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Implementation

Mercurial (d38398e5144e)

VCS Links

DefineAsIntrinsic

GenerateShape

IntegrityLevel

JSObject

JSObject_Slots0

JSObject_Slots12

JSObject_Slots16

JSObject_Slots2

JSObject_Slots4

JSObject_Slots8

JSValueArray

ValueArray

Macros

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/* -*- 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 jsobj_h
#define jsobj_h

/*
 * JS object definitions.
 *
 * A JS object consists of a possibly-shared object descriptor containing
 * ordered property names, called the map; and a dense vector of property
 * values, called slots.  The map/slot pointer pair is GC'ed, while the map
 * is reference counted and the slot vector is malloc'ed.
 */

#include "mozilla/MemoryReporting.h"

#include "gc/Barrier.h"
#include "gc/Marking.h"
#include "js/Conversions.h"
#include "js/GCAPI.h"
#include "js/GCVector.h"
#include "js/HeapAPI.h"
#include "vm/Shape.h"
#include "vm/String.h"
#include "vm/Xdr.h"

namespace JS {
struct ClassInfo;
} // namespace JS

namespace js {

using PropertyDescriptorVector = JS::GCVector<JS::PropertyDescriptor>;
class GCMarker;
class Nursery;

namespace gc {
class RelocationOverlay;
} // namespace gc

inline JSObject*
CastAsObject(GetterOp op)
{
    return JS_FUNC_TO_DATA_PTR(JSObject*, op);
}

inline JSObject*
CastAsObject(SetterOp op)
{
    return JS_FUNC_TO_DATA_PTR(JSObject*, op);
}

inline Value
CastAsObjectJsval(GetterOp op)
{
    return ObjectOrNullValue(CastAsObject(op));
}

inline Value
CastAsObjectJsval(SetterOp op)
{
    return ObjectOrNullValue(CastAsObject(op));
}

/******************************************************************************/

extern const Class IntlClass;
extern const Class JSONClass;
extern const Class MathClass;

class GlobalObject;
class NewObjectCache;

enum class IntegrityLevel {
    Sealed,
    Frozen
};

// Forward declarations, required for later friend declarations.
bool PreventExtensions(JSContext* cx, JS::HandleObject obj, JS::ObjectOpResult& result, IntegrityLevel level = IntegrityLevel::Sealed);
bool SetImmutablePrototype(JSContext* cx, JS::HandleObject obj, bool* succeeded);

}  /* namespace js */

/*
 * A JavaScript object. The members common to all objects are as follows:
 *
 * - The |group_| member stores the group of the object, which contains its
 *   prototype object, its class and the possible types of its properties.
 *
 * Subclasses of JSObject --- mainly NativeObject and JSFunction --- add more
 * members. Notable among these is the object's shape, which stores flags and
 * some other state, and, for native objects, the layout of all its properties.
 * The second word of a JSObject generally stores its shape; if the second word
 * stores anything else, the value stored cannot be a valid Shape* pointer, so
 * that shape guards can be performed on objects without regard to the specific
 * layout in use.
 */
class JSObject : public js::gc::Cell
{
  protected:
    js::GCPtrObjectGroup group_;

  private:
    friend class js::Shape;
    friend class js::GCMarker;
    friend class js::NewObjectCache;
    friend class js::Nursery;
    friend class js::gc::RelocationOverlay;
    friend bool js::PreventExtensions(JSContext* cx, JS::HandleObject obj, JS::ObjectOpResult& result, js::IntegrityLevel level);
    friend bool js::SetImmutablePrototype(JSContext* cx, JS::HandleObject obj,
                                          bool* succeeded);

    // Make a new group to use for a singleton object.
    static js::ObjectGroup* makeLazyGroup(JSContext* cx, js::HandleObject obj);

  public:
    bool isNative() const {
        return getClass()->isNative();
    }

    const js::Class* getClass() const {
        return group_->clasp();
    }
    const JSClass* getJSClass() const {
        return Jsvalify(getClass());
    }
    bool hasClass(const js::Class* c) const {
        return getClass() == c;
    }

    js::LookupPropertyOp getOpsLookupProperty() const { return getClass()->getOpsLookupProperty(); }
    js::DefinePropertyOp getOpsDefineProperty() const { return getClass()->getOpsDefineProperty(); }
    js::HasPropertyOp    getOpsHasProperty()    const { return getClass()->getOpsHasProperty(); }
    js::GetPropertyOp    getOpsGetProperty()    const { return getClass()->getOpsGetProperty(); }
    js::SetPropertyOp    getOpsSetProperty()    const { return getClass()->getOpsSetProperty(); }
    js::GetOwnPropertyOp getOpsGetOwnPropertyDescriptor()
                                                const { return getClass()->getOpsGetOwnPropertyDescriptor(); }
    js::DeletePropertyOp getOpsDeleteProperty() const { return getClass()->getOpsDeleteProperty(); }
    js::WatchOp          getOpsWatch()          const { return getClass()->getOpsWatch(); }
    js::UnwatchOp        getOpsUnwatch()        const { return getClass()->getOpsUnwatch(); }
    js::GetElementsOp    getOpsGetElements()    const { return getClass()->getOpsGetElements(); }
    JSNewEnumerateOp     getOpsEnumerate()      const { return getClass()->getOpsEnumerate(); }
    JSFunToStringOp      getOpsFunToString()    const { return getClass()->getOpsFunToString(); }

    js::ObjectGroup* group() const {
        MOZ_ASSERT(!hasLazyGroup());
        return groupRaw();
    }

    js::ObjectGroup* groupRaw() const {
        return group_;
    }

    /*
     * Whether this is the only object which has its specified group. This
     * object will have its group constructed lazily as needed by analysis.
     */
    bool isSingleton() const {
        return group_->singleton();
    }

    /*
     * Whether the object's group has not been constructed yet. If an object
     * might have a lazy group, use getGroup() below, otherwise group().
     */
    bool hasLazyGroup() const {
        return group_->lazy();
    }

    JSCompartment* compartment() const { return group_->compartment(); }
    JSCompartment* maybeCompartment() const { return compartment(); }

    inline js::Shape* maybeShape() const;
    inline js::Shape* ensureShape(JSContext* cx);

    // Set the initial slots and elements of an object. These pointers are only
    // valid for native objects, but during initialization are set for all
    // objects. For non-native objects, these must not be dynamically allocated
    // pointers which leak when the non-native object finishes initialization.
    inline void setInitialSlotsMaybeNonNative(js::HeapSlot* slots);
    inline void setInitialElementsMaybeNonNative(js::HeapSlot* elements);

    enum GenerateShape {
        GENERATE_NONE,
        GENERATE_SHAPE
    };

    static bool setFlags(JSContext* cx, JS::HandleObject obj, js::BaseShape::Flag flags,
                         GenerateShape generateShape = GENERATE_NONE);
    inline bool hasAllFlags(js::BaseShape::Flag flags) const;

    /*
     * An object is a delegate if it is on another object's prototype or scope
     * chain, and therefore the delegate might be asked implicitly to get or
     * set a property on behalf of another object. Delegates may be accessed
     * directly too, as may any object, but only those objects linked after the
     * head of any prototype or scope chain are flagged as delegates. This
     * definition helps to optimize shape-based property cache invalidation
     * (see Purge{Scope,Proto}Chain in jsobj.cpp).
     */
    inline bool isDelegate() const;
    static bool setDelegate(JSContext* cx, JS::HandleObject obj) {
        return setFlags(cx, obj, js::BaseShape::DELEGATE, GENERATE_SHAPE);
    }

    inline bool isBoundFunction() const;
    inline bool hasSpecialEquality() const;

    inline bool watched() const;
    static bool setWatched(JSContext* cx, JS::HandleObject obj) {
        return setFlags(cx, obj, js::BaseShape::WATCHED, GENERATE_SHAPE);
    }

    // A "qualified" varobj is the object on which "qualified" variable
    // declarations (i.e., those defined with "var") are kept.
    //
    // Conceptually, when a var binding is defined, it is defined on the
    // innermost qualified varobj on the scope chain.
    //
    // Function scopes (CallObjects) are qualified varobjs, and there can be
    // no other qualified varobj that is more inner for var bindings in that
    // function. As such, all references to local var bindings in a function
    // may be statically bound to the function scope. This is subject to
    // further optimization. Unaliased bindings inside functions reside
    // entirely on the frame, not in CallObjects.
    //
    // Global scopes are also qualified varobjs. It is possible to statically
    // know, for a given script, that are no more inner qualified varobjs, so
    // free variable references can be statically bound to the global.
    //
    // Finally, there are non-syntactic qualified varobjs used by embedders
    // (e.g., Gecko and XPConnect), as they often wish to run scripts under a
    // scope that captures var bindings.
    inline bool isQualifiedVarObj() const;
    static bool setQualifiedVarObj(JSContext* cx, JS::HandleObject obj) {
        return setFlags(cx, obj, js::BaseShape::QUALIFIED_VAROBJ);
    }

    // An "unqualified" varobj is the object on which "unqualified"
    // assignments (i.e., bareword assignments for which the LHS does not
    // exist on the scope chain) are kept.
    inline bool isUnqualifiedVarObj() const;

    // Objects with an uncacheable proto can have their prototype mutated
    // without inducing a shape change on the object. JIT inline caches should
    // do an explicit group guard to guard against this. Singletons always
    // generate a new shape when their prototype changes, regardless of this
    // hasUncacheableProto flag.
    inline bool hasUncacheableProto() const;
    static bool setUncacheableProto(JSContext* cx, JS::HandleObject obj) {
        MOZ_ASSERT(obj->hasStaticPrototype(),
                   "uncacheability as a concept is only applicable to static "
                   "(not dynamically-computed) prototypes");
        return setFlags(cx, obj, js::BaseShape::UNCACHEABLE_PROTO, GENERATE_SHAPE);
    }

    /*
     * Whether SETLELEM was used to access this object. See also the comment near
     * PropertyTree::MAX_HEIGHT.
     */
    inline bool hadElementsAccess() const;
    static bool setHadElementsAccess(JSContext* cx, JS::HandleObject obj) {
        return setFlags(cx, obj, js::BaseShape::HAD_ELEMENTS_ACCESS);
    }

    /*
     * Whether there may be indexed properties on this object, excluding any in
     * the object's elements.
     */
    inline bool isIndexed() const;

    /*
     * If this object was instantiated with `new Ctor`, return the constructor's
     * display atom. Otherwise, return nullptr.
     */
    static bool constructorDisplayAtom(JSContext* cx, js::HandleObject obj,
                                       js::MutableHandleAtom name);

    /*
     * The same as constructorDisplayAtom above, however if this object has a
     * lazy group, nullptr is returned. This allows for use in situations that
     * cannot GC and where having some information, even if it is inconsistently
     * available, is better than no information.
     */
    JSAtom* maybeConstructorDisplayAtom() const;

    /* GC support. */

    void traceChildren(JSTracer* trc);

    void fixupAfterMovingGC();

    static const JS::TraceKind TraceKind = JS::TraceKind::Object;
    static const size_t MaxTagBits = 3;

    MOZ_ALWAYS_INLINE JS::Zone* zone() const {
        return group_->zone();
    }
    MOZ_ALWAYS_INLINE JS::shadow::Zone* shadowZone() const {
        return JS::shadow::Zone::asShadowZone(zone());
    }
    MOZ_ALWAYS_INLINE JS::Zone* zoneFromAnyThread() const {
        return group_->zoneFromAnyThread();
    }
    MOZ_ALWAYS_INLINE JS::shadow::Zone* shadowZoneFromAnyThread() const {
        return JS::shadow::Zone::asShadowZone(zoneFromAnyThread());
    }
    static MOZ_ALWAYS_INLINE void readBarrier(JSObject* obj);
    static MOZ_ALWAYS_INLINE void writeBarrierPre(JSObject* obj);
    static MOZ_ALWAYS_INLINE void writeBarrierPost(void* cellp, JSObject* prev, JSObject* next);

    /* Return the allocKind we would use if we were to tenure this object. */
    js::gc::AllocKind allocKindForTenure(const js::Nursery& nursery) const;

    size_t tenuredSizeOfThis() const {
        MOZ_ASSERT(isTenured());
        return js::gc::Arena::thingSize(asTenured().getAllocKind());
    }

    void addSizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf, JS::ClassInfo* info);

    // We can only use addSizeOfExcludingThis on tenured objects: it assumes it
    // can apply mallocSizeOf to bits and pieces of the object, whereas objects
    // in the nursery may have those bits and pieces allocated in the nursery
    // along with them, and are not each their own malloc blocks.
    size_t sizeOfIncludingThisInNursery() const;

    // Marks this object as having a singleton group, and leave the group lazy.
    // Constructs a new, unique shape for the object. This should only be
    // called for an object that was just created.
    static inline bool setSingleton(JSContext* cx, js::HandleObject obj);

    // Change an existing object to have a singleton group.
    static bool changeToSingleton(JSContext* cx, js::HandleObject obj);

    static inline js::ObjectGroup* getGroup(JSContext* cx, js::HandleObject obj);

    const js::GCPtrObjectGroup& groupFromGC() const {
        /* Direct field access for use by GC. */
        return group_;
    }

#ifdef DEBUG
    static void debugCheckNewObject(js::ObjectGroup* group, js::Shape* shape,
                                    js::gc::AllocKind allocKind, js::gc::InitialHeap heap);
#else
    static void debugCheckNewObject(js::ObjectGroup* group, js::Shape* shape,
                                    js::gc::AllocKind allocKind, js::gc::InitialHeap heap)
    {}
#endif

    /*
     * We permit proxies to dynamically compute their prototype if desired.
     * (Not all proxies will so desire: in particular, most DOM proxies can
     * track their prototype with a single, nullable JSObject*.)  If a proxy
     * so desires, we store (JSObject*)0x1 in the proto field of the object's
     * group.
     *
     * We offer three ways to get an object's prototype:
     *
     * 1. obj->staticPrototype() returns the prototype, but it asserts if obj
     *    is a proxy, and the proxy has opted to dynamically compute its
     *    prototype using a getPrototype() handler.
     * 2. obj->taggedProto() returns a TaggedProto, which can be tested to
     *    check if the proto is an object, nullptr, or lazily computed.
     * 3. js::GetPrototype(cx, obj, &proto) computes the proto of an object.
     *    If obj is a proxy with dynamically-computed prototype, this code may
     *    perform arbitrary behavior (allocation, GC, run JS) while computing
     *    the proto.
     */

    js::TaggedProto taggedProto() const {
        return group_->proto();
    }

    bool hasTenuredProto() const;

    bool uninlinedIsProxy() const;

    JSObject* staticPrototype() const {
        MOZ_ASSERT(hasStaticPrototype());
        return taggedProto().toObjectOrNull();
    }

    // Normal objects and a subset of proxies have an uninteresting, static
    // (albeit perhaps mutable) [[Prototype]].  For such objects the
    // [[Prototype]] is just a value returned when needed for accesses, or
    // modified in response to requests.  These objects store the
    // [[Prototype]] directly within |obj->group_|.
    bool hasStaticPrototype() const {
        return !hasDynamicPrototype();
    }

    // The remaining proxies have a [[Prototype]] requiring dynamic computation
    // for every access, going through the proxy handler {get,set}Prototype and
    // setImmutablePrototype methods.  (Wrappers particularly use this to keep
    // the wrapper/wrappee [[Prototype]]s consistent.)
    bool hasDynamicPrototype() const {
        bool dynamic = taggedProto().isDynamic();
        MOZ_ASSERT_IF(dynamic, uninlinedIsProxy());
        MOZ_ASSERT_IF(dynamic, !isNative());
        return dynamic;
    }

    // True iff this object's [[Prototype]] is immutable.  Must be called only
    // on objects with a static [[Prototype]]!
    inline bool staticPrototypeIsImmutable() const;

    inline void setGroup(js::ObjectGroup* group);

    /*
     * Mark an object that has been iterated over and is a singleton. We need
     * to recover this information in the object's type information after it
     * is purged on GC.
     */
    inline bool isIteratedSingleton() const;
    static bool setIteratedSingleton(JSContext* cx, JS::HandleObject obj) {
        return setFlags(cx, obj, js::BaseShape::ITERATED_SINGLETON);
    }

    /*
     * Mark an object as requiring its default 'new' type to have unknown
     * properties.
     */
    inline bool isNewGroupUnknown() const;
    static bool setNewGroupUnknown(JSContext* cx, const js::Class* clasp, JS::HandleObject obj);

    // Mark an object as having its 'new' script information cleared.
    inline bool wasNewScriptCleared() const;
    static bool setNewScriptCleared(JSContext* cx, JS::HandleObject obj) {
        return setFlags(cx, obj, js::BaseShape::NEW_SCRIPT_CLEARED);
    }

    /* Set a new prototype for an object with a singleton type. */
    static bool splicePrototype(JSContext* cx, js::HandleObject obj, const js::Class* clasp,
                                js::Handle<js::TaggedProto> proto);

    /*
     * For bootstrapping, whether to splice a prototype for Function.prototype
     * or the global object.
     */
    bool shouldSplicePrototype();

    /*
     * Environment chains.
     *
     * The environment chain of an object is the link in the search path when
     * a script does a name lookup on an environment object. For JS internal
     * environment objects --- Call, LexicalEnvironment, and WithEnvironment
     * --- the chain is stored in the first fixed slot of the object.  For
     * other environment objects, the chain goes directly to the global.
     *
     * In code which is not marked hasNonSyntacticScope, environment chains
     * can contain only syntactic environment objects (see
     * IsSyntacticEnvironment) with a global object at the root as the
     * environment of the outermost non-function script. In
     * hasNonSyntacticScope code, the environment of the outermost
     * non-function script might not be a global object, and can have a mix of
     * other objects above it before the global object is reached.
     */

    /*
     * Get the enclosing environment of an object. When called on a
     * non-EnvironmentObject, this will just be the global (the name
     * "enclosing environment" still applies in this situation because
     * non-EnvironmentObjects can be on the environment chain).
     */
    inline JSObject* enclosingEnvironment() const;

    inline js::GlobalObject& global() const;

    // In some rare cases the global object's compartment's global may not be
    // the same global object. For this reason, we need to take extra care when
    // tracing.
    //
    // These cases are:
    //  1) The off-thread parsing task uses a dummy global since it cannot
    //     share with the actual global being used concurrently on the active
    //     thread.
    //  2) A GC may occur when creating the GlobalObject, in which case the
    //     compartment global pointer may not yet be set. In this case there is
    //     nothing interesting to trace in the compartment.
    inline bool isOwnGlobal(JSTracer*) const;
    inline js::GlobalObject* globalForTracing(JSTracer*) const;

    /*
     * ES5 meta-object properties and operations.
     */

  public:
    // Indicates whether a non-proxy is extensible.  Don't call on proxies!
    // This method really shouldn't exist -- but there are a few internal
    // places that want it (JITs and the like), and it'd be a pain to mark them
    // all as friends.
    inline bool nonProxyIsExtensible() const;

  public:
    /*
     * Iterator-specific getters and setters.
     */

    static const uint32_t ITER_CLASS_NFIXED_SLOTS = 1;

    /*
     * Back to generic stuff.
     */
    bool isCallable() const;
    bool isConstructor() const;
    JSNative callHook() const;
    JSNative constructHook() const;

    MOZ_ALWAYS_INLINE void finalize(js::FreeOp* fop);

  public:
    static bool reportReadOnly(JSContext* cx, jsid id, unsigned report = JSREPORT_ERROR);
    static bool reportNotConfigurable(JSContext* cx, jsid id, unsigned report = JSREPORT_ERROR);
    static bool reportNotExtensible(JSContext* cx, js::HandleObject obj,
                                    unsigned report = JSREPORT_ERROR);

    static bool nonNativeSetProperty(JSContext* cx, js::HandleObject obj, js::HandleId id,
                                     js::HandleValue v, js::HandleValue receiver,
                                     JS::ObjectOpResult& result);
    static bool nonNativeSetElement(JSContext* cx, js::HandleObject obj, uint32_t index,
                                    js::HandleValue v, js::HandleValue receiver,
                                    JS::ObjectOpResult& result);

    static bool swap(JSContext* cx, JS::HandleObject a, JS::HandleObject b);

  private:
    void fixDictionaryShapeAfterSwap();

  public:
    inline void initArrayClass();

    /*
     * In addition to the generic object interface provided by JSObject,
     * specific types of objects may provide additional operations. To access,
     * these addition operations, callers should use the pattern:
     *
     *   if (obj.is<XObject>()) {
     *     XObject& x = obj.as<XObject>();
     *     x.foo();
     *   }
     *
     * These XObject classes form a hierarchy. For example, for a cloned block
     * object, the following predicates are true: is<ClonedBlockObject>,
     * is<NestedScopeObject> and is<ScopeObject>. Each of these has a
     * respective class that derives and adds operations.
     *
     * A class XObject is defined in a vm/XObject{.h, .cpp, -inl.h} file
     * triplet (along with any class YObject that derives XObject).
     *
     * Note that X represents a low-level representation and does not query the
     * [[Class]] property of object defined by the spec (for this, see
     * js::GetBuiltinClass).
     */

    template <class T>
    inline bool is() const { return getClass() == &T::class_; }

    template <class T>
    T& as() {
        MOZ_ASSERT(this->is<T>());
        return *static_cast<T*>(this);
    }

    template <class T>
    const T& as() const {
        MOZ_ASSERT(this->is<T>());
        return *static_cast<const T*>(this);
    }

#ifdef DEBUG
    void dump(FILE* fp) const;
    void dump() const;
#endif

    /* JIT Accessors */

    static size_t offsetOfGroup() { return offsetof(JSObject, group_); }

    // Maximum size in bytes of a JSObject.
    static const size_t MAX_BYTE_SIZE = 4 * sizeof(void*) + 16 * sizeof(JS::Value);

  private:
    JSObject() = delete;
    JSObject(const JSObject& other) = delete;
    void operator=(const JSObject& other) = delete;
};

template <typename Wrapper>
template <typename U>
MOZ_ALWAYS_INLINE JS::Handle<U*>
js::RootedBase<JSObject*, Wrapper>::as() const
{
    const Wrapper& self = *static_cast<const Wrapper*>(this);
    MOZ_ASSERT(self->template is<U>());
    return Handle<U*>::fromMarkedLocation(reinterpret_cast<U* const*>(self.address()));
}

template <typename Wrapper>
template <class U>
MOZ_ALWAYS_INLINE JS::Handle<U*>
js::HandleBase<JSObject*, Wrapper>::as() const
{
    const JS::Handle<JSObject*>& self = *static_cast<const JS::Handle<JSObject*>*>(this);
    MOZ_ASSERT(self->template is<U>());
    return Handle<U*>::fromMarkedLocation(reinterpret_cast<U* const*>(self.address()));
}

/*
 * The only sensible way to compare JSObject with == is by identity. We use
 * const& instead of * as a syntactic way to assert non-null. This leads to an
 * abundance of address-of operators to identity. Hence this overload.
 */
static MOZ_ALWAYS_INLINE bool
operator==(const JSObject& lhs, const JSObject& rhs)
{
    return &lhs == &rhs;
}

static MOZ_ALWAYS_INLINE bool
operator!=(const JSObject& lhs, const JSObject& rhs)
{
    return &lhs != &rhs;
}

// Size of the various GC thing allocation sizes used for objects.
struct JSObject_Slots0 : JSObject { void* data[3]; };
struct JSObject_Slots2 : JSObject { void* data[3]; js::Value fslots[2]; };
struct JSObject_Slots4 : JSObject { void* data[3]; js::Value fslots[4]; };
struct JSObject_Slots8 : JSObject { void* data[3]; js::Value fslots[8]; };
struct JSObject_Slots12 : JSObject { void* data[3]; js::Value fslots[12]; };
struct JSObject_Slots16 : JSObject { void* data[3]; js::Value fslots[16]; };

/* static */ MOZ_ALWAYS_INLINE void
JSObject::readBarrier(JSObject* obj)
{
    if (obj && obj->isTenured())
        obj->asTenured().readBarrier(&obj->asTenured());
}

/* static */ MOZ_ALWAYS_INLINE void
JSObject::writeBarrierPre(JSObject* obj)
{
    if (obj && obj->isTenured())
        obj->asTenured().writeBarrierPre(&obj->asTenured());
}

/* static */ MOZ_ALWAYS_INLINE void
JSObject::writeBarrierPost(void* cellp, JSObject* prev, JSObject* next)
{
    MOZ_ASSERT(cellp);

    // If the target needs an entry, add it.
    js::gc::StoreBuffer* buffer;
    if (next && (buffer = next->storeBuffer())) {
        // If we know that the prev has already inserted an entry, we can skip
        // doing the lookup to add the new entry. Note that we cannot safely
        // assert the presence of the entry because it may have been added
        // via a different store buffer.
        if (prev && prev->storeBuffer())
            return;
        buffer->putCell(static_cast<js::gc::Cell**>(cellp));
        return;
    }

    // Remove the prev entry if the new value does not need it.
    if (prev && (buffer = prev->storeBuffer()))
        buffer->unputCell(static_cast<js::gc::Cell**>(cellp));
}

namespace js {

inline bool
IsCallable(const Value& v)
{
    return v.isObject() && v.toObject().isCallable();
}

// ES6 rev 24 (2014 April 27) 7.2.5 IsConstructor
inline bool
IsConstructor(const Value& v)
{
    return v.