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NativeObject.cpp
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NativeObject.cpp
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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 http://mozilla.org/MPL/2.0/. */
#include "vm/NativeObject-inl.h"
#include "mozilla/ArrayUtils.h"
#include "mozilla/Casting.h"
#include "mozilla/CheckedInt.h"
#include "mozilla/DebugOnly.h"
#include "mozilla/Maybe.h"
#include <algorithm>
#include "debugger/DebugAPI.h"
#include "gc/Marking.h"
#include "jit/BaselineIC.h"
#include "js/CharacterEncoding.h"
#include "js/Result.h"
#include "js/Value.h"
#include "util/Memory.h"
#include "vm/EqualityOperations.h" // js::SameValue
#include "vm/TypedArrayObject.h"
#include "gc/Nursery-inl.h"
#include "vm/ArrayObject-inl.h"
#include "vm/BytecodeLocation-inl.h"
#include "vm/EnvironmentObject-inl.h"
#include "vm/JSObject-inl.h"
#include "vm/JSScript-inl.h"
#include "vm/Shape-inl.h"
#include "vm/TypeInference-inl.h"
using namespace js;
using JS::AutoCheckCannotGC;
using mozilla::ArrayLength;
using mozilla::CheckedInt;
using mozilla::DebugOnly;
using mozilla::PodCopy;
using mozilla::RoundUpPow2;
struct EmptyObjectElements {
const ObjectElements emptyElementsHeader;
// Add an extra (unused) Value to make sure an out-of-bounds index when
// masked (resulting in index 0) accesses valid memory.
const Value val;
public:
constexpr EmptyObjectElements()
: emptyElementsHeader(0, 0), val(UndefinedValue()) {}
explicit constexpr EmptyObjectElements(ObjectElements::SharedMemory shmem)
: emptyElementsHeader(0, 0, shmem), val(UndefinedValue()) {}
};
static constexpr EmptyObjectElements emptyElementsHeader;
/* Objects with no elements share one empty set of elements. */
HeapSlot* const js::emptyObjectElements = reinterpret_cast<HeapSlot*>(
uintptr_t(&emptyElementsHeader) + sizeof(ObjectElements));
static constexpr EmptyObjectElements emptyElementsHeaderShared(
ObjectElements::SharedMemory::IsShared);
/* Objects with no elements share one empty set of elements. */
HeapSlot* const js::emptyObjectElementsShared = reinterpret_cast<HeapSlot*>(
uintptr_t(&emptyElementsHeaderShared) + sizeof(ObjectElements));
#ifdef DEBUG
bool NativeObject::canHaveNonEmptyElements() {
return !this->is<TypedArrayObject>();
}
#endif // DEBUG
/* static */
void ObjectElements::ConvertElementsToDoubles(JSContext* cx,
uintptr_t elementsPtr) {
/*
* This function has an otherwise unused JSContext argument so that it can
* be called directly from Ion code. Only arrays can have their dense
* elements converted to doubles, and arrays never have empty elements.
*/
HeapSlot* elementsHeapPtr = (HeapSlot*)elementsPtr;
MOZ_ASSERT(elementsHeapPtr != emptyObjectElements &&
elementsHeapPtr != emptyObjectElementsShared);
ObjectElements* header = ObjectElements::fromElements(elementsHeapPtr);
MOZ_ASSERT(!header->shouldConvertDoubleElements());
// Note: the elements can be mutated in place even for copy on write
// arrays. See comment on ObjectElements.
Value* vp = (Value*)elementsPtr;
for (size_t i = 0; i < header->initializedLength; i++) {
if (vp[i].isInt32()) {
vp[i].setDouble(vp[i].toInt32());
}
}
header->setShouldConvertDoubleElements();
}
/* static */
bool ObjectElements::MakeElementsCopyOnWrite(JSContext* cx, NativeObject* obj) {
static_assert(sizeof(HeapSlot) >= sizeof(GCPtrObject),
"there must be enough room for the owner object pointer at "
"the end of the elements");
if (!obj->ensureElements(cx, obj->getDenseInitializedLength() + 1)) {
return false;
}
ObjectElements* header = obj->getElementsHeader();
// Note: this method doesn't update type information to indicate that the
// elements might be copy on write. Handling this is left to the caller.
MOZ_ASSERT(!header->isCopyOnWrite());
MOZ_ASSERT(obj->isExtensible());
header->flags |= COPY_ON_WRITE;
header->ownerObject().init(obj);
return true;
}
/* static */
bool ObjectElements::PreventExtensions(JSContext* cx, NativeObject* obj) {
if (!obj->maybeCopyElementsForWrite(cx)) {
return false;
}
if (!obj->hasEmptyElements()) {
obj->shrinkCapacityToInitializedLength(cx);
MarkObjectGroupFlags(cx, obj, OBJECT_FLAG_NON_EXTENSIBLE_ELEMENTS);
}
// shrinkCapacityToInitializedLength ensures there are no shifted elements.
MOZ_ASSERT(obj->getElementsHeader()->numShiftedElements() == 0);
return true;
}
/* static */
void ObjectElements::FreezeOrSeal(JSContext* cx, NativeObject* obj,
IntegrityLevel level) {
MOZ_ASSERT_IF(level == IntegrityLevel::Frozen && obj->is<ArrayObject>(),
!obj->as<ArrayObject>().lengthIsWritable());
MOZ_ASSERT(!obj->denseElementsAreCopyOnWrite());
MOZ_ASSERT(!obj->isExtensible());
MOZ_ASSERT(obj->getElementsHeader()->numShiftedElements() == 0);
if (obj->hasEmptyElements() || obj->denseElementsAreFrozen()) {
return;
}
if (!obj->denseElementsAreSealed()) {
obj->getElementsHeader()->seal();
}
if (level == IntegrityLevel::Frozen) {
obj->getElementsHeader()->freeze();
}
}
#ifdef DEBUG
static mozilla::Atomic<bool, mozilla::Relaxed,
mozilla::recordreplay::Behavior::DontPreserve>
gShapeConsistencyChecksEnabled(false);
/* static */
void js::NativeObject::enableShapeConsistencyChecks() {
gShapeConsistencyChecksEnabled = true;
}
void js::NativeObject::checkShapeConsistency() {
if (!gShapeConsistencyChecksEnabled) {
return;
}
MOZ_ASSERT(isNative());
Shape* shape = lastProperty();
Shape* prev = nullptr;
AutoCheckCannotGC nogc;
if (inDictionaryMode()) {
if (ShapeTable* table = shape->maybeTable(nogc)) {
for (uint32_t fslot = table->freeList(); fslot != SHAPE_INVALID_SLOT;
fslot = getSlot(fslot).toPrivateUint32()) {
MOZ_ASSERT(fslot < slotSpan());
}
while (shape->parent) {
MOZ_ASSERT_IF(lastProperty() != shape, !shape->hasTable());
ShapeTable::Entry& entry =
table->search<MaybeAdding::NotAdding>(shape->propid(), nogc);
MOZ_ASSERT(entry.shape() == shape);
shape = shape->parent;
}
}
shape = lastProperty();
while (shape) {
MOZ_ASSERT_IF(!shape->isEmptyShape() && shape->isDataProperty(),
shape->slot() < slotSpan());
if (!prev) {
MOZ_ASSERT(lastProperty() == shape);
MOZ_ASSERT(shape->dictNext.toObject() == this);
} else {
MOZ_ASSERT(shape->dictNext.toShape() == prev);
}
prev = shape;
shape = shape->parent;
}
} else {
while (shape->parent) {
if (ShapeTable* table = shape->maybeTable(nogc)) {
MOZ_ASSERT(shape->parent);
for (Shape::Range<NoGC> r(shape); !r.empty(); r.popFront()) {
ShapeTable::Entry& entry =
table->search<MaybeAdding::NotAdding>(r.front().propid(), nogc);
MOZ_ASSERT(entry.shape() == &r.front());
}
}
if (prev) {
MOZ_ASSERT_IF(shape->isDataProperty(),
prev->maybeSlot() >= shape->maybeSlot());
shape->children.checkHasChild(prev);
}
prev = shape;
shape = shape->parent;
}
}
}
#endif
void js::NativeObject::initializeSlotRange(uint32_t start, uint32_t length) {
/*
* No bounds check, as this is used when the object's shape does not
* reflect its allocated slots (updateSlotsForSpan).
*/
HeapSlot* fixedStart;
HeapSlot* fixedEnd;
HeapSlot* slotsStart;
HeapSlot* slotsEnd;
getSlotRangeUnchecked(start, length, &fixedStart, &fixedEnd, &slotsStart,
&slotsEnd);
uint32_t offset = start;
for (HeapSlot* sp = fixedStart; sp < fixedEnd; sp++) {
sp->init(this, HeapSlot::Slot, offset++, UndefinedValue());
}
for (HeapSlot* sp = slotsStart; sp < slotsEnd; sp++) {
sp->init(this, HeapSlot::Slot, offset++, UndefinedValue());
}
}
void js::NativeObject::initSlotRange(uint32_t start, const Value* vector,
uint32_t length) {
HeapSlot* fixedStart;
HeapSlot* fixedEnd;
HeapSlot* slotsStart;
HeapSlot* slotsEnd;
getSlotRange(start, length, &fixedStart, &fixedEnd, &slotsStart, &slotsEnd);
for (HeapSlot* sp = fixedStart; sp < fixedEnd; sp++) {
sp->init(this, HeapSlot::Slot, start++, *vector++);
}
for (HeapSlot* sp = slotsStart; sp < slotsEnd; sp++) {
sp->init(this, HeapSlot::Slot, start++, *vector++);
}
}
#ifdef DEBUG
bool js::NativeObject::slotInRange(uint32_t slot,
SentinelAllowed sentinel) const {
MOZ_ASSERT(!gc::IsForwarded(lastProperty()));
uint32_t capacity = numFixedSlots() + numDynamicSlots();
if (sentinel == SENTINEL_ALLOWED) {
return slot <= capacity;
}
return slot < capacity;
}
bool js::NativeObject::slotIsFixed(uint32_t slot) const {
// We call numFixedSlotsMaybeForwarded() to allow reading slots of
// associated objects in trace hooks that may be called during a moving GC.
return slot < numFixedSlotsMaybeForwarded();
}
bool js::NativeObject::isNumFixedSlots(uint32_t nfixed) const {
// We call numFixedSlotsMaybeForwarded() to allow reading slots of
// associated objects in trace hooks that may be called during a moving GC.
return nfixed == numFixedSlotsMaybeForwarded();
}
#endif /* DEBUG */
Shape* js::NativeObject::lookup(JSContext* cx, jsid id) {
MOZ_ASSERT(isNative());
return Shape::search(cx, lastProperty(), id);
}
Shape* js::NativeObject::lookupPure(jsid id) {
MOZ_ASSERT(isNative());
return Shape::searchNoHashify(lastProperty(), id);
}
void NativeObject::setLastPropertyShrinkFixedSlots(Shape* shape) {
MOZ_ASSERT(!inDictionaryMode());
MOZ_ASSERT(!shape->inDictionary());
MOZ_ASSERT(shape->zone() == zone());
MOZ_ASSERT(lastProperty()->slotSpan() == shape->slotSpan());
MOZ_ASSERT(shape->getObjectClass() == getClass());
DebugOnly<size_t> oldFixed = numFixedSlots();
DebugOnly<size_t> newFixed = shape->numFixedSlots();
MOZ_ASSERT(newFixed < oldFixed);
MOZ_ASSERT(shape->slotSpan() <= oldFixed);
MOZ_ASSERT(shape->slotSpan() <= newFixed);
MOZ_ASSERT(dynamicSlotsCount(oldFixed, shape->slotSpan(), getClass()) == 0);
MOZ_ASSERT(dynamicSlotsCount(newFixed, shape->slotSpan(), getClass()) == 0);
setShape(shape);
}
bool NativeObject::setSlotSpan(JSContext* cx, uint32_t span) {
MOZ_ASSERT(inDictionaryMode());
size_t oldSpan = lastProperty()->base()->slotSpan();
if (oldSpan == span) {
return true;
}
if (!updateSlotsForSpan(cx, oldSpan, span)) {
return false;
}
lastProperty()->base()->setSlotSpan(span);
return true;
}
bool NativeObject::growSlots(JSContext* cx, uint32_t oldCount,
uint32_t newCount) {
MOZ_ASSERT(newCount > oldCount);
MOZ_ASSERT_IF(!is<ArrayObject>(), newCount >= SLOT_CAPACITY_MIN);
/*
* Slot capacities are determined by the span of allocated objects. Due to
* the limited number of bits to store shape slots, object growth is
* throttled well before the slot capacity can overflow.
*/
NativeObject::slotsSizeMustNotOverflow();
MOZ_ASSERT(newCount <= MAX_SLOTS_COUNT);
if (!oldCount) {
MOZ_ASSERT(!slots_);
slots_ = AllocateObjectBuffer<HeapSlot>(cx, this, newCount);
if (!slots_) {
return false;
}
Debug_SetSlotRangeToCrashOnTouch(slots_, newCount);
AddCellMemory(this, newCount * sizeof(HeapSlot), MemoryUse::ObjectSlots);
return true;
}
HeapSlot* newslots =
ReallocateObjectBuffer<HeapSlot>(cx, this, slots_, oldCount, newCount);
if (!newslots) {
return false; /* Leave slots at its old size. */
}
RemoveCellMemory(this, oldCount * sizeof(HeapSlot), MemoryUse::ObjectSlots);
AddCellMemory(this, newCount * sizeof(HeapSlot), MemoryUse::ObjectSlots);
slots_ = newslots;
Debug_SetSlotRangeToCrashOnTouch(slots_ + oldCount, newCount - oldCount);
return true;
}
/* static */
bool NativeObject::growSlotsPure(JSContext* cx, NativeObject* obj,
uint32_t newCount) {
// IC code calls this directly.
AutoUnsafeCallWithABI unsafe;
if (!obj->growSlots(cx, obj->numDynamicSlots(), newCount)) {
cx->recoverFromOutOfMemory();
return false;
}
return true;
}
/* static */
bool NativeObject::addDenseElementPure(JSContext* cx, NativeObject* obj) {
// IC code calls this directly.
AutoUnsafeCallWithABI unsafe;
MOZ_ASSERT(obj->getDenseInitializedLength() == obj->getDenseCapacity());
MOZ_ASSERT(!obj->denseElementsAreCopyOnWrite());
MOZ_ASSERT(obj->isExtensible());
MOZ_ASSERT(!obj->isIndexed());
MOZ_ASSERT(!obj->is<TypedArrayObject>());
MOZ_ASSERT_IF(obj->is<ArrayObject>(),
obj->as<ArrayObject>().lengthIsWritable());
// growElements will report OOM also if the number of dense elements will
// exceed MAX_DENSE_ELEMENTS_COUNT. See goodElementsAllocationAmount.
uint32_t oldCapacity = obj->getDenseCapacity();
if (MOZ_UNLIKELY(!obj->growElements(cx, oldCapacity + 1))) {
cx->recoverFromOutOfMemory();
return false;
}
MOZ_ASSERT(obj->getDenseCapacity() > oldCapacity);
MOZ_ASSERT(obj->getDenseCapacity() <= MAX_DENSE_ELEMENTS_COUNT);
return true;
}
static inline void FreeSlots(JSContext* cx, NativeObject* obj,
HeapSlot* slots) {
if (cx->isHelperThreadContext()) {
js_free(slots);
} else if (obj->isTenured()) {
MOZ_ASSERT(!cx->nursery().isInside(slots));
js_free(slots);
} else {
cx->nursery().freeBuffer(slots);
}
}
void NativeObject::shrinkSlots(JSContext* cx, uint32_t oldCount,
uint32_t newCount) {
MOZ_ASSERT(newCount < oldCount);
if (newCount == 0) {
RemoveCellMemory(this, numDynamicSlots() * sizeof(HeapSlot),
MemoryUse::ObjectSlots);
FreeSlots(cx, this, slots_);
slots_ = nullptr;
return;
}
MOZ_ASSERT_IF(!is<ArrayObject>(), newCount >= SLOT_CAPACITY_MIN);
// Update the memory tracking whether or not the reallocation succeeds,
// because we have no way to tell the buffer size if it fails.
RemoveCellMemory(this, oldCount * sizeof(HeapSlot), MemoryUse::ObjectSlots);
AddCellMemory(this, newCount * sizeof(HeapSlot), MemoryUse::ObjectSlots);
HeapSlot* newslots =
ReallocateObjectBuffer<HeapSlot>(cx, this, slots_, oldCount, newCount);
if (!newslots) {
cx->recoverFromOutOfMemory();
return; // Leave slots at its old size.
}
slots_ = newslots;
}
bool NativeObject::willBeSparseElements(uint32_t requiredCapacity,
uint32_t newElementsHint) {
MOZ_ASSERT(isNative());
MOZ_ASSERT(requiredCapacity > MIN_SPARSE_INDEX);
uint32_t cap = getDenseCapacity();
MOZ_ASSERT(requiredCapacity >= cap);
if (requiredCapacity > MAX_DENSE_ELEMENTS_COUNT) {
return true;
}
uint32_t minimalDenseCount = requiredCapacity / SPARSE_DENSITY_RATIO;
if (newElementsHint >= minimalDenseCount) {
return false;
}
minimalDenseCount -= newElementsHint;
if (minimalDenseCount > cap) {
return true;
}
uint32_t len = getDenseInitializedLength();
const Value* elems = getDenseElements();
for (uint32_t i = 0; i < len; i++) {
if (!elems[i].isMagic(JS_ELEMENTS_HOLE) && !--minimalDenseCount) {
return false;
}
}
return true;
}
/* static */
DenseElementResult NativeObject::maybeDensifySparseElements(
JSContext* cx, HandleNativeObject obj) {
/*
* Wait until after the object goes into dictionary mode, which must happen
* when sparsely packing any array with more than MIN_SPARSE_INDEX elements
* (see PropertyTree::MAX_HEIGHT).
*/
if (!obj->inDictionaryMode()) {
return DenseElementResult::Incomplete;
}
/*
* Only measure the number of indexed properties every log(n) times when
* populating the object.
*/
uint32_t slotSpan = obj->slotSpan();
if (slotSpan != RoundUpPow2(slotSpan)) {
return DenseElementResult::Incomplete;
}
/* Watch for conditions under which an object's elements cannot be dense. */
if (!obj->isExtensible()) {
return DenseElementResult::Incomplete;
}
/*
* The indexes in the object need to be sufficiently dense before they can
* be converted to dense mode.
*/
uint32_t numDenseElements = 0;
uint32_t newInitializedLength = 0;
RootedShape shape(cx, obj->lastProperty());
while (!shape->isEmptyShape()) {
uint32_t index;
if (IdIsIndex(shape->propid(), &index)) {
if (shape->attributes() == JSPROP_ENUMERATE &&
shape->hasDefaultGetter() && shape->hasDefaultSetter()) {
numDenseElements++;
newInitializedLength = std::max(newInitializedLength, index + 1);
} else {
/*
* For simplicity, only densify the object if all indexed
* properties can be converted to dense elements.
*/
return DenseElementResult::Incomplete;
}
}
shape = shape->previous();
}
if (numDenseElements * SPARSE_DENSITY_RATIO < newInitializedLength) {
return DenseElementResult::Incomplete;
}
if (newInitializedLength > MAX_DENSE_ELEMENTS_COUNT) {
return DenseElementResult::Incomplete;
}
/*
* This object meets all necessary restrictions, convert all indexed
* properties into dense elements.
*/
if (!obj->maybeCopyElementsForWrite(cx)) {
return DenseElementResult::Failure;
}
if (newInitializedLength > obj->getDenseCapacity()) {
if (!obj->growElements(cx, newInitializedLength)) {
return DenseElementResult::Failure;
}
}
obj->ensureDenseInitializedLength(cx, newInitializedLength, 0);
RootedValue value(cx);
shape = obj->lastProperty();
while (!shape->isEmptyShape()) {
jsid id = shape->propid();
uint32_t index;
if (IdIsIndex(id, &index)) {
value = obj->getSlot(shape->slot());
/*
* When removing a property from a dictionary, the specified
* property will be removed from the dictionary list and the
* last property will then be changed due to reshaping the object.
* Compute the next shape in the traverse, watching for such
* removals from the list.
*/
if (shape != obj->lastProperty()) {
shape = shape->previous();
if (!NativeObject::removeProperty(cx, obj, id)) {
return DenseElementResult::Failure;
}
} else {
if (!NativeObject::removeProperty(cx, obj, id)) {
return DenseElementResult::Failure;
}
shape = obj->lastProperty();
}
obj->setDenseElement(index, value);
} else {
shape = shape->previous();
}
}
/*
* All indexed properties on the object are now dense, clear the indexed
* flag so that we will not start using sparse indexes again if we need
* to grow the object.
*/
if (!NativeObject::clearFlag(cx, obj, BaseShape::INDEXED)) {
return DenseElementResult::Failure;
}
return DenseElementResult::Success;
}
void NativeObject::moveShiftedElements() {
MOZ_ASSERT(isExtensible());
ObjectElements* header = getElementsHeader();
uint32_t numShifted = header->numShiftedElements();
MOZ_ASSERT(numShifted > 0);
uint32_t initLength = header->initializedLength;
ObjectElements* newHeader =
static_cast<ObjectElements*>(getUnshiftedElementsHeader());
memmove(newHeader, header, sizeof(ObjectElements));
newHeader->clearShiftedElements();
newHeader->capacity += numShifted;
elements_ = newHeader->elements();
// To move the elements, temporarily update initializedLength to include
// the shifted elements.
newHeader->initializedLength += numShifted;
// Move the elements. Initialize to |undefined| to ensure pre-barriers
// don't see garbage.
for (size_t i = 0; i < numShifted; i++) {
initDenseElement(i, UndefinedValue());
}
moveDenseElements(0, numShifted, initLength);
// Restore the initialized length. We use setDenseInitializedLength to
// make sure prepareElementRangeForOverwrite is called on the shifted
// elements.
setDenseInitializedLength(initLength);
}
void NativeObject::maybeMoveShiftedElements() {
MOZ_ASSERT(isExtensible());
ObjectElements* header = getElementsHeader();
MOZ_ASSERT(header->numShiftedElements() > 0);
// Move the elements if less than a third of the allocated space is in use.
if (header->capacity < header->numAllocatedElements() / 3) {
moveShiftedElements();
}
}
bool NativeObject::tryUnshiftDenseElements(uint32_t count) {
MOZ_ASSERT(isExtensible());
MOZ_ASSERT(count > 0);
ObjectElements* header = getElementsHeader();
uint32_t numShifted = header->numShiftedElements();
if (count > numShifted) {
// We need more elements than are easily available. Try to make space
// for more elements than we need (and shift the remaining ones) so
// that unshifting more elements later will be fast.
// Don't bother reserving elements if the number of elements is small.
// Note that there's no technical reason for using this particular
// limit.
if (header->initializedLength <= 10 || header->isCopyOnWrite() ||
header->hasNonwritableArrayLength() ||
MOZ_UNLIKELY(count > ObjectElements::MaxShiftedElements)) {
return false;
}
MOZ_ASSERT(header->capacity >= header->initializedLength);
uint32_t unusedCapacity = header->capacity - header->initializedLength;
// Determine toShift, the number of extra elements we want to make
// available.
uint32_t toShift = count - numShifted;
MOZ_ASSERT(toShift <= ObjectElements::MaxShiftedElements,
"count <= MaxShiftedElements so toShift <= MaxShiftedElements");
// Give up if we need to allocate more elements.
if (toShift > unusedCapacity) {
return false;
}
// Move more elements than we need, so that other unshift calls will be
// fast. We just have to make sure we don't exceed unusedCapacity.
toShift = std::min(toShift + unusedCapacity / 2, unusedCapacity);
// Ensure |numShifted + toShift| does not exceed MaxShiftedElements.
if (numShifted + toShift > ObjectElements::MaxShiftedElements) {
toShift = ObjectElements::MaxShiftedElements - numShifted;
}
MOZ_ASSERT(count <= numShifted + toShift);
MOZ_ASSERT(numShifted + toShift <= ObjectElements::MaxShiftedElements);
MOZ_ASSERT(toShift <= unusedCapacity);
// Now move/unshift the elements.
uint32_t initLen = header->initializedLength;
setDenseInitializedLength(initLen + toShift);
for (uint32_t i = 0; i < toShift; i++) {
initDenseElement(initLen + i, UndefinedValue());
}
moveDenseElements(toShift, 0, initLen);
// Shift the elements we just prepended.
shiftDenseElementsUnchecked(toShift);
// We can now fall-through to the fast path below.
header = getElementsHeader();
MOZ_ASSERT(header->numShiftedElements() == numShifted + toShift);
numShifted = header->numShiftedElements();
MOZ_ASSERT(count <= numShifted);
}
elements_ -= count;
ObjectElements* newHeader = getElementsHeader();
memmove(newHeader, header, sizeof(ObjectElements));
newHeader->unshiftShiftedElements(count);
// Initialize to |undefined| to ensure pre-barriers don't see garbage.
for (uint32_t i = 0; i < count; i++) {
initDenseElement(i, UndefinedValue());
}
return true;
}
// Given a requested capacity (in elements) and (potentially) the length of an
// array for which elements are being allocated, compute an actual allocation
// amount (in elements). (Allocation amounts include space for an
// ObjectElements instance, so a return value of |N| implies
// |N - ObjectElements::VALUES_PER_HEADER| usable elements.)
//
// The requested/actual allocation distinction is meant to:
//
// * preserve amortized O(N) time to add N elements;
// * minimize the number of unused elements beyond an array's length, and
// * provide at least SLOT_CAPACITY_MIN elements no matter what (so adding
// the first several elements to small arrays only needs one allocation).
//
// Note: the structure and behavior of this method follow along with
// UnboxedArrayObject::chooseCapacityIndex. Changes to the allocation strategy
// in one should generally be matched by the other.
/* static */
bool NativeObject::goodElementsAllocationAmount(JSContext* cx,
uint32_t reqCapacity,
uint32_t length,
uint32_t* goodAmount) {
if (reqCapacity > MAX_DENSE_ELEMENTS_COUNT) {
ReportOutOfMemory(cx);
return false;
}
uint32_t reqAllocated = reqCapacity + ObjectElements::VALUES_PER_HEADER;
// Handle "small" requests primarily by doubling.
const uint32_t Mebi = 1 << 20;
if (reqAllocated < Mebi) {
uint32_t amount =
mozilla::AssertedCast<uint32_t>(RoundUpPow2(reqAllocated));
// If |amount| would be 2/3 or more of the array's length, adjust
// it (up or down) to be equal to the array's length. This avoids
// allocating excess elements that aren't likely to be needed, either
// in this resizing or a subsequent one. The 2/3 factor is chosen so
// that exceptional resizings will at most triple the capacity, as
// opposed to the usual doubling.
uint32_t goodCapacity = amount - ObjectElements::VALUES_PER_HEADER;
if (length >= reqCapacity && goodCapacity > (length / 3) * 2) {
amount = length + ObjectElements::VALUES_PER_HEADER;
}
if (amount < SLOT_CAPACITY_MIN) {
amount = SLOT_CAPACITY_MIN;
}
*goodAmount = amount;
return true;
}
// The almost-doubling above wastes a lot of space for larger bucket sizes.
// For large amounts, switch to bucket sizes that obey this formula:
//
// count(n+1) = Math.ceil(count(n) * 1.125)
//
// where |count(n)| is the size of the nth bucket, measured in 2**20 slots.
// These bucket sizes still preserve amortized O(N) time to add N elements,
// just with a larger constant factor.
//
// The bucket size table below was generated with this JavaScript (and
// manual reformatting):
//
// for (let n = 1, i = 0; i < 34; i++) {
// print('0x' + (n * (1 << 20)).toString(16) + ', ');
// n = Math.ceil(n * 1.125);
// }
static const uint32_t BigBuckets[] = {
0x100000, 0x200000, 0x300000, 0x400000, 0x500000, 0x600000,
0x700000, 0x800000, 0x900000, 0xb00000, 0xd00000, 0xf00000,
0x1100000, 0x1400000, 0x1700000, 0x1a00000, 0x1e00000, 0x2200000,
0x2700000, 0x2c00000, 0x3200000, 0x3900000, 0x4100000, 0x4a00000,
0x5400000, 0x5f00000, 0x6b00000, 0x7900000, 0x8900000, 0x9b00000,
0xaf00000, 0xc500000, 0xde00000, 0xfa00000};
MOZ_ASSERT(BigBuckets[ArrayLength(BigBuckets) - 1] <=
MAX_DENSE_ELEMENTS_ALLOCATION);
// Pick the first bucket that'll fit |reqAllocated|.
for (uint32_t b : BigBuckets) {
if (b >= reqAllocated) {
*goodAmount = b;
return true;
}
}
// Otherwise, return the maximum bucket size.
*goodAmount = MAX_DENSE_ELEMENTS_ALLOCATION;
return true;
}
bool NativeObject::growElements(JSContext* cx, uint32_t reqCapacity) {
MOZ_ASSERT(isExtensible());
MOZ_ASSERT(canHaveNonEmptyElements());
if (denseElementsAreCopyOnWrite()) {
MOZ_CRASH();
}
// If there are shifted elements, consider moving them first. If we don't
// move them here, the code below will include the shifted elements in the
// resize.
uint32_t numShifted = getElementsHeader()->numShiftedElements();
if (numShifted > 0) {
// If the number of elements is small, it's cheaper to just move them as
// it may avoid a malloc/realloc. Note that there's no technical reason
// for using this particular value, but it works well in real-world use
// cases.
static const size_t MaxElementsToMoveEagerly = 20;
if (getElementsHeader()->initializedLength <= MaxElementsToMoveEagerly) {
moveShiftedElements();
} else {
maybeMoveShiftedElements();
}
if (getDenseCapacity() >= reqCapacity) {
return true;
}
numShifted = getElementsHeader()->numShiftedElements();
// If |reqCapacity + numShifted| overflows, we just move all shifted
// elements to avoid the problem.
CheckedInt<uint32_t> checkedReqCapacity(reqCapacity);
checkedReqCapacity += numShifted;
if (MOZ_UNLIKELY(!checkedReqCapacity.isValid())) {
moveShiftedElements();
numShifted = 0;
}
}
uint32_t oldCapacity = getDenseCapacity();
MOZ_ASSERT(oldCapacity < reqCapacity);
uint32_t newAllocated = 0;
if (is<ArrayObject>() && !as<ArrayObject>().lengthIsWritable()) {
MOZ_ASSERT(reqCapacity <= as<ArrayObject>().length());
MOZ_ASSERT(reqCapacity <= MAX_DENSE_ELEMENTS_COUNT);
// Preserve the |capacity <= length| invariant for arrays with
// non-writable length. See also js::ArraySetLength which initially
// enforces this requirement.
newAllocated = reqCapacity + numShifted + ObjectElements::VALUES_PER_HEADER;
} else {
if (!goodElementsAllocationAmount(cx, reqCapacity + numShifted,
getElementsHeader()->length,
&newAllocated)) {
return false;
}
}
uint32_t newCapacity =
newAllocated - ObjectElements::VALUES_PER_HEADER - numShifted;
MOZ_ASSERT(newCapacity > oldCapacity && newCapacity >= reqCapacity);
// If newCapacity exceeds MAX_DENSE_ELEMENTS_COUNT, the array should become
// sparse.
MOZ_ASSERT(newCapacity <= MAX_DENSE_ELEMENTS_COUNT);
uint32_t initlen = getDenseInitializedLength();
HeapSlot* oldHeaderSlots =
reinterpret_cast<HeapSlot*>(getUnshiftedElementsHeader());
HeapSlot* newHeaderSlots;
uint32_t oldAllocated = 0;
if (hasDynamicElements()) {
MOZ_ASSERT(oldCapacity <= MAX_DENSE_ELEMENTS_COUNT);
oldAllocated = oldCapacity + ObjectElements::VALUES_PER_HEADER + numShifted;
newHeaderSlots = ReallocateObjectBuffer<HeapSlot>(
cx, this, oldHeaderSlots, oldAllocated, newAllocated);
if (!newHeaderSlots) {
return false; // Leave elements at its old size.
}
} else {
newHeaderSlots = AllocateObjectBuffer<HeapSlot>(cx, this, newAllocated);
if (!newHeaderSlots) {
return false; // Leave elements at its old size.
}
PodCopy(newHeaderSlots, oldHeaderSlots,
ObjectElements::VALUES_PER_HEADER + initlen + numShifted);
}
if (oldAllocated) {
RemoveCellMemory(this, oldAllocated * sizeof(HeapSlot),
MemoryUse::ObjectElements);
}
ObjectElements* newheader = reinterpret_cast<ObjectElements*>(newHeaderSlots);
elements_ = newheader->elements() + numShifted;
getElementsHeader()->capacity = newCapacity;
Debug_SetSlotRangeToCrashOnTouch(elements_ + initlen, newCapacity - initlen);
AddCellMemory(this, newAllocated * sizeof(HeapSlot),
MemoryUse::ObjectElements);
return true;
}
void NativeObject::shrinkElements(JSContext* cx, uint32_t reqCapacity) {
MOZ_ASSERT(canHaveNonEmptyElements());
MOZ_ASSERT(reqCapacity >= getDenseInitializedLength());
if (denseElementsAreCopyOnWrite()) {
MOZ_CRASH();
}
if (!hasDynamicElements()) {
return;
}
// If we have shifted elements, consider moving them.
uint32_t numShifted = getElementsHeader()->numShiftedElements();
if (numShifted > 0) {
maybeMoveShiftedElements();
numShifted = getElementsHeader()->numShiftedElements();
}
uint32_t oldCapacity = getDenseCapacity();
MOZ_ASSERT(reqCapacity < oldCapacity);
uint32_t newAllocated = 0;
MOZ_ALWAYS_TRUE(goodElementsAllocationAmount(cx, reqCapacity + numShifted, 0,
&newAllocated));
MOZ_ASSERT(oldCapacity <= MAX_DENSE_ELEMENTS_COUNT);
uint32_t oldAllocated =
oldCapacity + ObjectElements::VALUES_PER_HEADER + numShifted;
if (newAllocated == oldAllocated) {
return; // Leave elements at its old size.
}
MOZ_ASSERT(newAllocated > ObjectElements::VALUES_PER_HEADER);
uint32_t newCapacity =
newAllocated - ObjectElements::VALUES_PER_HEADER - numShifted;
MOZ_ASSERT(newCapacity <= MAX_DENSE_ELEMENTS_COUNT);
HeapSlot* oldHeaderSlots =
reinterpret_cast<HeapSlot*>(getUnshiftedElementsHeader());
HeapSlot* newHeaderSlots = ReallocateObjectBuffer<HeapSlot>(
cx, this, oldHeaderSlots, oldAllocated, newAllocated);
if (!newHeaderSlots) {
cx->recoverFromOutOfMemory();
return; // Leave elements at its old size.
}
RemoveCellMemory(this, oldAllocated * sizeof(HeapSlot),
MemoryUse::ObjectElements);
ObjectElements* newheader = reinterpret_cast<ObjectElements*>(newHeaderSlots);
elements_ = newheader->elements() + numShifted;