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diff --git a/js/src/ds/InlineTable.h b/js/src/ds/InlineTable.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 ds_InlineTable_h
+#define ds_InlineTable_h
+
+#include "mozilla/Maybe.h"
+
+#include <utility>
+
+#include "js/AllocPolicy.h"
+#include "js/HashTable.h"
+
+namespace js {
+
+namespace detail {
+
+// The InlineTable below needs an abstract way of testing keys for
+// tombstone values, and to set a key in an entry to a tombstone.
+// This is provided by the KeyPolicy generic type argument, which
+// has a default implementation for pointers provided below.
+
+// A default implementation of a KeyPolicy for some types (only pointer
+// types for now).
+//
+// The `KeyPolicy` type parameter informs an InlineTable of how to
+// check for tombstone values and to set tombstone values within
+// the domain of key (entry).
+//
+// A `KeyPolicy` for some key type `K` must provide two static methods:
+//   static bool isTombstone(const K& key);
+//   static void setToTombstone(K& key);
+template <typename K>
+class DefaultKeyPolicy;
+
+template <typename T>
+class DefaultKeyPolicy<T*> {
+  DefaultKeyPolicy() = delete;
+  DefaultKeyPolicy(const T*&) = delete;
+
+ public:
+  static bool isTombstone(T* const& ptr) { return ptr == nullptr; }
+  static void setToTombstone(T*& ptr) { ptr = nullptr; }
+};
+
+template <typename InlineEntry, typename Entry, typename Table,
+          typename HashPolicy, typename AllocPolicy, typename KeyPolicy,
+          size_t InlineEntries>
+class InlineTable : private AllocPolicy {
+ private:
+  using TablePtr = typename Table::Ptr;
+  using TableAddPtr = typename Table::AddPtr;
+  using TableRange = typename Table::Range;
+  using Lookup = typename HashPolicy::Lookup;
+
+  size_t inlNext_;
+  size_t inlCount_;
+  InlineEntry inl_[InlineEntries];
+  Table table_;
+
+#ifdef DEBUG
+  template <typename Key>
+  static bool keyNonZero(const Key& key) {
+    // Zero as tombstone means zero keys are invalid.
+    return !!key;
+  }
+#endif
+
+  InlineEntry* inlineStart() {
+    MOZ_ASSERT(!usingTable());
+    return inl_;
+  }
+
+  const InlineEntry* inlineStart() const {
+    MOZ_ASSERT(!usingTable());
+    return inl_;
+  }
+
+  InlineEntry* inlineEnd() {
+    MOZ_ASSERT(!usingTable());
+    return inl_ + inlNext_;
+  }
+
+  const InlineEntry* inlineEnd() const {
+    MOZ_ASSERT(!usingTable());
+    return inl_ + inlNext_;
+  }
+
+  bool usingTable() const { return inlNext_ > InlineEntries; }
+
+  [[nodiscard]] bool switchToTable() {
+    MOZ_ASSERT(inlNext_ == InlineEntries);
+
+    table_.clearAndCompact();
+
+    InlineEntry* end = inlineEnd();
+    for (InlineEntry* it = inlineStart(); it != end; ++it) {
+      if (it->key && !it->moveTo(table_)) {
+        return false;
+      }
+    }
+
+    inlNext_ = InlineEntries + 1;
+    MOZ_ASSERT(table_.count() == inlCount_);
+    MOZ_ASSERT(usingTable());
+    return true;
+  }
+
+  [[nodiscard]] MOZ_NEVER_INLINE bool switchAndAdd(const InlineEntry& entry) {
+    if (!switchToTable()) {
+      return false;
+    }
+
+    return entry.putNew(table_);
+  }
+
+ public:
+  static const size_t SizeOfInlineEntries = sizeof(InlineEntry) * InlineEntries;
+
+  explicit InlineTable(AllocPolicy a = AllocPolicy())
+      : AllocPolicy(std::move(a)), inlNext_(0), inlCount_(0), table_(a) {}
+
+  class Ptr {
+    friend class InlineTable;
+
+   protected:
+    MOZ_INIT_OUTSIDE_CTOR Entry entry_;
+    MOZ_INIT_OUTSIDE_CTOR TablePtr tablePtr_;
+    MOZ_INIT_OUTSIDE_CTOR InlineEntry* inlPtr_;
+    MOZ_INIT_OUTSIDE_CTOR bool isInlinePtr_;
+
+    explicit Ptr(TablePtr p)
+        : entry_(p.found() ? &*p : nullptr),
+          tablePtr_(p),
+          isInlinePtr_(false) {}
+
+    explicit Ptr(InlineEntry* inlineEntry)
+        : entry_(inlineEntry), inlPtr_(inlineEntry), isInlinePtr_(true) {}
+
+    void operator==(const Ptr& other);
+
+   public:
+    // Leaves Ptr uninitialized.
+    Ptr() {
+#ifdef DEBUG
+      inlPtr_ = (InlineEntry*)0xbad;
+      isInlinePtr_ = true;
+#endif
+    }
+
+    // Default copy constructor works for this structure.
+
+    bool found() const {
+      return isInlinePtr_ ? bool(inlPtr_) : tablePtr_.found();
+    }
+
+    explicit operator bool() const { return found(); }
+
+    bool operator==(const Ptr& other) const {
+      MOZ_ASSERT(found() && other.found());
+      if (isInlinePtr_ != other.isInlinePtr_) {
+        return false;
+      }
+      if (isInlinePtr_) {
+        return inlPtr_ == other.inlPtr_;
+      }
+      return tablePtr_ == other.tablePtr_;
+    }
+
+    bool operator!=(const Ptr& other) const { return !(*this == other); }
+
+    Entry& operator*() {
+      MOZ_ASSERT(found());
+      return entry_;
+    }
+
+    Entry* operator->() {
+      MOZ_ASSERT(found());
+      return &entry_;
+    }
+  };
+
+  class AddPtr {
+    friend class InlineTable;
+
+   protected:
+    MOZ_INIT_OUTSIDE_CTOR Entry entry_;
+    MOZ_INIT_OUTSIDE_CTOR TableAddPtr tableAddPtr_;
+    MOZ_INIT_OUTSIDE_CTOR InlineEntry* inlAddPtr_;
+    MOZ_INIT_OUTSIDE_CTOR bool isInlinePtr_;
+    // Indicates whether inlAddPtr is a found result or an add pointer.
+    MOZ_INIT_OUTSIDE_CTOR bool inlPtrFound_;
+
+    AddPtr(InlineEntry* ptr, bool found)
+        : entry_(ptr),
+          inlAddPtr_(ptr),
+          isInlinePtr_(true),
+          inlPtrFound_(found) {}
+
+    explicit AddPtr(const TableAddPtr& p)
+        : entry_(p.found() ? &*p : nullptr),
+          tableAddPtr_(p),
+          isInlinePtr_(false) {}
+
+   public:
+    AddPtr() = default;
+
+    bool found() const {
+      return isInlinePtr_ ? inlPtrFound_ : tableAddPtr_.found();
+    }
+
+    explicit operator bool() const { return found(); }
+
+    bool operator==(const AddPtr& other) const {
+      MOZ_ASSERT(found() && other.found());
+      if (isInlinePtr_ != other.isInlinePtr_) {
+        return false;
+      }
+      if (isInlinePtr_) {
+        return inlAddPtr_ == other.inlAddPtr_;
+      }
+      return tableAddPtr_ == other.tableAddPtr_;
+    }
+
+    bool operator!=(const AddPtr& other) const { return !(*this == other); }
+
+    Entry& operator*() {
+      MOZ_ASSERT(found());
+      return entry_;
+    }
+
+    Entry* operator->() {
+      MOZ_ASSERT(found());
+      return &entry_;
+    }
+  };
+
+  size_t count() const { return usingTable() ? table_.count() : inlCount_; }
+
+  bool empty() const { return usingTable() ? table_.empty() : !inlCount_; }
+
+  void clear() {
+    inlNext_ = 0;
+    inlCount_ = 0;
+  }
+
+  MOZ_ALWAYS_INLINE
+  Ptr lookup(const Lookup& l) {
+    MOZ_ASSERT(keyNonZero(l));
+
+    if (usingTable()) {
+      return Ptr(table_.lookup(l));
+    }
+
+    InlineEntry* end = inlineEnd();
+    for (InlineEntry* it = inlineStart(); it != end; ++it) {
+      if (it->key && HashPolicy::match(it->key, l)) {
+        return Ptr(it);
+      }
+    }
+
+    return Ptr(nullptr);
+  }
+
+  MOZ_ALWAYS_INLINE
+  AddPtr lookupForAdd(const Lookup& l) {
+    MOZ_ASSERT(keyNonZero(l));
+
+    if (usingTable()) {
+      return AddPtr(table_.lookupForAdd(l));
+    }
+
+    InlineEntry* end = inlineEnd();
+    for (InlineEntry* it = inlineStart(); it != end; ++it) {
+      if (it->key && HashPolicy::match(it->key, l)) {
+        return AddPtr(it, true);
+      }
+    }
+
+    // The add pointer that's returned here may indicate the limit entry of
+    // the linear space, in which case the |add| operation will initialize
+    // the table if necessary and add the entry there.
+    return AddPtr(inlineEnd(), false);
+  }
+
+  template <typename KeyInput, typename... Args>
+  [[nodiscard]] MOZ_ALWAYS_INLINE bool add(AddPtr& p, KeyInput&& key,
+                                           Args&&... args) {
+    MOZ_ASSERT(!p);
+    MOZ_ASSERT(keyNonZero(key));
+
+    if (p.isInlinePtr_) {
+      InlineEntry* addPtr = p.inlAddPtr_;
+      MOZ_ASSERT(addPtr == inlineEnd());
+
+      // Switching to table mode before we add this pointer.
+      if (addPtr == inlineStart() + InlineEntries) {
+        if (!switchToTable()) {
+          return false;
+        }
+        return table_.putNew(std::forward<KeyInput>(key),
+                             std::forward<Args>(args)...);
+      }
+
+      MOZ_ASSERT(!p.found());
+      MOZ_ASSERT(uintptr_t(inlineEnd()) == uintptr_t(p.inlAddPtr_));
+
+      if (!this->checkSimulatedOOM()) {
+        return false;
+      }
+
+      addPtr->update(std::forward<KeyInput>(key), std::forward<Args>(args)...);
+      ++inlCount_;
+      ++inlNext_;
+      return true;
+    }
+
+    return table_.add(p.tableAddPtr_, std::forward<KeyInput>(key),
+                      std::forward<Args>(args)...);
+  }
+
+  void remove(Ptr& p) {
+    MOZ_ASSERT(p);
+    if (p.isInlinePtr_) {
+      MOZ_ASSERT(inlCount_ > 0);
+      MOZ_ASSERT(!KeyPolicy::isTombstone(p.inlPtr_->key));
+      KeyPolicy::setToTombstone(p.inlPtr_->key);
+      --inlCount_;
+      return;
+    }
+    MOZ_ASSERT(usingTable());
+    table_.remove(p.tablePtr_);
+  }
+
+  void remove(const Lookup& l) {
+    if (Ptr p = lookup(l)) {
+      remove(p);
+    }
+  }
+
+  class Range {
+    friend class InlineTable;
+
+   protected:
+    mozilla::Maybe<TableRange> tableRange_;  // `Nothing` if `isInline_==true`
+    InlineEntry* cur_;
+    InlineEntry* end_;
+    bool isInline_;
+
+    explicit Range(TableRange r)
+        : tableRange_(mozilla::Some(r)),
+          cur_(nullptr),
+          end_(nullptr),
+          isInline_(false) {
+      MOZ_ASSERT(!isInlineRange());
+    }
+
+    Range(const InlineEntry* begin, const InlineEntry* end)
+        : tableRange_(mozilla::Nothing()),
+          cur_(const_cast<InlineEntry*>(begin)),
+          end_(const_cast<InlineEntry*>(end)),
+          isInline_(true) {
+      advancePastNulls(cur_);
+      MOZ_ASSERT(isInlineRange());
+    }
+
+    bool assertInlineRangeInvariants() const {
+      MOZ_ASSERT(uintptr_t(cur_) <= uintptr_t(end_));
+      MOZ_ASSERT_IF(cur_ != end_, !KeyPolicy::isTombstone(cur_->key));
+      return true;
+    }
+
+    bool isInlineRange() const {
+      MOZ_ASSERT_IF(isInline_, assertInlineRangeInvariants());
+      return isInline_;
+    }
+
+    void advancePastNulls(InlineEntry* begin) {
+      InlineEntry* newCur = begin;
+      while (newCur < end_ && KeyPolicy::isTombstone(newCur->key)) {
+        ++newCur;
+      }
+      MOZ_ASSERT(uintptr_t(newCur) <= uintptr_t(end_));
+      cur_ = newCur;
+    }
+
+    void bumpCurPtr() {
+      MOZ_ASSERT(isInlineRange());
+      advancePastNulls(cur_ + 1);
+    }
+
+   public:
+    bool empty() const {
+      return isInlineRange() ? cur_ == end_ : tableRange_->empty();
+    }
+
+    Entry front() {
+      MOZ_ASSERT(!empty());
+      if (isInlineRange()) {
+        return Entry(cur_);
+      }
+      return Entry(&tableRange_->front());
+    }
+
+    void popFront() {
+      MOZ_ASSERT(!empty());
+      if (isInlineRange()) {
+        bumpCurPtr();
+      } else {
+        tableRange_->popFront();
+      }
+    }
+  };
+
+  Range all() const {
+    return usingTable() ? Range(table_.all())
+                        : Range(inlineStart(), inlineEnd());
+  }
+};
+
+}  // namespace detail
+
+// A map with InlineEntries number of entries kept inline in an array.
+//
+// The Key type must be zeroable as zeros are used as tombstone keys.
+// The Value type must have a default constructor.
+//
+// The API is very much like HashMap's.
+template <typename Key, typename Value, size_t InlineEntries,
+          typename HashPolicy = DefaultHasher<Key>,
+          typename AllocPolicy = TempAllocPolicy,
+          typename KeyPolicy = detail::DefaultKeyPolicy<Key>>
+class InlineMap {
+  using Map = HashMap<Key, Value, HashPolicy, AllocPolicy>;
+
+  struct InlineEntry {
+    Key key;
+    Value value;
+
+    template <typename KeyInput, typename ValueInput>
+    void update(KeyInput&& key, ValueInput&& value) {
+      this->key = std::forward<KeyInput>(key);
+      this->value = std::forward<ValueInput>(value);
+    }
+
+    [[nodiscard]] bool moveTo(Map& map) {
+      return map.putNew(std::move(key), std::move(value));
+    }
+  };
+
+  class Entry {
+    using MapEntry = typename Map::Entry;
+
+    MapEntry* mapEntry_;
+    InlineEntry* inlineEntry_;
+
+   public:
+    Entry() = default;
+
+    explicit Entry(MapEntry* mapEntry)
+        : mapEntry_(mapEntry), inlineEntry_(nullptr) {}
+
+    explicit Entry(InlineEntry* inlineEntry)
+        : mapEntry_(nullptr), inlineEntry_(inlineEntry) {}
+
+    const Key& key() const {
+      MOZ_ASSERT(!!mapEntry_ != !!inlineEntry_);
+      if (mapEntry_) {
+        return mapEntry_->key();
+      }
+      return inlineEntry_->key;
+    }
+
+    Value& value() {
+      MOZ_ASSERT(!!mapEntry_ != !!inlineEntry_);
+      if (mapEntry_) {
+        return mapEntry_->value();
+      }
+      return inlineEntry_->value;
+    }
+  };
+
+  using Impl = detail::InlineTable<InlineEntry, Entry, Map, HashPolicy,
+                                   AllocPolicy, KeyPolicy, InlineEntries>;
+
+  Impl impl_;
+
+ public:
+  using Table = Map;
+  using Ptr = typename Impl::Ptr;
+  using AddPtr = typename Impl::AddPtr;
+  using Range = typename Impl::Range;
+  using Lookup = typename HashPolicy::Lookup;
+
+  static const size_t SizeOfInlineEntries = Impl::SizeOfInlineEntries;
+
+  explicit InlineMap(AllocPolicy a = AllocPolicy()) : impl_(std::move(a)) {}
+
+  size_t count() const { return impl_.count(); }
+
+  bool empty() const { return impl_.empty(); }
+
+  void clear() { impl_.clear(); }
+
+  Range all() const { return impl_.all(); }
+
+  MOZ_ALWAYS_INLINE
+  Ptr lookup(const Lookup& l) { return impl_.lookup(l); }
+
+  MOZ_ALWAYS_INLINE
+  bool has(const Lookup& l) const {
+    return const_cast<InlineMap*>(this)->lookup(l).found();
+  }
+
+  MOZ_ALWAYS_INLINE
+  AddPtr lookupForAdd(const Lookup& l) { return impl_.lookupForAdd(l); }
+
+  template <typename KeyInput, typename ValueInput>
+  [[nodiscard]] MOZ_ALWAYS_INLINE bool add(AddPtr& p, KeyInput&& key,
+                                           ValueInput&& value) {
+    return impl_.add(p, std::forward<KeyInput>(key),
+                     std::forward<ValueInput>(value));
+  }
+
+  template <typename KeyInput, typename ValueInput>
+  [[nodiscard]] bool put(KeyInput&& key, ValueInput&& value) {
+    AddPtr p = lookupForAdd(key);
+    if (p) {
+      p->value() = std::forward<ValueInput>(value);
+      return true;
+    }
+    return add(p, std::forward<KeyInput>(key), std::forward<ValueInput>(value));
+  }
+
+  void remove(Ptr& p) { impl_.remove(p); }
+
+  void remove(const Lookup& l) { impl_.remove(l); }
+};
+
+// A set with InlineEntries number of entries kept inline in an array.
+//
+// The T type must be zeroable as zeros are used as tombstone keys.
+// The T type must have a default constructor.
+//
+// The API is very much like HashMap's.
+template <typename T, size_t InlineEntries,
+          typename HashPolicy = DefaultHasher<T>,
+          typename AllocPolicy = TempAllocPolicy,
+          typename KeyPolicy = detail::DefaultKeyPolicy<T>>
+class InlineSet {
+  using Set = HashSet<T, HashPolicy, AllocPolicy>;
+
+  struct InlineEntry {
+    T key;
+
+    template <typename TInput>
+    void update(TInput&& key) {
+      this->key = std::forward<TInput>(key);
+    }
+
+    [[nodiscard]] bool moveTo(Set& set) { return set.putNew(std::move(key)); }
+  };
+
+  class Entry {
+    using SetEntry = typename Set::Entry;
+
+    SetEntry* setEntry_;
+    InlineEntry* inlineEntry_;
+
+   public:
+    Entry() = default;
+
+    explicit Entry(const SetEntry* setEntry)
+        : setEntry_(const_cast<SetEntry*>(setEntry)), inlineEntry_(nullptr) {}
+
+    explicit Entry(InlineEntry* inlineEntry)
+        : setEntry_(nullptr), inlineEntry_(inlineEntry) {}
+
+    operator T() const {
+      MOZ_ASSERT(!!setEntry_ != !!inlineEntry_);
+      if (setEntry_) {
+        return *setEntry_;
+      }
+      return inlineEntry_->key;
+    }
+  };
+
+  using Impl = detail::InlineTable<InlineEntry, Entry, Set, HashPolicy,
+                                   AllocPolicy, KeyPolicy, InlineEntries>;
+
+  Impl impl_;
+
+ public:
+  using Table = Set;
+  using Ptr = typename Impl::Ptr;
+  using AddPtr = typename Impl::AddPtr;
+  using Range = typename Impl::Range;
+  using Lookup = typename HashPolicy::Lookup;
+
+  static const size_t SizeOfInlineEntries = Impl::SizeOfInlineEntries;
+
+  explicit InlineSet(AllocPolicy a = AllocPolicy()) : impl_(std::move(a)) {}
+
+  size_t count() const { return impl_.count(); }
+
+  bool empty() const { return impl_.empty(); }
+
+  void clear() { impl_.clear(); }
+
+  Range all() const { return impl_.all(); }
+
+  MOZ_ALWAYS_INLINE
+  Ptr lookup(const Lookup& l) { return impl_.lookup(l); }
+
+  MOZ_ALWAYS_INLINE
+  bool has(const Lookup& l) const {
+    return const_cast<InlineSet*>(this)->lookup(l).found();
+  }
+
+  MOZ_ALWAYS_INLINE
+  AddPtr lookupForAdd(const Lookup& l) { return impl_.lookupForAdd(l); }
+
+  template <typename TInput>
+  [[nodiscard]] MOZ_ALWAYS_INLINE bool add(AddPtr& p, TInput&& key) {
+    return impl_.add(p, std::forward<TInput>(key));
+  }
+
+  template <typename TInput>
+  [[nodiscard]] bool put(TInput&& key) {
+    AddPtr p = lookupForAdd(key);
+    return p ? true : add(p, std::forward<TInput>(key));
+  }
+
+  void remove(Ptr& p) { impl_.remove(p); }
+
+  void remove(const Lookup& l) { impl_.remove(l); }
+};
+
+}  // namespace js
+
+#endif  // ds_InlineTable_h