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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
#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;
inline ChunkBase* chunk() 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;
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;
}
ArenaChunk* chunk() const { return static_cast<ArenaChunk*>(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(ChunkMarkBitmap* 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(ArenaChunk::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(ChunkMarkBitmap* 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;
}
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 = zoneFromAnyThread();
MOZ_ASSERT(CurrentThreadIsGCMarking() || CurrentThreadCanAccessZone(zone));
return zone;
}
JS::Zone* TenuredCell::zoneFromAnyThread() const { return arena()->zone(); }
bool TenuredCell::isInsideZone(JS::Zone* zone) const {
return zone == zoneFromAnyThread();
}
// 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 */