Add m-esr52 at 52.6.0

1 files changed, 2152 insertions, 0 deletions

diff --git a/js/src/vm/UnboxedObject.cpp b/js/src/vm/UnboxedObject.cpp
new file mode 100644
index 000000000..3018ace67
--- /dev/null
+++ b/js/src/vm/UnboxedObject.cpp
@@ -0,0 +1,2152 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ * vim: set ts=8 sts=4 et sw=4 tw=99:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "vm/UnboxedObject-inl.h"
+
+#include "jit/BaselineIC.h"
+#include "jit/ExecutableAllocator.h"
+#include "jit/JitCommon.h"
+#include "jit/Linker.h"
+
+#include "jsobjinlines.h"
+
+#include "gc/Nursery-inl.h"
+#include "jit/MacroAssembler-inl.h"
+#include "vm/Shape-inl.h"
+
+using mozilla::ArrayLength;
+using mozilla::DebugOnly;
+using mozilla::PodCopy;
+
+using namespace js;
+
+/////////////////////////////////////////////////////////////////////
+// UnboxedLayout
+/////////////////////////////////////////////////////////////////////
+
+void
+UnboxedLayout::trace(JSTracer* trc)
+{
+    for (size_t i = 0; i < properties_.length(); i++)
+        TraceManuallyBarrieredEdge(trc, &properties_[i].name, "unboxed_layout_name");
+
+    if (newScript())
+        newScript()->trace(trc);
+
+    TraceNullableEdge(trc, &nativeGroup_, "unboxed_layout_nativeGroup");
+    TraceNullableEdge(trc, &nativeShape_, "unboxed_layout_nativeShape");
+    TraceNullableEdge(trc, &allocationScript_, "unboxed_layout_allocationScript");
+    TraceNullableEdge(trc, &replacementGroup_, "unboxed_layout_replacementGroup");
+    TraceNullableEdge(trc, &constructorCode_, "unboxed_layout_constructorCode");
+}
+
+size_t
+UnboxedLayout::sizeOfIncludingThis(mozilla::MallocSizeOf mallocSizeOf)
+{
+    return mallocSizeOf(this)
+         + properties_.sizeOfExcludingThis(mallocSizeOf)
+         + (newScript() ? newScript()->sizeOfIncludingThis(mallocSizeOf) : 0)
+         + mallocSizeOf(traceList());
+}
+
+void
+UnboxedLayout::setNewScript(TypeNewScript* newScript, bool writeBarrier /* = true */)
+{
+    if (newScript_ && writeBarrier)
+        TypeNewScript::writeBarrierPre(newScript_);
+    newScript_ = newScript;
+}
+
+// Constructor code returns a 0x1 value to indicate the constructor code should
+// be cleared.
+static const uintptr_t CLEAR_CONSTRUCTOR_CODE_TOKEN = 0x1;
+
+/* static */ bool
+UnboxedLayout::makeConstructorCode(JSContext* cx, HandleObjectGroup group)
+{
+    gc::AutoSuppressGC suppress(cx);
+
+    using namespace jit;
+
+    if (!cx->compartment()->ensureJitCompartmentExists(cx))
+        return false;
+
+    UnboxedLayout& layout = group->unboxedLayout();
+    MOZ_ASSERT(!layout.constructorCode());
+
+    UnboxedPlainObject* templateObject = UnboxedPlainObject::create(cx, group, TenuredObject);
+    if (!templateObject)
+        return false;
+
+    JitContext jitContext(cx, nullptr);
+
+    MacroAssembler masm;
+
+    Register propertiesReg, newKindReg;
+#ifdef JS_CODEGEN_X86
+    propertiesReg = eax;
+    newKindReg = ecx;
+    masm.loadPtr(Address(masm.getStackPointer(), sizeof(void*)), propertiesReg);
+    masm.loadPtr(Address(masm.getStackPointer(), 2 * sizeof(void*)), newKindReg);
+#else
+    propertiesReg = IntArgReg0;
+    newKindReg = IntArgReg1;
+#endif
+
+#ifdef JS_CODEGEN_ARM64
+    // ARM64 communicates stack address via sp, but uses a pseudo-sp for addressing.
+    masm.initStackPtr();
+#endif
+
+    MOZ_ASSERT(propertiesReg.volatile_());
+    MOZ_ASSERT(newKindReg.volatile_());
+
+    AllocatableGeneralRegisterSet regs(GeneralRegisterSet::All());
+    regs.take(propertiesReg);
+    regs.take(newKindReg);
+    Register object = regs.takeAny(), scratch1 = regs.takeAny(), scratch2 = regs.takeAny();
+
+    LiveGeneralRegisterSet savedNonVolatileRegisters = SavedNonVolatileRegisters(regs);
+    masm.PushRegsInMask(savedNonVolatileRegisters);
+
+    // The scratch double register might be used by MacroAssembler methods.
+    if (ScratchDoubleReg.volatile_())
+        masm.push(ScratchDoubleReg);
+
+    Label failure, tenuredObject, allocated;
+    masm.branch32(Assembler::NotEqual, newKindReg, Imm32(GenericObject), &tenuredObject);
+    masm.branchTest32(Assembler::NonZero, AbsoluteAddress(group->addressOfFlags()),
+                      Imm32(OBJECT_FLAG_PRE_TENURE), &tenuredObject);
+
+    // Allocate an object in the nursery
+    masm.createGCObject(object, scratch1, templateObject, gc::DefaultHeap, &failure,
+                        /* initFixedSlots = */ false);
+
+    masm.jump(&allocated);
+    masm.bind(&tenuredObject);
+
+    // Allocate an object in the tenured heap.
+    masm.createGCObject(object, scratch1, templateObject, gc::TenuredHeap, &failure,
+                        /* initFixedSlots = */ false);
+
+    // If any of the properties being stored are in the nursery, add a store
+    // buffer entry for the new object.
+    Label postBarrier;
+    for (size_t i = 0; i < layout.properties().length(); i++) {
+        const UnboxedLayout::Property& property = layout.properties()[i];
+        if (property.type == JSVAL_TYPE_OBJECT) {
+            Address valueAddress(propertiesReg, i * sizeof(IdValuePair) + offsetof(IdValuePair, value));
+            Label notObject;
+            masm.branchTestObject(Assembler::NotEqual, valueAddress, &notObject);
+            Register valueObject = masm.extractObject(valueAddress, scratch1);
+            masm.branchPtrInNurseryChunk(Assembler::Equal, valueObject, scratch2, &postBarrier);
+            masm.bind(&notObject);
+        }
+    }
+
+    masm.jump(&allocated);
+    masm.bind(&postBarrier);
+
+    LiveGeneralRegisterSet liveVolatileRegisters;
+    liveVolatileRegisters.add(propertiesReg);
+    if (object.volatile_())
+        liveVolatileRegisters.add(object);
+    masm.PushRegsInMask(liveVolatileRegisters);
+
+    masm.mov(ImmPtr(cx->runtime()), scratch1);
+    masm.setupUnalignedABICall(scratch2);
+    masm.passABIArg(scratch1);
+    masm.passABIArg(object);
+    masm.callWithABI(JS_FUNC_TO_DATA_PTR(void*, PostWriteBarrier));
+
+    masm.PopRegsInMask(liveVolatileRegisters);
+
+    masm.bind(&allocated);
+
+    ValueOperand valueOperand;
+#ifdef JS_NUNBOX32
+    valueOperand = ValueOperand(scratch1, scratch2);
+#else
+    valueOperand = ValueOperand(scratch1);
+#endif
+
+    Label failureStoreOther, failureStoreObject;
+
+    for (size_t i = 0; i < layout.properties().length(); i++) {
+        const UnboxedLayout::Property& property = layout.properties()[i];
+        Address valueAddress(propertiesReg, i * sizeof(IdValuePair) + offsetof(IdValuePair, value));
+        Address targetAddress(object, UnboxedPlainObject::offsetOfData() + property.offset);
+
+        masm.loadValue(valueAddress, valueOperand);
+
+        if (property.type == JSVAL_TYPE_OBJECT) {
+            HeapTypeSet* types = group->maybeGetProperty(IdToTypeId(NameToId(property.name)));
+
+            Label notObject;
+            masm.branchTestObject(Assembler::NotEqual, valueOperand,
+                                  types->mightBeMIRType(MIRType::Null) ? &notObject : &failureStoreObject);
+
+            Register payloadReg = masm.extractObject(valueOperand, scratch1);
+
+            if (!types->hasType(TypeSet::AnyObjectType())) {
+                Register scratch = (payloadReg == scratch1) ? scratch2 : scratch1;
+                masm.guardObjectType(payloadReg, types, scratch, &failureStoreObject);
+            }
+
+            masm.storeUnboxedProperty(targetAddress, JSVAL_TYPE_OBJECT,
+                                      TypedOrValueRegister(MIRType::Object,
+                                                           AnyRegister(payloadReg)), nullptr);
+
+            if (notObject.used()) {
+                Label done;
+                masm.jump(&done);
+                masm.bind(&notObject);
+                masm.branchTestNull(Assembler::NotEqual, valueOperand, &failureStoreOther);
+                masm.storeUnboxedProperty(targetAddress, JSVAL_TYPE_OBJECT, NullValue(), nullptr);
+                masm.bind(&done);
+            }
+        } else {
+            masm.storeUnboxedProperty(targetAddress, property.type,
+                                      ConstantOrRegister(valueOperand), &failureStoreOther);
+        }
+    }
+
+    Label done;
+    masm.bind(&done);
+
+    if (object != ReturnReg)
+        masm.movePtr(object, ReturnReg);
+
+    // Restore non-volatile registers which were saved on entry.
+    if (ScratchDoubleReg.volatile_())
+        masm.pop(ScratchDoubleReg);
+    masm.PopRegsInMask(savedNonVolatileRegisters);
+
+    masm.abiret();
+
+    masm.bind(&failureStoreOther);
+
+    // There was a failure while storing a value which cannot be stored at all
+    // in the unboxed object. Initialize the object so it is safe for GC and
+    // return null.
+    masm.initUnboxedObjectContents(object, templateObject);
+
+    masm.bind(&failure);
+
+    masm.movePtr(ImmWord(0), object);
+    masm.jump(&done);
+
+    masm.bind(&failureStoreObject);
+
+    // There was a failure while storing a value to an object slot of the
+    // unboxed object. If the value is storable, the failure occurred due to
+    // incomplete type information in the object, so return a token to trigger
+    // regeneration of the jitcode after a new object is created in the VM.
+    {
+        Label isObject;
+        masm.branchTestObject(Assembler::Equal, valueOperand, &isObject);
+        masm.branchTestNull(Assembler::NotEqual, valueOperand, &failureStoreOther);
+        masm.bind(&isObject);
+    }
+
+    // Initialize the object so it is safe for GC.
+    masm.initUnboxedObjectContents(object, templateObject);
+
+    masm.movePtr(ImmWord(CLEAR_CONSTRUCTOR_CODE_TOKEN), object);
+    masm.jump(&done);
+
+    Linker linker(masm);
+    AutoFlushICache afc("UnboxedObject");
+    JitCode* code = linker.newCode<NoGC>(cx, OTHER_CODE);
+    if (!code)
+        return false;
+
+    layout.setConstructorCode(code);
+    return true;
+}
+
+void
+UnboxedLayout::detachFromCompartment()
+{
+    if (isInList())
+        remove();
+}
+
+/////////////////////////////////////////////////////////////////////
+// UnboxedPlainObject
+/////////////////////////////////////////////////////////////////////
+
+bool
+UnboxedPlainObject::setValue(ExclusiveContext* cx, const UnboxedLayout::Property& property,
+                             const Value& v)
+{
+    uint8_t* p = &data_[property.offset];
+    return SetUnboxedValue(cx, this, NameToId(property.name), p, property.type, v,
+                           /* preBarrier = */ true);
+}
+
+Value
+UnboxedPlainObject::getValue(const UnboxedLayout::Property& property,
+                             bool maybeUninitialized /* = false */)
+{
+    uint8_t* p = &data_[property.offset];
+    return GetUnboxedValue(p, property.type, maybeUninitialized);
+}
+
+void
+UnboxedPlainObject::trace(JSTracer* trc, JSObject* obj)
+{
+    if (obj->as<UnboxedPlainObject>().expando_) {
+        TraceManuallyBarrieredEdge(trc,
+            reinterpret_cast<NativeObject**>(&obj->as<UnboxedPlainObject>().expando_),
+            "unboxed_expando");
+    }
+
+    const UnboxedLayout& layout = obj->as<UnboxedPlainObject>().layoutDontCheckGeneration();
+    const int32_t* list = layout.traceList();
+    if (!list)
+        return;
+
+    uint8_t* data = obj->as<UnboxedPlainObject>().data();
+    while (*list != -1) {
+        GCPtrString* heap = reinterpret_cast<GCPtrString*>(data + *list);
+        TraceEdge(trc, heap, "unboxed_string");
+        list++;
+    }
+    list++;
+    while (*list != -1) {
+        GCPtrObject* heap = reinterpret_cast<GCPtrObject*>(data + *list);
+        TraceNullableEdge(trc, heap, "unboxed_object");
+        list++;
+    }
+
+    // Unboxed objects don't have Values to trace.
+    MOZ_ASSERT(*(list + 1) == -1);
+}
+
+/* static */ UnboxedExpandoObject*
+UnboxedPlainObject::ensureExpando(JSContext* cx, Handle<UnboxedPlainObject*> obj)
+{
+    if (obj->expando_)
+        return obj->expando_;
+
+    UnboxedExpandoObject* expando =
+        NewObjectWithGivenProto<UnboxedExpandoObject>(cx, nullptr, gc::AllocKind::OBJECT4);
+    if (!expando)
+        return nullptr;
+
+    // Don't track property types for expando objects. This allows Baseline
+    // and Ion AddSlot ICs to guard on the unboxed group without guarding on
+    // the expando group.
+    MarkObjectGroupUnknownProperties(cx, expando->group());
+
+    // If the expando is tenured then the original object must also be tenured.
+    // Otherwise barriers triggered on the original object for writes to the
+    // expando (as can happen in the JIT) won't see the tenured->nursery edge.
+    // See WholeCellEdges::mark.
+    MOZ_ASSERT_IF(!IsInsideNursery(expando), !IsInsideNursery(obj));
+
+    // As with setValue(), we need to manually trigger post barriers on the
+    // whole object. If we treat the field as a GCPtrObject and later
+    // convert the object to its native representation, we will end up with a
+    // corrupted store buffer entry.
+    if (IsInsideNursery(expando) && !IsInsideNursery(obj))
+        cx->runtime()->gc.storeBuffer.putWholeCell(obj);
+
+    obj->expando_ = expando;
+    return expando;
+}
+
+bool
+UnboxedPlainObject::containsUnboxedOrExpandoProperty(ExclusiveContext* cx, jsid id) const
+{
+    if (layout().lookup(id))
+        return true;
+
+    if (maybeExpando() && maybeExpando()->containsShapeOrElement(cx, id))
+        return true;
+
+    return false;
+}
+
+static bool
+PropagatePropertyTypes(JSContext* cx, jsid id, ObjectGroup* oldGroup, ObjectGroup* newGroup)
+{
+    HeapTypeSet* typeProperty = oldGroup->maybeGetProperty(id);
+    TypeSet::TypeList types;
+    if (!typeProperty->enumerateTypes(&types)) {
+        ReportOutOfMemory(cx);
+        return false;
+    }
+    for (size_t j = 0; j < types.length(); j++)
+        AddTypePropertyId(cx, newGroup, nullptr, id, types[j]);
+    return true;
+}
+
+static PlainObject*
+MakeReplacementTemplateObject(JSContext* cx, HandleObjectGroup group, const UnboxedLayout &layout)
+{
+    PlainObject* obj = NewObjectWithGroup<PlainObject>(cx, group, layout.getAllocKind(),
+                                                       TenuredObject);
+    if (!obj)
+        return nullptr;
+
+    for (size_t i = 0; i < layout.properties().length(); i++) {
+        const UnboxedLayout::Property& property = layout.properties()[i];
+        if (!obj->addDataProperty(cx, NameToId(property.name), i, JSPROP_ENUMERATE))
+            return nullptr;
+        MOZ_ASSERT(obj->slotSpan() == i + 1);
+        MOZ_ASSERT(!obj->inDictionaryMode());
+    }
+
+    return obj;
+}
+
+/* static */ bool
+UnboxedLayout::makeNativeGroup(JSContext* cx, ObjectGroup* group)
+{
+    AutoEnterAnalysis enter(cx);
+
+    UnboxedLayout& layout = group->unboxedLayout();
+    Rooted<TaggedProto> proto(cx, group->proto());
+
+    MOZ_ASSERT(!layout.nativeGroup());
+
+    RootedObjectGroup replacementGroup(cx);
+
+    const Class* clasp = layout.isArray() ? &ArrayObject::class_ : &PlainObject::class_;
+
+    // Immediately clear any new script on the group. This is done by replacing
+    // the existing new script with one for a replacement default new group.
+    // This is done so that the size of the replacment group's objects is the
+    // same as that for the unboxed group, so that we do not see polymorphic
+    // slot accesses later on for sites that see converted objects from this
+    // group and objects that were allocated using the replacement new group.
+    if (layout.newScript()) {
+        MOZ_ASSERT(!layout.isArray());
+
+        replacementGroup = ObjectGroupCompartment::makeGroup(cx, &PlainObject::class_, proto);
+        if (!replacementGroup)
+            return false;
+
+        PlainObject* templateObject = MakeReplacementTemplateObject(cx, replacementGroup, layout);
+        if (!templateObject)
+            return false;
+
+        TypeNewScript* replacementNewScript =
+            TypeNewScript::makeNativeVersion(cx, layout.newScript(), templateObject);
+        if (!replacementNewScript)
+            return false;
+
+        replacementGroup->setNewScript(replacementNewScript);
+        gc::TraceTypeNewScript(replacementGroup);
+
+        group->clearNewScript(cx, replacementGroup);
+    }
+
+    // Similarly, if this group is keyed to an allocation site, replace its
+    // entry with a new group that has no unboxed layout.
+    if (layout.allocationScript()) {
+        RootedScript script(cx, layout.allocationScript());
+        jsbytecode* pc = layout.allocationPc();
+
+        replacementGroup = ObjectGroupCompartment::makeGroup(cx, clasp, proto);
+        if (!replacementGroup)
+            return false;
+
+        PlainObject* templateObject = &script->getObject(pc)->as<PlainObject>();
+        replacementGroup->addDefiniteProperties(cx, templateObject->lastProperty());
+
+        JSProtoKey key = layout.isArray() ? JSProto_Array : JSProto_Object;
+        cx->compartment()->objectGroups.replaceAllocationSiteGroup(script, pc, key,
+                                                                   replacementGroup);
+
+        // Clear any baseline information at this opcode which might use the old group.
+        if (script->hasBaselineScript()) {
+            jit::ICEntry& entry = script->baselineScript()->icEntryFromPCOffset(script->pcToOffset(pc));
+            jit::ICFallbackStub* fallback = entry.fallbackStub();
+            for (jit::ICStubIterator iter = fallback->beginChain(); !iter.atEnd(); iter++)
+                iter.unlink(cx);
+            if (fallback->isNewObject_Fallback())
+                fallback->toNewObject_Fallback()->setTemplateObject(nullptr);
+            else if (fallback->isNewArray_Fallback())
+                fallback->toNewArray_Fallback()->setTemplateGroup(replacementGroup);
+        }
+    }
+
+    size_t nfixed = layout.isArray() ? 0 : gc::GetGCKindSlots(layout.getAllocKind());
+
+    if (layout.isArray()) {
+        // The length shape to use for arrays is cached via a modified initial
+        // shape for array objects. Create an array now to make sure this entry
+        // is instantiated.
+        if (!NewDenseEmptyArray(cx))
+            return false;
+    }
+
+    RootedShape shape(cx, EmptyShape::getInitialShape(cx, clasp, proto, nfixed, 0));
+    if (!shape)
+        return false;
+
+    MOZ_ASSERT_IF(layout.isArray(), !shape->isEmptyShape() && shape->slotSpan() == 0);
+
+    // Add shapes for each property, if this is for a plain object.
+    for (size_t i = 0; i < layout.properties().length(); i++) {
+        const UnboxedLayout::Property& property = layout.properties()[i];
+
+        Rooted<StackShape> child(cx, StackShape(shape->base()->unowned(), NameToId(property.name),
+                                                i, JSPROP_ENUMERATE, 0));
+        shape = cx->zone()->propertyTree.getChild(cx, shape, child);
+        if (!shape)
+            return false;
+    }
+
+    ObjectGroup* nativeGroup =
+        ObjectGroupCompartment::makeGroup(cx, clasp, proto,
+                                          group->flags() & OBJECT_FLAG_DYNAMIC_MASK);
+    if (!nativeGroup)
+        return false;
+
+    // No sense propagating if we don't know what we started with.
+    if (!group->unknownProperties()) {
+        // Propagate all property types from the old group to the new group.
+        if (layout.isArray()) {
+            if (!PropagatePropertyTypes(cx, JSID_VOID, group, nativeGroup))
+                return false;
+        } else {
+            for (size_t i = 0; i < layout.properties().length(); i++) {
+                const UnboxedLayout::Property& property = layout.properties()[i];
+                jsid id = NameToId(property.name);
+                if (!PropagatePropertyTypes(cx, id, group, nativeGroup))
+                    return false;
+
+                // If we are OOM we may not be able to propagate properties.
+                if (nativeGroup->unknownProperties())
+                    break;
+
+                HeapTypeSet* nativeProperty = nativeGroup->maybeGetProperty(id);
+                if (nativeProperty && nativeProperty->canSetDefinite(i))
+                    nativeProperty->setDefinite(i);
+            }
+        }
+    } else {
+        // If we skip, though, the new group had better agree.
+        MOZ_ASSERT(nativeGroup->unknownProperties());
+    }
+
+    layout.nativeGroup_ = nativeGroup;
+    layout.nativeShape_ = shape;
+    layout.replacementGroup_ = replacementGroup;
+
+    nativeGroup->setOriginalUnboxedGroup(group);
+
+    group->markStateChange(cx);
+
+    return true;
+}
+
+/* static */ bool
+UnboxedPlainObject::convertToNative(JSContext* cx, JSObject* obj)
+{
+    const UnboxedLayout& layout = obj->as<UnboxedPlainObject>().layout();
+    UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando();
+
+    if (!layout.nativeGroup()) {
+        if (!UnboxedLayout::makeNativeGroup(cx, obj->group()))
+            return false;
+
+        // makeNativeGroup can reentrantly invoke this method.
+        if (obj->is<PlainObject>())
+            return true;
+    }
+
+    AutoValueVector values(cx);
+    for (size_t i = 0; i < layout.properties().length(); i++) {
+        // We might be reading properties off the object which have not been
+        // initialized yet. Make sure any double values we read here are
+        // canonicalized.
+        if (!values.append(obj->as<UnboxedPlainObject>().getValue(layout.properties()[i], true)))
+            return false;
+    }
+
+    // We are eliminating the expando edge with the conversion, so trigger a
+    // pre barrier.
+    JSObject::writeBarrierPre(expando);
+
+    // Additionally trigger a post barrier on the expando itself. Whole cell
+    // store buffer entries can be added on the original unboxed object for
+    // writes to the expando (see WholeCellEdges::trace), so after conversion
+    // we need to make sure the expando itself will still be traced.
+    if (expando && !IsInsideNursery(expando))
+        cx->runtime()->gc.storeBuffer.putWholeCell(expando);
+
+    obj->setGroup(layout.nativeGroup());
+    obj->as<PlainObject>().setLastPropertyMakeNative(cx, layout.nativeShape());
+
+    for (size_t i = 0; i < values.length(); i++)
+        obj->as<PlainObject>().initSlotUnchecked(i, values[i]);
+
+    if (expando) {
+        // Add properties from the expando object to the object, in order.
+        // Suppress GC here, so that callers don't need to worry about this
+        // method collecting. The stuff below can only fail due to OOM, in
+        // which case the object will not have been completely filled back in.
+        gc::AutoSuppressGC suppress(cx);
+
+        Vector<jsid> ids(cx);
+        for (Shape::Range<NoGC> r(expando->lastProperty()); !r.empty(); r.popFront()) {
+            if (!ids.append(r.front().propid()))
+                return false;
+        }
+        for (size_t i = 0; i < expando->getDenseInitializedLength(); i++) {
+            if (!expando->getDenseElement(i).isMagic(JS_ELEMENTS_HOLE)) {
+                if (!ids.append(INT_TO_JSID(i)))
+                    return false;
+            }
+        }
+        ::Reverse(ids.begin(), ids.end());
+
+        RootedPlainObject nobj(cx, &obj->as<PlainObject>());
+        Rooted<UnboxedExpandoObject*> nexpando(cx, expando);
+        RootedId id(cx);
+        Rooted<PropertyDescriptor> desc(cx);
+        for (size_t i = 0; i < ids.length(); i++) {
+            id = ids[i];
+            if (!GetOwnPropertyDescriptor(cx, nexpando, id, &desc))
+                return false;
+            ObjectOpResult result;
+            if (!DefineProperty(cx, nobj, id, desc, result))
+                return false;
+            MOZ_ASSERT(result.ok());
+        }
+    }
+
+    return true;
+}
+
+/* static */
+UnboxedPlainObject*
+UnboxedPlainObject::create(ExclusiveContext* cx, HandleObjectGroup group, NewObjectKind newKind)
+{
+    AutoSetNewObjectMetadata metadata(cx);
+
+    MOZ_ASSERT(group->clasp() == &class_);
+    gc::AllocKind allocKind = group->unboxedLayout().getAllocKind();
+
+    UnboxedPlainObject* res =
+        NewObjectWithGroup<UnboxedPlainObject>(cx, group, allocKind, newKind);
+    if (!res)
+        return nullptr;
+
+    // Overwrite the dummy shape which was written to the object's expando field.
+    res->initExpando();
+
+    // Initialize reference fields of the object. All fields in the object will
+    // be overwritten shortly, but references need to be safe for the GC.
+    const int32_t* list = res->layout().traceList();
+    if (list) {
+        uint8_t* data = res->data();
+        while (*list != -1) {
+            GCPtrString* heap = reinterpret_cast<GCPtrString*>(data + *list);
+            heap->init(cx->names().empty);
+            list++;
+        }
+        list++;
+        while (*list != -1) {
+            GCPtrObject* heap = reinterpret_cast<GCPtrObject*>(data + *list);
+            heap->init(nullptr);
+            list++;
+        }
+        // Unboxed objects don't have Values to initialize.
+        MOZ_ASSERT(*(list + 1) == -1);
+    }
+
+    return res;
+}
+
+/* static */ JSObject*
+UnboxedPlainObject::createWithProperties(ExclusiveContext* cx, HandleObjectGroup group,
+                                         NewObjectKind newKind, IdValuePair* properties)
+{
+    MOZ_ASSERT(newKind == GenericObject || newKind == TenuredObject);
+
+    UnboxedLayout& layout = group->unboxedLayout();
+
+    if (layout.constructorCode()) {
+        MOZ_ASSERT(cx->isJSContext());
+
+        typedef JSObject* (*ConstructorCodeSignature)(IdValuePair*, NewObjectKind);
+        ConstructorCodeSignature function =
+            reinterpret_cast<ConstructorCodeSignature>(layout.constructorCode()->raw());
+
+        JSObject* obj;
+        {
+            JS::AutoSuppressGCAnalysis nogc;
+            obj = reinterpret_cast<JSObject*>(CALL_GENERATED_2(function, properties, newKind));
+        }
+        if (obj > reinterpret_cast<JSObject*>(CLEAR_CONSTRUCTOR_CODE_TOKEN))
+            return obj;
+
+        if (obj == reinterpret_cast<JSObject*>(CLEAR_CONSTRUCTOR_CODE_TOKEN))
+            layout.setConstructorCode(nullptr);
+    }
+
+    UnboxedPlainObject* obj = UnboxedPlainObject::create(cx, group, newKind);
+    if (!obj)
+        return nullptr;
+
+    for (size_t i = 0; i < layout.properties().length(); i++) {
+        if (!obj->setValue(cx, layout.properties()[i], properties[i].value))
+            return NewPlainObjectWithProperties(cx, properties, layout.properties().length(), newKind);
+    }
+
+#ifndef JS_CODEGEN_NONE
+    if (cx->isJSContext() &&
+        !group->unknownProperties() &&
+        !layout.constructorCode() &&
+        cx->asJSContext()->runtime()->jitSupportsFloatingPoint &&
+        jit::CanLikelyAllocateMoreExecutableMemory())
+    {
+        if (!UnboxedLayout::makeConstructorCode(cx->asJSContext(), group))
+            return nullptr;
+    }
+#endif
+
+    return obj;
+}
+
+/* static */ bool
+UnboxedPlainObject::obj_lookupProperty(JSContext* cx, HandleObject obj,
+                                       HandleId id, MutableHandleObject objp,
+                                       MutableHandleShape propp)
+{
+    if (obj->as<UnboxedPlainObject>().containsUnboxedOrExpandoProperty(cx, id)) {
+        MarkNonNativePropertyFound<CanGC>(propp);
+        objp.set(obj);
+        return true;
+    }
+
+    RootedObject proto(cx, obj->staticPrototype());
+    if (!proto) {
+        objp.set(nullptr);
+        propp.set(nullptr);
+        return true;
+    }
+
+    return LookupProperty(cx, proto, id, objp, propp);
+}
+
+/* static */ bool
+UnboxedPlainObject::obj_defineProperty(JSContext* cx, HandleObject obj, HandleId id,
+                                       Handle<PropertyDescriptor> desc,
+                                       ObjectOpResult& result)
+{
+    const UnboxedLayout& layout = obj->as<UnboxedPlainObject>().layout();
+
+    if (const UnboxedLayout::Property* property = layout.lookup(id)) {
+        if (!desc.getter() && !desc.setter() && desc.attributes() == JSPROP_ENUMERATE) {
+            // This define is equivalent to setting an existing property.
+            if (obj->as<UnboxedPlainObject>().setValue(cx, *property, desc.value()))
+                return result.succeed();
+        }
+
+        // Trying to incompatibly redefine an existing property requires the
+        // object to be converted to a native object.
+        if (!convertToNative(cx, obj))
+            return false;
+
+        return DefineProperty(cx, obj, id, desc, result);
+    }
+
+    // Define the property on the expando object.
+    Rooted<UnboxedExpandoObject*> expando(cx, ensureExpando(cx, obj.as<UnboxedPlainObject>()));
+    if (!expando)
+        return false;
+
+    // Update property types on the unboxed object as well.
+    AddTypePropertyId(cx, obj, id, desc.value());
+
+    return DefineProperty(cx, expando, id, desc, result);
+}
+
+/* static */ bool
+UnboxedPlainObject::obj_hasProperty(JSContext* cx, HandleObject obj, HandleId id, bool* foundp)
+{
+    if (obj->as<UnboxedPlainObject>().containsUnboxedOrExpandoProperty(cx, id)) {
+        *foundp = true;
+        return true;
+    }
+
+    RootedObject proto(cx, obj->staticPrototype());
+    if (!proto) {
+        *foundp = false;
+        return true;
+    }
+
+    return HasProperty(cx, proto, id, foundp);
+}
+
+/* static */ bool
+UnboxedPlainObject::obj_getProperty(JSContext* cx, HandleObject obj, HandleValue receiver,
+                                    HandleId id, MutableHandleValue vp)
+{
+    const UnboxedLayout& layout = obj->as<UnboxedPlainObject>().layout();
+
+    if (const UnboxedLayout::Property* property = layout.lookup(id)) {
+        vp.set(obj->as<UnboxedPlainObject>().getValue(*property));
+        return true;
+    }
+
+    if (UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando()) {
+        if (expando->containsShapeOrElement(cx, id)) {
+            RootedObject nexpando(cx, expando);
+            return GetProperty(cx, nexpando, receiver, id, vp);
+        }
+    }
+
+    RootedObject proto(cx, obj->staticPrototype());
+    if (!proto) {
+        vp.setUndefined();
+        return true;
+    }
+
+    return GetProperty(cx, proto, receiver, id, vp);
+}
+
+/* static */ bool
+UnboxedPlainObject::obj_setProperty(JSContext* cx, HandleObject obj, HandleId id, HandleValue v,
+                                    HandleValue receiver, ObjectOpResult& result)
+{
+    const UnboxedLayout& layout = obj->as<UnboxedPlainObject>().layout();
+
+    if (const UnboxedLayout::Property* property = layout.lookup(id)) {
+        if (receiver.isObject() && obj == &receiver.toObject()) {
+            if (obj->as<UnboxedPlainObject>().setValue(cx, *property, v))
+                return result.succeed();
+
+            if (!convertToNative(cx, obj))
+                return false;
+            return SetProperty(cx, obj, id, v, receiver, result);
+        }
+
+        return SetPropertyByDefining(cx, id, v, receiver, result);
+    }
+
+    if (UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando()) {
+        if (expando->containsShapeOrElement(cx, id)) {
+            // Update property types on the unboxed object as well.
+            AddTypePropertyId(cx, obj, id, v);
+
+            RootedObject nexpando(cx, expando);
+            return SetProperty(cx, nexpando, id, v, receiver, result);
+        }
+    }
+
+    return SetPropertyOnProto(cx, obj, id, v, receiver, result);
+}
+
+/* static */ bool
+UnboxedPlainObject::obj_getOwnPropertyDescriptor(JSContext* cx, HandleObject obj, HandleId id,
+                                                 MutableHandle<PropertyDescriptor> desc)
+{
+    const UnboxedLayout& layout = obj->as<UnboxedPlainObject>().layout();
+
+    if (const UnboxedLayout::Property* property = layout.lookup(id)) {
+        desc.value().set(obj->as<UnboxedPlainObject>().getValue(*property));
+        desc.setAttributes(JSPROP_ENUMERATE);
+        desc.object().set(obj);
+        return true;
+    }
+
+    if (UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando()) {
+        if (expando->containsShapeOrElement(cx, id)) {
+            RootedObject nexpando(cx, expando);
+            if (!GetOwnPropertyDescriptor(cx, nexpando, id, desc))
+                return false;
+            if (desc.object() == nexpando)
+                desc.object().set(obj);
+            return true;
+        }
+    }
+
+    desc.object().set(nullptr);
+    return true;
+}
+
+/* static */ bool
+UnboxedPlainObject::obj_deleteProperty(JSContext* cx, HandleObject obj, HandleId id,
+                                       ObjectOpResult& result)
+{
+    if (!convertToNative(cx, obj))
+        return false;
+    return DeleteProperty(cx, obj, id, result);
+}
+
+/* static */ bool
+UnboxedPlainObject::obj_watch(JSContext* cx, HandleObject obj, HandleId id, HandleObject callable)
+{
+    if (!convertToNative(cx, obj))
+        return false;
+    return WatchProperty(cx, obj, id, callable);
+}
+
+/* static */ bool
+UnboxedPlainObject::obj_enumerate(JSContext* cx, HandleObject obj, AutoIdVector& properties,
+                                  bool enumerableOnly)
+{
+    // Ignore expando properties here, they are special-cased by the property
+    // enumeration code.
+
+    const UnboxedLayout::PropertyVector& unboxed = obj->as<UnboxedPlainObject>().layout().properties();
+    for (size_t i = 0; i < unboxed.length(); i++) {
+        if (!properties.append(NameToId(unboxed[i].name)))
+            return false;
+    }
+
+    return true;
+}
+
+const Class UnboxedExpandoObject::class_ = {
+    "UnboxedExpandoObject",
+    0
+};
+
+static const ClassOps UnboxedPlainObjectClassOps = {
+    nullptr,        /* addProperty */
+    nullptr,        /* delProperty */
+    nullptr,        /* getProperty */
+    nullptr,        /* setProperty */
+    nullptr,        /* enumerate   */
+    nullptr,        /* resolve     */
+    nullptr,        /* mayResolve  */
+    nullptr,        /* finalize    */
+    nullptr,        /* call        */
+    nullptr,        /* hasInstance */
+    nullptr,        /* construct   */
+    UnboxedPlainObject::trace,
+};
+
+static const ObjectOps UnboxedPlainObjectObjectOps = {
+    UnboxedPlainObject::obj_lookupProperty,
+    UnboxedPlainObject::obj_defineProperty,
+    UnboxedPlainObject::obj_hasProperty,
+    UnboxedPlainObject::obj_getProperty,
+    UnboxedPlainObject::obj_setProperty,
+    UnboxedPlainObject::obj_getOwnPropertyDescriptor,
+    UnboxedPlainObject::obj_deleteProperty,
+    UnboxedPlainObject::obj_watch,
+    nullptr,   /* No unwatch needed, as watch() converts the object to native */
+    nullptr,   /* getElements */
+    UnboxedPlainObject::obj_enumerate,
+    nullptr    /* funToString */
+};
+
+const Class UnboxedPlainObject::class_ = {
+    js_Object_str,
+    Class::NON_NATIVE |
+    JSCLASS_HAS_CACHED_PROTO(JSProto_Object) |
+    JSCLASS_DELAY_METADATA_BUILDER,
+    &UnboxedPlainObjectClassOps,
+    JS_NULL_CLASS_SPEC,
+    JS_NULL_CLASS_EXT,
+    &UnboxedPlainObjectObjectOps
+};
+
+/////////////////////////////////////////////////////////////////////
+// UnboxedArrayObject
+/////////////////////////////////////////////////////////////////////
+
+template <JSValueType Type>
+DenseElementResult
+AppendUnboxedDenseElements(UnboxedArrayObject* obj, uint32_t initlen,
+                           MutableHandle<GCVector<Value>> values)
+{
+    for (size_t i = 0; i < initlen; i++)
+        values.infallibleAppend(obj->getElementSpecific<Type>(i));
+    return DenseElementResult::Success;
+}
+
+DefineBoxedOrUnboxedFunctor3(AppendUnboxedDenseElements,
+                             UnboxedArrayObject*, uint32_t, MutableHandle<GCVector<Value>>);
+
+/* static */ bool
+UnboxedArrayObject::convertToNativeWithGroup(ExclusiveContext* cx, JSObject* obj,
+                                             ObjectGroup* group, Shape* shape)
+{
+    size_t length = obj->as<UnboxedArrayObject>().length();
+    size_t initlen = obj->as<UnboxedArrayObject>().initializedLength();
+
+    Rooted<GCVector<Value>> values(cx, GCVector<Value>(cx));
+    if (!values.reserve(initlen))
+        return false;
+
+    AppendUnboxedDenseElementsFunctor functor(&obj->as<UnboxedArrayObject>(), initlen, &values);
+    DebugOnly<DenseElementResult> result = CallBoxedOrUnboxedSpecialization(functor, obj);
+    MOZ_ASSERT(result.value == DenseElementResult::Success);
+
+    obj->setGroup(group);
+
+    ArrayObject* aobj = &obj->as<ArrayObject>();
+    aobj->setLastPropertyMakeNative(cx, shape);
+
+    // Make sure there is at least one element, so that this array does not
+    // use emptyObjectElements / emptyObjectElementsShared.
+    if (!aobj->ensureElements(cx, Max<size_t>(initlen, 1)))
+        return false;
+
+    MOZ_ASSERT(!aobj->getDenseInitializedLength());
+    aobj->setDenseInitializedLength(initlen);
+    aobj->initDenseElements(0, values.begin(), initlen);
+    aobj->setLengthInt32(length);
+
+    return true;
+}
+
+/* static */ bool
+UnboxedArrayObject::convertToNative(JSContext* cx, JSObject* obj)
+{
+    const UnboxedLayout& layout = obj->as<UnboxedArrayObject>().layout();
+
+    if (!layout.nativeGroup()) {
+        if (!UnboxedLayout::makeNativeGroup(cx, obj->group()))
+            return false;
+    }
+
+    return convertToNativeWithGroup(cx, obj, layout.nativeGroup(), layout.nativeShape());
+}
+
+bool
+UnboxedArrayObject::convertInt32ToDouble(ExclusiveContext* cx, ObjectGroup* group)
+{
+    MOZ_ASSERT(elementType() == JSVAL_TYPE_INT32);
+    MOZ_ASSERT(group->unboxedLayout().elementType() == JSVAL_TYPE_DOUBLE);
+
+    Vector<int32_t> values(cx);
+    if (!values.reserve(initializedLength()))
+        return false;
+    for (size_t i = 0; i < initializedLength(); i++)
+        values.infallibleAppend(getElementSpecific<JSVAL_TYPE_INT32>(i).toInt32());
+
+    uint8_t* newElements;
+    if (hasInlineElements()) {
+        newElements = AllocateObjectBuffer<uint8_t>(cx, this, capacity() * sizeof(double));
+    } else {
+        newElements = ReallocateObjectBuffer<uint8_t>(cx, this, elements(),
+                                                      capacity() * sizeof(int32_t),
+                                                      capacity() * sizeof(double));
+    }
+    if (!newElements)
+        return false;
+
+    setGroup(group);
+    elements_ = newElements;
+
+    for (size_t i = 0; i < initializedLength(); i++)
+        setElementNoTypeChangeSpecific<JSVAL_TYPE_DOUBLE>(i, DoubleValue(values[i]));
+
+    return true;
+}
+
+/* static */ UnboxedArrayObject*
+UnboxedArrayObject::create(ExclusiveContext* cx, HandleObjectGroup group, uint32_t length,
+                           NewObjectKind newKind, uint32_t maxLength)
+{
+    MOZ_ASSERT(length <= MaximumCapacity);
+
+    MOZ_ASSERT(group->clasp() == &class_);
+    uint32_t elementSize = UnboxedTypeSize(group->unboxedLayout().elementType());
+    uint32_t capacity = Min(length, maxLength);
+    uint32_t nbytes = offsetOfInlineElements() + elementSize * capacity;
+
+    UnboxedArrayObject* res;
+    if (nbytes <= JSObject::MAX_BYTE_SIZE) {
+        gc::AllocKind allocKind = gc::GetGCObjectKindForBytes(nbytes);
+
+        // If there was no provided length information, pick an allocation kind
+        // to accommodate small arrays (as is done for normal native arrays).
+        if (capacity == 0)
+            allocKind = gc::AllocKind::OBJECT8;
+
+        res = NewObjectWithGroup<UnboxedArrayObject>(cx, group, allocKind, newKind);
+        if (!res)
+            return nullptr;
+        res->setInitializedLengthNoBarrier(0);
+        res->setInlineElements();
+
+        size_t actualCapacity = (GetGCKindBytes(allocKind) - offsetOfInlineElements()) / elementSize;
+        MOZ_ASSERT(actualCapacity >= capacity);
+        res->setCapacityIndex(exactCapacityIndex(actualCapacity));
+    } else {
+        res = NewObjectWithGroup<UnboxedArrayObject>(cx, group, gc::AllocKind::OBJECT0, newKind);
+        if (!res)
+            return nullptr;
+        res->setInitializedLengthNoBarrier(0);
+
+        uint32_t capacityIndex = (capacity == length)
+                                 ? CapacityMatchesLengthIndex
+                                 : chooseCapacityIndex(capacity, length);
+        uint32_t actualCapacity = computeCapacity(capacityIndex, length);
+
+        res->elements_ = AllocateObjectBuffer<uint8_t>(cx, res, actualCapacity * elementSize);
+        if (!res->elements_) {
+            // Make the object safe for GC.
+            res->setInlineElements();
+            return nullptr;
+        }
+
+        res->setCapacityIndex(capacityIndex);
+    }
+
+    res->setLength(cx, length);
+    return res;
+}
+
+bool
+UnboxedArrayObject::setElement(ExclusiveContext* cx, size_t index, const Value& v)
+{
+    MOZ_ASSERT(index < initializedLength());
+    uint8_t* p = elements() + index * elementSize();
+    return SetUnboxedValue(cx, this, JSID_VOID, p, elementType(), v, /* preBarrier = */ true);
+}
+
+bool
+UnboxedArrayObject::initElement(ExclusiveContext* cx, size_t index, const Value& v)
+{
+    MOZ_ASSERT(index < initializedLength());
+    uint8_t* p = elements() + index * elementSize();
+    return SetUnboxedValue(cx, this, JSID_VOID, p, elementType(), v, /* preBarrier = */ false);
+}
+
+void
+UnboxedArrayObject::initElementNoTypeChange(size_t index, const Value& v)
+{
+    MOZ_ASSERT(index < initializedLength());
+    uint8_t* p = elements() + index * elementSize();
+    if (UnboxedTypeNeedsPreBarrier(elementType()))
+        *reinterpret_cast<void**>(p) = nullptr;
+    SetUnboxedValueNoTypeChange(this, p, elementType(), v, /* preBarrier = */ false);
+}
+
+Value
+UnboxedArrayObject::getElement(size_t index)
+{
+    MOZ_ASSERT(index < initializedLength());
+    uint8_t* p = elements() + index * elementSize();
+    return GetUnboxedValue(p, elementType(), /* maybeUninitialized = */ false);
+}
+
+/* static */ void
+UnboxedArrayObject::trace(JSTracer* trc, JSObject* obj)
+{
+    JSValueType type = obj->as<UnboxedArrayObject>().elementType();
+    if (!UnboxedTypeNeedsPreBarrier(type))
+        return;
+
+    MOZ_ASSERT(obj->as<UnboxedArrayObject>().elementSize() == sizeof(uintptr_t));
+    size_t initlen = obj->as<UnboxedArrayObject>().initializedLength();
+    void** elements = reinterpret_cast<void**>(obj->as<UnboxedArrayObject>().elements());
+
+    switch (type) {
+      case JSVAL_TYPE_OBJECT:
+        for (size_t i = 0; i < initlen; i++) {
+            GCPtrObject* heap = reinterpret_cast<GCPtrObject*>(elements + i);
+            TraceNullableEdge(trc, heap, "unboxed_object");
+        }
+        break;
+
+      case JSVAL_TYPE_STRING:
+        for (size_t i = 0; i < initlen; i++) {
+            GCPtrString* heap = reinterpret_cast<GCPtrString*>(elements + i);
+            TraceEdge(trc, heap, "unboxed_string");
+        }
+        break;
+
+      default:
+        MOZ_CRASH();
+    }
+}
+
+/* static */ void
+UnboxedArrayObject::objectMoved(JSObject* obj, const JSObject* old)
+{
+    UnboxedArrayObject& dst = obj->as<UnboxedArrayObject>();
+    const UnboxedArrayObject& src = old->as<UnboxedArrayObject>();
+
+    // Fix up possible inline data pointer.
+    if (src.hasInlineElements())
+        dst.setInlineElements();
+}
+
+/* static */ void
+UnboxedArrayObject::finalize(FreeOp* fop, JSObject* obj)
+{
+    MOZ_ASSERT(!IsInsideNursery(obj));
+    if (!obj->as<UnboxedArrayObject>().hasInlineElements())
+        js_free(obj->as<UnboxedArrayObject>().elements());
+}
+
+/* static */ size_t
+UnboxedArrayObject::objectMovedDuringMinorGC(JSTracer* trc, JSObject* dst, JSObject* src,
+                                             gc::AllocKind allocKind)
+{
+    UnboxedArrayObject* ndst = &dst->as<UnboxedArrayObject>();
+    UnboxedArrayObject* nsrc = &src->as<UnboxedArrayObject>();
+    MOZ_ASSERT(ndst->elements() == nsrc->elements());
+
+    Nursery& nursery = trc->runtime()->gc.nursery;
+
+    if (!nursery.isInside(nsrc->elements())) {
+        nursery.removeMallocedBuffer(nsrc->elements());
+        return 0;
+    }
+
+    // Determine if we can use inline data for the target array. If this is
+    // possible, the nursery will have picked an allocation size that is large
+    // enough.
+    size_t nbytes = nsrc->capacity() * nsrc->elementSize();
+    if (offsetOfInlineElements() + nbytes <= GetGCKindBytes(allocKind)) {
+        ndst->setInlineElements();
+    } else {
+        MOZ_ASSERT(allocKind == gc::AllocKind::OBJECT0);
+
+        AutoEnterOOMUnsafeRegion oomUnsafe;
+        uint8_t* data = nsrc->zone()->pod_malloc<uint8_t>(nbytes);
+        if (!data)
+            oomUnsafe.crash("Failed to allocate unboxed array elements while tenuring.");
+        ndst->elements_ = data;
+    }
+
+    PodCopy(ndst->elements(), nsrc->elements(), nsrc->initializedLength() * nsrc->elementSize());
+
+    // Set a forwarding pointer for the element buffers in case they were
+    // preserved on the stack by Ion.
+    bool direct = nsrc->capacity() * nsrc->elementSize() >= sizeof(uintptr_t);
+    nursery.maybeSetForwardingPointer(trc, nsrc->elements(), ndst->elements(), direct);
+
+    return ndst->hasInlineElements() ? 0 : nbytes;
+}
+
+// Possible capacities for unboxed arrays. Some of these capacities might seem
+// a little weird, but were chosen to allow the inline data of objects of each
+// size to be fully utilized for arrays of the various types on both 32 bit and
+// 64 bit platforms.
+//
+// To find the possible inline capacities, the following script was used:
+//
+// var fixedSlotCapacities = [0, 2, 4, 8, 12, 16];
+// var dataSizes = [1, 4, 8];
+// var header32 = 4 * 2 + 4 * 2;
+// var header64 = 8 * 2 + 4 * 2;
+//
+// for (var i = 0; i < fixedSlotCapacities.length; i++) {
+//    var nfixed = fixedSlotCapacities[i];
+//    var size32 = 4 * 4 + 8 * nfixed - header32;
+//    var size64 = 8 * 4 + 8 * nfixed - header64;
+//    for (var j = 0; j < dataSizes.length; j++) {
+//        print(size32 / dataSizes[j]);
+//        print(size64 / dataSizes[j]);
+//    }
+// }
+//
+/* static */ const uint32_t
+UnboxedArrayObject::CapacityArray[] = {
+    UINT32_MAX, // For CapacityMatchesLengthIndex.
+    0, 1, 2, 3, 4, 5, 6, 8, 9, 10, 12, 13, 16, 17, 18, 24, 26, 32, 34, 40, 64, 72, 96, 104, 128, 136,
+    256, 512, 1024, 2048, 4096, 8192, 16384, 32768, 65536, 131072, 262144, 524288,
+    1048576, 2097152, 3145728, 4194304, 5242880, 6291456, 7340032, 8388608, 9437184, 11534336,
+    13631488, 15728640, 17825792, 20971520, 24117248, 27262976, 31457280, 35651584, 40894464,
+    46137344, 52428800, 59768832, MaximumCapacity
+};
+
+static const uint32_t
+Pow2CapacityIndexes[] = {
+    2,  // 1
+    3,  // 2
+    5,  // 4
+    8,  // 8
+    13, // 16
+    18, // 32
+    21, // 64
+    25, // 128
+    27, // 256
+    28, // 512
+    29, // 1024
+    30, // 2048
+    31, // 4096
+    32, // 8192
+    33, // 16384
+    34, // 32768
+    35, // 65536
+    36, // 131072
+    37, // 262144
+    38, // 524288
+    39  // 1048576
+};
+
+static const uint32_t MebiCapacityIndex = 39;
+
+/* static */ uint32_t
+UnboxedArrayObject::chooseCapacityIndex(uint32_t capacity, uint32_t length)
+{
+    // Note: the structure and behavior of this method follow along with
+    // NativeObject::goodAllocated. Changes to the allocation strategy in one
+    // should generally be matched by the other.
+
+    // Make sure we have enough space to store all possible values for the capacity index.
+    // This ought to be a static_assert, but MSVC doesn't like that.
+    MOZ_ASSERT(mozilla::ArrayLength(CapacityArray) - 1 <= (CapacityMask >> CapacityShift));
+
+    // The caller should have ensured the capacity is possible for an unboxed array.
+    MOZ_ASSERT(capacity <= MaximumCapacity);
+
+    static const uint32_t Mebi = 1024 * 1024;
+
+    if (capacity <= Mebi) {
+        capacity = mozilla::RoundUpPow2(capacity);
+
+        // When the required capacity is close to the array length, then round
+        // up to the array length itself, as for NativeObject.
+        if (length >= capacity && capacity > (length / 3) * 2)
+            return CapacityMatchesLengthIndex;
+
+        if (capacity < MinimumDynamicCapacity)
+            capacity = MinimumDynamicCapacity;
+
+        uint32_t bit = mozilla::FloorLog2Size(capacity);
+        MOZ_ASSERT(capacity == uint32_t(1 << bit));
+        MOZ_ASSERT(bit <= 20);
+        MOZ_ASSERT(mozilla::ArrayLength(Pow2CapacityIndexes) == 21);
+
+        uint32_t index = Pow2CapacityIndexes[bit];
+        MOZ_ASSERT(CapacityArray[index] == capacity);
+
+        return index;
+    }
+
+    MOZ_ASSERT(CapacityArray[MebiCapacityIndex] == Mebi);
+
+    for (uint32_t i = MebiCapacityIndex + 1;; i++) {
+        if (CapacityArray[i] >= capacity)
+            return i;
+    }
+
+    MOZ_CRASH("Invalid capacity");
+}
+
+/* static */ uint32_t
+UnboxedArrayObject::exactCapacityIndex(uint32_t capacity)
+{
+    for (size_t i = CapacityMatchesLengthIndex + 1; i < ArrayLength(CapacityArray); i++) {
+        if (CapacityArray[i] == capacity)
+            return i;
+    }
+    MOZ_CRASH();
+}
+
+bool
+UnboxedArrayObject::growElements(ExclusiveContext* cx, size_t cap)
+{
+    // The caller should have checked if this capacity is possible for an
+    // unboxed array, so the only way this call can fail is from OOM.
+    MOZ_ASSERT(cap <= MaximumCapacity);
+
+    uint32_t oldCapacity = capacity();
+    uint32_t newCapacityIndex = chooseCapacityIndex(cap, length());
+    uint32_t newCapacity = computeCapacity(newCapacityIndex, length());
+
+    MOZ_ASSERT(oldCapacity < cap);
+    MOZ_ASSERT(cap <= newCapacity);
+
+    // The allocation size computation below cannot have integer overflows.
+    JS_STATIC_ASSERT(MaximumCapacity < UINT32_MAX / sizeof(double));
+
+    uint8_t* newElements;
+    if (hasInlineElements()) {
+        newElements = AllocateObjectBuffer<uint8_t>(cx, this, newCapacity * elementSize());
+        if (!newElements)
+            return false;
+        js_memcpy(newElements, elements(), initializedLength() * elementSize());
+    } else {
+        newElements = ReallocateObjectBuffer<uint8_t>(cx, this, elements(),
+                                                      oldCapacity * elementSize(),
+                                                      newCapacity * elementSize());
+        if (!newElements)
+            return false;
+    }
+
+    elements_ = newElements;
+    setCapacityIndex(newCapacityIndex);
+
+    return true;
+}
+
+void
+UnboxedArrayObject::shrinkElements(ExclusiveContext* cx, size_t cap)
+{
+    if (hasInlineElements())
+        return;
+
+    uint32_t oldCapacity = capacity();
+    uint32_t newCapacityIndex = chooseCapacityIndex(cap, 0);
+    uint32_t newCapacity = computeCapacity(newCapacityIndex, 0);
+
+    MOZ_ASSERT(cap < oldCapacity);
+    MOZ_ASSERT(cap <= newCapacity);
+
+    if (newCapacity >= oldCapacity)
+        return;
+
+    uint8_t* newElements = ReallocateObjectBuffer<uint8_t>(cx, this, elements(),
+                                                           oldCapacity * elementSize(),
+                                                           newCapacity * elementSize());
+    if (!newElements)
+        return;
+
+    elements_ = newElements;
+    setCapacityIndex(newCapacityIndex);
+}
+
+bool
+UnboxedArrayObject::containsProperty(ExclusiveContext* cx, jsid id)
+{
+    if (JSID_IS_INT(id) && uint32_t(JSID_TO_INT(id)) < initializedLength())
+        return true;
+    if (JSID_IS_ATOM(id) && JSID_TO_ATOM(id) == cx->names().length)
+        return true;
+    return false;
+}
+
+/* static */ bool
+UnboxedArrayObject::obj_lookupProperty(JSContext* cx, HandleObject obj,
+                                       HandleId id, MutableHandleObject objp,
+                                       MutableHandleShape propp)
+{
+    if (obj->as<UnboxedArrayObject>().containsProperty(cx, id)) {
+        MarkNonNativePropertyFound<CanGC>(propp);
+        objp.set(obj);
+        return true;
+    }
+
+    RootedObject proto(cx, obj->staticPrototype());
+    if (!proto) {
+        objp.set(nullptr);
+        propp.set(nullptr);
+        return true;
+    }
+
+    return LookupProperty(cx, proto, id, objp, propp);
+}
+
+/* static */ bool
+UnboxedArrayObject::obj_defineProperty(JSContext* cx, HandleObject obj, HandleId id,
+                                       Handle<PropertyDescriptor> desc,
+                                       ObjectOpResult& result)
+{
+    if (JSID_IS_INT(id) && !desc.getter() && !desc.setter() && desc.attributes() == JSPROP_ENUMERATE) {
+        UnboxedArrayObject* nobj = &obj->as<UnboxedArrayObject>();
+
+        uint32_t index = JSID_TO_INT(id);
+        if (index < nobj->initializedLength()) {
+            if (nobj->setElement(cx, index, desc.value()))
+                return result.succeed();
+        } else if (index == nobj->initializedLength() && index < MaximumCapacity) {
+            if (nobj->initializedLength() == nobj->capacity()) {
+                if (!nobj->growElements(cx, index + 1))
+                    return false;
+            }
+            nobj->setInitializedLength(index + 1);
+            if (nobj->initElement(cx, index, desc.value())) {
+                if (nobj->length() <= index)
+                    nobj->setLengthInt32(index + 1);
+                return result.succeed();
+            }
+            nobj->setInitializedLengthNoBarrier(index);
+        }
+    }
+
+    if (!convertToNative(cx, obj))
+        return false;
+
+    return DefineProperty(cx, obj, id, desc, result);
+}
+
+/* static */ bool
+UnboxedArrayObject::obj_hasProperty(JSContext* cx, HandleObject obj, HandleId id, bool* foundp)
+{
+    if (obj->as<UnboxedArrayObject>().containsProperty(cx, id)) {
+        *foundp = true;
+        return true;
+    }
+
+    RootedObject proto(cx, obj->staticPrototype());
+    if (!proto) {
+        *foundp = false;
+        return true;
+    }
+
+    return HasProperty(cx, proto, id, foundp);
+}
+
+/* static */ bool
+UnboxedArrayObject::obj_getProperty(JSContext* cx, HandleObject obj, HandleValue receiver,
+                                    HandleId id, MutableHandleValue vp)
+{
+    if (obj->as<UnboxedArrayObject>().containsProperty(cx, id)) {
+        if (JSID_IS_INT(id))
+            vp.set(obj->as<UnboxedArrayObject>().getElement(JSID_TO_INT(id)));
+        else
+            vp.set(Int32Value(obj->as<UnboxedArrayObject>().length()));
+        return true;
+    }
+
+    RootedObject proto(cx, obj->staticPrototype());
+    if (!proto) {
+        vp.setUndefined();
+        return true;
+    }
+
+    return GetProperty(cx, proto, receiver, id, vp);
+}
+
+/* static */ bool
+UnboxedArrayObject::obj_setProperty(JSContext* cx, HandleObject obj, HandleId id, HandleValue v,
+                                    HandleValue receiver, ObjectOpResult& result)
+{
+    if (obj->as<UnboxedArrayObject>().containsProperty(cx, id)) {
+        if (receiver.isObject() && obj == &receiver.toObject()) {
+            if (JSID_IS_INT(id)) {
+                if (obj->as<UnboxedArrayObject>().setElement(cx, JSID_TO_INT(id), v))
+                    return result.succeed();
+            } else {
+                uint32_t len;
+                if (!CanonicalizeArrayLengthValue(cx, v, &len))
+                    return false;
+                UnboxedArrayObject* nobj = &obj->as<UnboxedArrayObject>();
+                if (len < nobj->initializedLength()) {
+                    nobj->setInitializedLength(len);
+                    nobj->shrinkElements(cx, len);
+                }
+                nobj->setLength(cx, len);
+                return result.succeed();
+            }
+
+            if (!convertToNative(cx, obj))
+                return false;
+            return SetProperty(cx, obj, id, v, receiver, result);
+        }
+
+        return SetPropertyByDefining(cx, id, v, receiver, result);
+    }
+
+    return SetPropertyOnProto(cx, obj, id, v, receiver, result);
+}
+
+/* static */ bool
+UnboxedArrayObject::obj_getOwnPropertyDescriptor(JSContext* cx, HandleObject obj, HandleId id,
+                                                 MutableHandle<PropertyDescriptor> desc)
+{
+    if (obj->as<UnboxedArrayObject>().containsProperty(cx, id)) {
+        if (JSID_IS_INT(id)) {
+            desc.value().set(obj->as<UnboxedArrayObject>().getElement(JSID_TO_INT(id)));
+            desc.setAttributes(JSPROP_ENUMERATE);
+        } else {
+            desc.value().set(Int32Value(obj->as<UnboxedArrayObject>().length()));
+            desc.setAttributes(JSPROP_PERMANENT);
+        }
+        desc.object().set(obj);
+        return true;
+    }
+
+    desc.object().set(nullptr);
+    return true;
+}
+
+/* static */ bool
+UnboxedArrayObject::obj_deleteProperty(JSContext* cx, HandleObject obj, HandleId id,
+                                       ObjectOpResult& result)
+{
+    if (obj->as<UnboxedArrayObject>().containsProperty(cx, id)) {
+        size_t initlen = obj->as<UnboxedArrayObject>().initializedLength();
+        if (JSID_IS_INT(id) && JSID_TO_INT(id) == int32_t(initlen - 1)) {
+            obj->as<UnboxedArrayObject>().setInitializedLength(initlen - 1);
+            obj->as<UnboxedArrayObject>().shrinkElements(cx, initlen - 1);
+            return result.succeed();
+        }
+    }
+
+    if (!convertToNative(cx, obj))
+        return false;
+    return DeleteProperty(cx, obj, id, result);
+}
+
+/* static */ bool
+UnboxedArrayObject::obj_watch(JSContext* cx, HandleObject obj, HandleId id, HandleObject callable)
+{
+    if (!convertToNative(cx, obj))
+        return false;
+    return WatchProperty(cx, obj, id, callable);
+}
+
+/* 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,
+    UnboxedArrayObject::obj_watch,
+    nullptr,   /* No unwatch needed, as watch() converts the object to native */
+    nullptr,   /* getElements */
+    UnboxedArrayObject::obj_enumerate,
+    nullptr    /* funToString */
+};
+
+const Class UnboxedArrayObject::class_ = {
+    "Array",
+    Class::NON_NATIVE |
+    JSCLASS_SKIP_NURSERY_FINALIZE |
+    JSCLASS_BACKGROUND_FINALIZE,
+    &UnboxedArrayObjectClassOps,
+    JS_NULL_CLASS_SPEC,
+    &UnboxedArrayObjectClassExtension,
+    &UnboxedArrayObjectObjectOps
+};
+
+/////////////////////////////////////////////////////////////////////
+// API
+/////////////////////////////////////////////////////////////////////
+
+static bool
+UnboxedTypeIncludes(JSValueType supertype, JSValueType subtype)
+{
+    if (supertype == JSVAL_TYPE_DOUBLE && subtype == JSVAL_TYPE_INT32)
+        return true;
+    if (supertype == JSVAL_TYPE_OBJECT && subtype == JSVAL_TYPE_NULL)
+        return true;
+    return false;
+}
+
+static bool
+CombineUnboxedTypes(const Value& value, JSValueType* existing)
+{
+    JSValueType type = value.isDouble() ? JSVAL_TYPE_DOUBLE : value.extractNonDoubleType();
+
+    if (*existing == JSVAL_TYPE_MAGIC || *existing == type || UnboxedTypeIncludes(type, *existing)) {
+        *existing = type;
+        return true;
+    }
+    if (UnboxedTypeIncludes(*existing, type))
+        return true;
+    return false;
+}
+
+// Return whether the property names and types in layout are a subset of the
+// specified vector.
+static bool
+PropertiesAreSuperset(const UnboxedLayout::PropertyVector& properties, UnboxedLayout* layout)
+{
+    for (size_t i = 0; i < layout->properties().length(); i++) {
+        const UnboxedLayout::Property& layoutProperty = layout->properties()[i];
+        bool found = false;
+        for (size_t j = 0; j < properties.length(); j++) {
+            if (layoutProperty.name == properties[j].name) {
+                found = (layoutProperty.type == properties[j].type);
+                break;
+            }
+        }
+        if (!found)
+            return false;
+    }
+    return true;
+}
+
+static bool
+CombinePlainObjectProperties(PlainObject* obj, Shape* templateShape,
+                             UnboxedLayout::PropertyVector& properties)
+{
+    // All preliminary objects must have been created with enough space to
+    // fill in their unboxed data inline. This is ensured either by using
+    // the largest allocation kind (which limits the maximum size of an
+    // unboxed object), or by using an allocation kind that covers all
+    // properties in the template, as the space used by unboxed properties
+    // is less than or equal to that used by boxed properties.
+    MOZ_ASSERT(gc::GetGCKindSlots(obj->asTenured().getAllocKind()) >=
+               Min(NativeObject::MAX_FIXED_SLOTS, templateShape->slotSpan()));
+
+    if (obj->lastProperty() != templateShape || obj->hasDynamicElements()) {
+        // Only use an unboxed representation if all created objects match
+        // the template shape exactly.
+        return false;
+    }
+
+    for (size_t i = 0; i < templateShape->slotSpan(); i++) {
+        Value val = obj->getSlot(i);
+
+        JSValueType& existing = properties[i].type;
+        if (!CombineUnboxedTypes(val, &existing))
+            return false;
+    }
+
+    return true;
+}
+
+static bool
+CombineArrayObjectElements(ExclusiveContext* cx, ArrayObject* obj, JSValueType* elementType)
+{
+    if (obj->inDictionaryMode() ||
+        obj->lastProperty()->propid() != AtomToId(cx->names().length) ||
+        !obj->lastProperty()->previous()->isEmptyShape())
+    {
+        // Only use an unboxed representation if the object has no properties.
+        return false;
+    }
+
+    for (size_t i = 0; i < obj->getDenseInitializedLength(); i++) {
+        Value val = obj->getDenseElement(i);
+
+        // For now, unboxed arrays cannot have holes.
+        if (val.isMagic(JS_ELEMENTS_HOLE))
+            return false;
+
+        if (!CombineUnboxedTypes(val, elementType))
+            return false;
+    }
+
+    return true;
+}
+
+static size_t
+ComputePlainObjectLayout(ExclusiveContext* cx, Shape* templateShape,
+                         UnboxedLayout::PropertyVector& properties)
+{
+    // Fill in the names for all the object's properties.
+    for (Shape::Range<NoGC> r(templateShape); !r.empty(); r.popFront()) {
+        size_t slot = r.front().slot();
+        MOZ_ASSERT(!properties[slot].name);
+        properties[slot].name = JSID_TO_ATOM(r.front().propid())->asPropertyName();
+    }
+
+    // Fill in all the unboxed object's property offsets.
+    uint32_t offset = 0;
+
+    // Search for an existing unboxed layout which is a subset of this one.
+    // If there are multiple such layouts, use the largest one. If we're able
+    // to find such a layout, use the same property offsets for the shared
+    // properties, which will allow us to generate better code if the objects
+    // have a subtype/supertype relation and are accessed at common sites.
+    UnboxedLayout* bestExisting = nullptr;
+    for (UnboxedLayout* existing : cx->compartment()->unboxedLayouts) {
+        if (PropertiesAreSuperset(properties, existing)) {
+            if (!bestExisting ||
+                existing->properties().length() > bestExisting->properties().length())
+            {
+                bestExisting = existing;
+            }
+        }
+    }
+    if (bestExisting) {
+        for (size_t i = 0; i < bestExisting->properties().length(); i++) {
+            const UnboxedLayout::Property& existingProperty = bestExisting->properties()[i];
+            for (size_t j = 0; j < templateShape->slotSpan(); j++) {
+                if (existingProperty.name == properties[j].name) {
+                    MOZ_ASSERT(existingProperty.type == properties[j].type);
+                    properties[j].offset = existingProperty.offset;
+                }
+            }
+        }
+        offset = bestExisting->size();
+    }
+
+    // Order remaining properties from the largest down for the best space
+    // utilization.
+    static const size_t typeSizes[] = { 8, 4, 1 };
+
+    for (size_t i = 0; i < ArrayLength(typeSizes); i++) {
+        size_t size = typeSizes[i];
+        for (size_t j = 0; j < templateShape->slotSpan(); j++) {
+            if (properties[j].offset != UINT32_MAX)
+                continue;
+            JSValueType type = properties[j].type;
+            if (UnboxedTypeSize(type) == size) {
+                offset = JS_ROUNDUP(offset, size);
+                properties[j].offset = offset;
+                offset += size;
+            }
+        }
+    }
+
+    // The final offset is the amount of data needed by the object.
+    return offset;
+}
+
+static bool
+SetLayoutTraceList(ExclusiveContext* cx, UnboxedLayout* layout)
+{
+    // Figure out the offsets of any objects or string properties.
+    Vector<int32_t, 8, SystemAllocPolicy> objectOffsets, stringOffsets;
+    for (size_t i = 0; i < layout->properties().length(); i++) {
+        const UnboxedLayout::Property& property = layout->properties()[i];
+        MOZ_ASSERT(property.offset != UINT32_MAX);
+        if (property.type == JSVAL_TYPE_OBJECT) {
+            if (!objectOffsets.append(property.offset))
+                return false;
+        } else if (property.type == JSVAL_TYPE_STRING) {
+            if (!stringOffsets.append(property.offset))
+                return false;
+        }
+    }
+
+    // Construct the layout's trace list.
+    if (!objectOffsets.empty() || !stringOffsets.empty()) {
+        Vector<int32_t, 8, SystemAllocPolicy> entries;
+        if (!entries.appendAll(stringOffsets) ||
+            !entries.append(-1) ||
+            !entries.appendAll(objectOffsets) ||
+            !entries.append(-1) ||
+            !entries.append(-1))
+        {
+            return false;
+        }
+        int32_t* traceList = cx->zone()->pod_malloc<int32_t>(entries.length());
+        if (!traceList)
+            return false;
+        PodCopy(traceList, entries.begin(), entries.length());
+        layout->setTraceList(traceList);
+    }
+
+    return true;
+}
+
+static inline Value
+NextValue(Handle<GCVector<Value>> values, size_t* valueCursor)
+{
+    return values[(*valueCursor)++];
+}
+
+static bool
+GetValuesFromPreliminaryArrayObject(ArrayObject* obj, MutableHandle<GCVector<Value>> values)
+{
+    if (!values.append(Int32Value(obj->length())))
+        return false;
+    if (!values.append(Int32Value(obj->getDenseInitializedLength())))
+        return false;
+    for (size_t i = 0; i < obj->getDenseInitializedLength(); i++) {
+        if (!values.append(obj->getDenseElement(i)))
+            return false;
+    }
+    return true;
+}
+
+void
+UnboxedArrayObject::fillAfterConvert(ExclusiveContext* cx,
+                                     Handle<GCVector<Value>> values, size_t* valueCursor)
+{
+    MOZ_ASSERT(CapacityArray[1] == 0);
+    setCapacityIndex(1);
+    setInitializedLengthNoBarrier(0);
+    setInlineElements();
+
+    setLength(cx, NextValue(values, valueCursor).toInt32());
+
+    int32_t initlen = NextValue(values, valueCursor).toInt32();
+    if (!initlen)
+        return;
+
+    AutoEnterOOMUnsafeRegion oomUnsafe;
+    if (!growElements(cx, initlen))
+        oomUnsafe.crash("UnboxedArrayObject::fillAfterConvert");
+
+    setInitializedLength(initlen);
+
+    for (size_t i = 0; i < size_t(initlen); i++)
+        JS_ALWAYS_TRUE(initElement(cx, i, NextValue(values, valueCursor)));
+}
+
+static bool
+GetValuesFromPreliminaryPlainObject(PlainObject* obj, MutableHandle<GCVector<Value>> values)
+{
+    for (size_t i = 0; i < obj->slotSpan(); i++) {
+        if (!values.append(obj->getSlot(i)))
+            return false;
+    }
+    return true;
+}
+
+void
+UnboxedPlainObject::fillAfterConvert(ExclusiveContext* cx,
+                                     Handle<GCVector<Value>> values, size_t* valueCursor)
+{
+    initExpando();
+    memset(data(), 0, layout().size());
+    for (size_t i = 0; i < layout().properties().length(); i++)
+        JS_ALWAYS_TRUE(setValue(cx, layout().properties()[i], NextValue(values, valueCursor)));
+}
+
+bool
+js::TryConvertToUnboxedLayout(ExclusiveContext* cx, AutoEnterAnalysis& enter, Shape* templateShape,
+                              ObjectGroup* group, PreliminaryObjectArray* objects)
+{
+    bool isArray = !templateShape;
+
+    // Unboxed arrays are nightly only for now. The getenv() call will be
+    // removed when they are on by default. See bug 1153266.
+    if (isArray) {
+#ifdef NIGHTLY_BUILD
+        if (!getenv("JS_OPTION_USE_UNBOXED_ARRAYS")) {
+            if (!cx->options().unboxedArrays())
+                return true;
+        }
+#else
+        return true;
+#endif
+    } else {
+        if (jit::JitOptions.disableUnboxedObjects)
+            return true;
+    }
+
+    MOZ_ASSERT_IF(templateShape, !templateShape->getObjectFlags());
+
+    if (group->runtimeFromAnyThread()->isSelfHostingGlobal(cx->global()))
+        return true;
+
+    if (!isArray && templateShape->slotSpan() == 0)
+        return true;
+
+    UnboxedLayout::PropertyVector properties;
+    if (!isArray) {
+        if (!properties.appendN(UnboxedLayout::Property(), templateShape->slotSpan()))
+            return false;
+    }
+    JSValueType elementType = JSVAL_TYPE_MAGIC;
+
+    size_t objectCount = 0;
+    for (size_t i = 0; i < PreliminaryObjectArray::COUNT; i++) {
+        JSObject* obj = objects->get(i);
+        if (!obj)
+            continue;
+
+        if (obj->isSingleton() || obj->group() != group)
+            return true;
+
+        objectCount++;
+
+        if (isArray) {
+            if (!CombineArrayObjectElements(cx, &obj->as<ArrayObject>(), &elementType))
+                return true;
+        } else {
+            if (!CombinePlainObjectProperties(&obj->as<PlainObject>(), templateShape, properties))
+                return true;
+        }
+    }
+
+    size_t layoutSize = 0;
+    if (isArray) {
+        // Don't use an unboxed representation if we couldn't determine an
+        // element type for the objects.
+        if (UnboxedTypeSize(elementType) == 0)
+            return true;
+    } else {
+        if (objectCount <= 1) {
+            // If only one of the objects has been created, it is more likely
+            // to have new properties added later. This heuristic is not used
+            // for array objects, where we might want an unboxed representation
+            // even if there is only one large array.
+            return true;
+        }
+
+        for (size_t i = 0; i < templateShape->slotSpan(); i++) {
+            // We can't use an unboxed representation if e.g. all the objects have
+            // a null value for one of the properties, as we can't decide what type
+            // it is supposed to have.
+            if (UnboxedTypeSize(properties[i].type) == 0)
+                return true;
+        }
+
+        // Make sure that all properties on the template shape are property
+        // names, and not indexes.
+        for (Shape::Range<NoGC> r(templateShape); !r.empty(); r.popFront()) {
+            jsid id = r.front().propid();
+            uint32_t dummy;
+            if (!JSID_IS_ATOM(id) || JSID_TO_ATOM(id)->isIndex(&dummy))
+                return true;
+        }
+
+        layoutSize = ComputePlainObjectLayout(cx, templateShape, properties);
+
+        // The entire object must be allocatable inline.
+        if (UnboxedPlainObject::offsetOfData() + layoutSize > JSObject::MAX_BYTE_SIZE)
+            return true;
+    }
+
+    UniquePtr<UnboxedLayout>& layout = enter.unboxedLayoutToCleanUp;
+    MOZ_ASSERT(!layout);
+    layout = group->zone()->make_unique<UnboxedLayout>();
+    if (!layout)
+        return false;
+
+    if (isArray) {
+        layout->initArray(elementType);
+    } else {
+        if (!layout->initProperties(properties, layoutSize))
+            return false;
+
+        // The unboxedLayouts list only tracks layouts for plain objects.
+        cx->compartment()->unboxedLayouts.insertFront(layout.get());
+
+        if (!SetLayoutTraceList(cx, layout.get()))
+            return false;
+    }
+
+    // We've determined that all the preliminary objects can use the new layout
+    // just constructed, so convert the existing group to use the unboxed class,
+    // and update the preliminary objects to use the new layout. Do the
+    // fallible stuff first before modifying any objects.
+
+    // Get an empty shape which we can use for the preliminary objects.
+    const Class* clasp = isArray ? &UnboxedArrayObject::class_ : &UnboxedPlainObject::class_;
+    Shape* newShape = EmptyShape::getInitialShape(cx, clasp, group->proto(), 0);
+    if (!newShape) {
+        cx->recoverFromOutOfMemory();
+        return false;
+    }
+
+    // Accumulate a list of all the values in each preliminary object, and
+    // update their shapes.
+    Rooted<GCVector<Value>> values(cx, GCVector<Value>(cx));
+    for (size_t i = 0; i < PreliminaryObjectArray::COUNT; i++) {
+        JSObject* obj = objects->get(i);
+        if (!obj)
+            continue;
+
+        bool ok;
+        if (isArray)
+            ok = GetValuesFromPreliminaryArrayObject(&obj->as<ArrayObject>(), &values);
+        else
+            ok = GetValuesFromPreliminaryPlainObject(&obj->as<PlainObject>(), &values);
+
+        if (!ok) {
+            cx->recoverFromOutOfMemory();
+            return false;
+        }
+    }
+
+    if (TypeNewScript* newScript = group->newScript())
+        layout->setNewScript(newScript);
+
+    for (size_t i = 0; i < PreliminaryObjectArray::COUNT; i++) {
+        if (JSObject* obj = objects->get(i))
+            obj->as<NativeObject>().setLastPropertyMakeNonNative(newShape);
+    }
+
+    group->setClasp(clasp);
+    group->setUnboxedLayout(layout.release());
+
+    size_t valueCursor = 0;
+    for (size_t i = 0; i < PreliminaryObjectArray::COUNT; i++) {
+        JSObject* obj = objects->get(i);
+        if (!obj)
+            continue;
+
+        if (isArray)
+            obj->as<UnboxedArrayObject>().fillAfterConvert(cx, values, &valueCursor);
+        else
+            obj->as<UnboxedPlainObject>().fillAfterConvert(cx, values, &valueCursor);
+    }
+
+    MOZ_ASSERT(valueCursor == values.length());
+    return true;
+}
+
+DefineBoxedOrUnboxedFunctor6(SetOrExtendBoxedOrUnboxedDenseElements,
+                             ExclusiveContext*, JSObject*, uint32_t, const Value*, uint32_t,
+                             ShouldUpdateTypes);
+
+DenseElementResult
+js::SetOrExtendAnyBoxedOrUnboxedDenseElements(ExclusiveContext* cx, JSObject* obj,
+                                              uint32_t start, const Value* vp, uint32_t count,
+                                              ShouldUpdateTypes updateTypes)
+{
+    SetOrExtendBoxedOrUnboxedDenseElementsFunctor functor(cx, obj, start, vp, count, updateTypes);
+    return CallBoxedOrUnboxedSpecialization(functor, obj);
+};
+
+DefineBoxedOrUnboxedFunctor5(MoveBoxedOrUnboxedDenseElements,
+                             JSContext*, JSObject*, uint32_t, uint32_t, uint32_t);
+
+DenseElementResult
+js::MoveAnyBoxedOrUnboxedDenseElements(JSContext* cx, JSObject* obj,
+                                       uint32_t dstStart, uint32_t srcStart, uint32_t length)
+{
+    MoveBoxedOrUnboxedDenseElementsFunctor functor(cx, obj, dstStart, srcStart, length);
+    return CallBoxedOrUnboxedSpecialization(functor, obj);
+}
+
+DefineBoxedOrUnboxedFunctorPair6(CopyBoxedOrUnboxedDenseElements,
+                                 JSContext*, JSObject*, JSObject*, uint32_t, uint32_t, uint32_t);
+
+DenseElementResult
+js::CopyAnyBoxedOrUnboxedDenseElements(JSContext* cx, JSObject* dst, JSObject* src,
+                                       uint32_t dstStart, uint32_t srcStart, uint32_t length)
+{
+    CopyBoxedOrUnboxedDenseElementsFunctor functor(cx, dst, src, dstStart, srcStart, length);
+    return CallBoxedOrUnboxedSpecialization(functor, dst, src);
+}
+
+DefineBoxedOrUnboxedFunctor3(SetBoxedOrUnboxedInitializedLength,
+                             JSContext*, JSObject*, size_t);
+
+void
+js::SetAnyBoxedOrUnboxedInitializedLength(JSContext* cx, JSObject* obj, size_t initlen)
+{
+    SetBoxedOrUnboxedInitializedLengthFunctor functor(cx, obj, initlen);
+    JS_ALWAYS_TRUE(CallBoxedOrUnboxedSpecialization(functor, obj) == DenseElementResult::Success);
+}
+
+DefineBoxedOrUnboxedFunctor3(EnsureBoxedOrUnboxedDenseElements,
+                             JSContext*, JSObject*, size_t);
+
+DenseElementResult
+js::EnsureAnyBoxedOrUnboxedDenseElements(JSContext* cx, JSObject* obj, size_t initlen)
+{
+    EnsureBoxedOrUnboxedDenseElementsFunctor functor(cx, obj, initlen);
+    return CallBoxedOrUnboxedSpecialization(functor, obj);
+}