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Diffstat (limited to '')
| -rw-r--r-- | js/src/gc/Cell.h | 780 |
1 files changed, 780 insertions, 0 deletions
diff --git a/js/src/gc/Cell.h b/js/src/gc/Cell.h new file mode 100644 index 0000000000..d9ebb3a751 --- /dev/null +++ b/js/src/gc/Cell.h @@ -0,0 +1,780 @@ +/* -*- 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/GCEnum.h" +#include "gc/Heap.h" +#include "js/GCAnnotations.h" +#include "js/shadow/Zone.h" // JS::shadow::Zone +#include "js/TraceKind.h" +#include "js/TypeDecls.h" + +namespace JS { +enum class TraceKind; +} /* namespace JS */ + +namespace js { + +class 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(); + +extern bool CurrentThreadIsGCMarking(); +extern bool CurrentThreadIsGCSweeping(); +extern bool CurrentThreadIsGCFinalizing(); +extern bool RuntimeIsVerifyingPreBarriers(JSRuntime* runtime); + +#endif + +extern void TraceManuallyBarrieredGenericPointerEdge(JSTracer* trc, + gc::Cell** thingp, + const char* name); + +namespace gc { + +class Arena; +enum class AllocKind : uint8_t; +class StoreBuffer; +class TenuredCell; + +extern void UnmarkGrayGCThingRecursively(TenuredCell* cell); + +// Like gc::MarkColor but allows the possibility of the cell being unmarked. +// +// This class mimics an enum class, but supports operator overloading. +class CellColor { + public: + enum Color { White = 0, Gray = 1, Black = 2 }; + + CellColor() : color(White) {} + + MOZ_IMPLICIT CellColor(MarkColor markColor) + : color(markColor == MarkColor::Black ? Black : Gray) {} + + MOZ_IMPLICIT constexpr CellColor(Color c) : color(c) {} + + MarkColor asMarkColor() const { + MOZ_ASSERT(color != White); + return color == Black ? MarkColor::Black : MarkColor::Gray; + } + + // Implement a total ordering for CellColor, with white being 'least marked' + // and black being 'most marked'. + bool operator<(const CellColor other) const { return color < other.color; } + bool operator>(const CellColor other) const { return color > other.color; } + bool operator<=(const CellColor other) const { return color <= other.color; } + bool operator>=(const CellColor other) const { return color >= other.color; } + bool operator!=(const CellColor other) const { return color != other.color; } + bool operator==(const CellColor other) const { return color == other.color; } + explicit operator bool() const { return color != White; } + +#if defined(JS_GC_ZEAL) || defined(DEBUG) + const char* name() const { + switch (color) { + case CellColor::White: + return "white"; + case CellColor::Black: + return "black"; + case CellColor::Gray: + return "gray"; + default: + MOZ_CRASH("Unexpected cell color"); + } + } +#endif + + private: + Color color; +}; + +// [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 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. It can also be used for temporary +// storage (see setTemporaryGCUnsafeData). 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 { + protected: + // Cell header word. Stores GC flags and derived class data. + // + // This is atomic since it can be read from and written to by different + // threads during compacting GC, in a limited way. Specifically, writes that + // update the derived class data can race with reads that check the forwarded + // flag. The writes do not change the forwarded flag (which is always false in + // this situation). + mozilla::Atomic<uintptr_t, mozilla::MemoryOrdering::Relaxed> header_; + + public: + static_assert(gc::CellFlagBitsReservedForGC >= 3, + "Not enough flag bits reserved for GC"); + static constexpr uintptr_t RESERVED_MASK = + BitMask(gc::CellFlagBitsReservedForGC); + + // 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 currently unused. + + bool isForwarded() const { return header_ & FORWARD_BIT; } + uintptr_t flags() const { return header_ & RESERVED_MASK; } + + 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 { + return isMarkedBlack() ? CellColor::Black + : isMarkedGray() ? CellColor::Gray + : CellColor::White; + } + + 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 TenuredChunk* 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; + } + + // Mark bit management. + MOZ_ALWAYS_INLINE bool isMarkedAny() const; + MOZ_ALWAYS_INLINE bool isMarkedBlack() const; + MOZ_ALWAYS_INLINE bool isMarkedGray() const; + + // Same as Cell::color, but skips nursery checks. + MOZ_ALWAYS_INLINE CellColor color() const { + return isMarkedBlack() ? CellColor::Black + : isMarkedGray() ? CellColor::Gray + : CellColor::White; + } + + // 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 void markBlack() 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() {} + +#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(); +} + +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; +} + +TenuredChunk* Cell::chunk() const { + uintptr_t addr = uintptr_t(this); + MOZ_ASSERT(addr % CellAlignBytes == 0); + addr &= ~ChunkMask; + return reinterpret_cast<TenuredChunk*>(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(); +} + +bool TenuredCell::isMarkedAny() const { + MOZ_ASSERT(arena()->allocated()); + return chunk()->markBits.isMarkedAny(this); +} + +bool TenuredCell::isMarkedBlack() const { + MOZ_ASSERT(arena()->allocated()); + return chunk()->markBits.isMarkedBlack(this); +} + +bool TenuredCell::isMarkedGray() const { + MOZ_ASSERT(arena()->allocated()); + return chunk()->markBits.isMarkedGray(this); +} + +bool TenuredCell::markIfUnmarked(MarkColor color /* = Black */) const { + return chunk()->markBits.markIfUnmarked(this, color); +} + +void TenuredCell::markBlack() const { chunk()->markBits.markBlack(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 && !thing->isPermanentAndMayBeShared()) { + ReadBarrierImpl(thing); + } +} + +MOZ_ALWAYS_INLINE void ReadBarrierImpl(TenuredCell* thing) { + MOZ_ASSERT(!CurrentThreadIsIonCompiling()); + MOZ_ASSERT(!CurrentThreadIsGCMarking()); + MOZ_ASSERT(thing); + MOZ_ASSERT(CurrentThreadCanAccessZone(thing->zoneFromAnyThread())); + + // Barriers should not be triggered on main thread while collecting. + mozilla::DebugOnly<JSRuntime*> runtime = thing->runtimeFromAnyThread(); + MOZ_ASSERT_IF(CurrentThreadCanAccessRuntime(runtime), + !JS::RuntimeHeapIsCollecting()); + + JS::shadow::Zone* shadowZone = thing->shadowZoneFromAnyThread(); + if (shadowZone->needsIncrementalBarrier()) { + // We should only observe barriers being enabled on the main thread. + MOZ_ASSERT(CurrentThreadCanAccessRuntime(runtime)); + Cell* tmp = thing; + TraceManuallyBarrieredGenericPointerEdge(shadowZone->barrierTracer(), &tmp, + "read barrier"); + MOZ_ASSERT(tmp == thing); + return; + } + + if (thing->isMarkedGray()) { + // There shouldn't be anything marked gray unless we're on the main thread. + MOZ_ASSERT(CurrentThreadCanAccessRuntime(runtime)); + UnmarkGrayGCThingRecursively(thing); + } +} + +MOZ_ALWAYS_INLINE void ReadBarrierImpl(Cell* thing) { + MOZ_ASSERT(!CurrentThreadIsGCMarking()); + if (thing->isTenured()) { + ReadBarrierImpl(&thing->asTenured()); + } +} + +MOZ_ALWAYS_INLINE void PreWriteBarrierImpl(TenuredCell* thing) { + MOZ_ASSERT(!CurrentThreadIsIonCompiling()); + MOZ_ASSERT(!CurrentThreadIsGCMarking()); + + if (!thing) { + return; + } + + // Barriers can be triggered on the main thread while collecting, but are + // disabled. For example, this happens when destroying HeapPtr wrappers. + + JS::shadow::Zone* zone = thing->shadowZoneFromAnyThread(); + if (!zone->needsIncrementalBarrier()) { + return; + } + + // Barriers can be triggered on off the main thread in two situations: + // - background finalization of HeapPtrs to the atoms zone + // - while we are verifying pre-barriers for a worker runtime + // The barrier is not required in either case. + bool checkThread = zone->isAtomsZone(); +#ifdef JS_GC_ZEAL + checkThread = checkThread || zone->isSelfHostingZone(); +#endif + JSRuntime* runtime = thing->runtimeFromAnyThread(); + if (checkThread && !CurrentThreadCanAccessRuntime(runtime)) { + MOZ_ASSERT(CurrentThreadIsGCFinalizing() || + RuntimeIsVerifyingPreBarriers(runtime)); + return; + } + + MOZ_ASSERT(CurrentThreadCanAccessRuntime(runtime)); + MOZ_ASSERT(!RuntimeFromMainThreadIsHeapMajorCollecting(zone)); + Cell* tmp = thing; + TraceManuallyBarrieredGenericPointerEdge(zone->barrierTracer(), &tmp, + "pre barrier"); + MOZ_ASSERT(tmp == thing); +} + +MOZ_ALWAYS_INLINE void PreWriteBarrierImpl(Cell* thing) { + MOZ_ASSERT(!CurrentThreadIsGCMarking()); + if (thing && 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 && !thing->isPermanentAndMayBeShared()) { + PreWriteBarrierImpl(thing); + } +} + +#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_ >> 32); +#endif + } + + uint32_t headerFlagsField() const { return uint32_t(header_); } + + void setHeaderFlagBit(uint32_t flag) { header_ |= uintptr_t(flag); } + void clearHeaderFlagBit(uint32_t flag) { header_ &= ~uintptr_t(flag); } + void toggleHeaderFlagBit(uint32_t flag) { header_ ^= uintptr_t(flag); } + + void setHeaderLengthAndFlags(uint32_t len, uint32_t flags) { +#if JS_BITS_PER_WORD == 32 + header_ = flags; + length_ = len; +#else + header_ = (uint64_t(len) << 32) | uint64_t(flags); +#endif + } + + // Sub classes can store temporary data in the flags word. This is not GC safe + // and users must ensure flags/length are never checked (including by asserts) + // while this data is stored. Use of this method is strongly discouraged! + void setTemporaryGCUnsafeData(uintptr_t data) { header_ = data; } + + // To get back the data, values to safely re-initialize clobbered flags + // must be provided. + uintptr_t unsetTemporaryGCUnsafeData(uint32_t len, uint32_t flags) { + uintptr_t data = header_; + setHeaderLengthAndFlags(len, flags); + return data; + } + + 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); + MOZ_ASSERT((data & RESERVED_MASK) == 0); + header_ = data; + } + + PtrT* headerPtr() const { + // Currently we never observe any flags set here because this base class is + // only used for JSObject (for which the nursery kind flags are always + // clear) or GC things that are always tenured (for which the nursery kind + // flags are also always clear). This means we don't need to use masking to + // get and set the pointer. + MOZ_ASSERT(flags() == 0); + return reinterpret_cast<PtrT*>(uintptr_t(header_)); + } + + void setHeaderPtr(PtrT* newValue) { + // As above, no flags are expected to be set here. + uintptr_t data = uintptr_t(newValue); + MOZ_ASSERT(flags() == 0); + MOZ_ASSERT((data & RESERVED_MASK) == 0); + header_ = data; + } + + public: + static constexpr size_t offsetOfHeaderPtr() { + return offsetof(TenuredCellWithNonGCPointer, header_); + } +}; + +// 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(!IsInsideNursery(initial)); + uintptr_t data = uintptr_t(initial); + MOZ_ASSERT((data & Cell::RESERVED_MASK) == 0); + this->header_ = data; + } + + void setHeaderPtr(PtrT* newValue) { + // As above, no flags are expected to be set here. + MOZ_ASSERT(!IsInsideNursery(newValue)); + PreWriteBarrier(headerPtr()); + unbarrieredSetHeaderPtr(newValue); + } + + public: + PtrT* headerPtr() const { + // Currently we never observe any flags set here because this base class is + // only used for GC things that are always tenured (for which the nursery + // kind flags are also always clear). This means we don't need to use + // masking to get and set the pointer. + staticAsserts(); + MOZ_ASSERT(this->flags() == 0); + return reinterpret_cast<PtrT*>(uintptr_t(this->header_)); + } + + void unbarrieredSetHeaderPtr(PtrT* newValue) { + uintptr_t data = uintptr_t(newValue); + MOZ_ASSERT(this->flags() == 0); + MOZ_ASSERT((data & Cell::RESERVED_MASK) == 0); + this->header_ = data; + } + + static constexpr size_t offsetOfHeaderPtr() { + return offsetof(CellWithTenuredGCPointer, header_); + } +}; + +} /* namespace gc */ +} /* namespace js */ + +#endif /* gc_Cell_h */ |