Adding upstream version 124.0.1.upstream/124.0.1

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

1 files changed, 765 insertions, 0 deletions

diff --git a/js/src/gc/StoreBuffer.h b/js/src/gc/StoreBuffer.h
new file mode 100644
index 0000000000..2e117f02c8
--- /dev/null
+++ b/js/src/gc/StoreBuffer.h
@@ -0,0 +1,765 @@
+/* -*- 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_StoreBuffer_h
+#define gc_StoreBuffer_h
+
+#include "mozilla/Attributes.h"
+#include "mozilla/HashFunctions.h"
+#include "mozilla/ReentrancyGuard.h"
+
+#include <algorithm>
+
+#include "ds/BitArray.h"
+#include "ds/LifoAlloc.h"
+#include "gc/Cell.h"
+#include "gc/Nursery.h"
+#include "gc/TraceKind.h"
+#include "js/AllocPolicy.h"
+#include "js/UniquePtr.h"
+#include "threading/Mutex.h"
+#include "wasm/WasmAnyRef.h"
+
+namespace JS {
+struct GCSizes;
+}
+
+namespace js {
+
+class NativeObject;
+
+#ifdef DEBUG
+extern bool CurrentThreadIsGCMarking();
+#endif
+
+namespace gc {
+
+class Arena;
+class ArenaCellSet;
+
+#ifdef DEBUG
+extern bool CurrentThreadHasLockedGC();
+#endif
+
+/*
+ * BufferableRef represents an abstract reference for use in the generational
+ * GC's remembered set. Entries in the store buffer that cannot be represented
+ * with the simple pointer-to-a-pointer scheme must derive from this class and
+ * use the generic store buffer interface.
+ *
+ * A single BufferableRef entry in the generic buffer can represent many entries
+ * in the remembered set.  For example js::OrderedHashTableRef represents all
+ * the incoming edges corresponding to keys in an ordered hash table.
+ */
+class BufferableRef {
+ public:
+  virtual void trace(JSTracer* trc) = 0;
+  bool maybeInRememberedSet(const Nursery&) const { return true; }
+};
+
+using EdgeSet = HashSet<void*, PointerHasher<void*>, SystemAllocPolicy>;
+
+/* The size of a single block of store buffer storage space. */
+static const size_t LifoAllocBlockSize = 8 * 1024;
+
+/*
+ * The StoreBuffer observes all writes that occur in the system and performs
+ * efficient filtering of them to derive a remembered set for nursery GC.
+ */
+class StoreBuffer {
+  friend class mozilla::ReentrancyGuard;
+
+  /* The size at which a block is about to overflow for the generic buffer. */
+  static const size_t GenericBufferLowAvailableThreshold =
+      LifoAllocBlockSize / 2;
+
+  /* The size at which other store buffers are about to overflow. */
+  static const size_t BufferOverflowThresholdBytes = 128 * 1024;
+
+  enum class PutResult { OK, AboutToOverflow };
+
+  /*
+   * This buffer holds only a single type of edge. Using this buffer is more
+   * efficient than the generic buffer when many writes will be to the same
+   * type of edge: e.g. Value or Cell*.
+   */
+  template <typename T>
+  struct MonoTypeBuffer {
+    /* The canonical set of stores. */
+    using StoreSet = HashSet<T, typename T::Hasher, SystemAllocPolicy>;
+    StoreSet stores_;
+
+    /*
+     * A one element cache in front of the canonical set to speed up
+     * temporary instances of HeapPtr.
+     */
+    T last_ = T();
+
+    /* Maximum number of entries before we request a minor GC. */
+    const static size_t MaxEntries = BufferOverflowThresholdBytes / sizeof(T);
+
+    MonoTypeBuffer() = default;
+
+    MonoTypeBuffer(const MonoTypeBuffer& other) = delete;
+    MonoTypeBuffer& operator=(const MonoTypeBuffer& other) = delete;
+
+    MonoTypeBuffer(MonoTypeBuffer&& other)
+        : stores_(std::move(other.stores_)), last_(std::move(other.last_)) {
+      other.clear();
+    }
+    MonoTypeBuffer& operator=(MonoTypeBuffer&& other) {
+      if (&other != this) {
+        this->~MonoTypeBuffer();
+        new (this) MonoTypeBuffer(std::move(other));
+      }
+      return *this;
+    }
+
+    void clear() {
+      last_ = T();
+      stores_.clear();
+    }
+
+    /* Add one item to the buffer. */
+    PutResult put(const T& t) {
+      PutResult r = sinkStore();
+      last_ = t;
+      return r;
+    }
+
+    /* Remove an item from the store buffer. */
+    void unput(const T& v) {
+      // Fast, hashless remove of last put.
+      if (last_ == v) {
+        last_ = T();
+        return;
+      }
+      stores_.remove(v);
+    }
+
+    /* Move any buffered stores to the canonical store set. */
+    PutResult sinkStore() {
+      if (last_) {
+        AutoEnterOOMUnsafeRegion oomUnsafe;
+        if (!stores_.put(last_)) {
+          oomUnsafe.crash("Failed to allocate for MonoTypeBuffer::put.");
+        }
+      }
+      last_ = T();
+
+      if (stores_.count() > MaxEntries) {
+        return PutResult::AboutToOverflow;
+      }
+
+      return PutResult::OK;
+    }
+
+    /* Trace the source of all edges in the store buffer. */
+    void trace(TenuringTracer& mover, StoreBuffer* owner);
+
+    size_t sizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf) {
+      return stores_.shallowSizeOfExcludingThis(mallocSizeOf);
+    }
+
+    bool isEmpty() const { return last_ == T() && stores_.empty(); }
+  };
+
+  struct WholeCellBuffer {
+    UniquePtr<LifoAlloc> storage_;
+    ArenaCellSet* stringHead_ = nullptr;
+    ArenaCellSet* nonStringHead_ = nullptr;
+    const Cell* last_ = nullptr;
+
+    WholeCellBuffer() = default;
+
+    WholeCellBuffer(const WholeCellBuffer& other) = delete;
+    WholeCellBuffer& operator=(const WholeCellBuffer& other) = delete;
+
+    WholeCellBuffer(WholeCellBuffer&& other)
+        : storage_(std::move(other.storage_)),
+          stringHead_(other.stringHead_),
+          nonStringHead_(other.nonStringHead_),
+          last_(other.last_) {
+      other.stringHead_ = nullptr;
+      other.nonStringHead_ = nullptr;
+      other.last_ = nullptr;
+    }
+    WholeCellBuffer& operator=(WholeCellBuffer&& other) {
+      if (&other != this) {
+        this->~WholeCellBuffer();
+        new (this) WholeCellBuffer(std::move(other));
+      }
+      return *this;
+    }
+
+    [[nodiscard]] bool init();
+
+    void clear();
+
+    bool isAboutToOverflow() const {
+      return !storage_->isEmpty() &&
+             storage_->used() > BufferOverflowThresholdBytes;
+    }
+
+    void trace(TenuringTracer& mover, StoreBuffer* owner);
+
+    inline void put(const Cell* cell);
+    inline void putDontCheckLast(const Cell* cell);
+
+    size_t sizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf) {
+      return storage_ ? storage_->sizeOfIncludingThis(mallocSizeOf) : 0;
+    }
+
+    bool isEmpty() const {
+      MOZ_ASSERT_IF(!stringHead_ && !nonStringHead_,
+                    !storage_ || storage_->isEmpty());
+      return !stringHead_ && !nonStringHead_;
+    }
+
+    const Cell** lastBufferedPtr() { return &last_; }
+
+   private:
+    ArenaCellSet* allocateCellSet(Arena* arena);
+  };
+
+  struct GenericBuffer {
+    UniquePtr<LifoAlloc> storage_;
+
+    GenericBuffer() = default;
+
+    GenericBuffer(const GenericBuffer& other) = delete;
+    GenericBuffer& operator=(const GenericBuffer& other) = delete;
+
+    GenericBuffer(GenericBuffer&& other)
+        : storage_(std::move(other.storage_)) {}
+    GenericBuffer& operator=(GenericBuffer&& other) {
+      if (&other != this) {
+        this->~GenericBuffer();
+        new (this) GenericBuffer(std::move(other));
+      }
+      return *this;
+    }
+
+    [[nodiscard]] bool init();
+
+    void clear() {
+      if (storage_) {
+        storage_->used() ? storage_->releaseAll() : storage_->freeAll();
+      }
+    }
+
+    bool isAboutToOverflow() const {
+      return !storage_->isEmpty() && storage_->availableInCurrentChunk() <
+                                         GenericBufferLowAvailableThreshold;
+    }
+
+    /* Trace all generic edges. */
+    void trace(JSTracer* trc, StoreBuffer* owner);
+
+    template <typename T>
+    PutResult put(const T& t) {
+      MOZ_ASSERT(storage_);
+
+      /* Ensure T is derived from BufferableRef. */
+      (void)static_cast<const BufferableRef*>(&t);
+
+      AutoEnterOOMUnsafeRegion oomUnsafe;
+      unsigned size = sizeof(T);
+      unsigned* sizep = storage_->pod_malloc<unsigned>();
+      if (!sizep) {
+        oomUnsafe.crash("Failed to allocate for GenericBuffer::put.");
+      }
+      *sizep = size;
+
+      T* tp = storage_->new_<T>(t);
+      if (!tp) {
+        oomUnsafe.crash("Failed to allocate for GenericBuffer::put.");
+      }
+
+      if (isAboutToOverflow()) {
+        return PutResult::AboutToOverflow;
+      }
+
+      return PutResult::OK;
+    }
+
+    size_t sizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf) {
+      return storage_ ? storage_->sizeOfIncludingThis(mallocSizeOf) : 0;
+    }
+
+    bool isEmpty() const { return !storage_ || storage_->isEmpty(); }
+  };
+
+  template <typename Edge>
+  struct PointerEdgeHasher {
+    using Lookup = Edge;
+    static HashNumber hash(const Lookup& l) {
+      return mozilla::HashGeneric(l.edge);
+    }
+    static bool match(const Edge& k, const Lookup& l) { return k == l; }
+  };
+
+  template <typename T>
+  struct CellPtrEdge {
+    T** edge = nullptr;
+
+    CellPtrEdge() = default;
+    explicit CellPtrEdge(T** v) : edge(v) {}
+    bool operator==(const CellPtrEdge& other) const {
+      return edge == other.edge;
+    }
+    bool operator!=(const CellPtrEdge& other) const {
+      return edge != other.edge;
+    }
+
+    bool maybeInRememberedSet(const Nursery& nursery) const {
+      MOZ_ASSERT(IsInsideNursery(*edge));
+      return !nursery.isInside(edge);
+    }
+
+    void trace(TenuringTracer& mover) const;
+
+    explicit operator bool() const { return edge != nullptr; }
+
+    using Hasher = PointerEdgeHasher<CellPtrEdge<T>>;
+  };
+
+  using ObjectPtrEdge = CellPtrEdge<JSObject>;
+  using StringPtrEdge = CellPtrEdge<JSString>;
+  using BigIntPtrEdge = CellPtrEdge<JS::BigInt>;
+
+  struct ValueEdge {
+    JS::Value* edge;
+
+    ValueEdge() : edge(nullptr) {}
+    explicit ValueEdge(JS::Value* v) : edge(v) {}
+    bool operator==(const ValueEdge& other) const { return edge == other.edge; }
+    bool operator!=(const ValueEdge& other) const { return edge != other.edge; }
+
+    Cell* deref() const {
+      return edge->isGCThing() ? static_cast<Cell*>(edge->toGCThing())
+                               : nullptr;
+    }
+
+    bool maybeInRememberedSet(const Nursery& nursery) const {
+      MOZ_ASSERT(IsInsideNursery(deref()));
+      return !nursery.isInside(edge);
+    }
+
+    void trace(TenuringTracer& mover) const;
+
+    explicit operator bool() const { return edge != nullptr; }
+
+    using Hasher = PointerEdgeHasher<ValueEdge>;
+  };
+
+  struct SlotsEdge {
+    // These definitions must match those in HeapSlot::Kind.
+    const static int SlotKind = 0;
+    const static int ElementKind = 1;
+
+    uintptr_t objectAndKind_;  // NativeObject* | Kind
+    uint32_t start_;
+    uint32_t count_;
+
+    SlotsEdge() : objectAndKind_(0), start_(0), count_(0) {}
+    SlotsEdge(NativeObject* object, int kind, uint32_t start, uint32_t count)
+        : objectAndKind_(uintptr_t(object) | kind),
+          start_(start),
+          count_(count) {
+      MOZ_ASSERT((uintptr_t(object) & 1) == 0);
+      MOZ_ASSERT(kind <= 1);
+      MOZ_ASSERT(count > 0);
+      MOZ_ASSERT(start + count > start);
+    }
+
+    NativeObject* object() const {
+      return reinterpret_cast<NativeObject*>(objectAndKind_ & ~1);
+    }
+    int kind() const { return (int)(objectAndKind_ & 1); }
+
+    bool operator==(const SlotsEdge& other) const {
+      return objectAndKind_ == other.objectAndKind_ && start_ == other.start_ &&
+             count_ == other.count_;
+    }
+
+    bool operator!=(const SlotsEdge& other) const { return !(*this == other); }
+
+    // True if this SlotsEdge range overlaps with the other SlotsEdge range,
+    // false if they do not overlap.
+    bool overlaps(const SlotsEdge& other) const {
+      if (objectAndKind_ != other.objectAndKind_) {
+        return false;
+      }
+
+      // Widen our range by one on each side so that we consider
+      // adjacent-but-not-actually-overlapping ranges as overlapping. This
+      // is particularly useful for coalescing a series of increasing or
+      // decreasing single index writes 0, 1, 2, ..., N into a SlotsEdge
+      // range of elements [0, N].
+      uint32_t end = start_ + count_ + 1;
+      uint32_t start = start_ > 0 ? start_ - 1 : 0;
+      MOZ_ASSERT(start < end);
+
+      uint32_t otherEnd = other.start_ + other.count_;
+      MOZ_ASSERT(other.start_ <= otherEnd);
+      return (start <= other.start_ && other.start_ <= end) ||
+             (start <= otherEnd && otherEnd <= end);
+    }
+
+    // Destructively make this SlotsEdge range the union of the other
+    // SlotsEdge range and this one. A precondition is that the ranges must
+    // overlap.
+    void merge(const SlotsEdge& other) {
+      MOZ_ASSERT(overlaps(other));
+      uint32_t end = std::max(start_ + count_, other.start_ + other.count_);
+      start_ = std::min(start_, other.start_);
+      count_ = end - start_;
+    }
+
+    bool maybeInRememberedSet(const Nursery& n) const {
+      return !IsInsideNursery(reinterpret_cast<Cell*>(object()));
+    }
+
+    void trace(TenuringTracer& mover) const;
+
+    explicit operator bool() const { return objectAndKind_ != 0; }
+
+    struct Hasher {
+      using Lookup = SlotsEdge;
+      static HashNumber hash(const Lookup& l) {
+        return mozilla::HashGeneric(l.objectAndKind_, l.start_, l.count_);
+      }
+      static bool match(const SlotsEdge& k, const Lookup& l) { return k == l; }
+    };
+  };
+
+  struct WasmAnyRefEdge {
+    wasm::AnyRef* edge;
+
+    WasmAnyRefEdge() : edge(nullptr) {}
+    explicit WasmAnyRefEdge(wasm::AnyRef* v) : edge(v) {}
+    bool operator==(const WasmAnyRefEdge& other) const {
+      return edge == other.edge;
+    }
+    bool operator!=(const WasmAnyRefEdge& other) const {
+      return edge != other.edge;
+    }
+
+    Cell* deref() const {
+      return edge->isGCThing() ? static_cast<Cell*>(edge->toGCThing())
+                               : nullptr;
+      return nullptr;
+    }
+
+    bool maybeInRememberedSet(const Nursery& nursery) const {
+      MOZ_ASSERT(IsInsideNursery(deref()));
+      return !nursery.isInside(edge);
+    }
+
+    void trace(TenuringTracer& mover) const;
+
+    explicit operator bool() const { return edge != nullptr; }
+
+    using Hasher = PointerEdgeHasher<WasmAnyRefEdge>;
+  };
+
+#ifdef DEBUG
+  void checkAccess() const;
+#else
+  void checkAccess() const {}
+#endif
+
+  template <typename Buffer, typename Edge>
+  void unput(Buffer& buffer, const Edge& edge) {
+    checkAccess();
+    if (!isEnabled()) {
+      return;
+    }
+
+    mozilla::ReentrancyGuard g(*this);
+
+    buffer.unput(edge);
+  }
+
+  template <typename Buffer, typename Edge>
+  void put(Buffer& buffer, const Edge& edge, JS::GCReason overflowReason) {
+    checkAccess();
+    if (!isEnabled()) {
+      return;
+    }
+
+    mozilla::ReentrancyGuard g(*this);
+
+    if (!edge.maybeInRememberedSet(nursery_)) {
+      return;
+    }
+
+    PutResult r = buffer.put(edge);
+
+    if (MOZ_UNLIKELY(r == PutResult::AboutToOverflow)) {
+      setAboutToOverflow(overflowReason);
+    }
+  }
+
+  MonoTypeBuffer<ValueEdge> bufferVal;
+  MonoTypeBuffer<StringPtrEdge> bufStrCell;
+  MonoTypeBuffer<BigIntPtrEdge> bufBigIntCell;
+  MonoTypeBuffer<ObjectPtrEdge> bufObjCell;
+  MonoTypeBuffer<SlotsEdge> bufferSlot;
+  MonoTypeBuffer<WasmAnyRefEdge> bufferWasmAnyRef;
+  WholeCellBuffer bufferWholeCell;
+  GenericBuffer bufferGeneric;
+
+  JSRuntime* runtime_;
+  Nursery& nursery_;
+
+  bool aboutToOverflow_;
+  bool enabled_;
+  bool mayHavePointersToDeadCells_;
+#ifdef DEBUG
+  bool mEntered; /* For ReentrancyGuard. */
+#endif
+
+ public:
+#ifdef DEBUG
+  bool markingNondeduplicatable;
+#endif
+
+  explicit StoreBuffer(JSRuntime* rt);
+
+  StoreBuffer(const StoreBuffer& other) = delete;
+  StoreBuffer& operator=(const StoreBuffer& other) = delete;
+
+  StoreBuffer(StoreBuffer&& other);
+  StoreBuffer& operator=(StoreBuffer&& other);
+
+  [[nodiscard]] bool enable();
+
+  void disable();
+  bool isEnabled() const { return enabled_; }
+
+  bool isEmpty() const;
+  void clear();
+
+  const Nursery& nursery() const { return nursery_; }
+
+  /* Get the overflowed status. */
+  bool isAboutToOverflow() const { return aboutToOverflow_; }
+
+  /*
+   * Brain transplants may add whole cell buffer entires for dead cells. We must
+   * evict the nursery prior to sweeping arenas if any such entries are present.
+   */
+  bool mayHavePointersToDeadCells() const {
+    return mayHavePointersToDeadCells_;
+  }
+
+  /* Insert a single edge into the buffer/remembered set. */
+  void putValue(JS::Value* vp) {
+    put(bufferVal, ValueEdge(vp), JS::GCReason::FULL_VALUE_BUFFER);
+  }
+  void unputValue(JS::Value* vp) { unput(bufferVal, ValueEdge(vp)); }
+
+  void putCell(JSString** strp) {
+    put(bufStrCell, StringPtrEdge(strp),
+        JS::GCReason::FULL_CELL_PTR_STR_BUFFER);
+  }
+  void unputCell(JSString** strp) { unput(bufStrCell, StringPtrEdge(strp)); }
+
+  void putCell(JS::BigInt** bip) {
+    put(bufBigIntCell, BigIntPtrEdge(bip),
+        JS::GCReason::FULL_CELL_PTR_BIGINT_BUFFER);
+  }
+  void unputCell(JS::BigInt** bip) { unput(bufBigIntCell, BigIntPtrEdge(bip)); }
+
+  void putCell(JSObject** strp) {
+    put(bufObjCell, ObjectPtrEdge(strp),
+        JS::GCReason::FULL_CELL_PTR_OBJ_BUFFER);
+  }
+  void unputCell(JSObject** strp) { unput(bufObjCell, ObjectPtrEdge(strp)); }
+
+  void putSlot(NativeObject* obj, int kind, uint32_t start, uint32_t count) {
+    SlotsEdge edge(obj, kind, start, count);
+    if (bufferSlot.last_.overlaps(edge)) {
+      bufferSlot.last_.merge(edge);
+    } else {
+      put(bufferSlot, edge, JS::GCReason::FULL_SLOT_BUFFER);
+    }
+  }
+
+  void putWasmAnyRef(wasm::AnyRef* vp) {
+    put(bufferWasmAnyRef, WasmAnyRefEdge(vp),
+        JS::GCReason::FULL_WASM_ANYREF_BUFFER);
+  }
+  void unputWasmAnyRef(wasm::AnyRef* vp) {
+    unput(bufferWasmAnyRef, WasmAnyRefEdge(vp));
+  }
+
+  inline void putWholeCell(Cell* cell);
+  inline void putWholeCellDontCheckLast(Cell* cell);
+  const void* addressOfLastBufferedWholeCell() {
+    return bufferWholeCell.lastBufferedPtr();
+  }
+
+  /* Insert an entry into the generic buffer. */
+  template <typename T>
+  void putGeneric(const T& t) {
+    put(bufferGeneric, t, JS::GCReason::FULL_GENERIC_BUFFER);
+  }
+
+  void setMayHavePointersToDeadCells() { mayHavePointersToDeadCells_ = true; }
+
+  /* Methods to trace the source of all edges in the store buffer. */
+  void traceValues(TenuringTracer& mover) { bufferVal.trace(mover, this); }
+  void traceCells(TenuringTracer& mover) {
+    bufStrCell.trace(mover, this);
+    bufBigIntCell.trace(mover, this);
+    bufObjCell.trace(mover, this);
+  }
+  void traceSlots(TenuringTracer& mover) { bufferSlot.trace(mover, this); }
+  void traceWasmAnyRefs(TenuringTracer& mover) {
+    bufferWasmAnyRef.trace(mover, this);
+  }
+  void traceWholeCells(TenuringTracer& mover) {
+    bufferWholeCell.trace(mover, this);
+  }
+  void traceGenericEntries(JSTracer* trc) { bufferGeneric.trace(trc, this); }
+
+  /* For use by our owned buffers and for testing. */
+  void setAboutToOverflow(JS::GCReason);
+
+  void addSizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf,
+                              JS::GCSizes* sizes);
+
+  void checkEmpty() const;
+};
+
+// A set of cells in an arena used to implement the whole cell store buffer.
+class ArenaCellSet {
+  friend class StoreBuffer;
+
+  using ArenaCellBits = BitArray<MaxArenaCellIndex>;
+
+  // The arena this relates to.
+  Arena* arena;
+
+  // Pointer to next set forming a linked list.
+  ArenaCellSet* next;
+
+  // Bit vector for each possible cell start position.
+  ArenaCellBits bits;
+
+#ifdef DEBUG
+  // The minor GC number when this was created. This object should not survive
+  // past the next minor collection.
+  const uint64_t minorGCNumberAtCreation;
+#endif
+
+  // Construct the empty sentinel object.
+  constexpr ArenaCellSet()
+      : arena(nullptr),
+        next(nullptr)
+#ifdef DEBUG
+        ,
+        minorGCNumberAtCreation(0)
+#endif
+  {
+  }
+
+ public:
+  using WordT = ArenaCellBits::WordT;
+  const size_t BitsPerWord = ArenaCellBits::bitsPerElement;
+  const size_t NumWords = ArenaCellBits::numSlots;
+
+  ArenaCellSet(Arena* arena, ArenaCellSet* next);
+
+  bool hasCell(const TenuredCell* cell) const {
+    return hasCell(getCellIndex(cell));
+  }
+
+  void putCell(const TenuredCell* cell) { putCell(getCellIndex(cell)); }
+
+  bool isEmpty() const { return this == &Empty; }
+
+  bool hasCell(size_t cellIndex) const;
+
+  void putCell(size_t cellIndex);
+
+  void check() const;
+
+  WordT getWord(size_t wordIndex) const { return bits.getWord(wordIndex); }
+
+  void trace(TenuringTracer& mover);
+
+  // Sentinel object used for all empty sets.
+  //
+  // We use a sentinel because it simplifies the JIT code slightly as we can
+  // assume all arenas have a cell set.
+  static ArenaCellSet Empty;
+
+  static size_t getCellIndex(const TenuredCell* cell);
+  static void getWordIndexAndMask(size_t cellIndex, size_t* wordp,
+                                  uint32_t* maskp);
+
+  // Attempt to trigger a minor GC if free space in the nursery (where these
+  // objects are allocated) falls below this threshold.
+  static const size_t NurseryFreeThresholdBytes = 64 * 1024;
+
+  static size_t offsetOfArena() { return offsetof(ArenaCellSet, arena); }
+  static size_t offsetOfBits() { return offsetof(ArenaCellSet, bits); }
+};
+
+// Post-write barrier implementation for GC cells.
+
+// Implement the post-write barrier for nursery allocateable cell type |T|. Call
+// this from |T::postWriteBarrier|.
+template <typename T>
+MOZ_ALWAYS_INLINE void PostWriteBarrierImpl(void* cellp, T* prev, T* next) {
+  MOZ_ASSERT(cellp);
+
+  // If the target needs an entry, add it.
+  StoreBuffer* buffer;
+  if (next && (buffer = next->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 && prev->storeBuffer()) {
+      return;
+    }
+    buffer->putCell(static_cast<T**>(cellp));
+    return;
+  }
+
+  // Remove the prev entry if the new value does not need it. There will only
+  // be a prev entry if the prev value was in the nursery.
+  if (prev && (buffer = prev->storeBuffer())) {
+    buffer->unputCell(static_cast<T**>(cellp));
+  }
+}
+
+template <typename T>
+MOZ_ALWAYS_INLINE void PostWriteBarrier(T** vp, T* prev, T* next) {
+  static_assert(std::is_base_of_v<Cell, T>);
+  static_assert(!std::is_same_v<Cell, T> && !std::is_same_v<TenuredCell, T>);
+
+  if constexpr (!GCTypeIsTenured<T>()) {
+    using BaseT = typename BaseGCType<T>::type;
+    PostWriteBarrierImpl<BaseT>(vp, prev, next);
+    return;
+  }
+
+  MOZ_ASSERT_IF(next, !IsInsideNursery(next));
+}
+
+// Used when we don't have a specific edge to put in the store buffer.
+void PostWriteBarrierCell(Cell* cell, Cell* prev, Cell* next);
+
+} /* namespace gc */
+} /* namespace js */
+
+#endif /* gc_StoreBuffer_h */