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diff --git a/js/src/vm/AtomsTable.h b/js/src/vm/AtomsTable.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/. */
+
+/*
+ * Implementation details of the atoms table.
+ */
+
+#ifndef vm_AtomsTable_h
+#define vm_AtomsTable_h
+
+#include "gc/Barrier.h"
+#include "js/GCHashTable.h"
+#include "js/TypeDecls.h"
+#include "js/Vector.h"
+#include "vm/StringType.h"
+
+/*
+ * The atoms table is a mapping from strings to JSAtoms that supports
+ * incremental sweeping.
+ */
+
+namespace js {
+
+struct AtomHasher {
+  struct Lookup;
+  static inline HashNumber hash(const Lookup& l);
+  static MOZ_ALWAYS_INLINE bool match(const WeakHeapPtr<JSAtom*>& entry,
+                                      const Lookup& lookup);
+  static void rekey(WeakHeapPtr<JSAtom*>& k,
+                    const WeakHeapPtr<JSAtom*>& newKey) {
+    k = newKey;
+  }
+};
+
+// Note: Use a 'class' here to make forward declarations easier to use.
+class AtomSet : public JS::GCHashSet<WeakHeapPtr<JSAtom*>, AtomHasher,
+                                     SystemAllocPolicy> {
+  using Base =
+      JS::GCHashSet<WeakHeapPtr<JSAtom*>, AtomHasher, SystemAllocPolicy>;
+
+ public:
+  AtomSet() = default;
+  explicit AtomSet(size_t length) : Base(length){};
+};
+
+// This class is a wrapper for AtomSet that is used to ensure the AtomSet is
+// not modified. It should only expose read-only methods from AtomSet.
+// Note however that the atoms within the table can be marked during GC.
+class FrozenAtomSet {
+  AtomSet* mSet;
+
+ public:
+  // This constructor takes ownership of the passed-in AtomSet.
+  explicit FrozenAtomSet(AtomSet* set) { mSet = set; }
+
+  ~FrozenAtomSet() { js_delete(mSet); }
+
+  MOZ_ALWAYS_INLINE AtomSet::Ptr readonlyThreadsafeLookup(
+      const AtomSet::Lookup& l) const;
+
+  size_t sizeOfIncludingThis(mozilla::MallocSizeOf mallocSizeOf) const {
+    return mSet->shallowSizeOfIncludingThis(mallocSizeOf);
+  }
+
+  using Range = AtomSet::Range;
+
+  AtomSet::Range all() const { return mSet->all(); }
+};
+
+class AtomsTable {
+  // Use a low initial capacity for atom hash tables to avoid penalizing
+  // runtimes which create a small number of atoms.
+  static const size_t InitialTableSize = 16;
+
+  // The main atoms set.
+  AtomSet atoms;
+
+  // Set of atoms added while the |atoms| set is being swept.
+  AtomSet* atomsAddedWhileSweeping;
+
+  // List of pinned atoms that are traced in every GC.
+  Vector<JSAtom*, 0, SystemAllocPolicy> pinnedAtoms;
+
+ public:
+  // An iterator used for sweeping atoms incrementally.
+  using SweepIterator = AtomSet::Enum;
+
+  AtomsTable();
+  ~AtomsTable();
+  bool init();
+
+  template <typename CharT>
+  MOZ_ALWAYS_INLINE JSAtom* atomizeAndCopyCharsNonStaticValidLength(
+      JSContext* cx, const CharT* chars, size_t length,
+      const mozilla::Maybe<uint32_t>& indexValue,
+      const AtomHasher::Lookup& lookup);
+
+  bool maybePinExistingAtom(JSContext* cx, JSAtom* atom);
+
+  void tracePinnedAtoms(JSTracer* trc);
+
+  // Sweep all atoms non-incrementally.
+  void traceWeak(JSTracer* trc);
+
+  bool startIncrementalSweep(mozilla::Maybe<SweepIterator>& atomsToSweepOut);
+
+  // Sweep some atoms incrementally and return whether we finished.
+  bool sweepIncrementally(SweepIterator& atomsToSweep, SliceBudget& budget);
+
+  size_t sizeOfIncludingThis(mozilla::MallocSizeOf mallocSizeOf) const;
+
+ private:
+  void mergeAtomsAddedWhileSweeping();
+};
+
+bool AtomIsPinned(JSContext* cx, JSAtom* atom);
+
+}  // namespace js
+
+#endif /* vm_AtomsTable_h */