The Tor Browser: js/src/gc/Barrier.h@6474c204b198 (annotated)

js/src/gc/Barrier.h@6474c204b198 (annotated)

js/src/gc/Barrier.h

Wed, 31 Dec 2014 06:09:35 +0100

author: Michael Schloh von Bennewitz <michael@schloh.com>
date: Wed, 31 Dec 2014 06:09:35 +0100
changeset 0: 6474c204b198
permissions: -rw-r--r--

Cloned upstream origin tor-browser at tor-browser-31.3.0esr-4.5-1-build1
revision ID fc1c9ff7c1b2defdbc039f12214767608f46423f for hacking purpose.

 /* -*- 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 gc_Barrier_h
 #define gc_Barrier_h
 #include "NamespaceImports.h"
 #include "gc/Heap.h"
 #ifdef JSGC_GENERATIONAL
 # include "gc/StoreBuffer.h"
 #endif
 #include "js/HashTable.h"
 #include "js/Id.h"
 #include "js/RootingAPI.h"
 /*
  * A write barrier is a mechanism used by incremental or generation GCs to
  * ensure that every value that needs to be marked is marked. In general, the
  * write barrier should be invoked whenever a write can cause the set of things
  * traced through by the GC to change. This includes:
  *   - writes to object properties
  *   - writes to array slots
  *   - writes to fields like JSObject::shape_ that we trace through
  *   - writes to fields in private data, like JSGenerator::obj
  *   - writes to non-markable fields like JSObject::private that point to
  *     markable data
  * The last category is the trickiest. Even though the private pointers does not
  * point to a GC thing, changing the private pointer may change the set of
  * objects that are traced by the GC. Therefore it needs a write barrier.
  *
  * Every barriered write should have the following form:
  *   <pre-barrier>
  *   obj->field = value; // do the actual write
  *   <post-barrier>
  * The pre-barrier is used for incremental GC and the post-barrier is for
  * generational GC.
  *
  *                               PRE-BARRIER
  *
  * To understand the pre-barrier, let's consider how incremental GC works. The
  * GC itself is divided into "slices". Between each slice, JS code is allowed to
  * run. Each slice should be short so that the user doesn't notice the
  * interruptions. In our GC, the structure of the slices is as follows:
  *
  * 1. ... JS work, which leads to a request to do GC ...
  * 2. [first GC slice, which performs all root marking and possibly more marking]
  * 3. ... more JS work is allowed to run ...
  * 4. [GC mark slice, which runs entirely in drainMarkStack]
  * 5. ... more JS work ...
  * 6. [GC mark slice, which runs entirely in drainMarkStack]
  * 7. ... more JS work ...
  * 8. [GC marking finishes; sweeping done non-incrementally; GC is done]
  * 9. ... JS continues uninterrupted now that GC is finishes ...
  *
  * Of course, there may be a different number of slices depending on how much
  * marking is to be done.
  *
  * The danger inherent in this scheme is that the JS code in steps 3, 5, and 7
  * might change the heap in a way that causes the GC to collect an object that
  * is actually reachable. The write barrier prevents this from happening. We use
  * a variant of incremental GC called "snapshot at the beginning." This approach
  * guarantees the invariant that if an object is reachable in step 2, then we
  * will mark it eventually. The name comes from the idea that we take a
  * theoretical "snapshot" of all reachable objects in step 2; all objects in
  * that snapshot should eventually be marked. (Note that the write barrier
  * verifier code takes an actual snapshot.)
  *
  * The basic correctness invariant of a snapshot-at-the-beginning collector is
  * that any object reachable at the end of the GC (step 9) must either:
  *   (1) have been reachable at the beginning (step 2) and thus in the snapshot
  *   (2) or must have been newly allocated, in steps 3, 5, or 7.
  * To deal with case (2), any objects allocated during an incremental GC are
  * automatically marked black.
  *
  * This strategy is actually somewhat conservative: if an object becomes
  * unreachable between steps 2 and 8, it would be safe to collect it. We won't,
  * mainly for simplicity. (Also, note that the snapshot is entirely
  * theoretical. We don't actually do anything special in step 2 that we wouldn't
  * do in a non-incremental GC.
  *
  * It's the pre-barrier's job to maintain the snapshot invariant. Consider the
  * write "obj->field = value". Let the prior value of obj->field be
  * value0. Since it's possible that value0 may have been what obj->field
  * contained in step 2, when the snapshot was taken, the barrier marks
  * value0. Note that it only does this if we're in the middle of an incremental
  * GC. Since this is rare, the cost of the write barrier is usually just an
  * extra branch.
  *
  * In practice, we implement the pre-barrier differently based on the type of
  * value0. E.g., see JSObject::writeBarrierPre, which is used if obj->field is
  * a JSObject*. It takes value0 as a parameter.
  *
  *                                POST-BARRIER
  *
  * For generational GC, we want to be able to quickly collect the nursery in a
  * minor collection.  Part of the way this is achieved is to only mark the
  * nursery itself; tenured things, which may form the majority of the heap, are
  * not traced through or marked.  This leads to the problem of what to do about
  * tenured objects that have pointers into the nursery: if such things are not
  * marked, they may be discarded while there are still live objects which
  * reference them. The solution is to maintain information about these pointers,
  * and mark their targets when we start a minor collection.
  *
  * The pointers can be thoughs of as edges in object graph, and the set of edges
  * from the tenured generation into the nursery is know as the remembered set.
  * Post barriers are used to track this remembered set.
  *
  * Whenever a slot which could contain such a pointer is written, we use a write
  * barrier to check if the edge created is in the remembered set, and if so we
  * insert it into the store buffer, which is the collector's representation of
  * the remembered set.  This means than when we come to do a minor collection we
  * can examine the contents of the store buffer and mark any edge targets that
  * are in the nursery.
  *
  *                            IMPLEMENTATION DETAILS
  *
  * Since it would be awkward to change every write to memory into a function
  * call, this file contains a bunch of C++ classes and templates that use
  * operator overloading to take care of barriers automatically. In many cases,
  * all that's necessary to make some field be barriered is to replace
  *     Type *field;
  * with
  *     HeapPtr<Type> field;
  * There are also special classes HeapValue and HeapId, which barrier js::Value
  * and jsid, respectively.
  *
  * One additional note: not all object writes need to be barriered. Writes to
  * newly allocated objects do not need a pre-barrier.  In these cases, we use
  * the "obj->field.init(value)" method instead of "obj->field = value". We use
  * the init naming idiom in many places to signify that a field is being
  * assigned for the first time.
  *
  * For each of pointers, Values and jsids this file implements four classes,
  * illustrated here for the pointer (Ptr) classes:
  *
  * BarrieredPtr           abstract base class which provides common operations
  *  |  |  |
  *  |  | EncapsulatedPtr  provides pre-barriers only
  *  |  |
  *  | HeapPtr             provides pre- and post-barriers
  *  |
  * RelocatablePtr         provides pre- and post-barriers and is relocatable
  *
  * These classes are designed to be used by the internals of the JS engine.
  * Barriers designed to be used externally are provided in
  * js/public/RootingAPI.h.
  */
 namespace js {
 class PropertyName;
 #ifdef DEBUG
 bool
 RuntimeFromMainThreadIsHeapMajorCollecting(JS::shadow::Zone *shadowZone);
 #endif
 namespace gc {
 template <typename T>
 void
 MarkUnbarriered(JSTracer *trc, T **thingp, const char *name);
 // Direct value access used by the write barriers and the jits.
 void
 MarkValueUnbarriered(JSTracer *trc, Value *v, const char *name);
 // These two declarations are also present in gc/Marking.h, via the DeclMarker
 // macro.  Not great, but hard to avoid.
 void
 MarkObjectUnbarriered(JSTracer *trc, JSObject **obj, const char *name);
 void
 MarkStringUnbarriered(JSTracer *trc, JSString **str, const char *name);
 // Note that some subclasses (e.g. ObjectImpl) specialize some of these
 // methods.
 template <typename T>
 class BarrieredCell : public gc::Cell
 {
   public:
     MOZ_ALWAYS_INLINE JS::Zone *zone() const { return tenuredZone(); }
     MOZ_ALWAYS_INLINE JS::shadow::Zone *shadowZone() const { return JS::shadow::Zone::asShadowZone(zone()); }
     MOZ_ALWAYS_INLINE JS::Zone *zoneFromAnyThread() const { return tenuredZoneFromAnyThread(); }
     MOZ_ALWAYS_INLINE JS::shadow::Zone *shadowZoneFromAnyThread() const {
         return JS::shadow::Zone::asShadowZone(zoneFromAnyThread());
     }
     static MOZ_ALWAYS_INLINE void readBarrier(T *thing) {
 #ifdef JSGC_INCREMENTAL
         JS::shadow::Zone *shadowZone = thing->shadowZoneFromAnyThread();
         if (shadowZone->needsBarrier()) {
             MOZ_ASSERT(!RuntimeFromMainThreadIsHeapMajorCollecting(shadowZone));
             T *tmp = thing;
             js::gc::MarkUnbarriered<T>(shadowZone->barrierTracer(), &tmp, "read barrier");
             JS_ASSERT(tmp == thing);
         }
 #endif
     }
     static MOZ_ALWAYS_INLINE bool needWriteBarrierPre(JS::Zone *zone) {
 #ifdef JSGC_INCREMENTAL
         return JS::shadow::Zone::asShadowZone(zone)->needsBarrier();
 #else
         return false;
 #endif
     }
     static MOZ_ALWAYS_INLINE bool isNullLike(T *thing) { return !thing; }
     static MOZ_ALWAYS_INLINE void writeBarrierPre(T *thing) {
 #ifdef JSGC_INCREMENTAL
         if (isNullLike(thing) || !thing->shadowRuntimeFromAnyThread()->needsBarrier())
             return;
         JS::shadow::Zone *shadowZone = thing->shadowZoneFromAnyThread();
         if (shadowZone->needsBarrier()) {
             MOZ_ASSERT(!RuntimeFromMainThreadIsHeapMajorCollecting(shadowZone));
             T *tmp = thing;
             js::gc::MarkUnbarriered<T>(shadowZone->barrierTracer(), &tmp, "write barrier");
             JS_ASSERT(tmp == thing);
         }
 #endif
     }
     static void writeBarrierPost(T *thing, void *addr) {}
     static void writeBarrierPostRelocate(T *thing, void *addr) {}
     static void writeBarrierPostRemove(T *thing, void *addr) {}
 };
 } // namespace gc
 // Note: the following Zone-getting functions must be equivalent to the zone()
 // and shadowZone() functions implemented by the subclasses of BarrieredCell.
 JS::Zone *
 ZoneOfObject(const JSObject &obj);
 static inline JS::shadow::Zone *
 ShadowZoneOfObject(JSObject *obj)
 {
     return JS::shadow::Zone::asShadowZone(ZoneOfObject(*obj));
 }
 static inline JS::shadow::Zone *
 ShadowZoneOfString(JSString *str)
 {
     return JS::shadow::Zone::asShadowZone(reinterpret_cast<const js::gc::Cell *>(str)->tenuredZone());
 }
 MOZ_ALWAYS_INLINE JS::Zone *
 ZoneOfValue(const JS::Value &value)
 {
     JS_ASSERT(value.isMarkable());
     if (value.isObject())
         return ZoneOfObject(value.toObject());
     return static_cast<js::gc::Cell *>(value.toGCThing())->tenuredZone();
 }
 JS::Zone *
 ZoneOfObjectFromAnyThread(const JSObject &obj);
 static inline JS::shadow::Zone *
 ShadowZoneOfObjectFromAnyThread(JSObject *obj)
 {
     return JS::shadow::Zone::asShadowZone(ZoneOfObjectFromAnyThread(*obj));
 }
 static inline JS::shadow::Zone *
 ShadowZoneOfStringFromAnyThread(JSString *str)
 {
     return JS::shadow::Zone::asShadowZone(
         reinterpret_cast<const js::gc::Cell *>(str)->tenuredZoneFromAnyThread());
 }
 MOZ_ALWAYS_INLINE JS::Zone *
 ZoneOfValueFromAnyThread(const JS::Value &value)
 {
     JS_ASSERT(value.isMarkable());
     if (value.isObject())
         return ZoneOfObjectFromAnyThread(value.toObject());
     return static_cast<js::gc::Cell *>(value.toGCThing())->tenuredZoneFromAnyThread();
 }
 /*
  * Base class for barriered pointer types.
  */
 template <class T, typename Unioned = uintptr_t>
 class BarrieredPtr
 {
   protected:
     union {
         T *value;
         Unioned other;
     };
     BarrieredPtr(T *v) : value(v) {}
     ~BarrieredPtr() { pre(); }
   public:
     void init(T *v) {
         JS_ASSERT(!IsPoisonedPtr<T>(v));
         this->value = v;
     }
     /* Use this if the automatic coercion to T* isn't working. */
     T *get() const { return value; }
     /*
      * Use these if you want to change the value without invoking the barrier.
      * Obviously this is dangerous unless you know the barrier is not needed.
      */
     T **unsafeGet() { return &value; }
     void unsafeSet(T *v) { value = v; }
     Unioned *unsafeGetUnioned() { return &other; }
     T &operator*() const { return *value; }
     T *operator->() const { return value; }
     operator T*() const { return value; }
   protected:
     void pre() { T::writeBarrierPre(value); }
 };
 /*
  * EncapsulatedPtr only automatically handles pre-barriers. Post-barriers must
  * be manually implemented when using this class. HeapPtr and RelocatablePtr
  * should be used in all cases that do not require explicit low-level control
  * of moving behavior, e.g. for HashMap keys.
  */
 template <class T, typename Unioned = uintptr_t>
 class EncapsulatedPtr : public BarrieredPtr<T, Unioned>
 {
   public:
     EncapsulatedPtr() : BarrieredPtr<T, Unioned>(nullptr) {}
     EncapsulatedPtr(T *v) : BarrieredPtr<T, Unioned>(v) {}
     explicit EncapsulatedPtr(const EncapsulatedPtr<T, Unioned> &v)
       : BarrieredPtr<T, Unioned>(v.value) {}
     /* Use to set the pointer to nullptr. */
     void clear() {
         this->pre();
         this->value = nullptr;
     }
     EncapsulatedPtr<T, Unioned> &operator=(T *v) {
         this->pre();
         JS_ASSERT(!IsPoisonedPtr<T>(v));
         this->value = v;
         return *this;
     }
     EncapsulatedPtr<T, Unioned> &operator=(const EncapsulatedPtr<T> &v) {
         this->pre();
         JS_ASSERT(!IsPoisonedPtr<T>(v.value));
         this->value = v.value;
         return *this;
     }
 };
 /*
  * A pre- and post-barriered heap pointer, for use inside the JS engine.
  *
  * Not to be confused with JS::Heap<T>. This is a different class from the
  * external interface and implements substantially different semantics.
  *
  * The post-barriers implemented by this class are faster than those
  * implemented by RelocatablePtr<T> or JS::Heap<T> at the cost of not
  * automatically handling deletion or movement. It should generally only be
  * stored in memory that has GC lifetime. HeapPtr must not be used in contexts
  * where it may be implicitly moved or deleted, e.g. most containers.
  */
 template <class T, class Unioned = uintptr_t>
 class HeapPtr : public BarrieredPtr<T, Unioned>
 {
   public:
     HeapPtr() : BarrieredPtr<T, Unioned>(nullptr) {}
     explicit HeapPtr(T *v) : BarrieredPtr<T, Unioned>(v) { post(); }
     explicit HeapPtr(const HeapPtr<T, Unioned> &v) : BarrieredPtr<T, Unioned>(v) { post(); }
     void init(T *v) {
         JS_ASSERT(!IsPoisonedPtr<T>(v));
         this->value = v;
         post();
     }
     HeapPtr<T, Unioned> &operator=(T *v) {
         this->pre();
         JS_ASSERT(!IsPoisonedPtr<T>(v));
         this->value = v;
         post();
         return *this;
     }
     HeapPtr<T, Unioned> &operator=(const HeapPtr<T, Unioned> &v) {
         this->pre();
         JS_ASSERT(!IsPoisonedPtr<T>(v.value));
         this->value = v.value;
         post();
         return *this;
     }
   protected:
     void post() { T::writeBarrierPost(this->value, (void *)&this->value); }
     /* Make this friend so it can access pre() and post(). */
     template <class T1, class T2>
     friend inline void
     BarrieredSetPair(Zone *zone,
                      HeapPtr<T1> &v1, T1 *val1,
                      HeapPtr<T2> &v2, T2 *val2);
   private:
     /*
      * Unlike RelocatablePtr<T>, HeapPtr<T> must be managed with GC lifetimes.
      * Specifically, the memory used by the pointer itself must be live until
      * at least the next minor GC. For that reason, move semantics are invalid
      * and are deleted here. Please note that not all containers support move
      * semantics, so this does not completely prevent invalid uses.
      */
     HeapPtr(HeapPtr<T> &&) MOZ_DELETE;
     HeapPtr<T, Unioned> &operator=(HeapPtr<T, Unioned> &&) MOZ_DELETE;
 };
 /*
  * FixedHeapPtr is designed for one very narrow case: replacing immutable raw
  * pointers to GC-managed things, implicitly converting to a handle type for
  * ease of use.  Pointers encapsulated by this type must:
  *
  *   be immutable (no incremental write barriers),
  *   never point into the nursery (no generational write barriers), and
  *   be traced via MarkRuntime (we use fromMarkedLocation).
  *
  * In short: you *really* need to know what you're doing before you use this
  * class!
  */
 template <class T>
 class FixedHeapPtr
 {
     T *value;
   public:
     operator T*() const { return value; }
     T * operator->() const { return value; }
     operator Handle<T*>() const {
         return Handle<T*>::fromMarkedLocation(&value);
     }
     void init(T *ptr) {
         value = ptr;
     }
 };
 /*
  * A pre- and post-barriered heap pointer, for use inside the JS engine.
  *
  * Unlike HeapPtr<T>, it can be used in memory that is not managed by the GC,
  * i.e. in C++ containers.  It is, however, somewhat slower, so should only be
  * used in contexts where this ability is necessary.
  */
 template <class T>
 class RelocatablePtr : public BarrieredPtr<T>
 {
   public:
     RelocatablePtr() : BarrieredPtr<T>(nullptr) {}
     explicit RelocatablePtr(T *v) : BarrieredPtr<T>(v) {
         if (v)
             post();
     }
     /*
      * For RelocatablePtr, move semantics are equivalent to copy semantics. In
      * C++, a copy constructor taking const-ref is the way to get a single
      * function that will be used for both lvalue and rvalue copies, so we can
      * simply omit the rvalue variant.
      */
     RelocatablePtr(const RelocatablePtr<T> &v) : BarrieredPtr<T>(v) {
         if (this->value)
             post();
     }
     ~RelocatablePtr() {
         if (this->value)
             relocate();
     }
     RelocatablePtr<T> &operator=(T *v) {
         this->pre();
         JS_ASSERT(!IsPoisonedPtr<T>(v));
         if (v) {
             this->value = v;
             post();
         } else if (this->value) {
             relocate();
             this->value = v;
         }
         return *this;
     }
     RelocatablePtr<T> &operator=(const RelocatablePtr<T> &v) {
         this->pre();
         JS_ASSERT(!IsPoisonedPtr<T>(v.value));
         if (v.value) {
             this->value = v.value;
             post();
         } else if (this->value) {
             relocate();
             this->value = v;
         }
         return *this;
     }
   protected:
     void post() {
 #ifdef JSGC_GENERATIONAL
         JS_ASSERT(this->value);
         T::writeBarrierPostRelocate(this->value, &this->value);
 #endif
     }
     void relocate() {
 #ifdef JSGC_GENERATIONAL
         JS_ASSERT(this->value);
         T::writeBarrierPostRemove(this->value, &this->value);
 #endif
     }
 };
 /*
  * This is a hack for RegExpStatics::updateFromMatch. It allows us to do two
  * barriers with only one branch to check if we're in an incremental GC.
  */
 template <class T1, class T2>
 static inline void
 BarrieredSetPair(Zone *zone,
                  HeapPtr<T1> &v1, T1 *val1,
                  HeapPtr<T2> &v2, T2 *val2)
 {
     if (T1::needWriteBarrierPre(zone)) {
         v1.pre();
         v2.pre();
     }
     v1.unsafeSet(val1);
     v2.unsafeSet(val2);
     v1.post();
     v2.post();
 }
 class Shape;
 class BaseShape;
 namespace types { struct TypeObject; }
 typedef BarrieredPtr<JSObject> BarrieredPtrObject;
 typedef BarrieredPtr<JSScript> BarrieredPtrScript;
 typedef EncapsulatedPtr<JSObject> EncapsulatedPtrObject;
 typedef EncapsulatedPtr<JSScript> EncapsulatedPtrScript;
 typedef RelocatablePtr<JSObject> RelocatablePtrObject;
 typedef RelocatablePtr<JSScript> RelocatablePtrScript;
 typedef HeapPtr<JSObject> HeapPtrObject;
 typedef HeapPtr<JSFunction> HeapPtrFunction;
 typedef HeapPtr<JSString> HeapPtrString;
 typedef HeapPtr<PropertyName> HeapPtrPropertyName;
 typedef HeapPtr<JSScript> HeapPtrScript;
 typedef HeapPtr<Shape> HeapPtrShape;
 typedef HeapPtr<BaseShape> HeapPtrBaseShape;
 typedef HeapPtr<types::TypeObject> HeapPtrTypeObject;
 /* Useful for hashtables with a HeapPtr as key. */
 template <class T>
 struct HeapPtrHasher
 {
     typedef HeapPtr<T> Key;
     typedef T *Lookup;
     static HashNumber hash(Lookup obj) { return DefaultHasher<T *>::hash(obj); }
     static bool match(const Key &k, Lookup l) { return k.get() == l; }
     static void rekey(Key &k, const Key& newKey) { k.unsafeSet(newKey); }
 };
 /* Specialized hashing policy for HeapPtrs. */
 template <class T>
 struct DefaultHasher< HeapPtr<T> > : HeapPtrHasher<T> { };
 template <class T>
 struct EncapsulatedPtrHasher
 {
     typedef EncapsulatedPtr<T> Key;
     typedef T *Lookup;
     static HashNumber hash(Lookup obj) { return DefaultHasher<T *>::hash(obj); }
     static bool match(const Key &k, Lookup l) { return k.get() == l; }
     static void rekey(Key &k, const Key& newKey) { k.unsafeSet(newKey); }
 };
 template <class T>
 struct DefaultHasher< EncapsulatedPtr<T> > : EncapsulatedPtrHasher<T> { };
 bool
 StringIsPermanentAtom(JSString *str);
 /*
  * Base class for barriered value types.
  */
 class BarrieredValue : public ValueOperations<BarrieredValue>
 {
   protected:
     Value value;
     /*
      * Ensure that EncapsulatedValue is not constructable, except by our
      * implementations.
      */
     BarrieredValue() MOZ_DELETE;
     BarrieredValue(const Value &v) : value(v) {
         JS_ASSERT(!IsPoisonedValue(v));
     }
     ~BarrieredValue() {
         pre();
     }
   public:
     void init(const Value &v) {
         JS_ASSERT(!IsPoisonedValue(v));
         value = v;
     }
     void init(JSRuntime *rt, const Value &v) {
         JS_ASSERT(!IsPoisonedValue(v));
         value = v;
     }
     bool operator==(const BarrieredValue &v) const { return value == v.value; }
     bool operator!=(const BarrieredValue &v) const { return value != v.value; }
     const Value &get() const { return value; }
     Value *unsafeGet() { return &value; }
     operator const Value &() const { return value; }
     JSGCTraceKind gcKind() const { return value.gcKind(); }
     uint64_t asRawBits() const { return value.asRawBits(); }
     static void writeBarrierPre(const Value &v) {
 #ifdef JSGC_INCREMENTAL
         if (v.isMarkable() && shadowRuntimeFromAnyThread(v)->needsBarrier())
             writeBarrierPre(ZoneOfValueFromAnyThread(v), v);
 #endif
     }
     static void writeBarrierPre(Zone *zone, const Value &v) {
 #ifdef JSGC_INCREMENTAL
         if (v.isString() && StringIsPermanentAtom(v.toString()))
             return;
         JS::shadow::Zone *shadowZone = JS::shadow::Zone::asShadowZone(zone);
         if (shadowZone->needsBarrier()) {
             JS_ASSERT_IF(v.isMarkable(), shadowRuntimeFromMainThread(v)->needsBarrier());
             Value tmp(v);
             js::gc::MarkValueUnbarriered(shadowZone->barrierTracer(), &tmp, "write barrier");
             JS_ASSERT(tmp == v);
         }
 #endif
     }
   protected:
     void pre() { writeBarrierPre(value); }
     void pre(Zone *zone) { writeBarrierPre(zone, value); }
     static JSRuntime *runtimeFromMainThread(const Value &v) {
         JS_ASSERT(v.isMarkable());
         return static_cast<js::gc::Cell *>(v.toGCThing())->runtimeFromMainThread();
     }
     static JSRuntime *runtimeFromAnyThread(const Value &v) {
         JS_ASSERT(v.isMarkable());
         return static_cast<js::gc::Cell *>(v.toGCThing())->runtimeFromAnyThread();
     }
     static JS::shadow::Runtime *shadowRuntimeFromMainThread(const Value &v) {
         return reinterpret_cast<JS::shadow::Runtime*>(runtimeFromMainThread(v));
     }
     static JS::shadow::Runtime *shadowRuntimeFromAnyThread(const Value &v) {
         return reinterpret_cast<JS::shadow::Runtime*>(runtimeFromAnyThread(v));
     }
   private:
     friend class ValueOperations<BarrieredValue>;
     const Value * extract() const { return &value; }
 };
 // Like EncapsulatedPtr, but specialized for Value.
 // See the comments on that class for details.
 class EncapsulatedValue : public BarrieredValue
 {
   public:
     EncapsulatedValue(const Value &v) : BarrieredValue(v) {}
     EncapsulatedValue(const EncapsulatedValue &v) : BarrieredValue(v) {}
     EncapsulatedValue &operator=(const Value &v) {
         pre();
         JS_ASSERT(!IsPoisonedValue(v));
         value = v;
         return *this;
     }
     EncapsulatedValue &operator=(const EncapsulatedValue &v) {
         pre();
         JS_ASSERT(!IsPoisonedValue(v));
         value = v.get();
         return *this;
     }
 };
 // Like HeapPtr, but specialized for Value.
 // See the comments on that class for details.
 class HeapValue : public BarrieredValue
 {
   public:
     explicit HeapValue()
       : BarrieredValue(UndefinedValue())
     {
         post();
     }
     explicit HeapValue(const Value &v)
       : BarrieredValue(v)
     {
         JS_ASSERT(!IsPoisonedValue(v));
         post();
     }
     explicit HeapValue(const HeapValue &v)
       : BarrieredValue(v.value)
     {
         JS_ASSERT(!IsPoisonedValue(v.value));
         post();
     }
     ~HeapValue() {
         pre();
     }
     void init(const Value &v) {
         JS_ASSERT(!IsPoisonedValue(v));
         value = v;
         post();
     }
     void init(JSRuntime *rt, const Value &v) {
         JS_ASSERT(!IsPoisonedValue(v));
         value = v;
         post(rt);
     }
     HeapValue &operator=(const Value &v) {
         pre();
         JS_ASSERT(!IsPoisonedValue(v));
         value = v;
         post();
         return *this;
     }
     HeapValue &operator=(const HeapValue &v) {
         pre();
         JS_ASSERT(!IsPoisonedValue(v.value));
         value = v.value;
         post();
         return *this;
     }
 #ifdef DEBUG
     bool preconditionForSet(Zone *zone);
 #endif
     /*
      * This is a faster version of operator=. Normally, operator= has to
      * determine the compartment of the value before it can decide whether to do
      * the barrier. If you already know the compartment, it's faster to pass it
      * in.
      */
     void set(Zone *zone, const Value &v) {
         JS::shadow::Zone *shadowZone = JS::shadow::Zone::asShadowZone(zone);
         JS_ASSERT(preconditionForSet(zone));
         pre(zone);
         JS_ASSERT(!IsPoisonedValue(v));
         value = v;
         post(shadowZone->runtimeFromAnyThread());
     }
     static void writeBarrierPost(const Value &value, Value *addr) {
 #ifdef JSGC_GENERATIONAL
         if (value.isMarkable())
             shadowRuntimeFromAnyThread(value)->gcStoreBufferPtr()->putValue(addr);
 #endif
     }
     static void writeBarrierPost(JSRuntime *rt, const Value &value, Value *addr) {
 #ifdef JSGC_GENERATIONAL
         if (value.isMarkable()) {
             JS::shadow::Runtime *shadowRuntime = JS::shadow::Runtime::asShadowRuntime(rt);
             shadowRuntime->gcStoreBufferPtr()->putValue(addr);
         }
 #endif
     }
   private:
     void post() {
         writeBarrierPost(value, &value);
     }
     void post(JSRuntime *rt) {
         writeBarrierPost(rt, value, &value);
     }
     HeapValue(HeapValue &&) MOZ_DELETE;
     HeapValue &operator=(HeapValue &&) MOZ_DELETE;
 };
 // Like RelocatablePtr, but specialized for Value.
 // See the comments on that class for details.
 class RelocatableValue : public BarrieredValue
 {
   public:
     explicit RelocatableValue() : BarrieredValue(UndefinedValue()) {}
     explicit RelocatableValue(const Value &v)
       : BarrieredValue(v)
     {
         if (v.isMarkable())
             post();
     }
     RelocatableValue(const RelocatableValue &v)
       : BarrieredValue(v.value)
     {
         JS_ASSERT(!IsPoisonedValue(v.value));
         if (v.value.isMarkable())
             post();
     }
     ~RelocatableValue()
     {
         if (value.isMarkable())
             relocate(runtimeFromAnyThread(value));
     }
     RelocatableValue &operator=(const Value &v) {
         pre();
         JS_ASSERT(!IsPoisonedValue(v));
         if (v.isMarkable()) {
             value = v;
             post();
         } else if (value.isMarkable()) {
             JSRuntime *rt = runtimeFromAnyThread(value);
             relocate(rt);
             value = v;
         } else {
             value = v;
         }
         return *this;
     }
     RelocatableValue &operator=(const RelocatableValue &v) {
         pre();
         JS_ASSERT(!IsPoisonedValue(v.value));
         if (v.value.isMarkable()) {
             value = v.value;
             post();
         } else if (value.isMarkable()) {
             JSRuntime *rt = runtimeFromAnyThread(value);
             relocate(rt);
             value = v.value;
         } else {
             value = v.value;
         }
         return *this;
     }
   private:
     void post() {
 #ifdef JSGC_GENERATIONAL
         JS_ASSERT(value.isMarkable());
         shadowRuntimeFromAnyThread(value)->gcStoreBufferPtr()->putRelocatableValue(&value);
 #endif
     }
     void relocate(JSRuntime *rt) {
 #ifdef JSGC_GENERATIONAL
         JS::shadow::Runtime *shadowRuntime = JS::shadow::Runtime::asShadowRuntime(rt);
         shadowRuntime->gcStoreBufferPtr()->removeRelocatableValue(&value);
 #endif
     }
 };
 // A pre- and post-barriered Value that is specialized to be aware that it
 // resides in a slots or elements vector. This allows it to be relocated in
 // memory, but with substantially less overhead than a RelocatablePtr.
 class HeapSlot : public BarrieredValue
 {
   public:
     enum Kind {
         Slot = 0,
         Element = 1
     };
     explicit HeapSlot() MOZ_DELETE;
     explicit HeapSlot(JSObject *obj, Kind kind, uint32_t slot, const Value &v)
       : BarrieredValue(v)
     {
         JS_ASSERT(!IsPoisonedValue(v));
         post(obj, kind, slot, v);
     }
     explicit HeapSlot(JSObject *obj, Kind kind, uint32_t slot, const HeapSlot &s)
       : BarrieredValue(s.value)
     {
         JS_ASSERT(!IsPoisonedValue(s.value));
         post(obj, kind, slot, s);
     }
     ~HeapSlot() {
         pre();
     }
     void init(JSObject *owner, Kind kind, uint32_t slot, const Value &v) {
         value = v;
         post(owner, kind, slot, v);
     }
     void init(JSRuntime *rt, JSObject *owner, Kind kind, uint32_t slot, const Value &v) {
         value = v;
         post(rt, owner, kind, slot, v);
     }
 #ifdef DEBUG
     bool preconditionForSet(JSObject *owner, Kind kind, uint32_t slot);
     bool preconditionForSet(Zone *zone, JSObject *owner, Kind kind, uint32_t slot);
     static void preconditionForWriteBarrierPost(JSObject *obj, Kind kind, uint32_t slot,
                                                 Value target);
 #endif
     void set(JSObject *owner, Kind kind, uint32_t slot, const Value &v) {
         JS_ASSERT(preconditionForSet(owner, kind, slot));
         pre();
         JS_ASSERT(!IsPoisonedValue(v));
         value = v;
         post(owner, kind, slot, v);
     }
     void set(Zone *zone, JSObject *owner, Kind kind, uint32_t slot, const Value &v) {
         JS_ASSERT(preconditionForSet(zone, owner, kind, slot));
         JS::shadow::Zone *shadowZone = JS::shadow::Zone::asShadowZone(zone);
         pre(zone);
         JS_ASSERT(!IsPoisonedValue(v));
         value = v;
         post(shadowZone->runtimeFromAnyThread(), owner, kind, slot, v);
     }
     static void writeBarrierPost(JSObject *obj, Kind kind, uint32_t slot, Value target)
     {
 #ifdef JSGC_GENERATIONAL
         js::gc::Cell *cell = reinterpret_cast<js::gc::Cell*>(obj);
         writeBarrierPost(cell->runtimeFromAnyThread(), obj, kind, slot, target);
 #endif
     }
     static void writeBarrierPost(JSRuntime *rt, JSObject *obj, Kind kind, uint32_t slot,
                                  Value target)
     {
 #ifdef DEBUG
         preconditionForWriteBarrierPost(obj, kind, slot, target);
 #endif
 #ifdef JSGC_GENERATIONAL
         if (target.isObject()) {
             JS::shadow::Runtime *shadowRuntime = JS::shadow::Runtime::asShadowRuntime(rt);
             shadowRuntime->gcStoreBufferPtr()->putSlot(obj, kind, slot, 1);
         }
 #endif
     }
   private:
     void post(JSObject *owner, Kind kind, uint32_t slot, Value target) {
         HeapSlot::writeBarrierPost(owner, kind, slot, target);
     }
     void post(JSRuntime *rt, JSObject *owner, Kind kind, uint32_t slot, Value target) {
         HeapSlot::writeBarrierPost(rt, owner, kind, slot, target);
     }
 };
 static inline const Value *
 Valueify(const BarrieredValue *array)
 {
     JS_STATIC_ASSERT(sizeof(HeapValue) == sizeof(Value));
     JS_STATIC_ASSERT(sizeof(HeapSlot) == sizeof(Value));
     return (const Value *)array;
 }
 static inline HeapValue *
 HeapValueify(Value *v)
 {
     JS_STATIC_ASSERT(sizeof(HeapValue) == sizeof(Value));
     JS_STATIC_ASSERT(sizeof(HeapSlot) == sizeof(Value));
     return (HeapValue *)v;
 }
 class HeapSlotArray
 {
     HeapSlot *array;
   public:
     HeapSlotArray(HeapSlot *array) : array(array) {}
     operator const Value *() const { return Valueify(array); }
     operator HeapSlot *() const { return array; }
     HeapSlotArray operator +(int offset) const { return HeapSlotArray(array + offset); }
     HeapSlotArray operator +(uint32_t offset) const { return HeapSlotArray(array + offset); }
 };
 /*
  * Base class for barriered jsid types.
  */
 class BarrieredId
 {
   protected:
     jsid value;
   private:
     BarrieredId(const BarrieredId &v) MOZ_DELETE;
   protected:
     explicit BarrieredId(jsid id) : value(id) {}
     ~BarrieredId() { pre(); }
   public:
     bool operator==(jsid id) const { return value == id; }
     bool operator!=(jsid id) const { return value != id; }
     jsid get() const { return value; }
     jsid *unsafeGet() { return &value; }
     void unsafeSet(jsid newId) { value = newId; }
     operator jsid() const { return value; }
   protected:
     void pre() {
 #ifdef JSGC_INCREMENTAL
         if (JSID_IS_OBJECT(value)) {
             JSObject *obj = JSID_TO_OBJECT(value);
             JS::shadow::Zone *shadowZone = ShadowZoneOfObjectFromAnyThread(obj);
             if (shadowZone->needsBarrier()) {
                 js::gc::MarkObjectUnbarriered(shadowZone->barrierTracer(), &obj, "write barrier");
                 JS_ASSERT(obj == JSID_TO_OBJECT(value));
             }
         } else if (JSID_IS_STRING(value)) {
             JSString *str = JSID_TO_STRING(value);
             JS::shadow::Zone *shadowZone = ShadowZoneOfStringFromAnyThread(str);
             if (shadowZone->needsBarrier()) {
                 js::gc::MarkStringUnbarriered(shadowZone->barrierTracer(), &str, "write barrier");
                 JS_ASSERT(str == JSID_TO_STRING(value));
             }
         }
 #endif
     }
 };
 // Like EncapsulatedPtr, but specialized for jsid.
 // See the comments on that class for details.
 class EncapsulatedId : public BarrieredId
 {
   public:
     explicit EncapsulatedId(jsid id) : BarrieredId(id) {}
     explicit EncapsulatedId() : BarrieredId(JSID_VOID) {}
     EncapsulatedId &operator=(const EncapsulatedId &v) {
         if (v.value != value)
             pre();
         JS_ASSERT(!IsPoisonedId(v.value));
         value = v.value;
         return *this;
     }
 };
 // Like RelocatablePtr, but specialized for jsid.
 // See the comments on that class for details.
 class RelocatableId : public BarrieredId
 {
   public:
     explicit RelocatableId() : BarrieredId(JSID_VOID) {}
     explicit inline RelocatableId(jsid id) : BarrieredId(id) {}
     ~RelocatableId() { pre(); }
     bool operator==(jsid id) const { return value == id; }
     bool operator!=(jsid id) const { return value != id; }
     jsid get() const { return value; }
     operator jsid() const { return value; }
     jsid *unsafeGet() { return &value; }
     RelocatableId &operator=(jsid id) {
         if (id != value)
             pre();
         JS_ASSERT(!IsPoisonedId(id));
         value = id;
         return *this;
     }
     RelocatableId &operator=(const RelocatableId &v) {
         if (v.value != value)
             pre();
         JS_ASSERT(!IsPoisonedId(v.value));
         value = v.value;
         return *this;
     }
 };
 // Like HeapPtr, but specialized for jsid.
 // See the comments on that class for details.
 class HeapId : public BarrieredId
 {
   public:
     explicit HeapId() : BarrieredId(JSID_VOID) {}
     explicit HeapId(jsid id)
       : BarrieredId(id)
     {
         JS_ASSERT(!IsPoisonedId(id));
         post();
     }
     ~HeapId() { pre(); }
     void init(jsid id) {
         JS_ASSERT(!IsPoisonedId(id));
         value = id;
         post();
     }
     HeapId &operator=(jsid id) {
         if (id != value)
             pre();
         JS_ASSERT(!IsPoisonedId(id));
         value = id;
         post();
         return *this;
     }
     HeapId &operator=(const HeapId &v) {
         if (v.value != value)
             pre();
         JS_ASSERT(!IsPoisonedId(v.value));
         value = v.value;
         post();
         return *this;
     }
   private:
     void post() {};
     HeapId(const HeapId &v) MOZ_DELETE;
     HeapId(HeapId &&) MOZ_DELETE;
     HeapId &operator=(HeapId &&) MOZ_DELETE;
 };
 /*
  * Incremental GC requires that weak pointers have read barriers. This is mostly
  * an issue for empty shapes stored in JSCompartment. The problem happens when,
  * during an incremental GC, some JS code stores one of the compartment's empty
  * shapes into an object already marked black. Normally, this would not be a
  * problem, because the empty shape would have been part of the initial snapshot
  * when the GC started. However, since this is a weak pointer, it isn't. So we
  * may collect the empty shape even though a live object points to it. To fix
  * this, we mark these empty shapes black whenever they get read out.
  */
 template <class T>
 class ReadBarriered
 {
     T *value;
   public:
     ReadBarriered() : value(nullptr) {}
     ReadBarriered(T *value) : value(value) {}
     ReadBarriered(const Rooted<T*> &rooted) : value(rooted) {}
     T *get() const {
         if (!value)
             return nullptr;
         T::readBarrier(value);
         return value;
     }
     operator T*() const { return get(); }
     T &operator*() const { return *get(); }
     T *operator->() const { return get(); }
     T **unsafeGet() { return &value; }
     T * const * unsafeGet() const { return &value; }
     void set(T *v) { value = v; }
     operator bool() { return !!value; }
 };
 class ReadBarrieredValue
 {
     Value value;
   public:
     ReadBarrieredValue() : value(UndefinedValue()) {}
     ReadBarrieredValue(const Value &value) : value(value) {}
     inline const Value &get() const;
     Value *unsafeGet() { return &value; }
     inline operator const Value &() const;
     inline JSObject &toObject() const;
 };
 /*
  * Operations on a Heap thing inside the GC need to strip the barriers from
  * pointer operations. This template helps do that in contexts where the type
  * is templatized.
  */
 template <typename T> struct Unbarriered {};
 template <typename S> struct Unbarriered< EncapsulatedPtr<S> > { typedef S *type; };
 template <typename S> struct Unbarriered< RelocatablePtr<S> > { typedef S *type; };
 template <> struct Unbarriered<EncapsulatedValue> { typedef Value type; };
 template <> struct Unbarriered<RelocatableValue> { typedef Value type; };
 template <typename S> struct Unbarriered< DefaultHasher< EncapsulatedPtr<S> > > {
     typedef DefaultHasher<S *> type;
 };
 } /* namespace js */
 #endif /* gc_Barrier_h */

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js/src/gc/Barrier.h