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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
* vim: set ts=8 sts=2 et sw=2 tw=80:
* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at */
#ifndef vm_SharedArrayObject_h
#define vm_SharedArrayObject_h
#include "mozilla/Atomics.h"
#include "jstypes.h"
#include "gc/Memory.h"
#include "vm/ArrayBufferObject.h"
#include "wasm/WasmMemory.h"
namespace js {
class FutexWaiter;
class WasmSharedArrayRawBuffer;
* SharedArrayRawBuffer
* A bookkeeping object always stored before the raw buffer. The buffer itself
* is refcounted. SharedArrayBufferObjects and structured clone objects may hold
* references.
* WasmSharedArrayRawBuffer is a derived class that's used for Wasm buffers.
* - Non-Wasm buffers are allocated with a single calloc allocation, like this:
* |<------ sizeof ------>|<- length ->|
* | SharedArrayRawBuffer | data array |
* - Wasm buffers are allocated with MapBufferMemory (mmap), like this:
* |<-------- sizeof -------->|<- length ->|
* | waste | WasmSharedArrayRawBuffer | data array | waste |
* Observe that if we want to map the data array on a specific address, such
* as absolute zero (bug 1056027), then the {Wasm}SharedArrayRawBuffer cannot be
* prefixed to the data array, it has to be a separate object, also in
* shared memory. (That would get rid of ~4KB of waste, as well.) Very little
* else would have to change throughout the engine, the SARB would point to
* the data array using a constant pointer, instead of computing its
* address.
* For Wasm buffers, length_ can change following initialization; it may grow
* toward sourceMaxPages_. See extensive comments above WasmArrayRawBuffer in
* ArrayBufferObject.cpp. length_ only grows when the lock is held.
class SharedArrayRawBuffer {
// Whether this is a WasmSharedArrayRawBuffer.
bool isWasm_;
// Whether this is a growable non-Wasm buffer.
bool isGrowable_;
mozilla::Atomic<uint32_t, mozilla::ReleaseAcquire> refcount_;
mozilla::Atomic<size_t, mozilla::SequentiallyConsistent> length_;
// A list of structures representing tasks waiting on some
// location within this buffer.
FutexWaiter* waiters_ = nullptr;
SharedArrayRawBuffer(bool isGrowable, uint8_t* buffer, size_t length)
: isWasm_(false), isGrowable_(isGrowable), refcount_(1), length_(length) {
MOZ_ASSERT(buffer == dataPointerShared());
enum class WasmBuffer {};
SharedArrayRawBuffer(WasmBuffer, uint8_t* buffer, size_t length)
: isWasm_(true), isGrowable_(false), refcount_(1), length_(length) {
MOZ_ASSERT(buffer == dataPointerShared());
static SharedArrayRawBuffer* Allocate(bool isGrowable, size_t length,
size_t maxLength);
inline WasmSharedArrayRawBuffer* toWasmBuffer();
// This may be called from multiple threads. The caller must take
// care of mutual exclusion.
FutexWaiter* waiters() const { return waiters_; }
// This may be called from multiple threads. The caller must take
// care of mutual exclusion.
void setWaiters(FutexWaiter* waiters) { waiters_ = waiters; }
inline SharedMem<uint8_t*> dataPointerShared() const;
size_t volatileByteLength() const { return length_; }
bool isWasm() const { return isWasm_; }
bool isGrowable() const { return isGrowable_; }
uint32_t refcount() const { return refcount_; }
[[nodiscard]] bool addReference();
void dropReference();
// Try to grow this buffer to |newByteLength| bytes. Returns false when the
// current byte length is larger than |newByteLength|. Otherwise atomically
// changes the byte length to |newByteLength| and then returns true.
// This method DOES NOT perform any memory operations to allocate additional
// space. The caller is responsible to ensure that the buffer has been
// allocated with enough space to hold at least |newByteLength| bytes. IOW
// this method merely sets the number of user accessible bytes of this buffer.
bool grow(size_t newByteLength);
static int32_t liveBuffers();
class WasmSharedArrayRawBuffer : public SharedArrayRawBuffer {
Mutex growLock_ MOZ_UNANNOTATED;
// The index type of this buffer.
wasm::IndexType indexType_;
// The maximum size of this buffer in wasm pages.
wasm::Pages clampedMaxPages_;
wasm::Pages sourceMaxPages_;
size_t mappedSize_; // Does not include the page for the header.
uint8_t* basePointer() {
SharedMem<uint8_t*> p = dataPointerShared() - gc::SystemPageSize();
MOZ_ASSERT(p.asValue() % gc::SystemPageSize() == 0);
return p.unwrap(/* we trust you won't abuse it */);
WasmSharedArrayRawBuffer(uint8_t* buffer, size_t length,
wasm::IndexType indexType,
wasm::Pages clampedMaxPages,
wasm::Pages sourceMaxPages, size_t mappedSize)
: SharedArrayRawBuffer(WasmBuffer{}, buffer, length),
mappedSize_(mappedSize) {}
friend class SharedArrayRawBuffer;
class Lock;
friend class Lock;
class MOZ_RAII Lock {
WasmSharedArrayRawBuffer* buf;
explicit Lock(WasmSharedArrayRawBuffer* buf) : buf(buf) {
~Lock() { buf->growLock_.unlock(); }
static WasmSharedArrayRawBuffer* AllocateWasm(
wasm::IndexType indexType, wasm::Pages initialPages,
wasm::Pages clampedMaxPages,
const mozilla::Maybe<wasm::Pages>& sourceMaxPages,
const mozilla::Maybe<size_t>& mappedSize);
static const WasmSharedArrayRawBuffer* fromDataPtr(const uint8_t* dataPtr) {
return reinterpret_cast<const WasmSharedArrayRawBuffer*>(
dataPtr - sizeof(WasmSharedArrayRawBuffer));
static WasmSharedArrayRawBuffer* fromDataPtr(uint8_t* dataPtr) {
return reinterpret_cast<WasmSharedArrayRawBuffer*>(
dataPtr - sizeof(WasmSharedArrayRawBuffer));
wasm::IndexType wasmIndexType() const { return indexType_; }
wasm::Pages volatileWasmPages() const {
return wasm::Pages::fromByteLengthExact(length_);
wasm::Pages wasmClampedMaxPages() const { return clampedMaxPages_; }
wasm::Pages wasmSourceMaxPages() const { return sourceMaxPages_; }
size_t mappedSize() const { return mappedSize_; }
void tryGrowMaxPagesInPlace(wasm::Pages deltaMaxPages);
bool wasmGrowToPagesInPlace(const Lock&, wasm::IndexType t,
wasm::Pages newPages);
// Discard a region of memory, zeroing the pages and releasing physical memory
// back to the operating system. byteOffset and byteLen must be wasm page
// aligned and in bounds. A discard of zero bytes will have no effect.
void discard(size_t byteOffset, size_t byteLen);
inline WasmSharedArrayRawBuffer* SharedArrayRawBuffer::toWasmBuffer() {
return static_cast<WasmSharedArrayRawBuffer*>(this);
inline SharedMem<uint8_t*> SharedArrayRawBuffer::dataPointerShared() const {
uint8_t* ptr =
ptr += isWasm() ? sizeof(WasmSharedArrayRawBuffer)
: sizeof(SharedArrayRawBuffer);
return SharedMem<uint8_t*>::shared(ptr);
class FixedLengthSharedArrayBufferObject;
class GrowableSharedArrayBufferObject;
* SharedArrayBufferObject
* When transferred to a WebWorker, the buffer is not detached on the
* parent side, and both child and parent reference the same buffer.
* The underlying memory is memory-mapped and reference counted
* (across workers and/or processes). The SharedArrayBuffer object
* has a finalizer that decrements the refcount, the last one to leave
* (globally) unmaps the memory. The sender ups the refcount before
* transmitting the memory to another worker.
* SharedArrayBufferObject (or really the underlying memory) /is
* racy/: more than one worker can access the memory at the same time.
* A TypedArrayObject (a view) references a SharedArrayBuffer
* and keeps it alive. The SharedArrayBuffer does /not/ reference its
* views.
* SharedArrayBufferObject is an abstract base class and has exactly two
* concrete subclasses, FixedLengthSharedArrayBufferObject and
* GrowableSharedArrayBufferObject.
class SharedArrayBufferObject : public ArrayBufferObjectMaybeShared {
static bool byteLengthGetterImpl(JSContext* cx, const CallArgs& args);
static bool maxByteLengthGetterImpl(JSContext* cx, const CallArgs& args);
static bool growableGetterImpl(JSContext* cx, const CallArgs& args);
static bool growImpl(JSContext* cx, const CallArgs& args);
// RAWBUF_SLOT holds a pointer (as "private" data) to the
// SharedArrayRawBuffer object, which is manually managed storage.
static const uint8_t RAWBUF_SLOT = 0;
// LENGTH_SLOT holds the length of the underlying buffer as it was when this
// object was created. For JS use cases this is the same length as the
// buffer, but for Wasm the buffer can grow, and the buffer's length may be
// greater than the object's length.
static const uint8_t LENGTH_SLOT = 1;
static_assert(LENGTH_SLOT == ArrayBufferObject::BYTE_LENGTH_SLOT,
"JIT code assumes the same slot is used for the length");
static const uint8_t RESERVED_SLOTS = 2;
static const JSClass protoClass_;
static bool byteLengthGetter(JSContext* cx, unsigned argc, Value* vp);
static bool maxByteLengthGetter(JSContext* cx, unsigned argc, Value* vp);
static bool growableGetter(JSContext* cx, unsigned argc, Value* vp);
static bool class_constructor(JSContext* cx, unsigned argc, Value* vp);
static bool grow(JSContext* cx, unsigned argc, Value* vp);
static bool isOriginalByteLengthGetter(Native native) {
return native == byteLengthGetter;
template <class SharedArrayBufferType>
static SharedArrayBufferType* NewWith(JSContext* cx,
SharedArrayRawBuffer* buffer,
size_t length, HandleObject proto);
// Create a SharedArrayBufferObject with a new SharedArrayRawBuffer.
static FixedLengthSharedArrayBufferObject* New(JSContext* cx, size_t length,
HandleObject proto = nullptr);
// Create a SharedArrayBufferObject using an existing SharedArrayRawBuffer,
// recording the given length in the SharedArrayBufferObject.
static FixedLengthSharedArrayBufferObject* New(JSContext* cx,
SharedArrayRawBuffer* buffer,
size_t length,
HandleObject proto = nullptr);
// Create a growable SharedArrayBufferObject with a new SharedArrayRawBuffer.
static GrowableSharedArrayBufferObject* NewGrowable(
JSContext* cx, size_t length, size_t maxLength,
HandleObject proto = nullptr);
// Create a growable SharedArrayBufferObject using an existing
// SharedArrayRawBuffer, recording the given length in the
// SharedArrayBufferObject.
static GrowableSharedArrayBufferObject* NewGrowable(
JSContext* cx, SharedArrayRawBuffer* buffer, size_t maxLength,
HandleObject proto = nullptr);
static void Finalize(JS::GCContext* gcx, JSObject* obj);
static void addSizeOfExcludingThis(JSObject* obj,
mozilla::MallocSizeOf mallocSizeOf,
JS::ClassInfo* info,
JS::RuntimeSizes* runtimeSizes);
static void copyData(Handle<ArrayBufferObjectMaybeShared*> toBuffer,
size_t toIndex,
Handle<ArrayBufferObjectMaybeShared*> fromBuffer,
size_t fromIndex, size_t count);
SharedArrayRawBuffer* rawBufferObject() const;
WasmSharedArrayRawBuffer* rawWasmBufferObject() const {
return rawBufferObject()->toWasmBuffer();
// Invariant: This method does not cause GC and can be called
// without anchoring the object it is called on.
uintptr_t globalID() const {
// The buffer address is good enough as an ID provided the memory is not
// shared between processes or, if it is, it is mapped to the same address
// in every process. (At the moment, shared memory cannot be shared between
// processes.)
return dataPointerShared().asValue();
size_t growableByteLength() const {
return rawBufferObject()->volatileByteLength();
// Returns either the byte length for fixed-length shared arrays. Or the
// maximum byte length for growable shared arrays.
size_t byteLengthOrMaxByteLength() const {
return size_t(getFixedSlot(LENGTH_SLOT).toPrivate());
size_t byteLength() const {
if (isGrowable()) {
return growableByteLength();
return byteLengthOrMaxByteLength();
bool isWasm() const { return rawBufferObject()->isWasm(); }
bool isGrowable() const { return rawBufferObject()->isGrowable(); }
SharedMem<uint8_t*> dataPointerShared() const {
return rawBufferObject()->dataPointerShared();
// WebAssembly support:
// Create a SharedArrayBufferObject using the provided buffer and size.
// Assumes ownership of a reference to |buffer| even in case of failure,
// i.e. on failure |buffer->dropReference()| is performed.
static SharedArrayBufferObject* createFromNewRawBuffer(
JSContext* cx, WasmSharedArrayRawBuffer* buffer, size_t initialSize);
wasm::Pages volatileWasmPages() const {
return rawWasmBufferObject()->volatileWasmPages();
wasm::Pages wasmClampedMaxPages() const {
return rawWasmBufferObject()->wasmClampedMaxPages();
wasm::Pages wasmSourceMaxPages() const {
return rawWasmBufferObject()->wasmSourceMaxPages();
size_t wasmMappedSize() const { return rawWasmBufferObject()->mappedSize(); }
static void wasmDiscard(Handle<SharedArrayBufferObject*> buf,
uint64_t byteOffset, uint64_t byteLength);
[[nodiscard]] bool acceptRawBuffer(SharedArrayRawBuffer* buffer,
size_t length);
void dropRawBuffer();
* FixedLengthSharedArrayBufferObject
* SharedArrayBuffer object with a fixed length. The JS exposed length is
* unmodifiable, but the underlying memory can still grow for WebAssembly.
* Fixed-length SharedArrayBuffers can be used for asm.js and WebAssembly.
class FixedLengthSharedArrayBufferObject : public SharedArrayBufferObject {
static const JSClass class_;
size_t byteLength() const { return byteLengthOrMaxByteLength(); }
* GrowableSharedArrayBufferObject
* SharedArrayBuffer object which can grow in size. The maximum byte length it
* can grow to is set when creating the object.
* Growable SharedArrayBuffers can neither be used for asm.js nor WebAssembly.
class GrowableSharedArrayBufferObject : public SharedArrayBufferObject {
static const JSClass class_;
size_t byteLength() const { return growableByteLength(); }
size_t maxByteLength() const { return byteLengthOrMaxByteLength(); }
} // namespace js
template <>
inline bool JSObject::is<js::SharedArrayBufferObject>() const {
return is<js::FixedLengthSharedArrayBufferObject>() ||
#endif // vm_SharedArrayObject_h