Adding upstream version 124.0.1.upstream/124.0.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 2278 insertions, 0 deletions

diff --git a/mfbt/HashTable.h b/mfbt/HashTable.h
new file mode 100644
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+++ b/mfbt/HashTable.h
@@ -0,0 +1,2278 @@
+/* -*- 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/. */
+
+//---------------------------------------------------------------------------
+// Overview
+//---------------------------------------------------------------------------
+//
+// This file defines HashMap<Key, Value> and HashSet<T>, hash tables that are
+// fast and have a nice API.
+//
+// Both hash tables have two optional template parameters.
+//
+// - HashPolicy. This defines the operations for hashing and matching keys. The
+//   default HashPolicy is appropriate when both of the following two
+//   conditions are true.
+//
+//   - The key type stored in the table (|Key| for |HashMap<Key, Value>|, |T|
+//     for |HashSet<T>|) is an integer, pointer, UniquePtr, float, or double.
+//
+//   - The type used for lookups (|Lookup|) is the same as the key type. This
+//     is usually the case, but not always.
+//
+//   There is also a |CStringHasher| policy for |char*| keys. If your keys
+//   don't match any of the above cases, you must provide your own hash policy;
+//   see the "Hash Policy" section below.
+//
+// - AllocPolicy. This defines how allocations are done by the table.
+//
+//   - |MallocAllocPolicy| is the default and is usually appropriate; note that
+//     operations (such as insertions) that might cause allocations are
+//     fallible and must be checked for OOM. These checks are enforced by the
+//     use of [[nodiscard]].
+//
+//   - |InfallibleAllocPolicy| is another possibility; it allows the
+//     abovementioned OOM checks to be done with MOZ_ALWAYS_TRUE().
+//
+//   Note that entry storage allocation is lazy, and not done until the first
+//   lookupForAdd(), put(), or putNew() is performed.
+//
+//  See AllocPolicy.h for more details.
+//
+// Documentation on how to use HashMap and HashSet, including examples, is
+// present within those classes. Search for "class HashMap" and "class
+// HashSet".
+//
+// Both HashMap and HashSet are implemented on top of a third class, HashTable.
+// You only need to look at HashTable if you want to understand the
+// implementation.
+//
+// How does mozilla::HashTable (this file) compare with PLDHashTable (and its
+// subclasses, such as nsTHashtable)?
+//
+// - mozilla::HashTable is a lot faster, largely because it uses templates
+//   throughout *and* inlines everything. PLDHashTable inlines operations much
+//   less aggressively, and also uses "virtual ops" for operations like hashing
+//   and matching entries that require function calls.
+//
+// - Correspondingly, mozilla::HashTable use is likely to increase executable
+//   size much more than PLDHashTable.
+//
+// - mozilla::HashTable has a nicer API, with a proper HashSet vs. HashMap
+//   distinction.
+//
+// - mozilla::HashTable requires more explicit OOM checking. As mentioned
+//   above, the use of |InfallibleAllocPolicy| can simplify things.
+//
+// - mozilla::HashTable has a default capacity on creation of 32 and a minimum
+//   capacity of 4. PLDHashTable has a default capacity on creation of 8 and a
+//   minimum capacity of 8.
+
+#ifndef mozilla_HashTable_h
+#define mozilla_HashTable_h
+
+#include <utility>
+#include <type_traits>
+
+#include "mozilla/AllocPolicy.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/Casting.h"
+#include "mozilla/HashFunctions.h"
+#include "mozilla/MathAlgorithms.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/MemoryChecking.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/Opaque.h"
+#include "mozilla/OperatorNewExtensions.h"
+#include "mozilla/ReentrancyGuard.h"
+#include "mozilla/UniquePtr.h"
+#include "mozilla/WrappingOperations.h"
+
+namespace mozilla {
+
+template <class, class = void>
+struct DefaultHasher;
+
+template <class, class>
+class HashMapEntry;
+
+namespace detail {
+
+template <typename T>
+class HashTableEntry;
+
+template <class T, class HashPolicy, class AllocPolicy>
+class HashTable;
+
+}  // namespace detail
+
+// The "generation" of a hash table is an opaque value indicating the state of
+// modification of the hash table through its lifetime.  If the generation of
+// a hash table compares equal at times T1 and T2, then lookups in the hash
+// table, pointers to (or into) hash table entries, etc. at time T1 are valid
+// at time T2.  If the generation compares unequal, these computations are all
+// invalid and must be performed again to be used.
+//
+// Generations are meaningfully comparable only with respect to a single hash
+// table.  It's always nonsensical to compare the generation of distinct hash
+// tables H1 and H2.
+using Generation = Opaque<uint64_t>;
+
+//---------------------------------------------------------------------------
+// HashMap
+//---------------------------------------------------------------------------
+
+// HashMap is a fast hash-based map from keys to values.
+//
+// Template parameter requirements:
+// - Key/Value: movable, destructible, assignable.
+// - HashPolicy: see the "Hash Policy" section below.
+// - AllocPolicy: see AllocPolicy.h.
+//
+// Note:
+// - HashMap is not reentrant: Key/Value/HashPolicy/AllocPolicy members
+//   called by HashMap must not call back into the same HashMap object.
+//
+template <class Key, class Value, class HashPolicy = DefaultHasher<Key>,
+          class AllocPolicy = MallocAllocPolicy>
+class HashMap {
+  // -- Implementation details -----------------------------------------------
+
+  // HashMap is not copyable or assignable.
+  HashMap(const HashMap& hm) = delete;
+  HashMap& operator=(const HashMap& hm) = delete;
+
+  using TableEntry = HashMapEntry<Key, Value>;
+
+  struct MapHashPolicy : HashPolicy {
+    using Base = HashPolicy;
+    using KeyType = Key;
+
+    static const Key& getKey(TableEntry& aEntry) { return aEntry.key(); }
+
+    static void setKey(TableEntry& aEntry, Key& aKey) {
+      HashPolicy::rekey(aEntry.mutableKey(), aKey);
+    }
+  };
+
+  using Impl = detail::HashTable<TableEntry, MapHashPolicy, AllocPolicy>;
+  Impl mImpl;
+
+  friend class Impl::Enum;
+
+ public:
+  using Lookup = typename HashPolicy::Lookup;
+  using Entry = TableEntry;
+
+  // -- Initialization -------------------------------------------------------
+
+  explicit HashMap(AllocPolicy aAllocPolicy = AllocPolicy(),
+                   uint32_t aLen = Impl::sDefaultLen)
+      : mImpl(std::move(aAllocPolicy), aLen) {}
+
+  explicit HashMap(uint32_t aLen) : mImpl(AllocPolicy(), aLen) {}
+
+  // HashMap is movable.
+  HashMap(HashMap&& aRhs) = default;
+  HashMap& operator=(HashMap&& aRhs) = default;
+
+  // -- Status and sizing ----------------------------------------------------
+
+  // The map's current generation.
+  Generation generation() const { return mImpl.generation(); }
+
+  // Is the map empty?
+  bool empty() const { return mImpl.empty(); }
+
+  // Number of keys/values in the map.
+  uint32_t count() const { return mImpl.count(); }
+
+  // Number of key/value slots in the map. Note: resize will happen well before
+  // count() == capacity().
+  uint32_t capacity() const { return mImpl.capacity(); }
+
+  // The size of the map's entry storage, in bytes. If the keys/values contain
+  // pointers to other heap blocks, you must iterate over the map and measure
+  // them separately; hence the "shallow" prefix.
+  size_t shallowSizeOfExcludingThis(MallocSizeOf aMallocSizeOf) const {
+    return mImpl.shallowSizeOfExcludingThis(aMallocSizeOf);
+  }
+  size_t shallowSizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const {
+    return aMallocSizeOf(this) +
+           mImpl.shallowSizeOfExcludingThis(aMallocSizeOf);
+  }
+
+  // Attempt to minimize the capacity(). If the table is empty, this will free
+  // the empty storage and upon regrowth it will be given the minimum capacity.
+  void compact() { mImpl.compact(); }
+
+  // Attempt to reserve enough space to fit at least |aLen| elements. This is
+  // total capacity, including elements already present. Does nothing if the
+  // map already has sufficient capacity.
+  [[nodiscard]] bool reserve(uint32_t aLen) { return mImpl.reserve(aLen); }
+
+  // -- Lookups --------------------------------------------------------------
+
+  // Does the map contain a key/value matching |aLookup|?
+  bool has(const Lookup& aLookup) const {
+    return mImpl.lookup(aLookup).found();
+  }
+
+  // Return a Ptr indicating whether a key/value matching |aLookup| is
+  // present in the map. E.g.:
+  //
+  //   using HM = HashMap<int,char>;
+  //   HM h;
+  //   if (HM::Ptr p = h.lookup(3)) {
+  //     assert(p->key() == 3);
+  //     char val = p->value();
+  //   }
+  //
+  using Ptr = typename Impl::Ptr;
+  MOZ_ALWAYS_INLINE Ptr lookup(const Lookup& aLookup) const {
+    return mImpl.lookup(aLookup);
+  }
+
+  // Like lookup(), but does not assert if two threads call it at the same
+  // time. Only use this method when none of the threads will modify the map.
+  MOZ_ALWAYS_INLINE Ptr readonlyThreadsafeLookup(const Lookup& aLookup) const {
+    return mImpl.readonlyThreadsafeLookup(aLookup);
+  }
+
+  // -- Insertions -----------------------------------------------------------
+
+  // Overwrite existing value with |aValue|, or add it if not present. Returns
+  // false on OOM.
+  template <typename KeyInput, typename ValueInput>
+  [[nodiscard]] bool put(KeyInput&& aKey, ValueInput&& aValue) {
+    return put(aKey, std::forward<KeyInput>(aKey),
+               std::forward<ValueInput>(aValue));
+  }
+
+  template <typename KeyInput, typename ValueInput>
+  [[nodiscard]] bool put(const Lookup& aLookup, KeyInput&& aKey,
+                         ValueInput&& aValue) {
+    AddPtr p = lookupForAdd(aLookup);
+    if (p) {
+      p->value() = std::forward<ValueInput>(aValue);
+      return true;
+    }
+    return add(p, std::forward<KeyInput>(aKey),
+               std::forward<ValueInput>(aValue));
+  }
+
+  // Like put(), but slightly faster. Must only be used when the given key is
+  // not already present. (In debug builds, assertions check this.)
+  template <typename KeyInput, typename ValueInput>
+  [[nodiscard]] bool putNew(KeyInput&& aKey, ValueInput&& aValue) {
+    return mImpl.putNew(aKey, std::forward<KeyInput>(aKey),
+                        std::forward<ValueInput>(aValue));
+  }
+
+  template <typename KeyInput, typename ValueInput>
+  [[nodiscard]] bool putNew(const Lookup& aLookup, KeyInput&& aKey,
+                            ValueInput&& aValue) {
+    return mImpl.putNew(aLookup, std::forward<KeyInput>(aKey),
+                        std::forward<ValueInput>(aValue));
+  }
+
+  // Like putNew(), but should be only used when the table is known to be big
+  // enough for the insertion, and hashing cannot fail. Typically this is used
+  // to populate an empty map with known-unique keys after reserving space with
+  // reserve(), e.g.
+  //
+  //   using HM = HashMap<int,char>;
+  //   HM h;
+  //   if (!h.reserve(3)) {
+  //     MOZ_CRASH("OOM");
+  //   }
+  //   h.putNewInfallible(1, 'a');    // unique key
+  //   h.putNewInfallible(2, 'b');    // unique key
+  //   h.putNewInfallible(3, 'c');    // unique key
+  //
+  template <typename KeyInput, typename ValueInput>
+  void putNewInfallible(KeyInput&& aKey, ValueInput&& aValue) {
+    mImpl.putNewInfallible(aKey, std::forward<KeyInput>(aKey),
+                           std::forward<ValueInput>(aValue));
+  }
+
+  // Like |lookup(l)|, but on miss, |p = lookupForAdd(l)| allows efficient
+  // insertion of Key |k| (where |HashPolicy::match(k,l) == true|) using
+  // |add(p,k,v)|. After |add(p,k,v)|, |p| points to the new key/value. E.g.:
+  //
+  //   using HM = HashMap<int,char>;
+  //   HM h;
+  //   HM::AddPtr p = h.lookupForAdd(3);
+  //   if (!p) {
+  //     if (!h.add(p, 3, 'a')) {
+  //       return false;
+  //     }
+  //   }
+  //   assert(p->key() == 3);
+  //   char val = p->value();
+  //
+  // N.B. The caller must ensure that no mutating hash table operations occur
+  // between a pair of lookupForAdd() and add() calls. To avoid looking up the
+  // key a second time, the caller may use the more efficient relookupOrAdd()
+  // method. This method reuses part of the hashing computation to more
+  // efficiently insert the key if it has not been added. For example, a
+  // mutation-handling version of the previous example:
+  //
+  //    HM::AddPtr p = h.lookupForAdd(3);
+  //    if (!p) {
+  //      call_that_may_mutate_h();
+  //      if (!h.relookupOrAdd(p, 3, 'a')) {
+  //        return false;
+  //      }
+  //    }
+  //    assert(p->key() == 3);
+  //    char val = p->value();
+  //
+  using AddPtr = typename Impl::AddPtr;
+  MOZ_ALWAYS_INLINE AddPtr lookupForAdd(const Lookup& aLookup) {
+    return mImpl.lookupForAdd(aLookup);
+  }
+
+  // Add a key/value. Returns false on OOM.
+  template <typename KeyInput, typename ValueInput>
+  [[nodiscard]] bool add(AddPtr& aPtr, KeyInput&& aKey, ValueInput&& aValue) {
+    return mImpl.add(aPtr, std::forward<KeyInput>(aKey),
+                     std::forward<ValueInput>(aValue));
+  }
+
+  // See the comment above lookupForAdd() for details.
+  template <typename KeyInput, typename ValueInput>
+  [[nodiscard]] bool relookupOrAdd(AddPtr& aPtr, KeyInput&& aKey,
+                                   ValueInput&& aValue) {
+    return mImpl.relookupOrAdd(aPtr, aKey, std::forward<KeyInput>(aKey),
+                               std::forward<ValueInput>(aValue));
+  }
+
+  // -- Removal --------------------------------------------------------------
+
+  // Lookup and remove the key/value matching |aLookup|, if present.
+  void remove(const Lookup& aLookup) {
+    if (Ptr p = lookup(aLookup)) {
+      remove(p);
+    }
+  }
+
+  // Remove a previously found key/value (assuming aPtr.found()). The map must
+  // not have been mutated in the interim.
+  void remove(Ptr aPtr) { mImpl.remove(aPtr); }
+
+  // Remove all keys/values without changing the capacity.
+  void clear() { mImpl.clear(); }
+
+  // Like clear() followed by compact().
+  void clearAndCompact() { mImpl.clearAndCompact(); }
+
+  // -- Rekeying -------------------------------------------------------------
+
+  // Infallibly rekey one entry, if necessary. Requires that template
+  // parameters Key and HashPolicy::Lookup are the same type.
+  void rekeyIfMoved(const Key& aOldKey, const Key& aNewKey) {
+    if (aOldKey != aNewKey) {
+      rekeyAs(aOldKey, aNewKey, aNewKey);
+    }
+  }
+
+  // Infallibly rekey one entry if present, and return whether that happened.
+  bool rekeyAs(const Lookup& aOldLookup, const Lookup& aNewLookup,
+               const Key& aNewKey) {
+    if (Ptr p = lookup(aOldLookup)) {
+      mImpl.rekeyAndMaybeRehash(p, aNewLookup, aNewKey);
+      return true;
+    }
+    return false;
+  }
+
+  // -- Iteration ------------------------------------------------------------
+
+  // |iter()| returns an Iterator:
+  //
+  //   HashMap<int, char> h;
+  //   for (auto iter = h.iter(); !iter.done(); iter.next()) {
+  //     char c = iter.get().value();
+  //   }
+  //
+  using Iterator = typename Impl::Iterator;
+  Iterator iter() const { return mImpl.iter(); }
+
+  // |modIter()| returns a ModIterator:
+  //
+  //   HashMap<int, char> h;
+  //   for (auto iter = h.modIter(); !iter.done(); iter.next()) {
+  //     if (iter.get().value() == 'l') {
+  //       iter.remove();
+  //     }
+  //   }
+  //
+  // Table resize may occur in ModIterator's destructor.
+  using ModIterator = typename Impl::ModIterator;
+  ModIterator modIter() { return mImpl.modIter(); }
+
+  // These are similar to Iterator/ModIterator/iter(), but use different
+  // terminology.
+  using Range = typename Impl::Range;
+  using Enum = typename Impl::Enum;
+  Range all() const { return mImpl.all(); }
+};
+
+//---------------------------------------------------------------------------
+// HashSet
+//---------------------------------------------------------------------------
+
+// HashSet is a fast hash-based set of values.
+//
+// Template parameter requirements:
+// - T: movable, destructible, assignable.
+// - HashPolicy: see the "Hash Policy" section below.
+// - AllocPolicy: see AllocPolicy.h
+//
+// Note:
+// - HashSet is not reentrant: T/HashPolicy/AllocPolicy members called by
+//   HashSet must not call back into the same HashSet object.
+//
+template <class T, class HashPolicy = DefaultHasher<T>,
+          class AllocPolicy = MallocAllocPolicy>
+class HashSet {
+  // -- Implementation details -----------------------------------------------
+
+  // HashSet is not copyable or assignable.
+  HashSet(const HashSet& hs) = delete;
+  HashSet& operator=(const HashSet& hs) = delete;
+
+  struct SetHashPolicy : HashPolicy {
+    using Base = HashPolicy;
+    using KeyType = T;
+
+    static const KeyType& getKey(const T& aT) { return aT; }
+
+    static void setKey(T& aT, KeyType& aKey) { HashPolicy::rekey(aT, aKey); }
+  };
+
+  using Impl = detail::HashTable<const T, SetHashPolicy, AllocPolicy>;
+  Impl mImpl;
+
+  friend class Impl::Enum;
+
+ public:
+  using Lookup = typename HashPolicy::Lookup;
+  using Entry = T;
+
+  // -- Initialization -------------------------------------------------------
+
+  explicit HashSet(AllocPolicy aAllocPolicy = AllocPolicy(),
+                   uint32_t aLen = Impl::sDefaultLen)
+      : mImpl(std::move(aAllocPolicy), aLen) {}
+
+  explicit HashSet(uint32_t aLen) : mImpl(AllocPolicy(), aLen) {}
+
+  // HashSet is movable.
+  HashSet(HashSet&& aRhs) = default;
+  HashSet& operator=(HashSet&& aRhs) = default;
+
+  // -- Status and sizing ----------------------------------------------------
+
+  // The set's current generation.
+  Generation generation() const { return mImpl.generation(); }
+
+  // Is the set empty?
+  bool empty() const { return mImpl.empty(); }
+
+  // Number of elements in the set.
+  uint32_t count() const { return mImpl.count(); }
+
+  // Number of element slots in the set. Note: resize will happen well before
+  // count() == capacity().
+  uint32_t capacity() const { return mImpl.capacity(); }
+
+  // The size of the set's entry storage, in bytes. If the elements contain
+  // pointers to other heap blocks, you must iterate over the set and measure
+  // them separately; hence the "shallow" prefix.
+  size_t shallowSizeOfExcludingThis(MallocSizeOf aMallocSizeOf) const {
+    return mImpl.shallowSizeOfExcludingThis(aMallocSizeOf);
+  }
+  size_t shallowSizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const {
+    return aMallocSizeOf(this) +
+           mImpl.shallowSizeOfExcludingThis(aMallocSizeOf);
+  }
+
+  // Attempt to minimize the capacity(). If the table is empty, this will free
+  // the empty storage and upon regrowth it will be given the minimum capacity.
+  void compact() { mImpl.compact(); }
+
+  // Attempt to reserve enough space to fit at least |aLen| elements. This is
+  // total capacity, including elements already present. Does nothing if the
+  // map already has sufficient capacity.
+  [[nodiscard]] bool reserve(uint32_t aLen) { return mImpl.reserve(aLen); }
+
+  // -- Lookups --------------------------------------------------------------
+
+  // Does the set contain an element matching |aLookup|?
+  bool has(const Lookup& aLookup) const {
+    return mImpl.lookup(aLookup).found();
+  }
+
+  // Return a Ptr indicating whether an element matching |aLookup| is present
+  // in the set. E.g.:
+  //
+  //   using HS = HashSet<int>;
+  //   HS h;
+  //   if (HS::Ptr p = h.lookup(3)) {
+  //     assert(*p == 3);   // p acts like a pointer to int
+  //   }
+  //
+  using Ptr = typename Impl::Ptr;
+  MOZ_ALWAYS_INLINE Ptr lookup(const Lookup& aLookup) const {
+    return mImpl.lookup(aLookup);
+  }
+
+  // Like lookup(), but does not assert if two threads call it at the same
+  // time. Only use this method when none of the threads will modify the set.
+  MOZ_ALWAYS_INLINE Ptr readonlyThreadsafeLookup(const Lookup& aLookup) const {
+    return mImpl.readonlyThreadsafeLookup(aLookup);
+  }
+
+  // -- Insertions -----------------------------------------------------------
+
+  // Add |aU| if it is not present already. Returns false on OOM.
+  template <typename U>
+  [[nodiscard]] bool put(U&& aU) {
+    AddPtr p = lookupForAdd(aU);
+    return p ? true : add(p, std::forward<U>(aU));
+  }
+
+  // Like put(), but slightly faster. Must only be used when the given element
+  // is not already present. (In debug builds, assertions check this.)
+  template <typename U>
+  [[nodiscard]] bool putNew(U&& aU) {
+    return mImpl.putNew(aU, std::forward<U>(aU));
+  }
+
+  // Like the other putNew(), but for when |Lookup| is different to |T|.
+  template <typename U>
+  [[nodiscard]] bool putNew(const Lookup& aLookup, U&& aU) {
+    return mImpl.putNew(aLookup, std::forward<U>(aU));
+  }
+
+  // Like putNew(), but should be only used when the table is known to be big
+  // enough for the insertion, and hashing cannot fail. Typically this is used
+  // to populate an empty set with known-unique elements after reserving space
+  // with reserve(), e.g.
+  //
+  //   using HS = HashMap<int>;
+  //   HS h;
+  //   if (!h.reserve(3)) {
+  //     MOZ_CRASH("OOM");
+  //   }
+  //   h.putNewInfallible(1);     // unique element
+  //   h.putNewInfallible(2);     // unique element
+  //   h.putNewInfallible(3);     // unique element
+  //
+  template <typename U>
+  void putNewInfallible(const Lookup& aLookup, U&& aU) {
+    mImpl.putNewInfallible(aLookup, std::forward<U>(aU));
+  }
+
+  // Like |lookup(l)|, but on miss, |p = lookupForAdd(l)| allows efficient
+  // insertion of T value |t| (where |HashPolicy::match(t,l) == true|) using
+  // |add(p,t)|. After |add(p,t)|, |p| points to the new element. E.g.:
+  //
+  //   using HS = HashSet<int>;
+  //   HS h;
+  //   HS::AddPtr p = h.lookupForAdd(3);
+  //   if (!p) {
+  //     if (!h.add(p, 3)) {
+  //       return false;
+  //     }
+  //   }
+  //   assert(*p == 3);   // p acts like a pointer to int
+  //
+  // N.B. The caller must ensure that no mutating hash table operations occur
+  // between a pair of lookupForAdd() and add() calls. To avoid looking up the
+  // key a second time, the caller may use the more efficient relookupOrAdd()
+  // method. This method reuses part of the hashing computation to more
+  // efficiently insert the key if it has not been added. For example, a
+  // mutation-handling version of the previous example:
+  //
+  //    HS::AddPtr p = h.lookupForAdd(3);
+  //    if (!p) {
+  //      call_that_may_mutate_h();
+  //      if (!h.relookupOrAdd(p, 3, 3)) {
+  //        return false;
+  //      }
+  //    }
+  //    assert(*p == 3);
+  //
+  // Note that relookupOrAdd(p,l,t) performs Lookup using |l| and adds the
+  // entry |t|, where the caller ensures match(l,t).
+  using AddPtr = typename Impl::AddPtr;
+  MOZ_ALWAYS_INLINE AddPtr lookupForAdd(const Lookup& aLookup) {
+    return mImpl.lookupForAdd(aLookup);
+  }
+
+  // Add an element. Returns false on OOM.
+  template <typename U>
+  [[nodiscard]] bool add(AddPtr& aPtr, U&& aU) {
+    return mImpl.add(aPtr, std::forward<U>(aU));
+  }
+
+  // See the comment above lookupForAdd() for details.
+  template <typename U>
+  [[nodiscard]] bool relookupOrAdd(AddPtr& aPtr, const Lookup& aLookup,
+                                   U&& aU) {
+    return mImpl.relookupOrAdd(aPtr, aLookup, std::forward<U>(aU));
+  }
+
+  // -- Removal --------------------------------------------------------------
+
+  // Lookup and remove the element matching |aLookup|, if present.
+  void remove(const Lookup& aLookup) {
+    if (Ptr p = lookup(aLookup)) {
+      remove(p);
+    }
+  }
+
+  // Remove a previously found element (assuming aPtr.found()). The set must
+  // not have been mutated in the interim.
+  void remove(Ptr aPtr) { mImpl.remove(aPtr); }
+
+  // Remove all keys/values without changing the capacity.
+  void clear() { mImpl.clear(); }
+
+  // Like clear() followed by compact().
+  void clearAndCompact() { mImpl.clearAndCompact(); }
+
+  // -- Rekeying -------------------------------------------------------------
+
+  // Infallibly rekey one entry, if present. Requires that template parameters
+  // T and HashPolicy::Lookup are the same type.
+  void rekeyIfMoved(const Lookup& aOldValue, const T& aNewValue) {
+    if (aOldValue != aNewValue) {
+      rekeyAs(aOldValue, aNewValue, aNewValue);
+    }
+  }
+
+  // Infallibly rekey one entry if present, and return whether that happened.
+  bool rekeyAs(const Lookup& aOldLookup, const Lookup& aNewLookup,
+               const T& aNewValue) {
+    if (Ptr p = lookup(aOldLookup)) {
+      mImpl.rekeyAndMaybeRehash(p, aNewLookup, aNewValue);
+      return true;
+    }
+    return false;
+  }
+
+  // Infallibly replace the current key at |aPtr| with an equivalent key.
+  // Specifically, both HashPolicy::hash and HashPolicy::match must return
+  // identical results for the new and old key when applied against all
+  // possible matching values.
+  void replaceKey(Ptr aPtr, const Lookup& aLookup, const T& aNewValue) {
+    MOZ_ASSERT(aPtr.found());
+    MOZ_ASSERT(*aPtr != aNewValue);
+    MOZ_ASSERT(HashPolicy::match(*aPtr, aLookup));
+    MOZ_ASSERT(HashPolicy::match(aNewValue, aLookup));
+    const_cast<T&>(*aPtr) = aNewValue;
+    MOZ_ASSERT(*lookup(aLookup) == aNewValue);
+  }
+  void replaceKey(Ptr aPtr, const T& aNewValue) {
+    replaceKey(aPtr, aNewValue, aNewValue);
+  }
+
+  // -- Iteration ------------------------------------------------------------
+
+  // |iter()| returns an Iterator:
+  //
+  //   HashSet<int> h;
+  //   for (auto iter = h.iter(); !iter.done(); iter.next()) {
+  //     int i = iter.get();
+  //   }
+  //
+  using Iterator = typename Impl::Iterator;
+  Iterator iter() const { return mImpl.iter(); }
+
+  // |modIter()| returns a ModIterator:
+  //
+  //   HashSet<int> h;
+  //   for (auto iter = h.modIter(); !iter.done(); iter.next()) {
+  //     if (iter.get() == 42) {
+  //       iter.remove();
+  //     }
+  //   }
+  //
+  // Table resize may occur in ModIterator's destructor.
+  using ModIterator = typename Impl::ModIterator;
+  ModIterator modIter() { return mImpl.modIter(); }
+
+  // These are similar to Iterator/ModIterator/iter(), but use different
+  // terminology.
+  using Range = typename Impl::Range;
+  using Enum = typename Impl::Enum;
+  Range all() const { return mImpl.all(); }
+};
+
+//---------------------------------------------------------------------------
+// Hash Policy
+//---------------------------------------------------------------------------
+
+// A hash policy |HP| for a hash table with key-type |Key| must provide:
+//
+//  - a type |HP::Lookup| to use to lookup table entries;
+//
+//  - a static member function |HP::hash| that hashes lookup values:
+//
+//      static mozilla::HashNumber hash(const Lookup&);
+//
+//  - a static member function |HP::match| that tests equality of key and
+//    lookup values:
+//
+//      static bool match(const Key& aKey, const Lookup& aLookup);
+//
+//    |aKey| and |aLookup| can have different hash numbers, only when a
+//    collision happens with |prepareHash| operation, which is less frequent.
+//    Thus, |HP::match| shouldn't assume the hash equality in the comparison,
+//    even if the hash numbers are almost always same between them.
+//
+// Normally, Lookup = Key. In general, though, different values and types of
+// values can be used to lookup and store. If a Lookup value |l| is not equal
+// to the added Key value |k|, the user must ensure that |HP::match(k,l)| is
+// true. E.g.:
+//
+//   mozilla::HashSet<Key, HP>::AddPtr p = h.lookup(l);
+//   if (!p) {
+//     assert(HP::match(k, l));  // must hold
+//     h.add(p, k);
+//   }
+
+// A pointer hashing policy that uses HashGeneric() to create good hashes for
+// pointers. Note that we don't shift out the lowest k bits because we don't
+// want to assume anything about the alignment of the pointers.
+template <typename Key>
+struct PointerHasher {
+  using Lookup = Key;
+
+  static HashNumber hash(const Lookup& aLookup) { return HashGeneric(aLookup); }
+
+  static bool match(const Key& aKey, const Lookup& aLookup) {
+    return aKey == aLookup;
+  }
+
+  static void rekey(Key& aKey, const Key& aNewKey) { aKey = aNewKey; }
+};
+
+// The default hash policy, which only works with integers.
+template <class Key, typename>
+struct DefaultHasher {
+  using Lookup = Key;
+
+  static HashNumber hash(const Lookup& aLookup) {
+    // Just convert the integer to a HashNumber and use that as is. (This
+    // discards the high 32-bits of 64-bit integers!) ScrambleHashCode() is
+    // subsequently called on the value to improve the distribution.
+    return aLookup;
+  }
+
+  static bool match(const Key& aKey, const Lookup& aLookup) {
+    // Use builtin or overloaded operator==.
+    return aKey == aLookup;
+  }
+
+  static void rekey(Key& aKey, const Key& aNewKey) { aKey = aNewKey; }
+};
+
+// A DefaultHasher specialization for enums.
+template <class T>
+struct DefaultHasher<T, std::enable_if_t<std::is_enum_v<T>>> {
+  using Key = T;
+  using Lookup = Key;
+
+  static HashNumber hash(const Lookup& aLookup) { return HashGeneric(aLookup); }
+
+  static bool match(const Key& aKey, const Lookup& aLookup) {
+    // Use builtin or overloaded operator==.
+    return aKey == static_cast<Key>(aLookup);
+  }
+
+  static void rekey(Key& aKey, const Key& aNewKey) { aKey = aNewKey; }
+};
+
+// A DefaultHasher specialization for pointers.
+template <class T>
+struct DefaultHasher<T*> : PointerHasher<T*> {};
+
+// A DefaultHasher specialization for mozilla::UniquePtr.
+template <class T, class D>
+struct DefaultHasher<UniquePtr<T, D>> {
+  using Key = UniquePtr<T, D>;
+  using Lookup = Key;
+  using PtrHasher = PointerHasher<T*>;
+
+  static HashNumber hash(const Lookup& aLookup) {
+    return PtrHasher::hash(aLookup.get());
+  }
+
+  static bool match(const Key& aKey, const Lookup& aLookup) {
+    return PtrHasher::match(aKey.get(), aLookup.get());
+  }
+
+  static void rekey(UniquePtr<T, D>& aKey, UniquePtr<T, D>&& aNewKey) {
+    aKey = std::move(aNewKey);
+  }
+};
+
+// A DefaultHasher specialization for doubles.
+template <>
+struct DefaultHasher<double> {
+  using Key = double;
+  using Lookup = Key;
+
+  static HashNumber hash(const Lookup& aLookup) {
+    // Just xor the high bits with the low bits, and then treat the bits of the
+    // result as a uint32_t.
+    static_assert(sizeof(HashNumber) == 4,
+                  "subsequent code assumes a four-byte hash");
+    uint64_t u = BitwiseCast<uint64_t>(aLookup);
+    return HashNumber(u ^ (u >> 32));
+  }
+
+  static bool match(const Key& aKey, const Lookup& aLookup) {
+    return BitwiseCast<uint64_t>(aKey) == BitwiseCast<uint64_t>(aLookup);
+  }
+};
+
+// A DefaultHasher specialization for floats.
+template <>
+struct DefaultHasher<float> {
+  using Key = float;
+  using Lookup = Key;
+
+  static HashNumber hash(const Lookup& aLookup) {
+    // Just use the value as if its bits form an integer. ScrambleHashCode() is
+    // subsequently called on the value to improve the distribution.
+    static_assert(sizeof(HashNumber) == 4,
+                  "subsequent code assumes a four-byte hash");
+    return HashNumber(BitwiseCast<uint32_t>(aLookup));
+  }
+
+  static bool match(const Key& aKey, const Lookup& aLookup) {
+    return BitwiseCast<uint32_t>(aKey) == BitwiseCast<uint32_t>(aLookup);
+  }
+};
+
+// A hash policy for C strings.
+struct CStringHasher {
+  using Key = const char*;
+  using Lookup = const char*;
+
+  static HashNumber hash(const Lookup& aLookup) { return HashString(aLookup); }
+
+  static bool match(const Key& aKey, const Lookup& aLookup) {
+    return strcmp(aKey, aLookup) == 0;
+  }
+};
+
+//---------------------------------------------------------------------------
+// Fallible Hashing Interface
+//---------------------------------------------------------------------------
+
+// Most of the time generating a hash code is infallible, but sometimes it is
+// necessary to generate hash codes on demand in a way that can fail. Specialize
+// this class for your own hash policy to provide fallible hashing.
+//
+// This is used by MovableCellHasher to handle the fact that generating a unique
+// ID for cell pointer may fail due to OOM.
+//
+// The default implementations of these methods delegate to the usual HashPolicy
+// implementation and always succeed.
+template <typename HashPolicy>
+struct FallibleHashMethods {
+  // Return true if a hashcode is already available for its argument, and
+  // sets |aHashOut|. Once this succeeds for a specific argument it
+  // must continue to do so.
+  //
+  // Return false if a hashcode is not already available. This implies that any
+  // lookup must fail, as the hash code would have to have been successfully
+  // created on insertion.
+  template <typename Lookup>
+  static bool maybeGetHash(Lookup&& aLookup, HashNumber* aHashOut) {
+    *aHashOut = HashPolicy::hash(aLookup);
+    return true;
+  }
+
+  // Fallible method to ensure a hashcode exists for its argument and create one
+  // if not. Sets |aHashOut| to the hashcode and retuns true on success. Returns
+  // false on error, e.g. out of memory.
+  template <typename Lookup>
+  static bool ensureHash(Lookup&& aLookup, HashNumber* aHashOut) {
+    *aHashOut = HashPolicy::hash(aLookup);
+    return true;
+  }
+};
+
+template <typename HashPolicy, typename Lookup>
+static bool MaybeGetHash(Lookup&& aLookup, HashNumber* aHashOut) {
+  return FallibleHashMethods<typename HashPolicy::Base>::maybeGetHash(
+      std::forward<Lookup>(aLookup), aHashOut);
+}
+
+template <typename HashPolicy, typename Lookup>
+static bool EnsureHash(Lookup&& aLookup, HashNumber* aHashOut) {
+  return FallibleHashMethods<typename HashPolicy::Base>::ensureHash(
+      std::forward<Lookup>(aLookup), aHashOut);
+}
+
+//---------------------------------------------------------------------------
+// Implementation Details (HashMapEntry, HashTableEntry, HashTable)
+//---------------------------------------------------------------------------
+
+// Both HashMap and HashSet are implemented by a single HashTable that is even
+// more heavily parameterized than the other two. This leaves HashTable gnarly
+// and extremely coupled to HashMap and HashSet; thus code should not use
+// HashTable directly.
+
+template <class Key, class Value>
+class HashMapEntry {
+  Key key_;
+  Value value_;
+
+  template <class, class, class>
+  friend class detail::HashTable;
+  template <class>
+  friend class detail::HashTableEntry;
+  template <class, class, class, class>
+  friend class HashMap;
+
+ public:
+  template <typename KeyInput, typename ValueInput>
+  HashMapEntry(KeyInput&& aKey, ValueInput&& aValue)
+      : key_(std::forward<KeyInput>(aKey)),
+        value_(std::forward<ValueInput>(aValue)) {}
+
+  HashMapEntry(HashMapEntry&& aRhs) = default;
+  HashMapEntry& operator=(HashMapEntry&& aRhs) = default;
+
+  using KeyType = Key;
+  using ValueType = Value;
+
+  const Key& key() const { return key_; }
+
+  // Use this method with caution! If the key is changed such that its hash
+  // value also changes, the map will be left in an invalid state.
+  Key& mutableKey() { return key_; }
+
+  const Value& value() const { return value_; }
+  Value& value() { return value_; }
+
+ private:
+  HashMapEntry(const HashMapEntry&) = delete;
+  void operator=(const HashMapEntry&) = delete;
+};
+
+namespace detail {
+
+template <class T, class HashPolicy, class AllocPolicy>
+class HashTable;
+
+template <typename T>
+class EntrySlot;
+
+template <typename T>
+class HashTableEntry {
+ private:
+  using NonConstT = std::remove_const_t<T>;
+
+  // Instead of having a hash table entry store that looks like this:
+  //
+  // +--------+--------+--------+--------+
+  // | entry0 | entry1 |  ....  | entryN |
+  // +--------+--------+--------+--------+
+  //
+  // where the entries contained their cached hash code, we're going to lay out
+  // the entry store thusly:
+  //
+  // +-------+-------+-------+-------+--------+--------+--------+--------+
+  // | hash0 | hash1 |  ...  | hashN | entry0 | entry1 |  ....  | entryN |
+  // +-------+-------+-------+-------+--------+--------+--------+--------+
+  //
+  // with all the cached hashes prior to the actual entries themselves.
+  //
+  // We do this because implementing the first strategy requires us to make
+  // HashTableEntry look roughly like:
+  //
+  // template <typename T>
+  // class HashTableEntry {
+  //   HashNumber mKeyHash;
+  //   T mValue;
+  // };
+  //
+  // The problem with this setup is that, depending on the layout of `T`, there
+  // may be platform ABI-mandated padding between `mKeyHash` and the first
+  // member of `T`. This ABI-mandated padding is wasted space, and can be
+  // surprisingly common, e.g. when `T` is a single pointer on 64-bit platforms.
+  // In such cases, we're throwing away a quarter of our entry store on padding,
+  // which is undesirable.
+  //
+  // The second layout above, namely:
+  //
+  // +-------+-------+-------+-------+--------+--------+--------+--------+
+  // | hash0 | hash1 |  ...  | hashN | entry0 | entry1 |  ....  | entryN |
+  // +-------+-------+-------+-------+--------+--------+--------+--------+
+  //
+  // means there is no wasted space between the hashes themselves, and no wasted
+  // space between the entries themselves.  However, we would also like there to
+  // be no gap between the last hash and the first entry. The memory allocator
+  // guarantees the alignment of the start of the hashes. The use of a
+  // power-of-two capacity of at least 4 guarantees that the alignment of the
+  // *end* of the hash array is no less than the alignment of the start.
+  // Finally, the static_asserts here guarantee that the entries themselves
+  // don't need to be any more aligned than the alignment of the entry store
+  // itself.
+  //
+  // This assertion is safe for 32-bit builds because on both Windows and Linux
+  // (including Android), the minimum alignment for allocations larger than 8
+  // bytes is 8 bytes, and the actual data for entries in our entry store is
+  // guaranteed to have that alignment as well, thanks to the power-of-two
+  // number of cached hash values stored prior to the entry data.
+
+  // The allocation policy must allocate a table with at least this much
+  // alignment.
+  static constexpr size_t kMinimumAlignment = 8;
+
+  static_assert(alignof(HashNumber) <= kMinimumAlignment,
+                "[N*2 hashes, N*2 T values] allocation's alignment must be "
+                "enough to align each hash");
+  static_assert(alignof(NonConstT) <= 2 * sizeof(HashNumber),
+                "subsequent N*2 T values must not require more than an even "
+                "number of HashNumbers provides");
+
+  static const HashNumber sFreeKey = 0;
+  static const HashNumber sRemovedKey = 1;
+  static const HashNumber sCollisionBit = 1;
+
+  alignas(NonConstT) unsigned char mValueData[sizeof(NonConstT)];
+
+ private:
+  template <class, class, class>
+  friend class HashTable;
+  template <typename>
+  friend class EntrySlot;
+
+  // Some versions of GCC treat it as a -Wstrict-aliasing violation (ergo a
+  // -Werror compile error) to reinterpret_cast<> |mValueData| to |T*|, even
+  // through |void*|.  Placing the latter cast in these separate functions
+  // breaks the chain such that affected GCC versions no longer warn/error.
+  void* rawValuePtr() { return mValueData; }
+
+  static bool isLiveHash(HashNumber hash) { return hash > sRemovedKey; }
+
+  HashTableEntry(const HashTableEntry&) = delete;
+  void operator=(const HashTableEntry&) = delete;
+
+  NonConstT* valuePtr() { return reinterpret_cast<NonConstT*>(rawValuePtr()); }
+
+  void destroyStoredT() {
+    NonConstT* ptr = valuePtr();
+    ptr->~T();
+    MOZ_MAKE_MEM_UNDEFINED(ptr, sizeof(*ptr));
+  }
+
+ public:
+  HashTableEntry() = default;
+
+  ~HashTableEntry() { MOZ_MAKE_MEM_UNDEFINED(this, sizeof(*this)); }
+
+  void destroy() { destroyStoredT(); }
+
+  void swap(HashTableEntry* aOther, bool aIsLive) {
+    // This allows types to use Argument-Dependent-Lookup, and thus use a custom
+    // std::swap, which is needed by types like JS::Heap and such.
+    using std::swap;
+
+    if (this == aOther) {
+      return;
+    }
+    if (aIsLive) {
+      swap(*valuePtr(), *aOther->valuePtr());
+    } else {
+      *aOther->valuePtr() = std::move(*valuePtr());
+      destroy();
+    }
+  }
+
+  T& get() { return *valuePtr(); }
+
+  NonConstT& getMutable() { return *valuePtr(); }
+};
+
+// A slot represents a cached hash value and its associated entry stored
+// in the hash table. These two things are not stored in contiguous memory.
+template <class T>
+class EntrySlot {
+  using NonConstT = std::remove_const_t<T>;
+
+  using Entry = HashTableEntry<T>;
+
+  Entry* mEntry;
+  HashNumber* mKeyHash;
+
+  template <class, class, class>
+  friend class HashTable;
+
+  EntrySlot(Entry* aEntry, HashNumber* aKeyHash)
+      : mEntry(aEntry), mKeyHash(aKeyHash) {}
+
+ public:
+  static bool isLiveHash(HashNumber hash) { return hash > Entry::sRemovedKey; }
+
+  EntrySlot(const EntrySlot&) = default;
+  EntrySlot(EntrySlot&& aOther) = default;
+
+  EntrySlot& operator=(const EntrySlot&) = default;
+  EntrySlot& operator=(EntrySlot&&) = default;
+
+  bool operator==(const EntrySlot& aRhs) const { return mEntry == aRhs.mEntry; }
+
+  bool operator<(const EntrySlot& aRhs) const { return mEntry < aRhs.mEntry; }
+
+  EntrySlot& operator++() {
+    ++mEntry;
+    ++mKeyHash;
+    return *this;
+  }
+
+  void destroy() { mEntry->destroy(); }
+
+  void swap(EntrySlot& aOther) {
+    mEntry->swap(aOther.mEntry, aOther.isLive());
+    std::swap(*mKeyHash, *aOther.mKeyHash);
+  }
+
+  T& get() const { return mEntry->get(); }
+
+  NonConstT& getMutable() { return mEntry->getMutable(); }
+
+  bool isFree() const { return *mKeyHash == Entry::sFreeKey; }
+
+  void clearLive() {
+    MOZ_ASSERT(isLive());
+    *mKeyHash = Entry::sFreeKey;
+    mEntry->destroyStoredT();
+  }
+
+  void clear() {
+    if (isLive()) {
+      mEntry->destroyStoredT();
+    }
+    MOZ_MAKE_MEM_UNDEFINED(mEntry, sizeof(*mEntry));
+    *mKeyHash = Entry::sFreeKey;
+  }
+
+  bool isRemoved() const { return *mKeyHash == Entry::sRemovedKey; }
+
+  void removeLive() {
+    MOZ_ASSERT(isLive());
+    *mKeyHash = Entry::sRemovedKey;
+    mEntry->destroyStoredT();
+  }
+
+  bool isLive() const { return isLiveHash(*mKeyHash); }
+
+  void setCollision() {
+    MOZ_ASSERT(isLive());
+    *mKeyHash |= Entry::sCollisionBit;
+  }
+  void unsetCollision() { *mKeyHash &= ~Entry::sCollisionBit; }
+  bool hasCollision() const { return *mKeyHash & Entry::sCollisionBit; }
+  bool matchHash(HashNumber hn) {
+    return (*mKeyHash & ~Entry::sCollisionBit) == hn;
+  }
+  HashNumber getKeyHash() const { return *mKeyHash & ~Entry::sCollisionBit; }
+
+  template <typename... Args>
+  void setLive(HashNumber aHashNumber, Args&&... aArgs) {
+    MOZ_ASSERT(!isLive());
+    *mKeyHash = aHashNumber;
+    new (KnownNotNull, mEntry->valuePtr()) T(std::forward<Args>(aArgs)...);
+    MOZ_ASSERT(isLive());
+  }
+
+  Entry* toEntry() const { return mEntry; }
+};
+
+template <class T, class HashPolicy, class AllocPolicy>
+class HashTable : private AllocPolicy {
+  friend class mozilla::ReentrancyGuard;
+
+  using NonConstT = std::remove_const_t<T>;
+  using Key = typename HashPolicy::KeyType;
+  using Lookup = typename HashPolicy::Lookup;
+
+ public:
+  using Entry = HashTableEntry<T>;
+  using Slot = EntrySlot<T>;
+
+  template <typename F>
+  static void forEachSlot(char* aTable, uint32_t aCapacity, F&& f) {
+    auto hashes = reinterpret_cast<HashNumber*>(aTable);
+    auto entries = reinterpret_cast<Entry*>(&hashes[aCapacity]);
+    Slot slot(entries, hashes);
+    for (size_t i = 0; i < size_t(aCapacity); ++i) {
+      f(slot);
+      ++slot;
+    }
+  }
+
+  // A nullable pointer to a hash table element. A Ptr |p| can be tested
+  // either explicitly |if (p.found()) p->...| or using boolean conversion
+  // |if (p) p->...|. Ptr objects must not be used after any mutating hash
+  // table operations unless |generation()| is tested.
+  class Ptr {
+    friend class HashTable;
+
+    Slot mSlot;
+#ifdef DEBUG
+    const HashTable* mTable;
+    Generation mGeneration;
+#endif
+
+   protected:
+    Ptr(Slot aSlot, const HashTable& aTable)
+        : mSlot(aSlot)
+#ifdef DEBUG
+          ,
+          mTable(&aTable),
+          mGeneration(aTable.generation())
+#endif
+    {
+    }
+
+    // This constructor is used only by AddPtr() within lookupForAdd().
+    explicit Ptr(const HashTable& aTable)
+        : mSlot(nullptr, nullptr)
+#ifdef DEBUG
+          ,
+          mTable(&aTable),
+          mGeneration(aTable.generation())
+#endif
+    {
+    }
+
+    bool isValid() const { return !!mSlot.toEntry(); }
+
+   public:
+    Ptr()
+        : mSlot(nullptr, nullptr)
+#ifdef DEBUG
+          ,
+          mTable(nullptr),
+          mGeneration(0)
+#endif
+    {
+    }
+
+    bool found() const {
+      if (!isValid()) {
+        return false;
+      }
+#ifdef DEBUG
+      MOZ_ASSERT(mGeneration == mTable->generation());
+#endif
+      return mSlot.isLive();
+    }
+
+    explicit operator bool() const { return found(); }
+
+    bool operator==(const Ptr& aRhs) const {
+      MOZ_ASSERT(found() && aRhs.found());
+      return mSlot == aRhs.mSlot;
+    }
+
+    bool operator!=(const Ptr& aRhs) const {
+#ifdef DEBUG
+      MOZ_ASSERT(mGeneration == mTable->generation());
+#endif
+      return !(*this == aRhs);
+    }
+
+    T& operator*() const {
+#ifdef DEBUG
+      MOZ_ASSERT(found());
+      MOZ_ASSERT(mGeneration == mTable->generation());
+#endif
+      return mSlot.get();
+    }
+
+    T* operator->() const {
+#ifdef DEBUG
+      MOZ_ASSERT(found());
+      MOZ_ASSERT(mGeneration == mTable->generation());
+#endif
+      return &mSlot.get();
+    }
+  };
+
+  // A Ptr that can be used to add a key after a failed lookup.
+  class AddPtr : public Ptr {
+    friend class HashTable;
+
+    HashNumber mKeyHash;
+#ifdef DEBUG
+    uint64_t mMutationCount;
+#endif
+
+    AddPtr(Slot aSlot, const HashTable& aTable, HashNumber aHashNumber)
+        : Ptr(aSlot, aTable),
+          mKeyHash(aHashNumber)
+#ifdef DEBUG
+          ,
+          mMutationCount(aTable.mMutationCount)
+#endif
+    {
+    }
+
+    // This constructor is used when lookupForAdd() is performed on a table
+    // lacking entry storage; it leaves mSlot null but initializes everything
+    // else.
+    AddPtr(const HashTable& aTable, HashNumber aHashNumber)
+        : Ptr(aTable),
+          mKeyHash(aHashNumber)
+#ifdef DEBUG
+          ,
+          mMutationCount(aTable.mMutationCount)
+#endif
+    {
+      MOZ_ASSERT(isLive());
+    }
+
+    bool isLive() const { return isLiveHash(mKeyHash); }
+
+   public:
+    AddPtr() : mKeyHash(0) {}
+  };
+
+  // A hash table iterator that (mostly) doesn't allow table modifications.
+  // As with Ptr/AddPtr, Iterator objects must not be used after any mutating
+  // hash table operation unless the |generation()| is tested.
+  class Iterator {
+    void moveToNextLiveEntry() {
+      while (++mCur < mEnd && !mCur.isLive()) {
+        continue;
+      }
+    }
+
+   protected:
+    friend class HashTable;
+
+    explicit Iterator(const HashTable& aTable)
+        : mCur(aTable.slotForIndex(0)),
+          mEnd(aTable.slotForIndex(aTable.capacity()))
+#ifdef DEBUG
+          ,
+          mTable(aTable),
+          mMutationCount(aTable.mMutationCount),
+          mGeneration(aTable.generation()),
+          mValidEntry(true)
+#endif
+    {
+      if (!done() && !mCur.isLive()) {
+        moveToNextLiveEntry();
+      }
+    }
+
+    Slot mCur;
+    Slot mEnd;
+#ifdef DEBUG
+    const HashTable& mTable;
+    uint64_t mMutationCount;
+    Generation mGeneration;
+    bool mValidEntry;
+#endif
+
+   public:
+    bool done() const {
+      MOZ_ASSERT(mGeneration == mTable.generation());
+      MOZ_ASSERT(mMutationCount == mTable.mMutationCount);
+      return mCur == mEnd;
+    }
+
+    T& get() const {
+      MOZ_ASSERT(!done());
+      MOZ_ASSERT(mValidEntry);
+      MOZ_ASSERT(mGeneration == mTable.generation());
+      MOZ_ASSERT(mMutationCount == mTable.mMutationCount);
+      return mCur.get();
+    }
+
+    void next() {
+      MOZ_ASSERT(!done());
+      MOZ_ASSERT(mGeneration == mTable.generation());
+      MOZ_ASSERT(mMutationCount == mTable.mMutationCount);
+      moveToNextLiveEntry();
+#ifdef DEBUG
+      mValidEntry = true;
+#endif
+    }
+  };
+
+  // A hash table iterator that permits modification, removal and rekeying.
+  // Since rehashing when elements were removed during enumeration would be
+  // bad, it is postponed until the ModIterator is destructed. Since the
+  // ModIterator's destructor touches the hash table, the user must ensure
+  // that the hash table is still alive when the destructor runs.
+  class ModIterator : public Iterator {
+    friend class HashTable;
+
+    HashTable& mTable;
+    bool mRekeyed;
+    bool mRemoved;
+
+    // ModIterator is movable but not copyable.
+    ModIterator(const ModIterator&) = delete;
+    void operator=(const ModIterator&) = delete;
+
+   protected:
+    explicit ModIterator(HashTable& aTable)
+        : Iterator(aTable), mTable(aTable), mRekeyed(false), mRemoved(false) {}
+
+   public:
+    MOZ_IMPLICIT ModIterator(ModIterator&& aOther)
+        : Iterator(aOther),
+          mTable(aOther.mTable),
+          mRekeyed(aOther.mRekeyed),
+          mRemoved(aOther.mRemoved) {
+      aOther.mRekeyed = false;
+      aOther.mRemoved = false;
+    }
+
+    // Removes the current element from the table, leaving |get()|
+    // invalid until the next call to |next()|.
+    void remove() {
+      mTable.remove(this->mCur);
+      mRemoved = true;
+#ifdef DEBUG
+      this->mValidEntry = false;
+      this->mMutationCount = mTable.mMutationCount;
+#endif
+    }
+
+    NonConstT& getMutable() {
+      MOZ_ASSERT(!this->done());
+      MOZ_ASSERT(this->mValidEntry);
+      MOZ_ASSERT(this->mGeneration == this->Iterator::mTable.generation());
+      MOZ_ASSERT(this->mMutationCount == this->Iterator::mTable.mMutationCount);
+      return this->mCur.getMutable();
+    }
+
+    // Removes the current element and re-inserts it into the table with
+    // a new key at the new Lookup position.  |get()| is invalid after
+    // this operation until the next call to |next()|.
+    void rekey(const Lookup& l, const Key& k) {
+      MOZ_ASSERT(&k != &HashPolicy::getKey(this->mCur.get()));
+      Ptr p(this->mCur, mTable);
+      mTable.rekeyWithoutRehash(p, l, k);
+      mRekeyed = true;
+#ifdef DEBUG
+      this->mValidEntry = false;
+      this->mMutationCount = mTable.mMutationCount;
+#endif
+    }
+
+    void rekey(const Key& k) { rekey(k, k); }
+
+    // Potentially rehashes the table.
+    ~ModIterator() {
+      if (mRekeyed) {
+        mTable.mGen++;
+        mTable.infallibleRehashIfOverloaded();
+      }
+
+      if (mRemoved) {
+        mTable.compact();
+      }
+    }
+  };
+
+  // Range is similar to Iterator, but uses different terminology.
+  class Range {
+    friend class HashTable;
+
+    Iterator mIter;
+
+   protected:
+    explicit Range(const HashTable& table) : mIter(table) {}
+
+   public:
+    bool empty() const { return mIter.done(); }
+
+    T& front() const { return mIter.get(); }
+
+    void popFront() { return mIter.next(); }
+  };
+
+  // Enum is similar to ModIterator, but uses different terminology.
+  class Enum {
+    ModIterator mIter;
+
+    // Enum is movable but not copyable.
+    Enum(const Enum&) = delete;
+    void operator=(const Enum&) = delete;
+
+   public:
+    template <class Map>
+    explicit Enum(Map& map) : mIter(map.mImpl) {}
+
+    MOZ_IMPLICIT Enum(Enum&& other) : mIter(std::move(other.mIter)) {}
+
+    bool empty() const { return mIter.done(); }
+
+    T& front() const { return mIter.get(); }
+
+    void popFront() { return mIter.next(); }
+
+    void removeFront() { mIter.remove(); }
+
+    NonConstT& mutableFront() { return mIter.getMutable(); }
+
+    void rekeyFront(const Lookup& aLookup, const Key& aKey) {
+      mIter.rekey(aLookup, aKey);
+    }
+
+    void rekeyFront(const Key& aKey) { mIter.rekey(aKey); }
+  };
+
+  // HashTable is movable
+  HashTable(HashTable&& aRhs) : AllocPolicy(std::move(aRhs)) { moveFrom(aRhs); }
+  HashTable& operator=(HashTable&& aRhs) {
+    MOZ_ASSERT(this != &aRhs, "self-move assignment is prohibited");
+    if (mTable) {
+      destroyTable(*this, mTable, capacity());
+    }
+    AllocPolicy::operator=(std::move(aRhs));
+    moveFrom(aRhs);
+    return *this;
+  }
+
+ private:
+  void moveFrom(HashTable& aRhs) {
+    mGen = aRhs.mGen;
+    mHashShift = aRhs.mHashShift;
+    mTable = aRhs.mTable;
+    mEntryCount = aRhs.mEntryCount;
+    mRemovedCount = aRhs.mRemovedCount;
+#ifdef DEBUG
+    mMutationCount = aRhs.mMutationCount;
+    mEntered = aRhs.mEntered;
+#endif
+    aRhs.mTable = nullptr;
+    aRhs.clearAndCompact();
+  }
+
+  // HashTable is not copyable or assignable
+  HashTable(const HashTable&) = delete;
+  void operator=(const HashTable&) = delete;
+
+  static const uint32_t CAP_BITS = 30;
+
+ public:
+  uint64_t mGen : 56;       // entry storage generation number
+  uint64_t mHashShift : 8;  // multiplicative hash shift
+  char* mTable;             // entry storage
+  uint32_t mEntryCount;     // number of entries in mTable
+  uint32_t mRemovedCount;   // removed entry sentinels in mTable
+
+#ifdef DEBUG
+  uint64_t mMutationCount;
+  mutable bool mEntered;
+#endif
+
+  // The default initial capacity is 32 (enough to hold 16 elements), but it
+  // can be as low as 4.
+  static const uint32_t sDefaultLen = 16;
+  static const uint32_t sMinCapacity = 4;
+  // See the comments in HashTableEntry about this value.
+  static_assert(sMinCapacity >= 4, "too-small sMinCapacity breaks assumptions");
+  static const uint32_t sMaxInit = 1u << (CAP_BITS - 1);
+  static const uint32_t sMaxCapacity = 1u << CAP_BITS;
+
+  // Hash-table alpha is conceptually a fraction, but to avoid floating-point
+  // math we implement it as a ratio of integers.
+  static const uint8_t sAlphaDenominator = 4;
+  static const uint8_t sMinAlphaNumerator = 1;  // min alpha: 1/4
+  static const uint8_t sMaxAlphaNumerator = 3;  // max alpha: 3/4
+
+  static const HashNumber sFreeKey = Entry::sFreeKey;
+  static const HashNumber sRemovedKey = Entry::sRemovedKey;
+  static const HashNumber sCollisionBit = Entry::sCollisionBit;
+
+  static uint32_t bestCapacity(uint32_t aLen) {
+    static_assert(
+        (sMaxInit * sAlphaDenominator) / sAlphaDenominator == sMaxInit,
+        "multiplication in numerator below could overflow");
+    static_assert(
+        sMaxInit * sAlphaDenominator <= UINT32_MAX - sMaxAlphaNumerator,
+        "numerator calculation below could potentially overflow");
+
+    // Callers should ensure this is true.
+    MOZ_ASSERT(aLen <= sMaxInit);
+
+    // Compute the smallest capacity allowing |aLen| elements to be
+    // inserted without rehashing: ceil(aLen / max-alpha).  (Ceiling
+    // integral division: <http://stackoverflow.com/a/2745086>.)
+    uint32_t capacity = (aLen * sAlphaDenominator + sMaxAlphaNumerator - 1) /
+                        sMaxAlphaNumerator;
+    capacity = (capacity < sMinCapacity) ? sMinCapacity : RoundUpPow2(capacity);
+
+    MOZ_ASSERT(capacity >= aLen);
+    MOZ_ASSERT(capacity <= sMaxCapacity);
+
+    return capacity;
+  }
+
+  static uint32_t hashShift(uint32_t aLen) {
+    // Reject all lengths whose initial computed capacity would exceed
+    // sMaxCapacity. Round that maximum aLen down to the nearest power of two
+    // for speedier code.
+    if (MOZ_UNLIKELY(aLen > sMaxInit)) {
+      MOZ_CRASH("initial length is too large");
+    }
+
+    return kHashNumberBits - mozilla::CeilingLog2(bestCapacity(aLen));
+  }
+
+  static bool isLiveHash(HashNumber aHash) { return Entry::isLiveHash(aHash); }
+
+  static HashNumber prepareHash(HashNumber aInputHash) {
+    HashNumber keyHash = ScrambleHashCode(aInputHash);
+
+    // Avoid reserved hash codes.
+    if (!isLiveHash(keyHash)) {
+      keyHash -= (sRemovedKey + 1);
+    }
+    return keyHash & ~sCollisionBit;
+  }
+
+  enum FailureBehavior { DontReportFailure = false, ReportFailure = true };
+
+  // Fake a struct that we're going to alloc. See the comments in
+  // HashTableEntry about how the table is laid out, and why it's safe.
+  struct FakeSlot {
+    unsigned char c[sizeof(HashNumber) + sizeof(typename Entry::NonConstT)];
+  };
+
+  static char* createTable(AllocPolicy& aAllocPolicy, uint32_t aCapacity,
+                           FailureBehavior aReportFailure = ReportFailure) {
+    FakeSlot* fake =
+        aReportFailure
+            ? aAllocPolicy.template pod_malloc<FakeSlot>(aCapacity)
+            : aAllocPolicy.template maybe_pod_malloc<FakeSlot>(aCapacity);
+
+    MOZ_ASSERT((reinterpret_cast<uintptr_t>(fake) % Entry::kMinimumAlignment) ==
+               0);
+
+    char* table = reinterpret_cast<char*>(fake);
+    if (table) {
+      forEachSlot(table, aCapacity, [&](Slot& slot) {
+        *slot.mKeyHash = sFreeKey;
+        new (KnownNotNull, slot.toEntry()) Entry();
+      });
+    }
+    return table;
+  }
+
+  static void destroyTable(AllocPolicy& aAllocPolicy, char* aOldTable,
+                           uint32_t aCapacity) {
+    forEachSlot(aOldTable, aCapacity, [&](const Slot& slot) {
+      if (slot.isLive()) {
+        slot.toEntry()->destroyStoredT();
+      }
+    });
+    freeTable(aAllocPolicy, aOldTable, aCapacity);
+  }
+
+  static void freeTable(AllocPolicy& aAllocPolicy, char* aOldTable,
+                        uint32_t aCapacity) {
+    FakeSlot* fake = reinterpret_cast<FakeSlot*>(aOldTable);
+    aAllocPolicy.free_(fake, aCapacity);
+  }
+
+ public:
+  HashTable(AllocPolicy aAllocPolicy, uint32_t aLen)
+      : AllocPolicy(std::move(aAllocPolicy)),
+        mGen(0),
+        mHashShift(hashShift(aLen)),
+        mTable(nullptr),
+        mEntryCount(0),
+        mRemovedCount(0)
+#ifdef DEBUG
+        ,
+        mMutationCount(0),
+        mEntered(false)
+#endif
+  {
+  }
+
+  explicit HashTable(AllocPolicy aAllocPolicy)
+      : HashTable(aAllocPolicy, sDefaultLen) {}
+
+  ~HashTable() {
+    if (mTable) {
+      destroyTable(*this, mTable, capacity());
+    }
+  }
+
+ private:
+  HashNumber hash1(HashNumber aHash0) const { return aHash0 >> mHashShift; }
+
+  struct DoubleHash {
+    HashNumber mHash2;
+    HashNumber mSizeMask;
+  };
+
+  DoubleHash hash2(HashNumber aCurKeyHash) const {
+    uint32_t sizeLog2 = kHashNumberBits - mHashShift;
+    DoubleHash dh = {((aCurKeyHash << sizeLog2) >> mHashShift) | 1,
+                     (HashNumber(1) << sizeLog2) - 1};
+    return dh;
+  }
+
+  static HashNumber applyDoubleHash(HashNumber aHash1,
+                                    const DoubleHash& aDoubleHash) {
+    return WrappingSubtract(aHash1, aDoubleHash.mHash2) & aDoubleHash.mSizeMask;
+  }
+
+  static MOZ_ALWAYS_INLINE bool match(T& aEntry, const Lookup& aLookup) {
+    return HashPolicy::match(HashPolicy::getKey(aEntry), aLookup);
+  }
+
+  enum LookupReason { ForNonAdd, ForAdd };
+
+  Slot slotForIndex(HashNumber aIndex) const {
+    auto hashes = reinterpret_cast<HashNumber*>(mTable);
+    auto entries = reinterpret_cast<Entry*>(&hashes[capacity()]);
+    return Slot(&entries[aIndex], &hashes[aIndex]);
+  }
+
+  // Warning: in order for readonlyThreadsafeLookup() to be safe this
+  // function must not modify the table in any way when Reason==ForNonAdd.
+  template <LookupReason Reason>
+  MOZ_ALWAYS_INLINE Slot lookup(const Lookup& aLookup,
+                                HashNumber aKeyHash) const {
+    MOZ_ASSERT(isLiveHash(aKeyHash));
+    MOZ_ASSERT(!(aKeyHash & sCollisionBit));
+    MOZ_ASSERT(mTable);
+
+    // Compute the primary hash address.
+    HashNumber h1 = hash1(aKeyHash);
+    Slot slot = slotForIndex(h1);
+
+    // Miss: return space for a new entry.
+    if (slot.isFree()) {
+      return slot;
+    }
+
+    // Hit: return entry.
+    if (slot.matchHash(aKeyHash) && match(slot.get(), aLookup)) {
+      return slot;
+    }
+
+    // Collision: double hash.
+    DoubleHash dh = hash2(aKeyHash);
+
+    // Save the first removed entry pointer so we can recycle later.
+    Maybe<Slot> firstRemoved;
+
+    while (true) {
+      if (Reason == ForAdd && !firstRemoved) {
+        if (MOZ_UNLIKELY(slot.isRemoved())) {
+          firstRemoved.emplace(slot);
+        } else {
+          slot.setCollision();
+        }
+      }
+
+      h1 = applyDoubleHash(h1, dh);
+
+      slot = slotForIndex(h1);
+      if (slot.isFree()) {
+        return firstRemoved.refOr(slot);
+      }
+
+      if (slot.matchHash(aKeyHash) && match(slot.get(), aLookup)) {
+        return slot;
+      }
+    }
+  }
+
+  // This is a copy of lookup() hardcoded to the assumptions:
+  //   1. the lookup is for an add;
+  //   2. the key, whose |keyHash| has been passed, is not in the table.
+  Slot findNonLiveSlot(HashNumber aKeyHash) {
+    MOZ_ASSERT(!(aKeyHash & sCollisionBit));
+    MOZ_ASSERT(mTable);
+
+    // We assume 'aKeyHash' has already been distributed.
+
+    // Compute the primary hash address.
+    HashNumber h1 = hash1(aKeyHash);
+    Slot slot = slotForIndex(h1);
+
+    // Miss: return space for a new entry.
+    if (!slot.isLive()) {
+      return slot;
+    }
+
+    // Collision: double hash.
+    DoubleHash dh = hash2(aKeyHash);
+
+    while (true) {
+      slot.setCollision();
+
+      h1 = applyDoubleHash(h1, dh);
+
+      slot = slotForIndex(h1);
+      if (!slot.isLive()) {
+        return slot;
+      }
+    }
+  }
+
+  enum RebuildStatus { NotOverloaded, Rehashed, RehashFailed };
+
+  RebuildStatus changeTableSize(
+      uint32_t newCapacity, FailureBehavior aReportFailure = ReportFailure) {
+    MOZ_ASSERT(IsPowerOfTwo(newCapacity));
+    MOZ_ASSERT(!!mTable == !!capacity());
+
+    // Look, but don't touch, until we succeed in getting new entry store.
+    char* oldTable = mTable;
+    uint32_t oldCapacity = capacity();
+    uint32_t newLog2 = mozilla::CeilingLog2(newCapacity);
+
+    if (MOZ_UNLIKELY(newCapacity > sMaxCapacity)) {
+      if (aReportFailure) {
+        this->reportAllocOverflow();
+      }
+      return RehashFailed;
+    }
+
+    char* newTable = createTable(*this, newCapacity, aReportFailure);
+    if (!newTable) {
+      return RehashFailed;
+    }
+
+    // We can't fail from here on, so update table parameters.
+    mHashShift = kHashNumberBits - newLog2;
+    mRemovedCount = 0;
+    mGen++;
+    mTable = newTable;
+
+    // Copy only live entries, leaving removed ones behind.
+    forEachSlot(oldTable, oldCapacity, [&](Slot& slot) {
+      if (slot.isLive()) {
+        HashNumber hn = slot.getKeyHash();
+        findNonLiveSlot(hn).setLive(
+            hn, std::move(const_cast<typename Entry::NonConstT&>(slot.get())));
+      }
+
+      slot.clear();
+    });
+
+    // All entries have been destroyed, no need to destroyTable.
+    freeTable(*this, oldTable, oldCapacity);
+    return Rehashed;
+  }
+
+  RebuildStatus rehashIfOverloaded(
+      FailureBehavior aReportFailure = ReportFailure) {
+    static_assert(sMaxCapacity <= UINT32_MAX / sMaxAlphaNumerator,
+                  "multiplication below could overflow");
+
+    // Note: if capacity() is zero, this will always succeed, which is
+    // what we want.
+    bool overloaded = mEntryCount + mRemovedCount >=
+                      capacity() * sMaxAlphaNumerator / sAlphaDenominator;
+
+    if (!overloaded) {
+      return NotOverloaded;
+    }
+
+    // Succeed if a quarter or more of all entries are removed. Note that this
+    // always succeeds if capacity() == 0 (i.e. entry storage has not been
+    // allocated), which is what we want, because it means changeTableSize()
+    // will allocate the requested capacity rather than doubling it.
+    bool manyRemoved = mRemovedCount >= (capacity() >> 2);
+    uint32_t newCapacity = manyRemoved ? rawCapacity() : rawCapacity() * 2;
+    return changeTableSize(newCapacity, aReportFailure);
+  }
+
+  void infallibleRehashIfOverloaded() {
+    if (rehashIfOverloaded(DontReportFailure) == RehashFailed) {
+      rehashTableInPlace();
+    }
+  }
+
+  void remove(Slot& aSlot) {
+    MOZ_ASSERT(mTable);
+
+    if (aSlot.hasCollision()) {
+      aSlot.removeLive();
+      mRemovedCount++;
+    } else {
+      aSlot.clearLive();
+    }
+    mEntryCount--;
+#ifdef DEBUG
+    mMutationCount++;
+#endif
+  }
+
+  void shrinkIfUnderloaded() {
+    static_assert(sMaxCapacity <= UINT32_MAX / sMinAlphaNumerator,
+                  "multiplication below could overflow");
+    bool underloaded =
+        capacity() > sMinCapacity &&
+        mEntryCount <= capacity() * sMinAlphaNumerator / sAlphaDenominator;
+
+    if (underloaded) {
+      (void)changeTableSize(capacity() / 2, DontReportFailure);
+    }
+  }
+
+  // This is identical to changeTableSize(currentSize), but without requiring
+  // a second table.  We do this by recycling the collision bits to tell us if
+  // the element is already inserted or still waiting to be inserted.  Since
+  // already-inserted elements win any conflicts, we get the same table as we
+  // would have gotten through random insertion order.
+  void rehashTableInPlace() {
+    mRemovedCount = 0;
+    mGen++;
+    forEachSlot(mTable, capacity(), [&](Slot& slot) { slot.unsetCollision(); });
+    for (uint32_t i = 0; i < capacity();) {
+      Slot src = slotForIndex(i);
+
+      if (!src.isLive() || src.hasCollision()) {
+        ++i;
+        continue;
+      }
+
+      HashNumber keyHash = src.getKeyHash();
+      HashNumber h1 = hash1(keyHash);
+      DoubleHash dh = hash2(keyHash);
+      Slot tgt = slotForIndex(h1);
+      while (true) {
+        if (!tgt.hasCollision()) {
+          src.swap(tgt);
+          tgt.setCollision();
+          break;
+        }
+
+        h1 = applyDoubleHash(h1, dh);
+        tgt = slotForIndex(h1);
+      }
+    }
+
+    // TODO: this algorithm leaves collision bits on *all* elements, even if
+    // they are on no collision path. We have the option of setting the
+    // collision bits correctly on a subsequent pass or skipping the rehash
+    // unless we are totally filled with tombstones: benchmark to find out
+    // which approach is best.
+  }
+
+  // Prefer to use putNewInfallible; this function does not check
+  // invariants.
+  template <typename... Args>
+  void putNewInfallibleInternal(HashNumber aKeyHash, Args&&... aArgs) {
+    MOZ_ASSERT(mTable);
+
+    Slot slot = findNonLiveSlot(aKeyHash);
+
+    if (slot.isRemoved()) {
+      mRemovedCount--;
+      aKeyHash |= sCollisionBit;
+    }
+
+    slot.setLive(aKeyHash, std::forward<Args>(aArgs)...);
+    mEntryCount++;
+#ifdef DEBUG
+    mMutationCount++;
+#endif
+  }
+
+ public:
+  void clear() {
+    forEachSlot(mTable, capacity(), [&](Slot& slot) { slot.clear(); });
+    mRemovedCount = 0;
+    mEntryCount = 0;
+#ifdef DEBUG
+    mMutationCount++;
+#endif
+  }
+
+  // Resize the table down to the smallest capacity that doesn't overload the
+  // table. Since we call shrinkIfUnderloaded() on every remove, you only need
+  // to call this after a bulk removal of items done without calling remove().
+  void compact() {
+    if (empty()) {
+      // Free the entry storage.
+      freeTable(*this, mTable, capacity());
+      mGen++;
+      mHashShift = hashShift(0);  // gives minimum capacity on regrowth
+      mTable = nullptr;
+      mRemovedCount = 0;
+      return;
+    }
+
+    uint32_t bestCapacity = this->bestCapacity(mEntryCount);
+    MOZ_ASSERT(bestCapacity <= capacity());
+
+    if (bestCapacity < capacity()) {
+      (void)changeTableSize(bestCapacity, DontReportFailure);
+    }
+  }
+
+  void clearAndCompact() {
+    clear();
+    compact();
+  }
+
+  [[nodiscard]] bool reserve(uint32_t aLen) {
+    if (aLen == 0) {
+      return true;
+    }
+
+    if (MOZ_UNLIKELY(aLen > sMaxInit)) {
+      this->reportAllocOverflow();
+      return false;
+    }
+
+    uint32_t bestCapacity = this->bestCapacity(aLen);
+    if (bestCapacity <= capacity()) {
+      return true;  // Capacity is already sufficient.
+    }
+
+    RebuildStatus status = changeTableSize(bestCapacity, ReportFailure);
+    MOZ_ASSERT(status != NotOverloaded);
+    return status != RehashFailed;
+  }
+
+  Iterator iter() const { return Iterator(*this); }
+
+  ModIterator modIter() { return ModIterator(*this); }
+
+  Range all() const { return Range(*this); }
+
+  bool empty() const { return mEntryCount == 0; }
+
+  uint32_t count() const { return mEntryCount; }
+
+  uint32_t rawCapacity() const { return 1u << (kHashNumberBits - mHashShift); }
+
+  uint32_t capacity() const { return mTable ? rawCapacity() : 0; }
+
+  Generation generation() const { return Generation(mGen); }
+
+  size_t shallowSizeOfExcludingThis(MallocSizeOf aMallocSizeOf) const {
+    return aMallocSizeOf(mTable);
+  }
+
+  size_t shallowSizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const {
+    return aMallocSizeOf(this) + shallowSizeOfExcludingThis(aMallocSizeOf);
+  }
+
+  MOZ_ALWAYS_INLINE Ptr readonlyThreadsafeLookup(const Lookup& aLookup) const {
+    if (empty()) {
+      return Ptr();
+    }
+
+    HashNumber inputHash;
+    if (!MaybeGetHash<HashPolicy>(aLookup, &inputHash)) {
+      return Ptr();
+    }
+
+    HashNumber keyHash = prepareHash(inputHash);
+    return Ptr(lookup<ForNonAdd>(aLookup, keyHash), *this);
+  }
+
+  MOZ_ALWAYS_INLINE Ptr lookup(const Lookup& aLookup) const {
+    ReentrancyGuard g(*this);
+    return readonlyThreadsafeLookup(aLookup);
+  }
+
+  MOZ_ALWAYS_INLINE AddPtr lookupForAdd(const Lookup& aLookup) {
+    ReentrancyGuard g(*this);
+
+    HashNumber inputHash;
+    if (!EnsureHash<HashPolicy>(aLookup, &inputHash)) {
+      return AddPtr();
+    }
+
+    HashNumber keyHash = prepareHash(inputHash);
+
+    if (!mTable) {
+      return AddPtr(*this, keyHash);
+    }
+
+    // Directly call the constructor in the return statement to avoid
+    // excess copying when building with Visual Studio 2017.
+    // See bug 1385181.
+    return AddPtr(lookup<ForAdd>(aLookup, keyHash), *this, keyHash);
+  }
+
+  template <typename... Args>
+  [[nodiscard]] bool add(AddPtr& aPtr, Args&&... aArgs) {
+    ReentrancyGuard g(*this);
+    MOZ_ASSERT_IF(aPtr.isValid(), mTable);
+    MOZ_ASSERT_IF(aPtr.isValid(), aPtr.mTable == this);
+    MOZ_ASSERT(!aPtr.found());
+    MOZ_ASSERT(!(aPtr.mKeyHash & sCollisionBit));
+
+    // Check for error from ensureHash() here.
+    if (!aPtr.isLive()) {
+      return false;
+    }
+
+    MOZ_ASSERT(aPtr.mGeneration == generation());
+#ifdef DEBUG
+    MOZ_ASSERT(aPtr.mMutationCount == mMutationCount);
+#endif
+
+    if (!aPtr.isValid()) {
+      MOZ_ASSERT(!mTable && mEntryCount == 0);
+      uint32_t newCapacity = rawCapacity();
+      RebuildStatus status = changeTableSize(newCapacity, ReportFailure);
+      MOZ_ASSERT(status != NotOverloaded);
+      if (status == RehashFailed) {
+        return false;
+      }
+      aPtr.mSlot = findNonLiveSlot(aPtr.mKeyHash);
+
+    } else if (aPtr.mSlot.isRemoved()) {
+      // Changing an entry from removed to live does not affect whether we are
+      // overloaded and can be handled separately.
+      if (!this->checkSimulatedOOM()) {
+        return false;
+      }
+      mRemovedCount--;
+      aPtr.mKeyHash |= sCollisionBit;
+
+    } else {
+      // Preserve the validity of |aPtr.mSlot|.
+      RebuildStatus status = rehashIfOverloaded();
+      if (status == RehashFailed) {
+        return false;
+      }
+      if (status == NotOverloaded && !this->checkSimulatedOOM()) {
+        return false;
+      }
+      if (status == Rehashed) {
+        aPtr.mSlot = findNonLiveSlot(aPtr.mKeyHash);
+      }
+    }
+
+    aPtr.mSlot.setLive(aPtr.mKeyHash, std::forward<Args>(aArgs)...);
+    mEntryCount++;
+#ifdef DEBUG
+    mMutationCount++;
+    aPtr.mGeneration = generation();
+    aPtr.mMutationCount = mMutationCount;
+#endif
+    return true;
+  }
+
+  // Note: |aLookup| may reference pieces of arguments in |aArgs|, so this
+  // function must take care not to use |aLookup| after moving |aArgs|.
+  template <typename... Args>
+  void putNewInfallible(const Lookup& aLookup, Args&&... aArgs) {
+    MOZ_ASSERT(!lookup(aLookup).found());
+    ReentrancyGuard g(*this);
+    HashNumber keyHash = prepareHash(HashPolicy::hash(aLookup));
+    putNewInfallibleInternal(keyHash, std::forward<Args>(aArgs)...);
+  }
+
+  // Note: |aLookup| may alias arguments in |aArgs|, so this function must take
+  // care not to use |aLookup| after moving |aArgs|.
+  template <typename... Args>
+  [[nodiscard]] bool putNew(const Lookup& aLookup, Args&&... aArgs) {
+    MOZ_ASSERT(!lookup(aLookup).found());
+    ReentrancyGuard g(*this);
+    if (!this->checkSimulatedOOM()) {
+      return false;
+    }
+    HashNumber inputHash;
+    if (!EnsureHash<HashPolicy>(aLookup, &inputHash)) {
+      return false;
+    }
+    HashNumber keyHash = prepareHash(inputHash);
+    if (rehashIfOverloaded() == RehashFailed) {
+      return false;
+    }
+    putNewInfallibleInternal(keyHash, std::forward<Args>(aArgs)...);
+    return true;
+  }
+
+  // Note: |aLookup| may be a reference pieces of arguments in |aArgs|, so this
+  // function must take care not to use |aLookup| after moving |aArgs|.
+  template <typename... Args>
+  [[nodiscard]] bool relookupOrAdd(AddPtr& aPtr, const Lookup& aLookup,
+                                   Args&&... aArgs) {
+    // Check for error from ensureHash() here.
+    if (!aPtr.isLive()) {
+      return false;
+    }
+#ifdef DEBUG
+    aPtr.mGeneration = generation();
+    aPtr.mMutationCount = mMutationCount;
+#endif
+    if (mTable) {
+      ReentrancyGuard g(*this);
+      // Check that aLookup has not been destroyed.
+      MOZ_ASSERT(prepareHash(HashPolicy::hash(aLookup)) == aPtr.mKeyHash);
+      aPtr.mSlot = lookup<ForAdd>(aLookup, aPtr.mKeyHash);
+      if (aPtr.found()) {
+        return true;
+      }
+    } else {
+      // Clear aPtr so it's invalid; add() will allocate storage and redo the
+      // lookup.
+      aPtr.mSlot = Slot(nullptr, nullptr);
+    }
+    return add(aPtr, std::forward<Args>(aArgs)...);
+  }
+
+  void remove(Ptr aPtr) {
+    MOZ_ASSERT(mTable);
+    ReentrancyGuard g(*this);
+    MOZ_ASSERT(aPtr.found());
+    MOZ_ASSERT(aPtr.mGeneration == generation());
+    remove(aPtr.mSlot);
+    shrinkIfUnderloaded();
+  }
+
+  void rekeyWithoutRehash(Ptr aPtr, const Lookup& aLookup, const Key& aKey) {
+    MOZ_ASSERT(mTable);
+    ReentrancyGuard g(*this);
+    MOZ_ASSERT(aPtr.found());
+    MOZ_ASSERT(aPtr.mGeneration == generation());
+    typename HashTableEntry<T>::NonConstT t(std::move(*aPtr));
+    HashPolicy::setKey(t, const_cast<Key&>(aKey));
+    remove(aPtr.mSlot);
+    HashNumber keyHash = prepareHash(HashPolicy::hash(aLookup));
+    putNewInfallibleInternal(keyHash, std::move(t));
+  }
+
+  void rekeyAndMaybeRehash(Ptr aPtr, const Lookup& aLookup, const Key& aKey) {
+    rekeyWithoutRehash(aPtr, aLookup, aKey);
+    infallibleRehashIfOverloaded();
+  }
+};
+
+}  // namespace detail
+}  // namespace mozilla
+
+#endif /* mozilla_HashTable_h */