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Diffstat (limited to 'js/src/gc/Heap.h')
| -rw-r--r-- | js/src/gc/Heap.h | 774 |
1 files changed, 774 insertions, 0 deletions
diff --git a/js/src/gc/Heap.h b/js/src/gc/Heap.h new file mode 100644 index 0000000000..daa20cbf0f --- /dev/null +++ b/js/src/gc/Heap.h @@ -0,0 +1,774 @@ +/* -*- 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_Heap_h +#define gc_Heap_h + +#include "mozilla/DebugOnly.h" + +#include "ds/BitArray.h" +#include "gc/AllocKind.h" +#include "gc/GCEnum.h" +#include "js/HeapAPI.h" +#include "js/TypeDecls.h" +#include "util/Poison.h" + +namespace js { + +class AutoLockGC; +class AutoLockGCBgAlloc; +class NurseryDecommitTask; + +namespace gc { + +class Arena; +class ArenaCellSet; +class ArenaList; +class GCRuntime; +class MarkingValidator; +class SortedArenaList; +class StoreBuffer; +class TenuredCell; + +// Cells are aligned to CellAlignShift, so the largest tagged null pointer is: +const uintptr_t LargestTaggedNullCellPointer = (1 << CellAlignShift) - 1; + +/* + * The minimum cell size ends up as twice the cell alignment because the mark + * bitmap contains one bit per CellBytesPerMarkBit bytes (which is equal to + * CellAlignBytes) and we need two mark bits per cell. + */ +const size_t MinCellSize = CellBytesPerMarkBit * MarkBitsPerCell; + +static_assert(ArenaSize % CellAlignBytes == 0, + "Arena size must be a multiple of cell alignment"); + +/* + * A FreeSpan represents a contiguous sequence of free cells in an Arena. It + * can take two forms. + * + * - In an empty span, |first| and |last| are both zero. + * + * - In a non-empty span, |first| is the address of the first free thing in the + * span, and |last| is the address of the last free thing in the span. + * Furthermore, the memory pointed to by |last| holds a FreeSpan structure + * that points to the next span (which may be empty); this works because + * sizeof(FreeSpan) is less than the smallest thingSize. + */ +class FreeSpan { + friend class Arena; + friend class ArenaCellIter; + friend class ArenaFreeCellIter; + + uint16_t first; + uint16_t last; + + public: + // This inits just |first| and |last|; if the span is non-empty it doesn't + // do anything with the next span stored at |last|. + void initBounds(uintptr_t firstArg, uintptr_t lastArg, const Arena* arena) { + checkRange(firstArg, lastArg, arena); + first = firstArg; + last = lastArg; + } + + void initAsEmpty() { + first = 0; + last = 0; + } + + // This sets |first| and |last|, and also sets the next span stored at + // |last| as empty. (As a result, |firstArg| and |lastArg| cannot represent + // an empty span.) + void initFinal(uintptr_t firstArg, uintptr_t lastArg, const Arena* arena) { + initBounds(firstArg, lastArg, arena); + FreeSpan* last = nextSpanUnchecked(arena); + last->initAsEmpty(); + checkSpan(arena); + } + + bool isEmpty() const { return !first; } + + Arena* getArenaUnchecked() { return reinterpret_cast<Arena*>(this); } + inline Arena* getArena(); + + static size_t offsetOfFirst() { return offsetof(FreeSpan, first); } + + static size_t offsetOfLast() { return offsetof(FreeSpan, last); } + + // Like nextSpan(), but no checking of the following span is done. + FreeSpan* nextSpanUnchecked(const Arena* arena) const { + MOZ_ASSERT(arena && !isEmpty()); + return reinterpret_cast<FreeSpan*>(uintptr_t(arena) + last); + } + + const FreeSpan* nextSpan(const Arena* arena) const { + checkSpan(arena); + return nextSpanUnchecked(arena); + } + + MOZ_ALWAYS_INLINE TenuredCell* allocate(size_t thingSize) { + // Eschew the usual checks, because this might be the placeholder span. + // If this is somehow an invalid, non-empty span, checkSpan() will catch it. + Arena* arena = getArenaUnchecked(); + checkSpan(arena); + uintptr_t thing = uintptr_t(arena) + first; + if (first < last) { + // We have space for at least two more things, so do a simple + // bump-allocate. + first += thingSize; + } else if (MOZ_LIKELY(first)) { + // The last space points to the next free span (which may be empty). + const FreeSpan* next = nextSpan(arena); + first = next->first; + last = next->last; + } else { + return nullptr; // The span is empty. + } + checkSpan(arena); + DebugOnlyPoison(reinterpret_cast<void*>(thing), + JS_ALLOCATED_TENURED_PATTERN, thingSize, + MemCheckKind::MakeUndefined); + return reinterpret_cast<TenuredCell*>(thing); + } + + inline void checkSpan(const Arena* arena) const; + inline void checkRange(uintptr_t first, uintptr_t last, + const Arena* arena) const; +}; + +/* + * Arenas are the allocation units of the tenured heap in the GC. An arena + * is 4kiB in size and 4kiB-aligned. It starts with several header fields + * followed by some bytes of padding. The remainder of the arena is filled + * with GC things of a particular AllocKind. The padding ensures that the + * GC thing array ends exactly at the end of the arena: + * + * <----------------------------------------------> = ArenaSize bytes + * +---------------+---------+----+----+-----+----+ + * | header fields | padding | T0 | T1 | ... | Tn | + * +---------------+---------+----+----+-----+----+ + * <-------------------------> = first thing offset + */ +class alignas(ArenaSize) Arena { + static JS_FRIEND_DATA const uint8_t ThingSizes[]; + static JS_FRIEND_DATA const uint8_t FirstThingOffsets[]; + static JS_FRIEND_DATA const uint8_t ThingsPerArena[]; + /* + * The first span of free things in the arena. Most of these spans are + * stored as offsets in free regions of the data array, and most operations + * on FreeSpans take an Arena pointer for safety. However, the FreeSpans + * used for allocation are stored here, at the start of an Arena, and use + * their own address to grab the next span within the same Arena. + */ + FreeSpan firstFreeSpan; + + public: + /* + * The zone that this Arena is contained within, when allocated. The offset + * of this field must match the ArenaZoneOffset stored in js/HeapAPI.h, + * as is statically asserted below. + */ + JS::Zone* zone; + + /* + * Arena::next has two purposes: when unallocated, it points to the next + * available Arena. When allocated, it points to the next Arena in the same + * zone and with the same alloc kind. + */ + Arena* next; + + private: + /* + * One of the AllocKind constants or AllocKind::LIMIT when the arena does + * not contain any GC things and is on the list of empty arenas in the GC + * chunk. + * + * We use 8 bits for the alloc kind so the compiler can use byte-level + * memory instructions to access it. + */ + size_t allocKind : 8; + + private: + /* + * When recursive marking uses too much stack we delay marking of + * arenas and link them into a list for later processing. This + * uses the following fields. + */ + static const size_t DELAYED_MARKING_FLAG_BITS = 3; + static const size_t DELAYED_MARKING_ARENA_BITS = + JS_BITS_PER_WORD - 8 - DELAYED_MARKING_FLAG_BITS; + size_t onDelayedMarkingList_ : 1; + size_t hasDelayedBlackMarking_ : 1; + size_t hasDelayedGrayMarking_ : 1; + size_t nextDelayedMarkingArena_ : DELAYED_MARKING_ARENA_BITS; + static_assert( + DELAYED_MARKING_ARENA_BITS >= JS_BITS_PER_WORD - ArenaShift, + "Arena::nextDelayedMarkingArena_ packing assumes that ArenaShift has " + "enough bits to cover allocKind and delayed marking state."); + + union { + /* + * For arenas in zones other than the atoms zone, if non-null, points + * to an ArenaCellSet that represents the set of cells in this arena + * that are in the nursery's store buffer. + */ + ArenaCellSet* bufferedCells_; + + /* + * For arenas in the atoms zone, the starting index into zone atom + * marking bitmaps (see AtomMarking.h) of the things in this zone. + * Atoms never refer to nursery things, so no store buffer index is + * needed. + */ + size_t atomBitmapStart_; + }; + + public: + /* + * The size of data should be |ArenaSize - offsetof(data)|, but the offset + * is not yet known to the compiler, so we do it by hand. |firstFreeSpan| + * takes up 8 bytes on 64-bit due to alignment requirements; the rest are + * obvious. This constant is stored in js/HeapAPI.h. + */ + uint8_t data[ArenaSize - ArenaHeaderSize]; + + void init(JS::Zone* zoneArg, AllocKind kind, const AutoLockGC& lock); + + // Sets |firstFreeSpan| to the Arena's entire valid range, and + // also sets the next span stored at |firstFreeSpan.last| as empty. + void setAsFullyUnused() { + AllocKind kind = getAllocKind(); + firstFreeSpan.first = firstThingOffset(kind); + firstFreeSpan.last = lastThingOffset(kind); + FreeSpan* last = firstFreeSpan.nextSpanUnchecked(this); + last->initAsEmpty(); + } + + // Initialize an arena to its unallocated state. For arenas that were + // previously allocated for some zone, use release() instead. + void setAsNotAllocated() { + firstFreeSpan.initAsEmpty(); + + // Poison zone pointer to highlight UAF on released arenas in crash data. + AlwaysPoison(&zone, JS_FREED_ARENA_PATTERN, sizeof(zone), + MemCheckKind::MakeNoAccess); + + allocKind = size_t(AllocKind::LIMIT); + onDelayedMarkingList_ = 0; + hasDelayedBlackMarking_ = 0; + hasDelayedGrayMarking_ = 0; + nextDelayedMarkingArena_ = 0; + bufferedCells_ = nullptr; + } + + // Return an allocated arena to its unallocated state. + inline void release(const AutoLockGC& lock); + + uintptr_t address() const { + checkAddress(); + return uintptr_t(this); + } + + inline void checkAddress() const; + + inline TenuredChunk* chunk() const; + + bool allocated() const { + MOZ_ASSERT(IsAllocKind(AllocKind(allocKind))); + return IsValidAllocKind(AllocKind(allocKind)); + } + + AllocKind getAllocKind() const { + MOZ_ASSERT(allocated()); + return AllocKind(allocKind); + } + + FreeSpan* getFirstFreeSpan() { return &firstFreeSpan; } + + static size_t thingSize(AllocKind kind) { return ThingSizes[size_t(kind)]; } + static size_t thingsPerArena(AllocKind kind) { + return ThingsPerArena[size_t(kind)]; + } + static size_t thingsSpan(AllocKind kind) { + return thingsPerArena(kind) * thingSize(kind); + } + + static size_t firstThingOffset(AllocKind kind) { + return FirstThingOffsets[size_t(kind)]; + } + static size_t lastThingOffset(AllocKind kind) { + return ArenaSize - thingSize(kind); + } + + size_t getThingSize() const { return thingSize(getAllocKind()); } + size_t getThingsPerArena() const { return thingsPerArena(getAllocKind()); } + size_t getThingsSpan() const { return getThingsPerArena() * getThingSize(); } + size_t getFirstThingOffset() const { + return firstThingOffset(getAllocKind()); + } + + uintptr_t thingsStart() const { return address() + getFirstThingOffset(); } + uintptr_t thingsEnd() const { return address() + ArenaSize; } + + bool isEmpty() const { + // Arena is empty if its first span covers the whole arena. + firstFreeSpan.checkSpan(this); + AllocKind kind = getAllocKind(); + return firstFreeSpan.first == firstThingOffset(kind) && + firstFreeSpan.last == lastThingOffset(kind); + } + + bool hasFreeThings() const { return !firstFreeSpan.isEmpty(); } + + size_t numFreeThings(size_t thingSize) const { + firstFreeSpan.checkSpan(this); + size_t numFree = 0; + const FreeSpan* span = &firstFreeSpan; + for (; !span->isEmpty(); span = span->nextSpan(this)) { + numFree += (span->last - span->first) / thingSize + 1; + } + return numFree; + } + + size_t countFreeCells() { return numFreeThings(getThingSize()); } + size_t countUsedCells() { return getThingsPerArena() - countFreeCells(); } + + bool inFreeList(uintptr_t thing) { + uintptr_t base = address(); + const FreeSpan* span = &firstFreeSpan; + for (; !span->isEmpty(); span = span->nextSpan(this)) { + /* If the thing comes before the current span, it's not free. */ + if (thing < base + span->first) { + return false; + } + + /* If we find it before the end of the span, it's free. */ + if (thing <= base + span->last) { + return true; + } + } + return false; + } + + static bool isAligned(uintptr_t thing, size_t thingSize) { + /* Things ends at the arena end. */ + uintptr_t tailOffset = ArenaSize - (thing & ArenaMask); + return tailOffset % thingSize == 0; + } + + bool onDelayedMarkingList() const { return onDelayedMarkingList_; } + + Arena* getNextDelayedMarking() const { + MOZ_ASSERT(onDelayedMarkingList_); + return reinterpret_cast<Arena*>(nextDelayedMarkingArena_ << ArenaShift); + } + + void setNextDelayedMarkingArena(Arena* arena) { + MOZ_ASSERT(!(uintptr_t(arena) & ArenaMask)); + MOZ_ASSERT(!onDelayedMarkingList_); + MOZ_ASSERT(!hasDelayedBlackMarking_); + MOZ_ASSERT(!hasDelayedGrayMarking_); + MOZ_ASSERT(!nextDelayedMarkingArena_); + onDelayedMarkingList_ = 1; + if (arena) { + nextDelayedMarkingArena_ = arena->address() >> ArenaShift; + } + } + + void updateNextDelayedMarkingArena(Arena* arena) { + MOZ_ASSERT(!(uintptr_t(arena) & ArenaMask)); + MOZ_ASSERT(onDelayedMarkingList_); + nextDelayedMarkingArena_ = arena ? arena->address() >> ArenaShift : 0; + } + + bool hasDelayedMarking(MarkColor color) const { + MOZ_ASSERT(onDelayedMarkingList_); + return color == MarkColor::Black ? hasDelayedBlackMarking_ + : hasDelayedGrayMarking_; + } + + bool hasAnyDelayedMarking() const { + MOZ_ASSERT(onDelayedMarkingList_); + return hasDelayedBlackMarking_ || hasDelayedGrayMarking_; + } + + void setHasDelayedMarking(MarkColor color, bool value) { + MOZ_ASSERT(onDelayedMarkingList_); + if (color == MarkColor::Black) { + hasDelayedBlackMarking_ = value; + } else { + hasDelayedGrayMarking_ = value; + } + } + + void clearDelayedMarkingState() { + MOZ_ASSERT(onDelayedMarkingList_); + onDelayedMarkingList_ = 0; + hasDelayedBlackMarking_ = 0; + hasDelayedGrayMarking_ = 0; + nextDelayedMarkingArena_ = 0; + } + + inline ArenaCellSet*& bufferedCells(); + inline size_t& atomBitmapStart(); + + template <typename T> + size_t finalize(JSFreeOp* fop, AllocKind thingKind, size_t thingSize); + + static void staticAsserts(); + static void checkLookupTables(); + + void unmarkAll(); + void unmarkPreMarkedFreeCells(); + + void arenaAllocatedDuringGC(); + +#ifdef DEBUG + void checkNoMarkedFreeCells(); + void checkAllCellsMarkedBlack(); +#endif + +#if defined(DEBUG) || defined(JS_GC_ZEAL) + void checkNoMarkedCells(); +#endif +}; + +static_assert(ArenaZoneOffset == offsetof(Arena, zone), + "The hardcoded API zone offset must match the actual offset."); + +static_assert(sizeof(Arena) == ArenaSize, + "ArenaSize must match the actual size of the Arena structure."); + +static_assert( + offsetof(Arena, data) == ArenaHeaderSize, + "ArenaHeaderSize must match the actual size of the header fields."); + +inline Arena* FreeSpan::getArena() { + Arena* arena = getArenaUnchecked(); + arena->checkAddress(); + return arena; +} + +inline void FreeSpan::checkSpan(const Arena* arena) const { +#ifdef DEBUG + if (!first) { + MOZ_ASSERT(!first && !last); + return; + } + + arena->checkAddress(); + checkRange(first, last, arena); + + // If there's a following span, it must have a higher address, + // and the gap must be at least 2 * thingSize. + const FreeSpan* next = nextSpanUnchecked(arena); + if (next->first) { + checkRange(next->first, next->last, arena); + size_t thingSize = arena->getThingSize(); + MOZ_ASSERT(last + 2 * thingSize <= next->first); + } +#endif +} + +inline void FreeSpan::checkRange(uintptr_t first, uintptr_t last, + const Arena* arena) const { +#ifdef DEBUG + MOZ_ASSERT(arena); + MOZ_ASSERT(first <= last); + AllocKind thingKind = arena->getAllocKind(); + MOZ_ASSERT(first >= Arena::firstThingOffset(thingKind)); + MOZ_ASSERT(last <= Arena::lastThingOffset(thingKind)); + MOZ_ASSERT((last - first) % Arena::thingSize(thingKind) == 0); +#endif +} + +// Mark bitmap API: + +MOZ_ALWAYS_INLINE bool MarkBitmap::markBit(const TenuredCell* cell, + ColorBit colorBit) { + MarkBitmapWord* word; + uintptr_t mask; + getMarkWordAndMask(cell, colorBit, &word, &mask); + return *word & mask; +} + +MOZ_ALWAYS_INLINE bool MarkBitmap::isMarkedAny(const TenuredCell* cell) { + return markBit(cell, ColorBit::BlackBit) || + markBit(cell, ColorBit::GrayOrBlackBit); +} + +MOZ_ALWAYS_INLINE bool MarkBitmap::isMarkedBlack(const TenuredCell* cell) { + return markBit(cell, ColorBit::BlackBit); +} + +MOZ_ALWAYS_INLINE bool MarkBitmap::isMarkedGray(const TenuredCell* cell) { + return !markBit(cell, ColorBit::BlackBit) && + markBit(cell, ColorBit::GrayOrBlackBit); +} + +// The return value indicates if the cell went from unmarked to marked. +MOZ_ALWAYS_INLINE bool MarkBitmap::markIfUnmarked(const TenuredCell* cell, + MarkColor color) { + MarkBitmapWord* word; + uintptr_t mask; + getMarkWordAndMask(cell, ColorBit::BlackBit, &word, &mask); + if (*word & mask) { + return false; + } + if (color == MarkColor::Black) { + *word |= mask; + } else { + /* + * We use getMarkWordAndMask to recalculate both mask and word as + * doing just mask << color may overflow the mask. + */ + getMarkWordAndMask(cell, ColorBit::GrayOrBlackBit, &word, &mask); + if (*word & mask) { + return false; + } + *word |= mask; + } + return true; +} + +MOZ_ALWAYS_INLINE void MarkBitmap::markBlack(const TenuredCell* cell) { + MarkBitmapWord* word; + uintptr_t mask; + getMarkWordAndMask(cell, ColorBit::BlackBit, &word, &mask); + *word |= mask; +} + +MOZ_ALWAYS_INLINE void MarkBitmap::copyMarkBit(TenuredCell* dst, + const TenuredCell* src, + ColorBit colorBit) { + TenuredChunkBase* srcChunk = detail::GetCellChunkBase(src); + MarkBitmapWord* srcWord; + uintptr_t srcMask; + srcChunk->markBits.getMarkWordAndMask(src, colorBit, &srcWord, &srcMask); + + MarkBitmapWord* dstWord; + uintptr_t dstMask; + getMarkWordAndMask(dst, colorBit, &dstWord, &dstMask); + + *dstWord &= ~dstMask; + if (*srcWord & srcMask) { + *dstWord |= dstMask; + } +} + +MOZ_ALWAYS_INLINE void MarkBitmap::unmark(const TenuredCell* cell) { + MarkBitmapWord* word; + uintptr_t mask; + getMarkWordAndMask(cell, ColorBit::BlackBit, &word, &mask); + *word &= ~mask; + getMarkWordAndMask(cell, ColorBit::GrayOrBlackBit, &word, &mask); + *word &= ~mask; +} + +inline void MarkBitmap::clear() { + for (size_t i = 0; i < MarkBitmap::WordCount; i++) { + bitmap[i] = 0; + } +} + +inline MarkBitmapWord* MarkBitmap::arenaBits(Arena* arena) { + static_assert( + ArenaBitmapBits == ArenaBitmapWords * JS_BITS_PER_WORD, + "We assume that the part of the bitmap corresponding to the arena " + "has the exact number of words so we do not need to deal with a word " + "that covers bits from two arenas."); + + MarkBitmapWord* word; + uintptr_t unused; + getMarkWordAndMask(reinterpret_cast<TenuredCell*>(arena->address()), + ColorBit::BlackBit, &word, &unused); + return word; +} + +/* + * A chunk in the tenured heap. TenuredChunks contain arenas and associated data + * structures (mark bitmap, delayed marking state). + */ +class TenuredChunk : public TenuredChunkBase { + Arena arenas[ArenasPerChunk]; + + friend class GCRuntime; + friend class MarkingValidator; + + public: + static TenuredChunk* fromAddress(uintptr_t addr) { + addr &= ~ChunkMask; + return reinterpret_cast<TenuredChunk*>(addr); + } + + static bool withinValidRange(uintptr_t addr) { + uintptr_t offset = addr & ChunkMask; + if (TenuredChunk::fromAddress(addr)->isNurseryChunk()) { + return offset >= sizeof(ChunkBase) && offset < ChunkSize; + } + return offset >= offsetof(TenuredChunk, arenas) && offset < ChunkSize; + } + + static size_t arenaIndex(uintptr_t addr) { + MOZ_ASSERT(!TenuredChunk::fromAddress(addr)->isNurseryChunk()); + MOZ_ASSERT(withinValidRange(addr)); + uintptr_t offset = addr & ChunkMask; + return (offset - offsetof(TenuredChunk, arenas)) >> ArenaShift; + } + + explicit TenuredChunk(JSRuntime* runtime) : TenuredChunkBase(runtime) {} + + uintptr_t address() const { + uintptr_t addr = reinterpret_cast<uintptr_t>(this); + MOZ_ASSERT(!(addr & ChunkMask)); + return addr; + } + + bool unused() const { return info.numArenasFree == ArenasPerChunk; } + + bool hasAvailableArenas() const { return info.numArenasFree != 0; } + + bool isNurseryChunk() const { return storeBuffer; } + + Arena* allocateArena(GCRuntime* gc, JS::Zone* zone, AllocKind kind, + const AutoLockGC& lock); + + void releaseArena(GCRuntime* gc, Arena* arena, const AutoLockGC& lock); + void recycleArena(Arena* arena, SortedArenaList& dest, size_t thingsPerArena); + + MOZ_MUST_USE bool decommitOneFreeArena(GCRuntime* gc, AutoLockGC& lock); + void decommitAllArenas(); + + // This will decommit each unused not-already decommitted arena. It performs a + // system call for each arena but is only used during OOM. + void decommitFreeArenasWithoutUnlocking(const AutoLockGC& lock); + + static TenuredChunk* allocate(GCRuntime* gc); + void init(GCRuntime* gc); + + /* Unlink and return the freeArenasHead. */ + Arena* fetchNextFreeArena(GCRuntime* gc); + + private: + /* Search for a decommitted arena to allocate. */ + unsigned findDecommittedArenaOffset(); + Arena* fetchNextDecommittedArena(); + + void addArenaToFreeList(GCRuntime* gc, Arena* arena); + void addArenaToDecommittedList(const Arena* arena); + + void updateChunkListAfterAlloc(GCRuntime* gc, const AutoLockGC& lock); + void updateChunkListAfterFree(GCRuntime* gc, const AutoLockGC& lock); +}; + +inline void Arena::checkAddress() const { + mozilla::DebugOnly<uintptr_t> addr = uintptr_t(this); + MOZ_ASSERT(addr); + MOZ_ASSERT(!(addr & ArenaMask)); + MOZ_ASSERT(TenuredChunk::withinValidRange(addr)); +} + +inline TenuredChunk* Arena::chunk() const { + return TenuredChunk::fromAddress(address()); +} + +inline bool InFreeList(Arena* arena, void* thing) { + uintptr_t addr = reinterpret_cast<uintptr_t>(thing); + MOZ_ASSERT(Arena::isAligned(addr, arena->getThingSize())); + return arena->inFreeList(addr); +} + +static const int32_t ChunkStoreBufferOffsetFromLastByte = + int32_t(gc::ChunkStoreBufferOffset) - int32_t(gc::ChunkMask); + +// Cell header stored before all nursery cells. +struct alignas(gc::CellAlignBytes) NurseryCellHeader { + // Store zone pointer with the trace kind in the lowest three bits. + const uintptr_t zoneAndTraceKind; + + // We only need to store a subset of trace kinds so this doesn't cover the + // full range. + static const uintptr_t TraceKindMask = 3; + + static uintptr_t MakeValue(JS::Zone* const zone, JS::TraceKind kind) { + MOZ_ASSERT(uintptr_t(kind) < TraceKindMask); + MOZ_ASSERT((uintptr_t(zone) & TraceKindMask) == 0); + return uintptr_t(zone) | uintptr_t(kind); + } + + NurseryCellHeader(JS::Zone* const zone, JS::TraceKind kind) + : zoneAndTraceKind(MakeValue(zone, kind)) {} + + JS::Zone* zone() const { + return reinterpret_cast<JS::Zone*>(zoneAndTraceKind & ~TraceKindMask); + } + + JS::TraceKind traceKind() const { + return JS::TraceKind(zoneAndTraceKind & TraceKindMask); + } + + static const NurseryCellHeader* from(const Cell* cell) { + MOZ_ASSERT(IsInsideNursery(cell)); + return reinterpret_cast<const NurseryCellHeader*>( + uintptr_t(cell) - sizeof(NurseryCellHeader)); + } +}; + +static_assert(uintptr_t(JS::TraceKind::Object) <= + NurseryCellHeader::TraceKindMask); +static_assert(uintptr_t(JS::TraceKind::String) <= + NurseryCellHeader::TraceKindMask); +static_assert(uintptr_t(JS::TraceKind::BigInt) <= + NurseryCellHeader::TraceKindMask); + +} /* namespace gc */ + +namespace debug { + +// Utility functions meant to be called from an interactive debugger. +enum class MarkInfo : int { + BLACK = 0, + GRAY = 1, + UNMARKED = -1, + NURSERY = -2, +}; + +// Get the mark color for a cell, in a way easily usable from a debugger. +MOZ_NEVER_INLINE MarkInfo GetMarkInfo(js::gc::Cell* cell); + +// Sample usage from gdb: +// +// (gdb) p $word = js::debug::GetMarkWordAddress(obj) +// $1 = (uintptr_t *) 0x7fa56d5fe360 +// (gdb) p/x $mask = js::debug::GetMarkMask(obj, js::gc::GRAY) +// $2 = 0x200000000 +// (gdb) watch *$word +// Hardware watchpoint 7: *$word +// (gdb) cond 7 *$word & $mask +// (gdb) cont +// +// Note that this is *not* a watchpoint on a single bit. It is a watchpoint on +// the whole word, which will trigger whenever the word changes and the +// selected bit is set after the change. +// +// So if the bit changing is the desired one, this is exactly what you want. +// But if a different bit changes (either set or cleared), you may still stop +// execution if the $mask bit happened to already be set. gdb does not expose +// enough information to restrict the watchpoint to just a single bit. + +// Return the address of the word containing the mark bits for the given cell, +// or nullptr if the cell is in the nursery. +MOZ_NEVER_INLINE uintptr_t* GetMarkWordAddress(js::gc::Cell* cell); + +// Return the mask for the given cell and color bit, or 0 if the cell is in the +// nursery. +MOZ_NEVER_INLINE uintptr_t GetMarkMask(js::gc::Cell* cell, uint32_t colorBit); + +} /* namespace debug */ +} /* namespace js */ + +#endif /* gc_Heap_h */ |