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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_StoreBuffer_h
#define gc_StoreBuffer_h
#include "mozilla/Attributes.h"
#include "mozilla/HashFunctions.h"
#include "mozilla/ReentrancyGuard.h"
#include <algorithm>
#include "ds/BitArray.h"
#include "ds/LifoAlloc.h"
#include "gc/Cell.h"
#include "gc/Nursery.h"
#include "gc/TraceKind.h"
#include "js/AllocPolicy.h"
#include "js/UniquePtr.h"
#include "threading/Mutex.h"
#include "wasm/WasmAnyRef.h"
namespace JS {
struct GCSizes;
}
namespace js {
class NativeObject;
#ifdef DEBUG
extern bool CurrentThreadIsGCMarking();
#endif
namespace gc {
class Arena;
class ArenaCellSet;
#ifdef DEBUG
extern bool CurrentThreadHasLockedGC();
#endif
/*
* BufferableRef represents an abstract reference for use in the generational
* GC's remembered set. Entries in the store buffer that cannot be represented
* with the simple pointer-to-a-pointer scheme must derive from this class and
* use the generic store buffer interface.
*
* A single BufferableRef entry in the generic buffer can represent many entries
* in the remembered set. For example js::OrderedHashTableRef represents all
* the incoming edges corresponding to keys in an ordered hash table.
*/
class BufferableRef {
public:
virtual void trace(JSTracer* trc) = 0;
bool maybeInRememberedSet(const Nursery&) const { return true; }
};
using EdgeSet = HashSet<void*, PointerHasher<void*>, SystemAllocPolicy>;
/* The size of a single block of store buffer storage space. */
static const size_t LifoAllocBlockSize = 8 * 1024;
/*
* The StoreBuffer observes all writes that occur in the system and performs
* efficient filtering of them to derive a remembered set for nursery GC.
*/
class StoreBuffer {
friend class mozilla::ReentrancyGuard;
/* The size at which a block is about to overflow for the generic buffer. */
static const size_t GenericBufferLowAvailableThreshold =
LifoAllocBlockSize / 2;
/* The size at which other store buffers are about to overflow. */
static const size_t BufferOverflowThresholdBytes = 128 * 1024;
enum class PutResult { OK, AboutToOverflow };
/*
* This buffer holds only a single type of edge. Using this buffer is more
* efficient than the generic buffer when many writes will be to the same
* type of edge: e.g. Value or Cell*.
*/
template <typename T>
struct MonoTypeBuffer {
/* The canonical set of stores. */
using StoreSet = HashSet<T, typename T::Hasher, SystemAllocPolicy>;
StoreSet stores_;
/*
* A one element cache in front of the canonical set to speed up
* temporary instances of HeapPtr.
*/
T last_ = T();
/* Maximum number of entries before we request a minor GC. */
const static size_t MaxEntries = BufferOverflowThresholdBytes / sizeof(T);
MonoTypeBuffer() = default;
MonoTypeBuffer(const MonoTypeBuffer& other) = delete;
MonoTypeBuffer& operator=(const MonoTypeBuffer& other) = delete;
MonoTypeBuffer(MonoTypeBuffer&& other)
: stores_(std::move(other.stores_)), last_(std::move(other.last_)) {
other.clear();
}
MonoTypeBuffer& operator=(MonoTypeBuffer&& other) {
if (&other != this) {
this->~MonoTypeBuffer();
new (this) MonoTypeBuffer(std::move(other));
}
return *this;
}
void clear() {
last_ = T();
stores_.clear();
}
/* Add one item to the buffer. */
PutResult put(const T& t) {
PutResult r = sinkStore();
last_ = t;
return r;
}
/* Remove an item from the store buffer. */
void unput(const T& v) {
// Fast, hashless remove of last put.
if (last_ == v) {
last_ = T();
return;
}
stores_.remove(v);
}
/* Move any buffered stores to the canonical store set. */
PutResult sinkStore() {
if (last_) {
AutoEnterOOMUnsafeRegion oomUnsafe;
if (!stores_.put(last_)) {
oomUnsafe.crash("Failed to allocate for MonoTypeBuffer::put.");
}
}
last_ = T();
if (stores_.count() > MaxEntries) {
return PutResult::AboutToOverflow;
}
return PutResult::OK;
}
/* Trace the source of all edges in the store buffer. */
void trace(TenuringTracer& mover, StoreBuffer* owner);
size_t sizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf) {
return stores_.shallowSizeOfExcludingThis(mallocSizeOf);
}
bool isEmpty() const { return last_ == T() && stores_.empty(); }
};
struct WholeCellBuffer {
UniquePtr<LifoAlloc> storage_;
ArenaCellSet* head_ = nullptr;
const Cell* last_ = nullptr;
WholeCellBuffer() = default;
WholeCellBuffer(const WholeCellBuffer& other) = delete;
WholeCellBuffer& operator=(const WholeCellBuffer& other) = delete;
WholeCellBuffer(WholeCellBuffer&& other)
: storage_(std::move(other.storage_)),
head_(other.head_),
last_(other.last_) {
other.head_ = nullptr;
other.last_ = nullptr;
}
WholeCellBuffer& operator=(WholeCellBuffer&& other) {
if (&other != this) {
this->~WholeCellBuffer();
new (this) WholeCellBuffer(std::move(other));
}
return *this;
}
[[nodiscard]] bool init();
void clear();
bool isAboutToOverflow() const {
return !storage_->isEmpty() &&
storage_->used() > BufferOverflowThresholdBytes;
}
void trace(TenuringTracer& mover, StoreBuffer* owner);
inline void put(const Cell* cell);
inline void putDontCheckLast(const Cell* cell);
size_t sizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf) {
return storage_ ? storage_->sizeOfIncludingThis(mallocSizeOf) : 0;
}
bool isEmpty() const {
MOZ_ASSERT_IF(!head_, !storage_ || storage_->isEmpty());
return !head_;
}
const Cell** lastBufferedPtr() { return &last_; }
CellSweepSet releaseCellSweepSet() {
CellSweepSet set;
std::swap(storage_, set.storage_);
std::swap(head_, set.head_);
last_ = nullptr;
return set;
}
private:
ArenaCellSet* allocateCellSet(Arena* arena);
};
struct GenericBuffer {
UniquePtr<LifoAlloc> storage_;
GenericBuffer() = default;
GenericBuffer(const GenericBuffer& other) = delete;
GenericBuffer& operator=(const GenericBuffer& other) = delete;
GenericBuffer(GenericBuffer&& other)
: storage_(std::move(other.storage_)) {}
GenericBuffer& operator=(GenericBuffer&& other) {
if (&other != this) {
this->~GenericBuffer();
new (this) GenericBuffer(std::move(other));
}
return *this;
}
[[nodiscard]] bool init();
void clear() {
if (storage_) {
storage_->used() ? storage_->releaseAll() : storage_->freeAll();
}
}
bool isAboutToOverflow() const {
return !storage_->isEmpty() && storage_->availableInCurrentChunk() <
GenericBufferLowAvailableThreshold;
}
/* Trace all generic edges. */
void trace(JSTracer* trc, StoreBuffer* owner);
template <typename T>
PutResult put(const T& t) {
MOZ_ASSERT(storage_);
/* Ensure T is derived from BufferableRef. */
(void)static_cast<const BufferableRef*>(&t);
AutoEnterOOMUnsafeRegion oomUnsafe;
unsigned size = sizeof(T);
unsigned* sizep = storage_->pod_malloc<unsigned>();
if (!sizep) {
oomUnsafe.crash("Failed to allocate for GenericBuffer::put.");
}
*sizep = size;
T* tp = storage_->new_<T>(t);
if (!tp) {
oomUnsafe.crash("Failed to allocate for GenericBuffer::put.");
}
if (isAboutToOverflow()) {
return PutResult::AboutToOverflow;
}
return PutResult::OK;
}
size_t sizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf) {
return storage_ ? storage_->sizeOfIncludingThis(mallocSizeOf) : 0;
}
bool isEmpty() const { return !storage_ || storage_->isEmpty(); }
};
template <typename Edge>
struct PointerEdgeHasher {
using Lookup = Edge;
static HashNumber hash(const Lookup& l) {
return mozilla::HashGeneric(l.edge);
}
static bool match(const Edge& k, const Lookup& l) { return k == l; }
};
template <typename T>
struct CellPtrEdge {
T** edge = nullptr;
CellPtrEdge() = default;
explicit CellPtrEdge(T** v) : edge(v) {}
bool operator==(const CellPtrEdge& other) const {
return edge == other.edge;
}
bool operator!=(const CellPtrEdge& other) const {
return edge != other.edge;
}
bool maybeInRememberedSet(const Nursery& nursery) const {
MOZ_ASSERT(IsInsideNursery(*edge));
return !nursery.isInside(edge);
}
void trace(TenuringTracer& mover) const;
explicit operator bool() const { return edge != nullptr; }
using Hasher = PointerEdgeHasher<CellPtrEdge<T>>;
};
using ObjectPtrEdge = CellPtrEdge<JSObject>;
using StringPtrEdge = CellPtrEdge<JSString>;
using BigIntPtrEdge = CellPtrEdge<JS::BigInt>;
struct ValueEdge {
JS::Value* edge;
ValueEdge() : edge(nullptr) {}
explicit ValueEdge(JS::Value* v) : edge(v) {}
bool operator==(const ValueEdge& other) const { return edge == other.edge; }
bool operator!=(const ValueEdge& other) const { return edge != other.edge; }
bool isGCThing() const { return edge->isGCThing(); }
Cell* deref() const {
return isGCThing() ? static_cast<Cell*>(edge->toGCThing()) : nullptr;
}
bool maybeInRememberedSet(const Nursery& nursery) const {
MOZ_ASSERT(IsInsideNursery(deref()));
return !nursery.isInside(edge);
}
void trace(TenuringTracer& mover) const;
explicit operator bool() const { return edge != nullptr; }
using Hasher = PointerEdgeHasher<ValueEdge>;
};
struct SlotsEdge {
// These definitions must match those in HeapSlot::Kind.
const static int SlotKind = 0;
const static int ElementKind = 1;
uintptr_t objectAndKind_; // NativeObject* | Kind
uint32_t start_;
uint32_t count_;
SlotsEdge() : objectAndKind_(0), start_(0), count_(0) {}
SlotsEdge(NativeObject* object, int kind, uint32_t start, uint32_t count)
: objectAndKind_(uintptr_t(object) | kind),
start_(start),
count_(count) {
MOZ_ASSERT((uintptr_t(object) & 1) == 0);
MOZ_ASSERT(kind <= 1);
MOZ_ASSERT(count > 0);
MOZ_ASSERT(start + count > start);
}
NativeObject* object() const {
return reinterpret_cast<NativeObject*>(objectAndKind_ & ~1);
}
int kind() const { return (int)(objectAndKind_ & 1); }
bool operator==(const SlotsEdge& other) const {
return objectAndKind_ == other.objectAndKind_ && start_ == other.start_ &&
count_ == other.count_;
}
bool operator!=(const SlotsEdge& other) const { return !(*this == other); }
// True if this SlotsEdge range overlaps with the other SlotsEdge range,
// false if they do not overlap.
bool overlaps(const SlotsEdge& other) const {
if (objectAndKind_ != other.objectAndKind_) {
return false;
}
// Widen our range by one on each side so that we consider
// adjacent-but-not-actually-overlapping ranges as overlapping. This
// is particularly useful for coalescing a series of increasing or
// decreasing single index writes 0, 1, 2, ..., N into a SlotsEdge
// range of elements [0, N].
uint32_t end = start_ + count_ + 1;
uint32_t start = start_ > 0 ? start_ - 1 : 0;
MOZ_ASSERT(start < end);
uint32_t otherEnd = other.start_ + other.count_;
MOZ_ASSERT(other.start_ <= otherEnd);
return (start <= other.start_ && other.start_ <= end) ||
(start <= otherEnd && otherEnd <= end);
}
// Destructively make this SlotsEdge range the union of the other
// SlotsEdge range and this one. A precondition is that the ranges must
// overlap.
void merge(const SlotsEdge& other) {
MOZ_ASSERT(overlaps(other));
uint32_t end = std::max(start_ + count_, other.start_ + other.count_);
start_ = std::min(start_, other.start_);
count_ = end - start_;
}
bool maybeInRememberedSet(const Nursery& n) const {
return !IsInsideNursery(reinterpret_cast<Cell*>(object()));
}
void trace(TenuringTracer& mover) const;
explicit operator bool() const { return objectAndKind_ != 0; }
struct Hasher {
using Lookup = SlotsEdge;
static HashNumber hash(const Lookup& l) {
return mozilla::HashGeneric(l.objectAndKind_, l.start_, l.count_);
}
static bool match(const SlotsEdge& k, const Lookup& l) { return k == l; }
};
};
struct WasmAnyRefEdge {
wasm::AnyRef* edge;
WasmAnyRefEdge() : edge(nullptr) {}
explicit WasmAnyRefEdge(wasm::AnyRef* v) : edge(v) {}
bool operator==(const WasmAnyRefEdge& other) const {
return edge == other.edge;
}
bool operator!=(const WasmAnyRefEdge& other) const {
return edge != other.edge;
}
bool isGCThing() const { return edge->isGCThing(); }
Cell* deref() const {
return isGCThing() ? static_cast<Cell*>(edge->toGCThing()) : nullptr;
}
bool maybeInRememberedSet(const Nursery& nursery) const {
MOZ_ASSERT(IsInsideNursery(deref()));
return !nursery.isInside(edge);
}
void trace(TenuringTracer& mover) const;
explicit operator bool() const { return edge != nullptr; }
using Hasher = PointerEdgeHasher<WasmAnyRefEdge>;
};
#ifdef DEBUG
void checkAccess() const;
#else
void checkAccess() const {}
#endif
template <typename Buffer, typename Edge>
void unput(Buffer& buffer, const Edge& edge) {
checkAccess();
if (!isEnabled()) {
return;
}
mozilla::ReentrancyGuard g(*this);
buffer.unput(edge);
}
template <typename Buffer, typename Edge>
void put(Buffer& buffer, const Edge& edge, JS::GCReason overflowReason) {
checkAccess();
if (!isEnabled()) {
return;
}
mozilla::ReentrancyGuard g(*this);
if (!edge.maybeInRememberedSet(nursery_)) {
return;
}
PutResult r = buffer.put(edge);
if (MOZ_UNLIKELY(r == PutResult::AboutToOverflow)) {
setAboutToOverflow(overflowReason);
}
}
MonoTypeBuffer<ValueEdge> bufferVal;
MonoTypeBuffer<StringPtrEdge> bufStrCell;
MonoTypeBuffer<BigIntPtrEdge> bufBigIntCell;
MonoTypeBuffer<ObjectPtrEdge> bufObjCell;
MonoTypeBuffer<SlotsEdge> bufferSlot;
MonoTypeBuffer<WasmAnyRefEdge> bufferWasmAnyRef;
WholeCellBuffer bufferWholeCell;
GenericBuffer bufferGeneric;
JSRuntime* runtime_;
Nursery& nursery_;
bool aboutToOverflow_;
bool enabled_;
bool mayHavePointersToDeadCells_;
#ifdef DEBUG
bool mEntered; /* For ReentrancyGuard. */
#endif
public:
explicit StoreBuffer(JSRuntime* rt);
StoreBuffer(const StoreBuffer& other) = delete;
StoreBuffer& operator=(const StoreBuffer& other) = delete;
StoreBuffer(StoreBuffer&& other);
StoreBuffer& operator=(StoreBuffer&& other);
[[nodiscard]] bool enable();
void disable();
bool isEnabled() const { return enabled_; }
bool isEmpty() const;
void clear();
const Nursery& nursery() const { return nursery_; }
/* Get the overflowed status. */
bool isAboutToOverflow() const { return aboutToOverflow_; }
/*
* Brain transplants may add whole cell buffer entires for dead cells. We must
* evict the nursery prior to sweeping arenas if any such entries are present.
*/
bool mayHavePointersToDeadCells() const {
return mayHavePointersToDeadCells_;
}
/* Insert a single edge into the buffer/remembered set. */
void putValue(JS::Value* vp) {
put(bufferVal, ValueEdge(vp), JS::GCReason::FULL_VALUE_BUFFER);
}
void unputValue(JS::Value* vp) { unput(bufferVal, ValueEdge(vp)); }
void putCell(JSString** strp) {
put(bufStrCell, StringPtrEdge(strp),
JS::GCReason::FULL_CELL_PTR_STR_BUFFER);
}
void unputCell(JSString** strp) { unput(bufStrCell, StringPtrEdge(strp)); }
void putCell(JS::BigInt** bip) {
put(bufBigIntCell, BigIntPtrEdge(bip),
JS::GCReason::FULL_CELL_PTR_BIGINT_BUFFER);
}
void unputCell(JS::BigInt** bip) { unput(bufBigIntCell, BigIntPtrEdge(bip)); }
void putCell(JSObject** strp) {
put(bufObjCell, ObjectPtrEdge(strp),
JS::GCReason::FULL_CELL_PTR_OBJ_BUFFER);
}
void unputCell(JSObject** strp) { unput(bufObjCell, ObjectPtrEdge(strp)); }
void putSlot(NativeObject* obj, int kind, uint32_t start, uint32_t count) {
SlotsEdge edge(obj, kind, start, count);
if (bufferSlot.last_.overlaps(edge)) {
bufferSlot.last_.merge(edge);
} else {
put(bufferSlot, edge, JS::GCReason::FULL_SLOT_BUFFER);
}
}
void putWasmAnyRef(wasm::AnyRef* vp) {
put(bufferWasmAnyRef, WasmAnyRefEdge(vp),
JS::GCReason::FULL_WASM_ANYREF_BUFFER);
}
void unputWasmAnyRef(wasm::AnyRef* vp) {
unput(bufferWasmAnyRef, WasmAnyRefEdge(vp));
}
inline void putWholeCell(Cell* cell);
inline void putWholeCellDontCheckLast(Cell* cell);
const void* addressOfLastBufferedWholeCell() {
return bufferWholeCell.lastBufferedPtr();
}
/* Insert an entry into the generic buffer. */
template <typename T>
void putGeneric(const T& t) {
put(bufferGeneric, t, JS::GCReason::FULL_GENERIC_BUFFER);
}
void setMayHavePointersToDeadCells() { mayHavePointersToDeadCells_ = true; }
/* Methods to trace the source of all edges in the store buffer. */
void traceValues(TenuringTracer& mover) { bufferVal.trace(mover, this); }
void traceCells(TenuringTracer& mover) {
bufStrCell.trace(mover, this);
bufBigIntCell.trace(mover, this);
bufObjCell.trace(mover, this);
}
void traceSlots(TenuringTracer& mover) { bufferSlot.trace(mover, this); }
void traceWasmAnyRefs(TenuringTracer& mover) {
bufferWasmAnyRef.trace(mover, this);
}
void traceWholeCells(TenuringTracer& mover) {
bufferWholeCell.trace(mover, this);
}
void traceGenericEntries(JSTracer* trc) { bufferGeneric.trace(trc, this); }
gc::CellSweepSet releaseCellSweepSet() {
return bufferWholeCell.releaseCellSweepSet();
}
/* For use by our owned buffers and for testing. */
void setAboutToOverflow(JS::GCReason);
void addSizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf,
JS::GCSizes* sizes);
void checkEmpty() const;
};
// A set of cells in an arena used to implement the whole cell store buffer.
// Also used to store a set of cells that need to be swept.
class ArenaCellSet {
friend class StoreBuffer;
using ArenaCellBits = BitArray<MaxArenaCellIndex>;
// The arena this relates to.
Arena* arena;
// Pointer to next set forming a linked list.
ArenaCellSet* next;
// Bit vector for each possible cell start position.
ArenaCellBits bits;
#ifdef DEBUG
// The minor GC number when this was created. This object should not survive
// past the next minor collection.
const uint64_t minorGCNumberAtCreation;
#endif
// Construct the empty sentinel object.
constexpr ArenaCellSet()
: arena(nullptr),
next(nullptr)
#ifdef DEBUG
,
minorGCNumberAtCreation(0)
#endif
{
}
public:
using WordT = ArenaCellBits::WordT;
const size_t BitsPerWord = ArenaCellBits::bitsPerElement;
const size_t NumWords = ArenaCellBits::numSlots;
ArenaCellSet(Arena* arena, ArenaCellSet* next);
bool hasCell(const TenuredCell* cell) const {
return hasCell(getCellIndex(cell));
}
void putCell(const TenuredCell* cell) { putCell(getCellIndex(cell)); }
bool isEmpty() const { return this == &Empty; }
bool hasCell(size_t cellIndex) const;
void putCell(size_t cellIndex);
void check() const;
WordT getWord(size_t wordIndex) const { return bits.getWord(wordIndex); }
void setWord(size_t wordIndex, WordT value) {
bits.setWord(wordIndex, value);
}
// Returns the list of ArenaCellSets that need to be swept.
ArenaCellSet* trace(TenuringTracer& mover);
// At the end of a minor GC, sweep through all tenured dependent strings that
// may point to nursery-allocated chars to update their pointers in case the
// base string moved its chars.
void sweepDependentStrings();
// Sentinel object used for all empty sets.
//
// We use a sentinel because it simplifies the JIT code slightly as we can
// assume all arenas have a cell set.
static ArenaCellSet Empty;
static size_t getCellIndex(const TenuredCell* cell);
static void getWordIndexAndMask(size_t cellIndex, size_t* wordp,
uint32_t* maskp);
// Attempt to trigger a minor GC if free space in the nursery (where these
// objects are allocated) falls below this threshold.
static const size_t NurseryFreeThresholdBytes = 64 * 1024;
static size_t offsetOfArena() { return offsetof(ArenaCellSet, arena); }
static size_t offsetOfBits() { return offsetof(ArenaCellSet, bits); }
};
// Post-write barrier implementation for GC cells.
// Implement the post-write barrier for nursery allocateable cell type |T|. Call
// this from |T::postWriteBarrier|.
template <typename T>
MOZ_ALWAYS_INLINE void PostWriteBarrierImpl(void* cellp, T* prev, T* next) {
MOZ_ASSERT(cellp);
// If the target needs an entry, add it.
StoreBuffer* buffer;
if (next && (buffer = next->storeBuffer())) {
// If we know that the prev has already inserted an entry, we can skip
// doing the lookup to add the new entry. Note that we cannot safely
// assert the presence of the entry because it may have been added
// via a different store buffer.
if (prev && prev->storeBuffer()) {
return;
}
buffer->putCell(static_cast<T**>(cellp));
return;
}
// Remove the prev entry if the new value does not need it. There will only
// be a prev entry if the prev value was in the nursery.
if (prev && (buffer = prev->storeBuffer())) {
buffer->unputCell(static_cast<T**>(cellp));
}
}
template <typename T>
MOZ_ALWAYS_INLINE void PostWriteBarrier(T** vp, T* prev, T* next) {
static_assert(std::is_base_of_v<Cell, T>);
static_assert(!std::is_same_v<Cell, T> && !std::is_same_v<TenuredCell, T>);
if constexpr (!GCTypeIsTenured<T>()) {
using BaseT = typename BaseGCType<T>::type;
PostWriteBarrierImpl<BaseT>(vp, prev, next);
return;
}
MOZ_ASSERT_IF(next, !IsInsideNursery(next));
}
// Used when we don't have a specific edge to put in the store buffer.
void PostWriteBarrierCell(Cell* cell, Cell* prev, Cell* next);
} /* namespace gc */
} /* namespace js */
#endif /* gc_StoreBuffer_h */