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diff --git a/js/public/GCHashTable.h b/js/public/GCHashTable.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 GCHashTable_h
+#define GCHashTable_h
+
+#include "mozilla/Maybe.h"
+
+#include "js/GCPolicyAPI.h"
+#include "js/HashTable.h"
+#include "js/RootingAPI.h"
+#include "js/SweepingAPI.h"
+#include "js/TracingAPI.h"
+
+namespace JS {
+
+// Define a reasonable default GC policy for GC-aware Maps.
+template <typename Key, typename Value>
+struct DefaultMapSweepPolicy {
+  static bool needsSweep(Key* key, Value* value) {
+    return GCPolicy<Key>::needsSweep(key) || GCPolicy<Value>::needsSweep(value);
+  }
+
+  static bool traceWeak(JSTracer* trc, Key* key, Value* value) {
+    return GCPolicy<Key>::traceWeak(trc, key) &&
+           GCPolicy<Value>::traceWeak(trc, value);
+  }
+};
+
+// A GCHashMap is a GC-aware HashMap, meaning that it has additional trace and
+// sweep methods that know how to visit all keys and values in the table.
+// HashMaps that contain GC pointers will generally want to use this GCHashMap
+// specialization instead of HashMap, because this conveniently supports tracing
+// keys and values, and cleaning up weak entries.
+//
+// GCHashMap::trace applies GCPolicy<T>::trace to each entry's key and value.
+// Most types of GC pointers already have appropriate specializations of
+// GCPolicy, so they should just work as keys and values. Any struct type with a
+// default constructor and trace and sweep functions should work as well. If you
+// need to define your own GCPolicy specialization, generic helpers can be found
+// in js/public/TracingAPI.h.
+//
+// The MapSweepPolicy template parameter controls how the table drops entries
+// when swept. GCHashMap::sweep applies MapSweepPolicy::needsSweep to each table
+// entry; if it returns true, the entry is dropped. The default MapSweepPolicy
+// drops the entry if either the key or value is about to be finalized,
+// according to its GCPolicy<T>::needsSweep method. (This default is almost
+// always fine: it's hard to imagine keeping such an entry around anyway.)
+//
+// Note that this HashMap only knows *how* to trace and sweep, but it does not
+// itself cause tracing or sweeping to be invoked. For tracing, it must be used
+// with Rooted or PersistentRooted, or barriered and traced manually. For
+// sweeping, currently it requires an explicit call to <map>.sweep().
+template <typename Key, typename Value,
+          typename HashPolicy = js::DefaultHasher<Key>,
+          typename AllocPolicy = js::TempAllocPolicy,
+          typename MapSweepPolicy = DefaultMapSweepPolicy<Key, Value>>
+class GCHashMap : public js::HashMap<Key, Value, HashPolicy, AllocPolicy> {
+  using Base = js::HashMap<Key, Value, HashPolicy, AllocPolicy>;
+
+ public:
+  explicit GCHashMap(AllocPolicy a = AllocPolicy()) : Base(std::move(a)) {}
+  explicit GCHashMap(size_t length) : Base(length) {}
+  GCHashMap(AllocPolicy a, size_t length) : Base(std::move(a), length) {}
+
+  void trace(JSTracer* trc) {
+    for (typename Base::Enum e(*this); !e.empty(); e.popFront()) {
+      GCPolicy<Value>::trace(trc, &e.front().value(), "hashmap value");
+      GCPolicy<Key>::trace(trc, &e.front().mutableKey(), "hashmap key");
+    }
+  }
+
+  bool needsSweep() const { return !this->empty(); }
+
+  void sweep() {
+    typename Base::Enum e(*this);
+    sweepEntries(e);
+  }
+
+  void sweepEntries(typename Base::Enum& e) {
+    for (; !e.empty(); e.popFront()) {
+      if (MapSweepPolicy::needsSweep(&e.front().mutableKey(),
+                                     &e.front().value())) {
+        e.removeFront();
+      }
+    }
+  }
+
+  void traceWeak(JSTracer* trc) {
+    for (typename Base::Enum e(*this); !e.empty(); e.popFront()) {
+      if (!MapSweepPolicy::traceWeak(trc, &e.front().mutableKey(),
+                                     &e.front().value())) {
+        e.removeFront();
+      }
+    }
+  }
+
+  // GCHashMap is movable
+  GCHashMap(GCHashMap&& rhs) : Base(std::move(rhs)) {}
+  void operator=(GCHashMap&& rhs) {
+    MOZ_ASSERT(this != &rhs, "self-move assignment is prohibited");
+    Base::operator=(std::move(rhs));
+  }
+
+ private:
+  // GCHashMap is not copyable or assignable
+  GCHashMap(const GCHashMap& hm) = delete;
+  GCHashMap& operator=(const GCHashMap& hm) = delete;
+} MOZ_INHERIT_TYPE_ANNOTATIONS_FROM_TEMPLATE_ARGS;
+
+}  // namespace JS
+
+namespace js {
+
+// HashMap that supports rekeying.
+//
+// If your keys are pointers to something like JSObject that can be tenured or
+// compacted, prefer to use GCHashMap with MovableCellHasher, which takes
+// advantage of the Zone's stable id table to make rekeying unnecessary.
+template <typename Key, typename Value,
+          typename HashPolicy = DefaultHasher<Key>,
+          typename AllocPolicy = TempAllocPolicy,
+          typename MapSweepPolicy = JS::DefaultMapSweepPolicy<Key, Value>>
+class GCRekeyableHashMap : public JS::GCHashMap<Key, Value, HashPolicy,
+                                                AllocPolicy, MapSweepPolicy> {
+  using Base = JS::GCHashMap<Key, Value, HashPolicy, AllocPolicy>;
+
+ public:
+  explicit GCRekeyableHashMap(AllocPolicy a = AllocPolicy())
+      : Base(std::move(a)) {}
+  explicit GCRekeyableHashMap(size_t length) : Base(length) {}
+  GCRekeyableHashMap(AllocPolicy a, size_t length)
+      : Base(std::move(a), length) {}
+
+  void sweep() {
+    for (typename Base::Enum e(*this); !e.empty(); e.popFront()) {
+      Key key(e.front().key());
+      if (MapSweepPolicy::needsSweep(&key, &e.front().value())) {
+        e.removeFront();
+      } else if (!HashPolicy::match(key, e.front().key())) {
+        e.rekeyFront(key);
+      }
+    }
+  }
+
+  void traceWeak(JSTracer* trc) {
+    for (typename Base::Enum e(*this); !e.empty(); e.popFront()) {
+      Key key(e.front().key());
+      if (!MapSweepPolicy::traceWeak(trc, &key, &e.front().value())) {
+        e.removeFront();
+      } else if (!HashPolicy::match(key, e.front().key())) {
+        e.rekeyFront(key);
+      }
+    }
+  }
+
+  // GCRekeyableHashMap is movable
+  GCRekeyableHashMap(GCRekeyableHashMap&& rhs) : Base(std::move(rhs)) {}
+  void operator=(GCRekeyableHashMap&& rhs) {
+    MOZ_ASSERT(this != &rhs, "self-move assignment is prohibited");
+    Base::operator=(std::move(rhs));
+  }
+} MOZ_INHERIT_TYPE_ANNOTATIONS_FROM_TEMPLATE_ARGS;
+
+template <typename Wrapper, typename... Args>
+class WrappedPtrOperations<JS::GCHashMap<Args...>, Wrapper> {
+  using Map = JS::GCHashMap<Args...>;
+  using Lookup = typename Map::Lookup;
+
+  const Map& map() const { return static_cast<const Wrapper*>(this)->get(); }
+
+ public:
+  using AddPtr = typename Map::AddPtr;
+  using Ptr = typename Map::Ptr;
+  using Range = typename Map::Range;
+
+  Ptr lookup(const Lookup& l) const { return map().lookup(l); }
+  Range all() const { return map().all(); }
+  bool empty() const { return map().empty(); }
+  uint32_t count() const { return map().count(); }
+  size_t capacity() const { return map().capacity(); }
+  bool has(const Lookup& l) const { return map().lookup(l).found(); }
+  size_t sizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf) const {
+    return map().sizeOfExcludingThis(mallocSizeOf);
+  }
+  size_t sizeOfIncludingThis(mozilla::MallocSizeOf mallocSizeOf) const {
+    return mallocSizeOf(this) + map().sizeOfExcludingThis(mallocSizeOf);
+  }
+};
+
+template <typename Wrapper, typename... Args>
+class MutableWrappedPtrOperations<JS::GCHashMap<Args...>, Wrapper>
+    : public WrappedPtrOperations<JS::GCHashMap<Args...>, Wrapper> {
+  using Map = JS::GCHashMap<Args...>;
+  using Lookup = typename Map::Lookup;
+
+  Map& map() { return static_cast<Wrapper*>(this)->get(); }
+
+ public:
+  using AddPtr = typename Map::AddPtr;
+  struct Enum : public Map::Enum {
+    explicit Enum(Wrapper& o) : Map::Enum(o.map()) {}
+  };
+  using Ptr = typename Map::Ptr;
+  using Range = typename Map::Range;
+
+  void clear() { map().clear(); }
+  void clearAndCompact() { map().clearAndCompact(); }
+  void remove(Ptr p) { map().remove(p); }
+  AddPtr lookupForAdd(const Lookup& l) { return map().lookupForAdd(l); }
+
+  template <typename KeyInput, typename ValueInput>
+  bool add(AddPtr& p, KeyInput&& k, ValueInput&& v) {
+    return map().add(p, std::forward<KeyInput>(k), std::forward<ValueInput>(v));
+  }
+
+  template <typename KeyInput>
+  bool add(AddPtr& p, KeyInput&& k) {
+    return map().add(p, std::forward<KeyInput>(k), Map::Value());
+  }
+
+  template <typename KeyInput, typename ValueInput>
+  bool relookupOrAdd(AddPtr& p, KeyInput&& k, ValueInput&& v) {
+    return map().relookupOrAdd(p, k, std::forward<KeyInput>(k),
+                               std::forward<ValueInput>(v));
+  }
+
+  template <typename KeyInput, typename ValueInput>
+  bool put(KeyInput&& k, ValueInput&& v) {
+    return map().put(std::forward<KeyInput>(k), std::forward<ValueInput>(v));
+  }
+
+  template <typename KeyInput, typename ValueInput>
+  bool putNew(KeyInput&& k, ValueInput&& v) {
+    return map().putNew(std::forward<KeyInput>(k), std::forward<ValueInput>(v));
+  }
+};
+
+}  // namespace js
+
+namespace JS {
+
+// A GCHashSet is a HashSet with an additional trace method that knows
+// be traced to be kept alive will generally want to use this GCHashSet
+// specialization in lieu of HashSet.
+//
+// Most types of GC pointers can be traced with no extra infrastructure. For
+// structs and non-gc-pointer members, ensure that there is a specialization of
+// GCPolicy<T> with an appropriate trace method available to handle the custom
+// type. Generic helpers can be found in js/public/TracingAPI.h.
+//
+// Note that although this HashSet's trace will deal correctly with moved
+// elements, it does not itself know when to barrier or trace elements. To
+// function properly it must either be used with Rooted or barriered and traced
+// manually.
+template <typename T, typename HashPolicy = js::DefaultHasher<T>,
+          typename AllocPolicy = js::TempAllocPolicy>
+class GCHashSet : public js::HashSet<T, HashPolicy, AllocPolicy> {
+  using Base = js::HashSet<T, HashPolicy, AllocPolicy>;
+
+ public:
+  explicit GCHashSet(AllocPolicy a = AllocPolicy()) : Base(std::move(a)) {}
+  explicit GCHashSet(size_t length) : Base(length) {}
+  GCHashSet(AllocPolicy a, size_t length) : Base(std::move(a), length) {}
+
+  void trace(JSTracer* trc) {
+    for (typename Base::Enum e(*this); !e.empty(); e.popFront()) {
+      GCPolicy<T>::trace(trc, &e.mutableFront(), "hashset element");
+    }
+  }
+
+  bool needsSweep() const { return !this->empty(); }
+
+  void sweep() {
+    typename Base::Enum e(*this);
+    sweepEntries(e);
+  }
+
+  void sweepEntries(typename Base::Enum& e) {
+    for (; !e.empty(); e.popFront()) {
+      if (GCPolicy<T>::needsSweep(&e.mutableFront())) {
+        e.removeFront();
+      }
+    }
+  }
+
+  void traceWeak(JSTracer* trc) {
+    for (typename Base::Enum e(*this); !e.empty(); e.popFront()) {
+      if (!GCPolicy<T>::traceWeak(trc, &e.mutableFront())) {
+        e.removeFront();
+      }
+    }
+  }
+
+  // GCHashSet is movable
+  GCHashSet(GCHashSet&& rhs) : Base(std::move(rhs)) {}
+  void operator=(GCHashSet&& rhs) {
+    MOZ_ASSERT(this != &rhs, "self-move assignment is prohibited");
+    Base::operator=(std::move(rhs));
+  }
+
+ private:
+  // GCHashSet is not copyable or assignable
+  GCHashSet(const GCHashSet& hs) = delete;
+  GCHashSet& operator=(const GCHashSet& hs) = delete;
+} MOZ_INHERIT_TYPE_ANNOTATIONS_FROM_TEMPLATE_ARGS;
+
+}  // namespace JS
+
+namespace js {
+
+template <typename Wrapper, typename... Args>
+class WrappedPtrOperations<JS::GCHashSet<Args...>, Wrapper> {
+  using Set = JS::GCHashSet<Args...>;
+
+  const Set& set() const { return static_cast<const Wrapper*>(this)->get(); }
+
+ public:
+  using Lookup = typename Set::Lookup;
+  using AddPtr = typename Set::AddPtr;
+  using Entry = typename Set::Entry;
+  using Ptr = typename Set::Ptr;
+  using Range = typename Set::Range;
+
+  Ptr lookup(const Lookup& l) const { return set().lookup(l); }
+  Range all() const { return set().all(); }
+  bool empty() const { return set().empty(); }
+  uint32_t count() const { return set().count(); }
+  size_t capacity() const { return set().capacity(); }
+  bool has(const Lookup& l) const { return set().lookup(l).found(); }
+  size_t sizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf) const {
+    return set().sizeOfExcludingThis(mallocSizeOf);
+  }
+  size_t sizeOfIncludingThis(mozilla::MallocSizeOf mallocSizeOf) const {
+    return mallocSizeOf(this) + set().sizeOfExcludingThis(mallocSizeOf);
+  }
+};
+
+template <typename Wrapper, typename... Args>
+class MutableWrappedPtrOperations<JS::GCHashSet<Args...>, Wrapper>
+    : public WrappedPtrOperations<JS::GCHashSet<Args...>, Wrapper> {
+  using Set = JS::GCHashSet<Args...>;
+  using Lookup = typename Set::Lookup;
+
+  Set& set() { return static_cast<Wrapper*>(this)->get(); }
+
+ public:
+  using AddPtr = typename Set::AddPtr;
+  using Entry = typename Set::Entry;
+  struct Enum : public Set::Enum {
+    explicit Enum(Wrapper& o) : Set::Enum(o.set()) {}
+  };
+  using Ptr = typename Set::Ptr;
+  using Range = typename Set::Range;
+
+  void clear() { set().clear(); }
+  void clearAndCompact() { set().clearAndCompact(); }
+  MOZ_MUST_USE bool reserve(uint32_t len) { return set().reserve(len); }
+  void remove(Ptr p) { set().remove(p); }
+  void remove(const Lookup& l) { set().remove(l); }
+  AddPtr lookupForAdd(const Lookup& l) { return set().lookupForAdd(l); }
+
+  template <typename TInput>
+  bool add(AddPtr& p, TInput&& t) {
+    return set().add(p, std::forward<TInput>(t));
+  }
+
+  template <typename TInput>
+  bool relookupOrAdd(AddPtr& p, const Lookup& l, TInput&& t) {
+    return set().relookupOrAdd(p, l, std::forward<TInput>(t));
+  }
+
+  template <typename TInput>
+  bool put(TInput&& t) {
+    return set().put(std::forward<TInput>(t));
+  }
+
+  template <typename TInput>
+  bool putNew(TInput&& t) {
+    return set().putNew(std::forward<TInput>(t));
+  }
+
+  template <typename TInput>
+  bool putNew(const Lookup& l, TInput&& t) {
+    return set().putNew(l, std::forward<TInput>(t));
+  }
+};
+
+} /* namespace js */
+
+namespace JS {
+
+// Specialize WeakCache for GCHashMap to provide a barriered map that does not
+// need to be swept immediately.
+template <typename Key, typename Value, typename HashPolicy,
+          typename AllocPolicy, typename MapSweepPolicy>
+class WeakCache<GCHashMap<Key, Value, HashPolicy, AllocPolicy, MapSweepPolicy>>
+    : protected detail::WeakCacheBase {
+  using Map = GCHashMap<Key, Value, HashPolicy, AllocPolicy, MapSweepPolicy>;
+  using Self = WeakCache<Map>;
+
+  Map map;
+  bool needsBarrier;
+
+ public:
+  template <typename... Args>
+  explicit WeakCache(Zone* zone, Args&&... args)
+      : WeakCacheBase(zone),
+        map(std::forward<Args>(args)...),
+        needsBarrier(false) {}
+  template <typename... Args>
+  explicit WeakCache(JSRuntime* rt, Args&&... args)
+      : WeakCacheBase(rt),
+        map(std::forward<Args>(args)...),
+        needsBarrier(false) {}
+  ~WeakCache() { MOZ_ASSERT(!needsBarrier); }
+
+  bool needsSweep() override { return map.needsSweep(); }
+
+  size_t sweep(js::gc::StoreBuffer* sbToLock) override {
+    size_t steps = map.count();
+
+    // Create an Enum and sweep the table entries.
+    mozilla::Maybe<typename Map::Enum> e;
+    e.emplace(map);
+    map.sweepEntries(e.ref());
+
+    // Potentially take a lock while the Enum's destructor is called as this can
+    // rehash/resize the table and access the store buffer.
+    mozilla::Maybe<js::gc::AutoLockStoreBuffer> lock;
+    if (sbToLock) {
+      lock.emplace(sbToLock);
+    }
+    e.reset();
+
+    return steps;
+  }
+
+  bool setNeedsIncrementalBarrier(bool needs) override {
+    MOZ_ASSERT(needsBarrier != needs);
+    needsBarrier = needs;
+    return true;
+  }
+
+  bool needsIncrementalBarrier() const override { return needsBarrier; }
+
+ private:
+  using Entry = typename Map::Entry;
+
+  static bool entryNeedsSweep(const Entry& prior) {
+    Key key(prior.key());
+    Value value(prior.value());
+    bool result = MapSweepPolicy::needsSweep(&key, &value);
+    MOZ_ASSERT(prior.key() == key);      // We shouldn't update here.
+    MOZ_ASSERT(prior.value() == value);  // We shouldn't update here.
+    return result;
+  }
+
+ public:
+  using Lookup = typename Map::Lookup;
+  using Ptr = typename Map::Ptr;
+  using AddPtr = typename Map::AddPtr;
+
+  struct Range {
+    explicit Range(const typename Map::Range& r) : range(r) { settle(); }
+    Range() = default;
+
+    bool empty() const { return range.empty(); }
+    const Entry& front() const { return range.front(); }
+
+    void popFront() {
+      range.popFront();
+      settle();
+    }
+
+   private:
+    typename Map::Range range;
+
+    void settle() {
+      while (!empty() && entryNeedsSweep(front())) {
+        popFront();
+      }
+    }
+  };
+
+  struct Enum : public Map::Enum {
+    explicit Enum(Self& cache) : Map::Enum(cache.map) {
+      // This operation is not allowed while barriers are in place as we
+      // may also need to enumerate the set for sweeping.
+      MOZ_ASSERT(!cache.needsBarrier);
+    }
+  };
+
+  Ptr lookup(const Lookup& l) const {
+    Ptr ptr = map.lookup(l);
+    if (needsBarrier && ptr && entryNeedsSweep(*ptr)) {
+      const_cast<Map&>(map).remove(ptr);
+      return Ptr();
+    }
+    return ptr;
+  }
+
+  AddPtr lookupForAdd(const Lookup& l) {
+    AddPtr ptr = map.lookupForAdd(l);
+    if (needsBarrier && ptr && entryNeedsSweep(*ptr)) {
+      const_cast<Map&>(map).remove(ptr);
+      return map.lookupForAdd(l);
+    }
+    return ptr;
+  }
+
+  Range all() const { return Range(map.all()); }
+
+  bool empty() const {
+    // This operation is not currently allowed while barriers are in place
+    // as it would require iterating the map and the caller expects a
+    // constant time operation.
+    MOZ_ASSERT(!needsBarrier);
+    return map.empty();
+  }
+
+  uint32_t count() const {
+    // This operation is not currently allowed while barriers are in place
+    // as it would require iterating the set and the caller expects a
+    // constant time operation.
+    MOZ_ASSERT(!needsBarrier);
+    return map.count();
+  }
+
+  size_t capacity() const { return map.capacity(); }
+
+  bool has(const Lookup& l) const { return lookup(l).found(); }
+
+  size_t sizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf) const {
+    return map.sizeOfExcludingThis(mallocSizeOf);
+  }
+  size_t sizeOfIncludingThis(mozilla::MallocSizeOf mallocSizeOf) const {
+    return mallocSizeOf(this) + map.shallowSizeOfExcludingThis(mallocSizeOf);
+  }
+
+  void clear() {
+    // This operation is not currently allowed while barriers are in place
+    // since it doesn't make sense to clear a cache while it is being swept.
+    MOZ_ASSERT(!needsBarrier);
+    map.clear();
+  }
+
+  void clearAndCompact() {
+    // This operation is not currently allowed while barriers are in place
+    // since it doesn't make sense to clear a cache while it is being swept.
+    MOZ_ASSERT(!needsBarrier);
+    map.clearAndCompact();
+  }
+
+  void remove(Ptr p) {
+    // This currently supports removing entries during incremental
+    // sweeping. If we allow these tables to be swept incrementally this may
+    // no longer be possible.
+    map.remove(p);
+  }
+
+  void remove(const Lookup& l) {
+    Ptr p = lookup(l);
+    if (p) {
+      remove(p);
+    }
+  }
+
+  template <typename KeyInput, typename ValueInput>
+  bool add(AddPtr& p, KeyInput&& k, ValueInput&& v) {
+    return map.add(p, std::forward<KeyInput>(k), std::forward<ValueInput>(v));
+  }
+
+  template <typename KeyInput, typename ValueInput>
+  bool relookupOrAdd(AddPtr& p, KeyInput&& k, ValueInput&& v) {
+    return map.relookupOrAdd(p, std::forward<KeyInput>(k),
+                             std::forward<ValueInput>(v));
+  }
+
+  template <typename KeyInput, typename ValueInput>
+  bool put(KeyInput&& k, ValueInput&& v) {
+    return map.put(std::forward<KeyInput>(k), std::forward<ValueInput>(v));
+  }
+
+  template <typename KeyInput, typename ValueInput>
+  bool putNew(KeyInput&& k, ValueInput&& v) {
+    return map.putNew(std::forward<KeyInput>(k), std::forward<ValueInput>(v));
+  }
+} JS_HAZ_NON_GC_POINTER;
+
+// Specialize WeakCache for GCHashSet to provide a barriered set that does not
+// need to be swept immediately.
+template <typename T, typename HashPolicy, typename AllocPolicy>
+class WeakCache<GCHashSet<T, HashPolicy, AllocPolicy>>
+    : protected detail::WeakCacheBase {
+  using Set = GCHashSet<T, HashPolicy, AllocPolicy>;
+  using Self = WeakCache<Set>;
+
+  Set set;
+  bool needsBarrier;
+
+ public:
+  using Entry = typename Set::Entry;
+
+  template <typename... Args>
+  explicit WeakCache(Zone* zone, Args&&... args)
+      : WeakCacheBase(zone),
+        set(std::forward<Args>(args)...),
+        needsBarrier(false) {}
+  template <typename... Args>
+  explicit WeakCache(JSRuntime* rt, Args&&... args)
+      : WeakCacheBase(rt),
+        set(std::forward<Args>(args)...),
+        needsBarrier(false) {}
+
+  size_t sweep(js::gc::StoreBuffer* sbToLock) override {
+    size_t steps = set.count();
+
+    // Create an Enum and sweep the table entries. It's not necessary to take
+    // the store buffer lock yet.
+    mozilla::Maybe<typename Set::Enum> e;
+    e.emplace(set);
+    set.sweepEntries(e.ref());
+
+    // Destroy the Enum, potentially rehashing or resizing the table. Since this
+    // can access the store buffer, we need to take a lock for this if we're
+    // called off main thread.
+    mozilla::Maybe<js::gc::AutoLockStoreBuffer> lock;
+    if (sbToLock) {
+      lock.emplace(sbToLock);
+    }
+    e.reset();
+
+    return steps;
+  }
+
+  bool needsSweep() override { return set.needsSweep(); }
+
+  bool setNeedsIncrementalBarrier(bool needs) override {
+    MOZ_ASSERT(needsBarrier != needs);
+    needsBarrier = needs;
+    return true;
+  }
+
+  bool needsIncrementalBarrier() const override { return needsBarrier; }
+
+ private:
+  static bool entryNeedsSweep(const Entry& prior) {
+    Entry entry(prior);
+    bool result = GCPolicy<T>::needsSweep(&entry);
+    MOZ_ASSERT(prior == entry);  // We shouldn't update here.
+    return result;
+  }
+
+ public:
+  using Lookup = typename Set::Lookup;
+  using Ptr = typename Set::Ptr;
+  using AddPtr = typename Set::AddPtr;
+
+  struct Range {
+    explicit Range(const typename Set::Range& r) : range(r) { settle(); }
+    Range() = default;
+
+    bool empty() const { return range.empty(); }
+    const Entry& front() const { return range.front(); }
+
+    void popFront() {
+      range.popFront();
+      settle();
+    }
+
+   private:
+    typename Set::Range range;
+
+    void settle() {
+      while (!empty() && entryNeedsSweep(front())) {
+        popFront();
+      }
+    }
+  };
+
+  struct Enum : public Set::Enum {
+    explicit Enum(Self& cache) : Set::Enum(cache.set) {
+      // This operation is not allowed while barriers are in place as we
+      // may also need to enumerate the set for sweeping.
+      MOZ_ASSERT(!cache.needsBarrier);
+    }
+  };
+
+  Ptr lookup(const Lookup& l) const {
+    Ptr ptr = set.lookup(l);
+    if (needsBarrier && ptr && entryNeedsSweep(*ptr)) {
+      const_cast<Set&>(set).remove(ptr);
+      return Ptr();
+    }
+    return ptr;
+  }
+
+  AddPtr lookupForAdd(const Lookup& l) {
+    AddPtr ptr = set.lookupForAdd(l);
+    if (needsBarrier && ptr && entryNeedsSweep(*ptr)) {
+      const_cast<Set&>(set).remove(ptr);
+      return set.lookupForAdd(l);
+    }
+    return ptr;
+  }
+
+  Range all() const { return Range(set.all()); }
+
+  bool empty() const {
+    // This operation is not currently allowed while barriers are in place
+    // as it would require iterating the set and the caller expects a
+    // constant time operation.
+    MOZ_ASSERT(!needsBarrier);
+    return set.empty();
+  }
+
+  uint32_t count() const {
+    // This operation is not currently allowed while barriers are in place
+    // as it would require iterating the set and the caller expects a
+    // constant time operation.
+    MOZ_ASSERT(!needsBarrier);
+    return set.count();
+  }
+
+  size_t capacity() const { return set.capacity(); }
+
+  bool has(const Lookup& l) const { return lookup(l).found(); }
+
+  size_t sizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf) const {
+    return set.shallowSizeOfExcludingThis(mallocSizeOf);
+  }
+  size_t sizeOfIncludingThis(mozilla::MallocSizeOf mallocSizeOf) const {
+    return mallocSizeOf(this) + set.shallowSizeOfExcludingThis(mallocSizeOf);
+  }
+
+  void clear() {
+    // This operation is not currently allowed while barriers are in place
+    // since it doesn't make sense to clear a cache while it is being swept.
+    MOZ_ASSERT(!needsBarrier);
+    set.clear();
+  }
+
+  void clearAndCompact() {
+    // This operation is not currently allowed while barriers are in place
+    // since it doesn't make sense to clear a cache while it is being swept.
+    MOZ_ASSERT(!needsBarrier);
+    set.clearAndCompact();
+  }
+
+  void remove(Ptr p) {
+    // This currently supports removing entries during incremental
+    // sweeping. If we allow these tables to be swept incrementally this may
+    // no longer be possible.
+    set.remove(p);
+  }
+
+  void remove(const Lookup& l) {
+    Ptr p = lookup(l);
+    if (p) {
+      remove(p);
+    }
+  }
+
+  template <typename TInput>
+  bool add(AddPtr& p, TInput&& t) {
+    return set.add(p, std::forward<TInput>(t));
+  }
+
+  template <typename TInput>
+  bool relookupOrAdd(AddPtr& p, const Lookup& l, TInput&& t) {
+    return set.relookupOrAdd(p, l, std::forward<TInput>(t));
+  }
+
+  template <typename TInput>
+  bool put(TInput&& t) {
+    return set.put(std::forward<TInput>(t));
+  }
+
+  template <typename TInput>
+  bool putNew(TInput&& t) {
+    return set.putNew(std::forward<TInput>(t));
+  }
+
+  template <typename TInput>
+  bool putNew(const Lookup& l, TInput&& t) {
+    return set.putNew(l, std::forward<TInput>(t));
+  }
+} JS_HAZ_NON_GC_POINTER;
+
+}  // namespace JS
+
+#endif /* GCHashTable_h */