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diff --git a/js/src/gc/Tenuring.h b/js/src/gc/Tenuring.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_Tenuring_h
+#define gc_Tenuring_h
+
+#include "mozilla/Maybe.h"
+
+#include "gc/AllocKind.h"
+#include "js/GCAPI.h"
+#include "js/TracingAPI.h"
+#include "util/Text.h"
+
+namespace js {
+
+class NativeObject;
+class Nursery;
+class PlainObject;
+
+namespace wasm {
+class AnyRef;
+}  // namespace wasm
+
+namespace gc {
+
+class RelocationOverlay;
+class StringRelocationOverlay;
+
+template <typename Key>
+struct DeduplicationStringHasher {
+  using Lookup = Key;
+  static inline HashNumber hash(const Lookup& lookup);
+  static MOZ_ALWAYS_INLINE bool match(const Key& key, const Lookup& lookup);
+};
+
+class TenuringTracer final : public JSTracer {
+  Nursery& nursery_;
+
+  // Amount of data moved to the tenured generation during collection.
+  size_t tenuredSize = 0;
+  // Number of cells moved to the tenured generation.
+  size_t tenuredCells = 0;
+
+  // These lists are threaded through the Nursery using the space from
+  // already moved things. The lists are used to fix up the moved things and
+  // to find things held live by intra-Nursery pointers.
+  gc::RelocationOverlay* objHead = nullptr;
+  gc::StringRelocationOverlay* stringHead = nullptr;
+
+  using StringDeDupSet =
+      HashSet<JSString*, DeduplicationStringHasher<JSString*>,
+              SystemAllocPolicy>;
+
+  // deDupSet is emplaced at the beginning of the nursery collection and reset
+  // at the end of the nursery collection. It can also be reset during nursery
+  // collection when out of memory to insert new entries.
+  mozilla::Maybe<StringDeDupSet> stringDeDupSet;
+
+#define DEFINE_ON_EDGE_METHOD(name, type, _1, _2) \
+  void on##name##Edge(type** thingp, const char* name) override;
+  JS_FOR_EACH_TRACEKIND(DEFINE_ON_EDGE_METHOD)
+#undef DEFINE_ON_EDGE_METHOD
+
+ public:
+  TenuringTracer(JSRuntime* rt, Nursery* nursery);
+
+  Nursery& nursery() { return nursery_; }
+
+  // Move all objects and everything they can reach to the tenured heap. Called
+  // after all roots have been traced.
+  void collectToObjectFixedPoint();
+
+  // Move all strings and all strings they can reach to the tenured heap, and
+  // additionally do any fixups for when strings are pointing into memory that
+  // was deduplicated. Called after collectToObjectFixedPoint().
+  void collectToStringFixedPoint();
+
+  size_t getTenuredSize() const;
+  size_t getTenuredCells() const;
+
+  void traverse(JS::Value* thingp);
+  void traverse(wasm::AnyRef* thingp);
+
+  // The store buffers need to be able to call these directly.
+  void traceObject(JSObject* obj);
+  void traceObjectSlots(NativeObject* nobj, uint32_t start, uint32_t end);
+  void traceSlots(JS::Value* vp, uint32_t nslots);
+  void traceString(JSString* str);
+  void traceBigInt(JS::BigInt* bi);
+
+ private:
+  // The dependent string chars needs to be relocated if the base which it's
+  // using chars from has been deduplicated.
+  template <typename CharT>
+  void relocateDependentStringChars(JSDependentString* tenuredDependentStr,
+                                    JSLinearString* baseOrRelocOverlay,
+                                    size_t* offset,
+                                    bool* rootBaseNotYetForwarded,
+                                    JSLinearString** rootBase);
+
+  inline void insertIntoObjectFixupList(gc::RelocationOverlay* entry);
+  inline void insertIntoStringFixupList(gc::StringRelocationOverlay* entry);
+
+  template <typename T>
+  inline T* allocTenured(JS::Zone* zone, gc::AllocKind kind);
+  JSString* allocTenuredString(JSString* src, JS::Zone* zone,
+                               gc::AllocKind dstKind);
+
+  inline JSObject* movePlainObjectToTenured(PlainObject* src);
+  JSObject* moveToTenuredSlow(JSObject* src);
+  JSString* moveToTenured(JSString* src);
+  JS::BigInt* moveToTenured(JS::BigInt* src);
+
+  size_t moveElementsToTenured(NativeObject* dst, NativeObject* src,
+                               gc::AllocKind dstKind);
+  size_t moveSlotsToTenured(NativeObject* dst, NativeObject* src);
+  size_t moveStringToTenured(JSString* dst, JSString* src,
+                             gc::AllocKind dstKind);
+  size_t moveBigIntToTenured(JS::BigInt* dst, JS::BigInt* src,
+                             gc::AllocKind dstKind);
+
+  void traceSlots(JS::Value* vp, JS::Value* end);
+};
+
+}  // namespace gc
+}  // namespace js
+
+#endif  // gc_Tenuring_h