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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_ArrayBufferObject_h
#define vm_ArrayBufferObject_h
#include "mozilla/Maybe.h"
#include <tuple> // std::tuple
#include "builtin/TypedArrayConstants.h"
#include "gc/Memory.h"
#include "gc/ZoneAllocator.h"
#include "js/ArrayBuffer.h"
#include "js/GCHashTable.h"
#include "vm/JSFunction.h"
#include "vm/JSObject.h"
#include "vm/SharedMem.h"
#include "wasm/WasmMemory.h"
namespace js {
class ArrayBufferViewObject;
class AutoSetNewObjectMetadata;
class WasmArrayRawBuffer;
namespace wasm {
struct MemoryDesc;
} // namespace wasm
// Create a new mapping of size `mappedSize` with an initially committed prefix
// of size `initialCommittedSize`. Both arguments denote bytes and must be
// multiples of the page size, with `initialCommittedSize` <= `mappedSize`.
// Returns nullptr on failure.
void* MapBufferMemory(wasm::IndexType, size_t mappedSize,
size_t initialCommittedSize);
// Commit additional memory in an existing mapping. `dataEnd` must be the
// correct value for the end of the existing committed area, and `delta` must be
// a byte amount to grow the mapping by, and must be a multiple of the page
// size. Returns false on failure.
bool CommitBufferMemory(void* dataEnd, size_t delta);
// Extend an existing mapping by adding uncommited pages to it. `dataStart`
// must be the pointer to the start of the existing mapping, `mappedSize` the
// size of the existing mapping, and `newMappedSize` the size of the extended
// mapping (sizes in bytes), with `mappedSize` <= `newMappedSize`. Both sizes
// must be divisible by the page size. Returns false on failure.
bool ExtendBufferMapping(void* dataStart, size_t mappedSize,
size_t newMappedSize);
// Remove an existing mapping. `dataStart` must be the pointer to the start of
// the mapping, and `mappedSize` the size of that mapping.
void UnmapBufferMemory(wasm::IndexType t, void* dataStart, size_t mappedSize);
// Return the number of bytes currently reserved for WebAssembly memory
uint64_t WasmReservedBytes();
// The inheritance hierarchy for the various classes relating to typed arrays
// is as follows.
// - JSObject
// - TypedObject (declared in wasm/TypedObject.h)
// - NativeObject
// - ArrayBufferObjectMaybeShared
// - ArrayBufferObject
// - SharedArrayBufferObject
// - ArrayBufferViewObject
// - DataViewObject
// - TypedArrayObject (declared in vm/TypedArrayObject.h)
// - TypedArrayObjectTemplate
// - Int8ArrayObject
// - Uint8ArrayObject
// - ...
// Note that |TypedArrayObjectTemplate| is just an implementation
// detail that makes implementing its various subclasses easier.
// ArrayBufferObject and SharedArrayBufferObject are unrelated data types:
// the racy memory of the latter cannot substitute for the non-racy memory of
// the former; the non-racy memory of the former cannot be used with the
// atomics; the former can be detached and the latter not. Hence they have been
// separated completely.
// Most APIs will only accept ArrayBufferObject. ArrayBufferObjectMaybeShared
// exists as a join point to allow APIs that can take or use either, notably
// AsmJS.
// In contrast with the separation of ArrayBufferObject and
// SharedArrayBufferObject, the TypedArray types can map either.
// The possible data ownership and reference relationships with ArrayBuffers
// and related classes are enumerated below. These are the possible locations
// for typed data:
// (1) malloc'ed or mmap'ed data owned by an ArrayBufferObject.
// (2) Data allocated inline with an ArrayBufferObject.
// (3) Data allocated inline with a TypedArrayObject.
// (4) Data allocated inline with an InlineTypedObject.
// An ArrayBufferObject may point to any of these sources of data, except (3).
// All array buffer views may point to any of these sources of data, except
// that (3) may only be pointed to by the typed array the data is inline with.
// During a minor GC, (3) and (4) may move. During a compacting GC, (2), (3),
// and (4) may move.
class ArrayBufferObjectMaybeShared;
wasm::IndexType WasmArrayBufferIndexType(
const ArrayBufferObjectMaybeShared* buf);
wasm::Pages WasmArrayBufferPages(const ArrayBufferObjectMaybeShared* buf);
wasm::Pages WasmArrayBufferClampedMaxPages(
const ArrayBufferObjectMaybeShared* buf);
mozilla::Maybe<wasm::Pages> WasmArrayBufferSourceMaxPages(
const ArrayBufferObjectMaybeShared* buf);
size_t WasmArrayBufferMappedSize(const ArrayBufferObjectMaybeShared* buf);
class ArrayBufferObjectMaybeShared : public NativeObject {
inline size_t byteLength() const;
inline bool isDetached() const;
inline SharedMem<uint8_t*> dataPointerEither();
// WebAssembly support:
// Note: the eventual goal is to remove this from ArrayBuffer and have
// (Shared)ArrayBuffers alias memory owned by some wasm::Memory object.
wasm::IndexType wasmIndexType() const {
return WasmArrayBufferIndexType(this);
wasm::Pages wasmPages() const { return WasmArrayBufferPages(this); }
wasm::Pages wasmClampedMaxPages() const {
return WasmArrayBufferClampedMaxPages(this);
mozilla::Maybe<wasm::Pages> wasmSourceMaxPages() const {
return WasmArrayBufferSourceMaxPages(this);
size_t wasmMappedSize() const { return WasmArrayBufferMappedSize(this); }
inline bool isPreparedForAsmJS() const;
inline bool isWasm() const;
using RootedArrayBufferObjectMaybeShared =
using HandleArrayBufferObjectMaybeShared =
using MutableHandleArrayBufferObjectMaybeShared =
* ArrayBufferObject
* This class holds the underlying raw buffer that the various ArrayBufferViews
* (eg DataViewObject, the TypedArrays, TypedObjects) access. It can be created
* explicitly and used to construct an ArrayBufferView, or can be created
* lazily when it is first accessed for a TypedArrayObject or TypedObject that
* doesn't have an explicit buffer.
* ArrayBufferObject (or really the underlying memory) /is not racy/: the
* memory is private to a single worker.
class ArrayBufferObject : public ArrayBufferObjectMaybeShared {
static bool byteLengthGetterImpl(JSContext* cx, const CallArgs& args);
static const uint8_t DATA_SLOT = 0;
static const uint8_t BYTE_LENGTH_SLOT = 1;
static const uint8_t FIRST_VIEW_SLOT = 2;
static const uint8_t FLAGS_SLOT = 3;
static const uint8_t RESERVED_SLOTS = 4;
static const size_t ARRAY_BUFFER_ALIGNMENT = 8;
"self-hosted code with burned-in constants must get the "
"right flags slot");
// The length of an ArrayBuffer or SharedArrayBuffer can be at most INT32_MAX
// on 32-bit platforms. Allow a larger limit on 64-bit platforms.
static constexpr size_t MaxByteLengthForSmallBuffer = INT32_MAX;
#ifdef JS_64BIT
static constexpr size_t MaxByteLength =
size_t(8) * 1024 * 1024 * 1024; // 8 GB.
static constexpr size_t MaxByteLength = MaxByteLengthForSmallBuffer;
/** The largest number of bytes that can be stored inline. */
static constexpr size_t MaxInlineBytes =
(NativeObject::MAX_FIXED_SLOTS - RESERVED_SLOTS) * sizeof(JS::Value);
enum BufferKind {
/** Inline data kept in the repurposed slots of this ArrayBufferObject. */
INLINE_DATA = 0b000,
/* Data allocated using the SpiderMonkey allocator. */
MALLOCED = 0b001,
* No bytes are associated with this buffer. (This could be because the
* buffer is detached, because it's an internal, newborn buffer not yet
* overwritten with user-exposable semantics, or some other reason. The
* point is, don't read precise language semantics into this kind.)
NO_DATA = 0b010,
* User-owned memory. The associated buffer must be manually detached
* before the user invalidates (deallocates, reuses the storage of, &c.)
* the user-owned memory.
USER_OWNED = 0b011,
WASM = 0b100,
MAPPED = 0b101,
EXTERNAL = 0b110,
// These kind-values are currently invalid. We intend to expand valid
// BufferKinds in the future to either partly or fully use these values.
BAD1 = 0b111,
KIND_MASK = 0b111
enum ArrayBufferFlags {
// The flags also store the BufferKind
DETACHED = 0b1000,
// This MALLOCED, MAPPED, or EXTERNAL buffer has been prepared for asm.js
// and cannot henceforth be transferred/detached. (WASM, USER_OWNED, and
// INLINE_DATA buffers can't be prepared for asm.js -- although if an
// INLINE_DATA buffer is used with asm.js, it's silently rewritten into a
// MALLOCED buffer which *can* be prepared.)
FOR_ASMJS = 0b10'0000,
"self-hosted code with burned-in constants must use the "
"correct DETACHED bit value");
enum class FillContents { Zero, Uninitialized };
template <FillContents FillType>
static std::tuple<ArrayBufferObject*, uint8_t*> createBufferAndData(
JSContext* cx, size_t nbytes, AutoSetNewObjectMetadata&,
JS::Handle<JSObject*> proto = nullptr);
class BufferContents {
uint8_t* data_;
BufferKind kind_;
JS::BufferContentsFreeFunc free_;
void* freeUserData_;
friend class ArrayBufferObject;
BufferContents(uint8_t* data, BufferKind kind,
JS::BufferContentsFreeFunc freeFunc = nullptr,
void* freeUserData = nullptr)
: data_(data),
freeUserData_(freeUserData) {
MOZ_ASSERT((kind_ & ~KIND_MASK) == 0);
MOZ_ASSERT_IF(free_ || freeUserData_, kind_ == EXTERNAL);
// It is the caller's responsibility to ensure that the
// BufferContents does not outlive the data.
static BufferContents createInlineData(void* data) {
return BufferContents(static_cast<uint8_t*>(data), INLINE_DATA);
static BufferContents createMalloced(void* data) {
return BufferContents(static_cast<uint8_t*>(data), MALLOCED);
static BufferContents createNoData() {
return BufferContents(nullptr, NO_DATA);
static BufferContents createUserOwned(void* data) {
return BufferContents(static_cast<uint8_t*>(data), USER_OWNED);
static BufferContents createWasm(void* data) {
return BufferContents(static_cast<uint8_t*>(data), WASM);
static BufferContents createMapped(void* data) {
return BufferContents(static_cast<uint8_t*>(data), MAPPED);
static BufferContents createExternal(void* data,
JS::BufferContentsFreeFunc freeFunc,
void* freeUserData = nullptr) {
return BufferContents(static_cast<uint8_t*>(data), EXTERNAL, freeFunc,
static BufferContents createFailed() {
// There's no harm in tagging this as MALLOCED, even tho obviously it
// isn't. And adding an extra tag purely for this case is a complication
// that presently appears avoidable.
return BufferContents(nullptr, MALLOCED);
uint8_t* data() const { return data_; }
BufferKind kind() const { return kind_; }
JS::BufferContentsFreeFunc freeFunc() const { return free_; }
void* freeUserData() const { return freeUserData_; }
explicit operator bool() const { return data_ != nullptr; }
WasmArrayRawBuffer* wasmBuffer() const;
static const JSClass class_;
static const JSClass protoClass_;
static bool byteLengthGetter(JSContext* cx, unsigned argc, Value* vp);
static bool fun_isView(JSContext* cx, unsigned argc, Value* vp);
static bool class_constructor(JSContext* cx, unsigned argc, Value* vp);
static bool isOriginalByteLengthGetter(Native native) {
return native == byteLengthGetter;
static ArrayBufferObject* createForContents(JSContext* cx, size_t nbytes,
BufferContents contents);
static ArrayBufferObject* copy(
JSContext* cx, JS::Handle<ArrayBufferObject*> unwrappedArrayBuffer);
static ArrayBufferObject* createZeroed(JSContext* cx, size_t nbytes,
HandleObject proto = nullptr);
// Create an ArrayBufferObject that is safely finalizable and can later be
// initialize()d to become a real, content-visible ArrayBufferObject.
static ArrayBufferObject* createEmpty(JSContext* cx);
// Create an ArrayBufferObject using the provided buffer and size. Assumes
// ownership of |buffer| even in case of failure, i.e. on failure |buffer|
// is deallocated.
static ArrayBufferObject* createFromNewRawBuffer(JSContext* cx,
WasmArrayRawBuffer* buffer,
size_t initialSize);
static void copyData(Handle<ArrayBufferObject*> toBuffer, size_t toIndex,
Handle<ArrayBufferObject*> fromBuffer, size_t fromIndex,
size_t count);
static size_t objectMoved(JSObject* obj, JSObject* old);
static uint8_t* stealMallocedContents(JSContext* cx,
Handle<ArrayBufferObject*> buffer);
static BufferContents extractStructuredCloneContents(
JSContext* cx, Handle<ArrayBufferObject*> buffer);
static void addSizeOfExcludingThis(JSObject* obj,
mozilla::MallocSizeOf mallocSizeOf,
JS::ClassInfo* info,
JS::RuntimeSizes* runtimeSizes);
// ArrayBufferObjects (strongly) store the first view added to them, while
// later views are (weakly) stored in the compartment's InnerViewTable
// below. Buffers usually only have one view, so this slot optimizes for
// the common case. Avoiding entries in the InnerViewTable saves memory and
// non-incrementalized sweep time.
JSObject* firstView();
bool addView(JSContext* cx, ArrayBufferViewObject* view);
// Detach this buffer from its original memory. (This necessarily makes
// views of this buffer unusable for modifying that original memory.)
static void detach(JSContext* cx, Handle<ArrayBufferObject*> buffer);
static constexpr size_t offsetOfByteLengthSlot() {
return getFixedSlotOffset(BYTE_LENGTH_SLOT);
static constexpr size_t offsetOfFlagsSlot() {
return getFixedSlotOffset(FLAGS_SLOT);
void setFirstView(ArrayBufferViewObject* view);
uint8_t* inlineDataPointer() const;
struct FreeInfo {
JS::BufferContentsFreeFunc freeFunc;
void* freeUserData;
FreeInfo* freeInfo() const;
uint8_t* dataPointer() const;
SharedMem<uint8_t*> dataPointerShared() const;
size_t byteLength() const;
BufferContents contents() const {
if (isExternal()) {
return BufferContents(dataPointer(), EXTERNAL, freeInfo()->freeFunc,
return BufferContents(dataPointer(), bufferKind());
bool hasInlineData() const { return dataPointer() == inlineDataPointer(); }
void releaseData(JS::GCContext* gcx);
BufferKind bufferKind() const {
return BufferKind(flags() & BUFFER_KIND_MASK);
bool isInlineData() const { return bufferKind() == INLINE_DATA; }
bool isMalloced() const { return bufferKind() == MALLOCED; }
bool isNoData() const { return bufferKind() == NO_DATA; }
bool hasUserOwnedData() const { return bufferKind() == USER_OWNED; }
bool isWasm() const { return bufferKind() == WASM; }
bool isMapped() const { return bufferKind() == MAPPED; }
bool isExternal() const { return bufferKind() == EXTERNAL; }
bool isDetached() const { return flags() & DETACHED; }
bool isPreparedForAsmJS() const { return flags() & FOR_ASMJS; }
// WebAssembly support:
* Prepare this ArrayBuffer for use with asm.js. Returns true on success,
* false on failure. This function reports no errors.
[[nodiscard]] bool prepareForAsmJS();
size_t wasmMappedSize() const;
wasm::IndexType wasmIndexType() const;
wasm::Pages wasmPages() const;
wasm::Pages wasmClampedMaxPages() const;
mozilla::Maybe<wasm::Pages> wasmSourceMaxPages() const;
[[nodiscard]] static bool wasmGrowToPagesInPlace(
wasm::IndexType t, wasm::Pages newPages,
Handle<ArrayBufferObject*> oldBuf,
MutableHandle<ArrayBufferObject*> newBuf, JSContext* cx);
[[nodiscard]] static bool wasmMovingGrowToPages(
wasm::IndexType t, wasm::Pages newPages,
Handle<ArrayBufferObject*> oldBuf,
MutableHandle<ArrayBufferObject*> newBuf, JSContext* cx);
static void wasmDiscard(Handle<ArrayBufferObject*> buf, uint64_t byteOffset,
uint64_t byteLength);
static void finalize(JS::GCContext* gcx, JSObject* obj);
static BufferContents createMappedContents(int fd, size_t offset,
size_t length);
void setDataPointer(BufferContents contents);
void setByteLength(size_t length);
size_t associatedBytes() const;
uint32_t flags() const;
void setFlags(uint32_t flags);
void setIsDetached() { setFlags(flags() | DETACHED); }
void setIsPreparedForAsmJS() {
MOZ_ASSERT(isMalloced() || isMapped() || isExternal());
setFlags(flags() | FOR_ASMJS);
void initialize(size_t byteLength, BufferContents contents) {
void* initializeToInlineData(size_t byteLength) {
void* data = inlineDataPointer();
initialize(byteLength, BufferContents::createInlineData(data));
return data;
using RootedArrayBufferObject = Rooted<ArrayBufferObject*>;
using HandleArrayBufferObject = Handle<ArrayBufferObject*>;
using MutableHandleArrayBufferObject = MutableHandle<ArrayBufferObject*>;
// Create a buffer for a wasm memory, whose type is determined by
// memory.indexType().
bool CreateWasmBuffer(JSContext* cx, const wasm::MemoryDesc& memory,
MutableHandleArrayBufferObjectMaybeShared buffer);
// Per-compartment table that manages the relationship between array buffers
// and the views that use their storage.
class InnerViewTable {
using ViewVector = GCVector<UnsafeBarePtr<JSObject*>, 1, ZoneAllocPolicy>;
friend class ArrayBufferObject;
// This key is a raw pointer and not a WeakHeapPtr because the post-barrier
// would hold nursery-allocated entries live unconditionally. It is a very
// common pattern in low-level and performance-oriented JavaScript to create
// hundreds or thousands of very short lived temporary views on a larger
// buffer; having to tenure all of these would be a catastrophic performance
// regression. Thus, it is vital that nursery pointers in this map not be held
// live. Special support is required in the minor GC, implemented in
// sweepAfterMinorGC.
using Map = GCHashMap<UnsafeBarePtr<JSObject*>, ViewVector,
StableCellHasher<JSObject*>, ZoneAllocPolicy>;
// For all objects sharing their storage with some other view, this maps
// the object to the list of such views. All entries in this map are weak.
Map map;
// List of keys from innerViews where either the source or at least one
// target is in the nursery. The raw pointer to a JSObject is allowed here
// because this vector is cleared after every minor collection. Users in
// sweepAfterMinorCollection must be careful to use MaybeForwarded before
// touching these pointers.
Vector<JSObject*, 0, SystemAllocPolicy> nurseryKeys;
// Whether nurseryKeys is a complete list.
bool nurseryKeysValid;
bool addView(JSContext* cx, ArrayBufferObject* buffer, JSObject* view);
ViewVector* maybeViewsUnbarriered(ArrayBufferObject* obj);
void removeViews(ArrayBufferObject* obj);
explicit InnerViewTable(Zone* zone) : map(zone), nurseryKeysValid(true) {}
// Remove references to dead objects in the table and update table entries
// to reflect moved objects.
bool traceWeak(JSTracer* trc);
void sweepAfterMinorGC(JSTracer* trc);
bool empty() const { return map.empty(); }
bool needsSweepAfterMinorGC() const {
return !nurseryKeys.empty() || !nurseryKeysValid;
size_t sizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf);
template <typename Wrapper>
class MutableWrappedPtrOperations<InnerViewTable, Wrapper>
: public WrappedPtrOperations<InnerViewTable, Wrapper> {
InnerViewTable& table() { return static_cast<Wrapper*>(this)->get(); }
size_t sizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf) {
return table().sizeOfExcludingThis(mallocSizeOf);
class WasmArrayRawBuffer {
wasm::IndexType indexType_;
wasm::Pages clampedMaxPages_;
mozilla::Maybe<wasm::Pages> sourceMaxPages_;
size_t mappedSize_; // Not including the header page
size_t length_;
WasmArrayRawBuffer(wasm::IndexType indexType, uint8_t* buffer,
wasm::Pages clampedMaxPages,
const mozilla::Maybe<wasm::Pages>& sourceMaxPages,
size_t mappedSize, size_t length)
: indexType_(indexType),
length_(length) {
MOZ_ASSERT(buffer == dataPointer());
static WasmArrayRawBuffer* AllocateWasm(
wasm::IndexType indexType, wasm::Pages initialPages,
wasm::Pages clampedMaxPages,
const mozilla::Maybe<wasm::Pages>& sourceMaxPages,
const mozilla::Maybe<size_t>& mappedSize);
static void Release(void* mem);
uint8_t* dataPointer() {
uint8_t* ptr = reinterpret_cast<uint8_t*>(this);
return ptr + sizeof(WasmArrayRawBuffer);
static const WasmArrayRawBuffer* fromDataPtr(const uint8_t* dataPtr) {
return reinterpret_cast<const WasmArrayRawBuffer*>(
dataPtr - sizeof(WasmArrayRawBuffer));
static WasmArrayRawBuffer* fromDataPtr(uint8_t* dataPtr) {
return reinterpret_cast<WasmArrayRawBuffer*>(dataPtr -
wasm::IndexType indexType() const { return indexType_; }
uint8_t* basePointer() { return dataPointer() - gc::SystemPageSize(); }
size_t mappedSize() const { return mappedSize_; }
size_t byteLength() const { return length_; }
wasm::Pages pages() const {
return wasm::Pages::fromByteLengthExact(length_);
wasm::Pages clampedMaxPages() const { return clampedMaxPages_; }
mozilla::Maybe<wasm::Pages> sourceMaxPages() const { return sourceMaxPages_; }
[[nodiscard]] bool growToPagesInPlace(wasm::Pages newPages);
[[nodiscard]] bool extendMappedSize(wasm::Pages maxPages);
// Try and grow the mapped region of memory. Does not change current size.
// Does not move memory if no space to grow.
void tryGrowMaxPagesInPlace(wasm::Pages deltaMaxPages);
// 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);
} // namespace js
template <>
bool JSObject::is<js::ArrayBufferObjectMaybeShared>() const;
#endif // vm_ArrayBufferObject_h