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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
* vim: set ts=8 sts=2 et sw=2 tw=80:
* 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 */
#ifndef gc_WeakMap_h
#define gc_WeakMap_h
#include "mozilla/Atomics.h"
#include "mozilla/LinkedList.h"
#include "gc/Barrier.h"
#include "gc/Marking.h"
#include "gc/Tracer.h"
#include "gc/Zone.h"
#include "gc/ZoneAllocator.h"
#include "js/HashTable.h"
#include "js/HeapAPI.h"
namespace JS {
class Zone;
namespace js {
class GCMarker;
class WeakMapBase;
struct WeakMapTracer;
extern void DumpWeakMapLog(JSRuntime* rt);
namespace gc {
struct WeakMarkable;
#if defined(JS_GC_ZEAL) || defined(DEBUG)
// Check whether a weak map entry is marked correctly.
bool CheckWeakMapEntryMarking(const WeakMapBase* map, Cell* key, Cell* value);
} // namespace gc
// A subclass template of js::HashMap whose keys and values may be
// garbage-collected. When a key is collected, the table entry disappears,
// dropping its reference to the value.
// More precisely:
// A WeakMap entry is live if and only if both the WeakMap and the entry's
// key are live. An entry holds a strong reference to its value.
// You must call this table's 'trace' method when its owning object is reached
// by the garbage collection tracer. Once a table is known to be live, the
// implementation takes care of the special weak marking (ie, marking through
// the implicit edges stored in the map) and of removing (sweeping) table
// entries when collection is complete.
// WeakMaps are marked with an incremental linear-time algorithm that handles
// all orderings of map and key marking. The basic algorithm is:
// At first while marking, do nothing special when marking WeakMap keys (there
// is no straightforward way to know whether a particular object is being used
// as a key in some weakmap.) When a WeakMap is marked, scan through it to mark
// all entries with live keys, and collect all unmarked keys into a "weak keys"
// table.
// At some point, everything reachable has been marked. At this point, enter
// "weak marking mode". In this mode, whenever any object is marked, look it up
// in the weak keys table to see if it is the key for any WeakMap entry and if
// so, mark the value. When entering weak marking mode, scan the weak key table
// to find all keys that have been marked since we added them to the table, and
// mark those entries.
// In addition, we want weakmap marking to work incrementally. So WeakMap
// mutations are barriered to keep the weak keys table up to date: entries are
// removed if their key is removed from the table, etc.
// You can break down various ways that WeakMap values get marked based on the
// order that the map and key are marked. All of these assume the map and key
// get marked at some point:
// key marked, then map marked:
// - value was marked with map in `markEntries()`
// map marked, key already in map, key marked before weak marking mode:
// - key added to gcEphemeronEdges when map marked in `markEntries()`
// - value marked during `enterWeakMarkingMode`
// map marked, key already in map, key marked after weak marking mode:
// - when key is marked, gcEphemeronEdges[key] triggers marking of value in
// `markImplicitEdges()`
// map marked, key inserted into map, key marked:
// - value was live when inserted and must get marked at some point
using WeakMapColors = HashMap<WeakMapBase*, js::gc::CellColor,
DefaultHasher<WeakMapBase*>, SystemAllocPolicy>;
// Common base class for all WeakMap specializations, used for calling
// subclasses' GC-related methods.
class WeakMapBase : public mozilla::LinkedListElement<WeakMapBase> {
friend class js::GCMarker;
using CellColor = js::gc::CellColor;
WeakMapBase(JSObject* memOf, JS::Zone* zone);
virtual ~WeakMapBase();
JS::Zone* zone() const { return zone_; }
// Garbage collector entry points.
// Unmark all weak maps in a zone.
static void unmarkZone(JS::Zone* zone);
// Check all weak maps in a zone that have been marked as live in this garbage
// collection, and mark the values of all entries that have become strong
// references to them. Return true if we marked any new values, indicating
// that we need to make another pass. In other words, mark my marked maps'
// marked members' mid-collection.
static bool markZoneIteratively(JS::Zone* zone, GCMarker* marker);
// Add zone edges for weakmaps with key delegates in a different zone.
[[nodiscard]] static bool findSweepGroupEdgesForZone(JS::Zone* zone);
// Sweep the marked weak maps in a zone, updating moved keys.
static void sweepZoneAfterMinorGC(JS::Zone* zone);
// Trace all weak map bindings. Used by the cycle collector.
static void traceAllMappings(WeakMapTracer* tracer);
// Save information about which weak maps are marked for a zone.
static bool saveZoneMarkedWeakMaps(JS::Zone* zone,
WeakMapColors& markedWeakMaps);
// Restore information about which weak maps are marked for many zones.
static void restoreMarkedWeakMaps(WeakMapColors& markedWeakMaps);
#if defined(JS_GC_ZEAL) || defined(DEBUG)
static bool checkMarkingForZone(JS::Zone* zone);
static void checkWeakMapsAfterMovingGC(JS::Zone* zone);
// Instance member functions called by the above. Instantiations of WeakMap
// override these with definitions appropriate for their Key and Value types.
virtual void trace(JSTracer* tracer) = 0;
virtual bool findSweepGroupEdges() = 0;
virtual void traceWeakEdges(JSTracer* trc) = 0;
virtual void traceMappings(WeakMapTracer* tracer) = 0;
virtual void clearAndCompact() = 0;
// We have a key that, if it or its delegate is marked, may lead to a WeakMap
// value getting marked. Insert the necessary edges into the appropriate
// zone's gcEphemeronEdges or gcNurseryEphemeronEdges tables.
[[nodiscard]] bool addEphemeronEdgesForEntry(gc::MarkColor mapColor,
gc::Cell* key,
gc::Cell* delegate,
gc::TenuredCell* value);
[[nodiscard]] bool addEphemeronEdge(gc::MarkColor color, gc::Cell* src,
gc::Cell* dst);
virtual bool markEntries(GCMarker* marker) = 0;
gc::CellColor mapColor() const { return gc::CellColor(uint32_t(mapColor_)); }
void setMapColor(gc::CellColor newColor) { mapColor_ = uint32_t(newColor); }
bool markMap(gc::MarkColor markColor);
#ifdef JS_GC_ZEAL
virtual bool checkMarking() const = 0;
virtual bool allowKeysInOtherZones() const { return false; }
friend bool gc::CheckWeakMapEntryMarking(const WeakMapBase*, gc::Cell*,
virtual void checkAfterMovingGC() const = 0;
// Object that this weak map is part of, if any.
HeapPtr<JSObject*> memberOf;
// Zone containing this weak map.
JS::Zone* zone_;
// Whether this object has been marked during garbage collection and which
// color it was marked.
mozilla::Atomic<uint32_t, mozilla::Relaxed> mapColor_;
friend class JS::Zone;
template <class Key, class Value>
class WeakMap
: private HashMap<Key, Value, StableCellHasher<Key>, ZoneAllocPolicy>,
public WeakMapBase {
using Base = HashMap<Key, Value, StableCellHasher<Key>, ZoneAllocPolicy>;
using Lookup = typename Base::Lookup;
using Entry = typename Base::Entry;
using Range = typename Base::Range;
using Ptr = typename Base::Ptr;
using AddPtr = typename Base::AddPtr;
struct Enum : public Base::Enum {
explicit Enum(WeakMap& map) : Base::Enum(static_cast<Base&>(map)) {}
using Base::all;
using Base::clear;
using Base::count;
using Base::empty;
using Base::has;
using Base::shallowSizeOfExcludingThis;
// Resolve ambiguity with LinkedListElement<>::remove.
using Base::remove;
using UnbarrieredKey = typename RemoveBarrier<Key>::Type;
explicit WeakMap(JSContext* cx, JSObject* memOf = nullptr);
explicit WeakMap(JS::Zone* zone, JSObject* memOf = nullptr);
// Add a read barrier to prevent an incorrectly gray value from escaping the
// weak map. See the UnmarkGrayTracer::onChild comment in gc/Marking.cpp.
Ptr lookup(const Lookup& l) const {
Ptr p = Base::lookup(l);
if (p) {
return p;
Ptr lookupUnbarriered(const Lookup& l) const { return Base::lookup(l); }
AddPtr lookupForAdd(const Lookup& l) {
AddPtr p = Base::lookupForAdd(l);
if (p) {
return p;
template <typename KeyInput, typename ValueInput>
[[nodiscard]] bool add(AddPtr& p, KeyInput&& k, ValueInput&& v) {
return Base::add(p, std::forward<KeyInput>(k), std::forward<ValueInput>(v));
template <typename KeyInput, typename ValueInput>
[[nodiscard]] bool relookupOrAdd(AddPtr& p, KeyInput&& k, ValueInput&& v) {
return Base::relookupOrAdd(p, std::forward<KeyInput>(k),
template <typename KeyInput, typename ValueInput>
[[nodiscard]] bool put(KeyInput&& k, ValueInput&& v) {
return Base::put(std::forward<KeyInput>(k), std::forward<ValueInput>(v));
template <typename KeyInput, typename ValueInput>
[[nodiscard]] bool putNew(KeyInput&& k, ValueInput&& v) {
return Base::putNew(std::forward<KeyInput>(k), std::forward<ValueInput>(v));
template <typename KeyInput, typename ValueInput>
void putNewInfallible(KeyInput&& k, ValueInput&& v) {
Base::putNewInfallible(std::forward(k), std::forward<KeyInput>(k));
#ifdef DEBUG
template <typename KeyInput, typename ValueInput>
bool hasEntry(KeyInput&& key, ValueInput&& value) {
Ptr p = Base::lookup(std::forward<KeyInput>(key));
return p && p->value() == value;
bool markEntry(GCMarker* marker, gc::CellColor mapColor, Key& key,
Value& value, bool populateWeakKeysTable);
void trace(JSTracer* trc) override;
size_t sizeOfIncludingThis(mozilla::MallocSizeOf mallocSizeOf);
inline void assertMapIsSameZoneWithValue(const Value& v);
bool markEntries(GCMarker* marker) override;
// Find sweep group edges for delegates, if the key type has delegates. (If
// not, the optimizer should make this a nop.)
bool findSweepGroupEdges() override;
* If a wrapper is used as a key in a weakmap, the garbage collector should
* keep that object around longer than it otherwise would. We want to avoid
* collecting the wrapper (and removing the weakmap entry) as long as the
* wrapped object is alive (because the object can be rewrapped and looked up
* again). As long as the wrapper is used as a weakmap key, it will not be
* collected (and remain in the weakmap) until the wrapped object is
* collected.
void exposeGCThingToActiveJS(const JS::Value& v) const {
void exposeGCThingToActiveJS(JSObject* obj) const {
void traceWeakEdges(JSTracer* trc) override;
void clearAndCompact() override {
// memberOf can be nullptr, which means that the map is not part of a
// JSObject.
void traceMappings(WeakMapTracer* tracer) override;
void assertEntriesNotAboutToBeFinalized();
#ifdef JS_GC_ZEAL
bool checkMarking() const override;
void checkAfterMovingGC() const override;
using ObjectValueWeakMap = WeakMap<HeapPtr<JSObject*>, HeapPtr<Value>>;
using ValueValueWeakMap = WeakMap<HeapPtr<Value>, HeapPtr<Value>>;
// Generic weak map for mapping objects to other objects.
class ObjectWeakMap {
ObjectValueWeakMap map;
explicit ObjectWeakMap(JSContext* cx);
JS::Zone* zone() const { return; }
JSObject* lookup(const JSObject* obj);
bool add(JSContext* cx, JSObject* obj, JSObject* target);
void remove(JSObject* key);
void clear();
void trace(JSTracer* trc);
size_t sizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf);
size_t sizeOfIncludingThis(mozilla::MallocSizeOf mallocSizeOf) {
return mallocSizeOf(this) + sizeOfExcludingThis(mallocSizeOf);
ObjectValueWeakMap& valueMap() { return map; }
// Get the hash from the Symbol.
HashNumber GetSymbolHash(JS::Symbol* sym);
// NB: The specialization works based on pointer equality and not on JS Value
// semantics, and it will assert if the Value's isGCThing() is false.
// When the JS Value is of type JS::Symbol, we cannot access uniqueIds when it
// runs on the worker thread, so we get the hashes from the Symbols directly
// instead.
template <>
struct StableCellHasher<HeapPtr<Value>> {
using Key = HeapPtr<Value>;
using Lookup = Value;
static bool maybeGetHash(const Lookup& l, HashNumber* hashOut) {
if (l.isSymbol()) {
*hashOut = GetSymbolHash(l.toSymbol());
return true;
return StableCellHasher<gc::Cell*>::maybeGetHash(l.toGCThing(), hashOut);
static bool ensureHash(const Lookup& l, HashNumber* hashOut) {
if (l.isSymbol()) {
*hashOut = GetSymbolHash(l.toSymbol());
return true;
return StableCellHasher<gc::Cell*>::ensureHash(l.toGCThing(), hashOut);
static HashNumber hash(const Lookup& l) {
if (l.isSymbol()) {
return GetSymbolHash(l.toSymbol());
return StableCellHasher<gc::Cell*>::hash(l.toGCThing());
static bool match(const Key& k, const Lookup& l) {
if (l.isSymbol()) {
return k.toSymbol() == l.toSymbol();
return StableCellHasher<gc::Cell*>::match(k.toGCThing(), l.toGCThing());
} /* namespace js */
#endif /* gc_WeakMap_h */