Revert #1142 - Remove unboxed objects

- accounting for removal of watch()/unwatch()

12 files changed, 1765 insertions, 20 deletions

diff --git a/js/src/vm/Interpreter.cpp b/js/src/vm/Interpreter.cpp
index 3cf9b57f6..b87d12924 100644
--- a/js/src/vm/Interpreter.cpp
+++ b/js/src/vm/Interpreter.cpp
@@ -5016,6 +5016,11 @@ js::NewObjectOperationWithTemplate(JSContext* cx, HandleObject templateObject)
 
     NewObjectKind newKind = templateObject->group()->shouldPreTenure() ? TenuredObject : GenericObject;
 
+    if (templateObject->group()->maybeUnboxedLayout()) {
+        RootedObjectGroup group(cx, templateObject->group());
+        return UnboxedPlainObject::create(cx, group, newKind);
+    }
+
     JSObject* obj = CopyInitializerObject(cx, templateObject.as<PlainObject>(), newKind);
     if (!obj)
         return nullptr;
diff --git a/js/src/vm/NativeObject.cpp b/js/src/vm/NativeObject.cpp
index d801fad06..bd7484e07 100644
--- a/js/src/vm/NativeObject.cpp
+++ b/js/src/vm/NativeObject.cpp
@@ -388,6 +388,33 @@ NativeObject::setLastPropertyMakeNonNative(Shape* shape)
     shape_ = shape;
 }
 
+void
+NativeObject::setLastPropertyMakeNative(ExclusiveContext* cx, Shape* shape)
+{
+    MOZ_ASSERT(getClass()->isNative());
+    MOZ_ASSERT(shape->getObjectClass()->isNative());
+    MOZ_ASSERT(!shape->inDictionary());
+
+    // This method is used to convert unboxed objects into native objects. In
+    // this case, the shape_ field was previously used to store other data and
+    // this should be treated as an initialization.
+    shape_.init(shape);
+
+    slots_ = nullptr;
+    elements_ = emptyObjectElements;
+
+    size_t oldSpan = shape->numFixedSlots();
+    size_t newSpan = shape->slotSpan();
+
+    initializeSlotRange(0, oldSpan);
+
+    // A failure at this point will leave the object as a mutant, and we
+    // can't recover.
+    AutoEnterOOMUnsafeRegion oomUnsafe;
+    if (oldSpan != newSpan && !updateSlotsForSpan(cx, oldSpan, newSpan))
+        oomUnsafe.crash("NativeObject::setLastPropertyMakeNative");
+}
+
 bool
 NativeObject::setSlotSpan(ExclusiveContext* cx, uint32_t span)
 {
diff --git a/js/src/vm/NativeObject.h b/js/src/vm/NativeObject.h
index 9cc6d5436..6595703dc 100644
--- a/js/src/vm/NativeObject.h
+++ b/js/src/vm/NativeObject.h
@@ -470,6 +470,11 @@ class NativeObject : public ShapedObject
     // that are (temporarily) inconsistent.
     void setLastPropertyMakeNonNative(Shape* shape);
 
+    // As for setLastProperty(), but changes the class associated with the
+    // object to a native one. The object's type has already been changed, and
+    // this brings the shape into sync with it.
+    void setLastPropertyMakeNative(ExclusiveContext* cx, Shape* shape);
+
     // Newly-created TypedArrays that map a SharedArrayBuffer are
     // marked as shared by giving them an ObjectElements that has the
     // ObjectElements::SHARED_MEMORY flag set.
diff --git a/js/src/vm/ObjectGroup-inl.h b/js/src/vm/ObjectGroup-inl.h
index d41343be6..9074f4d97 100644
--- a/js/src/vm/ObjectGroup-inl.h
+++ b/js/src/vm/ObjectGroup-inl.h
@@ -108,6 +108,20 @@ ObjectGroup::maybePreliminaryObjects()
     return maybePreliminaryObjectsDontCheckGeneration();
 }
 
+inline UnboxedLayout*
+ObjectGroup::maybeUnboxedLayout()
+{
+    maybeSweep(nullptr);
+    return maybeUnboxedLayoutDontCheckGeneration();
+}
+
+inline UnboxedLayout&
+ObjectGroup::unboxedLayout()
+{
+    maybeSweep(nullptr);
+    return unboxedLayoutDontCheckGeneration();
+}
+
 } // namespace js
 
 #endif /* vm_ObjectGroup_inl_h */
diff --git a/js/src/vm/ObjectGroup.cpp b/js/src/vm/ObjectGroup.cpp
index 91070b3f6..95fcada94 100644
--- a/js/src/vm/ObjectGroup.cpp
+++ b/js/src/vm/ObjectGroup.cpp
@@ -18,10 +18,11 @@
 #include "vm/ArrayObject.h"
 #include "vm/Shape.h"
 #include "vm/TaggedProto.h"
+#include "vm/UnboxedObject.h"
 
 #include "jsobjinlines.h"
 
-#include "vm/NativeObject-inl.h"
+#include "vm/UnboxedObject-inl.h"
 
 using namespace js;
 
@@ -55,6 +56,7 @@ ObjectGroup::finalize(FreeOp* fop)
     if (newScriptDontCheckGeneration())
         newScriptDontCheckGeneration()->clear();
     fop->delete_(newScriptDontCheckGeneration());
+    fop->delete_(maybeUnboxedLayoutDontCheckGeneration());
     if (maybePreliminaryObjectsDontCheckGeneration())
         maybePreliminaryObjectsDontCheckGeneration()->clear();
     fop->delete_(maybePreliminaryObjectsDontCheckGeneration());
@@ -81,6 +83,8 @@ ObjectGroup::sizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf) const
     size_t n = 0;
     if (TypeNewScript* newScript = newScriptDontCheckGeneration())
         n += newScript->sizeOfIncludingThis(mallocSizeOf);
+    if (UnboxedLayout* layout = maybeUnboxedLayoutDontCheckGeneration())
+        n += layout->sizeOfIncludingThis(mallocSizeOf);
     return n;
 }
 
@@ -529,7 +533,8 @@ ObjectGroup::defaultNewGroup(ExclusiveContext* cx, const Class* clasp,
     if (p) {
         ObjectGroup* group = p->group;
         MOZ_ASSERT_IF(clasp, group->clasp() == clasp);
-        MOZ_ASSERT_IF(!clasp, group->clasp() == &PlainObject::class_);
+        MOZ_ASSERT_IF(!clasp, group->clasp() == &PlainObject::class_ ||
+                              group->clasp() == &UnboxedPlainObject::class_);
         MOZ_ASSERT(group->proto() == proto);
         return group;
     }
@@ -969,6 +974,46 @@ js::CombinePlainObjectPropertyTypes(ExclusiveContext* cx, JSObject* newObj,
                 }
             }
         }
+    } else if (newObj->is<UnboxedPlainObject>()) {
+        const UnboxedLayout& layout = newObj->as<UnboxedPlainObject>().layout();
+        const int32_t* traceList = layout.traceList();
+        if (!traceList)
+            return true;
+
+        uint8_t* newData = newObj->as<UnboxedPlainObject>().data();
+        uint8_t* oldData = oldObj->as<UnboxedPlainObject>().data();
+
+        for (; *traceList != -1; traceList++) {}
+        traceList++;
+        for (; *traceList != -1; traceList++) {
+            JSObject* newInnerObj = *reinterpret_cast<JSObject**>(newData + *traceList);
+            JSObject* oldInnerObj = *reinterpret_cast<JSObject**>(oldData + *traceList);
+
+            if (!newInnerObj || !oldInnerObj || SameGroup(oldInnerObj, newInnerObj))
+                continue;
+
+            if (!GiveObjectGroup(cx, newInnerObj, oldInnerObj))
+                return false;
+
+            if (SameGroup(oldInnerObj, newInnerObj))
+                continue;
+
+            if (!GiveObjectGroup(cx, oldInnerObj, newInnerObj))
+                return false;
+
+            if (SameGroup(oldInnerObj, newInnerObj)) {
+                for (size_t i = 1; i < ncompare; i++) {
+                    if (compare[i].isObject() && SameGroup(&compare[i].toObject(), newObj)) {
+                        uint8_t* otherData = compare[i].toObject().as<UnboxedPlainObject>().data();
+                        JSObject* otherInnerObj = *reinterpret_cast<JSObject**>(otherData + *traceList);
+                        if (otherInnerObj && !SameGroup(otherInnerObj, newInnerObj)) {
+                            if (!GiveObjectGroup(cx, otherInnerObj, newInnerObj))
+                                return false;
+                        }
+                    }
+                }
+            }
+        }
     }
 
     return true;
@@ -1192,6 +1237,12 @@ ObjectGroup::newPlainObject(ExclusiveContext* cx, IdValuePair* properties, size_
 
     RootedObjectGroup group(cx, p->value().group);
 
+    // Watch for existing groups which now use an unboxed layout.
+    if (group->maybeUnboxedLayout()) {
+        MOZ_ASSERT(group->unboxedLayout().properties().length() == nproperties);
+        return UnboxedPlainObject::createWithProperties(cx, group, newKind, properties);
+    }
+
     // Update property types according to the properties we are about to add.
     // Do this before we do anything which can GC, which might move or remove
     // this table entry.
diff --git a/js/src/vm/ObjectGroup.h b/js/src/vm/ObjectGroup.h
index 0b6eaee51..553cb8366 100644
--- a/js/src/vm/ObjectGroup.h
+++ b/js/src/vm/ObjectGroup.h
@@ -20,6 +20,7 @@
 namespace js {
 
 class TypeDescr;
+class UnboxedLayout;
 
 class PreliminaryObjectArrayWithTemplate;
 class TypeNewScript;
@@ -153,6 +154,16 @@ class ObjectGroup : public gc::TenuredCell
         // For some plain objects, the addendum stores a PreliminaryObjectArrayWithTemplate.
         Addendum_PreliminaryObjects,
 
+        // When objects in this group have an unboxed representation, the
+        // addendum stores an UnboxedLayout (which might have a TypeNewScript
+        // as well, if the group is also constructed using 'new').
+        Addendum_UnboxedLayout,
+
+        // If this group is used by objects that have been converted from an
+        // unboxed representation and/or have the same allocation kind as such
+        // objects, the addendum points to that unboxed group.
+        Addendum_OriginalUnboxedGroup,
+
         // When used by typed objects, the addendum stores a TypeDescr.
         Addendum_TypeDescr
     };
@@ -174,6 +185,7 @@ class ObjectGroup : public gc::TenuredCell
         return nullptr;
     }
 
+    TypeNewScript* anyNewScript();
     void detachNewScript(bool writeBarrier, ObjectGroup* replacement);
 
     ObjectGroupFlags flagsDontCheckGeneration() const {
@@ -213,6 +225,34 @@ class ObjectGroup : public gc::TenuredCell
                maybePreliminaryObjectsDontCheckGeneration();
     }
 
+    inline UnboxedLayout* maybeUnboxedLayout();
+    inline UnboxedLayout& unboxedLayout();
+
+    UnboxedLayout* maybeUnboxedLayoutDontCheckGeneration() const {
+        if (addendumKind() == Addendum_UnboxedLayout)
+            return reinterpret_cast<UnboxedLayout*>(addendum_);
+        return nullptr;
+    }
+
+    UnboxedLayout& unboxedLayoutDontCheckGeneration() const {
+        MOZ_ASSERT(addendumKind() == Addendum_UnboxedLayout);
+        return *maybeUnboxedLayoutDontCheckGeneration();
+    }
+
+    void setUnboxedLayout(UnboxedLayout* layout) {
+        setAddendum(Addendum_UnboxedLayout, layout);
+    }
+
+    ObjectGroup* maybeOriginalUnboxedGroup() const {
+        if (addendumKind() == Addendum_OriginalUnboxedGroup)
+            return reinterpret_cast<ObjectGroup*>(addendum_);
+        return nullptr;
+    }
+
+    void setOriginalUnboxedGroup(ObjectGroup* group) {
+        setAddendum(Addendum_OriginalUnboxedGroup, group);
+    }
+
     TypeDescr* maybeTypeDescr() {
         // Note: there is no need to sweep when accessing the type descriptor
         // of an object, as it is strongly held and immutable.
@@ -273,8 +313,9 @@ class ObjectGroup : public gc::TenuredCell
      * that can be read out of that property in actual JS objects. In native
      * objects, property types account for plain data properties (those with a
      * slot and no getter or setter hook) and dense elements. In typed objects
-     * property types account for object and value properties and elements in
-     * the object.
+     * and unboxed objects, property types account for object and value
+     * properties and elements in the object, and expando properties in unboxed
+     * objects.
      *
      * For accesses on these properties, the correspondence is as follows:
      *
@@ -297,9 +338,10 @@ class ObjectGroup : public gc::TenuredCell
      * 2. Array lengths are special cased by the compiler and VM and are not
      *    reflected in property types.
      *
-     * 3. In typed objects, the initial values of properties (null pointers and
-     *    undefined values) are not reflected in the property types. These
-     *    values are always possible when reading the property.
+     * 3. In typed objects (but not unboxed objects), the initial values of
+     *    properties (null pointers and undefined values) are not reflected in
+     *    the property types. These values are always possible when reading the
+     *    property.
      *
      * We establish these by using write barriers on calls to setProperty and
      * defineProperty which are on native properties, and on any jitcode which
@@ -413,6 +455,12 @@ class ObjectGroup : public gc::TenuredCell
         return &flags_;
     }
 
+    // Get the bit pattern stored in an object's addendum when it has an
+    // original unboxed group.
+    static inline int32_t addendumOriginalUnboxedGroupValue() {
+        return Addendum_OriginalUnboxedGroup << OBJECT_FLAG_ADDENDUM_SHIFT;
+    }
+
     inline uint32_t basePropertyCount();
 
   private:
@@ -463,8 +511,8 @@ class ObjectGroup : public gc::TenuredCell
                                        NewObjectKind newKind,
                                        NewArrayKind arrayKind = NewArrayKind::Normal);
 
-    // Create a PlainObject with the specified properties and a group specialized
-    // for those properties.
+    // Create a PlainObject or UnboxedPlainObject with the specified properties
+    // and a group specialized for those properties.
     static JSObject* newPlainObject(ExclusiveContext* cx,
                                     IdValuePair* properties, size_t nproperties,
                                     NewObjectKind newKind);
diff --git a/js/src/vm/ReceiverGuard.cpp b/js/src/vm/ReceiverGuard.cpp
index e95e8a208..e37bf8ee5 100644
--- a/js/src/vm/ReceiverGuard.cpp
+++ b/js/src/vm/ReceiverGuard.cpp
@@ -15,7 +15,11 @@ ReceiverGuard::ReceiverGuard(JSObject* obj)
   : group(nullptr), shape(nullptr)
 {
     if (obj) {
-        if (obj->is<TypedObject>()) {
+        if (obj->is<UnboxedPlainObject>()) {
+            group = obj->group();
+            if (UnboxedExpandoObject* expando = obj->as<UnboxedPlainObject>().maybeExpando())
+                shape = expando->lastProperty();
+        } else if (obj->is<TypedObject>()) {
             group = obj->group();
         } else {
             shape = obj->maybeShape();
@@ -28,7 +32,9 @@ ReceiverGuard::ReceiverGuard(ObjectGroup* group, Shape* shape)
 {
     if (group) {
         const Class* clasp = group->clasp();
-        if (IsTypedObjectClass(clasp)) {
+        if (clasp == &UnboxedPlainObject::class_) {
+            // Keep both group and shape.
+        } else if (IsTypedObjectClass(clasp)) {
             this->shape = nullptr;
         } else {
             this->group = nullptr;
@@ -39,6 +45,10 @@ ReceiverGuard::ReceiverGuard(ObjectGroup* group, Shape* shape)
 /* static */ int32_t
 HeapReceiverGuard::keyBits(JSObject* obj)
 {
+    if (obj->is<UnboxedPlainObject>()) {
+        // 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.
         return 2;
diff --git a/js/src/vm/TypeInference-inl.h b/js/src/vm/TypeInference-inl.h
index 2af252cea..da47fa898 100644
--- a/js/src/vm/TypeInference-inl.h
+++ b/js/src/vm/TypeInference-inl.h
@@ -23,6 +23,7 @@
 #include "vm/SharedArrayObject.h"
 #include "vm/StringObject.h"
 #include "vm/TypedArrayObject.h"
+#include "vm/UnboxedObject.h"
 
 #include "jscntxtinlines.h"
 
@@ -284,6 +285,10 @@ TypeIdString(jsid id)
  */
 struct AutoEnterAnalysis
 {
+    // For use when initializing an UnboxedLayout.  The UniquePtr's destructor
+    // must run when GC is not suppressed.
+    UniquePtr<UnboxedLayout> unboxedLayoutToCleanUp;
+
     // Prevent GC activity in the middle of analysis.
     gc::AutoSuppressGC suppressGC;
 
diff --git a/js/src/vm/TypeInference.cpp b/js/src/vm/TypeInference.cpp
index 2b1fa0e3b..39206539b 100644
--- a/js/src/vm/TypeInference.cpp
+++ b/js/src/vm/TypeInference.cpp
@@ -35,6 +35,7 @@
 #include "vm/Opcodes.h"
 #include "vm/Shape.h"
 #include "vm/Time.h"
+#include "vm/UnboxedObject.h"
 
 #include "jsatominlines.h"
 #include "jsscriptinlines.h"
@@ -296,6 +297,9 @@ js::ObjectGroupHasProperty(JSContext* cx, ObjectGroup* group, jsid id, const Val
                         return true;
                 }
             }
+            JSObject* obj = &value.toObject();
+            if (!obj->hasLazyGroup() && obj->group()->maybeOriginalUnboxedGroup())
+                return true;
         }
 
         if (!types->hasType(type)) {
@@ -1944,6 +1948,33 @@ class ConstraintDataFreezeObjectForTypedArrayData
     }
 };
 
+// Constraint which triggers recompilation if an unboxed object in some group
+// is converted to a native object.
+class ConstraintDataFreezeObjectForUnboxedConvertedToNative
+{
+  public:
+    ConstraintDataFreezeObjectForUnboxedConvertedToNative()
+    {}
+
+    const char* kind() { return "freezeObjectForUnboxedConvertedToNative"; }
+
+    bool invalidateOnNewType(TypeSet::Type type) { return false; }
+    bool invalidateOnNewPropertyState(TypeSet* property) { return false; }
+    bool invalidateOnNewObjectState(ObjectGroup* group) {
+        return group->unboxedLayout().nativeGroup() != nullptr;
+    }
+
+    bool constraintHolds(JSContext* cx,
+                         const HeapTypeSetKey& property, TemporaryTypeSet* expected)
+    {
+        return !invalidateOnNewObjectState(property.object()->maybeGroup());
+    }
+
+    bool shouldSweep() { return false; }
+
+    JSCompartment* maybeCompartment() { return nullptr; }
+};
+
 } /* anonymous namespace */
 
 void
@@ -2478,6 +2509,8 @@ TemporaryTypeSet::propertyNeedsBarrier(CompilerConstraintList* constraints, jsid
 bool
 js::ClassCanHaveExtraProperties(const Class* clasp)
 {
+    if (clasp == &UnboxedPlainObject::class_)
+        return false;
     return clasp->getResolve()
         || clasp->getOpsLookupProperty()
         || clasp->getOpsGetProperty()
@@ -2768,6 +2801,15 @@ js::AddTypePropertyId(ExclusiveContext* cx, ObjectGroup* group, JSObject* obj, j
     // from acquiring the fully initialized group.
     if (group->newScript() && group->newScript()->initializedGroup())
         AddTypePropertyId(cx, group->newScript()->initializedGroup(), nullptr, id, type);
+
+    // Maintain equivalent type information for unboxed object groups and their
+    // corresponding native group. Since type sets might contain the unboxed
+    // group but not the native group, this ensures optimizations based on the
+    // unboxed group are valid for the native group.
+    if (group->maybeUnboxedLayout() && group->maybeUnboxedLayout()->nativeGroup())
+        AddTypePropertyId(cx, group->maybeUnboxedLayout()->nativeGroup(), nullptr, id, type);
+    if (ObjectGroup* unboxedGroup = group->maybeOriginalUnboxedGroup())
+        AddTypePropertyId(cx, unboxedGroup, nullptr, id, type);
 }
 
 void
@@ -2839,6 +2881,12 @@ ObjectGroup::setFlags(ExclusiveContext* cx, ObjectGroupFlags flags)
     // acquired properties analysis.
     if (newScript() && newScript()->initializedGroup())
         newScript()->initializedGroup()->setFlags(cx, flags);
+
+    // Propagate flag changes between unboxed and corresponding native groups.
+    if (maybeUnboxedLayout() && maybeUnboxedLayout()->nativeGroup())
+        maybeUnboxedLayout()->nativeGroup()->setFlags(cx, flags);
+    if (ObjectGroup* unboxedGroup = maybeOriginalUnboxedGroup())
+        unboxedGroup->setFlags(cx, flags);
 }
 
 void
@@ -2871,6 +2919,23 @@ ObjectGroup::markUnknown(ExclusiveContext* cx)
             prop->types.setNonDataProperty(cx);
         }
     }
+
+    if (ObjectGroup* unboxedGroup = maybeOriginalUnboxedGroup())
+        MarkObjectGroupUnknownProperties(cx, unboxedGroup);
+    if (maybeUnboxedLayout() && maybeUnboxedLayout()->nativeGroup())
+        MarkObjectGroupUnknownProperties(cx, maybeUnboxedLayout()->nativeGroup());
+    if (ObjectGroup* unboxedGroup = maybeOriginalUnboxedGroup())
+        MarkObjectGroupUnknownProperties(cx, unboxedGroup);
+}
+
+TypeNewScript*
+ObjectGroup::anyNewScript()
+{
+    if (newScript())
+        return newScript();
+    if (maybeUnboxedLayout())
+        return unboxedLayout().newScript();
+    return nullptr;
 }
 
 void
@@ -2880,7 +2945,7 @@ ObjectGroup::detachNewScript(bool writeBarrier, ObjectGroup* replacement)
     // analyzed, remove it from the newObjectGroups table so that it will not be
     // produced by calling 'new' on the associated function anymore.
     // The TypeNewScript is not actually destroyed.
-    TypeNewScript* newScript = this->newScript();
+    TypeNewScript* newScript = anyNewScript();
     MOZ_ASSERT(newScript);
 
     if (newScript->analyzed()) {
@@ -2899,7 +2964,10 @@ ObjectGroup::detachNewScript(bool writeBarrier, ObjectGroup* replacement)
         MOZ_ASSERT(!replacement);
     }
 
-    setAddendum(Addendum_None, nullptr, writeBarrier);
+    if (this->newScript())
+        setAddendum(Addendum_None, nullptr, writeBarrier);
+    else
+        unboxedLayout().setNewScript(nullptr, writeBarrier);
 }
 
 void
@@ -2910,7 +2978,7 @@ ObjectGroup::maybeClearNewScriptOnOOM()
     if (!isMarked())
         return;
 
-    TypeNewScript* newScript = this->newScript();
+    TypeNewScript* newScript = anyNewScript();
     if (!newScript)
         return;
 
@@ -2925,7 +2993,7 @@ ObjectGroup::maybeClearNewScriptOnOOM()
 void
 ObjectGroup::clearNewScript(ExclusiveContext* cx, ObjectGroup* replacement /* = nullptr*/)
 {
-    TypeNewScript* newScript = this->newScript();
+    TypeNewScript* newScript = anyNewScript();
     if (!newScript)
         return;
 
@@ -3390,6 +3458,22 @@ PreliminaryObjectArray::sweep()
     for (size_t i = 0; i < COUNT; i++) {
         JSObject** ptr = &objects[i];
         if (*ptr && IsAboutToBeFinalizedUnbarriered(ptr)) {
+            // Before we clear this reference, change the object's group to the
+            // Object.prototype group. This is done to ensure JSObject::finalize
+            // sees a NativeObject Class even if we change the current group's
+            // Class to one of the unboxed object classes in the meantime. If
+            // the compartment's global is dead, we don't do anything as the
+            // group's Class is not going to change in that case.
+            JSObject* obj = *ptr;
+            GlobalObject* global = obj->compartment()->unsafeUnbarrieredMaybeGlobal();
+            if (global && !obj->isSingleton()) {
+                JSObject* objectProto = GetBuiltinPrototypePure(global, JSProto_Object);
+                obj->setGroup(objectProto->groupRaw());
+                MOZ_ASSERT(obj->is<NativeObject>());
+                MOZ_ASSERT(obj->getClass() == objectProto->getClass());
+                MOZ_ASSERT(!obj->getClass()->hasFinalize());
+            }
+
             *ptr = nullptr;
         }
     }
@@ -3489,11 +3573,16 @@ PreliminaryObjectArrayWithTemplate::maybeAnalyze(ExclusiveContext* cx, ObjectGro
         }
     }
 
-    // Since the preliminary objects still reflect the template object's
-    // properties, and all objects in the future will be created with those
-    // properties, the properties can be marked as definitive for objects in
-    // the group.
-    group->addDefiniteProperties(cx, shape());
+    if (group->maybeUnboxedLayout())
+        return;
+
+    if (shape()) {
+        // We weren't able to use an unboxed layout, but since the preliminary
+        // objects still reflect the template object's properties, and all
+        // objects in the future will be created with those properties, the
+        // properties can be marked as definite for objects in the group.
+        group->addDefiniteProperties(cx, shape());
+    }
 }
 
 /////////////////////////////////////////////////////////////////////
@@ -3507,6 +3596,7 @@ TypeNewScript::make(JSContext* cx, ObjectGroup* group, JSFunction* fun)
 {
     MOZ_ASSERT(cx->zone()->types.activeAnalysis);
     MOZ_ASSERT(!group->newScript());
+    MOZ_ASSERT(!group->maybeUnboxedLayout());
 
     // rollbackPartiallyInitializedObjects expects function_ to be
     // canonicalized.
@@ -3814,6 +3904,27 @@ TypeNewScript::maybeAnalyze(JSContext* cx, ObjectGroup* group, bool* regenerate,
     js_delete(preliminaryObjects);
     preliminaryObjects = nullptr;
 
+    if (group->maybeUnboxedLayout()) {
+        // An unboxed layout was constructed for the group, and this has already
+        // been hooked into it.
+        MOZ_ASSERT(group->unboxedLayout().newScript() == this);
+        destroyNewScript.group = nullptr;
+
+        // Clear out the template object, which is not used for TypeNewScripts
+        // with an unboxed layout. Currently it is a mutant object with a
+        // non-native group and native shape, so make it safe for GC by changing
+        // its group to the default for its prototype.
+        AutoEnterOOMUnsafeRegion oomUnsafe;
+        ObjectGroup* plainGroup = ObjectGroup::defaultNewGroup(cx, &PlainObject::class_,
+                                                               group->proto());
+        if (!plainGroup)
+            oomUnsafe.crash("TypeNewScript::maybeAnalyze");
+        templateObject_->setGroup(plainGroup);
+        templateObject_ = nullptr;
+
+        return true;
+    }
+
     if (prefixShape->slotSpan() == templateObject()->slotSpan()) {
         // The definite properties analysis found exactly the properties that
         // are held in common by the preliminary objects. No further analysis
@@ -3927,6 +4038,12 @@ TypeNewScript::rollbackPartiallyInitializedObjects(JSContext* cx, ObjectGroup* g
             continue;
         }
 
+        if (thisv.toObject().is<UnboxedPlainObject>()) {
+            AutoEnterOOMUnsafeRegion oomUnsafe;
+            if (!UnboxedPlainObject::convertToNative(cx, &thisv.toObject()))
+                oomUnsafe.crash("rollbackPartiallyInitializedObjects");
+        }
+
         // Found a matching frame.
         RootedPlainObject obj(cx, &thisv.toObject().as<PlainObject>());
 
@@ -4120,6 +4237,12 @@ ConstraintTypeSet::sweep(Zone* zone, AutoClearTypeInferenceStateOnOOM& oom)
                 // Object sets containing objects with unknown properties might
                 // not be complete. Mark the type set as unknown, which it will
                 // be treated as during Ion compilation.
+                //
+                // Note that we don't have to do this when the type set might
+                // be missing the native group corresponding to an unboxed
+                // object group. In this case, the native group points to the
+                // unboxed object group via its addendum, so as long as objects
+                // with either group exist, neither group will be finalized.
                 flags |= TYPE_FLAG_ANYOBJECT;
                 clearObjects();
                 objectCount = 0;
@@ -4203,6 +4326,21 @@ ObjectGroup::sweep(AutoClearTypeInferenceStateOnOOM* oom)
     Maybe<AutoClearTypeInferenceStateOnOOM> fallbackOOM;
     EnsureHasAutoClearTypeInferenceStateOnOOM(oom, zone(), fallbackOOM);
 
+    if (maybeUnboxedLayout()) {
+        // Remove unboxed layouts that are about to be finalized from the
+        // compartment wide list while we are still on the main thread.
+        ObjectGroup* group = this;
+        if (IsAboutToBeFinalizedUnbarriered(&group))
+            unboxedLayout().detachFromCompartment();
+
+        if (unboxedLayout().newScript())
+            unboxedLayout().newScript()->sweep();
+
+        // Discard constructor code to avoid holding onto ExecutablePools.
+        if (zone()->isGCCompacting())
+            unboxedLayout().setConstructorCode(nullptr);
+    }
+
     if (maybePreliminaryObjects())
         maybePreliminaryObjects()->sweep();
 
diff --git a/js/src/vm/UnboxedObject-inl.h b/js/src/vm/UnboxedObject-inl.h
new file mode 100644
index 000000000..c1468a5b1
--- /dev/null
+++ b/js/src/vm/UnboxedObject-inl.h
@@ -0,0 +1,177 @@
+/* -*- 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/. */
+
+#ifndef vm_UnboxedObject_inl_h
+#define vm_UnboxedObject_inl_h
+
+#include "vm/UnboxedObject.h"
+
+#include "gc/StoreBuffer-inl.h"
+#include "vm/ArrayObject-inl.h"
+#include "vm/NativeObject-inl.h"
+
+namespace js {
+
+static inline Value
+GetUnboxedValue(uint8_t* p, JSValueType type, bool maybeUninitialized)
+{
+    switch (type) {
+      case JSVAL_TYPE_BOOLEAN:
+        return BooleanValue(*p != 0);
+
+      case JSVAL_TYPE_INT32:
+        return Int32Value(*reinterpret_cast<int32_t*>(p));
+
+      case JSVAL_TYPE_DOUBLE: {
+        // During unboxed plain object creation, non-GC thing properties are
+        // left uninitialized. This is normally fine, since the properties will
+        // be filled in shortly, but if they are read before that happens we
+        // need to make sure that doubles are canonical.
+        double d = *reinterpret_cast<double*>(p);
+        if (maybeUninitialized)
+            return DoubleValue(JS::CanonicalizeNaN(d));
+        return DoubleValue(d);
+      }
+
+      case JSVAL_TYPE_STRING:
+        return StringValue(*reinterpret_cast<JSString**>(p));
+
+      case JSVAL_TYPE_OBJECT:
+        return ObjectOrNullValue(*reinterpret_cast<JSObject**>(p));
+
+      default:
+        MOZ_CRASH("Invalid type for unboxed value");
+    }
+}
+
+static inline void
+SetUnboxedValueNoTypeChange(JSObject* unboxedObject,
+                            uint8_t* p, JSValueType type, const Value& v,
+                            bool preBarrier)
+{
+    switch (type) {
+      case JSVAL_TYPE_BOOLEAN:
+        *p = v.toBoolean();
+        return;
+
+      case JSVAL_TYPE_INT32:
+        *reinterpret_cast<int32_t*>(p) = v.toInt32();
+        return;
+
+      case JSVAL_TYPE_DOUBLE:
+        *reinterpret_cast<double*>(p) = v.toNumber();
+        return;
+
+      case JSVAL_TYPE_STRING: {
+        MOZ_ASSERT(!IsInsideNursery(v.toString()));
+        JSString** np = reinterpret_cast<JSString**>(p);
+        if (preBarrier)
+            JSString::writeBarrierPre(*np);
+        *np = v.toString();
+        return;
+      }
+
+      case JSVAL_TYPE_OBJECT: {
+        JSObject** np = reinterpret_cast<JSObject**>(p);
+
+        // Manually trigger post barriers on the whole object. If we treat
+        // the pointer as a HeapPtrObject we will get confused later if the
+        // object is converted to its native representation.
+        JSObject* obj = v.toObjectOrNull();
+        if (IsInsideNursery(obj) && !IsInsideNursery(unboxedObject)) {
+            JSRuntime* rt = unboxedObject->runtimeFromMainThread();
+            rt->gc.storeBuffer.putWholeCell(unboxedObject);
+        }
+
+        if (preBarrier)
+            JSObject::writeBarrierPre(*np);
+        *np = obj;
+        return;
+      }
+
+      default:
+        MOZ_CRASH("Invalid type for unboxed value");
+    }
+}
+
+static inline bool
+SetUnboxedValue(ExclusiveContext* cx, JSObject* unboxedObject, jsid id,
+                uint8_t* p, JSValueType type, const Value& v, bool preBarrier)
+{
+    switch (type) {
+      case JSVAL_TYPE_BOOLEAN:
+        if (v.isBoolean()) {
+            *p = v.toBoolean();
+            return true;
+        }
+        return false;
+
+      case JSVAL_TYPE_INT32:
+        if (v.isInt32()) {
+            *reinterpret_cast<int32_t*>(p) = v.toInt32();
+            return true;
+        }
+        return false;
+
+      case JSVAL_TYPE_DOUBLE:
+        if (v.isNumber()) {
+            *reinterpret_cast<double*>(p) = v.toNumber();
+            return true;
+        }
+        return false;
+
+      case JSVAL_TYPE_STRING:
+        if (v.isString()) {
+            MOZ_ASSERT(!IsInsideNursery(v.toString()));
+            JSString** np = reinterpret_cast<JSString**>(p);
+            if (preBarrier)
+                JSString::writeBarrierPre(*np);
+            *np = v.toString();
+            return true;
+        }
+        return false;
+
+      case JSVAL_TYPE_OBJECT:
+        if (v.isObjectOrNull()) {
+            JSObject** np = reinterpret_cast<JSObject**>(p);
+
+            // Update property types when writing object properties. Types for
+            // other properties were captured when the unboxed layout was
+            // created.
+            AddTypePropertyId(cx, unboxedObject, id, v);
+
+            // As above, trigger post barriers on the whole object.
+            JSObject* obj = v.toObjectOrNull();
+            if (IsInsideNursery(v.toObjectOrNull()) && !IsInsideNursery(unboxedObject)) {
+                JSRuntime* rt = unboxedObject->runtimeFromMainThread();
+                rt->gc.storeBuffer.putWholeCell(unboxedObject);
+            }
+
+            if (preBarrier)
+                JSObject::writeBarrierPre(*np);
+            *np = obj;
+            return true;
+        }
+        return false;
+
+      default:
+        MOZ_CRASH("Invalid type for unboxed value");
+    }
+}
+
+/////////////////////////////////////////////////////////////////////
+// UnboxedPlainObject
+/////////////////////////////////////////////////////////////////////
+
+inline const UnboxedLayout&
+UnboxedPlainObject::layout() const
+{
+    return group()->unboxedLayout();
+}
+
+} // namespace js
+
+#endif // vm_UnboxedObject_inl_h
diff --git a/js/src/vm/UnboxedObject.cpp b/js/src/vm/UnboxedObject.cpp
new file mode 100644
index 000000000..2e017ca3b
--- /dev/null
+++ b/js/src/vm/UnboxedObject.cpp
@@ -0,0 +1,946 @@
+/* -*- 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);
+
+    // 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()) {
+        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, &PlainObject::class_, 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,
+                                                                   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 = gc::GetGCKindSlots(layout.getAllocKind());
+
+    RootedShape shape(cx, EmptyShape::getInitialShape(cx, &PlainObject::class_, proto, nfixed, 0));
+    if (!shape)
+        return false;
+
+    // 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, &PlainObject::class_, 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.
+        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_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,
+    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
+};
+
+/////////////////////////////////////////////////////////////////////
+// API
+/////////////////////////////////////////////////////////////////////
+
+static inline Value
+NextValue(Handle<GCVector<Value>> values, size_t* valueCursor)
+{
+    return values[(*valueCursor)++];
+}
+
+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)));
+}
diff --git a/js/src/vm/UnboxedObject.h b/js/src/vm/UnboxedObject.h
new file mode 100644
index 000000000..ba66434bc
--- /dev/null
+++ b/js/src/vm/UnboxedObject.h
@@ -0,0 +1,319 @@
+/* -*- 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/. */
+
+#ifndef vm_UnboxedObject_h
+#define vm_UnboxedObject_h
+
+#include "jsgc.h"
+#include "jsobj.h"
+
+#include "vm/Runtime.h"
+#include "vm/TypeInference.h"
+
+namespace js {
+
+// Memory required for an unboxed value of a given type. Returns zero for types
+// which can't be used for unboxed objects.
+static inline size_t
+UnboxedTypeSize(JSValueType type)
+{
+    switch (type) {
+      case JSVAL_TYPE_BOOLEAN: return 1;
+      case JSVAL_TYPE_INT32:   return 4;
+      case JSVAL_TYPE_DOUBLE:  return 8;
+      case JSVAL_TYPE_STRING:  return sizeof(void*);
+      case JSVAL_TYPE_OBJECT:  return sizeof(void*);
+      default:                 return 0;
+    }
+}
+
+static inline bool
+UnboxedTypeNeedsPreBarrier(JSValueType type)
+{
+    return type == JSVAL_TYPE_STRING || type == JSVAL_TYPE_OBJECT;
+}
+
+static inline bool
+UnboxedTypeNeedsPostBarrier(JSValueType type)
+{
+    return type == JSVAL_TYPE_OBJECT;
+}
+
+// Class tracking information specific to unboxed objects.
+class UnboxedLayout : public mozilla::LinkedListElement<UnboxedLayout>
+{
+  public:
+    struct Property {
+        PropertyName* name;
+        uint32_t offset;
+        JSValueType type;
+
+        Property()
+          : name(nullptr), offset(UINT32_MAX), type(JSVAL_TYPE_MAGIC)
+        {}
+    };
+
+    typedef Vector<Property, 0, SystemAllocPolicy> PropertyVector;
+
+  private:
+    // If objects in this group have ever been converted to native objects,
+    // these store the corresponding native group and initial shape for such
+    // objects. Type information for this object is reflected in nativeGroup.
+    GCPtrObjectGroup nativeGroup_;
+    GCPtrShape nativeShape_;
+
+    // Any script/pc which the associated group is created for.
+    GCPtrScript allocationScript_;
+    jsbytecode* allocationPc_;
+
+    // If nativeGroup is set and this object originally had a TypeNewScript or
+    // was keyed to an allocation site, this points to the group which replaced
+    // this one. This link is only needed to keep the replacement group from
+    // being GC'ed. If it were GC'ed and a new one regenerated later, that new
+    // group might have a different allocation kind from this group.
+    GCPtrObjectGroup replacementGroup_;
+
+    // The following members are only used for unboxed plain objects.
+
+    // All properties on objects with this layout, in enumeration order.
+    PropertyVector properties_;
+
+    // Byte size of the data for objects with this layout.
+    size_t size_;
+
+    // Any 'new' script information associated with this layout.
+    TypeNewScript* newScript_;
+
+    // List for use in tracing objects with this layout. This has the same
+    // structure as the trace list on a TypeDescr.
+    int32_t* traceList_;
+
+    // If this layout has been used to construct script or JSON constant
+    // objects, this code might be filled in to more quickly fill in objects
+    // from an array of values.
+    GCPtrJitCode constructorCode_;
+
+  public:
+    UnboxedLayout()
+      : nativeGroup_(nullptr), nativeShape_(nullptr),
+        allocationScript_(nullptr), allocationPc_(nullptr), replacementGroup_(nullptr),
+        size_(0), newScript_(nullptr), traceList_(nullptr), constructorCode_(nullptr)
+    {}
+
+    bool initProperties(const PropertyVector& properties, size_t size) {
+        size_ = size;
+        return properties_.appendAll(properties);
+    }
+
+    ~UnboxedLayout() {
+        if (newScript_)
+            newScript_->clear();
+        js_delete(newScript_);
+        js_free(traceList_);
+
+        nativeGroup_.init(nullptr);
+        nativeShape_.init(nullptr);
+        replacementGroup_.init(nullptr);
+        constructorCode_.init(nullptr);
+    }
+
+    void detachFromCompartment();
+
+    const PropertyVector& properties() const {
+        return properties_;
+    }
+
+    TypeNewScript* newScript() const {
+        return newScript_;
+    }
+
+    void setNewScript(TypeNewScript* newScript, bool writeBarrier = true);
+
+    JSScript* allocationScript() const {
+        return allocationScript_;
+    }
+
+    jsbytecode* allocationPc() const {
+        return allocationPc_;
+    }
+
+    void setAllocationSite(JSScript* script, jsbytecode* pc) {
+        allocationScript_ = script;
+        allocationPc_ = pc;
+    }
+
+    const int32_t* traceList() const {
+        return traceList_;
+    }
+
+    void setTraceList(int32_t* traceList) {
+        traceList_ = traceList;
+    }
+
+    const Property* lookup(JSAtom* atom) const {
+        for (size_t i = 0; i < properties_.length(); i++) {
+            if (properties_[i].name == atom)
+                return &properties_[i];
+        }
+        return nullptr;
+    }
+
+    const Property* lookup(jsid id) const {
+        if (JSID_IS_STRING(id))
+            return lookup(JSID_TO_ATOM(id));
+        return nullptr;
+    }
+
+    size_t size() const {
+        return size_;
+    }
+
+    ObjectGroup* nativeGroup() const {
+        return nativeGroup_;
+    }
+
+    Shape* nativeShape() const {
+        return nativeShape_;
+    }
+
+    jit::JitCode* constructorCode() const {
+        return constructorCode_;
+    }
+
+    void setConstructorCode(jit::JitCode* code) {
+        constructorCode_ = code;
+    }
+
+    inline gc::AllocKind getAllocKind() const;
+
+    void trace(JSTracer* trc);
+
+    size_t sizeOfIncludingThis(mozilla::MallocSizeOf mallocSizeOf);
+
+    static bool makeNativeGroup(JSContext* cx, ObjectGroup* group);
+    static bool makeConstructorCode(JSContext* cx, HandleObjectGroup group);
+};
+
+// Class for expando objects holding extra properties given to an unboxed plain
+// object. These objects behave identically to normal native plain objects, and
+// have a separate Class to distinguish them for memory usage reporting.
+class UnboxedExpandoObject : public NativeObject
+{
+  public:
+    static const Class class_;
+};
+
+// Class for a plain object using an unboxed representation. The physical
+// layout of these objects is identical to that of an InlineTypedObject, though
+// these objects use an UnboxedLayout instead of a TypeDescr to keep track of
+// how their properties are stored.
+class UnboxedPlainObject : public JSObject
+{
+    // Optional object which stores extra properties on this object. This is
+    // not automatically barriered to avoid problems if the object is converted
+    // to a native. See ensureExpando().
+    UnboxedExpandoObject* expando_;
+
+    // Start of the inline data, which immediately follows the group and extra properties.
+    uint8_t data_[1];
+
+  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();
+    }
+
+    uint8_t* data() {
+        return &data_[0];
+    }
+
+    UnboxedExpandoObject* maybeExpando() const {
+        return expando_;
+    }
+
+    void initExpando() {
+        expando_ = nullptr;
+    }
+
+    // For use during GC.
+    JSObject** addressOfExpando() {
+        return reinterpret_cast<JSObject**>(&expando_);
+    }
+
+    bool containsUnboxedOrExpandoProperty(ExclusiveContext* cx, jsid id) const;
+
+    static UnboxedExpandoObject* ensureExpando(JSContext* cx, Handle<UnboxedPlainObject*> obj);
+
+    bool setValue(ExclusiveContext* cx, const UnboxedLayout::Property& property, const Value& v);
+    Value getValue(const UnboxedLayout::Property& property, bool maybeUninitialized = false);
+
+    static bool convertToNative(JSContext* cx, JSObject* obj);
+    static UnboxedPlainObject* create(ExclusiveContext* cx, HandleObjectGroup group,
+                                      NewObjectKind newKind);
+    static JSObject* createWithProperties(ExclusiveContext* cx, HandleObjectGroup group,
+                                          NewObjectKind newKind, IdValuePair* properties);
+
+    void fillAfterConvert(ExclusiveContext* cx,
+                          Handle<GCVector<Value>> values, size_t* valueCursor);
+
+    static void trace(JSTracer* trc, JSObject* object);
+
+    static size_t offsetOfExpando() {
+        return offsetof(UnboxedPlainObject, expando_);
+    }
+
+    static size_t offsetOfData() {
+        return offsetof(UnboxedPlainObject, data_[0]);
+    }
+};
+
+inline gc::AllocKind
+UnboxedLayout::getAllocKind() const
+{
+    MOZ_ASSERT(size());
+    return gc::GetGCObjectKindForBytes(UnboxedPlainObject::offsetOfData() + size());
+}
+
+} // namespace js
+
+namespace JS {
+
+template <>
+struct DeletePolicy<js::UnboxedLayout> : public js::GCManagedDeletePolicy<js::UnboxedLayout>
+{};
+
+} /* namespace JS */
+
+#endif /* vm_UnboxedObject_h */