Revert #1137 - Remove unboxed arrays

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

16 files changed, 1814 insertions, 88 deletions

diff --git a/js/src/vm/Interpreter-inl.h b/js/src/vm/Interpreter-inl.h
index acfa8f74b..adefa6e93 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 b87d12924..834084c4d 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 e50da3bc4..01883bb15 100644
--- a/js/src/vm/JSONParser.cpp
+++ b/js/src/vm/JSONParser.cpp
@@ -606,8 +606,8 @@ JSONParserBase::finishArray(MutableHandleValue vp, ElementVector& elements)
 {
     MOZ_ASSERT(&elements == &stack.back().elements());
 
-    ArrayObject* obj = ObjectGroup::newArrayObject(cx, elements.begin(), elements.length(),
-                                                   GenericObject);
+    JSObject* obj = ObjectGroup::newArrayObject(cx, elements.begin(), elements.length(),
+                                                GenericObject);
     if (!obj)
         return false;
 
diff --git a/js/src/vm/NativeObject-inl.h b/js/src/vm/NativeObject-inl.h
index 030d92c12..205d99aa2 100644
--- a/js/src/vm/NativeObject-inl.h
+++ b/js/src/vm/NativeObject-inl.h
@@ -235,38 +235,6 @@ NativeObject::ensureDenseElements(ExclusiveContext* cx, uint32_t index, uint32_t
     return DenseElementResult::Success;
 }
 
-inline DenseElementResult
-NativeObject::setOrExtendDenseElements(ExclusiveContext* cx, uint32_t start, const Value* vp,
-                                       uint32_t count,
-                                       ShouldUpdateTypes updateTypes)
-{
-    if (denseElementsAreFrozen())
-        return DenseElementResult::Incomplete;
-
-    if (is<ArrayObject>() &&
-        !as<ArrayObject>().lengthIsWritable() &&
-        start + count >= as<ArrayObject>().length())
-    {
-        return DenseElementResult::Incomplete;
-    }
-
-    DenseElementResult result = ensureDenseElements(cx, start, count);
-    if (result != DenseElementResult::Success)
-        return result;
-
-    if (is<ArrayObject>() && start + count >= as<ArrayObject>().length())
-        as<ArrayObject>().setLengthInt32(start + count);
-
-    if (updateTypes == ShouldUpdateTypes::DontUpdate && !shouldConvertDoubleElements()) {
-        copyDenseElements(start, vp, count);
-    } else {
-        for (size_t i = 0; i < count; i++)
-            setDenseElementWithType(cx, start + i, vp[i]);
-    }
-
-    return DenseElementResult::Success;
-}
-
 inline Value
 NativeObject::getDenseOrTypedArrayElement(uint32_t idx)
 {
diff --git a/js/src/vm/NativeObject.h b/js/src/vm/NativeObject.h
index 6595703dc..107fade8c 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 95fcada94..d05a48646 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 553cb8366..4e24de9f1 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 e13cd6221..b26336815 100644
--- a/js/src/vm/Opcodes.h
+++ b/js/src/vm/Opcodes.h
@@ -1281,7 +1281,17 @@
      *   Stack: receiver, obj, propval => obj[propval]
      */ \
     macro(JSOP_GETELEM_SUPER, 125, "getelem-super", NULL, 1,  3,  1, JOF_BYTE |JOF_ELEM|JOF_LEFTASSOC) \
-    macro(JSOP_UNUSED126, 126, "unused126", NULL, 5,  0,  1, JOF_UINT32) \
+    /*
+     * Pushes newly created array for a spread call onto the stack. This has
+     * the same semantics as JSOP_NEWARRAY, but is distinguished to avoid
+     * using unboxed arrays in spread calls, which would make compiling spread
+     * calls in baseline more complex.
+     *   Category: Literals
+     *   Type: Array
+     *   Operands: uint32_t length
+     *   Stack: => obj
+     */ \
+    macro(JSOP_SPREADCALLARRAY, 126, "spreadcallarray", NULL, 5,  0,  1, JOF_UINT32) \
     \
     /*
      * Defines the given function on the current scope.
diff --git a/js/src/vm/ReceiverGuard.cpp b/js/src/vm/ReceiverGuard.cpp
index e37bf8ee5..11c2d0727 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 95940eeaf..7eb8f4ab8 100644
--- a/js/src/vm/Stack.cpp
+++ b/js/src/vm/Stack.cpp
@@ -82,7 +82,7 @@ InterpreterFrame::isNonGlobalEvalFrame() const
     return isEvalFrame() && script()->bodyScope()->as<EvalScope>().isNonGlobal();
 }
 
-ArrayObject*
+JSObject*
 InterpreterFrame::createRestParameter(JSContext* cx)
 {
     MOZ_ASSERT(script()->hasRest());
diff --git a/js/src/vm/Stack.h b/js/src/vm/Stack.h
index fe04a00f2..d08544213 100644
--- a/js/src/vm/Stack.h
+++ b/js/src/vm/Stack.h
@@ -523,7 +523,7 @@ class InterpreterFrame
     ArgumentsObject& argsObj() const;
     void initArgsObj(ArgumentsObject& argsobj);
 
-    ArrayObject* createRestParameter(JSContext* cx);
+    JSObject* createRestParameter(JSContext* cx);
 
     /*
      * Environment chain
diff --git a/js/src/vm/TypeInference.cpp b/js/src/vm/TypeInference.cpp
index 39206539b..ee5bbef5d 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 fd021fc96..537baa21f 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 c1468a5b1..93ad7bf28 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 2e017ca3b..059293a2d 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 ba66434bc..779dd14c7 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 {