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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 */
#ifndef js_HeapAPI_h
#define js_HeapAPI_h
#include "mozilla/Atomics.h"
#include "mozilla/BitSet.h"
#include <limits.h>
#include <type_traits>
#include "js/AllocPolicy.h"
#include "js/GCAnnotations.h"
#include "js/HashTable.h"
#include "js/shadow/String.h" // JS::shadow::String
#include "js/shadow/Symbol.h" // JS::shadow::Symbol
#include "js/shadow/Zone.h" // JS::shadow::Zone
#include "js/TraceKind.h"
#include "js/TypeDecls.h"
/* These values are private to the JS engine. */
namespace js {
class NurseryDecommitTask;
JS_PUBLIC_API bool CurrentThreadCanAccessZone(JS::Zone* zone);
namespace gc {
class Arena;
struct Cell;
class TenuredChunk;
class StoreBuffer;
class TenuredCell;
const size_t ArenaShift = 12;
const size_t ArenaSize = size_t(1) << ArenaShift;
const size_t ArenaMask = ArenaSize - 1;
#if defined(XP_MACOSX) && defined(__aarch64__)
const size_t PageShift = 14;
const size_t PageShift = 12;
// Expected page size, so we could initialze ArenasPerPage at compile-time.
// The actual system page size should be queried by SystemPageSize().
const size_t PageSize = size_t(1) << PageShift;
constexpr size_t ArenasPerPage = PageSize / ArenaSize;
const size_t ChunkShift = 18;
const size_t ChunkShift = 20;
const size_t ChunkSize = size_t(1) << ChunkShift;
const size_t ChunkMask = ChunkSize - 1;
const size_t CellAlignShift = 3;
const size_t CellAlignBytes = size_t(1) << CellAlignShift;
const size_t CellAlignMask = CellAlignBytes - 1;
const size_t CellBytesPerMarkBit = CellAlignBytes;
const size_t MarkBitsPerCell = 2;
* The mark bitmap has one bit per each possible cell start position. This
* wastes some space for larger GC things but allows us to avoid division by the
* cell's size when accessing the bitmap.
const size_t ArenaBitmapBits = ArenaSize / CellBytesPerMarkBit;
const size_t ArenaBitmapBytes = HowMany(ArenaBitmapBits, 8);
const size_t ArenaBitmapWords = HowMany(ArenaBitmapBits, JS_BITS_PER_WORD);
// The base class for all GC chunks, either in the nursery or in the tenured
// heap memory. This structure is locatable from any GC pointer by aligning to
// the chunk size.
class alignas(CellAlignBytes) ChunkBase {
ChunkBase(JSRuntime* rt, StoreBuffer* sb) {
MOZ_ASSERT((uintptr_t(this) & ChunkMask) == 0);
initBase(rt, sb);
void initBase(JSRuntime* rt, StoreBuffer* sb) {
runtime = rt;
storeBuffer = sb;
// The store buffer for pointers from tenured things to things in this
// chunk. Will be non-null if and only if this is a nursery chunk.
StoreBuffer* storeBuffer;
// Provide quick access to the runtime from absolutely anywhere.
JSRuntime* runtime;
// Information about tenured heap chunks.
struct TenuredChunkInfo {
friend class ChunkPool;
TenuredChunk* next = nullptr;
TenuredChunk* prev = nullptr;
/* Number of free arenas, either committed or decommitted. */
uint32_t numArenasFree;
/* Number of free, committed arenas. */
uint32_t numArenasFreeCommitted;
* Calculating ArenasPerChunk:
* To figure out how many Arenas will fit in a chunk we need to know how much
* extra space is available after we allocate the header data. This is a problem
* because the header size depends on the number of arenas in the chunk.
* The dependent fields are markBits, decommittedPages and
* freeCommittedArenas. markBits needs ArenaBitmapBytes bytes per arena,
* decommittedPages needs one bit per page and freeCommittedArenas needs one
* bit per arena.
* We can calculate an approximate value by dividing the number of bits of free
* space in the chunk by the number of bits needed per arena. This is an
* approximation because it doesn't take account of the fact that the variable
* sized fields must be rounded up to a whole number of words, or any padding
* the compiler adds between fields.
* Fortunately, for the chunk and arena size parameters we use this
* approximation turns out to be correct. If it were not we might need to adjust
* the arena count down by one to allow more space for the padding.
const size_t BitsPerPageWithHeaders =
(ArenaSize + ArenaBitmapBytes) * ArenasPerPage * CHAR_BIT + ArenasPerPage +
const size_t ChunkBitsAvailable =
(ChunkSize - sizeof(ChunkBase) - sizeof(TenuredChunkInfo)) * CHAR_BIT;
const size_t PagesPerChunk = ChunkBitsAvailable / BitsPerPageWithHeaders;
const size_t ArenasPerChunk = PagesPerChunk * ArenasPerPage;
const size_t FreeCommittedBits = ArenasPerChunk;
const size_t DecommitBits = PagesPerChunk;
const size_t BitsPerArenaWithHeaders =
(ArenaSize + ArenaBitmapBytes) * CHAR_BIT +
(DecommitBits / ArenasPerChunk) + 1;
const size_t CalculatedChunkSizeRequired =
sizeof(ChunkBase) + sizeof(TenuredChunkInfo) +
RoundUp(ArenasPerChunk * ArenaBitmapBytes, sizeof(uintptr_t)) +
RoundUp(FreeCommittedBits, sizeof(uint32_t) * CHAR_BIT) / CHAR_BIT +
RoundUp(DecommitBits, sizeof(uint32_t) * CHAR_BIT) / CHAR_BIT +
ArenasPerChunk * ArenaSize;
static_assert(CalculatedChunkSizeRequired <= ChunkSize,
"Calculated ArenasPerChunk is too large");
const size_t CalculatedChunkPadSize = ChunkSize - CalculatedChunkSizeRequired;
static_assert(CalculatedChunkPadSize * CHAR_BIT < BitsPerArenaWithHeaders,
"Calculated ArenasPerChunk is too small");
// Define a macro for the expected number of arenas so its value appears in the
// error message if the assertion fails.
static_assert(ArenasPerChunk == EXPECTED_ARENA_COUNT,
"Do not accidentally change our heap's density.");
// Mark bitmaps are atomic because they can be written by gray unmarking on the
// main thread while read by sweeping on a background thread. The former does
// not affect the result of the latter.
using MarkBitmapWord = mozilla::Atomic<uintptr_t, mozilla::Relaxed>;
* Live objects are marked black or gray. Everything reachable from a JS root is
* marked black. Objects marked gray are eligible for cycle collection.
* BlackBit: GrayOrBlackBit: Color:
* 0 0 white
* 0 1 gray
* 1 0 black
* 1 1 black
enum class ColorBit : uint32_t { BlackBit = 0, GrayOrBlackBit = 1 };
// Mark colors. Order is important here: the greater value the 'more marked' a
// cell is.
enum class MarkColor : uint8_t { Gray = 1, Black = 2 };
// Mark bitmap for a tenured heap chunk.
struct MarkBitmap {
static constexpr size_t WordCount = ArenaBitmapWords * ArenasPerChunk;
MarkBitmapWord bitmap[WordCount];
inline void getMarkWordAndMask(const TenuredCell* cell, ColorBit colorBit,
MarkBitmapWord** wordp, uintptr_t* maskp);
// The following are not exported and are defined in gc/Heap.h:
inline bool markBit(const TenuredCell* cell, ColorBit colorBit);
inline bool isMarkedAny(const TenuredCell* cell);
inline bool isMarkedBlack(const TenuredCell* cell);
inline bool isMarkedGray(const TenuredCell* cell);
inline bool markIfUnmarked(const TenuredCell* cell, MarkColor color);
inline void markBlack(const TenuredCell* cell);
inline void copyMarkBit(TenuredCell* dst, const TenuredCell* src,
ColorBit colorBit);
inline void unmark(const TenuredCell* cell);
inline MarkBitmapWord* arenaBits(Arena* arena);
static_assert(ArenaBitmapBytes * ArenasPerChunk == sizeof(MarkBitmap),
"Ensure our MarkBitmap actually covers all arenas.");
// Bitmap with one bit per page used for decommitted page set.
using ChunkPageBitmap = mozilla::BitSet<PagesPerChunk, uint32_t>;
// Bitmap with one bit per arena used for free committed arena set.
using ChunkArenaBitmap = mozilla::BitSet<ArenasPerChunk, uint32_t>;
// Base class containing data members for a tenured heap chunk.
class TenuredChunkBase : public ChunkBase {
TenuredChunkInfo info;
MarkBitmap markBits;
ChunkArenaBitmap freeCommittedArenas;
ChunkPageBitmap decommittedPages;
explicit TenuredChunkBase(JSRuntime* runtime) : ChunkBase(runtime, nullptr) {
info.numArenasFree = ArenasPerChunk;
void initAsDecommitted();
* We sometimes use an index to refer to a cell in an arena. The index for a
* cell is found by dividing by the cell alignment so not all indices refer to
* valid cells.
const size_t ArenaCellIndexBytes = CellAlignBytes;
const size_t MaxArenaCellIndex = ArenaSize / CellAlignBytes;
const size_t MarkBitmapWordBits = sizeof(MarkBitmapWord) * CHAR_BIT;
constexpr size_t FirstArenaAdjustmentBits =
RoundUp(sizeof(gc::TenuredChunkBase), ArenaSize) / gc::CellBytesPerMarkBit;
static_assert((FirstArenaAdjustmentBits % MarkBitmapWordBits) == 0);
constexpr size_t FirstArenaAdjustmentWords =
FirstArenaAdjustmentBits / MarkBitmapWordBits;
const size_t ChunkStoreBufferOffset = offsetof(ChunkBase, storeBuffer);
const size_t ChunkMarkBitmapOffset = offsetof(TenuredChunkBase, markBits);
// Hardcoded offsets into Arena class.
const size_t ArenaZoneOffset = 2 * sizeof(uint32_t);
const size_t ArenaHeaderSize = ArenaZoneOffset + 2 * sizeof(uintptr_t) +
sizeof(size_t) + sizeof(uintptr_t);
// The first word of a GC thing has certain requirements from the GC and is used
// to store flags in the low bits.
const size_t CellFlagBitsReservedForGC = 3;
// The first word can be used to store JSClass pointers for some thing kinds, so
// these must be suitably aligned.
const size_t JSClassAlignBytes = size_t(1) << CellFlagBitsReservedForGC;
#ifdef JS_DEBUG
/* When downcasting, ensure we are actually the right type. */
extern JS_PUBLIC_API void AssertGCThingHasType(js::gc::Cell* cell,
JS::TraceKind kind);
inline void AssertGCThingHasType(js::gc::Cell* cell, JS::TraceKind kind) {}
MOZ_ALWAYS_INLINE bool IsInsideNursery(const js::gc::Cell* cell);
MOZ_ALWAYS_INLINE bool IsInsideNursery(const js::gc::TenuredCell* cell);
} /* namespace gc */
} /* namespace js */
namespace JS {
enum class HeapState {
Idle, // doing nothing with the GC heap
Tracing, // tracing the GC heap without collecting, e.g.
// IterateCompartments()
MajorCollecting, // doing a GC of the major heap
MinorCollecting, // doing a GC of the minor heap (nursery)
CycleCollecting // in the "Unlink" phase of cycle collection
JS_PUBLIC_API HeapState RuntimeHeapState();
static inline bool RuntimeHeapIsBusy() {
return RuntimeHeapState() != HeapState::Idle;
static inline bool RuntimeHeapIsTracing() {
return RuntimeHeapState() == HeapState::Tracing;
static inline bool RuntimeHeapIsMajorCollecting() {
return RuntimeHeapState() == HeapState::MajorCollecting;
static inline bool RuntimeHeapIsMinorCollecting() {
return RuntimeHeapState() == HeapState::MinorCollecting;
static inline bool RuntimeHeapIsCollecting(HeapState state) {
return state == HeapState::MajorCollecting ||
state == HeapState::MinorCollecting;
static inline bool RuntimeHeapIsCollecting() {
return RuntimeHeapIsCollecting(RuntimeHeapState());
static inline bool RuntimeHeapIsCycleCollecting() {
return RuntimeHeapState() == HeapState::CycleCollecting;
* This list enumerates the different types of conceptual stacks we have in
* SpiderMonkey. In reality, they all share the C stack, but we allow different
* stack limits depending on the type of code running.
enum StackKind {
StackForSystemCode, // C++, such as the GC, running on behalf of the VM.
StackForTrustedScript, // Script running with trusted principals.
StackForUntrustedScript, // Script running with untrusted principals.
* Default maximum size for the generational nursery in bytes. This is the
* initial value. In the browser this configured by the
* javascript.options.mem.nursery.max_kb pref.
const uint32_t DefaultNurseryMaxBytes = 16 * js::gc::ChunkSize;
/* Default maximum heap size in bytes to pass to JS_NewContext(). */
const uint32_t DefaultHeapMaxBytes = 32 * 1024 * 1024;
* A GC pointer, tagged with the trace kind.
* In general, a GC pointer should be stored with an exact type. This class
* is for use when that is not possible because a single pointer must point
* to several kinds of GC thing.
class JS_PUBLIC_API GCCellPtr {
GCCellPtr() : GCCellPtr(nullptr) {}
// Construction from a void* and trace kind.
GCCellPtr(void* gcthing, JS::TraceKind traceKind)
: ptr(checkedCast(gcthing, traceKind)) {}
// Automatically construct a null GCCellPtr from nullptr.
MOZ_IMPLICIT GCCellPtr(decltype(nullptr))
: ptr(checkedCast(nullptr, JS::TraceKind::Null)) {}
// Construction from an explicit type.
template <typename T>
explicit GCCellPtr(T* p)
: ptr(checkedCast(p, JS::MapTypeToTraceKind<T>::kind)) {}
explicit GCCellPtr(JSFunction* p)
: ptr(checkedCast(p, JS::TraceKind::Object)) {}
explicit GCCellPtr(JSScript* p)
: ptr(checkedCast(p, JS::TraceKind::Script)) {}
explicit GCCellPtr(const Value& v);
JS::TraceKind kind() const {
uintptr_t kindBits = ptr & OutOfLineTraceKindMask;
if (kindBits != OutOfLineTraceKindMask) {
return JS::TraceKind(kindBits);
return outOfLineKind();
// Allow GCCellPtr to be used in a boolean context.
explicit operator bool() const {
MOZ_ASSERT(bool(asCell()) == (kind() != JS::TraceKind::Null));
return asCell();
// Simplify checks to the kind.
template <typename T, typename = std::enable_if_t<JS::IsBaseTraceType_v<T>>>
bool is() const {
return kind() == JS::MapTypeToTraceKind<T>::kind;
// Conversions to more specific types must match the kind. Access to
// further refined types is not allowed directly from a GCCellPtr.
template <typename T, typename = std::enable_if_t<JS::IsBaseTraceType_v<T>>>
T& as() const {
MOZ_ASSERT(kind() == JS::MapTypeToTraceKind<T>::kind);
// We can't use static_cast here, because the fact that JSObject
// inherits from js::gc::Cell is not part of the public API.
return *reinterpret_cast<T*>(asCell());
// Return a pointer to the cell this |GCCellPtr| refers to, or |nullptr|.
// (It would be more symmetrical with |to| for this to return a |Cell&|, but
// the result can be |nullptr|, and null references are undefined behavior.)
js::gc::Cell* asCell() const {
return reinterpret_cast<js::gc::Cell*>(ptr & ~OutOfLineTraceKindMask);
// The CC's trace logger needs an identity that is XPIDL serializable.
uint64_t unsafeAsInteger() const {
return static_cast<uint64_t>(unsafeAsUIntPtr());
// Inline mark bitmap access requires direct pointer arithmetic.
uintptr_t unsafeAsUIntPtr() const {
return reinterpret_cast<uintptr_t>(asCell());
MOZ_ALWAYS_INLINE bool mayBeOwnedByOtherRuntime() const {
if (!is<JSString>() && !is<JS::Symbol>()) {
return false;
if (is<JSString>()) {
return JS::shadow::String::isPermanentAtom(asCell());
return JS::shadow::Symbol::isWellKnownSymbol(asCell());
static uintptr_t checkedCast(void* p, JS::TraceKind traceKind) {
auto* cell = static_cast<js::gc::Cell*>(p);
MOZ_ASSERT((uintptr_t(p) & OutOfLineTraceKindMask) == 0);
AssertGCThingHasType(cell, traceKind);
// Store trace in the bottom bits of pointer for common kinds.
uintptr_t kindBits = uintptr_t(traceKind);
if (kindBits >= OutOfLineTraceKindMask) {
kindBits = OutOfLineTraceKindMask;
return uintptr_t(p) | kindBits;
JS::TraceKind outOfLineKind() const;
uintptr_t ptr;
// Unwraps the given GCCellPtr, calls the functor |f| with a template argument
// of the actual type of the pointer, and returns the result.
template <typename F>
auto MapGCThingTyped(GCCellPtr thing, F&& f) {
switch (thing.kind()) {
#define JS_EXPAND_DEF(name, type, _, _1) \
case JS::TraceKind::name: \
return f(&<type>());
MOZ_CRASH("Invalid trace kind in MapGCThingTyped for GCCellPtr.");
// Unwraps the given GCCellPtr and calls the functor |f| with a template
// argument of the actual type of the pointer. Doesn't return anything.
template <typename F>
void ApplyGCThingTyped(GCCellPtr thing, F&& f) {
// This function doesn't do anything but is supplied for symmetry with other
// MapGCThingTyped/ApplyGCThingTyped implementations that have to wrap the
// functor to return a dummy value that is ignored.
MapGCThingTyped(thing, f);
} /* namespace JS */
// These are defined in the toplevel namespace instead of within JS so that
// they won't shadow other operator== overloads (see bug 1456512.)
inline bool operator==(JS::GCCellPtr ptr1, JS::GCCellPtr ptr2) {
return ptr1.asCell() == ptr2.asCell();
inline bool operator!=(JS::GCCellPtr ptr1, JS::GCCellPtr ptr2) {
return !(ptr1 == ptr2);
namespace js {
namespace gc {
/* static */
MOZ_ALWAYS_INLINE void MarkBitmap::getMarkWordAndMask(const TenuredCell* cell,
ColorBit colorBit,
MarkBitmapWord** wordp,
uintptr_t* maskp) {
// Note: the JIT pre-barrier trampolines inline this code. Update
// MacroAssembler::emitPreBarrierFastPath code too when making changes here!
MOZ_ASSERT(size_t(colorBit) < MarkBitsPerCell);
size_t offset = uintptr_t(cell) & ChunkMask;
const size_t bit = offset / CellBytesPerMarkBit + size_t(colorBit);
size_t word = bit / MarkBitmapWordBits - FirstArenaAdjustmentWords;
MOZ_ASSERT(word < WordCount);
*wordp = &bitmap[word];
*maskp = uintptr_t(1) << (bit % MarkBitmapWordBits);
namespace detail {
static MOZ_ALWAYS_INLINE ChunkBase* GetCellChunkBase(const Cell* cell) {
return reinterpret_cast<ChunkBase*>(uintptr_t(cell) & ~ChunkMask);
static MOZ_ALWAYS_INLINE TenuredChunkBase* GetCellChunkBase(
const TenuredCell* cell) {
return reinterpret_cast<TenuredChunkBase*>(uintptr_t(cell) & ~ChunkMask);
static MOZ_ALWAYS_INLINE JS::Zone* GetTenuredGCThingZone(const uintptr_t addr) {
const uintptr_t zone_addr = (addr & ~ArenaMask) | ArenaZoneOffset;
return *reinterpret_cast<JS::Zone**>(zone_addr);
static MOZ_ALWAYS_INLINE bool TenuredCellIsMarkedBlack(
const TenuredCell* cell) {
// Return true if BlackBit is set.
MarkBitmapWord* blackWord;
uintptr_t blackMask;
TenuredChunkBase* chunk = GetCellChunkBase(cell);
chunk->markBits.getMarkWordAndMask(cell, js::gc::ColorBit::BlackBit,
&blackWord, &blackMask);
return *blackWord & blackMask;
static MOZ_ALWAYS_INLINE bool NonBlackCellIsMarkedGray(
const TenuredCell* cell) {
// Return true if GrayOrBlackBit is set. Callers should check BlackBit first.
MarkBitmapWord* grayWord;
uintptr_t grayMask;
TenuredChunkBase* chunk = GetCellChunkBase(cell);
chunk->markBits.getMarkWordAndMask(cell, js::gc::ColorBit::GrayOrBlackBit,
&grayWord, &grayMask);
return *grayWord & grayMask;
static MOZ_ALWAYS_INLINE bool TenuredCellIsMarkedGray(const TenuredCell* cell) {
return !TenuredCellIsMarkedBlack(cell) && NonBlackCellIsMarkedGray(cell);
static MOZ_ALWAYS_INLINE bool CellIsMarkedGray(const Cell* cell) {
if (js::gc::IsInsideNursery(cell)) {
return false;
return TenuredCellIsMarkedGray(reinterpret_cast<const TenuredCell*>(cell));
extern JS_PUBLIC_API bool CanCheckGrayBits(const TenuredCell* cell);
extern JS_PUBLIC_API bool CellIsMarkedGrayIfKnown(const TenuredCell* cell);
#ifdef DEBUG
extern JS_PUBLIC_API void AssertCellIsNotGray(const Cell* cell);
extern JS_PUBLIC_API bool ObjectIsMarkedBlack(const JSObject* obj);
MOZ_ALWAYS_INLINE bool CellHasStoreBuffer(const Cell* cell) {
return GetCellChunkBase(cell)->storeBuffer;
} /* namespace detail */
MOZ_ALWAYS_INLINE bool IsInsideNursery(const Cell* cell) {
return detail::CellHasStoreBuffer(cell);
MOZ_ALWAYS_INLINE bool IsInsideNursery(const TenuredCell* cell) {
MOZ_ASSERT(!IsInsideNursery(reinterpret_cast<const Cell*>(cell)));
return false;
// Allow use before the compiler knows the derivation of JSObject, JSString, and
// JS::BigInt.
MOZ_ALWAYS_INLINE bool IsInsideNursery(const JSObject* obj) {
return IsInsideNursery(reinterpret_cast<const Cell*>(obj));
MOZ_ALWAYS_INLINE bool IsInsideNursery(const JSString* str) {
return IsInsideNursery(reinterpret_cast<const Cell*>(str));
MOZ_ALWAYS_INLINE bool IsInsideNursery(const JS::BigInt* bi) {
return IsInsideNursery(reinterpret_cast<const Cell*>(bi));
MOZ_ALWAYS_INLINE bool IsCellPointerValid(const void* ptr) {
auto addr = uintptr_t(ptr);
if (addr < ChunkSize || addr % CellAlignBytes != 0) {
return false;
auto* cell = reinterpret_cast<const Cell*>(ptr);
if (!IsInsideNursery(cell)) {
return detail::GetTenuredGCThingZone(addr) != nullptr;
return true;
MOZ_ALWAYS_INLINE bool IsCellPointerValidOrNull(const void* cell) {
if (!cell) {
return true;
return IsCellPointerValid(cell);
} /* namespace gc */
} /* namespace js */
namespace JS {
static MOZ_ALWAYS_INLINE Zone* GetTenuredGCThingZone(GCCellPtr thing) {
return js::gc::detail::GetTenuredGCThingZone(thing.unsafeAsUIntPtr());
extern JS_PUBLIC_API Zone* GetNurseryCellZone(js::gc::Cell* cell);
static MOZ_ALWAYS_INLINE Zone* GetGCThingZone(GCCellPtr thing) {
if (!js::gc::IsInsideNursery(thing.asCell())) {
return js::gc::detail::GetTenuredGCThingZone(thing.unsafeAsUIntPtr());
return GetNurseryCellZone(thing.asCell());
static MOZ_ALWAYS_INLINE Zone* GetStringZone(JSString* str) {
if (!js::gc::IsInsideNursery(str)) {
return js::gc::detail::GetTenuredGCThingZone(
return GetNurseryCellZone(reinterpret_cast<js::gc::Cell*>(str));
extern JS_PUBLIC_API Zone* GetObjectZone(JSObject* obj);
static MOZ_ALWAYS_INLINE bool GCThingIsMarkedGray(GCCellPtr thing) {
js::gc::Cell* cell = thing.asCell();
if (IsInsideNursery(cell)) {
return false;
auto* tenuredCell = reinterpret_cast<js::gc::TenuredCell*>(cell);
return js::gc::detail::CellIsMarkedGrayIfKnown(tenuredCell);
// Specialised gray marking check for use by the cycle collector. This is not
// called during incremental GC or when the gray bits are invalid.
static MOZ_ALWAYS_INLINE bool GCThingIsMarkedGrayInCC(GCCellPtr thing) {
js::gc::Cell* cell = thing.asCell();
if (IsInsideNursery(cell)) {
return false;
auto* tenuredCell = reinterpret_cast<js::gc::TenuredCell*>(cell);
if (!js::gc::detail::TenuredCellIsMarkedGray(tenuredCell)) {
return false;
return true;
extern JS_PUBLIC_API JS::TraceKind GCThingTraceKind(void* thing);
extern JS_PUBLIC_API void EnableNurseryStrings(JSContext* cx);
extern JS_PUBLIC_API void DisableNurseryStrings(JSContext* cx);
extern JS_PUBLIC_API void EnableNurseryBigInts(JSContext* cx);
extern JS_PUBLIC_API void DisableNurseryBigInts(JSContext* cx);
* Returns true when writes to GC thing pointers (and reads from weak pointers)
* must call an incremental barrier. This is generally only true when running
* mutator code in-between GC slices. At other times, the barrier may be elided
* for performance.
extern JS_PUBLIC_API bool IsIncrementalBarrierNeeded(JSContext* cx);
* Notify the GC that a reference to a JSObject is about to be overwritten.
* This method must be called if IsIncrementalBarrierNeeded.
extern JS_PUBLIC_API void IncrementalPreWriteBarrier(JSObject* obj);
* Notify the GC that a reference to a tenured GC cell is about to be
* overwritten. This method must be called if IsIncrementalBarrierNeeded.
extern JS_PUBLIC_API void IncrementalPreWriteBarrier(GCCellPtr thing);
* Unsets the gray bit for anything reachable from |thing|. |kind| should not be
* JS::TraceKind::Shape. |thing| should be non-null. The return value indicates
* if anything was unmarked.
extern JS_PUBLIC_API bool UnmarkGrayGCThingRecursively(GCCellPtr thing);
} // namespace JS
namespace js {
namespace gc {
extern JS_PUBLIC_API void PerformIncrementalReadBarrier(JS::GCCellPtr thing);
static MOZ_ALWAYS_INLINE void ExposeGCThingToActiveJS(JS::GCCellPtr thing) {
// TODO: I'd like to assert !RuntimeHeapIsBusy() here but this gets
// called while we are tracing the heap, e.g. during memory reporting
// (see bug 1313318).
// GC things residing in the nursery cannot be gray: they have no mark bits.
// All live objects in the nursery are moved to tenured at the beginning of
// each GC slice, so the gray marker never sees nursery things.
if (IsInsideNursery(thing.asCell())) {
auto* cell = reinterpret_cast<TenuredCell*>(thing.asCell());
if (detail::TenuredCellIsMarkedBlack(cell)) {
// GC things owned by other runtimes are always black.
auto* zone = JS::shadow::Zone::from(JS::GetTenuredGCThingZone(thing));
if (zone->needsIncrementalBarrier()) {
} else if (!zone->isGCPreparing() && detail::NonBlackCellIsMarkedGray(cell)) {
MOZ_ASSERT_IF(!zone->isGCPreparing(), !detail::TenuredCellIsMarkedGray(cell));
static MOZ_ALWAYS_INLINE void IncrementalReadBarrier(JS::GCCellPtr thing) {
// This is a lighter version of ExposeGCThingToActiveJS that doesn't do gray
// unmarking.
if (IsInsideNursery(thing.asCell())) {
auto* zone = JS::shadow::Zone::from(JS::GetTenuredGCThingZone(thing));
auto* cell = reinterpret_cast<TenuredCell*>(thing.asCell());
if (zone->needsIncrementalBarrier() &&
!detail::TenuredCellIsMarkedBlack(cell)) {
// GC things owned by other runtimes are always black.
template <typename T>
extern JS_PUBLIC_API bool EdgeNeedsSweepUnbarrieredSlow(T* thingp);
static MOZ_ALWAYS_INLINE bool EdgeNeedsSweepUnbarriered(JSObject** objp) {
// This function does not handle updating nursery pointers. Raw JSObject
// pointers should be updated separately or replaced with
// JS::Heap<JSObject*> which handles this automatically.
if (IsInsideNursery(*objp)) {
return false;
auto zone =
if (!zone->isGCSweepingOrCompacting()) {
return false;
return EdgeNeedsSweepUnbarrieredSlow(objp);
} // namespace gc
} // namespace js
namespace JS {
* This should be called when an object that is marked gray is exposed to the JS
* engine (by handing it to running JS code or writing it into live JS
* data). During incremental GC, since the gray bits haven't been computed yet,
* we conservatively mark the object black.
static MOZ_ALWAYS_INLINE void ExposeObjectToActiveJS(JSObject* obj) {
} /* namespace JS */
#endif /* js_HeapAPI_h */