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diff --git a/js/src/gc/GCContext.h b/js/src/gc/GCContext.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_GCContext_h
+#define gc_GCContext_h
+
+#include "mozilla/Assertions.h"  // MOZ_ASSERT
+#include "mozilla/ThreadLocal.h"
+
+#include "jspubtd.h"
+#include "jstypes.h"                  // JS_PUBLIC_API
+#include "gc/GCEnum.h"                // js::MemoryUse
+#include "jit/ExecutableAllocator.h"  // jit::JitPoisonRangeVector
+#include "js/Utility.h"               // js_free
+
+struct JS_PUBLIC_API JSRuntime;
+
+namespace js {
+
+class AutoTouchingGrayThings;
+
+namespace gc {
+
+class AutoSetThreadGCUse;
+class AutoSetThreadIsSweeping;
+
+enum class GCUse {
+  // This thread is not running in the garbage collector.
+  None,
+
+  // This thread is currently collecting. Used when no finer detail is known.
+  Unspecified,
+
+  // This thread is currently marking GC things. This thread could be the main
+  // thread or a helper thread doing sweep-marking.
+  Marking,
+
+  // This thread is currently sweeping GC things. This thread could be the
+  // main thread or a helper thread while the main thread is running the
+  // mutator.
+  Sweeping,
+
+  // Whether this thread is currently finalizing GC things. This thread could
+  // be the main thread or a helper thread doing finalization while the main
+  // thread is running the mutator.
+  Finalizing
+};
+
+}  // namespace gc
+}  // namespace js
+
+namespace JS {
+
+/*
+ * GCContext is by GC operations that can run on or off the main thread.
+ *
+ * Its main function is to provide methods to free memory and update memory
+ * accounting. For convenience, it also has delete_ convenience methods that
+ * also call destructors.
+ *
+ * It is passed to finalizers and other sweep-phase hooks as JSContext is not
+ * available off the main thread.
+ */
+class GCContext {
+  using Cell = js::gc::Cell;
+  using MemoryUse = js::MemoryUse;
+
+  JSRuntime* const runtime_;
+
+  js::jit::JitPoisonRangeVector jitPoisonRanges;
+
+  // Which part of the garbage collector this context is running at the moment.
+  js::gc::GCUse gcUse_ = js::gc::GCUse::None;
+  friend class js::gc::AutoSetThreadGCUse;
+  friend class js::gc::AutoSetThreadIsSweeping;
+
+#ifdef DEBUG
+  // The specific zone currently being swept, if any.
+  Zone* gcSweepZone_ = nullptr;
+
+  // Whether this thread is currently manipulating possibly-gray GC things.
+  size_t isTouchingGrayThings_ = false;
+  friend class js::AutoTouchingGrayThings;
+#endif
+
+ public:
+  explicit GCContext(JSRuntime* maybeRuntime);
+  ~GCContext();
+
+  JSRuntime* runtime() const {
+    MOZ_ASSERT(onMainThread());
+    return runtimeFromAnyThread();
+  }
+  JSRuntime* runtimeFromAnyThread() const {
+    MOZ_ASSERT(runtime_);
+    return runtime_;
+  }
+
+  js::gc::GCUse gcUse() const { return gcUse_; }
+  bool isCollecting() const { return gcUse() != js::gc::GCUse::None; }
+  bool isFinalizing() const { return gcUse_ == js::gc::GCUse::Finalizing; }
+
+#ifdef DEBUG
+  bool onMainThread() const {
+    return js::CurrentThreadCanAccessRuntime(runtime_);
+  }
+
+  Zone* gcSweepZone() const { return gcSweepZone_; }
+  bool isTouchingGrayThings() const { return isTouchingGrayThings_; }
+#endif
+
+  // Deprecated. Where possible, memory should be tracked against the owning GC
+  // thing by calling js::AddCellMemory and the memory freed with free_() below.
+  void freeUntracked(void* p) { js_free(p); }
+
+  // Free memory associated with a GC thing and update the memory accounting.
+  //
+  // The memory should have been associated with the GC thing using
+  // js::InitReservedSlot or js::InitObjectPrivate, or possibly
+  // js::AddCellMemory.
+  void free_(Cell* cell, void* p, size_t nbytes, MemoryUse use);
+
+  bool appendJitPoisonRange(const js::jit::JitPoisonRange& range) {
+    return jitPoisonRanges.append(range);
+  }
+  bool hasJitCodeToPoison() const { return !jitPoisonRanges.empty(); }
+  void poisonJitCode();
+
+  // Deprecated. Where possible, memory should be tracked against the owning GC
+  // thing by calling js::AddCellMemory and the memory freed with delete_()
+  // below.
+  template <class T>
+  void deleteUntracked(T* p) {
+    if (p) {
+      p->~T();
+      js_free(p);
+    }
+  }
+
+  // Delete a C++ object that was associated with a GC thing and update the
+  // memory accounting. The size is determined by the type T.
+  //
+  // The memory should have been associated with the GC thing using
+  // js::InitReservedSlot or js::InitObjectPrivate, or possibly
+  // js::AddCellMemory.
+  template <class T>
+  void delete_(Cell* cell, T* p, MemoryUse use) {
+    delete_(cell, p, sizeof(T), use);
+  }
+
+  // Delete a C++ object that was associated with a GC thing and update the
+  // memory accounting.
+  //
+  // The memory should have been associated with the GC thing using
+  // js::InitReservedSlot or js::InitObjectPrivate, or possibly
+  // js::AddCellMemory.
+  template <class T>
+  void delete_(Cell* cell, T* p, size_t nbytes, MemoryUse use) {
+    if (p) {
+      p->~T();
+      free_(cell, p, nbytes, use);
+    }
+  }
+
+  // Release a RefCounted object that was associated with a GC thing and update
+  // the memory accounting.
+  //
+  // The memory should have been associated with the GC thing using
+  // js::InitReservedSlot or js::InitObjectPrivate, or possibly
+  // js::AddCellMemory.
+  //
+  // This counts the memory once per association with a GC thing. It's not
+  // expected that the same object is associated with more than one GC thing in
+  // each zone. If this is the case then some other form of accounting would be
+  // more appropriate.
+  template <class T>
+  void release(Cell* cell, T* p, MemoryUse use) {
+    release(cell, p, sizeof(T), use);
+  }
+
+  // Release a RefCounted object and that was associated with a GC thing and
+  // update the memory accounting.
+  //
+  // The memory should have been associated with the GC thing using
+  // js::InitReservedSlot or js::InitObjectPrivate, or possibly
+  // js::AddCellMemory.
+  template <class T>
+  void release(Cell* cell, T* p, size_t nbytes, MemoryUse use);
+
+  // Update the memory accounting for a GC for memory freed by some other
+  // method.
+  void removeCellMemory(Cell* cell, size_t nbytes, MemoryUse use);
+};
+
+}  // namespace JS
+
+namespace js {
+
+/* Thread Local Storage for storing the GCContext for a thread. */
+extern MOZ_THREAD_LOCAL(JS::GCContext*) TlsGCContext;
+
+inline JS::GCContext* MaybeGetGCContext() {
+  if (!TlsGCContext.init()) {
+    return nullptr;
+  }
+  return TlsGCContext.get();
+}
+
+class MOZ_RAII AutoTouchingGrayThings {
+ public:
+#ifdef DEBUG
+  AutoTouchingGrayThings() { TlsGCContext.get()->isTouchingGrayThings_++; }
+  ~AutoTouchingGrayThings() {
+    JS::GCContext* gcx = TlsGCContext.get();
+    MOZ_ASSERT(gcx->isTouchingGrayThings_);
+    gcx->isTouchingGrayThings_--;
+  }
+#else
+  AutoTouchingGrayThings() {}
+#endif
+};
+
+#ifdef DEBUG
+
+inline bool CurrentThreadIsGCMarking() {
+  JS::GCContext* gcx = MaybeGetGCContext();
+  return gcx && gcx->gcUse() == gc::GCUse::Marking;
+}
+
+inline bool CurrentThreadIsGCSweeping() {
+  JS::GCContext* gcx = MaybeGetGCContext();
+  return gcx && gcx->gcUse() == gc::GCUse::Sweeping;
+}
+
+inline bool CurrentThreadIsGCFinalizing() {
+  JS::GCContext* gcx = MaybeGetGCContext();
+  return gcx && gcx->gcUse() == gc::GCUse::Finalizing;
+}
+
+inline bool CurrentThreadIsTouchingGrayThings() {
+  JS::GCContext* gcx = MaybeGetGCContext();
+  return gcx && gcx->isTouchingGrayThings();
+}
+
+inline bool CurrentThreadIsPerformingGC() {
+  JS::GCContext* gcx = MaybeGetGCContext();
+  return gcx && gcx->isCollecting();
+}
+
+#endif
+
+}  // namespace js
+
+#endif  // gc_GCContext_h