Adding upstream version 124.0.1.upstream/124.0.1

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

1 files changed, 900 insertions, 0 deletions

diff --git a/js/src/gc/Cell.h b/js/src/gc/Cell.h
new file mode 100644
index 0000000000..5a89ad794d
--- /dev/null
+++ b/js/src/gc/Cell.h
@@ -0,0 +1,900 @@
+/* -*- 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_Cell_h
+#define gc_Cell_h
+
+#include "mozilla/Atomics.h"
+#include "mozilla/EndianUtils.h"
+
+#include <type_traits>
+
+#include "gc/GCContext.h"
+#include "gc/Heap.h"
+#include "gc/TraceKind.h"
+#include "js/GCAnnotations.h"
+#include "js/shadow/Zone.h"  // JS::shadow::Zone
+#include "js/TypeDecls.h"
+
+namespace JS {
+enum class TraceKind;
+} /* namespace JS */
+
+namespace js {
+
+class JS_PUBLIC_API GenericPrinter;
+
+extern bool RuntimeFromMainThreadIsHeapMajorCollecting(
+    JS::shadow::Zone* shadowZone);
+
+#ifdef DEBUG
+// Barriers can't be triggered during backend Ion compilation, which may run on
+// a helper thread.
+extern bool CurrentThreadIsIonCompiling();
+#endif
+
+extern void TraceManuallyBarrieredGenericPointerEdge(JSTracer* trc,
+                                                     gc::Cell** thingp,
+                                                     const char* name);
+
+namespace gc {
+
+enum class AllocKind : uint8_t;
+class CellAllocator;  // Declared so subtypes of Cell can friend it easily.
+class StoreBuffer;
+class TenuredCell;
+
+extern void PerformIncrementalReadBarrier(TenuredCell* cell);
+extern void PerformIncrementalPreWriteBarrier(TenuredCell* cell);
+extern void PerformIncrementalBarrierDuringFlattening(JSString* str);
+extern void UnmarkGrayGCThingRecursively(TenuredCell* cell);
+
+// Like gc::MarkColor but allows the possibility of the cell being unmarked.
+enum class CellColor : uint8_t { White = 0, Gray = 1, Black = 2 };
+static_assert(uint8_t(CellColor::Gray) == uint8_t(MarkColor::Gray));
+static_assert(uint8_t(CellColor::Black) == uint8_t(MarkColor::Black));
+
+inline bool IsMarked(CellColor color) { return color != CellColor::White; }
+inline MarkColor AsMarkColor(CellColor color) {
+  MOZ_ASSERT(IsMarked(color));
+  return MarkColor(color);
+}
+inline CellColor AsCellColor(MarkColor color) { return CellColor(color); }
+extern const char* CellColorName(CellColor color);
+
+// Cell header word. Stores GC flags and derived class data.
+//
+// Loads of GC flags + all stores are marked as (relaxed) atomic operations,
+// to deal with the following benign data race during compacting GC:
+//
+// - Thread 1 checks isForwarded (which is always false in this situation).
+// - Thread 2 updates the derived class data (without changing the forwarded
+//   flag).
+//
+// To improve performance, we don't use atomic operations for get() because
+// atomic operations inhibit certain compiler optimizations: GCC and Clang are
+// unable to fold multiple loads even if they're both relaxed atomics. This is
+// especially a problem for chained loads such as obj->shape->base->clasp.
+class HeaderWord {
+  // Indicates whether the cell has been forwarded (moved) by generational or
+  // compacting GC and is now a RelocationOverlay.
+  static constexpr uintptr_t FORWARD_BIT = Bit(0);
+  // Bits 1 and 2 are reserved for future use by the GC.
+
+  uintptr_t value_;
+
+  void setAtomic(uintptr_t value) {
+    __atomic_store_n(&value_, value, __ATOMIC_RELAXED);
+  }
+
+ public:
+  static constexpr uintptr_t RESERVED_MASK =
+      BitMask(gc::CellFlagBitsReservedForGC);
+  static_assert(gc::CellFlagBitsReservedForGC >= 3,
+                "Not enough flag bits reserved for GC");
+
+  uintptr_t getAtomic() const {
+    return __atomic_load_n(&value_, __ATOMIC_RELAXED);
+  }
+
+  // Accessors for derived class data.
+  uintptr_t get() const {
+    // Note: non-atomic load. See class comment.
+    uintptr_t value = value_;
+    MOZ_ASSERT((value & RESERVED_MASK) == 0);
+    return value;
+  }
+  void set(uintptr_t value) {
+    MOZ_ASSERT((value & RESERVED_MASK) == 0);
+    setAtomic(value);
+  }
+
+  // Accessors for GC data.
+  uintptr_t flags() const { return getAtomic() & RESERVED_MASK; }
+  bool isForwarded() const { return flags() & FORWARD_BIT; }
+  void setForwardingAddress(uintptr_t ptr) {
+    MOZ_ASSERT((ptr & RESERVED_MASK) == 0);
+    setAtomic(ptr | FORWARD_BIT);
+  }
+  uintptr_t getForwardingAddress() const {
+    MOZ_ASSERT(isForwarded());
+    return getAtomic() & ~RESERVED_MASK;
+  }
+};
+
+// [SMDOC] GC Cell
+//
+// A GC cell is the ultimate base class for all GC things. All types allocated
+// on the GC heap extend either gc::Cell or gc::TenuredCell. If a type is always
+// tenured, prefer the TenuredCell class as base.
+//
+// The first word of Cell is a HeaderWord (a uintptr_t) that reserves the low
+// three bits for GC purposes. The remaining bits are available to sub-classes
+// and can be used store a pointer to another gc::Cell. To make use of the
+// remaining space, sub-classes derive from a helper class such as
+// TenuredCellWithNonGCPointer.
+//
+// During moving GC operation a Cell may be marked as forwarded. This indicates
+// that a gc::RelocationOverlay is currently stored in the Cell's memory and
+// should be used to find the new location of the Cell.
+struct Cell {
+  // Cell header word. Stores GC flags and derived class data.
+  HeaderWord header_;
+
+ public:
+  Cell() = default;
+
+  Cell(const Cell&) = delete;
+  void operator=(const Cell&) = delete;
+
+  bool isForwarded() const { return header_.isForwarded(); }
+  uintptr_t flags() const { return header_.flags(); }
+
+  MOZ_ALWAYS_INLINE bool isTenured() const { return !IsInsideNursery(this); }
+  MOZ_ALWAYS_INLINE const TenuredCell& asTenured() const;
+  MOZ_ALWAYS_INLINE TenuredCell& asTenured();
+
+  MOZ_ALWAYS_INLINE bool isMarkedAny() const;
+  MOZ_ALWAYS_INLINE bool isMarkedBlack() const;
+  MOZ_ALWAYS_INLINE bool isMarkedGray() const;
+  MOZ_ALWAYS_INLINE bool isMarked(gc::MarkColor color) const;
+  MOZ_ALWAYS_INLINE bool isMarkedAtLeast(gc::MarkColor color) const;
+  MOZ_ALWAYS_INLINE CellColor color() const;
+
+  inline JSRuntime* runtimeFromMainThread() const;
+
+  // Note: Unrestricted access to the runtime of a GC thing from an arbitrary
+  // thread can easily lead to races. Use this method very carefully.
+  inline JSRuntime* runtimeFromAnyThread() const;
+
+  // May be overridden by GC thing kinds that have a compartment pointer.
+  inline JS::Compartment* maybeCompartment() const { return nullptr; }
+
+  // The StoreBuffer used to record incoming pointers from the tenured heap.
+  // This will return nullptr for a tenured cell.
+  inline StoreBuffer* storeBuffer() const;
+
+  inline JS::TraceKind getTraceKind() const;
+
+  static MOZ_ALWAYS_INLINE bool needPreWriteBarrier(JS::Zone* zone);
+
+  template <typename T, typename = std::enable_if_t<JS::IsBaseTraceType_v<T>>>
+  inline bool is() const {
+    return getTraceKind() == JS::MapTypeToTraceKind<T>::kind;
+  }
+
+  template <typename T, typename = std::enable_if_t<JS::IsBaseTraceType_v<T>>>
+  inline T* as() {
+    // |this|-qualify the |is| call below to avoid compile errors with even
+    // fairly recent versions of gcc, e.g. 7.1.1 according to bz.
+    MOZ_ASSERT(this->is<T>());
+    return static_cast<T*>(this);
+  }
+
+  template <typename T, typename = std::enable_if_t<JS::IsBaseTraceType_v<T>>>
+  inline const T* as() const {
+    // |this|-qualify the |is| call below to avoid compile errors with even
+    // fairly recent versions of gcc, e.g. 7.1.1 according to bz.
+    MOZ_ASSERT(this->is<T>());
+    return static_cast<const T*>(this);
+  }
+
+  inline JS::Zone* zone() const;
+  inline JS::Zone* zoneFromAnyThread() const;
+
+  // Get the zone for a cell known to be in the nursery.
+  inline JS::Zone* nurseryZone() const;
+  inline JS::Zone* nurseryZoneFromAnyThread() const;
+
+  // Default implementation for kinds that cannot be permanent. This may be
+  // overriden by derived classes.
+  MOZ_ALWAYS_INLINE bool isPermanentAndMayBeShared() const { return false; }
+
+#ifdef DEBUG
+  static inline void assertThingIsNotGray(Cell* cell);
+  inline bool isAligned() const;
+  void dump(GenericPrinter& out) const;
+  void dump() const;
+#endif
+
+ protected:
+  uintptr_t address() const;
+  inline ChunkBase* chunk() const;
+
+ private:
+  // Cells are destroyed by the GC. Do not delete them directly.
+  void operator delete(void*) = delete;
+} JS_HAZ_GC_THING;
+
+// A GC TenuredCell gets behaviors that are valid for things in the Tenured
+// heap, such as access to the arena and mark bits.
+class TenuredCell : public Cell {
+ public:
+  MOZ_ALWAYS_INLINE bool isTenured() const {
+    MOZ_ASSERT(!IsInsideNursery(this));
+    return true;
+  }
+
+  TenuredChunk* chunk() const {
+    return static_cast<TenuredChunk*>(Cell::chunk());
+  }
+
+  // Mark bit management.
+  MOZ_ALWAYS_INLINE bool isMarkedAny() const;
+  MOZ_ALWAYS_INLINE bool isMarkedBlack() const;
+  MOZ_ALWAYS_INLINE bool isMarkedGray() const;
+  MOZ_ALWAYS_INLINE CellColor color() const;
+
+  // The return value indicates if the cell went from unmarked to marked.
+  MOZ_ALWAYS_INLINE bool markIfUnmarked(
+      MarkColor color = MarkColor::Black) const;
+  MOZ_ALWAYS_INLINE bool markIfUnmarkedAtomic(MarkColor color) const;
+  MOZ_ALWAYS_INLINE void markBlack() const;
+  MOZ_ALWAYS_INLINE void markBlackAtomic() const;
+  MOZ_ALWAYS_INLINE void copyMarkBitsFrom(const TenuredCell* src);
+  MOZ_ALWAYS_INLINE void unmark();
+
+  // Access to the arena.
+  inline Arena* arena() const;
+  inline AllocKind getAllocKind() const;
+  inline JS::TraceKind getTraceKind() const;
+  inline JS::Zone* zone() const;
+  inline JS::Zone* zoneFromAnyThread() const;
+  inline bool isInsideZone(JS::Zone* zone) const;
+
+  MOZ_ALWAYS_INLINE JS::shadow::Zone* shadowZone() const {
+    return JS::shadow::Zone::from(zone());
+  }
+  MOZ_ALWAYS_INLINE JS::shadow::Zone* shadowZoneFromAnyThread() const {
+    return JS::shadow::Zone::from(zoneFromAnyThread());
+  }
+
+  template <typename T, typename = std::enable_if_t<JS::IsBaseTraceType_v<T>>>
+  inline bool is() const {
+    return getTraceKind() == JS::MapTypeToTraceKind<T>::kind;
+  }
+
+  template <typename T, typename = std::enable_if_t<JS::IsBaseTraceType_v<T>>>
+  inline T* as() {
+    // |this|-qualify the |is| call below to avoid compile errors with even
+    // fairly recent versions of gcc, e.g. 7.1.1 according to bz.
+    MOZ_ASSERT(this->is<T>());
+    return static_cast<T*>(this);
+  }
+
+  template <typename T, typename = std::enable_if_t<JS::IsBaseTraceType_v<T>>>
+  inline const T* as() const {
+    // |this|-qualify the |is| call below to avoid compile errors with even
+    // fairly recent versions of gcc, e.g. 7.1.1 according to bz.
+    MOZ_ASSERT(this->is<T>());
+    return static_cast<const T*>(this);
+  }
+
+  // Default implementation for kinds that don't require fixup.
+  void fixupAfterMovingGC() {}
+
+  static inline CellColor getColor(MarkBitmap* bitmap, const TenuredCell* cell);
+
+#ifdef DEBUG
+  inline bool isAligned() const;
+#endif
+};
+
+MOZ_ALWAYS_INLINE const TenuredCell& Cell::asTenured() const {
+  MOZ_ASSERT(isTenured());
+  return *static_cast<const TenuredCell*>(this);
+}
+
+MOZ_ALWAYS_INLINE TenuredCell& Cell::asTenured() {
+  MOZ_ASSERT(isTenured());
+  return *static_cast<TenuredCell*>(this);
+}
+
+MOZ_ALWAYS_INLINE bool Cell::isMarkedAny() const {
+  return !isTenured() || asTenured().isMarkedAny();
+}
+
+MOZ_ALWAYS_INLINE bool Cell::isMarkedBlack() const {
+  return !isTenured() || asTenured().isMarkedBlack();
+}
+
+MOZ_ALWAYS_INLINE bool Cell::isMarkedGray() const {
+  return isTenured() && asTenured().isMarkedGray();
+}
+
+MOZ_ALWAYS_INLINE bool Cell::isMarked(gc::MarkColor color) const {
+  return color == MarkColor::Gray ? isMarkedGray() : isMarkedBlack();
+}
+
+MOZ_ALWAYS_INLINE bool Cell::isMarkedAtLeast(gc::MarkColor color) const {
+  return color == MarkColor::Gray ? isMarkedAny() : isMarkedBlack();
+}
+
+MOZ_ALWAYS_INLINE CellColor Cell::color() const {
+  return isTenured() ? asTenured().color() : CellColor::Black;
+}
+
+inline JSRuntime* Cell::runtimeFromMainThread() const {
+  JSRuntime* rt = chunk()->runtime;
+  MOZ_ASSERT(CurrentThreadCanAccessRuntime(rt));
+  return rt;
+}
+
+inline JSRuntime* Cell::runtimeFromAnyThread() const {
+  return chunk()->runtime;
+}
+
+inline uintptr_t Cell::address() const {
+  uintptr_t addr = uintptr_t(this);
+  MOZ_ASSERT(addr % CellAlignBytes == 0);
+  MOZ_ASSERT(TenuredChunk::withinValidRange(addr));
+  return addr;
+}
+
+ChunkBase* Cell::chunk() const {
+  uintptr_t addr = uintptr_t(this);
+  MOZ_ASSERT(addr % CellAlignBytes == 0);
+  addr &= ~ChunkMask;
+  return reinterpret_cast<ChunkBase*>(addr);
+}
+
+inline StoreBuffer* Cell::storeBuffer() const { return chunk()->storeBuffer; }
+
+JS::Zone* Cell::zone() const {
+  if (isTenured()) {
+    return asTenured().zone();
+  }
+
+  return nurseryZone();
+}
+
+JS::Zone* Cell::zoneFromAnyThread() const {
+  if (isTenured()) {
+    return asTenured().zoneFromAnyThread();
+  }
+
+  return nurseryZoneFromAnyThread();
+}
+
+JS::Zone* Cell::nurseryZone() const {
+  JS::Zone* zone = nurseryZoneFromAnyThread();
+  MOZ_ASSERT(CurrentThreadIsGCMarking() || CurrentThreadCanAccessZone(zone));
+  return zone;
+}
+
+JS::Zone* Cell::nurseryZoneFromAnyThread() const {
+  return NurseryCellHeader::from(this)->zone();
+}
+
+#ifdef DEBUG
+extern Cell* UninlinedForwarded(const Cell* cell);
+#endif
+
+inline JS::TraceKind Cell::getTraceKind() const {
+  if (isTenured()) {
+    MOZ_ASSERT_IF(isForwarded(), UninlinedForwarded(this)->getTraceKind() ==
+                                     asTenured().getTraceKind());
+    return asTenured().getTraceKind();
+  }
+
+  return NurseryCellHeader::from(this)->traceKind();
+}
+
+/* static */ MOZ_ALWAYS_INLINE bool Cell::needPreWriteBarrier(JS::Zone* zone) {
+  return JS::shadow::Zone::from(zone)->needsIncrementalBarrier();
+}
+
+MOZ_ALWAYS_INLINE bool TenuredCell::isMarkedAny() const {
+  MOZ_ASSERT(arena()->allocated());
+  return chunk()->markBits.isMarkedAny(this);
+}
+
+MOZ_ALWAYS_INLINE bool TenuredCell::isMarkedBlack() const {
+  MOZ_ASSERT(arena()->allocated());
+  return chunk()->markBits.isMarkedBlack(this);
+}
+
+MOZ_ALWAYS_INLINE bool TenuredCell::isMarkedGray() const {
+  MOZ_ASSERT(arena()->allocated());
+  return chunk()->markBits.isMarkedGray(this);
+}
+
+MOZ_ALWAYS_INLINE CellColor TenuredCell::color() const {
+  return getColor(&chunk()->markBits, this);
+}
+
+/* static */
+inline CellColor TenuredCell::getColor(MarkBitmap* bitmap,
+                                       const TenuredCell* cell) {
+  // Note that this method isn't synchronised so may give surprising results if
+  // the mark bitmap is being modified concurrently.
+
+  if (bitmap->isMarkedBlack(cell)) {
+    return CellColor::Black;
+  }
+
+  if (bitmap->isMarkedGray(cell)) {
+    return CellColor::Gray;
+  }
+
+  return CellColor::White;
+}
+
+bool TenuredCell::markIfUnmarked(MarkColor color /* = Black */) const {
+  return chunk()->markBits.markIfUnmarked(this, color);
+}
+
+bool TenuredCell::markIfUnmarkedAtomic(MarkColor color) const {
+  return chunk()->markBits.markIfUnmarkedAtomic(this, color);
+}
+
+void TenuredCell::markBlack() const { chunk()->markBits.markBlack(this); }
+void TenuredCell::markBlackAtomic() const {
+  chunk()->markBits.markBlackAtomic(this);
+}
+
+void TenuredCell::copyMarkBitsFrom(const TenuredCell* src) {
+  MarkBitmap& markBits = chunk()->markBits;
+  markBits.copyMarkBit(this, src, ColorBit::BlackBit);
+  markBits.copyMarkBit(this, src, ColorBit::GrayOrBlackBit);
+}
+
+void TenuredCell::unmark() { chunk()->markBits.unmark(this); }
+
+inline Arena* TenuredCell::arena() const {
+  MOZ_ASSERT(isTenured());
+  uintptr_t addr = address();
+  addr &= ~ArenaMask;
+  return reinterpret_cast<Arena*>(addr);
+}
+
+AllocKind TenuredCell::getAllocKind() const { return arena()->getAllocKind(); }
+
+JS::TraceKind TenuredCell::getTraceKind() const {
+  return MapAllocToTraceKind(getAllocKind());
+}
+
+JS::Zone* TenuredCell::zone() const {
+  JS::Zone* zone = arena()->zone;
+  MOZ_ASSERT(CurrentThreadIsGCMarking() || CurrentThreadCanAccessZone(zone));
+  return zone;
+}
+
+JS::Zone* TenuredCell::zoneFromAnyThread() const { return arena()->zone; }
+
+bool TenuredCell::isInsideZone(JS::Zone* zone) const {
+  return zone == arena()->zone;
+}
+
+// Read barrier and pre-write barrier implementation for GC cells.
+
+template <typename T>
+MOZ_ALWAYS_INLINE void ReadBarrier(T* thing) {
+  static_assert(std::is_base_of_v<Cell, T>);
+  static_assert(!std::is_same_v<Cell, T> && !std::is_same_v<TenuredCell, T>);
+
+  if (thing) {
+    ReadBarrierImpl(thing);
+  }
+}
+
+MOZ_ALWAYS_INLINE void ReadBarrierImpl(TenuredCell* thing) {
+  MOZ_ASSERT(CurrentThreadIsMainThread());
+  MOZ_ASSERT(!JS::RuntimeHeapIsCollecting());
+  MOZ_ASSERT(thing);
+
+  JS::shadow::Zone* shadowZone = thing->shadowZoneFromAnyThread();
+  if (shadowZone->needsIncrementalBarrier()) {
+    PerformIncrementalReadBarrier(thing);
+    return;
+  }
+
+  if (thing->isMarkedGray()) {
+    UnmarkGrayGCThingRecursively(thing);
+  }
+}
+
+MOZ_ALWAYS_INLINE void ReadBarrierImpl(Cell* thing) {
+  MOZ_ASSERT(!CurrentThreadIsGCMarking());
+  MOZ_ASSERT(thing);
+
+  if (thing->isTenured()) {
+    ReadBarrierImpl(&thing->asTenured());
+  }
+}
+
+MOZ_ALWAYS_INLINE void PreWriteBarrierImpl(TenuredCell* thing) {
+  MOZ_ASSERT(CurrentThreadIsMainThread() || CurrentThreadIsGCSweeping() ||
+             CurrentThreadIsGCFinalizing());
+  MOZ_ASSERT(thing);
+
+  // Barriers can be triggered on the main thread while collecting, but are
+  // disabled. For example, this happens when sweeping HeapPtr wrappers. See
+  // AutoDisableBarriers.
+
+  JS::shadow::Zone* zone = thing->shadowZoneFromAnyThread();
+  if (zone->needsIncrementalBarrier()) {
+    PerformIncrementalPreWriteBarrier(thing);
+  }
+}
+
+MOZ_ALWAYS_INLINE void PreWriteBarrierImpl(Cell* thing) {
+  MOZ_ASSERT(!CurrentThreadIsGCMarking());
+  MOZ_ASSERT(thing);
+
+  if (thing->isTenured()) {
+    PreWriteBarrierImpl(&thing->asTenured());
+  }
+}
+
+template <typename T>
+MOZ_ALWAYS_INLINE void PreWriteBarrier(T* thing) {
+  static_assert(std::is_base_of_v<Cell, T>);
+  static_assert(!std::is_same_v<Cell, T> && !std::is_same_v<TenuredCell, T>);
+
+  if (thing) {
+    PreWriteBarrierImpl(thing);
+  }
+}
+
+// Pre-write barrier implementation for structures containing GC cells, taking a
+// functor to trace the structure.
+template <typename T, typename F>
+MOZ_ALWAYS_INLINE void PreWriteBarrier(JS::Zone* zone, T* data,
+                                       const F& traceFn) {
+  MOZ_ASSERT(data);
+  MOZ_ASSERT(!CurrentThreadIsIonCompiling());
+  MOZ_ASSERT(!CurrentThreadIsGCMarking());
+
+  auto* shadowZone = JS::shadow::Zone::from(zone);
+  if (!shadowZone->needsIncrementalBarrier()) {
+    return;
+  }
+
+  MOZ_ASSERT(CurrentThreadCanAccessRuntime(shadowZone->runtimeFromAnyThread()));
+  MOZ_ASSERT(!RuntimeFromMainThreadIsHeapMajorCollecting(shadowZone));
+
+  traceFn(shadowZone->barrierTracer(), data);
+}
+
+// Pre-write barrier implementation for structures containing GC cells. T must
+// support a |trace| method.
+template <typename T>
+MOZ_ALWAYS_INLINE void PreWriteBarrier(JS::Zone* zone, T* data) {
+  MOZ_ASSERT(data);
+  PreWriteBarrier(zone, data, [](JSTracer* trc, T* data) { data->trace(trc); });
+}
+
+#ifdef DEBUG
+
+/* static */ void Cell::assertThingIsNotGray(Cell* cell) {
+  JS::AssertCellIsNotGray(cell);
+}
+
+bool Cell::isAligned() const {
+  if (!isTenured()) {
+    return true;
+  }
+  return asTenured().isAligned();
+}
+
+bool TenuredCell::isAligned() const {
+  return Arena::isAligned(address(), arena()->getThingSize());
+}
+
+#endif
+
+// Base class for nusery-allocatable GC things that have 32-bit length and
+// 32-bit flags (currently JSString and BigInt).
+//
+// This tries to store both in Cell::header_, but if that isn't large enough the
+// length is stored separately.
+//
+//          32       0
+//  ------------------
+//  | Length | Flags |
+//  ------------------
+//
+// The low bits of the flags word (see CellFlagBitsReservedForGC) are reserved
+// for GC. Derived classes must ensure they don't use these flags for non-GC
+// purposes.
+class alignas(gc::CellAlignBytes) CellWithLengthAndFlags : public Cell {
+#if JS_BITS_PER_WORD == 32
+  // Additional storage for length if |header_| is too small to fit both.
+  uint32_t length_;
+#endif
+
+ protected:
+  uint32_t headerLengthField() const {
+#if JS_BITS_PER_WORD == 32
+    return length_;
+#else
+    return uint32_t(header_.get() >> 32);
+#endif
+  }
+
+  uint32_t headerFlagsField() const { return uint32_t(header_.get()); }
+
+  void setHeaderFlagBit(uint32_t flag) {
+    header_.set(header_.get() | uintptr_t(flag));
+  }
+  void clearHeaderFlagBit(uint32_t flag) {
+    header_.set(header_.get() & ~uintptr_t(flag));
+  }
+  void toggleHeaderFlagBit(uint32_t flag) {
+    header_.set(header_.get() ^ uintptr_t(flag));
+  }
+
+  void setHeaderLengthAndFlags(uint32_t len, uint32_t flags) {
+#if JS_BITS_PER_WORD == 32
+    header_.set(flags);
+    length_ = len;
+#else
+    header_.set((uint64_t(len) << 32) | uint64_t(flags));
+#endif
+  }
+
+ public:
+  // Returns the offset of header_. JIT code should use offsetOfFlags
+  // below.
+  static constexpr size_t offsetOfRawHeaderFlagsField() {
+    return offsetof(CellWithLengthAndFlags, header_);
+  }
+
+  // Offsets for direct field from jit code. A number of places directly
+  // access 32-bit length and flags fields so do endian trickery here.
+#if JS_BITS_PER_WORD == 32
+  static constexpr size_t offsetOfHeaderFlags() {
+    return offsetof(CellWithLengthAndFlags, header_);
+  }
+  static constexpr size_t offsetOfHeaderLength() {
+    return offsetof(CellWithLengthAndFlags, length_);
+  }
+#elif MOZ_LITTLE_ENDIAN()
+  static constexpr size_t offsetOfHeaderFlags() {
+    return offsetof(CellWithLengthAndFlags, header_);
+  }
+  static constexpr size_t offsetOfHeaderLength() {
+    return offsetof(CellWithLengthAndFlags, header_) + sizeof(uint32_t);
+  }
+#else
+  static constexpr size_t offsetOfHeaderFlags() {
+    return offsetof(CellWithLengthAndFlags, header_) + sizeof(uint32_t);
+  }
+  static constexpr size_t offsetOfHeaderLength() {
+    return offsetof(CellWithLengthAndFlags, header_);
+  }
+#endif
+};
+
+// Base class for non-nursery-allocatable GC things that allows storing a non-GC
+// thing pointer in the first word.
+//
+// The low bits of the word (see CellFlagBitsReservedForGC) are reserved for GC.
+template <class PtrT>
+class alignas(gc::CellAlignBytes) TenuredCellWithNonGCPointer
+    : public TenuredCell {
+  static_assert(!std::is_pointer_v<PtrT>,
+                "PtrT should be the type of the referent, not of the pointer");
+  static_assert(
+      !std::is_base_of_v<Cell, PtrT>,
+      "Don't use TenuredCellWithNonGCPointer for pointers to GC things");
+
+ protected:
+  TenuredCellWithNonGCPointer() = default;
+  explicit TenuredCellWithNonGCPointer(PtrT* initial) {
+    uintptr_t data = uintptr_t(initial);
+    header_.set(data);
+  }
+
+  PtrT* headerPtr() const {
+    MOZ_ASSERT(flags() == 0);
+    return reinterpret_cast<PtrT*>(uintptr_t(header_.get()));
+  }
+
+  void setHeaderPtr(PtrT* newValue) {
+    // As above, no flags are expected to be set here.
+    uintptr_t data = uintptr_t(newValue);
+    MOZ_ASSERT(flags() == 0);
+    header_.set(data);
+  }
+
+ public:
+  static constexpr size_t offsetOfHeaderPtr() {
+    return offsetof(TenuredCellWithNonGCPointer, header_);
+  }
+};
+
+// Base class for non-nursery-allocatable GC things that allows storing flags
+// in the first word.
+//
+// The low bits of the flags word (see CellFlagBitsReservedForGC) are reserved
+// for GC.
+class alignas(gc::CellAlignBytes) TenuredCellWithFlags : public TenuredCell {
+ protected:
+  TenuredCellWithFlags() { header_.set(0); }
+  explicit TenuredCellWithFlags(uintptr_t initial) { header_.set(initial); }
+
+  uintptr_t headerFlagsField() const {
+    MOZ_ASSERT(flags() == 0);
+    return header_.get();
+  }
+
+  void setHeaderFlagBits(uintptr_t flags) {
+    header_.set(header_.get() | flags);
+  }
+  void clearHeaderFlagBits(uintptr_t flags) {
+    header_.set(header_.get() & ~flags);
+  }
+};
+
+// Base class for GC things that have a tenured GC pointer as their first word.
+//
+// The low bits of the first word (see CellFlagBitsReservedForGC) are reserved
+// for GC.
+//
+// This includes a pre write barrier when the pointer is update. No post barrier
+// is necessary as the pointer is always tenured.
+template <class BaseCell, class PtrT>
+class alignas(gc::CellAlignBytes) CellWithTenuredGCPointer : public BaseCell {
+  static void staticAsserts() {
+    // These static asserts are not in class scope because the PtrT may not be
+    // defined when this class template is instantiated.
+    static_assert(
+        std::is_same_v<BaseCell, Cell> || std::is_same_v<BaseCell, TenuredCell>,
+        "BaseCell must be either Cell or TenuredCell");
+    static_assert(
+        !std::is_pointer_v<PtrT>,
+        "PtrT should be the type of the referent, not of the pointer");
+    static_assert(
+        std::is_base_of_v<Cell, PtrT>,
+        "Only use CellWithTenuredGCPointer for pointers to GC things");
+  }
+
+ protected:
+  CellWithTenuredGCPointer() = default;
+  explicit CellWithTenuredGCPointer(PtrT* initial) { initHeaderPtr(initial); }
+
+  void initHeaderPtr(PtrT* initial) {
+    MOZ_ASSERT_IF(initial, !IsInsideNursery(initial));
+    uintptr_t data = uintptr_t(initial);
+    this->header_.set(data);
+  }
+
+  void setHeaderPtr(PtrT* newValue) {
+    // As above, no flags are expected to be set here.
+    MOZ_ASSERT_IF(newValue, !IsInsideNursery(newValue));
+    PreWriteBarrier(headerPtr());
+    unbarrieredSetHeaderPtr(newValue);
+  }
+
+ public:
+  PtrT* headerPtr() const {
+    staticAsserts();
+    MOZ_ASSERT(this->flags() == 0);
+    return reinterpret_cast<PtrT*>(uintptr_t(this->header_.get()));
+  }
+  PtrT* headerPtrAtomic() const {
+    staticAsserts();
+    MOZ_ASSERT(this->flags() == 0);
+    return reinterpret_cast<PtrT*>(uintptr_t(this->header_.getAtomic()));
+  }
+
+  void unbarrieredSetHeaderPtr(PtrT* newValue) {
+    uintptr_t data = uintptr_t(newValue);
+    MOZ_ASSERT(this->flags() == 0);
+    this->header_.set(data);
+  }
+
+  static constexpr size_t offsetOfHeaderPtr() {
+    return offsetof(CellWithTenuredGCPointer, header_);
+  }
+};
+
+void CellHeaderPostWriteBarrier(JSObject** ptr, JSObject* prev, JSObject* next);
+
+template <typename T>
+constexpr inline bool GCTypeIsTenured() {
+  static_assert(std::is_base_of_v<Cell, T>);
+  static_assert(!std::is_same_v<Cell, T> && !std::is_same_v<TenuredCell, T>);
+
+  return std::is_base_of_v<TenuredCell, T> || std::is_base_of_v<JSAtom, T>;
+}
+
+template <class PtrT>
+class alignas(gc::CellAlignBytes) TenuredCellWithGCPointer
+    : public TenuredCell {
+  static void staticAsserts() {
+    // These static asserts are not in class scope because the PtrT may not be
+    // defined when this class template is instantiated.
+    static_assert(
+        !std::is_pointer_v<PtrT>,
+        "PtrT should be the type of the referent, not of the pointer");
+    static_assert(
+        std::is_base_of_v<Cell, PtrT>,
+        "Only use TenuredCellWithGCPointer for pointers to GC things");
+    static_assert(
+        !GCTypeIsTenured<PtrT>,
+        "Don't use TenuredCellWithGCPointer for always-tenured GC things");
+  }
+
+ protected:
+  TenuredCellWithGCPointer() = default;
+  explicit TenuredCellWithGCPointer(PtrT* initial) { initHeaderPtr(initial); }
+
+  void initHeaderPtr(PtrT* initial) {
+    uintptr_t data = uintptr_t(initial);
+    this->header_.set(data);
+    if (initial && IsInsideNursery(initial)) {
+      CellHeaderPostWriteBarrier(headerPtrAddress(), nullptr, initial);
+    }
+  }
+
+  PtrT** headerPtrAddress() {
+    MOZ_ASSERT(this->flags() == 0);
+    return reinterpret_cast<PtrT**>(&this->header_);
+  }
+
+ public:
+  PtrT* headerPtr() const {
+    MOZ_ASSERT(this->flags() == 0);
+    return reinterpret_cast<PtrT*>(uintptr_t(this->header_.get()));
+  }
+
+  void unbarrieredSetHeaderPtr(PtrT* newValue) {
+    uintptr_t data = uintptr_t(newValue);
+    MOZ_ASSERT(this->flags() == 0);
+    this->header_.set(data);
+  }
+
+  static constexpr size_t offsetOfHeaderPtr() {
+    return offsetof(TenuredCellWithGCPointer, header_);
+  }
+};
+
+// Check whether a typed GC thing is marked at all. Doesn't check gray bits for
+// kinds that can't be marked gray.
+template <typename T>
+static inline bool TenuredThingIsMarkedAny(T* thing) {
+  using BaseT = typename BaseGCType<T>::type;
+  TenuredCell* cell = &thing->asTenured();
+  if constexpr (TraceKindCanBeGray<BaseT>::value) {
+    return cell->isMarkedAny();
+  } else {
+    MOZ_ASSERT(!cell->isMarkedGray());
+    return cell->isMarkedBlack();
+  }
+}
+
+template <>
+inline bool TenuredThingIsMarkedAny<Cell>(Cell* thing) {
+  return thing->asTenured().isMarkedAny();
+}
+
+} /* namespace gc */
+} /* namespace js */
+
+#endif /* gc_Cell_h */