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diff --git a/js/src/gc/WeakMap.h b/js/src/gc/WeakMap.h
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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 http://mozilla.org/MPL/2.0/. */
+
+#ifndef gc_WeakMap_h
+#define gc_WeakMap_h
+
+#include "mozilla/LinkedList.h"
+
+#include "gc/Barrier.h"
+#include "gc/Tracer.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);
+#endif
+
+}  // 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;
+
+ public:
+  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);
+#endif
+
+ protected:
+  // 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 it or its delegate (if any) into the
+  // appropriate zone's gcEphemeronEdges or gcNurseryEphemeronEdges.
+  inline bool addImplicitEdges(gc::Cell* key, gc::Cell* delegate,
+                               gc::TenuredCell* value);
+
+  virtual bool markEntries(GCMarker* marker) = 0;
+
+#ifdef JS_GC_ZEAL
+  virtual bool checkMarking() const = 0;
+  virtual bool allowKeysInOtherZones() const { return false; }
+  friend bool gc::CheckWeakMapEntryMarking(const WeakMapBase*, gc::Cell*,
+                                           gc::Cell*);
+#endif
+
+  // 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.
+  gc::CellColor mapColor;
+
+  friend class JS::Zone;
+};
+
+template <class Key, class Value>
+class WeakMap
+    : private HashMap<Key, Value, StableCellHasher<Key>, ZoneAllocPolicy>,
+      public WeakMapBase {
+ public:
+  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) {
+      exposeGCThingToActiveJS(p->value());
+    }
+    return p;
+  }
+
+  Ptr lookupUnbarriered(const Lookup& l) const { return Base::lookup(l); }
+
+  AddPtr lookupForAdd(const Lookup& l) {
+    AddPtr p = Base::lookupForAdd(l);
+    if (p) {
+      exposeGCThingToActiveJS(p->value());
+    }
+    return p;
+  }
+
+  template <typename KeyInput, typename ValueInput>
+  [[nodiscard]] bool add(AddPtr& p, KeyInput&& k, ValueInput&& v) {
+    MOZ_ASSERT(k);
+    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) {
+    MOZ_ASSERT(k);
+    return Base::relookupOrAdd(p, std::forward<KeyInput>(k),
+                               std::forward<ValueInput>(v));
+  }
+
+  template <typename KeyInput, typename ValueInput>
+  [[nodiscard]] bool put(KeyInput&& k, ValueInput&& v) {
+    MOZ_ASSERT(k);
+    return Base::put(std::forward<KeyInput>(k), std::forward<ValueInput>(v));
+  }
+
+  template <typename KeyInput, typename ValueInput>
+  [[nodiscard]] bool putNew(KeyInput&& k, ValueInput&& v) {
+    MOZ_ASSERT(k);
+    return Base::putNew(std::forward<KeyInput>(k), std::forward<ValueInput>(v));
+  }
+
+  template <typename KeyInput, typename ValueInput>
+  void putNewInfallible(KeyInput&& k, ValueInput&& v) {
+    MOZ_ASSERT(k);
+    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;
+  }
+#endif
+
+  bool markEntry(GCMarker* marker, Key& key, Value& value,
+                 bool populateWeakKeysTable);
+
+  void trace(JSTracer* trc) override;
+
+  size_t sizeOfIncludingThis(mozilla::MallocSizeOf mallocSizeOf);
+
+ protected:
+  inline void assertMapIsSameZoneWithValue(const Value& v);
+
+  bool markEntries(GCMarker* marker) override;
+
+ protected:
+  // 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.
+   */
+ private:
+  void exposeGCThingToActiveJS(const JS::Value& v) const {
+    JS::ExposeValueToActiveJS(v);
+  }
+  void exposeGCThingToActiveJS(JSObject* obj) const {
+    JS::ExposeObjectToActiveJS(obj);
+  }
+
+  void traceWeakEdges(JSTracer* trc) override;
+
+  void clearAndCompact() override {
+    Base::clear();
+    Base::compact();
+  }
+
+  // memberOf can be nullptr, which means that the map is not part of a
+  // JSObject.
+  void traceMappings(WeakMapTracer* tracer) override;
+
+ protected:
+#if DEBUG
+  void assertEntriesNotAboutToBeFinalized();
+#endif
+
+#ifdef JS_GC_ZEAL
+  bool checkMarking() const override;
+#endif
+};
+
+using ObjectValueWeakMap = WeakMap<HeapPtr<JSObject*>, HeapPtr<Value>>;
+
+// Generic weak map for mapping objects to other objects.
+class ObjectWeakMap {
+  ObjectValueWeakMap map;
+
+ public:
+  explicit ObjectWeakMap(JSContext* cx);
+
+  JS::Zone* zone() const { return map.zone(); }
+
+  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; }
+
+#ifdef JSGC_HASH_TABLE_CHECKS
+  void checkAfterMovingGC();
+#endif
+};
+
+} /* namespace js */
+
+#endif /* gc_WeakMap_h */