TYPE_MAGIC) {
+        obj->as<NativeObject>().moveDenseElementsNoPreBarrier(0, 1, initlen);
+    } else {
+        uint8_t* data = obj->as<UnboxedArrayObject>().elements();
+        size_t elementSize = UnboxedTypeSize(Type);
+        memmove(data, data + elementSize, initlen * elementSize);
+    }
 
-static inline void
-SetInitializedLength(JSContext* cx, NativeObject* obj, size_t initlen)
-{
-    size_t oldInitlen = obj->getDenseInitializedLength();
-    obj->setDenseInitializedLength(initlen);
-    if (initlen < oldInitlen)
-        obj->shrinkElements(cx, initlen);
+    return DenseElementResult::Success;
 }
 
-static DenseElementResult
-MoveDenseElements(JSContext* cx, NativeObject* obj, uint32_t dstStart, uint32_t srcStart,
-                  uint32_t length)
-{
-    if (obj->denseElementsAreFrozen())
-        return DenseElementResult::Incomplete;
+DefineBoxedOrUnboxedFunctor1(ShiftMoveBoxedOrUnboxedDenseElements, JSObject*);
 
-    if (!obj->maybeCopyElementsForWrite(cx))
-        return DenseElementResult::Failure;
-    obj->moveDenseElements(dstStart, srcStart, length);
+void
+js::ArrayShiftMoveElements(JSObject* obj)
+{
+    MOZ_ASSERT_IF(obj->is<ArrayObject>(), obj->as<ArrayObject>().lengthIsWritable());
 
-    return DenseElementResult::Success;
+    ShiftMoveBoxedOrUnboxedDenseElementsFunctor functor(obj);
+    JS_ALWAYS_TRUE(CallBoxedOrUnboxedSpecialization(functor, obj) == DenseElementResult::Success);
 }
 
-static DenseElementResult
+template <JSValueType Type>
+DenseElementResult
 ArrayShiftDenseKernel(JSContext* cx, HandleObject obj, MutableHandleValue rval)
 {
     if (ObjectMayHaveExtraIndexedProperties(obj))
@@ -2152,22 +2203,25 @@ ArrayShiftDenseKernel(JSContext* cx, HandleObject obj, MutableHandleValue rval)
     if (MOZ_UNLIKELY(group->hasAllFlags(OBJECT_FLAG_ITERATED)))
         return DenseElementResult::Incomplete;
 
-    size_t initlen = obj->as<NativeObject>().getDenseInitializedLength();
+    size_t initlen = GetBoxedOrUnboxedInitializedLength<Type>(obj);
     if (initlen == 0)
         return DenseElementResult::Incomplete;
 
-    rval.set(obj->as<NativeObject>().getDenseElement(0));
+    rval.set(GetBoxedOrUnboxedDenseElement<Type>(obj, 0));
     if (rval.isMagic(JS_ELEMENTS_HOLE))
         rval.setUndefined();
 
-    DenseElementResult result = MoveDenseElements(cx, &obj->as<NativeObject>(), 0, 1, initlen - 1);
+    DenseElementResult result = MoveBoxedOrUnboxedDenseElements<Type>(cx, obj, 0, 1, initlen - 1);
     if (result != DenseElementResult::Success)
         return result;
 
-    SetInitializedLength(cx, obj.as<NativeObject>(), initlen - 1);
+    SetBoxedOrUnboxedInitializedLength<Type>(cx, obj, initlen - 1);
     return DenseElementResult::Success;
 }
 
+DefineBoxedOrUnboxedFunctor3(ArrayShiftDenseKernel,
+                             JSContext*, HandleObject, MutableHandleValue);
+
 /* ES5 15.4.4.9 */
 bool
 js::array_shift(JSContext* cx, unsigned argc, Value* vp)
@@ -2199,7 +2253,8 @@ js::array_shift(JSContext* cx, unsigned argc, Value* vp)
     uint32_t newlen = len - 1;
 
     /* Fast paths. */
-    DenseElementResult result = ArrayShiftDenseKernel(cx, obj, args.rval());
+    ArrayShiftDenseKernelFunctor functor(cx, obj, args.rval());
+    DenseElementResult result = CallBoxedOrUnboxedSpecialization(functor, obj);
     if (result != DenseElementResult::Incomplete) {
         if (result == DenseElementResult::Failure)
             return false;
@@ -2253,6 +2308,9 @@ js::array_unshift(JSContext* cx, unsigned argc, Value* vp)
     if (args.length() > 0) {
         /* Slide up the array to make room for all args at the bottom. */
         if (length > 0) {
+            // Only include a fast path for boxed arrays. Unboxed arrays can'nt
+            // be optimized here because unshifting temporarily places holes at
+            // the start of the array.
             bool optimized = false;
             do {
                 if (!obj->is<ArrayObject>())
@@ -2312,10 +2370,10 @@ js::array_unshift(JSContext* cx, unsigned argc, Value* vp)
 }
 
 /*
- * Returns true if this is a dense array whose properties ending at |endIndex|
- * (exclusive) may be accessed (get, set, delete) directly through its
- * contiguous vector of elements without fear of getters, setters, etc. along
- * the prototype chain, or of enumerators requiring notification of
+ * Returns true if this is a dense or unboxed array whose |count| properties
+ * starting from |startingIndex| may be accessed (get, set, delete) directly
+ * through its contiguous vector of elements without fear of getters, setters,
+ * etc. along the prototype chain, or of enumerators requiring notification of
  * modifications.
  */
 static inline bool
@@ -2326,11 +2384,11 @@ CanOptimizeForDenseStorage(HandleObject arr, uint32_t startingIndex, uint32_t co
         return false;
 
     /* There's no optimizing possible if it's not an array. */
-    if (!arr->is<ArrayObject>())
+    if (!arr->is<ArrayObject>() && !arr->is<UnboxedArrayObject>())
         return false;
 
     /* If it's a frozen array, always pick the slow path */
-    if (arr->as<ArrayObject>().denseElementsAreFrozen())
+    if (arr->is<ArrayObject>() && arr->as<ArrayObject>().denseElementsAreFrozen())
         return false;
 
     /*
@@ -2362,23 +2420,7 @@ CanOptimizeForDenseStorage(HandleObject arr, uint32_t startingIndex, uint32_t co
      * is subsumed by the initializedLength comparison.)
      */
     return !ObjectMayHaveExtraIndexedProperties(arr) &&
-           startingIndex + count <= arr->as<NativeObject>().getDenseInitializedLength();
-}
-
-static inline DenseElementResult
-CopyDenseElements(JSContext* cx, NativeObject* dst, NativeObject* src,
-                      uint32_t dstStart, uint32_t srcStart, uint32_t length)
-{
-    MOZ_ASSERT(dst->getDenseInitializedLength() == dstStart);
-    MOZ_ASSERT(src->getDenseInitializedLength() >= srcStart + length);
-    MOZ_ASSERT(dst->getDenseCapacity() >= dstStart + length);
-
-    dst->setDenseInitializedLength(dstStart + length);
-
-    const Value* vp = src->getDenseElements() + srcStart;
-    dst->initDenseElements(dstStart, vp, length);
-
-    return DenseElementResult::Success;
+           startingIndex + count <= GetAnyBoxedOrUnboxedInitializedLength(arr);
 }
 
 static inline bool
@@ -2472,9 +2514,7 @@ array_splice_impl(JSContext* cx, unsigned argc, Value* vp, bool returnValueIsUse
 
                 /* Steps 10-11. */
                 DebugOnly<DenseElementResult> result =
-                    CopyDenseElements(cx, &arr->as<NativeObject>(), 
-                                      &obj->as<NativeObject>(), 0, 
-                                      actualStart, actualDeleteCount);
+                    CopyAnyBoxedOrUnboxedDenseElements(cx, arr, obj, 0, actualStart, actualDeleteCount);
                 MOZ_ASSERT(result.value == DenseElementResult::Success);
 
                 /* Step 12 (implicit). */
@@ -2511,13 +2551,14 @@ array_splice_impl(JSContext* cx, unsigned argc, Value* vp, bool returnValueIsUse
         if (CanOptimizeForDenseStorage(obj, 0, len, cx)) {
             /* Steps 15.a-b. */
             DenseElementResult result =
-                MoveDenseElements(cx, &obj->as<NativeObject>(), targetIndex, sourceIndex, len - sourceIndex);
+                MoveAnyBoxedOrUnboxedDenseElements(cx, obj, targetIndex, sourceIndex,
+                                                   len - sourceIndex);
             MOZ_ASSERT(result != DenseElementResult::Incomplete);
             if (result == DenseElementResult::Failure)
                 return false;
 
             /* Steps 15.c-d. */
-            SetInitializedLength(cx, obj.as<NativeObject>(), finalLength);
+            SetAnyBoxedOrUnboxedInitializedLength(cx, obj, finalLength);
         } else {
             /*
              * This is all very slow if the length is very large. We don't yet
@@ -2597,15 +2638,15 @@ array_splice_impl(JSContext* cx, unsigned argc, Value* vp, bool returnValueIsUse
 
         if (CanOptimizeForDenseStorage(obj, len, itemCount - actualDeleteCount, cx)) {
             DenseElementResult result =
-                MoveDenseElements(cx, &obj->as<NativeObject>(), actualStart + itemCount,
-                                  actualStart + actualDeleteCount,
-                                  len - (actualStart + actualDeleteCount));
+                MoveAnyBoxedOrUnboxedDenseElements(cx, obj, actualStart + itemCount,
+                                                   actualStart + actualDeleteCount,
+                                                   len - (actualStart + actualDeleteCount));
             MOZ_ASSERT(result != DenseElementResult::Incomplete);
             if (result == DenseElementResult::Failure)
                 return false;
 
             /* Steps 16.a-b. */
-            SetInitializedLength(cx, obj.as<NativeObject>(), len + itemCount - actualDeleteCount);
+            SetAnyBoxedOrUnboxedInitializedLength(cx, obj, len + itemCount - actualDeleteCount);
         } else {
             RootedValue fromValue(cx);
             for (double k = len - actualDeleteCount; k > actualStart; k--) {
@@ -2790,7 +2831,7 @@ SliceSlowly(JSContext* cx, HandleObject obj, HandleObject receiver,
 }
 
 static bool
-SliceSparse(JSContext* cx, HandleObject obj, uint32_t begin, uint32_t end, HandleArrayObject result)
+SliceSparse(JSContext* cx, HandleObject obj, uint32_t begin, uint32_t end, HandleObject result)
 {
     MOZ_ASSERT(begin <= end);
 
@@ -2840,28 +2881,26 @@ ArraySliceOrdinary(JSContext* cx, HandleObject obj, uint32_t length, uint32_t be
         begin = end;
 
     if (!ObjectMayHaveExtraIndexedProperties(obj)) {
-        size_t initlen = obj->as<NativeObject>().getDenseInitializedLength();
+        size_t initlen = GetAnyBoxedOrUnboxedInitializedLength(obj);
         size_t count = 0;
         if (initlen > begin)
             count = Min<size_t>(initlen - begin, end - begin);
 
-        RootedArrayObject narr(cx, NewFullyAllocatedArrayTryReuseGroup(cx, obj, count));
+        RootedObject narr(cx, NewFullyAllocatedArrayTryReuseGroup(cx, obj, count));
         if (!narr)
             return false;
-
-        MOZ_ASSERT(count >= narr->as<ArrayObject>().length());
-        narr->as<ArrayObject>().setLength(cx, count);
+        SetAnyBoxedOrUnboxedArrayLength(cx, narr, end - begin);
 
         if (count) {
             DebugOnly<DenseElementResult> result =
-                CopyDenseElements(cx, &narr->as<NativeObject>(), &obj->as<NativeObject>(), 0, begin, count);
+                CopyAnyBoxedOrUnboxedDenseElements(cx, narr, obj, 0, begin, count);
             MOZ_ASSERT(result.value == DenseElementResult::Success);
         }
         arr.set(narr);
         return true;
     }
 
-    RootedArrayObject narr(cx, NewPartlyAllocatedArrayTryReuseGroup(cx, obj, end - begin));
+    RootedObject narr(cx, NewPartlyAllocatedArrayTryReuseGroup(cx, obj, end - begin));
     if (!narr)
         return false;
 
@@ -2978,10 +3017,11 @@ js::array_slice(JSContext* cx, unsigned argc, Value* vp)
     return true;
 }
 
-static bool
-ArraySliceDenseKernel(JSContext* cx, ArrayObject* arr, int32_t beginArg, int32_t endArg, ArrayObject* result)
+template <JSValueType Type>
+DenseElementResult
+ArraySliceDenseKernel(JSContext* cx, JSObject* obj, int32_t beginArg, int32_t endArg, JSObject* result)
 {
-    int32_t length = arr->length();
+    int32_t length = GetAnyBoxedOrUnboxedArrayLength(obj);
 
     uint32_t begin = NormalizeSliceTerm(beginArg, length);
     uint32_t end = NormalizeSliceTerm(endArg, length);
@@ -2989,33 +3029,33 @@ ArraySliceDenseKernel(JSContext* cx, ArrayObject* arr, int32_t beginArg, int32_t
     if (begin > end)
         begin = end;
 
-    size_t initlen = arr->getDenseInitializedLength();
-    size_t count = Min<size_t>(initlen - begin, end - begin);
+    size_t initlen = GetBoxedOrUnboxedInitializedLength<Type>(obj);
     if (initlen > begin) {
+        size_t count = Min<size_t>(initlen - begin, end - begin);
         if (count) {
-            if (!result->ensureElements(cx, count))
-                return false;
-            CopyDenseElements(cx, &result->as<NativeObject>(), &arr->as<NativeObject>(), 0, begin, count);
+            DenseElementResult rv = EnsureBoxedOrUnboxedDenseElements<Type>(cx, result, count);
+            if (rv != DenseElementResult::Success)
+                return rv;
+            CopyBoxedOrUnboxedDenseElements<Type, Type>(cx, result, obj, 0, begin, count);
         }
     }
 
-    MOZ_ASSERT(count >= result->length());
-    result->setLength(cx, count);
-
-    return true;
+    SetAnyBoxedOrUnboxedArrayLength(cx, result, end - begin);
+    return DenseElementResult::Success;
 }
 
+DefineBoxedOrUnboxedFunctor5(ArraySliceDenseKernel,
+                             JSContext*, JSObject*, int32_t, int32_t, JSObject*);
+
 JSObject*
 js::array_slice_dense(JSContext* cx, HandleObject obj, int32_t begin, int32_t end,
                       HandleObject result)
 {
     if (result && IsArraySpecies(cx, obj)) {
-        if (!ArraySliceDenseKernel(cx, &obj->as<ArrayObject>(), begin, end,
-                                   &result->as<ArrayObject>()))
-        {
-            return nullptr;
-        }
-        return result;
+        ArraySliceDenseKernelFunctor functor(cx, obj, begin, end, result);
+        DenseElementResult rv = CallBoxedOrUnboxedSpecialization(functor, result);
+        MOZ_ASSERT(rv != DenseElementResult::Incomplete);
+        return rv == DenseElementResult::Success ? result : nullptr;
     }
 
     // Slower path if the JIT wasn't able to allocate an object inline.
@@ -3046,7 +3086,7 @@ array_isArray(JSContext* cx, unsigned argc, Value* vp)
 static bool
 ArrayFromCallArgs(JSContext* cx, CallArgs& args, HandleObject proto = nullptr)
 {
-    ArrayObject* obj = NewCopiedArrayForCallingAllocationSite(cx, args.array(), args.length(), proto);
+    JSObject* obj = NewCopiedArrayForCallingAllocationSite(cx, args.array(), args.length(), proto);
     if (!obj)
         return false;
 
@@ -3220,7 +3260,7 @@ ArrayConstructorImpl(JSContext* cx, CallArgs& args, bool isConstructor)
         }
     }
 
-    ArrayObject* obj = NewPartlyAllocatedArrayForCallingAllocationSite(cx, length, proto);
+    JSObject* obj = NewPartlyAllocatedArrayForCallingAllocationSite(cx, length, proto);
     if (!obj)
         return false;
 
@@ -3246,7 +3286,7 @@ js::array_construct(JSContext* cx, unsigned argc, Value* vp)
     return ArrayConstructorImpl(cx, args, /* isConstructor = */ false);
 }
 
-ArrayObject*
+JSObject*
 js::ArrayConstructorOneArg(JSContext* cx, HandleObjectGroup group, int32_t lengthInt)
 {
     if (lengthInt < 0) {
@@ -3541,7 +3581,7 @@ js::NewDenseFullyAllocatedArrayWithTemplate(JSContext* cx, uint32_t length, JSOb
     return arr;
 }
 
-ArrayObject*
+JSObject*
 js::NewDenseCopyOnWriteArray(JSContext* cx, HandleArrayObject templateObject, gc::InitialHeap heap)
 {
     MOZ_ASSERT(!gc::IsInsideNursery(templateObject));
@@ -3554,21 +3594,30 @@ js::NewDenseCopyOnWriteArray(JSContext* cx, HandleArrayObject templateObject, gc
     return arr;
 }
 
-// Return a new array with the specified length and allocated capacity (up to
-// maxLength), using the specified group if possible. If the specified group
-// cannot be used, ensure that the created array at least has the given
-// [[Prototype]].
+// Return a new boxed or unboxed array with the specified length and allocated
+// capacity (up to maxLength), using the specified group if possible. If the
+// specified group cannot be used, ensure that the created array at least has
+// the given [[Prototype]].
 template <uint32_t maxLength>
-static inline ArrayObject*
+static inline JSObject*
 NewArrayTryUseGroup(ExclusiveContext* cx, HandleObjectGroup group, size_t length,
                     NewObjectKind newKind = GenericObject)
 {
     MOZ_ASSERT(newKind != SingletonObject);
 
-    if (group->shouldPreTenure())
+    if (group->maybePreliminaryObjects())
+        group->maybePreliminaryObjects()->maybeAnalyze(cx, group);
+
+    if (group->shouldPreTenure() || group->maybePreliminaryObjects())
         newKind = TenuredObject;
 
     RootedObject proto(cx, group->proto().toObject());
+    if (group->maybeUnboxedLayout()) {
+        if (length > UnboxedArrayObject::MaximumCapacity)
+            return NewArray<maxLength>(cx, length, proto, newKind);
+        return UnboxedArrayObject::create(cx, group, length, newKind, maxLength);
+    }
+
     ArrayObject* res = NewArray<maxLength>(cx, length, proto, newKind);
     if (!res)
         return nullptr;
@@ -3580,17 +3629,20 @@ NewArrayTryUseGroup(ExclusiveContext* cx, HandleObjectGroup group, size_t length
     if (res->length() > INT32_MAX)
         res->setLength(cx, res->length());
 
+    if (PreliminaryObjectArray* preliminaryObjects = group->maybePreliminaryObjects())
+        preliminaryObjects->registerNewObject(res);
+
     return res;
 }
 
-ArrayObject*
+JSObject*
 js::NewFullyAllocatedArrayTryUseGroup(ExclusiveContext* cx, HandleObjectGroup group, size_t length,
                                       NewObjectKind newKind)
 {
     return NewArrayTryUseGroup<UINT32_MAX>(cx, group, length, newKind);
 }
 
-ArrayObject*
+JSObject*
 js::NewPartlyAllocatedArrayTryUseGroup(ExclusiveContext* cx, HandleObjectGroup group, size_t length)
 {
     return NewArrayTryUseGroup<ArrayObject::EagerAllocationMaxLength>(cx, group, length);
@@ -3599,13 +3651,16 @@ js::NewPartlyAllocatedArrayTryUseGroup(ExclusiveContext* cx, HandleObjectGroup g
 // Return a new array with the default prototype and specified allocated
 // capacity and length. If possible, try to reuse the group of the input
 // object. The resulting array will either reuse the input object's group or
-// will have unknown property types.
+// will have unknown property types. Additionally, the result will have the
+// same boxed/unboxed elements representation as the input object, unless
+// |length| is larger than the input object's initialized length (in which case
+// UnboxedArrayObject::MaximumCapacity might be exceeded).
 template <uint32_t maxLength>
-static inline ArrayObject*
+static inline JSObject*
 NewArrayTryReuseGroup(JSContext* cx, HandleObject obj, size_t length,
                       NewObjectKind newKind = GenericObject)
 {
-    if (!obj->is<ArrayObject>())
+    if (!obj->is<ArrayObject>() && !obj->is<UnboxedArrayObject>())
         return NewArray<maxLength>(cx, length, nullptr, newKind);
 
     if (obj->staticPrototype() != cx->global()->maybeGetArrayPrototype())
@@ -3618,20 +3673,20 @@ NewArrayTryReuseGroup(JSContext* cx, HandleObject obj, size_t length,
     return NewArrayTryUseGroup<maxLength>(cx, group, length, newKind);
 }
 
-ArrayObject*
+JSObject*
 js::NewFullyAllocatedArrayTryReuseGroup(JSContext* cx, HandleObject obj, size_t length,
                                         NewObjectKind newKind)
 {
     return NewArrayTryReuseGroup<UINT32_MAX>(cx, obj, length, newKind);
 }
 
-ArrayObject*
+JSObject*
 js::NewPartlyAllocatedArrayTryReuseGroup(JSContext* cx, HandleObject obj, size_t length)
 {
     return NewArrayTryReuseGroup<ArrayObject::EagerAllocationMaxLength>(cx, obj, length);
 }
 
-ArrayObject*
+JSObject*
 js::NewFullyAllocatedArrayForCallingAllocationSite(JSContext* cx, size_t length,
                                                    NewObjectKind newKind)
 {
@@ -3641,7 +3696,7 @@ js::NewFullyAllocatedArrayForCallingAllocationSite(JSContext* cx, size_t length,
     return NewArrayTryUseGroup<UINT32_MAX>(cx, group, length, newKind);
 }
 
-ArrayObject*
+JSObject*
 js::NewPartlyAllocatedArrayForCallingAllocationSite(JSContext* cx, size_t length, HandleObject proto)
 {
     RootedObjectGroup group(cx, ObjectGroup::callingAllocationSiteGroup(cx, JSProto_Array, proto));
@@ -3650,23 +3705,68 @@ js::NewPartlyAllocatedArrayForCallingAllocationSite(JSContext* cx, size_t length
     return NewArrayTryUseGroup<ArrayObject::EagerAllocationMaxLength>(cx, group, length);
 }
 
-ArrayObject*
+bool
+js::MaybeAnalyzeBeforeCreatingLargeArray(ExclusiveContext* cx, HandleObjectGroup group,
+                                         const Value* vp, size_t length)
+{
+    static const size_t EagerPreliminaryObjectAnalysisThreshold = 800;
+
+    // Force analysis to see if an unboxed array can be used when making a
+    // sufficiently large array, to avoid excessive analysis and copying later
+    // on. If this is the first array of its group that is being created, first
+    // make a dummy array with the initial elements of the array we are about
+    // to make, so there is some basis for the unboxed array analysis.
+    if (length > EagerPreliminaryObjectAnalysisThreshold) {
+        if (PreliminaryObjectArrayWithTemplate* objects = group->maybePreliminaryObjects()) {
+            if (objects->empty()) {
+                size_t nlength = Min<size_t>(length, 100);
+                JSObject* obj = NewFullyAllocatedArrayTryUseGroup(cx, group, nlength);
+                if (!obj)
+                    return false;
+                DebugOnly<DenseElementResult> result =
+                    SetOrExtendAnyBoxedOrUnboxedDenseElements(cx, obj, 0, vp, nlength,
+                                                              ShouldUpdateTypes::Update);
+                MOZ_ASSERT(result.value == DenseElementResult::Success);
+            }
+            objects->maybeAnalyze(cx, group, /* forceAnalyze = */ true);
+        }
+    }
+    return true;
+}
+
+JSObject*
 js::NewCopiedArrayTryUseGroup(ExclusiveContext* cx, HandleObjectGroup group,
                               const Value* vp, size_t length, NewObjectKind newKind,
                               ShouldUpdateTypes updateTypes)
 {
-    ArrayObject* obj = NewFullyAllocatedArrayTryUseGroup(cx, group, length, newKind);
+    if (!MaybeAnalyzeBeforeCreatingLargeArray(cx, group, vp, length))
+        return nullptr;
+
+    JSObject* obj = NewFullyAllocatedArrayTryUseGroup(cx, group, length, newKind);
     if (!obj)
         return nullptr;
 
-    DenseElementResult result = obj->setOrExtendDenseElements(cx, 0, vp, length, updateTypes);
+    DenseElementResult result =
+        SetOrExtendAnyBoxedOrUnboxedDenseElements(cx, obj, 0, vp, length, updateTypes);
+    if (result == DenseElementResult::Failure)
+        return nullptr;
+    if (result == DenseElementResult::Success)
+        return obj;
+
+    MOZ_ASSERT(obj->is<UnboxedArrayObject>());
+    if (!UnboxedArrayObject::convertToNative(cx->asJSContext(), obj))
+        return nullptr;
+
+    result = SetOrExtendBoxedOrUnboxedDenseElements<JSVAL_TYPE_MAGIC>(cx, obj, 0, vp, length,
+                                                                      updateTypes);
+    MOZ_ASSERT(result != DenseElementResult::Incomplete);
     if (result == DenseElementResult::Failure)
         return nullptr;
-    MOZ_ASSERT(result == DenseElementResult::Success);
+
     return obj;
 }
 
-ArrayObject*
+JSObject*
 js::NewCopiedArrayForCallingAllocationSite(JSContext* cx, const Value* vp, size_t length,
                                            HandleObject proto /* = nullptr */)
 {
diff --git a/js/src/jsarray.h b/js/src/jsarray.h
index d0084731f9..0ca580ba72 100644
--- a/js/src/jsarray.h
+++ b/js/src/jsarray.h
@@ -72,37 +72,49 @@ extern ArrayObject*
 NewDenseFullyAllocatedArrayWithTemplate(JSContext* cx, uint32_t length, JSObject* templateObject);
 
 /* Create a dense array with the same copy-on-write elements as another object. */
-extern ArrayObject*
+extern JSObject*
 NewDenseCopyOnWriteArray(JSContext* cx, HandleArrayObject templateObject, gc::InitialHeap heap);
 
-extern ArrayObject*
+// The methods below can create either boxed or unboxed arrays.
+
+extern JSObject*
 NewFullyAllocatedArrayTryUseGroup(ExclusiveContext* cx, HandleObjectGroup group, size_t length,
                                   NewObjectKind newKind = GenericObject);
 
-extern ArrayObject*
+extern JSObject*
 NewPartlyAllocatedArrayTryUseGroup(ExclusiveContext* cx, HandleObjectGroup group, size_t length);
 
-extern ArrayObject*
+extern JSObject*
 NewFullyAllocatedArrayTryReuseGroup(JSContext* cx, HandleObject obj, size_t length,
                                     NewObjectKind newKind = GenericObject);
 
-extern ArrayObject*
+extern JSObject*
 NewPartlyAllocatedArrayTryReuseGroup(JSContext* cx, HandleObject obj, size_t length);
 
-extern ArrayObject*
+extern JSObject*
 NewFullyAllocatedArrayForCallingAllocationSite(JSContext* cx, size_t length,
                                                NewObjectKind newKind = GenericObject);
 
-extern ArrayObject*
+extern JSObject*
 NewPartlyAllocatedArrayForCallingAllocationSite(JSContext* cx, size_t length, HandleObject proto);
 
-extern ArrayObject*
+enum class ShouldUpdateTypes
+{
+    Update,
+    DontUpdate
+};
+
+extern bool
+MaybeAnalyzeBeforeCreatingLargeArray(ExclusiveContext* cx, HandleObjectGroup group,
+                                     const Value* vp, size_t length);
+
+extern JSObject*
 NewCopiedArrayTryUseGroup(ExclusiveContext* cx, HandleObjectGroup group,
                           const Value* vp, size_t length,
                           NewObjectKind newKind = GenericObject,
                           ShouldUpdateTypes updateTypes = ShouldUpdateTypes::Update);
 
-extern ArrayObject*
+extern JSObject*
 NewCopiedArrayForCallingAllocationSite(JSContext* cx, const Value* vp, size_t length,
                                        HandleObject proto = nullptr);
 
@@ -117,6 +129,13 @@ NewValuePair(JSContext* cx, const Value& val1, const Value& val2, MutableHandleV
 extern bool
 WouldDefinePastNonwritableLength(HandleNativeObject obj, uint32_t index);
 
+/*
+ * Canonicalize |vp| to a uint32_t value potentially suitable for use as an
+ * array length.
+ */
+extern bool
+CanonicalizeArrayLengthValue(JSContext* cx, HandleValue v, uint32_t* canonicalized);
+
 extern bool
 GetLengthProperty(JSContext* cx, HandleObject obj, uint32_t* lengthp);
 
@@ -150,7 +169,7 @@ extern bool
 array_join(JSContext* cx, unsigned argc, js::Value* vp);
 
 extern void
-ArrayShiftMoveElements(NativeObject* obj);
+ArrayShiftMoveElements(JSObject* obj);
 
 extern bool
 array_shift(JSContext* cx, unsigned argc, js::Value* vp);
@@ -182,7 +201,7 @@ extern const JSJitInfo array_splice_info;
 extern bool
 NewbornArrayPush(JSContext* cx, HandleObject obj, const Value& v);
 
-extern ArrayObject*
+extern JSObject*
 ArrayConstructorOneArg(JSContext* cx, HandleObjectGroup group, int32_t lengthInt);
 
 #ifdef DEBUG
diff --git a/js/src/jsfriendapi.cpp b/js/src/jsfriendapi.cpp
index f818bb2902..515f622130 100644
--- a/js/src/jsfriendapi.cpp
+++ b/js/src/jsfriendapi.cpp
@@ -270,7 +270,7 @@ js::GetBuiltinClass(JSContext* cx, HandleObject obj, ESClass* cls)
 
     if (obj->is<PlainObject>() || obj->is<UnboxedPlainObject>())
         *cls = ESClass::Object;
-    else if (obj->is<ArrayObject>())
+    else if (obj->is<ArrayObject>() || obj->is<UnboxedArrayObject>())
         *cls = ESClass::Array;
     else if (obj->is<NumberObject>())
         *cls = ESClass::Number;
diff --git a/js/src/jsobj.cpp b/js/src/jsobj.cpp
index 3d7f294fe7..901a9f5054 100644
--- a/js/src/jsobj.cpp
+++ b/js/src/jsobj.cpp
@@ -1136,18 +1136,19 @@ js::CloneObject(JSContext* cx, HandleObject obj, Handle<js::TaggedProto> proto)
 }
 
 static bool
-GetScriptArrayObjectElements(JSContext* cx, HandleArrayObject arr, MutableHandle<GCVector<Value>> values)
+GetScriptArrayObjectElements(JSContext* cx, HandleObject obj, MutableHandle<GCVector<Value>> values)
 {
-    MOZ_ASSERT(!arr->isSingleton());
-    MOZ_ASSERT(!arr->isIndexed());
+    MOZ_ASSERT(!obj->isSingleton());
+    MOZ_ASSERT(obj->is<ArrayObject>() || obj->is<UnboxedArrayObject>());
+    MOZ_ASSERT(!obj->isIndexed());
 
-    size_t length = arr->length();
+    size_t length = GetAnyBoxedOrUnboxedArrayLength(obj);
     if (!values.appendN(MagicValue(JS_ELEMENTS_HOLE), length))
         return false;
 
-    size_t initlen = arr->getDenseInitializedLength();
+    size_t initlen = GetAnyBoxedOrUnboxedInitializedLength(obj);
     for (size_t i = 0; i < initlen; i++)
-        values[i].set(arr->getDenseElement(i));
+        values[i].set(GetAnyBoxedOrUnboxedDenseElement(obj, i));
 
     return true;
 }
@@ -1199,12 +1200,13 @@ js::DeepCloneObjectLiteral(JSContext* cx, HandleObject obj, NewObjectKind newKin
     MOZ_ASSERT_IF(obj->isSingleton(),
                   cx->compartment()->behaviors().getSingletonsAsTemplates());
     MOZ_ASSERT(obj->is<PlainObject>() ||
-               obj->is<ArrayObject>());
+               obj->is<ArrayObject>() ||
+			   obj->is<UnboxedArrayObject>());
     MOZ_ASSERT(newKind != SingletonObject);
 
-    if (obj->is<ArrayObject>()) {
+    if (obj->is<ArrayObject>() || obj->is<UnboxedArrayObject>()) {
         Rooted<GCVector<Value>> values(cx, GCVector<Value>(cx));
-        if (!GetScriptArrayObjectElements(cx, obj.as<ArrayObject>(), &values))
+        if (!GetScriptArrayObjectElements(cx, obj, &values))
             return nullptr;
 
         // Deep clone any elements.
@@ -1318,8 +1320,9 @@ js::XDRObjectLiteral(XDRState<mode>* xdr, MutableHandleObject obj)
     {
         if (mode == XDR_ENCODE) {
             MOZ_ASSERT(obj->is<PlainObject>() ||
-                       obj->is<ArrayObject>());
-            isArray = obj->is<ArrayObject>() ? 1 : 0;
+                       obj->is<ArrayObject>() ||
+                       obj->is<UnboxedArrayObject>());
+            isArray = (obj->is<ArrayObject>() || obj->is<UnboxedArrayObject>()) ? 1 : 0;
         }
 
         if (!xdr->codeUint32(&isArray))
@@ -1331,11 +1334,8 @@ js::XDRObjectLiteral(XDRState<mode>* xdr, MutableHandleObject obj)
 
     if (isArray) {
         Rooted<GCVector<Value>> values(cx, GCVector<Value>(cx));
-        if (mode == XDR_ENCODE) {
-            RootedArrayObject arr(cx, &obj->as<ArrayObject>());
-            if (!GetScriptArrayObjectElements(cx, arr, &values))
-                return false;
-        }
+        if (mode == XDR_ENCODE && !GetScriptArrayObjectElements(cx, obj, &values))
+            return false;
 
         uint32_t initialized;
         if (mode == XDR_ENCODE)
@@ -2315,6 +2315,11 @@ js::LookupOwnPropertyPure(ExclusiveContext* cx, JSObject* obj, jsid id, Shape**
         // us the resolve hook won't define a property with this id.
         if (ClassMayResolveId(cx->names(), obj->getClass(), id, obj))
             return false;
+    } else if (obj->is<UnboxedArrayObject>()) {
+        if (obj->as<UnboxedArrayObject>().containsProperty(cx, id)) {
+            MarkNonNativePropertyFound<NoGC>(propp);
+            return true;
+        }
     } else if (obj->is<TypedObject>()) {
         if (obj->as<TypedObject>().typeDescr().hasProperty(cx->names(), id)) {
             MarkNonNativePropertyFound<NoGC>(propp);
@@ -3606,6 +3611,16 @@ JSObject::allocKindForTenure(const js::Nursery& nursery) const
     if (IsProxy(this))
         return as<ProxyObject>().allocKindForTenure();
 
+    // Unboxed arrays use inline data if their size is small enough.
+    if (is<UnboxedArrayObject>()) {
+        const UnboxedArrayObject* nobj = &as<UnboxedArrayObject>();
+        size_t nbytes = UnboxedArrayObject::offsetOfInlineElements() +
+                        nobj->capacity() * nobj->elementSize();
+        if (nbytes <= JSObject::MAX_BYTE_SIZE)
+            return GetGCObjectKindForBytes(nbytes);
+        return AllocKind::OBJECT0;
+    }
+
     // Inlined typed objects are followed by their data, so make sure we copy
     // it all over to the new object.
     if (is<InlineTypedObject>()) {
diff --git a/js/src/jsobjinlines.h b/js/src/jsobjinlines.h
index 889033143e..a82725ee86 100644
--- a/js/src/jsobjinlines.h
+++ b/js/src/jsobjinlines.h
@@ -40,6 +40,8 @@ MaybeConvertUnboxedObjectToNative(ExclusiveContext* cx, JSObject* obj)
 {
     if (obj->is<UnboxedPlainObject>())
         return UnboxedPlainObject::convertToNative(cx->asJSContext(), obj);
+    if (obj->is<UnboxedArrayObject>())
+        return UnboxedArrayObject::convertToNative(cx->asJSContext(), obj);
     return true;
 }
 
diff --git a/js/src/jsstr.cpp b/js/src/jsstr.cpp
index a482263e31..5e593846ed 100644
--- a/js/src/jsstr.cpp
+++ b/js/src/jsstr.cpp
@@ -2373,7 +2373,7 @@ js::str_replace_string_raw(JSContext* cx, HandleString string, HandleString patt
 }
 
 // ES 2016 draft Mar 25, 2016 21.1.3.17 steps 4, 8, 12-18.
-static ArrayObject*
+static JSObject*
 SplitHelper(JSContext* cx, HandleLinearString str, uint32_t limit, HandleLinearString sep,
             HandleObjectGroup group)
 {
@@ -2470,7 +2470,7 @@ SplitHelper(JSContext* cx, HandleLinearString str, uint32_t limit, HandleLinearS
 }
 
 // Fast-path for splitting a string into a character array via split("").
-static ArrayObject*
+static JSObject*
 CharSplitHelper(JSContext* cx, HandleLinearString str, uint32_t limit, HandleObjectGroup group)
 {
     size_t strLength = str->length();
@@ -2495,7 +2495,7 @@ CharSplitHelper(JSContext* cx, HandleLinearString str, uint32_t limit, HandleObj
 }
 
 // ES 2016 draft Mar 25, 2016 21.1.3.17 steps 4, 8, 12-18.
-ArrayObject*
+JSObject*
 js::str_split_string(JSContext* cx, HandleObjectGroup group, HandleString str, HandleString sep, uint32_t limit)
 
 {
diff --git a/js/src/jsstr.h b/js/src/jsstr.h
index 68175c8268..38fbfa85ed 100644
--- a/js/src/jsstr.h
+++ b/js/src/jsstr.h
@@ -465,7 +465,7 @@ FileEscapedString(FILE* fp, const char* chars, size_t length, uint32_t quote)
     return res;
 }
 
-ArrayObject*
+JSObject*
 str_split_string(JSContext* cx, HandleObjectGroup group, HandleString str, HandleString sep,
                  uint32_t limit);
 
diff --git a/js/src/shell/js.cpp b/js/src/shell/js.cpp
index e077bbbf88..35a527c5e4 100644
--- a/js/src/shell/js.cpp
+++ b/js/src/shell/js.cpp
@@ -317,6 +317,7 @@ static bool enableIon = false;
 static bool enableAsmJS = false;
 static bool enableWasm = false;
 static bool enableNativeRegExp = false;
+static bool enableUnboxedArrays = false;
 static bool enableSharedMemory = SHARED_MEMORY_DEFAULT;
 static bool enableWasmAlwaysBaseline = false;
 static bool enableArrayProtoValues = true;
@@ -7262,6 +7263,7 @@ SetContextOptions(JSContext* cx, const OptionParser& op)
     enableAsmJS = !op.getBoolOption("no-asmjs");
     enableWasm = !op.getBoolOption("no-wasm");
     enableNativeRegExp = !op.getBoolOption("no-native-regexp");
+    enableUnboxedArrays = op.getBoolOption("unboxed-arrays");
     enableWasmAlwaysBaseline = op.getBoolOption("wasm-always-baseline");
     enableArrayProtoValues = !op.getBoolOption("no-array-proto-values");
 
@@ -7271,6 +7273,7 @@ SetContextOptions(JSContext* cx, const OptionParser& op)
                              .setWasm(enableWasm)
                              .setWasmAlwaysBaseline(enableWasmAlwaysBaseline)
                              .setNativeRegExp(enableNativeRegExp)
+                             .setUnboxedArrays(enableUnboxedArrays)
                              .setArrayProtoValues(enableArrayProtoValues);
 
     if (op.getBoolOption("wasm-check-bce"))
@@ -7710,6 +7713,7 @@ main(int argc, char** argv, char** envp)
         || !op.addBoolOption('\0', "no-wasm", "Disable WebAssembly compilation")
         || !op.addBoolOption('\0', "no-native-regexp", "Disable native regexp compilation")
         || !op.addBoolOption('\0', "no-unboxed-objects", "Disable creating unboxed plain objects")
+        || !op.addBoolOption('\0', "unboxed-arrays", "Allow creating unboxed arrays")
         || !op.addBoolOption('\0', "wasm-always-baseline", "Enable wasm baseline compiler when possible")
         || !op.addBoolOption('\0', "wasm-check-bce", "Always generate wasm bounds check, even redundant ones.")
         || !op.addBoolOption('\0', "no-array-proto-values", "Remove Array.prototype.values")
diff --git a/js/src/vm/Interpreter-inl.h b/js/src/vm/Interpreter-inl.h
index acfa8f74bc..adefa6e93c 100644
--- a/js/src/vm/Interpreter-inl.h
+++ b/js/src/vm/Interpreter-inl.h
@@ -593,7 +593,7 @@ InitArrayElemOperation(JSContext* cx, jsbytecode* pc, HandleObject obj, uint32_t
     JSOp op = JSOp(*pc);
     MOZ_ASSERT(op == JSOP_INITELEM_ARRAY || op == JSOP_INITELEM_INC);
 
-    MOZ_ASSERT(obj->is<ArrayObject>());
+    MOZ_ASSERT(obj->is<ArrayObject>() || obj->is<UnboxedArrayObject>());
 
     if (op == JSOP_INITELEM_INC && index == INT32_MAX) {
         JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, JSMSG_SPREAD_TOO_LARGE);
diff --git a/js/src/vm/Interpreter.cpp b/js/src/vm/Interpreter.cpp
index b87d12924d..834084c4d5 100644
--- a/js/src/vm/Interpreter.cpp
+++ b/js/src/vm/Interpreter.cpp
@@ -1939,7 +1939,6 @@ CASE(EnableInterruptsPseudoOpcode)
 /* Various 1-byte no-ops. */
 CASE(JSOP_NOP)
 CASE(JSOP_NOP_DESTRUCTURING)
-CASE(JSOP_UNUSED126)
 CASE(JSOP_UNUSED211)
 CASE(JSOP_TRY_DESTRUCTURING_ITERCLOSE)
 CASE(JSOP_UNUSED221)
@@ -3682,6 +3681,7 @@ CASE(JSOP_NEWINIT)
 END_CASE(JSOP_NEWINIT)
 
 CASE(JSOP_NEWARRAY)
+CASE(JSOP_SPREADCALLARRAY)
 {
     uint32_t length = GET_UINT32(REGS.pc);
     JSObject* obj = NewArrayOperation(cx, script, REGS.pc, length);
@@ -4979,7 +4979,7 @@ js::NewObjectOperation(JSContext* cx, HandleScript script, jsbytecode* pc,
             newKind = TenuredObject;
     }
 
-    RootedPlainObject obj(cx);
+    RootedObject obj(cx);
 
     if (*pc == JSOP_NEWOBJECT) {
         RootedPlainObject baseObject(cx, &script->getObject(pc)->as<PlainObject>());
@@ -5047,6 +5047,9 @@ js::NewArrayOperation(JSContext* cx, HandleScript script, jsbytecode* pc, uint32
 
         if (group->shouldPreTenure() || group->maybePreliminaryObjects())
             newKind = TenuredObject;
+
+        if (group->maybeUnboxedLayout())
+            return UnboxedArrayObject::create(cx, group, length, newKind);
     }
 
     ArrayObject* obj = NewDenseFullyAllocatedArray(cx, length, nullptr, newKind);
@@ -5057,6 +5060,9 @@ js::NewArrayOperation(JSContext* cx, HandleScript script, jsbytecode* pc, uint32
         MOZ_ASSERT(obj->isSingleton());
     } else {
         obj->setGroup(group);
+
+        if (PreliminaryObjectArray* preliminaryObjects = group->maybePreliminaryObjects())
+            preliminaryObjects->registerNewObject(obj);
     }
 
     return obj;
@@ -5069,6 +5075,12 @@ js::NewArrayOperationWithTemplate(JSContext* cx, HandleObject templateObject)
 
     NewObjectKind newKind = templateObject->group()->shouldPreTenure() ? TenuredObject : GenericObject;
 
+    if (templateObject->is<UnboxedArrayObject>()) {
+        uint32_t length = templateObject->as<UnboxedArrayObject>().length();
+        RootedObjectGroup group(cx, templateObject->group());
+        return UnboxedArrayObject::create(cx, group, length, newKind);
+    }
+
     ArrayObject* obj = NewDenseFullyAllocatedArray(cx, templateObject->as<ArrayObject>().length(),
                                                    nullptr, newKind);
     if (!obj)
diff --git a/js/src/vm/JSONParser.cpp b/js/src/vm/JSONParser.cpp
index e50da3bc46..01883bb155 100644
--- a/js/src/vm/JSONParser.cpp
+++ b/js/src/vm/JSONParser.cpp
@@ -606,8 +606,8 @@ JSONParserBase::finishArray(MutableHandleValue vp, ElementVector& elements)
 {
     MOZ_ASSERT(&elements == &stack.back().elements());
 
-    ArrayObject* obj = ObjectGroup::newArrayObject(cx, elements.begin(), elements.length(),
-                                                   GenericObject);
+    JSObject* obj = ObjectGroup::newArrayObject(cx, elements.begin(), elements.length(),
+                                                GenericObject);
     if (!obj)
         return false;
 
diff --git a/js/src/vm/NativeObject-inl.h b/js/src/vm/NativeObject-inl.h
index 030d92c127..205d99aa2c 100644
--- a/js/src/vm/NativeObject-inl.h
+++ b/js/src/vm/NativeObject-inl.h
@@ -235,38 +235,6 @@ NativeObject::ensureDenseElements(ExclusiveContext* cx, uint32_t index, uint32_t
     return DenseElementResult::Success;
 }
 
-inline DenseElementResult
-NativeObject::setOrExtendDenseElements(ExclusiveContext* cx, uint32_t start, const Value* vp,
-                                       uint32_t count,
-                                       ShouldUpdateTypes updateTypes)
-{
-    if (denseElementsAreFrozen())
-        return DenseElementResult::Incomplete;
-
-    if (is<ArrayObject>() &&
-        !as<ArrayObject>().lengthIsWritable() &&
-        start + count >= as<ArrayObject>().length())
-    {
-        return DenseElementResult::Incomplete;
-    }
-
-    DenseElementResult result = ensureDenseElements(cx, start, count);
-    if (result != DenseElementResult::Success)
-        return result;
-
-    if (is<ArrayObject>() && start + count >= as<ArrayObject>().length())
-        as<ArrayObject>().setLengthInt32(start + count);
-
-    if (updateTypes == ShouldUpdateTypes::DontUpdate && !shouldConvertDoubleElements()) {
-        copyDenseElements(start, vp, count);
-    } else {
-        for (size_t i = 0; i < count; i++)
-            setDenseElementWithType(cx, start + i, vp[i]);
-    }
-
-    return DenseElementResult::Success;
-}
-
 inline Value
 NativeObject::getDenseOrTypedArrayElement(uint32_t idx)
 {
diff --git a/js/src/vm/NativeObject.h b/js/src/vm/NativeObject.h
index c774308aff..3a3e50244f 100644
--- a/js/src/vm/NativeObject.h
+++ b/js/src/vm/NativeObject.h
@@ -339,19 +339,16 @@ IsObjectValueInCompartment(const Value& v, JSCompartment* comp);
 #endif
 
 // Operations which change an object's dense elements can either succeed, fail,
-// or be unable to complete. The latter is used when the object's elements must
-// become sparse instead. The enum below is used for such operations.
+// or be unable to complete. For native objects, the latter is used when the
+// object's elements must become sparse instead. The enum below is used for
+// such operations, and for similar operations on unboxed arrays and methods
+// that work on both kinds of objects.
 enum class DenseElementResult {
     Failure,
     Success,
     Incomplete
 };
 
-enum class ShouldUpdateTypes {
-    Update,
-    DontUpdate
-};
-
 /*
  * NativeObject specifies the internal implementation of a native object.
  *
@@ -1154,10 +1151,6 @@ class NativeObject : public ShapedObject
         elementsRangeWriteBarrierPost(dstStart, count);
     }
 
-    inline DenseElementResult
-    setOrExtendDenseElements(ExclusiveContext* cx, uint32_t start, const Value* vp, uint32_t count,
-                             ShouldUpdateTypes updateTypes = ShouldUpdateTypes::Update);
-
     bool shouldConvertDoubleElements() {
         return getElementsHeader()->shouldConvertDoubleElements();
     }
diff --git a/js/src/vm/ObjectGroup.cpp b/js/src/vm/ObjectGroup.cpp
index 95fcada946..d05a48646b 100644
--- a/js/src/vm/ObjectGroup.cpp
+++ b/js/src/vm/ObjectGroup.cpp
@@ -777,7 +777,7 @@ GetValueTypeForTable(const Value& v)
     return type;
 }
 
-/* static */ ArrayObject*
+/* static */ JSObject*
 ObjectGroup::newArrayObject(ExclusiveContext* cx,
                             const Value* vp, size_t length,
                             NewObjectKind newKind, NewArrayKind arrayKind)
@@ -841,13 +841,56 @@ ObjectGroup::newArrayObject(ExclusiveContext* cx,
 
         AddTypePropertyId(cx, group, nullptr, JSID_VOID, elementType);
 
+        if (elementType != TypeSet::UnknownType()) {
+            // Keep track of the initial objects we create with this type.
+            // If the initial ones have a consistent shape and property types, we
+            // will try to use an unboxed layout for the group.
+            PreliminaryObjectArrayWithTemplate* preliminaryObjects =
+                cx->new_<PreliminaryObjectArrayWithTemplate>(nullptr);
+            if (!preliminaryObjects)
+                return nullptr;
+            group->setPreliminaryObjects(preliminaryObjects);
+        }
+
         if (!p.add(cx, *table, ObjectGroupCompartment::ArrayObjectKey(elementType), group))
             return nullptr;
     }
 
     // The type of the elements being added will already be reflected in type
-    // information.
+    // information, but make sure when creating an unboxed array that the
+    // common element type is suitable for the unboxed representation.
     ShouldUpdateTypes updateTypes = ShouldUpdateTypes::DontUpdate;
+    if (!MaybeAnalyzeBeforeCreatingLargeArray(cx, group, vp, length))
+        return nullptr;
+    if (group->maybePreliminaryObjects())
+        group->maybePreliminaryObjects()->maybeAnalyze(cx, group);
+    if (group->maybeUnboxedLayout()) {
+        switch (group->unboxedLayout().elementType()) {
+          case JSVAL_TYPE_BOOLEAN:
+            if (elementType != TypeSet::BooleanType())
+                updateTypes = ShouldUpdateTypes::Update;
+            break;
+          case JSVAL_TYPE_INT32:
+            if (elementType != TypeSet::Int32Type())
+                updateTypes = ShouldUpdateTypes::Update;
+            break;
+          case JSVAL_TYPE_DOUBLE:
+            if (elementType != TypeSet::Int32Type() && elementType != TypeSet::DoubleType())
+                updateTypes = ShouldUpdateTypes::Update;
+            break;
+          case JSVAL_TYPE_STRING:
+            if (elementType != TypeSet::StringType())
+                updateTypes = ShouldUpdateTypes::Update;
+            break;
+          case JSVAL_TYPE_OBJECT:
+            if (elementType != TypeSet::NullType() && !elementType.get().isObjectUnchecked())
+                updateTypes = ShouldUpdateTypes::Update;
+            break;
+          default:
+            MOZ_CRASH();
+        }
+    }
+
     return NewCopiedArrayTryUseGroup(cx, group, vp, length, newKind, updateTypes);
 }
 
@@ -857,15 +900,49 @@ GiveObjectGroup(ExclusiveContext* cx, JSObject* source, JSObject* target)
 {
     MOZ_ASSERT(source->group() != target->group());
 
-    if (!target->is<ArrayObject>() || !source->is<ArrayObject>()) {
+    if (!target->is<ArrayObject>() && !target->is<UnboxedArrayObject>())
         return true;
+
+    if (target->group()->maybePreliminaryObjects()) {
+        bool force = IsInsideNursery(source);
+        target->group()->maybePreliminaryObjects()->maybeAnalyze(cx, target->group(), force);
     }
 
-    source->setGroup(target->group());
+    if (target->is<ArrayObject>()) {
+        ObjectGroup* sourceGroup = source->group();
 
-    for (size_t i = 0; i < source->as<ArrayObject>().getDenseInitializedLength(); i++) {
-        Value v = source->as<ArrayObject>().getDenseElement(i);
-        AddTypePropertyId(cx, source->group(), source, JSID_VOID, v);
+        if (source->is<UnboxedArrayObject>()) {
+            Shape* shape = target->as<ArrayObject>().lastProperty();
+            if (!UnboxedArrayObject::convertToNativeWithGroup(cx, source, target->group(), shape))
+                return false;
+        } else if (source->is<ArrayObject>()) {
+            source->setGroup(target->group());
+        } else {
+            return true;
+        }
+
+        if (sourceGroup->maybePreliminaryObjects())
+            sourceGroup->maybePreliminaryObjects()->unregisterObject(source);
+        if (target->group()->maybePreliminaryObjects())
+            target->group()->maybePreliminaryObjects()->registerNewObject(source);
+
+        for (size_t i = 0; i < source->as<ArrayObject>().getDenseInitializedLength(); i++) {
+            Value v = source->as<ArrayObject>().getDenseElement(i);
+            AddTypePropertyId(cx, source->group(), source, JSID_VOID, v);
+        }
+
+        return true;
+    }
+
+    if (target->is<UnboxedArrayObject>()) {
+        if (!source->is<UnboxedArrayObject>())
+            return true;
+        if (source->as<UnboxedArrayObject>().elementType() != JSVAL_TYPE_INT32)
+            return true;
+        if (target->as<UnboxedArrayObject>().elementType() != JSVAL_TYPE_DOUBLE)
+            return true;
+
+        return source->as<UnboxedArrayObject>().convertInt32ToDouble(cx, target->group());
     }
 
     return true;
@@ -1429,6 +1506,18 @@ ObjectGroup::allocationSiteGroup(JSContext* cx, JSScript* scriptArg, jsbytecode*
         }
     }
 
+    if (kind == JSProto_Array &&
+        (JSOp(*pc) == JSOP_NEWARRAY || IsCallPC(pc)) &&
+        cx->options().unboxedArrays())
+    {
+        PreliminaryObjectArrayWithTemplate* preliminaryObjects =
+            cx->new_<PreliminaryObjectArrayWithTemplate>(nullptr);
+        if (preliminaryObjects)
+            res->setPreliminaryObjects(preliminaryObjects);
+        else
+            cx->recoverFromOutOfMemory();
+    }
+
     if (!table->add(p, key, res)) {
         ReportOutOfMemory(cx);
         return nullptr;
diff --git a/js/src/vm/ObjectGroup.h b/js/src/vm/ObjectGroup.h
index 553cb83667..4e24de9f14 100644
--- a/js/src/vm/ObjectGroup.h
+++ b/js/src/vm/ObjectGroup.h
@@ -505,11 +505,11 @@ class ObjectGroup : public gc::TenuredCell
         UnknownIndex  // Make an array with an unknown element type.
     };
 
-    // Create an ArrayObject with the specified elements and a group specialized
-    // for the elements.
-    static ArrayObject* newArrayObject(ExclusiveContext* cx, const Value* vp, size_t length,
-                                       NewObjectKind newKind,
-                                       NewArrayKind arrayKind = NewArrayKind::Normal);
+    // Create an ArrayObject or UnboxedArrayObject with the specified elements
+    // and a group specialized for the elements.
+    static JSObject* newArrayObject(ExclusiveContext* cx, const Value* vp, size_t length,
+                                    NewObjectKind newKind,
+                                    NewArrayKind arrayKind = NewArrayKind::Normal);
 
     // Create a PlainObject or UnboxedPlainObject with the specified properties
     // and a group specialized for those properties.
diff --git a/js/src/vm/Opcodes.h b/js/src/vm/Opcodes.h
index e13cd62219..b26336815e 100644
--- a/js/src/vm/Opcodes.h
+++ b/js/src/vm/Opcodes.h
@@ -1281,7 +1281,17 @@
      *   Stack: receiver, obj, propval => obj[propval]
      */ \
     macro(JSOP_GETELEM_SUPER, 125, "getelem-super", NULL, 1,  3,  1, JOF_BYTE |JOF_ELEM|JOF_LEFTASSOC) \
-    macro(JSOP_UNUSED126, 126, "unused126", NULL, 5,  0,  1, JOF_UINT32) \
+    /*
+     * Pushes newly created array for a spread call onto the stack. This has
+     * the same semantics as JSOP_NEWARRAY, but is distinguished to avoid
+     * using unboxed arrays in spread calls, which would make compiling spread
+     * calls in baseline more complex.
+     *   Category: Literals
+     *   Type: Array
+     *   Operands: uint32_t length
+     *   Stack: => obj
+     */ \
+    macro(JSOP_SPREADCALLARRAY, 126, "spreadcallarray", NULL, 5,  0,  1, JOF_UINT32) \
     \
     /*
      * Defines the given function on the current scope.
diff --git a/js/src/vm/ReceiverGuard.cpp b/js/src/vm/ReceiverGuard.cpp
index e37bf8ee52..11c2d07278 100644
--- a/js/src/vm/ReceiverGuard.cpp
+++ b/js/src/vm/ReceiverGuard.cpp
@@ -19,7 +19,7 @@ ReceiverGuard::ReceiverGuard(JSObject* obj)
             group = obj->group();
             if (UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando())
                 shape = expando->lastProperty();
-        } else if (obj->is<TypedObject>()) {
+        } else if (obj->is<UnboxedArrayObject>() || obj->is<TypedObject>()) {
             group = obj->group();
         } else {
             shape = obj->maybeShape();
@@ -34,7 +34,7 @@ ReceiverGuard::ReceiverGuard(ObjectGroup* group, Shape* shape)
         const Class* clasp = group->clasp();
         if (clasp == &UnboxedPlainObject::class_) {
             // Keep both group and shape.
-        } else if (IsTypedObjectClass(clasp)) {
+        } else if (clasp == &UnboxedArrayObject::class_ || IsTypedObjectClass(clasp)) {
             this->shape = nullptr;
         } else {
             this->group = nullptr;
@@ -49,8 +49,8 @@ HeapReceiverGuard::keyBits(JSObject* obj)
         // Both the group and shape need to be guarded for unboxed plain objects.
         return obj->as<UnboxedPlainObject>().maybeExpando() ? 0 : 1;
     }
-    if (obj->is<TypedObject>()) {
-        // Only the group needs to be guarded for typed objects.
+    if (obj->is<UnboxedArrayObject>() || obj->is<TypedObject>()) {
+        // Only the group needs to be guarded for unboxed arrays and typed objects.
         return 2;
     }
     // Other objects only need the shape to be guarded.
diff --git a/js/src/vm/Stack.cpp b/js/src/vm/Stack.cpp
index 95940eeaff..7eb8f4ab8b 100644
--- a/js/src/vm/Stack.cpp
+++ b/js/src/vm/Stack.cpp
@@ -82,7 +82,7 @@ InterpreterFrame::isNonGlobalEvalFrame() const
     return isEvalFrame() && script()->bodyScope()->as<EvalScope>().isNonGlobal();
 }
 
-ArrayObject*
+JSObject*
 InterpreterFrame::createRestParameter(JSContext* cx)
 {
     MOZ_ASSERT(script()->hasRest());
diff --git a/js/src/vm/Stack.h b/js/src/vm/Stack.h
index fe04a00f26..d085442135 100644
--- a/js/src/vm/Stack.h
+++ b/js/src/vm/Stack.h
@@ -523,7 +523,7 @@ class InterpreterFrame
     ArgumentsObject& argsObj() const;
     void initArgsObj(ArgumentsObject& argsobj);
 
-    ArrayObject* createRestParameter(JSContext* cx);
+    JSObject* createRestParameter(JSContext* cx);
 
     /*
      * Environment chain
diff --git a/js/src/vm/TypeInference.cpp b/js/src/vm/TypeInference.cpp
index 39206539be..ee5bbef5d2 100644
--- a/js/src/vm/TypeInference.cpp
+++ b/js/src/vm/TypeInference.cpp
@@ -2509,7 +2509,7 @@ TemporaryTypeSet::propertyNeedsBarrier(CompilerConstraintList* constraints, jsid
 bool
 js::ClassCanHaveExtraProperties(const Class* clasp)
 {
-    if (clasp == &UnboxedPlainObject::class_)
+    if (clasp == &UnboxedPlainObject::class_ || clasp == &UnboxedArrayObject::class_)
         return false;
     return clasp->getResolve()
         || clasp->getOpsLookupProperty()
@@ -3400,7 +3400,7 @@ JSFunction::setTypeForScriptedFunction(ExclusiveContext* cx, HandleFunction fun,
 /////////////////////////////////////////////////////////////////////
 
 void
-PreliminaryObjectArray::registerNewObject(PlainObject* res)
+PreliminaryObjectArray::registerNewObject(JSObject* res)
 {
     // The preliminary object pointers are weak, and won't be swept properly
     // during nursery collections, so the preliminary objects need to be
@@ -3418,7 +3418,7 @@ PreliminaryObjectArray::registerNewObject(PlainObject* res)
 }
 
 void
-PreliminaryObjectArray::unregisterObject(PlainObject* obj)
+PreliminaryObjectArray::unregisterObject(JSObject* obj)
 {
     for (size_t i = 0; i < COUNT; i++) {
         if (objects[i] == obj) {
diff --git a/js/src/vm/TypeInference.h b/js/src/vm/TypeInference.h
index fd021fc96c..537baa21f4 100644
--- a/js/src/vm/TypeInference.h
+++ b/js/src/vm/TypeInference.h
@@ -871,8 +871,8 @@ class PreliminaryObjectArray
   public:
     PreliminaryObjectArray() = default;
 
-    void registerNewObject(PlainObject* res);
-    void unregisterObject(PlainObject* obj);
+    void registerNewObject(JSObject* res);
+    void unregisterObject(JSObject* obj);
 
     JSObject* get(size_t i) const {
         MOZ_ASSERT(i < COUNT);
diff --git a/js/src/vm/UnboxedObject-inl.h b/js/src/vm/UnboxedObject-inl.h
index c1468a5b18..93ad7bf28a 100644
--- a/js/src/vm/UnboxedObject-inl.h
+++ b/js/src/vm/UnboxedObject-inl.h
@@ -172,6 +172,669 @@ UnboxedPlainObject::layout() const
     return group()->unboxedLayout();
 }
 
+/////////////////////////////////////////////////////////////////////
+// UnboxedArrayObject
+/////////////////////////////////////////////////////////////////////
+
+inline const UnboxedLayout&
+UnboxedArrayObject::layout() const
+{
+    return group()->unboxedLayout();
+}
+
+inline void
+UnboxedArrayObject::setLength(ExclusiveContext* cx, uint32_t length)
+{
+    if (length > INT32_MAX) {
+        // Track objects with overflowing lengths in type information.
+        MarkObjectGroupFlags(cx, this, OBJECT_FLAG_LENGTH_OVERFLOW);
+    }
+
+    length_ = length;
+}
+
+inline void
+UnboxedArrayObject::setInitializedLength(uint32_t initlen)
+{
+    if (initlen < initializedLength()) {
+        switch (elementType()) {
+          case JSVAL_TYPE_STRING:
+            for (size_t i = initlen; i < initializedLength(); i++)
+                triggerPreBarrier<JSVAL_TYPE_STRING>(i);
+            break;
+          case JSVAL_TYPE_OBJECT:
+            for (size_t i = initlen; i < initializedLength(); i++)
+                triggerPreBarrier<JSVAL_TYPE_OBJECT>(i);
+            break;
+          default:
+            MOZ_ASSERT(!UnboxedTypeNeedsPreBarrier(elementType()));
+        }
+    }
+    setInitializedLengthNoBarrier(initlen);
+}
+
+template <JSValueType Type>
+inline bool
+UnboxedArrayObject::setElementSpecific(ExclusiveContext* cx, size_t index, const Value& v)
+{
+    MOZ_ASSERT(index < initializedLength());
+    MOZ_ASSERT(Type == elementType());
+    uint8_t* p = elements() + index * UnboxedTypeSize(Type);
+    return SetUnboxedValue(cx, this, JSID_VOID, p, elementType(), v, /* preBarrier = */ true);
+}
+
+template <JSValueType Type>
+inline void
+UnboxedArrayObject::setElementNoTypeChangeSpecific(size_t index, const Value& v)
+{
+    MOZ_ASSERT(index < initializedLength());
+    MOZ_ASSERT(Type == elementType());
+    uint8_t* p = elements() + index * UnboxedTypeSize(Type);
+    return SetUnboxedValueNoTypeChange(this, p, elementType(), v, /* preBarrier = */ true);
+}
+
+template <JSValueType Type>
+inline bool
+UnboxedArrayObject::initElementSpecific(ExclusiveContext* cx, size_t index, const Value& v)
+{
+    MOZ_ASSERT(index < initializedLength());
+    MOZ_ASSERT(Type == elementType());
+    uint8_t* p = elements() + index * UnboxedTypeSize(Type);
+    return SetUnboxedValue(cx, this, JSID_VOID, p, elementType(), v, /* preBarrier = */ false);
+}
+
+template <JSValueType Type>
+inline void
+UnboxedArrayObject::initElementNoTypeChangeSpecific(size_t index, const Value& v)
+{
+    MOZ_ASSERT(index < initializedLength());
+    MOZ_ASSERT(Type == elementType());
+    uint8_t* p = elements() + index * UnboxedTypeSize(Type);
+    return SetUnboxedValueNoTypeChange(this, p, elementType(), v, /* preBarrier = */ false);
+}
+
+template <JSValueType Type>
+inline Value
+UnboxedArrayObject::getElementSpecific(size_t index)
+{
+    MOZ_ASSERT(index < initializedLength());
+    MOZ_ASSERT(Type == elementType());
+    uint8_t* p = elements() + index * UnboxedTypeSize(Type);
+    return GetUnboxedValue(p, Type, /* maybeUninitialized = */ false);
+}
+
+template <JSValueType Type>
+inline void
+UnboxedArrayObject::triggerPreBarrier(size_t index)
+{
+    MOZ_ASSERT(UnboxedTypeNeedsPreBarrier(Type));
+
+    uint8_t* p = elements() + index * UnboxedTypeSize(Type);
+
+    switch (Type) {
+      case JSVAL_TYPE_STRING: {
+        JSString** np = reinterpret_cast<JSString**>(p);
+        JSString::writeBarrierPre(*np);
+        break;
+      }
+
+      case JSVAL_TYPE_OBJECT: {
+        JSObject** np = reinterpret_cast<JSObject**>(p);
+        JSObject::writeBarrierPre(*np);
+        break;
+      }
+
+      default:
+        MOZ_CRASH("Bad type");
+    }
+}
+
+/////////////////////////////////////////////////////////////////////
+// Combined methods for NativeObject and UnboxedArrayObject accesses.
+/////////////////////////////////////////////////////////////////////
+
+static inline bool
+HasAnyBoxedOrUnboxedDenseElements(JSObject* obj)
+{
+    return obj->isNative() || obj->is<UnboxedArrayObject>();
+}
+
+static inline size_t
+GetAnyBoxedOrUnboxedInitializedLength(JSObject* obj)
+{
+    if (obj->isNative())
+        return obj->as<NativeObject>().getDenseInitializedLength();
+    if (obj->is<UnboxedArrayObject>())
+        return obj->as<UnboxedArrayObject>().initializedLength();
+    return 0;
+}
+
+static inline size_t
+GetAnyBoxedOrUnboxedCapacity(JSObject* obj)
+{
+    if (obj->isNative())
+        return obj->as<NativeObject>().getDenseCapacity();
+    if (obj->is<UnboxedArrayObject>())
+        return obj->as<UnboxedArrayObject>().capacity();
+    return 0;
+}
+
+static inline Value
+GetAnyBoxedOrUnboxedDenseElement(JSObject* obj, size_t index)
+{
+    if (obj->isNative())
+        return obj->as<NativeObject>().getDenseElement(index);
+    return obj->as<UnboxedArrayObject>().getElement(index);
+}
+
+static inline size_t
+GetAnyBoxedOrUnboxedArrayLength(JSObject* obj)
+{
+    if (obj->is<ArrayObject>())
+        return obj->as<ArrayObject>().length();
+    return obj->as<UnboxedArrayObject>().length();
+}
+
+static inline void
+SetAnyBoxedOrUnboxedArrayLength(JSContext* cx, JSObject* obj, size_t length)
+{
+    if (obj->is<ArrayObject>()) {
+        MOZ_ASSERT(length >= obj->as<ArrayObject>().length());
+        obj->as<ArrayObject>().setLength(cx, length);
+    } else {
+        MOZ_ASSERT(length >= obj->as<UnboxedArrayObject>().length());
+        obj->as<UnboxedArrayObject>().setLength(cx, length);
+    }
+}
+
+static inline bool
+SetAnyBoxedOrUnboxedDenseElement(JSContext* cx, JSObject* obj, size_t index, const Value& value)
+{
+    if (obj->isNative()) {
+        obj->as<NativeObject>().setDenseElementWithType(cx, index, value);
+        return true;
+    }
+    return obj->as<UnboxedArrayObject>().setElement(cx, index, value);
+}
+
+static inline bool
+InitAnyBoxedOrUnboxedDenseElement(JSContext* cx, JSObject* obj, size_t index, const Value& value)
+{
+    if (obj->isNative()) {
+        obj->as<NativeObject>().initDenseElementWithType(cx, index, value);
+        return true;
+    }
+    return obj->as<UnboxedArrayObject>().initElement(cx, index, value);
+}
+
+/////////////////////////////////////////////////////////////////////
+// Template methods for NativeObject and UnboxedArrayObject accesses.
+/////////////////////////////////////////////////////////////////////
+
+static inline JSValueType
+GetBoxedOrUnboxedType(JSObject* obj)
+{
+    if (obj->isNative())
+        return JSVAL_TYPE_MAGIC;
+    return obj->as<UnboxedArrayObject>().elementType();
+}
+
+template <JSValueType Type>
+static inline bool
+HasBoxedOrUnboxedDenseElements(JSObject* obj)
+{
+    if (Type == JSVAL_TYPE_MAGIC)
+        return obj->isNative();
+    return obj->is<UnboxedArrayObject>() && obj->as<UnboxedArrayObject>().elementType() == Type;
+}
+
+template <JSValueType Type>
+static inline size_t
+GetBoxedOrUnboxedInitializedLength(JSObject* obj)
+{
+    if (Type == JSVAL_TYPE_MAGIC)
+        return obj->as<NativeObject>().getDenseInitializedLength();
+    return obj->as<UnboxedArrayObject>().initializedLength();
+}
+
+template <JSValueType Type>
+static inline DenseElementResult
+SetBoxedOrUnboxedInitializedLength(JSContext* cx, JSObject* obj, size_t initlen)
+{
+    size_t oldInitlen = GetBoxedOrUnboxedInitializedLength<Type>(obj);
+    if (Type == JSVAL_TYPE_MAGIC) {
+        obj->as<NativeObject>().setDenseInitializedLength(initlen);
+        if (initlen < oldInitlen)
+            obj->as<NativeObject>().shrinkElements(cx, initlen);
+    } else {
+        obj->as<UnboxedArrayObject>().setInitializedLength(initlen);
+        if (initlen < oldInitlen)
+            obj->as<UnboxedArrayObject>().shrinkElements(cx, initlen);
+    }
+    return DenseElementResult::Success;
+}
+
+template <JSValueType Type>
+static inline size_t
+GetBoxedOrUnboxedCapacity(JSObject* obj)
+{
+    if (Type == JSVAL_TYPE_MAGIC)
+        return obj->as<NativeObject>().getDenseCapacity();
+    return obj->as<UnboxedArrayObject>().capacity();
+}
+
+template <JSValueType Type>
+static inline Value
+GetBoxedOrUnboxedDenseElement(JSObject* obj, size_t index)
+{
+    if (Type == JSVAL_TYPE_MAGIC)
+        return obj->as<NativeObject>().getDenseElement(index);
+    return obj->as<UnboxedArrayObject>().getElementSpecific<Type>(index);
+}
+
+template <JSValueType Type>
+static inline void
+SetBoxedOrUnboxedDenseElementNoTypeChange(JSObject* obj, size_t index, const Value& value)
+{
+    if (Type == JSVAL_TYPE_MAGIC)
+        obj->as<NativeObject>().setDenseElement(index, value);
+    else
+        obj->as<UnboxedArrayObject>().setElementNoTypeChangeSpecific<Type>(index, value);
+}
+
+template <JSValueType Type>
+static inline bool
+SetBoxedOrUnboxedDenseElement(JSContext* cx, JSObject* obj, size_t index, const Value& value)
+{
+    if (Type == JSVAL_TYPE_MAGIC) {
+        obj->as<NativeObject>().setDenseElementWithType(cx, index, value);
+        return true;
+    }
+    return obj->as<UnboxedArrayObject>().setElementSpecific<Type>(cx, index, value);
+}
+
+template <JSValueType Type>
+static inline DenseElementResult
+EnsureBoxedOrUnboxedDenseElements(JSContext* cx, JSObject* obj, size_t count)
+{
+    if (Type == JSVAL_TYPE_MAGIC) {
+        if (!obj->as<ArrayObject>().ensureElements(cx, count))
+            return DenseElementResult::Failure;
+    } else {
+        if (obj->as<UnboxedArrayObject>().capacity() < count) {
+            if (!obj->as<UnboxedArrayObject>().growElements(cx, count))
+                return DenseElementResult::Failure;
+        }
+    }
+    return DenseElementResult::Success;
+}
+
+template <JSValueType Type>
+static inline DenseElementResult
+SetOrExtendBoxedOrUnboxedDenseElements(ExclusiveContext* cx, JSObject* obj,
+                                       uint32_t start, const Value* vp, uint32_t count,
+                                       ShouldUpdateTypes updateTypes = ShouldUpdateTypes::Update)
+{
+    if (Type == JSVAL_TYPE_MAGIC) {
+        NativeObject* nobj = &obj->as<NativeObject>();
+
+        if (nobj->denseElementsAreFrozen())
+            return DenseElementResult::Incomplete;
+
+        if (obj->is<ArrayObject>() &&
+            !obj->as<ArrayObject>().lengthIsWritable() &&
+            start + count >= obj->as<ArrayObject>().length())
+        {
+            return DenseElementResult::Incomplete;
+        }
+
+        DenseElementResult result = nobj->ensureDenseElements(cx, start, count);
+        if (result != DenseElementResult::Success)
+            return result;
+
+        if (obj->is<ArrayObject>() && start + count >= obj->as<ArrayObject>().length())
+            obj->as<ArrayObject>().setLengthInt32(start + count);
+
+        if (updateTypes == ShouldUpdateTypes::DontUpdate && !nobj->shouldConvertDoubleElements()) {
+            nobj->copyDenseElements(start, vp, count);
+        } else {
+            for (size_t i = 0; i < count; i++)
+                nobj->setDenseElementWithType(cx, start + i, vp[i]);
+        }
+
+        return DenseElementResult::Success;
+    }
+
+    UnboxedArrayObject* nobj = &obj->as<UnboxedArrayObject>();
+
+    if (start > nobj->initializedLength())
+        return DenseElementResult::Incomplete;
+
+    if (start + count >= UnboxedArrayObject::MaximumCapacity)
+        return DenseElementResult::Incomplete;
+
+    if (start + count > nobj->capacity() && !nobj->growElements(cx, start + count))
+        return DenseElementResult::Failure;
+
+    size_t oldInitlen = nobj->initializedLength();
+
+    // Overwrite any existing elements covered by the new range. If we fail
+    // after this point due to some incompatible type being written to the
+    // object's elements, afterwards the contents will be different from when
+    // we started. The caller must retry the operation using a generic path,
+    // which will overwrite the already-modified elements as well as the ones
+    // that were left alone.
+    size_t i = 0;
+    if (updateTypes == ShouldUpdateTypes::DontUpdate) {
+        for (size_t j = start; i < count && j < oldInitlen; i++, j++)
+            nobj->setElementNoTypeChangeSpecific<Type>(j, vp[i]);
+    } else {
+        for (size_t j = start; i < count && j < oldInitlen; i++, j++) {
+            if (!nobj->setElementSpecific<Type>(cx, j, vp[i]))
+                return DenseElementResult::Incomplete;
+        }
+    }
+
+    if (i != count) {
+        obj->as<UnboxedArrayObject>().setInitializedLength(start + count);
+        if (updateTypes == ShouldUpdateTypes::DontUpdate) {
+            for (; i < count; i++)
+                nobj->initElementNoTypeChangeSpecific<Type>(start + i, vp[i]);
+        } else {
+            for (; i < count; i++) {
+                if (!nobj->initElementSpecific<Type>(cx, start + i, vp[i])) {
+                    nobj->setInitializedLengthNoBarrier(oldInitlen);
+                    return DenseElementResult::Incomplete;
+                }
+            }
+        }
+    }
+
+    if (start + count >= nobj->length())
+        nobj->setLength(cx, start + count);
+
+    return DenseElementResult::Success;
+}
+
+template <JSValueType Type>
+static inline DenseElementResult
+MoveBoxedOrUnboxedDenseElements(JSContext* cx, JSObject* obj, uint32_t dstStart, uint32_t srcStart,
+                                uint32_t length)
+{
+    MOZ_ASSERT(HasBoxedOrUnboxedDenseElements<Type>(obj));
+
+    if (Type == JSVAL_TYPE_MAGIC) {
+        if (obj->as<NativeObject>().denseElementsAreFrozen())
+            return DenseElementResult::Incomplete;
+
+        if (!obj->as<NativeObject>().maybeCopyElementsForWrite(cx))
+            return DenseElementResult::Failure;
+        obj->as<NativeObject>().moveDenseElements(dstStart, srcStart, length);
+    } else {
+        uint8_t* data = obj->as<UnboxedArrayObject>().elements();
+        size_t elementSize = UnboxedTypeSize(Type);
+
+        if (UnboxedTypeNeedsPreBarrier(Type) &&
+            JS::shadow::Zone::asShadowZone(obj->zone())->needsIncrementalBarrier())
+        {
+            // Trigger pre barriers on any elements we are overwriting. See
+            // NativeObject::moveDenseElements. No post barrier is needed as
+            // only whole cell post barriers are used with unboxed objects.
+            for (size_t i = 0; i < length; i++)
+                obj->as<UnboxedArrayObject>().triggerPreBarrier<Type>(dstStart + i);
+        }
+
+        memmove(data + dstStart * elementSize,
+                data + srcStart * elementSize,
+                length * elementSize);
+    }
+
+    return DenseElementResult::Success;
+}
+
+template <JSValueType DstType, JSValueType SrcType>
+static inline DenseElementResult
+CopyBoxedOrUnboxedDenseElements(JSContext* cx, JSObject* dst, JSObject* src,
+                                uint32_t dstStart, uint32_t srcStart, uint32_t length)
+{
+    MOZ_ASSERT(HasBoxedOrUnboxedDenseElements<SrcType>(src));
+    MOZ_ASSERT(HasBoxedOrUnboxedDenseElements<DstType>(dst));
+    MOZ_ASSERT(GetBoxedOrUnboxedInitializedLength<DstType>(dst) == dstStart);
+    MOZ_ASSERT(GetBoxedOrUnboxedInitializedLength<SrcType>(src) >= srcStart + length);
+    MOZ_ASSERT(GetBoxedOrUnboxedCapacity<DstType>(dst) >= dstStart + length);
+
+    SetBoxedOrUnboxedInitializedLength<DstType>(cx, dst, dstStart + length);
+
+    if (DstType == JSVAL_TYPE_MAGIC) {
+        if (SrcType == JSVAL_TYPE_MAGIC) {
+            const Value* vp = src->as<NativeObject>().getDenseElements() + srcStart;
+            dst->as<NativeObject>().initDenseElements(dstStart, vp, length);
+        } else {
+            for (size_t i = 0; i < length; i++) {
+                Value v = GetBoxedOrUnboxedDenseElement<SrcType>(src, srcStart + i);
+                dst->as<NativeObject>().initDenseElement(dstStart + i, v);
+            }
+        }
+    } else if (DstType == SrcType) {
+        uint8_t* dstData = dst->as<UnboxedArrayObject>().elements();
+        uint8_t* srcData = src->as<UnboxedArrayObject>().elements();
+        size_t elementSize = UnboxedTypeSize(DstType);
+
+        memcpy(dstData + dstStart * elementSize,
+               srcData + srcStart * elementSize,
+               length * elementSize);
+
+        // Add a store buffer entry if we might have copied a nursery pointer to dst.
+        if (UnboxedTypeNeedsPostBarrier(DstType) && !IsInsideNursery(dst))
+            dst->runtimeFromMainThread()->gc.storeBuffer.putWholeCell(dst);
+    } else if (DstType == JSVAL_TYPE_DOUBLE && SrcType == JSVAL_TYPE_INT32) {
+        uint8_t* dstData = dst->as<UnboxedArrayObject>().elements();
+        uint8_t* srcData = src->as<UnboxedArrayObject>().elements();
+
+        for (size_t i = 0; i < length; i++) {
+            int32_t v = *reinterpret_cast<int32_t*>(srcData + (srcStart + i) * sizeof(int32_t));
+            *reinterpret_cast<double*>(dstData + (dstStart + i) * sizeof(double)) = v;
+        }
+    } else {
+        for (size_t i = 0; i < length; i++) {
+            Value v = GetBoxedOrUnboxedDenseElement<SrcType>(src, srcStart + i);
+            dst->as<UnboxedArrayObject>().initElementNoTypeChangeSpecific<DstType>(dstStart + i, v);
+        }
+    }
+
+    return DenseElementResult::Success;
+}
+
+/////////////////////////////////////////////////////////////////////
+// Dispatch to specialized methods based on the type of an object.
+/////////////////////////////////////////////////////////////////////
+
+// Goop to fix MSVC. See DispatchTraceKindTyped in TraceKind.h.
+// The clang-cl front end defines _MSC_VER, but still requires the explicit
+// template declaration, so we must test for __clang__ here as well.
+#if defined(_MSC_VER) && !defined(__clang__)
+# define DEPENDENT_TEMPLATE_HINT
+#else
+# define DEPENDENT_TEMPLATE_HINT template
+#endif
+
+// Function to dispatch a method specialized to whatever boxed or unboxed dense
+// elements which an input object has.
+template <typename F>
+DenseElementResult
+CallBoxedOrUnboxedSpecialization(F f, JSObject* obj)
+{
+    if (!HasAnyBoxedOrUnboxedDenseElements(obj))
+        return DenseElementResult::Incomplete;
+    switch (GetBoxedOrUnboxedType(obj)) {
+      case JSVAL_TYPE_MAGIC:
+        return f. DEPENDENT_TEMPLATE_HINT operator()<JSVAL_TYPE_MAGIC>();
+      case JSVAL_TYPE_BOOLEAN:
+        return f. DEPENDENT_TEMPLATE_HINT operator()<JSVAL_TYPE_BOOLEAN>();
+      case JSVAL_TYPE_INT32:
+        return f. DEPENDENT_TEMPLATE_HINT operator()<JSVAL_TYPE_INT32>();
+      case JSVAL_TYPE_DOUBLE:
+        return f. DEPENDENT_TEMPLATE_HINT operator()<JSVAL_TYPE_DOUBLE>();
+      case JSVAL_TYPE_STRING:
+        return f. DEPENDENT_TEMPLATE_HINT operator()<JSVAL_TYPE_STRING>();
+      case JSVAL_TYPE_OBJECT:
+        return f. DEPENDENT_TEMPLATE_HINT operator()<JSVAL_TYPE_OBJECT>();
+      default:
+        MOZ_CRASH();
+    }
+}
+
+// As above, except the specialization can reflect the unboxed type of two objects.
+template <typename F>
+DenseElementResult
+CallBoxedOrUnboxedSpecialization(F f, JSObject* obj1, JSObject* obj2)
+{
+    if (!HasAnyBoxedOrUnboxedDenseElements(obj1) || !HasAnyBoxedOrUnboxedDenseElements(obj2))
+        return DenseElementResult::Incomplete;
+
+#define SPECIALIZE_OBJ2(TYPE)                                                     \
+    switch (GetBoxedOrUnboxedType(obj2)) {                                        \
+      case JSVAL_TYPE_MAGIC:                                                      \
+        return f. DEPENDENT_TEMPLATE_HINT operator()<TYPE, JSVAL_TYPE_MAGIC>();   \
+      case JSVAL_TYPE_BOOLEAN:                                                    \
+        return f. DEPENDENT_TEMPLATE_HINT operator()<TYPE, JSVAL_TYPE_BOOLEAN>(); \
+      case JSVAL_TYPE_INT32:                                                      \
+        return f. DEPENDENT_TEMPLATE_HINT operator()<TYPE, JSVAL_TYPE_INT32>();   \
+      case JSVAL_TYPE_DOUBLE:                                                     \
+        return f. DEPENDENT_TEMPLATE_HINT operator()<TYPE, JSVAL_TYPE_DOUBLE>();  \
+      case JSVAL_TYPE_STRING:                                                     \
+        return f. DEPENDENT_TEMPLATE_HINT operator()<TYPE, JSVAL_TYPE_STRING>();  \
+      case JSVAL_TYPE_OBJECT:                                                     \
+        return f. DEPENDENT_TEMPLATE_HINT operator()<TYPE, JSVAL_TYPE_OBJECT>();  \
+      default:                                                                    \
+        MOZ_CRASH();                                                              \
+    }
+
+    switch (GetBoxedOrUnboxedType(obj1)) {
+      case JSVAL_TYPE_MAGIC:
+        SPECIALIZE_OBJ2(JSVAL_TYPE_MAGIC)
+      case JSVAL_TYPE_BOOLEAN:
+        SPECIALIZE_OBJ2(JSVAL_TYPE_BOOLEAN)
+      case JSVAL_TYPE_INT32:
+        SPECIALIZE_OBJ2(JSVAL_TYPE_INT32)
+      case JSVAL_TYPE_DOUBLE:
+        SPECIALIZE_OBJ2(JSVAL_TYPE_DOUBLE)
+      case JSVAL_TYPE_STRING:
+        SPECIALIZE_OBJ2(JSVAL_TYPE_STRING)
+      case JSVAL_TYPE_OBJECT:
+        SPECIALIZE_OBJ2(JSVAL_TYPE_OBJECT)
+      default:
+        MOZ_CRASH();
+    }
+
+#undef SPECIALIZE_OBJ2
+}
+
+#undef DEPENDENT_TEMPLATE_HINT
+
+#define DefineBoxedOrUnboxedFunctor1(Signature, A)                      \
+struct Signature ## Functor {                                           \
+    A a;                                                                \
+    explicit Signature ## Functor(A a)                                  \
+      : a(a)                                                            \
+    {}                                                                  \
+    template <JSValueType Type>                                         \
+    DenseElementResult operator()() {                                   \
+        return Signature<Type>(a);                                      \
+    }                                                                   \
+}
+
+#define DefineBoxedOrUnboxedFunctor3(Signature, A, B, C)                \
+struct Signature ## Functor {                                           \
+    A a; B b; C c;                                                      \
+    Signature ## Functor(A a, B b, C c)                                 \
+      : a(a), b(b), c(c)                                                \
+    {}                                                                  \
+    template <JSValueType Type>                                         \
+    DenseElementResult operator()() {                                   \
+        return Signature<Type>(a, b, c);                                \
+    }                                                                   \
+}
+
+#define DefineBoxedOrUnboxedFunctor4(Signature, A, B, C, D)             \
+struct Signature ## Functor {                                           \
+    A a; B b; C c; D d;                                                 \
+    Signature ## Functor(A a, B b, C c, D d)                            \
+      : a(a), b(b), c(c), d(d)                                          \
+    {}                                                                  \
+    template <JSValueType Type>                                         \
+    DenseElementResult operator()() {                                   \
+        return Signature<Type>(a, b, c, d);                             \
+    }                                                                   \
+}
+
+#define DefineBoxedOrUnboxedFunctorPair4(Signature, A, B, C, D)         \
+struct Signature ## Functor {                                           \
+    A a; B b; C c; D d;                                                 \
+    Signature ## Functor(A a, B b, C c, D d)                            \
+      : a(a), b(b), c(c), d(d)                                          \
+    {}                                                                  \
+    template <JSValueType TypeOne, JSValueType TypeTwo>                 \
+    DenseElementResult operator()() {                                   \
+        return Signature<TypeOne, TypeTwo>(a, b, c, d);                 \
+    }                                                                   \
+}
+
+#define DefineBoxedOrUnboxedFunctor5(Signature, A, B, C, D, E)          \
+struct Signature ## Functor {                                           \
+    A a; B b; C c; D d; E e;                                            \
+    Signature ## Functor(A a, B b, C c, D d, E e)                       \
+      : a(a), b(b), c(c), d(d), e(e)                                    \
+    {}                                                                  \
+    template <JSValueType Type>                                         \
+    DenseElementResult operator()() {                                   \
+        return Signature<Type>(a, b, c, d, e);                          \
+    }                                                                   \
+}
+
+#define DefineBoxedOrUnboxedFunctor6(Signature, A, B, C, D, E, F)       \
+struct Signature ## Functor {                                           \
+    A a; B b; C c; D d; E e; F f;                                       \
+    Signature ## Functor(A a, B b, C c, D d, E e, F f)                  \
+      : a(a), b(b), c(c), d(d), e(e), f(f)                              \
+    {}                                                                  \
+    template <JSValueType Type>                                         \
+    DenseElementResult operator()() {                                   \
+        return Signature<Type>(a, b, c, d, e, f);                       \
+    }                                                                   \
+}
+
+#define DefineBoxedOrUnboxedFunctorPair6(Signature, A, B, C, D, E, F)   \
+struct Signature ## Functor {                                           \
+    A a; B b; C c; D d; E e; F f;                                       \
+    Signature ## Functor(A a, B b, C c, D d, E e, F f)                  \
+      : a(a), b(b), c(c), d(d), e(e), f(f)                              \
+    {}                                                                  \
+    template <JSValueType TypeOne, JSValueType TypeTwo>                 \
+    DenseElementResult operator()() {                                   \
+        return Signature<TypeOne, TypeTwo>(a, b, c, d, e, f);           \
+    }                                                                   \
+}
+
+DenseElementResult
+SetOrExtendAnyBoxedOrUnboxedDenseElements(ExclusiveContext* cx, JSObject* obj,
+                                          uint32_t start, const Value* vp, uint32_t count,
+                                          ShouldUpdateTypes updateTypes = ShouldUpdateTypes::Update);
+
+DenseElementResult
+MoveAnyBoxedOrUnboxedDenseElements(JSContext* cx, JSObject* obj,
+                                   uint32_t dstStart, uint32_t srcStart, uint32_t length);
+
+DenseElementResult
+CopyAnyBoxedOrUnboxedDenseElements(JSContext* cx, JSObject* dst, JSObject* src,
+                                   uint32_t dstStart, uint32_t srcStart, uint32_t length);
+
+void
+SetAnyBoxedOrUnboxedInitializedLength(JSContext* cx, JSObject* obj, size_t initlen);
+
+DenseElementResult
+EnsureAnyBoxedOrUnboxedDenseElements(JSContext* cx, JSObject* obj, size_t count);
+
 } // namespace js
 
 #endif // vm_UnboxedObject_inl_h
diff --git a/js/src/vm/UnboxedObject.cpp b/js/src/vm/UnboxedObject.cpp
index 2e017ca3bd..059293a2df 100644
--- a/js/src/vm/UnboxedObject.cpp
+++ b/js/src/vm/UnboxedObject.cpp
@@ -417,6 +417,8 @@ UnboxedLayout::makeNativeGroup(JSContext* cx, ObjectGroup* group)
 
     RootedObjectGroup replacementGroup(cx);
 
+    const Class* clasp = layout.isArray() ? &ArrayObject::class_ : &PlainObject::class_;
+
     // Immediately clear any new script on the group. This is done by replacing
     // the existing new script with one for a replacement default new group.
     // This is done so that the size of the replacment group's objects is the
@@ -424,6 +426,8 @@ UnboxedLayout::makeNativeGroup(JSContext* cx, ObjectGroup* group)
     // slot accesses later on for sites that see converted objects from this
     // group and objects that were allocated using the replacement new group.
     if (layout.newScript()) {
+        MOZ_ASSERT(!layout.isArray());
+
         replacementGroup = ObjectGroupCompartment::makeGroup(cx, &PlainObject::class_, proto);
         if (!replacementGroup)
             return false;
@@ -449,14 +453,15 @@ UnboxedLayout::makeNativeGroup(JSContext* cx, ObjectGroup* group)
         RootedScript script(cx, layout.allocationScript());
         jsbytecode* pc = layout.allocationPc();
 
-        replacementGroup = ObjectGroupCompartment::makeGroup(cx, &PlainObject::class_, proto);
+        replacementGroup = ObjectGroupCompartment::makeGroup(cx, clasp, proto);
         if (!replacementGroup)
             return false;
 
         PlainObject* templateObject = &script->getObject(pc)->as<PlainObject>();
         replacementGroup->addDefiniteProperties(cx, templateObject->lastProperty());
 
-        cx->compartment()->objectGroups.replaceAllocationSiteGroup(script, pc, JSProto_Object,
+        JSProtoKey key = layout.isArray() ? JSProto_Array : JSProto_Object;
+        cx->compartment()->objectGroups.replaceAllocationSiteGroup(script, pc, key,
                                                                    replacementGroup);
 
         // Clear any baseline information at this opcode which might use the old group.
@@ -472,12 +477,22 @@ UnboxedLayout::makeNativeGroup(JSContext* cx, ObjectGroup* group)
         }
     }
 
-    size_t nfixed = gc::GetGCKindSlots(layout.getAllocKind());
+    size_t nfixed = layout.isArray() ? 0 : gc::GetGCKindSlots(layout.getAllocKind());
+
+    if (layout.isArray()) {
+        // The length shape to use for arrays is cached via a modified initial
+        // shape for array objects. Create an array now to make sure this entry
+        // is instantiated.
+        if (!NewDenseEmptyArray(cx))
+            return false;
+    }
 
-    RootedShape shape(cx, EmptyShape::getInitialShape(cx, &PlainObject::class_, proto, nfixed, 0));
+    RootedShape shape(cx, EmptyShape::getInitialShape(cx, clasp, proto, nfixed, 0));
     if (!shape)
         return false;
 
+    MOZ_ASSERT_IF(layout.isArray(), !shape->isEmptyShape() && shape->slotSpan() == 0);
+
     // Add shapes for each property, if this is for a plain object.
     for (size_t i = 0; i < layout.properties().length(); i++) {
         const UnboxedLayout::Property& property = layout.properties()[i];
@@ -490,7 +505,7 @@ UnboxedLayout::makeNativeGroup(JSContext* cx, ObjectGroup* group)
     }
 
     ObjectGroup* nativeGroup =
-        ObjectGroupCompartment::makeGroup(cx, &PlainObject::class_, proto,
+        ObjectGroupCompartment::makeGroup(cx, clasp, proto,
                                           group->flags() & OBJECT_FLAG_DYNAMIC_MASK);
     if (!nativeGroup)
         return false;
@@ -498,19 +513,24 @@ UnboxedLayout::makeNativeGroup(JSContext* cx, ObjectGroup* group)
     // No sense propagating if we don't know what we started with.
     if (!group->unknownProperties()) {
         // Propagate all property types from the old group to the new group.
-        for (size_t i = 0; i < layout.properties().length(); i++) {
-            const UnboxedLayout::Property& property = layout.properties()[i];
-            jsid id = NameToId(property.name);
-            if (!PropagatePropertyTypes(cx, id, group, nativeGroup))
+        if (layout.isArray()) {
+            if (!PropagatePropertyTypes(cx, JSID_VOID, group, nativeGroup))
                 return false;
+        } else {
+            for (size_t i = 0; i < layout.properties().length(); i++) {
+                const UnboxedLayout::Property& property = layout.properties()[i];
+                jsid id = NameToId(property.name);
+                if (!PropagatePropertyTypes(cx, id, group, nativeGroup))
+                    return false;
 
-            // If we are OOM we may not be able to propagate properties.
-            if (nativeGroup->unknownProperties())
-                break;
+                // If we are OOM we may not be able to propagate properties.
+                if (nativeGroup->unknownProperties())
+                    break;
 
-            HeapTypeSet* nativeProperty = nativeGroup->maybeGetProperty(id);
-            if (nativeProperty && nativeProperty->canSetDefinite(i))
-                nativeProperty->setDefinite(i);
+                HeapTypeSet* nativeProperty = nativeGroup->maybeGetProperty(id);
+                if (nativeProperty && nativeProperty->canSetDefinite(i))
+                    nativeProperty->setDefinite(i);
+            }
         }
     } else {
         // If we skip, though, the new group had better agree.
@@ -926,6 +946,692 @@ const Class UnboxedPlainObject::class_ = {
 };
 
 /////////////////////////////////////////////////////////////////////
+// UnboxedArrayObject
+/////////////////////////////////////////////////////////////////////
+
+template <JSValueType Type>
+DenseElementResult
+AppendUnboxedDenseElements(UnboxedArrayObject* obj, uint32_t initlen,
+                           MutableHandle<GCVector<Value>> values)
+{
+    for (size_t i = 0; i < initlen; i++)
+        values.infallibleAppend(obj->getElementSpecific<Type>(i));
+    return DenseElementResult::Success;
+}
+
+DefineBoxedOrUnboxedFunctor3(AppendUnboxedDenseElements,
+                             UnboxedArrayObject*, uint32_t, MutableHandle<GCVector<Value>>);
+
+/* static */ bool
+UnboxedArrayObject::convertToNativeWithGroup(ExclusiveContext* cx, JSObject* obj,
+                                             ObjectGroup* group, Shape* shape)
+{
+    size_t length = obj->as<UnboxedArrayObject>().length();
+    size_t initlen = obj->as<UnboxedArrayObject>().initializedLength();
+
+    Rooted<GCVector<Value>> values(cx, GCVector<Value>(cx));
+    if (!values.reserve(initlen))
+        return false;
+
+    AppendUnboxedDenseElementsFunctor functor(&obj->as<UnboxedArrayObject>(), initlen, &values);
+    DebugOnly<DenseElementResult> result = CallBoxedOrUnboxedSpecialization(functor, obj);
+    MOZ_ASSERT(result.value == DenseElementResult::Success);
+
+    obj->setGroup(group);
+
+    ArrayObject* aobj = &obj->as<ArrayObject>();
+    aobj->setLastPropertyMakeNative(cx, shape);
+
+    // Make sure there is at least one element, so that this array does not
+    // use emptyObjectElements / emptyObjectElementsShared.
+    if (!aobj->ensureElements(cx, Max<size_t>(initlen, 1)))
+        return false;
+
+    MOZ_ASSERT(!aobj->getDenseInitializedLength());
+    aobj->setDenseInitializedLength(initlen);
+    aobj->initDenseElements(0, values.begin(), initlen);
+    aobj->setLengthInt32(length);
+
+    return true;
+}
+
+/* static */ bool
+UnboxedArrayObject::convertToNative(JSContext* cx, JSObject* obj)
+{
+    const UnboxedLayout& layout = obj->as<UnboxedArrayObject>().layout();
+
+    if (!layout.nativeGroup()) {
+        if (!UnboxedLayout::makeNativeGroup(cx, obj->group()))
+            return false;
+    }
+
+    return convertToNativeWithGroup(cx, obj, layout.nativeGroup(), layout.nativeShape());
+}
+
+bool
+UnboxedArrayObject::convertInt32ToDouble(ExclusiveContext* cx, ObjectGroup* group)
+{
+    MOZ_ASSERT(elementType() == JSVAL_TYPE_INT32);
+    MOZ_ASSERT(group->unboxedLayout().elementType() == JSVAL_TYPE_DOUBLE);
+
+    Vector<int32_t> values(cx);
+    if (!values.reserve(initializedLength()))
+        return false;
+    for (size_t i = 0; i < initializedLength(); i++)
+        values.infallibleAppend(getElementSpecific<JSVAL_TYPE_INT32>(i).toInt32());
+
+    uint8_t* newElements;
+    if (hasInlineElements()) {
+        newElements = AllocateObjectBuffer<uint8_t>(cx, this, capacity() * sizeof(double));
+    } else {
+        newElements = ReallocateObjectBuffer<uint8_t>(cx, this, elements(),
+                                                      capacity() * sizeof(int32_t),
+                                                      capacity() * sizeof(double));
+    }
+    if (!newElements)
+        return false;
+
+    setGroup(group);
+    elements_ = newElements;
+
+    for (size_t i = 0; i < initializedLength(); i++)
+        setElementNoTypeChangeSpecific<JSVAL_TYPE_DOUBLE>(i, DoubleValue(values[i]));
+
+    return true;
+}
+
+/* static */ UnboxedArrayObject*
+UnboxedArrayObject::create(ExclusiveContext* cx, HandleObjectGroup group, uint32_t length,
+                           NewObjectKind newKind, uint32_t maxLength)
+{
+    MOZ_ASSERT(length <= MaximumCapacity);
+
+    MOZ_ASSERT(group->clasp() == &class_);
+    uint32_t elementSize = UnboxedTypeSize(group->unboxedLayout().elementType());
+    uint32_t capacity = Min(length, maxLength);
+    uint32_t nbytes = offsetOfInlineElements() + elementSize * capacity;
+
+    UnboxedArrayObject* res;
+    if (nbytes <= JSObject::MAX_BYTE_SIZE) {
+        gc::AllocKind allocKind = gc::GetGCObjectKindForBytes(nbytes);
+
+        // If there was no provided length information, pick an allocation kind
+        // to accommodate small arrays (as is done for normal native arrays).
+        if (capacity == 0)
+            allocKind = gc::AllocKind::OBJECT8;
+
+        res = NewObjectWithGroup<UnboxedArrayObject>(cx, group, allocKind, newKind);
+        if (!res)
+            return nullptr;
+        res->setInitializedLengthNoBarrier(0);
+        res->setInlineElements();
+
+        size_t actualCapacity = (GetGCKindBytes(allocKind) - offsetOfInlineElements()) / elementSize;
+        MOZ_ASSERT(actualCapacity >= capacity);
+        res->setCapacityIndex(exactCapacityIndex(actualCapacity));
+    } else {
+        res = NewObjectWithGroup<UnboxedArrayObject>(cx, group, gc::AllocKind::OBJECT0, newKind);
+        if (!res)
+            return nullptr;
+        res->setInitializedLengthNoBarrier(0);
+
+        uint32_t capacityIndex = (capacity == length)
+                                 ? CapacityMatchesLengthIndex
+                                 : chooseCapacityIndex(capacity, length);
+        uint32_t actualCapacity = computeCapacity(capacityIndex, length);
+
+        res->elements_ = AllocateObjectBuffer<uint8_t>(cx, res, actualCapacity * elementSize);
+        if (!res->elements_) {
+            // Make the object safe for GC.
+            res->setInlineElements();
+            return nullptr;
+        }
+
+        res->setCapacityIndex(capacityIndex);
+    }
+
+    res->setLength(cx, length);
+    return res;
+}
+
+bool
+UnboxedArrayObject::setElement(ExclusiveContext* cx, size_t index, const Value& v)
+{
+    MOZ_ASSERT(index < initializedLength());
+    uint8_t* p = elements() + index * elementSize();
+    return SetUnboxedValue(cx, this, JSID_VOID, p, elementType(), v, /* preBarrier = */ true);
+}
+
+bool
+UnboxedArrayObject::initElement(ExclusiveContext* cx, size_t index, const Value& v)
+{
+    MOZ_ASSERT(index < initializedLength());
+    uint8_t* p = elements() + index * elementSize();
+    return SetUnboxedValue(cx, this, JSID_VOID, p, elementType(), v, /* preBarrier = */ false);
+}
+
+void
+UnboxedArrayObject::initElementNoTypeChange(size_t index, const Value& v)
+{
+    MOZ_ASSERT(index < initializedLength());
+    uint8_t* p = elements() + index * elementSize();
+    if (UnboxedTypeNeedsPreBarrier(elementType()))
+        *reinterpret_cast<void**>(p) = nullptr;
+    SetUnboxedValueNoTypeChange(this, p, elementType(), v, /* preBarrier = */ false);
+}
+
+Value
+UnboxedArrayObject::getElement(size_t index)
+{
+    MOZ_ASSERT(index < initializedLength());
+    uint8_t* p = elements() + index * elementSize();
+    return GetUnboxedValue(p, elementType(), /* maybeUninitialized = */ false);
+}
+
+/* static */ void
+UnboxedArrayObject::trace(JSTracer* trc, JSObject* obj)
+{
+    JSValueType type = obj->as<UnboxedArrayObject>().elementType();
+    if (!UnboxedTypeNeedsPreBarrier(type))
+        return;
+
+    MOZ_ASSERT(obj->as<UnboxedArrayObject>().elementSize() == sizeof(uintptr_t));
+    size_t initlen = obj->as<UnboxedArrayObject>().initializedLength();
+    void** elements = reinterpret_cast<void**>(obj->as<UnboxedArrayObject>().elements());
+
+    switch (type) {
+      case JSVAL_TYPE_OBJECT:
+        for (size_t i = 0; i < initlen; i++) {
+            GCPtrObject* heap = reinterpret_cast<GCPtrObject*>(elements + i);
+            TraceNullableEdge(trc, heap, "unboxed_object");
+        }
+        break;
+
+      case JSVAL_TYPE_STRING:
+        for (size_t i = 0; i < initlen; i++) {
+            GCPtrString* heap = reinterpret_cast<GCPtrString*>(elements + i);
+            TraceEdge(trc, heap, "unboxed_string");
+        }
+        break;
+
+      default:
+        MOZ_CRASH();
+    }
+}
+
+/* static */ void
+UnboxedArrayObject::objectMoved(JSObject* obj, const JSObject* old)
+{
+    UnboxedArrayObject& dst = obj->as<UnboxedArrayObject>();
+    const UnboxedArrayObject& src = old->as<UnboxedArrayObject>();
+
+    // Fix up possible inline data pointer.
+    if (src.hasInlineElements())
+        dst.setInlineElements();
+}
+
+/* static */ void
+UnboxedArrayObject::finalize(FreeOp* fop, JSObject* obj)
+{
+    MOZ_ASSERT(!IsInsideNursery(obj));
+    if (!obj->as<UnboxedArrayObject>().hasInlineElements())
+        js_free(obj->as<UnboxedArrayObject>().elements());
+}
+
+/* static */ size_t
+UnboxedArrayObject::objectMovedDuringMinorGC(JSTracer* trc, JSObject* dst, JSObject* src,
+                                             gc::AllocKind allocKind)
+{
+    UnboxedArrayObject* ndst = &dst->as<UnboxedArrayObject>();
+    UnboxedArrayObject* nsrc = &src->as<UnboxedArrayObject>();
+    MOZ_ASSERT(ndst->elements() == nsrc->elements());
+
+    Nursery& nursery = trc->runtime()->gc.nursery;
+
+    if (!nursery.isInside(nsrc->elements())) {
+        nursery.removeMallocedBuffer(nsrc->elements());
+        return 0;
+    }
+
+    // Determine if we can use inline data for the target array. If this is
+    // possible, the nursery will have picked an allocation size that is large
+    // enough.
+    size_t nbytes = nsrc->capacity() * nsrc->elementSize();
+    if (offsetOfInlineElements() + nbytes <= GetGCKindBytes(allocKind)) {
+        ndst->setInlineElements();
+    } else {
+        MOZ_ASSERT(allocKind == gc::AllocKind::OBJECT0);
+
+        AutoEnterOOMUnsafeRegion oomUnsafe;
+        uint8_t* data = nsrc->zone()->pod_malloc<uint8_t>(nbytes);
+        if (!data)
+            oomUnsafe.crash("Failed to allocate unboxed array elements while tenuring.");
+        ndst->elements_ = data;
+    }
+
+    PodCopy(ndst->elements(), nsrc->elements(), nsrc->initializedLength() * nsrc->elementSize());
+
+    // Set a forwarding pointer for the element buffers in case they were
+    // preserved on the stack by Ion.
+    bool direct = nsrc->capacity() * nsrc->elementSize() >= sizeof(uintptr_t);
+    nursery.maybeSetForwardingPointer(trc, nsrc->elements(), ndst->elements(), direct);
+
+    return ndst->hasInlineElements() ? 0 : nbytes;
+}
+
+// Possible capacities for unboxed arrays. Some of these capacities might seem
+// a little weird, but were chosen to allow the inline data of objects of each
+// size to be fully utilized for arrays of the various types on both 32 bit and
+// 64 bit platforms.
+//
+// To find the possible inline capacities, the following script was used:
+//
+// var fixedSlotCapacities = [0, 2, 4, 8, 12, 16];
+// var dataSizes = [1, 4, 8];
+// var header32 = 4 * 2 + 4 * 2;
+// var header64 = 8 * 2 + 4 * 2;
+//
+// for (var i = 0; i < fixedSlotCapacities.length; i++) {
+//    var nfixed = fixedSlotCapacities[i];
+//    var size32 = 4 * 4 + 8 * nfixed - header32;
+//    var size64 = 8 * 4 + 8 * nfixed - header64;
+//    for (var j = 0; j < dataSizes.length; j++) {
+//        print(size32 / dataSizes[j]);
+//        print(size64 / dataSizes[j]);
+//    }
+// }
+//
+/* static */ const uint32_t
+UnboxedArrayObject::CapacityArray[] = {
+    UINT32_MAX, // For CapacityMatchesLengthIndex.
+    0, 1, 2, 3, 4, 5, 6, 8, 9, 10, 12, 13, 16, 17, 18, 24, 26, 32, 34, 40, 64, 72, 96, 104, 128, 136,
+    256, 512, 1024, 2048, 4096, 8192, 16384, 32768, 65536, 131072, 262144, 524288,
+    1048576, 2097152, 3145728, 4194304, 5242880, 6291456, 7340032, 8388608, 9437184, 11534336,
+    13631488, 15728640, 17825792, 20971520, 24117248, 27262976, 31457280, 35651584, 40894464,
+    46137344, 52428800, 59768832, MaximumCapacity
+};
+
+static const uint32_t
+Pow2CapacityIndexes[] = {
+    2,  // 1
+    3,  // 2
+    5,  // 4
+    8,  // 8
+    13, // 16
+    18, // 32
+    21, // 64
+    25, // 128
+    27, // 256
+    28, // 512
+    29, // 1024
+    30, // 2048
+    31, // 4096
+    32, // 8192
+    33, // 16384
+    34, // 32768
+    35, // 65536
+    36, // 131072
+    37, // 262144
+    38, // 524288
+    39  // 1048576
+};
+
+static const uint32_t MebiCapacityIndex = 39;
+
+/* static */ uint32_t
+UnboxedArrayObject::chooseCapacityIndex(uint32_t capacity, uint32_t length)
+{
+    // Note: the structure and behavior of this method follow along with
+    // NativeObject::goodAllocated. Changes to the allocation strategy in one
+    // should generally be matched by the other.
+
+    // Make sure we have enough space to store all possible values for the capacity index.
+    // This ought to be a static_assert, but MSVC doesn't like that.
+    MOZ_ASSERT(mozilla::ArrayLength(CapacityArray) - 1 <= (CapacityMask >> CapacityShift));
+
+    // The caller should have ensured the capacity is possible for an unboxed array.
+    MOZ_ASSERT(capacity <= MaximumCapacity);
+
+    static const uint32_t Mebi = 1024 * 1024;
+
+    if (capacity <= Mebi) {
+        capacity = mozilla::RoundUpPow2(capacity);
+
+        // When the required capacity is close to the array length, then round
+        // up to the array length itself, as for NativeObject.
+        if (length >= capacity && capacity > (length / 3) * 2)
+            return CapacityMatchesLengthIndex;
+
+        if (capacity < MinimumDynamicCapacity)
+            capacity = MinimumDynamicCapacity;
+
+        uint32_t bit = mozilla::FloorLog2Size(capacity);
+        MOZ_ASSERT(capacity == uint32_t(1 << bit));
+        MOZ_ASSERT(bit <= 20);
+        MOZ_ASSERT(mozilla::ArrayLength(Pow2CapacityIndexes) == 21);
+
+        uint32_t index = Pow2CapacityIndexes[bit];
+        MOZ_ASSERT(CapacityArray[index] == capacity);
+
+        return index;
+    }
+
+    MOZ_ASSERT(CapacityArray[MebiCapacityIndex] == Mebi);
+
+    for (uint32_t i = MebiCapacityIndex + 1;; i++) {
+        if (CapacityArray[i] >= capacity)
+            return i;
+    }
+
+    MOZ_CRASH("Invalid capacity");
+}
+
+/* static */ uint32_t
+UnboxedArrayObject::exactCapacityIndex(uint32_t capacity)
+{
+    for (size_t i = CapacityMatchesLengthIndex + 1; i < ArrayLength(CapacityArray); i++) {
+        if (CapacityArray[i] == capacity)
+            return i;
+    }
+    MOZ_CRASH();
+}
+
+bool
+UnboxedArrayObject::growElements(ExclusiveContext* cx, size_t cap)
+{
+    // The caller should have checked if this capacity is possible for an
+    // unboxed array, so the only way this call can fail is from OOM.
+    MOZ_ASSERT(cap <= MaximumCapacity);
+
+    uint32_t oldCapacity = capacity();
+    uint32_t newCapacityIndex = chooseCapacityIndex(cap, length());
+    uint32_t newCapacity = computeCapacity(newCapacityIndex, length());
+
+    MOZ_ASSERT(oldCapacity < cap);
+    MOZ_ASSERT(cap <= newCapacity);
+
+    // The allocation size computation below cannot have integer overflows.
+    JS_STATIC_ASSERT(MaximumCapacity < UINT32_MAX / sizeof(double));
+
+    uint8_t* newElements;
+    if (hasInlineElements()) {
+        newElements = AllocateObjectBuffer<uint8_t>(cx, this, newCapacity * elementSize());
+        if (!newElements)
+            return false;
+        js_memcpy(newElements, elements(), initializedLength() * elementSize());
+    } else {
+        newElements = ReallocateObjectBuffer<uint8_t>(cx, this, elements(),
+                                                      oldCapacity * elementSize(),
+                                                      newCapacity * elementSize());
+        if (!newElements)
+            return false;
+    }
+
+    elements_ = newElements;
+    setCapacityIndex(newCapacityIndex);
+
+    return true;
+}
+
+void
+UnboxedArrayObject::shrinkElements(ExclusiveContext* cx, size_t cap)
+{
+    if (hasInlineElements())
+        return;
+
+    uint32_t oldCapacity = capacity();
+    uint32_t newCapacityIndex = chooseCapacityIndex(cap, 0);
+    uint32_t newCapacity = computeCapacity(newCapacityIndex, 0);
+
+    MOZ_ASSERT(cap < oldCapacity);
+    MOZ_ASSERT(cap <= newCapacity);
+
+    if (newCapacity >= oldCapacity)
+        return;
+
+    uint8_t* newElements = ReallocateObjectBuffer<uint8_t>(cx, this, elements(),
+                                                           oldCapacity * elementSize(),
+                                                           newCapacity * elementSize());
+    if (!newElements)
+        return;
+
+    elements_ = newElements;
+    setCapacityIndex(newCapacityIndex);
+}
+
+bool
+UnboxedArrayObject::containsProperty(ExclusiveContext* cx, jsid id)
+{
+    if (JSID_IS_INT(id) && uint32_t(JSID_TO_INT(id)) < initializedLength())
+        return true;
+    if (JSID_IS_ATOM(id) && JSID_TO_ATOM(id) == cx->names().length)
+        return true;
+    return false;
+}
+
+/* static */ bool
+UnboxedArrayObject::obj_lookupProperty(JSContext* cx, HandleObject obj,
+                                       HandleId id, MutableHandleObject objp,
+                                       MutableHandleShape propp)
+{
+    if (obj->as<UnboxedArrayObject>().containsProperty(cx, id)) {
+        MarkNonNativePropertyFound<CanGC>(propp);
+        objp.set(obj);
+        return true;
+    }
+
+    RootedObject proto(cx, obj->staticPrototype());
+    if (!proto) {
+        objp.set(nullptr);
+        propp.set(nullptr);
+        return true;
+    }
+
+    return LookupProperty(cx, proto, id, objp, propp);
+}
+
+/* static */ bool
+UnboxedArrayObject::obj_defineProperty(JSContext* cx, HandleObject obj, HandleId id,
+                                       Handle<PropertyDescriptor> desc,
+                                       ObjectOpResult& result)
+{
+    if (JSID_IS_INT(id) && !desc.getter() && !desc.setter() && desc.attributes() == JSPROP_ENUMERATE) {
+        UnboxedArrayObject* nobj = &obj->as<UnboxedArrayObject>();
+
+        uint32_t index = JSID_TO_INT(id);
+        if (index < nobj->initializedLength()) {
+            if (nobj->setElement(cx, index, desc.value()))
+                return result.succeed();
+        } else if (index == nobj->initializedLength() && index < MaximumCapacity) {
+            if (nobj->initializedLength() == nobj->capacity()) {
+                if (!nobj->growElements(cx, index + 1))
+                    return false;
+            }
+            nobj->setInitializedLength(index + 1);
+            if (nobj->initElement(cx, index, desc.value())) {
+                if (nobj->length() <= index)
+                    nobj->setLengthInt32(index + 1);
+                return result.succeed();
+            }
+            nobj->setInitializedLengthNoBarrier(index);
+        }
+    }
+
+    if (!convertToNative(cx, obj))
+        return false;
+
+    return DefineProperty(cx, obj, id, desc, result);
+}
+
+/* static */ bool
+UnboxedArrayObject::obj_hasProperty(JSContext* cx, HandleObject obj, HandleId id, bool* foundp)
+{
+    if (obj->as<UnboxedArrayObject>().containsProperty(cx, id)) {
+        *foundp = true;
+        return true;
+    }
+
+    RootedObject proto(cx, obj->staticPrototype());
+    if (!proto) {
+        *foundp = false;
+        return true;
+    }
+
+    return HasProperty(cx, proto, id, foundp);
+}
+
+/* static */ bool
+UnboxedArrayObject::obj_getProperty(JSContext* cx, HandleObject obj, HandleValue receiver,
+                                    HandleId id, MutableHandleValue vp)
+{
+    if (obj->as<UnboxedArrayObject>().containsProperty(cx, id)) {
+        if (JSID_IS_INT(id))
+            vp.set(obj->as<UnboxedArrayObject>().getElement(JSID_TO_INT(id)));
+        else
+            vp.set(Int32Value(obj->as<UnboxedArrayObject>().length()));
+        return true;
+    }
+
+    RootedObject proto(cx, obj->staticPrototype());
+    if (!proto) {
+        vp.setUndefined();
+        return true;
+    }
+
+    return GetProperty(cx, proto, receiver, id, vp);
+}
+
+/* static */ bool
+UnboxedArrayObject::obj_setProperty(JSContext* cx, HandleObject obj, HandleId id, HandleValue v,
+                                    HandleValue receiver, ObjectOpResult& result)
+{
+    if (obj->as<UnboxedArrayObject>().containsProperty(cx, id)) {
+        if (receiver.isObject() && obj == &receiver.toObject()) {
+            if (JSID_IS_INT(id)) {
+                if (obj->as<UnboxedArrayObject>().setElement(cx, JSID_TO_INT(id), v))
+                    return result.succeed();
+            } else {
+                uint32_t len;
+                if (!CanonicalizeArrayLengthValue(cx, v, &len))
+                    return false;
+                UnboxedArrayObject* nobj = &obj->as<UnboxedArrayObject>();
+                if (len < nobj->initializedLength()) {
+                    nobj->setInitializedLength(len);
+                    nobj->shrinkElements(cx, len);
+                }
+                nobj->setLength(cx, len);
+                return result.succeed();
+            }
+
+            if (!convertToNative(cx, obj))
+                return false;
+            return SetProperty(cx, obj, id, v, receiver, result);
+        }
+
+        return SetPropertyByDefining(cx, id, v, receiver, result);
+    }
+
+    return SetPropertyOnProto(cx, obj, id, v, receiver, result);
+}
+
+/* static */ bool
+UnboxedArrayObject::obj_getOwnPropertyDescriptor(JSContext* cx, HandleObject obj, HandleId id,
+                                                 MutableHandle<PropertyDescriptor> desc)
+{
+    if (obj->as<UnboxedArrayObject>().containsProperty(cx, id)) {
+        if (JSID_IS_INT(id)) {
+            desc.value().set(obj->as<UnboxedArrayObject>().getElement(JSID_TO_INT(id)));
+            desc.setAttributes(JSPROP_ENUMERATE);
+        } else {
+            desc.value().set(Int32Value(obj->as<UnboxedArrayObject>().length()));
+            desc.setAttributes(JSPROP_PERMANENT);
+        }
+        desc.object().set(obj);
+        return true;
+    }
+
+    desc.object().set(nullptr);
+    return true;
+}
+
+/* static */ bool
+UnboxedArrayObject::obj_deleteProperty(JSContext* cx, HandleObject obj, HandleId id,
+                                       ObjectOpResult& result)
+{
+    if (obj->as<UnboxedArrayObject>().containsProperty(cx, id)) {
+        size_t initlen = obj->as<UnboxedArrayObject>().initializedLength();
+        if (JSID_IS_INT(id) && JSID_TO_INT(id) == int32_t(initlen - 1)) {
+            obj->as<UnboxedArrayObject>().setInitializedLength(initlen - 1);
+            obj->as<UnboxedArrayObject>().shrinkElements(cx, initlen - 1);
+            return result.succeed();
+        }
+    }
+
+    if (!convertToNative(cx, obj))
+        return false;
+    return DeleteProperty(cx, obj, id, result);
+}
+
+/* static */ bool
+UnboxedArrayObject::obj_enumerate(JSContext* cx, HandleObject obj, AutoIdVector& properties,
+                                  bool enumerableOnly)
+{
+    for (size_t i = 0; i < obj->as<UnboxedArrayObject>().initializedLength(); i++) {
+        if (!properties.append(INT_TO_JSID(i)))
+            return false;
+    }
+
+    if (!enumerableOnly && !properties.append(NameToId(cx->names().length)))
+        return false;
+
+    return true;
+}
+
+static const ClassOps UnboxedArrayObjectClassOps = {
+    nullptr,        /* addProperty */
+    nullptr,        /* delProperty */
+    nullptr,        /* getProperty */
+    nullptr,        /* setProperty */
+    nullptr,        /* enumerate   */
+    nullptr,        /* resolve     */
+    nullptr,        /* mayResolve  */
+    UnboxedArrayObject::finalize,
+    nullptr,        /* call        */
+    nullptr,        /* hasInstance */
+    nullptr,        /* construct   */
+    UnboxedArrayObject::trace,
+};
+
+static const ClassExtension UnboxedArrayObjectClassExtension = {
+    nullptr,    /* weakmapKeyDelegateOp */
+    UnboxedArrayObject::objectMoved
+};
+
+static const ObjectOps UnboxedArrayObjectObjectOps = {
+    UnboxedArrayObject::obj_lookupProperty,
+    UnboxedArrayObject::obj_defineProperty,
+    UnboxedArrayObject::obj_hasProperty,
+    UnboxedArrayObject::obj_getProperty,
+    UnboxedArrayObject::obj_setProperty,
+    UnboxedArrayObject::obj_getOwnPropertyDescriptor,
+    UnboxedArrayObject::obj_deleteProperty,
+    nullptr,   /* getElements */
+    UnboxedArrayObject::obj_enumerate,
+    nullptr    /* funToString */
+};
+
+const Class UnboxedArrayObject::class_ = {
+    "Array",
+    Class::NON_NATIVE |
+    JSCLASS_SKIP_NURSERY_FINALIZE |
+    JSCLASS_BACKGROUND_FINALIZE,
+    &UnboxedArrayObjectClassOps,
+    JS_NULL_CLASS_SPEC,
+    &UnboxedArrayObjectClassExtension,
+    &UnboxedArrayObjectObjectOps
+};
+
+/////////////////////////////////////////////////////////////////////
 // API
 /////////////////////////////////////////////////////////////////////
 
@@ -936,6 +1642,31 @@ NextValue(Handle<GCVector<Value>> values, size_t* valueCursor)
 }
 
 void
+UnboxedArrayObject::fillAfterConvert(ExclusiveContext* cx,
+                                     Handle<GCVector<Value>> values, size_t* valueCursor)
+{
+    MOZ_ASSERT(CapacityArray[1] == 0);
+    setCapacityIndex(1);
+    setInitializedLengthNoBarrier(0);
+    setInlineElements();
+
+    setLength(cx, NextValue(values, valueCursor).toInt32());
+
+    int32_t initlen = NextValue(values, valueCursor).toInt32();
+    if (!initlen)
+        return;
+
+    AutoEnterOOMUnsafeRegion oomUnsafe;
+    if (!growElements(cx, initlen))
+        oomUnsafe.crash("UnboxedArrayObject::fillAfterConvert");
+
+    setInitializedLength(initlen);
+
+    for (size_t i = 0; i < size_t(initlen); i++)
+        JS_ALWAYS_TRUE(initElement(cx, i, NextValue(values, valueCursor)));
+}
+
+void
 UnboxedPlainObject::fillAfterConvert(ExclusiveContext* cx,
                                      Handle<GCVector<Value>> values, size_t* valueCursor)
 {
@@ -944,3 +1675,58 @@ UnboxedPlainObject::fillAfterConvert(ExclusiveContext* cx,
     for (size_t i = 0; i < layout().properties().length(); i++)
         JS_ALWAYS_TRUE(setValue(cx, layout().properties()[i], NextValue(values, valueCursor)));
 }
+
+DefineBoxedOrUnboxedFunctor6(SetOrExtendBoxedOrUnboxedDenseElements,
+                             ExclusiveContext*, JSObject*, uint32_t, const Value*, uint32_t,
+                             ShouldUpdateTypes);
+
+DenseElementResult
+js::SetOrExtendAnyBoxedOrUnboxedDenseElements(ExclusiveContext* cx, JSObject* obj,
+                                              uint32_t start, const Value* vp, uint32_t count,
+                                              ShouldUpdateTypes updateTypes)
+{
+    SetOrExtendBoxedOrUnboxedDenseElementsFunctor functor(cx, obj, start, vp, count, updateTypes);
+    return CallBoxedOrUnboxedSpecialization(functor, obj);
+};
+
+DefineBoxedOrUnboxedFunctor5(MoveBoxedOrUnboxedDenseElements,
+                             JSContext*, JSObject*, uint32_t, uint32_t, uint32_t);
+
+DenseElementResult
+js::MoveAnyBoxedOrUnboxedDenseElements(JSContext* cx, JSObject* obj,
+                                       uint32_t dstStart, uint32_t srcStart, uint32_t length)
+{
+    MoveBoxedOrUnboxedDenseElementsFunctor functor(cx, obj, dstStart, srcStart, length);
+    return CallBoxedOrUnboxedSpecialization(functor, obj);
+}
+
+DefineBoxedOrUnboxedFunctorPair6(CopyBoxedOrUnboxedDenseElements,
+                                 JSContext*, JSObject*, JSObject*, uint32_t, uint32_t, uint32_t);
+
+DenseElementResult
+js::CopyAnyBoxedOrUnboxedDenseElements(JSContext* cx, JSObject* dst, JSObject* src,
+                                       uint32_t dstStart, uint32_t srcStart, uint32_t length)
+{
+    CopyBoxedOrUnboxedDenseElementsFunctor functor(cx, dst, src, dstStart, srcStart, length);
+    return CallBoxedOrUnboxedSpecialization(functor, dst, src);
+}
+
+DefineBoxedOrUnboxedFunctor3(SetBoxedOrUnboxedInitializedLength,
+                             JSContext*, JSObject*, size_t);
+
+void
+js::SetAnyBoxedOrUnboxedInitializedLength(JSContext* cx, JSObject* obj, size_t initlen)
+{
+    SetBoxedOrUnboxedInitializedLengthFunctor functor(cx, obj, initlen);
+    JS_ALWAYS_TRUE(CallBoxedOrUnboxedSpecialization(functor, obj) == DenseElementResult::Success);
+}
+
+DefineBoxedOrUnboxedFunctor3(EnsureBoxedOrUnboxedDenseElements,
+                             JSContext*, JSObject*, size_t);
+
+DenseElementResult
+js::EnsureAnyBoxedOrUnboxedDenseElements(JSContext* cx, JSObject* obj, size_t initlen)
+{
+    EnsureBoxedOrUnboxedDenseElementsFunctor functor(cx, obj, initlen);
+    return CallBoxedOrUnboxedSpecialization(functor, obj);
+}
diff --git a/js/src/vm/UnboxedObject.h b/js/src/vm/UnboxedObject.h
index ba66434bc2..779dd14c7e 100644
--- a/js/src/vm/UnboxedObject.h
+++ b/js/src/vm/UnboxedObject.h
@@ -96,11 +96,17 @@ class UnboxedLayout : public mozilla::LinkedListElement<UnboxedLayout>
     // from an array of values.
     GCPtrJitCode constructorCode_;
 
+    // The following members are only used for unboxed arrays.
+
+    // The type of array elements.
+    JSValueType elementType_;
+
   public:
     UnboxedLayout()
       : nativeGroup_(nullptr), nativeShape_(nullptr),
         allocationScript_(nullptr), allocationPc_(nullptr), replacementGroup_(nullptr),
-        size_(0), newScript_(nullptr), traceList_(nullptr), constructorCode_(nullptr)
+        size_(0), newScript_(nullptr), traceList_(nullptr), constructorCode_(nullptr),
+        elementType_(JSVAL_TYPE_MAGIC)
     {}
 
     bool initProperties(const PropertyVector& properties, size_t size) {
@@ -108,6 +114,10 @@ class UnboxedLayout : public mozilla::LinkedListElement<UnboxedLayout>
         return properties_.appendAll(properties);
     }
 
+    void initArray(JSValueType elementType) {
+        elementType_ = elementType;
+    }
+
     ~UnboxedLayout() {
         if (newScript_)
             newScript_->clear();
@@ -120,6 +130,10 @@ class UnboxedLayout : public mozilla::LinkedListElement<UnboxedLayout>
         constructorCode_.init(nullptr);
     }
 
+    bool isArray() const {
+        return elementType_ != JSVAL_TYPE_MAGIC;
+    }
+
     void detachFromCompartment();
 
     const PropertyVector& properties() const {
@@ -187,6 +201,10 @@ class UnboxedLayout : public mozilla::LinkedListElement<UnboxedLayout>
         constructorCode_ = code;
     }
 
+    JSValueType elementType() const {
+        return elementType_;
+    }
+
     inline gc::AllocKind getAllocKind() const;
 
     void trace(JSTracer* trc);
@@ -306,6 +324,193 @@ UnboxedLayout::getAllocKind() const
     return gc::GetGCObjectKindForBytes(UnboxedPlainObject::offsetOfData() + size());
 }
 
+// Class for an array object using an unboxed representation.
+class UnboxedArrayObject : public JSObject
+{
+    // Elements pointer for the object.
+    uint8_t* elements_;
+
+    // The nominal array length. This always fits in an int32_t.
+    uint32_t length_;
+
+    // Value indicating the allocated capacity and initialized length of the
+    // array. The top CapacityBits bits are an index into CapacityArray, which
+    // indicates the elements capacity. The low InitializedLengthBits store the
+    // initialized length of the array.
+    uint32_t capacityIndexAndInitializedLength_;
+
+    // If the elements are inline, they will point here.
+    uint8_t inlineElements_[1];
+
+  public:
+    static const uint32_t CapacityBits = 6;
+    static const uint32_t CapacityShift = 26;
+
+    static const uint32_t CapacityMask = uint32_t(-1) << CapacityShift;
+    static const uint32_t InitializedLengthMask = (1 << CapacityShift) - 1;
+
+    static const uint32_t MaximumCapacity = InitializedLengthMask;
+    static const uint32_t MinimumDynamicCapacity = 8;
+
+    static const uint32_t CapacityArray[];
+
+    // Capacity index which indicates the array's length is also its capacity.
+    static const uint32_t CapacityMatchesLengthIndex = 0;
+
+  private:
+    static inline uint32_t computeCapacity(uint32_t index, uint32_t length) {
+        if (index == CapacityMatchesLengthIndex)
+            return length;
+        return CapacityArray[index];
+    }
+
+    static uint32_t chooseCapacityIndex(uint32_t capacity, uint32_t length);
+    static uint32_t exactCapacityIndex(uint32_t capacity);
+
+  public:
+    static const Class class_;
+
+    static bool obj_lookupProperty(JSContext* cx, HandleObject obj,
+                                   HandleId id, MutableHandleObject objp,
+                                   MutableHandleShape propp);
+
+    static bool obj_defineProperty(JSContext* cx, HandleObject obj, HandleId id,
+                                   Handle<PropertyDescriptor> desc,
+                                   ObjectOpResult& result);
+
+    static bool obj_hasProperty(JSContext* cx, HandleObject obj, HandleId id, bool* foundp);
+
+    static bool obj_getProperty(JSContext* cx, HandleObject obj, HandleValue receiver,
+                                HandleId id, MutableHandleValue vp);
+
+    static bool obj_setProperty(JSContext* cx, HandleObject obj, HandleId id, HandleValue v,
+                                HandleValue receiver, ObjectOpResult& result);
+
+    static bool obj_getOwnPropertyDescriptor(JSContext* cx, HandleObject obj, HandleId id,
+                                             MutableHandle<PropertyDescriptor> desc);
+
+    static bool obj_deleteProperty(JSContext* cx, HandleObject obj, HandleId id,
+                                   ObjectOpResult& result);
+
+    static bool obj_enumerate(JSContext* cx, HandleObject obj, AutoIdVector& properties,
+                              bool enumerableOnly);
+    static bool obj_watch(JSContext* cx, HandleObject obj, HandleId id, HandleObject callable);
+
+    inline const UnboxedLayout& layout() const;
+
+    const UnboxedLayout& layoutDontCheckGeneration() const {
+        return group()->unboxedLayoutDontCheckGeneration();
+    }
+
+    JSValueType elementType() const {
+        return layoutDontCheckGeneration().elementType();
+    }
+
+    uint32_t elementSize() const {
+        return UnboxedTypeSize(elementType());
+    }
+
+    static bool convertToNative(JSContext* cx, JSObject* obj);
+    static UnboxedArrayObject* create(ExclusiveContext* cx, HandleObjectGroup group,
+                                      uint32_t length, NewObjectKind newKind,
+                                      uint32_t maxLength = MaximumCapacity);
+
+    static bool convertToNativeWithGroup(ExclusiveContext* cx, JSObject* obj,
+                                         ObjectGroup* group, Shape* shape);
+    bool convertInt32ToDouble(ExclusiveContext* cx, ObjectGroup* group);
+
+    void fillAfterConvert(ExclusiveContext* cx,
+                          Handle<GCVector<Value>> values, size_t* valueCursor);
+
+    static void trace(JSTracer* trc, JSObject* object);
+    static void objectMoved(JSObject* obj, const JSObject* old);
+    static void finalize(FreeOp* fop, JSObject* obj);
+
+    static size_t objectMovedDuringMinorGC(JSTracer* trc, JSObject* dst, JSObject* src,
+                                           gc::AllocKind allocKind);
+
+    uint8_t* elements() {
+        return elements_;
+    }
+
+    bool hasInlineElements() const {
+        return elements_ == &inlineElements_[0];
+    }
+
+    uint32_t length() const {
+        return length_;
+    }
+
+    uint32_t initializedLength() const {
+        return capacityIndexAndInitializedLength_ & InitializedLengthMask;
+    }
+
+    uint32_t capacityIndex() const {
+        return (capacityIndexAndInitializedLength_ & CapacityMask) >> CapacityShift;
+    }
+
+    uint32_t capacity() const {
+        return computeCapacity(capacityIndex(), length());
+    }
+
+    bool containsProperty(ExclusiveContext* cx, jsid id);
+
+    bool setElement(ExclusiveContext* cx, size_t index, const Value& v);
+    bool initElement(ExclusiveContext* cx, size_t index, const Value& v);
+    void initElementNoTypeChange(size_t index, const Value& v);
+    Value getElement(size_t index);
+
+    template <JSValueType Type> inline bool setElementSpecific(ExclusiveContext* cx, size_t index,
+                                                               const Value& v);
+    template <JSValueType Type> inline void setElementNoTypeChangeSpecific(size_t index, const Value& v);
+    template <JSValueType Type> inline bool initElementSpecific(ExclusiveContext* cx, size_t index,
+                                                                const Value& v);
+    template <JSValueType Type> inline void initElementNoTypeChangeSpecific(size_t index, const Value& v);
+    template <JSValueType Type> inline Value getElementSpecific(size_t index);
+    template <JSValueType Type> inline void triggerPreBarrier(size_t index);
+
+    bool growElements(ExclusiveContext* cx, size_t cap);
+    void shrinkElements(ExclusiveContext* cx, size_t cap);
+
+    static uint32_t offsetOfElements() {
+        return offsetof(UnboxedArrayObject, elements_);
+    }
+    static uint32_t offsetOfLength() {
+        return offsetof(UnboxedArrayObject, length_);
+    }
+    static uint32_t offsetOfCapacityIndexAndInitializedLength() {
+        return offsetof(UnboxedArrayObject, capacityIndexAndInitializedLength_);
+    }
+    static uint32_t offsetOfInlineElements() {
+        return offsetof(UnboxedArrayObject, inlineElements_);
+    }
+
+    void setLengthInt32(uint32_t length) {
+        MOZ_ASSERT(length <= INT32_MAX);
+        length_ = length;
+    }
+
+    inline void setLength(ExclusiveContext* cx, uint32_t len);
+    inline void setInitializedLength(uint32_t initlen);
+
+    inline void setInitializedLengthNoBarrier(uint32_t initlen) {
+        MOZ_ASSERT(initlen <= InitializedLengthMask);
+        capacityIndexAndInitializedLength_ =
+            (capacityIndexAndInitializedLength_ & CapacityMask) | initlen;
+    }
+
+  private:
+    void setInlineElements() {
+        elements_ = &inlineElements_[0];
+    }
+
+    void setCapacityIndex(uint32_t index) {
+        MOZ_ASSERT(index <= (CapacityMask >> CapacityShift));
+        capacityIndexAndInitializedLength_ =
+            (index << CapacityShift) | initializedLength();
+    }
+};
+
 } // namespace js
 
 namespace JS {