Adding upstream version 115.7.0esr.upstream/115.7.0esrupstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 1257 insertions, 0 deletions

diff --git a/js/src/gc/Barrier.h b/js/src/gc/Barrier.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_Barrier_h
+#define gc_Barrier_h
+
+#include <type_traits>  // std::true_type
+
+#include "NamespaceImports.h"
+
+#include "gc/Cell.h"
+#include "gc/GCContext.h"
+#include "gc/StoreBuffer.h"
+#include "js/ComparisonOperators.h"     // JS::detail::DefineComparisonOps
+#include "js/experimental/TypedData.h"  // js::EnableIfABOVType
+#include "js/HeapAPI.h"
+#include "js/Id.h"
+#include "js/RootingAPI.h"
+#include "js/Value.h"
+#include "util/Poison.h"
+
+/*
+ * [SMDOC] GC Barriers
+ *
+ * Several kinds of barrier are necessary to allow the GC to function correctly.
+ * These are triggered by reading or writing to GC pointers in the heap and
+ * serve to tell the collector about changes to the graph of reachable GC
+ * things.
+ *
+ * Since it would be awkward to change every write to memory into a function
+ * call, this file contains a bunch of C++ classes and templates that use
+ * operator overloading to take care of barriers automatically. In most cases,
+ * all that's necessary is to replace:
+ *
+ *     Type* field;
+ *
+ * with:
+ *
+ *     HeapPtr<Type> field;
+ *
+ * All heap-based GC pointers and tagged pointers must use one of these classes,
+ * except in a couple of exceptional cases.
+ *
+ * These classes are designed to be used by the internals of the JS engine.
+ * Barriers designed to be used externally are provided in js/RootingAPI.h.
+ *
+ * Overview
+ * ========
+ *
+ * This file implements the following concrete classes:
+ *
+ * HeapPtr       General wrapper for heap-based pointers that provides pre- and
+ *               post-write barriers. Most clients should use this.
+ *
+ * GCPtr         An optimisation of HeapPtr for objects which are only destroyed
+ *               by GC finalization (this rules out use in Vector, for example).
+ *
+ * PreBarriered  Provides a pre-barrier but not a post-barrier. Necessary when
+ *               generational GC updates are handled manually, e.g. for hash
+ *               table keys that don't use StableCellHasher.
+ *
+ * HeapSlot      Provides pre and post-barriers, optimised for use in JSObject
+ *               slots and elements.
+ *
+ * WeakHeapPtr   Provides read and post-write barriers, for use with weak
+ *               pointers.
+ *
+ * UnsafeBarePtr Provides no barriers. Don't add new uses of this, or only if
+ *               you really know what you are doing.
+ *
+ * The following classes are implemented in js/RootingAPI.h (in the JS
+ * namespace):
+ *
+ * Heap          General wrapper for external clients. Like HeapPtr but also
+ *               handles cycle collector concerns. Most external clients should
+ *               use this.
+ *
+ * Heap::Tenured   Like Heap but doesn't allow nursery pointers. Allows storing
+ *               flags in unused lower bits of the pointer.
+ *
+ * Which class to use?
+ * -------------------
+ *
+ * Answer the following questions to decide which barrier class is right for
+ * your use case:
+ *
+ * Is your code part of the JS engine?
+ *   Yes, it's internal =>
+ *     Is your pointer weak or strong?
+ *       Strong =>
+ *         Do you want automatic handling of nursery pointers?
+ *           Yes, of course =>
+ *             Can your object be destroyed outside of a GC?
+ *               Yes => Use HeapPtr<T>
+ *               No => Use GCPtr<T> (optimization)
+ *           No, I'll do this myself =>
+ *             Do you want pre-barriers so incremental marking works?
+ *               Yes, of course => Use PreBarriered<T>
+ *               No, and I'll fix all the bugs myself => Use UnsafeBarePtr<T>
+ *       Weak => Use WeakHeapPtr<T>
+ *   No, it's external =>
+ *     Can your pointer refer to nursery objects?
+ *       Yes => Use JS::Heap<T>
+ *       Never => Use JS::Heap::Tenured<T> (optimization)
+ *
+ * If in doubt, use HeapPtr<T>.
+ *
+ * Write barriers
+ * ==============
+ *
+ * A write barrier is a mechanism used by incremental or generational GCs to
+ * ensure that every value that needs to be marked is marked. In general, the
+ * write barrier should be invoked whenever a write can cause the set of things
+ * traced through by the GC to change. This includes:
+ *
+ *   - writes to object properties
+ *   - writes to array slots
+ *   - writes to fields like JSObject::shape_ that we trace through
+ *   - writes to fields in private data
+ *   - writes to non-markable fields like JSObject::private that point to
+ *     markable data
+ *
+ * The last category is the trickiest. Even though the private pointer does not
+ * point to a GC thing, changing the private pointer may change the set of
+ * objects that are traced by the GC. Therefore it needs a write barrier.
+ *
+ * Every barriered write should have the following form:
+ *
+ *   <pre-barrier>
+ *   obj->field = value; // do the actual write
+ *   <post-barrier>
+ *
+ * The pre-barrier is used for incremental GC and the post-barrier is for
+ * generational GC.
+ *
+ * Pre-write barrier
+ * -----------------
+ *
+ * To understand the pre-barrier, let's consider how incremental GC works. The
+ * GC itself is divided into "slices". Between each slice, JS code is allowed to
+ * run. Each slice should be short so that the user doesn't notice the
+ * interruptions. In our GC, the structure of the slices is as follows:
+ *
+ * 1. ... JS work, which leads to a request to do GC ...
+ * 2. [first GC slice, which performs all root marking and (maybe) more marking]
+ * 3. ... more JS work is allowed to run ...
+ * 4. [GC mark slice, which runs entirely in
+ *    GCRuntime::markUntilBudgetExhausted]
+ * 5. ... more JS work ...
+ * 6. [GC mark slice, which runs entirely in
+ *    GCRuntime::markUntilBudgetExhausted]
+ * 7. ... more JS work ...
+ * 8. [GC marking finishes; sweeping done non-incrementally; GC is done]
+ * 9. ... JS continues uninterrupted now that GC is finishes ...
+ *
+ * Of course, there may be a different number of slices depending on how much
+ * marking is to be done.
+ *
+ * The danger inherent in this scheme is that the JS code in steps 3, 5, and 7
+ * might change the heap in a way that causes the GC to collect an object that
+ * is actually reachable. The write barrier prevents this from happening. We use
+ * a variant of incremental GC called "snapshot at the beginning." This approach
+ * guarantees the invariant that if an object is reachable in step 2, then we
+ * will mark it eventually. The name comes from the idea that we take a
+ * theoretical "snapshot" of all reachable objects in step 2; all objects in
+ * that snapshot should eventually be marked. (Note that the write barrier
+ * verifier code takes an actual snapshot.)
+ *
+ * The basic correctness invariant of a snapshot-at-the-beginning collector is
+ * that any object reachable at the end of the GC (step 9) must either:
+ *   (1) have been reachable at the beginning (step 2) and thus in the snapshot
+ *   (2) or must have been newly allocated, in steps 3, 5, or 7.
+ * To deal with case (2), any objects allocated during an incremental GC are
+ * automatically marked black.
+ *
+ * This strategy is actually somewhat conservative: if an object becomes
+ * unreachable between steps 2 and 8, it would be safe to collect it. We won't,
+ * mainly for simplicity. (Also, note that the snapshot is entirely
+ * theoretical. We don't actually do anything special in step 2 that we wouldn't
+ * do in a non-incremental GC.
+ *
+ * It's the pre-barrier's job to maintain the snapshot invariant. Consider the
+ * write "obj->field = value". Let the prior value of obj->field be
+ * value0. Since it's possible that value0 may have been what obj->field
+ * contained in step 2, when the snapshot was taken, the barrier marks
+ * value0. Note that it only does this if we're in the middle of an incremental
+ * GC. Since this is rare, the cost of the write barrier is usually just an
+ * extra branch.
+ *
+ * In practice, we implement the pre-barrier differently based on the type of
+ * value0. E.g., see JSObject::preWriteBarrier, which is used if obj->field is
+ * a JSObject*. It takes value0 as a parameter.
+ *
+ * Post-write barrier
+ * ------------------
+ *
+ * For generational GC, we want to be able to quickly collect the nursery in a
+ * minor collection.  Part of the way this is achieved is to only mark the
+ * nursery itself; tenured things, which may form the majority of the heap, are
+ * not traced through or marked.  This leads to the problem of what to do about
+ * tenured objects that have pointers into the nursery: if such things are not
+ * marked, they may be discarded while there are still live objects which
+ * reference them. The solution is to maintain information about these pointers,
+ * and mark their targets when we start a minor collection.
+ *
+ * The pointers can be thought of as edges in an object graph, and the set of
+ * edges from the tenured generation into the nursery is known as the remembered
+ * set. Post barriers are used to track this remembered set.
+ *
+ * Whenever a slot which could contain such a pointer is written, we check
+ * whether the pointed-to thing is in the nursery (if storeBuffer() returns a
+ * buffer).  If so we add the cell into the store buffer, which is the
+ * collector's representation of the remembered set.  This means that when we
+ * come to do a minor collection we can examine the contents of the store buffer
+ * and mark any edge targets that are in the nursery.
+ *
+ * Read barriers
+ * =============
+ *
+ * Weak pointer read barrier
+ * -------------------------
+ *
+ * Weak pointers must have a read barrier to prevent the referent from being
+ * collected if it is read after the start of an incremental GC.
+ *
+ * The problem happens when, during an incremental GC, some code reads a weak
+ * pointer and writes it somewhere on the heap that has been marked black in a
+ * previous slice. Since the weak pointer will not otherwise be marked and will
+ * be swept and finalized in the last slice, this will leave the pointer just
+ * written dangling after the GC. To solve this, we immediately mark black all
+ * weak pointers that get read between slices so that it is safe to store them
+ * in an already marked part of the heap, e.g. in Rooted.
+ *
+ * Cycle collector read barrier
+ * ----------------------------
+ *
+ * Heap pointers external to the engine may be marked gray. The JS API has an
+ * invariant that no gray pointers may be passed, and this maintained by a read
+ * barrier that calls ExposeGCThingToActiveJS on such pointers. This is
+ * implemented by JS::Heap<T> in js/RootingAPI.h.
+ *
+ * Implementation Details
+ * ======================
+ *
+ * One additional note: not all object writes need to be pre-barriered. Writes
+ * to newly allocated objects do not need a pre-barrier. In these cases, we use
+ * the "obj->field.init(value)" method instead of "obj->field = value". We use
+ * the init naming idiom in many places to signify that a field is being
+ * assigned for the first time.
+ *
+ * This file implements the following hierarchy of classes:
+ *
+ * BarrieredBase             base class of all barriers
+ *  |  |
+ *  | WriteBarriered         base class which provides common write operations
+ *  |  |  |  |  |
+ *  |  |  |  | PreBarriered  provides pre-barriers only
+ *  |  |  |  |
+ *  |  |  | GCPtr            provides pre- and post-barriers
+ *  |  |  |
+ *  |  | HeapPtr             provides pre- and post-barriers; is relocatable
+ *  |  |                     and deletable for use inside C++ managed memory
+ *  |  |
+ *  | HeapSlot               similar to GCPtr, but tailored to slots storage
+ *  |
+ * ReadBarriered             base class which provides common read operations
+ *  |
+ * WeakHeapPtr               provides read barriers only
+ *
+ *
+ * The implementation of the barrier logic is implemented in the
+ * Cell/TenuredCell base classes, which are called via:
+ *
+ * WriteBarriered<T>::pre
+ *  -> InternalBarrierMethods<T*>::preBarrier
+ *      -> Cell::preWriteBarrier
+ *  -> InternalBarrierMethods<Value>::preBarrier
+ *  -> InternalBarrierMethods<jsid>::preBarrier
+ *      -> InternalBarrierMethods<T*>::preBarrier
+ *          -> Cell::preWriteBarrier
+ *
+ * GCPtr<T>::post and HeapPtr<T>::post
+ *  -> InternalBarrierMethods<T*>::postBarrier
+ *      -> gc::PostWriteBarrierImpl
+ *  -> InternalBarrierMethods<Value>::postBarrier
+ *      -> StoreBuffer::put
+ *
+ * Barriers for use outside of the JS engine call into the same barrier
+ * implementations at InternalBarrierMethods<T>::post via an indirect call to
+ * Heap(.+)PostWriteBarrier.
+ *
+ * These clases are designed to be used to wrap GC thing pointers or values that
+ * act like them (i.e. JS::Value and jsid).  It is possible to use them for
+ * other types by supplying the necessary barrier implementations but this
+ * is not usually necessary and should be done with caution.
+ */
+
+namespace js {
+
+class NativeObject;
+
+namespace gc {
+
+inline void ValueReadBarrier(const Value& v) {
+  MOZ_ASSERT(v.isGCThing());
+  ReadBarrierImpl(v.toGCThing());
+}
+
+inline void ValuePreWriteBarrier(const Value& v) {
+  MOZ_ASSERT(v.isGCThing());
+  PreWriteBarrierImpl(v.toGCThing());
+}
+
+inline void IdPreWriteBarrier(jsid id) {
+  MOZ_ASSERT(id.isGCThing());
+  PreWriteBarrierImpl(&id.toGCThing()->asTenured());
+}
+
+inline void CellPtrPreWriteBarrier(JS::GCCellPtr thing) {
+  MOZ_ASSERT(thing);
+  PreWriteBarrierImpl(thing.asCell());
+}
+
+}  // namespace gc
+
+#ifdef DEBUG
+
+bool CurrentThreadIsTouchingGrayThings();
+
+bool IsMarkedBlack(JSObject* obj);
+
+#endif
+
+template <typename T, typename Enable = void>
+struct InternalBarrierMethods {};
+
+template <typename T>
+struct InternalBarrierMethods<T*> {
+  static_assert(std::is_base_of_v<gc::Cell, T>, "Expected a GC thing type");
+
+  static bool isMarkable(const T* v) { return v != nullptr; }
+
+  static void preBarrier(T* v) { gc::PreWriteBarrier(v); }
+
+  static void postBarrier(T** vp, T* prev, T* next) {
+    gc::PostWriteBarrier(vp, prev, next);
+  }
+
+  static void readBarrier(T* v) { gc::ReadBarrier(v); }
+
+#ifdef DEBUG
+  static void assertThingIsNotGray(T* v) { return T::assertThingIsNotGray(v); }
+#endif
+};
+
+template <>
+struct InternalBarrierMethods<Value> {
+  static bool isMarkable(const Value& v) { return v.isGCThing(); }
+
+  static void preBarrier(const Value& v) {
+    if (v.isGCThing()) {
+      gc::ValuePreWriteBarrier(v);
+    }
+  }
+
+  static MOZ_ALWAYS_INLINE void postBarrier(Value* vp, const Value& prev,
+                                            const Value& next) {
+    MOZ_ASSERT(!CurrentThreadIsIonCompiling());
+    MOZ_ASSERT(vp);
+
+    // If the target needs an entry, add it.
+    js::gc::StoreBuffer* sb;
+    if (next.isGCThing() && (sb = next.toGCThing()->storeBuffer())) {
+      // If we know that the prev has already inserted an entry, we can
+      // skip doing the lookup to add the new entry. Note that we cannot
+      // safely assert the presence of the entry because it may have been
+      // added via a different store buffer.
+      if (prev.isGCThing() && prev.toGCThing()->storeBuffer()) {
+        return;
+      }
+      sb->putValue(vp);
+      return;
+    }
+    // Remove the prev entry if the new value does not need it.
+    if (prev.isGCThing() && (sb = prev.toGCThing()->storeBuffer())) {
+      sb->unputValue(vp);
+    }
+  }
+
+  static void readBarrier(const Value& v) {
+    if (v.isGCThing()) {
+      gc::ValueReadBarrier(v);
+    }
+  }
+
+#ifdef DEBUG
+  static void assertThingIsNotGray(const Value& v) {
+    JS::AssertValueIsNotGray(v);
+  }
+#endif
+};
+
+template <>
+struct InternalBarrierMethods<jsid> {
+  static bool isMarkable(jsid id) { return id.isGCThing(); }
+  static void preBarrier(jsid id) {
+    if (id.isGCThing()) {
+      gc::IdPreWriteBarrier(id);
+    }
+  }
+  static void postBarrier(jsid* idp, jsid prev, jsid next) {}
+#ifdef DEBUG
+  static void assertThingIsNotGray(jsid id) { JS::AssertIdIsNotGray(id); }
+#endif
+};
+
+// Specialization for JS::ArrayBufferOrView subclasses.
+template <typename T>
+struct InternalBarrierMethods<T, EnableIfABOVType<T>> {
+  using BM = BarrierMethods<T>;
+
+  static bool isMarkable(const T& thing) { return bool(thing); }
+  static void preBarrier(const T& thing) {
+    gc::PreWriteBarrier(thing.asObjectUnbarriered());
+  }
+  static void postBarrier(T* tp, const T& prev, const T& next) {
+    BM::postWriteBarrier(tp, prev, next);
+  }
+  static void readBarrier(const T& thing) { BM::readBarrier(thing); }
+#ifdef DEBUG
+  static void assertThingIsNotGray(const T& thing) {
+    JSObject* obj = thing.asObjectUnbarriered();
+    if (obj) {
+      JS::AssertValueIsNotGray(JS::ObjectValue(*obj));
+    }
+  }
+#endif
+};
+
+template <typename T>
+static inline void AssertTargetIsNotGray(const T& v) {
+#ifdef DEBUG
+  if (!CurrentThreadIsTouchingGrayThings()) {
+    InternalBarrierMethods<T>::assertThingIsNotGray(v);
+  }
+#endif
+}
+
+// Base class of all barrier types.
+//
+// This is marked non-memmovable since post barriers added by derived classes
+// can add pointers to class instances to the store buffer.
+template <typename T>
+class MOZ_NON_MEMMOVABLE BarrieredBase {
+ protected:
+  // BarrieredBase is not directly instantiable.
+  explicit BarrieredBase(const T& v) : value(v) {}
+
+  // BarrieredBase subclasses cannot be copy constructed by default.
+  BarrieredBase(const BarrieredBase<T>& other) = default;
+
+  // Storage for all barrier classes. |value| must be a GC thing reference
+  // type: either a direct pointer to a GC thing or a supported tagged
+  // pointer that can reference GC things, such as JS::Value or jsid. Nested
+  // barrier types are NOT supported. See assertTypeConstraints.
+  T value;
+
+ public:
+  using ElementType = T;
+
+  // Note: this is public because C++ cannot friend to a specific template
+  // instantiation. Friending to the generic template leads to a number of
+  // unintended consequences, including template resolution ambiguity and a
+  // circular dependency with Tracing.h.
+  T* unbarrieredAddress() const { return const_cast<T*>(&value); }
+};
+
+// Base class for barriered pointer types that intercept only writes.
+template <class T>
+class WriteBarriered : public BarrieredBase<T>,
+                       public WrappedPtrOperations<T, WriteBarriered<T>> {
+ protected:
+  using BarrieredBase<T>::value;
+
+  // WriteBarriered is not directly instantiable.
+  explicit WriteBarriered(const T& v) : BarrieredBase<T>(v) {}
+
+ public:
+  DECLARE_POINTER_CONSTREF_OPS(T);
+
+  // Use this if the automatic coercion to T isn't working.
+  const T& get() const { return this->value; }
+
+  // Use this if you want to change the value without invoking barriers.
+  // Obviously this is dangerous unless you know the barrier is not needed.
+  void unbarrieredSet(const T& v) { this->value = v; }
+
+  // For users who need to manually barrier the raw types.
+  static void preWriteBarrier(const T& v) {
+    InternalBarrierMethods<T>::preBarrier(v);
+  }
+
+ protected:
+  void pre() { InternalBarrierMethods<T>::preBarrier(this->value); }
+  MOZ_ALWAYS_INLINE void post(const T& prev, const T& next) {
+    InternalBarrierMethods<T>::postBarrier(&this->value, prev, next);
+  }
+};
+
+#define DECLARE_POINTER_ASSIGN_AND_MOVE_OPS(Wrapper, T) \
+  DECLARE_POINTER_ASSIGN_OPS(Wrapper, T)                \
+  Wrapper<T>& operator=(Wrapper<T>&& other) {           \
+    setUnchecked(other.release());                      \
+    return *this;                                       \
+  }
+
+/*
+ * PreBarriered only automatically handles pre-barriers. Post-barriers must be
+ * manually implemented when using this class. GCPtr and HeapPtr should be used
+ * in all cases that do not require explicit low-level control of moving
+ * behavior.
+ *
+ * This class is useful for example for HashMap keys where automatically
+ * updating a moved nursery pointer would break the hash table.
+ */
+template <class T>
+class PreBarriered : public WriteBarriered<T> {
+ public:
+  PreBarriered() : WriteBarriered<T>(JS::SafelyInitialized<T>::create()) {}
+  /*
+   * Allow implicit construction for use in generic contexts.
+   */
+  MOZ_IMPLICIT PreBarriered(const T& v) : WriteBarriered<T>(v) {}
+
+  explicit PreBarriered(const PreBarriered<T>& other)
+      : WriteBarriered<T>(other.value) {}
+
+  PreBarriered(PreBarriered<T>&& other) : WriteBarriered<T>(other.release()) {}
+
+  ~PreBarriered() { this->pre(); }
+
+  void init(const T& v) { this->value = v; }
+
+  /* Use to set the pointer to nullptr. */
+  void clear() { set(JS::SafelyInitialized<T>::create()); }
+
+  DECLARE_POINTER_ASSIGN_AND_MOVE_OPS(PreBarriered, T);
+
+  void set(const T& v) {
+    AssertTargetIsNotGray(v);
+    setUnchecked(v);
+  }
+
+ private:
+  void setUnchecked(const T& v) {
+    this->pre();
+    this->value = v;
+  }
+
+  T release() {
+    T tmp = this->value;
+    this->value = JS::SafelyInitialized<T>::create();
+    return tmp;
+  }
+};
+
+}  // namespace js
+
+namespace JS {
+
+namespace detail {
+
+template <typename T>
+struct DefineComparisonOps<js::PreBarriered<T>> : std::true_type {
+  static const T& get(const js::PreBarriered<T>& v) { return v.get(); }
+};
+
+}  // namespace detail
+
+}  // namespace JS
+
+namespace js {
+
+/*
+ * A pre- and post-barriered heap pointer, for use inside the JS engine.
+ *
+ * It must only be stored in memory that has GC lifetime. GCPtr must not be
+ * used in contexts where it may be implicitly moved or deleted, e.g. most
+ * containers.
+ *
+ * The post-barriers implemented by this class are faster than those
+ * implemented by js::HeapPtr<T> or JS::Heap<T> at the cost of not
+ * automatically handling deletion or movement.
+ */
+template <class T>
+class GCPtr : public WriteBarriered<T> {
+ public:
+  GCPtr() : WriteBarriered<T>(JS::SafelyInitialized<T>::create()) {}
+
+  explicit GCPtr(const T& v) : WriteBarriered<T>(v) {
+    this->post(JS::SafelyInitialized<T>::create(), v);
+  }
+
+  explicit GCPtr(const GCPtr<T>& v) : WriteBarriered<T>(v) {
+    this->post(JS::SafelyInitialized<T>::create(), v);
+  }
+
+#ifdef DEBUG
+  ~GCPtr() {
+    // No barriers are necessary as this only happens when the GC is sweeping.
+    //
+    // If this assertion fails you may need to make the containing object use a
+    // HeapPtr instead, as this can be deleted from outside of GC.
+    MOZ_ASSERT(CurrentThreadIsGCSweeping() || CurrentThreadIsGCFinalizing());
+
+    Poison(this, JS_FREED_HEAP_PTR_PATTERN, sizeof(*this),
+           MemCheckKind::MakeNoAccess);
+  }
+#endif
+
+  void init(const T& v) {
+    AssertTargetIsNotGray(v);
+    this->value = v;
+    this->post(JS::SafelyInitialized<T>::create(), v);
+  }
+
+  DECLARE_POINTER_ASSIGN_OPS(GCPtr, T);
+
+  void set(const T& v) {
+    AssertTargetIsNotGray(v);
+    setUnchecked(v);
+  }
+
+ private:
+  void setUnchecked(const T& v) {
+    this->pre();
+    T tmp = this->value;
+    this->value = v;
+    this->post(tmp, this->value);
+  }
+
+  /*
+   * Unlike HeapPtr<T>, GCPtr<T> must be managed with GC lifetimes.
+   * Specifically, the memory used by the pointer itself must be live until
+   * at least the next minor GC. For that reason, move semantics are invalid
+   * and are deleted here. Please note that not all containers support move
+   * semantics, so this does not completely prevent invalid uses.
+   */
+  GCPtr(GCPtr<T>&&) = delete;
+  GCPtr<T>& operator=(GCPtr<T>&&) = delete;
+};
+
+}  // namespace js
+
+namespace JS {
+
+namespace detail {
+
+template <typename T>
+struct DefineComparisonOps<js::GCPtr<T>> : std::true_type {
+  static const T& get(const js::GCPtr<T>& v) { return v.get(); }
+};
+
+}  // namespace detail
+
+}  // namespace JS
+
+namespace js {
+
+/*
+ * A pre- and post-barriered heap pointer, for use inside the JS engine. These
+ * heap pointers can be stored in C++ containers like GCVector and GCHashMap.
+ *
+ * The GC sometimes keeps pointers to pointers to GC things --- for example, to
+ * track references into the nursery. However, C++ containers like GCVector and
+ * GCHashMap usually reserve the right to relocate their elements any time
+ * they're modified, invalidating all pointers to the elements. HeapPtr
+ * has a move constructor which knows how to keep the GC up to date if it is
+ * moved to a new location.
+ *
+ * However, because of this additional communication with the GC, HeapPtr
+ * is somewhat slower, so it should only be used in contexts where this ability
+ * is necessary.
+ *
+ * Obviously, JSObjects, JSStrings, and the like get tenured and compacted, so
+ * whatever pointers they contain get relocated, in the sense used here.
+ * However, since the GC itself is moving those values, it takes care of its
+ * internal pointers to those pointers itself. HeapPtr is only necessary
+ * when the relocation would otherwise occur without the GC's knowledge.
+ */
+template <class T>
+class HeapPtr : public WriteBarriered<T> {
+ public:
+  HeapPtr() : WriteBarriered<T>(JS::SafelyInitialized<T>::create()) {}
+
+  // Implicitly adding barriers is a reasonable default.
+  MOZ_IMPLICIT HeapPtr(const T& v) : WriteBarriered<T>(v) {
+    this->post(JS::SafelyInitialized<T>::create(), this->value);
+  }
+
+  MOZ_IMPLICIT HeapPtr(const HeapPtr<T>& other) : WriteBarriered<T>(other) {
+    this->post(JS::SafelyInitialized<T>::create(), this->value);
+  }
+
+  HeapPtr(HeapPtr<T>&& other) : WriteBarriered<T>(other.release()) {
+    this->post(JS::SafelyInitialized<T>::create(), this->value);
+  }
+
+  ~HeapPtr() {
+    this->pre();
+    this->post(this->value, JS::SafelyInitialized<T>::create());
+  }
+
+  void init(const T& v) {
+    MOZ_ASSERT(this->value == JS::SafelyInitialized<T>::create());
+    AssertTargetIsNotGray(v);
+    this->value = v;
+    this->post(JS::SafelyInitialized<T>::create(), this->value);
+  }
+
+  DECLARE_POINTER_ASSIGN_AND_MOVE_OPS(HeapPtr, T);
+
+  void set(const T& v) {
+    AssertTargetIsNotGray(v);
+    setUnchecked(v);
+  }
+
+  /* Make this friend so it can access pre() and post(). */
+  template <class T1, class T2>
+  friend inline void BarrieredSetPair(Zone* zone, HeapPtr<T1*>& v1, T1* val1,
+                                      HeapPtr<T2*>& v2, T2* val2);
+
+ protected:
+  void setUnchecked(const T& v) {
+    this->pre();
+    postBarrieredSet(v);
+  }
+
+  void postBarrieredSet(const T& v) {
+    T tmp = this->value;
+    this->value = v;
+    this->post(tmp, this->value);
+  }
+
+  T release() {
+    T tmp = this->value;
+    postBarrieredSet(JS::SafelyInitialized<T>::create());
+    return tmp;
+  }
+};
+
+/*
+ * A pre-barriered heap pointer, for use inside the JS engine.
+ *
+ * Similar to GCPtr, but used for a pointer to a malloc-allocated structure
+ * containing GC thing pointers.
+ *
+ * It must only be stored in memory that has GC lifetime. It must not be used in
+ * contexts where it may be implicitly moved or deleted, e.g. most containers.
+ *
+ * A post-barrier is unnecessary since malloc-allocated structures cannot be in
+ * the nursery.
+ */
+template <class T>
+class GCStructPtr : public BarrieredBase<T> {
+ public:
+  // This is sometimes used to hold tagged pointers.
+  static constexpr uintptr_t MaxTaggedPointer = 0x2;
+
+  GCStructPtr() : BarrieredBase<T>(JS::SafelyInitialized<T>::create()) {}
+
+  // Implicitly adding barriers is a reasonable default.
+  MOZ_IMPLICIT GCStructPtr(const T& v) : BarrieredBase<T>(v) {}
+
+  GCStructPtr(const GCStructPtr<T>& other) : BarrieredBase<T>(other) {}
+
+  GCStructPtr(GCStructPtr<T>&& other) : BarrieredBase<T>(other.release()) {}
+
+  ~GCStructPtr() {
+    // No barriers are necessary as this only happens when the GC is sweeping.
+    MOZ_ASSERT_IF(isTraceable(),
+                  CurrentThreadIsGCSweeping() || CurrentThreadIsGCFinalizing());
+  }
+
+  void init(const T& v) {
+    MOZ_ASSERT(this->get() == JS::SafelyInitialized<T>());
+    AssertTargetIsNotGray(v);
+    this->value = v;
+  }
+
+  void set(JS::Zone* zone, const T& v) {
+    pre(zone);
+    this->value = v;
+  }
+
+  T get() const { return this->value; }
+  operator T() const { return get(); }
+  T operator->() const { return get(); }
+
+ protected:
+  bool isTraceable() const { return uintptr_t(get()) > MaxTaggedPointer; }
+
+  void pre(JS::Zone* zone) {
+    if (isTraceable()) {
+      PreWriteBarrier(zone, get());
+    }
+  }
+};
+
+}  // namespace js
+
+namespace JS {
+
+namespace detail {
+
+template <typename T>
+struct DefineComparisonOps<js::HeapPtr<T>> : std::true_type {
+  static const T& get(const js::HeapPtr<T>& v) { return v.get(); }
+};
+
+}  // namespace detail
+
+}  // namespace JS
+
+namespace js {
+
+// Base class for barriered pointer types that intercept reads and writes.
+template <typename T>
+class ReadBarriered : public BarrieredBase<T> {
+ protected:
+  // ReadBarriered is not directly instantiable.
+  explicit ReadBarriered(const T& v) : BarrieredBase<T>(v) {}
+
+  void read() const { InternalBarrierMethods<T>::readBarrier(this->value); }
+  void post(const T& prev, const T& next) {
+    InternalBarrierMethods<T>::postBarrier(&this->value, prev, next);
+  }
+};
+
+// Incremental GC requires that weak pointers have read barriers. See the block
+// comment at the top of Barrier.h for a complete discussion of why.
+//
+// Note that this class also has post-barriers, so is safe to use with nursery
+// pointers. However, when used as a hashtable key, care must still be taken to
+// insert manual post-barriers on the table for rekeying if the key is based in
+// any way on the address of the object.
+template <typename T>
+class WeakHeapPtr : public ReadBarriered<T>,
+                    public WrappedPtrOperations<T, WeakHeapPtr<T>> {
+ protected:
+  using ReadBarriered<T>::value;
+
+ public:
+  WeakHeapPtr() : ReadBarriered<T>(JS::SafelyInitialized<T>::create()) {}
+
+  // It is okay to add barriers implicitly.
+  MOZ_IMPLICIT WeakHeapPtr(const T& v) : ReadBarriered<T>(v) {
+    this->post(JS::SafelyInitialized<T>::create(), v);
+  }
+
+  // The copy constructor creates a new weak edge but the wrapped pointer does
+  // not escape, so no read barrier is necessary.
+  explicit WeakHeapPtr(const WeakHeapPtr& other) : ReadBarriered<T>(other) {
+    this->post(JS::SafelyInitialized<T>::create(), value);
+  }
+
+  // Move retains the lifetime status of the source edge, so does not fire
+  // the read barrier of the defunct edge.
+  WeakHeapPtr(WeakHeapPtr&& other) : ReadBarriered<T>(other.release()) {
+    this->post(JS::SafelyInitialized<T>::create(), value);
+  }
+
+  ~WeakHeapPtr() {
+    this->post(this->value, JS::SafelyInitialized<T>::create());
+  }
+
+  WeakHeapPtr& operator=(const WeakHeapPtr& v) {
+    AssertTargetIsNotGray(v.value);
+    T prior = this->value;
+    this->value = v.value;
+    this->post(prior, v.value);
+    return *this;
+  }
+
+  const T& get() const {
+    if (InternalBarrierMethods<T>::isMarkable(this->value)) {
+      this->read();
+    }
+    return this->value;
+  }
+
+  const T& unbarrieredGet() const { return this->value; }
+
+  explicit operator bool() const { return bool(this->value); }
+
+  operator const T&() const { return get(); }
+
+  const T& operator->() const { return get(); }
+
+  void set(const T& v) {
+    AssertTargetIsNotGray(v);
+    setUnchecked(v);
+  }
+
+  void unbarrieredSet(const T& v) {
+    AssertTargetIsNotGray(v);
+    this->value = v;
+  }
+
+ private:
+  void setUnchecked(const T& v) {
+    T tmp = this->value;
+    this->value = v;
+    this->post(tmp, v);
+  }
+
+  T release() {
+    T tmp = value;
+    set(JS::SafelyInitialized<T>::create());
+    return tmp;
+  }
+};
+
+// A wrapper for a bare pointer, with no barriers.
+//
+// This should only be necessary in a limited number of cases. Please don't add
+// more uses of this if at all possible.
+template <typename T>
+class UnsafeBarePtr : public BarrieredBase<T> {
+ public:
+  UnsafeBarePtr() : BarrieredBase<T>(JS::SafelyInitialized<T>::create()) {}
+  MOZ_IMPLICIT UnsafeBarePtr(T v) : BarrieredBase<T>(v) {}
+  const T& get() const { return this->value; }
+  void set(T newValue) { this->value = newValue; }
+  DECLARE_POINTER_CONSTREF_OPS(T);
+};
+
+}  // namespace js
+
+namespace JS {
+
+namespace detail {
+
+template <typename T>
+struct DefineComparisonOps<js::WeakHeapPtr<T>> : std::true_type {
+  static const T& get(const js::WeakHeapPtr<T>& v) {
+    return v.unbarrieredGet();
+  }
+};
+
+}  // namespace detail
+
+}  // namespace JS
+
+namespace js {
+
+// A pre- and post-barriered Value that is specialized to be aware that it
+// resides in a slots or elements vector. This allows it to be relocated in
+// memory, but with substantially less overhead than a HeapPtr.
+class HeapSlot : public WriteBarriered<Value> {
+ public:
+  enum Kind { Slot = 0, Element = 1 };
+
+  void init(NativeObject* owner, Kind kind, uint32_t slot, const Value& v) {
+    value = v;
+    post(owner, kind, slot, v);
+  }
+
+  void initAsUndefined() { value.setUndefined(); }
+
+  void destroy() { pre(); }
+
+  void setUndefinedUnchecked() {
+    pre();
+    value.setUndefined();
+  }
+
+#ifdef DEBUG
+  bool preconditionForSet(NativeObject* owner, Kind kind, uint32_t slot) const;
+  void assertPreconditionForPostWriteBarrier(NativeObject* obj, Kind kind,
+                                             uint32_t slot,
+                                             const Value& target) const;
+#endif
+
+  MOZ_ALWAYS_INLINE void set(NativeObject* owner, Kind kind, uint32_t slot,
+                             const Value& v) {
+    MOZ_ASSERT(preconditionForSet(owner, kind, slot));
+    pre();
+    value = v;
+    post(owner, kind, slot, v);
+  }
+
+ private:
+  void post(NativeObject* owner, Kind kind, uint32_t slot,
+            const Value& target) {
+#ifdef DEBUG
+    assertPreconditionForPostWriteBarrier(owner, kind, slot, target);
+#endif
+    if (this->value.isGCThing()) {
+      gc::Cell* cell = this->value.toGCThing();
+      if (cell->storeBuffer()) {
+        cell->storeBuffer()->putSlot(owner, kind, slot, 1);
+      }
+    }
+  }
+};
+
+}  // namespace js
+
+namespace JS {
+
+namespace detail {
+
+template <>
+struct DefineComparisonOps<js::HeapSlot> : std::true_type {
+  static const Value& get(const js::HeapSlot& v) { return v.get(); }
+};
+
+}  // namespace detail
+
+}  // namespace JS
+
+namespace js {
+
+class HeapSlotArray {
+  HeapSlot* array;
+
+ public:
+  explicit HeapSlotArray(HeapSlot* array) : array(array) {}
+
+  HeapSlot* begin() const { return array; }
+
+  operator const Value*() const {
+    static_assert(sizeof(GCPtr<Value>) == sizeof(Value));
+    static_assert(sizeof(HeapSlot) == sizeof(Value));
+    return reinterpret_cast<const Value*>(array);
+  }
+  operator HeapSlot*() const { return begin(); }
+
+  HeapSlotArray operator+(int offset) const {
+    return HeapSlotArray(array + offset);
+  }
+  HeapSlotArray operator+(uint32_t offset) const {
+    return HeapSlotArray(array + offset);
+  }
+};
+
+/*
+ * This is a hack for RegExpStatics::updateFromMatch. It allows us to do two
+ * barriers with only one branch to check if we're in an incremental GC.
+ */
+template <class T1, class T2>
+static inline void BarrieredSetPair(Zone* zone, HeapPtr<T1*>& v1, T1* val1,
+                                    HeapPtr<T2*>& v2, T2* val2) {
+  AssertTargetIsNotGray(val1);
+  AssertTargetIsNotGray(val2);
+  if (T1::needPreWriteBarrier(zone)) {
+    v1.pre();
+    v2.pre();
+  }
+  v1.postBarrieredSet(val1);
+  v2.postBarrieredSet(val2);
+}
+
+/*
+ * ImmutableTenuredPtr is designed for one very narrow case: replacing
+ * immutable raw pointers to GC-managed things, implicitly converting to a
+ * handle type for ease of use. Pointers encapsulated by this type must:
+ *
+ *   be immutable (no incremental write barriers),
+ *   never point into the nursery (no generational write barriers), and
+ *   be traced via MarkRuntime (we use fromMarkedLocation).
+ *
+ * In short: you *really* need to know what you're doing before you use this
+ * class!
+ */
+template <typename T>
+class MOZ_HEAP_CLASS ImmutableTenuredPtr {
+  T value;
+
+ public:
+  operator T() const { return value; }
+  T operator->() const { return value; }
+
+  // `ImmutableTenuredPtr<T>` is implicitly convertible to `Handle<T>`.
+  //
+  // In case you need to convert to `Handle<U>` where `U` is base class of `T`,
+  // convert this to `Handle<T>` by `toHandle()` and then use implicit
+  // conversion from `Handle<T>` to `Handle<U>`.
+  operator Handle<T>() const { return toHandle(); }
+  Handle<T> toHandle() const { return Handle<T>::fromMarkedLocation(&value); }
+
+  void init(T ptr) {
+    MOZ_ASSERT(ptr->isTenured());
+    AssertTargetIsNotGray(ptr);
+    value = ptr;
+  }
+
+  T get() const { return value; }
+  const T* address() { return &value; }
+};
+
+// Template to remove any barrier wrapper and get the underlying type.
+template <typename T>
+struct RemoveBarrier {
+  using Type = T;
+};
+template <typename T>
+struct RemoveBarrier<HeapPtr<T>> {
+  using Type = T;
+};
+template <typename T>
+struct RemoveBarrier<GCPtr<T>> {
+  using Type = T;
+};
+template <typename T>
+struct RemoveBarrier<PreBarriered<T>> {
+  using Type = T;
+};
+template <typename T>
+struct RemoveBarrier<WeakHeapPtr<T>> {
+  using Type = T;
+};
+
+#if MOZ_IS_GCC
+template struct JS_PUBLIC_API StableCellHasher<JSObject*>;
+#endif
+
+template <typename T>
+struct StableCellHasher<PreBarriered<T>> {
+  using Key = PreBarriered<T>;
+  using Lookup = T;
+
+  static bool maybeGetHash(const Lookup& l, HashNumber* hashOut) {
+    return StableCellHasher<T>::maybeGetHash(l, hashOut);
+  }
+  static bool ensureHash(const Lookup& l, HashNumber* hashOut) {
+    return StableCellHasher<T>::ensureHash(l, hashOut);
+  }
+  static HashNumber hash(const Lookup& l) {
+    return StableCellHasher<T>::hash(l);
+  }
+  static bool match(const Key& k, const Lookup& l) {
+    return StableCellHasher<T>::match(k, l);
+  }
+};
+
+template <typename T>
+struct StableCellHasher<HeapPtr<T>> {
+  using Key = HeapPtr<T>;
+  using Lookup = T;
+
+  static bool maybeGetHash(const Lookup& l, HashNumber* hashOut) {
+    return StableCellHasher<T>::maybeGetHash(l, hashOut);
+  }
+  static bool ensureHash(const Lookup& l, HashNumber* hashOut) {
+    return StableCellHasher<T>::ensureHash(l, hashOut);
+  }
+  static HashNumber hash(const Lookup& l) {
+    return StableCellHasher<T>::hash(l);
+  }
+  static bool match(const Key& k, const Lookup& l) {
+    return StableCellHasher<T>::match(k, l);
+  }
+};
+
+template <typename T>
+struct StableCellHasher<WeakHeapPtr<T>> {
+  using Key = WeakHeapPtr<T>;
+  using Lookup = T;
+
+  static bool maybeGetHash(const Lookup& l, HashNumber* hashOut) {
+    return StableCellHasher<T>::maybeGetHash(l, hashOut);
+  }
+  static bool ensureHash(const Lookup& l, HashNumber* hashOut) {
+    return StableCellHasher<T>::ensureHash(l, hashOut);
+  }
+  static HashNumber hash(const Lookup& l) {
+    return StableCellHasher<T>::hash(l);
+  }
+  static bool match(const Key& k, const Lookup& l) {
+    return StableCellHasher<T>::match(k.unbarrieredGet(), l);
+  }
+};
+
+/* Useful for hashtables with a HeapPtr as key. */
+template <class T>
+struct HeapPtrHasher {
+  using Key = HeapPtr<T>;
+  using Lookup = T;
+
+  static HashNumber hash(Lookup obj) { return DefaultHasher<T>::hash(obj); }
+  static bool match(const Key& k, Lookup l) { return k.get() == l; }
+  static void rekey(Key& k, const Key& newKey) { k.unbarrieredSet(newKey); }
+};
+
+template <class T>
+struct PreBarrieredHasher {
+  using Key = PreBarriered<T>;
+  using Lookup = T;
+
+  static HashNumber hash(Lookup obj) { return DefaultHasher<T>::hash(obj); }
+  static bool match(const Key& k, Lookup l) { return k.get() == l; }
+  static void rekey(Key& k, const Key& newKey) { k.unbarrieredSet(newKey); }
+};
+
+/* Useful for hashtables with a WeakHeapPtr as key. */
+template <class T>
+struct WeakHeapPtrHasher {
+  using Key = WeakHeapPtr<T>;
+  using Lookup = T;
+
+  static HashNumber hash(Lookup obj) { return DefaultHasher<T>::hash(obj); }
+  static bool match(const Key& k, Lookup l) { return k.unbarrieredGet() == l; }
+  static void rekey(Key& k, const Key& newKey) {
+    k.set(newKey.unbarrieredGet());
+  }
+};
+
+template <class T>
+struct UnsafeBarePtrHasher {
+  using Key = UnsafeBarePtr<T>;
+  using Lookup = T;
+
+  static HashNumber hash(const Lookup& l) { return DefaultHasher<T>::hash(l); }
+  static bool match(const Key& k, Lookup l) { return k.get() == l; }
+  static void rekey(Key& k, const Key& newKey) { k.set(newKey.get()); }
+};
+
+}  // namespace js
+
+namespace mozilla {
+
+template <class T>
+struct DefaultHasher<js::HeapPtr<T>> : js::HeapPtrHasher<T> {};
+
+template <class T>
+struct DefaultHasher<js::GCPtr<T>> {
+  // Not implemented. GCPtr can't be used as a hash table key because it has a
+  // post barrier but doesn't support relocation.
+};
+
+template <class T>
+struct DefaultHasher<js::PreBarriered<T>> : js::PreBarrieredHasher<T> {};
+
+template <class T>
+struct DefaultHasher<js::WeakHeapPtr<T>> : js::WeakHeapPtrHasher<T> {};
+
+template <class T>
+struct DefaultHasher<js::UnsafeBarePtr<T>> : js::UnsafeBarePtrHasher<T> {};
+
+}  // namespace mozilla
+
+#endif /* gc_Barrier_h */