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diff --git a/js/src/gc/AtomMarking.h b/js/src/gc/AtomMarking.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef gc_AtomMarking_h
+#define gc_AtomMarking_h
+
+#include "NamespaceImports.h"
+#include "ds/Bitmap.h"
+#include "threading/ProtectedData.h"
+
+namespace js {
+
+class AutoLockGC;
+
+namespace gc {
+
+class Arena;
+
+// This class manages state used for marking atoms during GCs.
+// See AtomMarking.cpp for details.
+class AtomMarkingRuntime {
+  // Unused arena atom bitmap indexes. Protected by the GC lock.
+  js::GCLockData<Vector<size_t, 0, SystemAllocPolicy>> freeArenaIndexes;
+
+  inline void markChildren(JSContext* cx, JSAtom*);
+  inline void markChildren(JSContext* cx, JS::Symbol* symbol);
+
+ public:
+  // The extent of all allocated and free words in atom mark bitmaps.
+  // This monotonically increases and may be read from without locking.
+  mozilla::Atomic<size_t, mozilla::SequentiallyConsistent> allocatedWords;
+
+  AtomMarkingRuntime() : allocatedWords(0) {}
+
+  // Mark an arena as holding things in the atoms zone.
+  void registerArena(Arena* arena, const AutoLockGC& lock);
+
+  // Mark an arena as no longer holding things in the atoms zone.
+  void unregisterArena(Arena* arena, const AutoLockGC& lock);
+
+  // Fill |bitmap| with an atom marking bitmap based on the things that are
+  // currently marked in the chunks used by atoms zone arenas. This returns
+  // false on an allocation failure (but does not report an exception).
+  bool computeBitmapFromChunkMarkBits(JSRuntime* runtime, DenseBitmap& bitmap);
+
+  // Update the atom marking bitmap in |zone| according to another
+  // overapproximation of the reachable atoms in |bitmap|.
+  void refineZoneBitmapForCollectedZone(Zone* zone, const DenseBitmap& bitmap);
+
+  // Set any bits in the chunk mark bitmaps for atoms which are marked in any
+  // uncollected zone in the runtime.
+  void markAtomsUsedByUncollectedZones(JSRuntime* runtime);
+
+  // Mark an atom or id as being newly reachable by the context's zone.
+  template <typename T>
+  void markAtom(JSContext* cx, T* thing);
+
+  // Version of markAtom that's always inlined, for performance-sensitive
+  // callers.
+  template <typename T, bool Fallible>
+  MOZ_ALWAYS_INLINE bool inlinedMarkAtomInternal(JSContext* cx, T* thing);
+  template <typename T>
+  MOZ_ALWAYS_INLINE void inlinedMarkAtom(JSContext* cx, T* thing);
+  template <typename T>
+  MOZ_ALWAYS_INLINE bool inlinedMarkAtomFallible(JSContext* cx, T* thing);
+
+  void markId(JSContext* cx, jsid id);
+  void markAtomValue(JSContext* cx, const Value& value);
+
+  // Mark all atoms in |source| as being reachable within |target|.
+  void adoptMarkedAtoms(Zone* target, Zone* source);
+
+#ifdef DEBUG
+  // Return whether |thing/id| is in the atom marking bitmap for |zone|.
+  template <typename T>
+  bool atomIsMarked(Zone* zone, T* thing);
+  bool idIsMarked(Zone* zone, jsid id);
+  bool valueIsMarked(Zone* zone, const Value& value);
+#endif
+};
+
+}  // namespace gc
+}  // namespace js
+
+#endif  // gc_AtomMarking_h