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// Copyright 2011 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef V8_HEAP_SPACES_H_
#define V8_HEAP_SPACES_H_
#include "src/allocation.h"
#include "src/atomic-utils.h"
#include "src/base/atomicops.h"
#include "src/base/bits.h"
#include "src/base/platform/mutex.h"
#include "src/flags.h"
#include "src/hashmap.h"
#include "src/list.h"
#include "src/objects.h"
#include "src/utils.h"
namespace v8 {
namespace internal {
class AllocationInfo;
class AllocationObserver;
class CompactionSpace;
class CompactionSpaceCollection;
class FreeList;
class Isolate;
class MemoryAllocator;
class MemoryChunk;
class Page;
class PagedSpace;
class SemiSpace;
class SkipList;
class SlotsBuffer;
class SlotSet;
class TypedSlotSet;
class Space;
// -----------------------------------------------------------------------------
// Heap structures:
// A JS heap consists of a young generation, an old generation, and a large
// object space. The young generation is divided into two semispaces. A
// scavenger implements Cheney's copying algorithm. The old generation is
// separated into a map space and an old object space. The map space contains
// all (and only) map objects, the rest of old objects go into the old space.
// The old generation is collected by a mark-sweep-compact collector.
// The semispaces of the young generation are contiguous. The old and map
// spaces consists of a list of pages. A page has a page header and an object
// area.
// There is a separate large object space for objects larger than
// Page::kMaxRegularHeapObjectSize, so that they do not have to move during
// collection. The large object space is paged. Pages in large object space
// may be larger than the page size.
// A store-buffer based write barrier is used to keep track of intergenerational
// references. See heap/store-buffer.h.
// During scavenges and mark-sweep collections we sometimes (after a store
// buffer overflow) iterate intergenerational pointers without decoding heap
// object maps so if the page belongs to old space or large object space
// it is essential to guarantee that the page does not contain any
// garbage pointers to new space: every pointer aligned word which satisfies
// the Heap::InNewSpace() predicate must be a pointer to a live heap object in
// new space. Thus objects in old space and large object spaces should have a
// special layout (e.g. no bare integer fields). This requirement does not
// apply to map space which is iterated in a special fashion. However we still
// require pointer fields of dead maps to be cleaned.
// To enable lazy cleaning of old space pages we can mark chunks of the page
// as being garbage. Garbage sections are marked with a special map. These
// sections are skipped when scanning the page, even if we are otherwise
// scanning without regard for object boundaries. Garbage sections are chained
// together to form a free list after a GC. Garbage sections created outside
// of GCs by object trunctation etc. may not be in the free list chain. Very
// small free spaces are ignored, they need only be cleaned of bogus pointers
// into new space.
// Each page may have up to one special garbage section. The start of this
// section is denoted by the top field in the space. The end of the section
// is denoted by the limit field in the space. This special garbage section
// is not marked with a free space map in the data. The point of this section
// is to enable linear allocation without having to constantly update the byte
// array every time the top field is updated and a new object is created. The
// special garbage section is not in the chain of garbage sections.
// Since the top and limit fields are in the space, not the page, only one page
// has a special garbage section, and if the top and limit are equal then there
// is no special garbage section.
// Some assertion macros used in the debugging mode.
#define DCHECK_PAGE_ALIGNED(address) \
DCHECK((OffsetFrom(address) & Page::kPageAlignmentMask) == 0)
#define DCHECK_OBJECT_ALIGNED(address) \
DCHECK((OffsetFrom(address) & kObjectAlignmentMask) == 0)
#define DCHECK_OBJECT_SIZE(size) \
DCHECK((0 < size) && (size <= Page::kMaxRegularHeapObjectSize))
#define DCHECK_CODEOBJECT_SIZE(size, code_space) \
DCHECK((0 < size) && (size <= code_space->AreaSize()))
#define DCHECK_PAGE_OFFSET(offset) \
DCHECK((Page::kObjectStartOffset <= offset) && (offset <= Page::kPageSize))
class MarkBit {
typedef uint32_t CellType;
inline MarkBit(CellType* cell, CellType mask) : cell_(cell), mask_(mask) {}
#ifdef DEBUG
bool operator==(const MarkBit& other) {
return cell_ == other.cell_ && mask_ == other.mask_;
inline CellType* cell() { return cell_; }
inline CellType mask() { return mask_; }
inline MarkBit Next() {
CellType new_mask = mask_ << 1;
if (new_mask == 0) {
return MarkBit(cell_ + 1, 1);
} else {
return MarkBit(cell_, new_mask);
inline void Set() { *cell_ |= mask_; }
inline bool Get() { return (*cell_ & mask_) != 0; }
inline void Clear() { *cell_ &= ~mask_; }
CellType* cell_;
CellType mask_;
friend class Marking;
// Bitmap is a sequence of cells each containing fixed number of bits.
class Bitmap {
static const uint32_t kBitsPerCell = 32;
static const uint32_t kBitsPerCellLog2 = 5;
static const uint32_t kBitIndexMask = kBitsPerCell - 1;
static const uint32_t kBytesPerCell = kBitsPerCell / kBitsPerByte;
static const uint32_t kBytesPerCellLog2 = kBitsPerCellLog2 - kBitsPerByteLog2;
static const size_t kLength = (1 << kPageSizeBits) >> (kPointerSizeLog2);
static const size_t kSize =
(1 << kPageSizeBits) >> (kPointerSizeLog2 + kBitsPerByteLog2);
static int CellsForLength(int length) {
return (length + kBitsPerCell - 1) >> kBitsPerCellLog2;
int CellsCount() { return CellsForLength(kLength); }
static int SizeFor(int cells_count) {
return sizeof(MarkBit::CellType) * cells_count;
INLINE(static uint32_t IndexToCell(uint32_t index)) {
return index >> kBitsPerCellLog2;
V8_INLINE static uint32_t IndexInCell(uint32_t index) {
return index & kBitIndexMask;
INLINE(static uint32_t CellToIndex(uint32_t index)) {
return index << kBitsPerCellLog2;
INLINE(static uint32_t CellAlignIndex(uint32_t index)) {
return (index + kBitIndexMask) & ~kBitIndexMask;
INLINE(MarkBit::CellType* cells()) {
return reinterpret_cast<MarkBit::CellType*>(this);
INLINE(Address address()) { return reinterpret_cast<Address>(this); }
INLINE(static Bitmap* FromAddress(Address addr)) {
return reinterpret_cast<Bitmap*>(addr);
inline MarkBit MarkBitFromIndex(uint32_t index) {
MarkBit::CellType mask = 1u << IndexInCell(index);
MarkBit::CellType* cell = this->cells() + (index >> kBitsPerCellLog2);
return MarkBit(cell, mask);
static inline void Clear(MemoryChunk* chunk);
static inline void SetAllBits(MemoryChunk* chunk);
static void PrintWord(uint32_t word, uint32_t himask = 0) {
for (uint32_t mask = 1; mask != 0; mask <<= 1) {
if ((mask & himask) != 0) PrintF("[");
PrintF((mask & word) ? "1" : "0");
if ((mask & himask) != 0) PrintF("]");
class CellPrinter {
CellPrinter() : seq_start(0), seq_type(0), seq_length(0) {}
void Print(uint32_t pos, uint32_t cell) {
if (cell == seq_type) {
if (IsSeq(cell)) {
seq_start = pos;
seq_length = 0;
seq_type = cell;
PrintF("%d: ", pos);
void Flush() {
if (seq_length > 0) {
PrintF("%d: %dx%d\n", seq_start, seq_type == 0 ? 0 : 1,
seq_length * kBitsPerCell);
seq_length = 0;
static bool IsSeq(uint32_t cell) { return cell == 0 || cell == 0xFFFFFFFF; }
uint32_t seq_start;
uint32_t seq_type;
uint32_t seq_length;
void Print() {
CellPrinter printer;
for (int i = 0; i < CellsCount(); i++) {
printer.Print(i, cells()[i]);
bool IsClean() {
for (int i = 0; i < CellsCount(); i++) {
if (cells()[i] != 0) {
return false;
return true;
// Clears all bits starting from {cell_base_index} up to and excluding
// {index}. Note that {cell_base_index} is required to be cell aligned.
void ClearRange(uint32_t cell_base_index, uint32_t index) {
DCHECK_EQ(IndexInCell(cell_base_index), 0u);
DCHECK_GE(index, cell_base_index);
uint32_t start_cell_index = IndexToCell(cell_base_index);
uint32_t end_cell_index = IndexToCell(index);
DCHECK_GE(end_cell_index, start_cell_index);
// Clear all cells till the cell containing the last index.
for (uint32_t i = start_cell_index; i < end_cell_index; i++) {
cells()[i] = 0;
// Clear all bits in the last cell till the last bit before index.
uint32_t clear_mask = ~((1u << IndexInCell(index)) - 1);
cells()[end_cell_index] &= clear_mask;
enum FreeListCategoryType {
kFirstCategory = kTiniest,
kLastCategory = kHuge,
kNumberOfCategories = kLastCategory + 1,
enum FreeMode { kLinkCategory, kDoNotLinkCategory };
// A free list category maintains a linked list of free memory blocks.
class FreeListCategory {
static const int kSize = kIntSize + // FreeListCategoryType type_
kIntSize + // int available_
kPointerSize + // FreeSpace* top_
kPointerSize + // FreeListCategory* prev_
kPointerSize; // FreeListCategory* next_
: type_(kInvalidCategory),
next_(nullptr) {}
void Initialize(FreeListCategoryType type) {
type_ = type;
available_ = 0;
top_ = nullptr;
prev_ = nullptr;
next_ = nullptr;
void Invalidate();
void Reset();
void ResetStats() { Reset(); }
void RepairFreeList(Heap* heap);
// Relinks the category into the currently owning free list. Requires that the
// category is currently unlinked.
void Relink();
bool Free(FreeSpace* node, int size_in_bytes, FreeMode mode);
// Picks a node from the list and stores its size in |node_size|. Returns
// nullptr if the category is empty.
FreeSpace* PickNodeFromList(int* node_size);
// Performs a single try to pick a node of at least |minimum_size| from the
// category. Stores the actual size in |node_size|. Returns nullptr if no
// node is found.
FreeSpace* TryPickNodeFromList(int minimum_size, int* node_size);
// Picks a node of at least |minimum_size| from the category. Stores the
// actual size in |node_size|. Returns nullptr if no node is found.
FreeSpace* SearchForNodeInList(int minimum_size, int* node_size);
inline FreeList* owner();
inline bool is_linked();
bool is_empty() { return top() == nullptr; }
int available() const { return available_; }
#ifdef DEBUG
intptr_t SumFreeList();
int FreeListLength();
// For debug builds we accurately compute free lists lengths up until
// {kVeryLongFreeList} by manually walking the list.
static const int kVeryLongFreeList = 500;
inline Page* page();
FreeSpace* top() { return top_; }
void set_top(FreeSpace* top) { top_ = top; }
FreeListCategory* prev() { return prev_; }
void set_prev(FreeListCategory* prev) { prev_ = prev; }
FreeListCategory* next() { return next_; }
void set_next(FreeListCategory* next) { next_ = next; }
// |type_|: The type of this free list category.
FreeListCategoryType type_;
// |available_|: Total available bytes in all blocks of this free list
// category.
int available_;
// |top_|: Points to the top FreeSpace* in the free list category.
FreeSpace* top_;
FreeListCategory* prev_;
FreeListCategory* next_;
friend class FreeList;
friend class PagedSpace;
// MemoryChunk represents a memory region owned by a specific space.
// It is divided into the header and the body. Chunk start is always
// 1MB aligned. Start of the body is aligned so it can accommodate
// any heap object.
class MemoryChunk {
enum MemoryChunkFlags {
IN_FROM_SPACE, // Mutually exclusive with IN_TO_SPACE.
IN_TO_SPACE, // All pages in new space has one of these two set.
NEVER_EVACUATE, // May contain immortal immutables.
// Large objects can have a progress bar in their page header. These object
// are scanned in increments and will be kept black while being scanned.
// Even if the mutator writes to them they will be kept black and a white
// to grey transition is performed in the value.
// |PAGE_NEW_OLD_PROMOTION|: A page tagged with this flag has been promoted
// from new to old space during evacuation.
// A black page has all mark bits set to 1 (black). A black page currently
// cannot be iterated because it is not swept. Moreover live bytes are also
// not updated.
// This flag is intended to be used for testing. Works only when both
// FLAG_stress_compaction and FLAG_manual_evacuation_candidates_selection
// are set. It forces the page to become an evacuation candidate at next
// candidates selection cycle.
// This flag is intended to be used for testing.
// The memory chunk is already logically freed, however the actual freeing
// still has to be performed.
// |COMPACTION_WAS_ABORTED|: Indicates that the compaction in this page
// has been aborted and needs special handling by the sweeper.
// |ANCHOR|: Flag is set if page is an anchor.
// Last flag, keep at bottom.
// |kSweepingDone|: The page state when sweeping is complete or sweeping must
// not be performed on that page. Sweeper threads that are done with their
// work will set this value and not touch the page anymore.
// |kSweepingPending|: This page is ready for parallel sweeping.
// |kSweepingInProgress|: This page is currently swept by a sweeper thread.
enum ConcurrentSweepingState {
// Every n write barrier invocations we go to runtime even though
// we could have handled it in generated code. This lets us check
// whether we have hit the limit and should do some more marking.
static const int kWriteBarrierCounterGranularity = 500;
static const int kPointersToHereAreInterestingMask =
static const int kPointersFromHereAreInterestingMask =
static const int kEvacuationCandidateMask = 1 << EVACUATION_CANDIDATE;
static const int kSkipEvacuationSlotsRecordingMask =
static const intptr_t kAlignment =
(static_cast<uintptr_t>(1) << kPageSizeBits);
static const intptr_t kAlignmentMask = kAlignment - 1;
static const intptr_t kSizeOffset = 0;
static const intptr_t kFlagsOffset = kSizeOffset + kPointerSize;
static const intptr_t kLiveBytesOffset =
kSizeOffset + kPointerSize // size_t size
+ kIntptrSize // intptr_t flags_
+ kPointerSize // Address area_start_
+ kPointerSize // Address area_end_
+ 2 * kPointerSize // base::VirtualMemory reservation_
+ kPointerSize // Address owner_
+ kPointerSize // Heap* heap_
+ kIntSize; // int progress_bar_
static const size_t kOldToNewSlotsOffset =
kLiveBytesOffset + kIntSize; // int live_byte_count_
static const size_t kWriteBarrierCounterOffset =
kOldToNewSlotsOffset + kPointerSize // SlotSet* old_to_new_slots_;
+ kPointerSize // SlotSet* old_to_old_slots_;
+ kPointerSize // TypedSlotSet* typed_old_to_old_slots_;
+ kPointerSize; // SkipList* skip_list_;
static const size_t kMinHeaderSize =
kWriteBarrierCounterOffset +
kIntptrSize // intptr_t write_barrier_counter_
+ kPointerSize // AtomicValue high_water_mark_
+ kPointerSize // base::Mutex* mutex_
+ kPointerSize // base::AtomicWord concurrent_sweeping_
+ 2 * kPointerSize // AtomicNumber free-list statistics
+ kPointerSize // AtomicValue next_chunk_
+ kPointerSize // AtomicValue prev_chunk_
// FreeListCategory categories_[kNumberOfCategories]
+ FreeListCategory::kSize * kNumberOfCategories;
// We add some more space to the computed header size to amount for missing
// alignment requirements in our computation.
// Try to get kHeaderSize properly aligned on 32-bit and 64-bit machines.
static const size_t kHeaderSize = kMinHeaderSize;
static const int kBodyOffset =
CODE_POINTER_ALIGN(kHeaderSize + Bitmap::kSize);
// The start offset of the object area in a page. Aligned to both maps and
// code alignment to be suitable for both. Also aligned to 32 words because
// the marking bitmap is arranged in 32 bit chunks.
static const int kObjectStartAlignment = 32 * kPointerSize;
static const int kObjectStartOffset =
kBodyOffset - 1 +
(kObjectStartAlignment - (kBodyOffset - 1) % kObjectStartAlignment);
// Page size in bytes. This must be a multiple of the OS page size.
static const int kPageSize = 1 << kPageSizeBits;
static const intptr_t kPageAlignmentMask = (1 << kPageSizeBits) - 1;
static const int kAllocatableMemory = kPageSize - kObjectStartOffset;
static inline void IncrementLiveBytesFromMutator(HeapObject* object, int by);
static inline void IncrementLiveBytesFromGC(HeapObject* object, int by);
// Only works if the pointer is in the first kPageSize of the MemoryChunk.
static MemoryChunk* FromAddress(Address a) {
return reinterpret_cast<MemoryChunk*>(OffsetFrom(a) & ~kAlignmentMask);
static intptr_t OffsetInPage(Address a) {
return reinterpret_cast<intptr_t>(a) & kPageAlignmentMask;
static inline MemoryChunk* FromAnyPointerAddress(Heap* heap, Address addr);
static inline void UpdateHighWaterMark(Address mark) {
if (mark == nullptr) return;
// Need to subtract one from the mark because when a chunk is full the
// top points to the next address after the chunk, which effectively belongs
// to another chunk. See the comment to Page::FromTopOrLimit.
MemoryChunk* chunk = MemoryChunk::FromAddress(mark - 1);
intptr_t new_mark = static_cast<intptr_t>(mark - chunk->address());
intptr_t old_mark = 0;
do {
old_mark = chunk->high_water_mark_.Value();
} while ((new_mark > old_mark) &&
!chunk->high_water_mark_.TrySetValue(old_mark, new_mark));
static bool IsValid(MemoryChunk* chunk) { return chunk != nullptr; }
Address address() { return reinterpret_cast<Address>(this); }
base::Mutex* mutex() { return mutex_; }
bool Contains(Address addr) {
return addr >= area_start() && addr < area_end();
// Checks whether |addr| can be a limit of addresses in this page. It's a
// limit if it's in the page, or if it's just after the last byte of the page.
bool ContainsLimit(Address addr) {
return addr >= area_start() && addr <= area_end();
AtomicValue<ConcurrentSweepingState>& concurrent_sweeping_state() {
return concurrent_sweeping_;
// Manage live byte count, i.e., count of bytes in black objects.
inline void ResetLiveBytes();
inline void IncrementLiveBytes(int by);
int LiveBytes() {
DCHECK_LE(static_cast<unsigned>(live_byte_count_), size_);
DCHECK(!IsFlagSet(BLACK_PAGE) || live_byte_count_ == 0);
return live_byte_count_;
void SetLiveBytes(int live_bytes) {
if (IsFlagSet(BLACK_PAGE)) return;
DCHECK_GE(live_bytes, 0);
DCHECK_LE(static_cast<size_t>(live_bytes), size_);
live_byte_count_ = live_bytes;
int write_barrier_counter() {
return static_cast<int>(write_barrier_counter_);
void set_write_barrier_counter(int counter) {
write_barrier_counter_ = counter;
size_t size() const { return size_; }
inline Heap* heap() const { return heap_; }
inline SkipList* skip_list() { return skip_list_; }
inline void set_skip_list(SkipList* skip_list) { skip_list_ = skip_list; }
inline SlotSet* old_to_new_slots() { return old_to_new_slots_; }
inline SlotSet* old_to_old_slots() { return old_to_old_slots_; }
inline TypedSlotSet* typed_old_to_old_slots() {
return typed_old_to_old_slots_;
void AllocateOldToNewSlots();
void ReleaseOldToNewSlots();
void AllocateOldToOldSlots();
void ReleaseOldToOldSlots();
void AllocateTypedOldToOldSlots();
void ReleaseTypedOldToOldSlots();
Address area_start() { return area_start_; }
Address area_end() { return area_end_; }
int area_size() { return static_cast<int>(area_end() - area_start()); }
bool CommitArea(size_t requested);
// Approximate amount of physical memory committed for this chunk.
size_t CommittedPhysicalMemory() { return high_water_mark_.Value(); }
int progress_bar() {
return progress_bar_;
void set_progress_bar(int progress_bar) {
progress_bar_ = progress_bar;
void ResetProgressBar() {
if (IsFlagSet(MemoryChunk::HAS_PROGRESS_BAR)) {
inline Bitmap* markbits() {
return Bitmap::FromAddress(address() + kHeaderSize);
inline uint32_t AddressToMarkbitIndex(Address addr) {
return static_cast<uint32_t>(addr - this->address()) >> kPointerSizeLog2;
inline Address MarkbitIndexToAddress(uint32_t index) {
return this->address() + (index << kPointerSizeLog2);
void PrintMarkbits() { markbits()->Print(); }
void SetFlag(int flag) { flags_ |= static_cast<uintptr_t>(1) << flag; }
void ClearFlag(int flag) { flags_ &= ~(static_cast<uintptr_t>(1) << flag); }
bool IsFlagSet(int flag) {
return (flags_ & (static_cast<uintptr_t>(1) << flag)) != 0;
// Set or clear multiple flags at a time. The flags in the mask are set to
// the value in "flags", the rest retain the current value in |flags_|.
void SetFlags(intptr_t flags, intptr_t mask) {
flags_ = (flags_ & ~mask) | (flags & mask);
// Return all current flags.
intptr_t GetFlags() { return flags_; }
bool NeverEvacuate() { return IsFlagSet(NEVER_EVACUATE); }
void MarkNeverEvacuate() { SetFlag(NEVER_EVACUATE); }
bool IsEvacuationCandidate() {
bool CanAllocate() {
return !IsEvacuationCandidate() && !IsFlagSet(NEVER_ALLOCATE_ON_PAGE);
bool ShouldSkipEvacuationSlotRecording() {
return (flags_ & kSkipEvacuationSlotsRecordingMask) != 0;
Executability executable() {
bool InNewSpace() {
return (flags_ & ((1 << IN_FROM_SPACE) | (1 << IN_TO_SPACE))) != 0;
bool InToSpace() { return IsFlagSet(IN_TO_SPACE); }
bool InFromSpace() { return IsFlagSet(IN_FROM_SPACE); }
MemoryChunk* next_chunk() { return next_chunk_.Value(); }
MemoryChunk* prev_chunk() { return prev_chunk_.Value(); }
void set_next_chunk(MemoryChunk* next) { next_chunk_.SetValue(next); }
void set_prev_chunk(MemoryChunk* prev) { prev_chunk_.SetValue(prev); }
Space* owner() const {
if ((reinterpret_cast<intptr_t>(owner_) & kPageHeaderTagMask) ==
kPageHeaderTag) {
return reinterpret_cast<Space*>(reinterpret_cast<intptr_t>(owner_) -
} else {
return nullptr;
void set_owner(Space* space) {
DCHECK((reinterpret_cast<intptr_t>(space) & kPageHeaderTagMask) == 0);
owner_ = reinterpret_cast<Address>(space) + kPageHeaderTag;
DCHECK((reinterpret_cast<intptr_t>(owner_) & kPageHeaderTagMask) ==
bool HasPageHeader() { return owner() != nullptr; }
void InsertAfter(MemoryChunk* other);
void Unlink();
static MemoryChunk* Initialize(Heap* heap, Address base, size_t size,
Address area_start, Address area_end,
Executability executable, Space* owner,
base::VirtualMemory* reservation);
// Should be called when memory chunk is about to be freed.
void ReleaseAllocatedMemory();
base::VirtualMemory* reserved_memory() { return &reservation_; }
size_t size_;
intptr_t flags_;
// Start and end of allocatable memory on this chunk.
Address area_start_;
Address area_end_;
// If the chunk needs to remember its memory reservation, it is stored here.
base::VirtualMemory reservation_;
// The identity of the owning space. This is tagged as a failure pointer, but
// no failure can be in an object, so this can be distinguished from any entry
// in a fixed array.
Address owner_;
Heap* heap_;
// Used by the incremental marker to keep track of the scanning progress in
// large objects that have a progress bar and are scanned in increments.
int progress_bar_;
// Count of bytes marked black on page.
int live_byte_count_;
// A single slot set for small pages (of size kPageSize) or an array of slot
// set for large pages. In the latter case the number of entries in the array
// is ceil(size() / kPageSize).
SlotSet* old_to_new_slots_;
SlotSet* old_to_old_slots_;
TypedSlotSet* typed_old_to_old_slots_;
SkipList* skip_list_;
intptr_t write_barrier_counter_;
// Assuming the initial allocation on a page is sequential,
// count highest number of bytes ever allocated on the page.
AtomicValue<intptr_t> high_water_mark_;
base::Mutex* mutex_;
AtomicValue<ConcurrentSweepingState> concurrent_sweeping_;
// PagedSpace free-list statistics.
AtomicNumber<intptr_t> available_in_free_list_;
AtomicNumber<intptr_t> wasted_memory_;
// next_chunk_ holds a pointer of type MemoryChunk
AtomicValue<MemoryChunk*> next_chunk_;
// prev_chunk_ holds a pointer of type MemoryChunk
AtomicValue<MemoryChunk*> prev_chunk_;
FreeListCategory categories_[kNumberOfCategories];
void InitializeReservedMemory() { reservation_.Reset(); }
friend class MemoryAllocator;
friend class MemoryChunkValidator;
// -----------------------------------------------------------------------------
// A page is a memory chunk of a size 1MB. Large object pages may be larger.
// The only way to get a page pointer is by calling factory methods:
// Page* p = Page::FromAddress(addr); or
// Page* p = Page::FromTopOrLimit(top);
class Page : public MemoryChunk {
static const intptr_t kCopyAllFlags = ~0;
// Page flags copied from from-space to to-space when flipping semispaces.
static const intptr_t kCopyOnFlipFlagsMask =
// Maximum object size that gets allocated into regular pages. Objects larger
// than that size are allocated in large object space and are never moved in
// memory. This also applies to new space allocation, since objects are never
// migrated from new space to large object space. Takes double alignment into
// account.
// TODO(hpayer): This limit should be way smaller but we currently have
// short living objects >256K.
static const int kMaxRegularHeapObjectSize = 600 * KB;
static inline Page* ConvertNewToOld(Page* old_page, PagedSpace* new_owner);
// Returns the page containing a given address. The address ranges
// from [page_addr .. page_addr + kPageSize[. This only works if the object
// is in fact in a page.
static Page* FromAddress(Address addr) {
return reinterpret_cast<Page*>(OffsetFrom(addr) & ~kPageAlignmentMask);
// Returns the page containing the address provided. The address can
// potentially point righter after the page. To be also safe for tagged values
// we subtract a hole word. The valid address ranges from
// [page_addr + kObjectStartOffset .. page_addr + kPageSize + kPointerSize].
static Page* FromAllocationAreaAddress(Address address) {
return Page::FromAddress(address - kPointerSize);
// Checks if address1 and address2 are on the same new space page.
static bool OnSamePage(Address address1, Address address2) {
return Page::FromAddress(address1) == Page::FromAddress(address2);
// Checks whether an address is page aligned.
static bool IsAlignedToPageSize(Address addr) {
return (OffsetFrom(addr) & kPageAlignmentMask) == 0;
static bool IsAtObjectStart(Address addr) {
return (reinterpret_cast<intptr_t>(addr) & kPageAlignmentMask) ==
inline static Page* FromAnyPointerAddress(Heap* heap, Address addr);
// Create a Page object that is only used as anchor for the doubly-linked
// list of real pages.
explicit Page(Space* owner) { InitializeAsAnchor(owner); }
inline void MarkNeverAllocateForTesting();
inline void MarkEvacuationCandidate();
inline void ClearEvacuationCandidate();
Page* next_page() { return static_cast<Page*>(next_chunk()); }
Page* prev_page() { return static_cast<Page*>(prev_chunk()); }
void set_next_page(Page* page) { set_next_chunk(page); }
void set_prev_page(Page* page) { set_prev_chunk(page); }
template <typename Callback>
inline void ForAllFreeListCategories(Callback callback) {
for (int i = kFirstCategory; i < kNumberOfCategories; i++) {
// Returns the offset of a given address to this page.
inline int Offset(Address a) {
int offset = static_cast<int>(a - address());
return offset;
// Returns the address for a given offset to the this page.
Address OffsetToAddress(int offset) {
return address() + offset;
// WaitUntilSweepingCompleted only works when concurrent sweeping is in
// progress. In particular, when we know that right before this call a
// sweeper thread was sweeping this page.
void WaitUntilSweepingCompleted() {
bool SweepingDone() {
return concurrent_sweeping_state().Value() == kSweepingDone;
void ResetFreeListStatistics();
int LiveBytesFromFreeList() {
return static_cast<int>(area_size() - wasted_memory() -
FreeListCategory* free_list_category(FreeListCategoryType type) {
return &categories_[type];
bool is_anchor() { return IsFlagSet(Page::ANCHOR); }
intptr_t wasted_memory() { return wasted_memory_.Value(); }
void add_wasted_memory(intptr_t waste) { wasted_memory_.Increment(waste); }
intptr_t available_in_free_list() { return available_in_free_list_.Value(); }
void add_available_in_free_list(intptr_t available) {
#ifdef DEBUG
void Print();
#endif // DEBUG
enum InitializationMode { kFreeMemory, kDoNotFreeMemory };
template <InitializationMode mode = kFreeMemory>
static inline Page* Initialize(Heap* heap, MemoryChunk* chunk,
Executability executable, PagedSpace* owner);
static inline Page* Initialize(Heap* heap, MemoryChunk* chunk,
Executability executable, SemiSpace* owner);
inline void InitializeFreeListCategories();
void InitializeAsAnchor(Space* owner);
friend class MemoryAllocator;
class LargePage : public MemoryChunk {
HeapObject* GetObject() { return HeapObject::FromAddress(area_start()); }
inline LargePage* next_page() {
return static_cast<LargePage*>(next_chunk());
inline void set_next_page(LargePage* page) { set_next_chunk(page); }
// A limit to guarantee that we do not overflow typed slot offset in
// the old to old remembered set.
// Note that this limit is higher than what assembler already imposes on
// x64 and ia32 architectures.
static const int kMaxCodePageSize = 512 * MB;
static inline LargePage* Initialize(Heap* heap, MemoryChunk* chunk,
Executability executable, Space* owner);
friend class MemoryAllocator;
// ----------------------------------------------------------------------------
// Space is the abstract superclass for all allocation spaces.
class Space : public Malloced {
Space(Heap* heap, AllocationSpace id, Executability executable)
: allocation_observers_(new List<AllocationObserver*>()),
max_committed_(0) {}
virtual ~Space() {}
Heap* heap() const { return heap_; }
// Does the space need executable memory?
Executability executable() { return executable_; }
// Identity used in error reporting.
AllocationSpace identity() { return id_; }
virtual void AddAllocationObserver(AllocationObserver* observer) {
virtual void RemoveAllocationObserver(AllocationObserver* observer) {
bool removed = allocation_observers_->RemoveElement(observer);
virtual void PauseAllocationObservers() {
allocation_observers_paused_ = true;
virtual void ResumeAllocationObservers() {
allocation_observers_paused_ = false;
void AllocationStep(Address soon_object, int size);
// Return the total amount committed memory for this space, i.e., allocatable
// memory and page headers.
virtual intptr_t CommittedMemory() { return committed_; }
virtual intptr_t MaximumCommittedMemory() { return max_committed_; }
// Returns allocated size.
virtual intptr_t Size() = 0;
// Returns size of objects. Can differ from the allocated size
// (e.g. see LargeObjectSpace).
virtual intptr_t SizeOfObjects() { return Size(); }
// Approximate amount of physical memory committed for this space.
virtual size_t CommittedPhysicalMemory() = 0;
// Return the available bytes without growing.
virtual intptr_t Available() = 0;
virtual int RoundSizeDownToObjectAlignment(int size) {
if (id_ == CODE_SPACE) {
return RoundDown(size, kCodeAlignment);
} else {
return RoundDown(size, kPointerSize);
void AccountCommitted(intptr_t bytes) {
DCHECK_GE(bytes, 0);
committed_ += bytes;
if (committed_ > max_committed_) {
max_committed_ = committed_;
void AccountUncommitted(intptr_t bytes) {
DCHECK_GE(bytes, 0);
committed_ -= bytes;
DCHECK_GE(committed_, 0);
#ifdef DEBUG
virtual void Print() = 0;
v8::base::SmartPointer<List<AllocationObserver*>> allocation_observers_;
bool allocation_observers_paused_;
Heap* heap_;
AllocationSpace id_;
Executability executable_;
// Keeps track of committed memory in a space.
intptr_t committed_;
intptr_t max_committed_;
class MemoryChunkValidator {
// Computed offsets should match the compiler generated ones.
STATIC_ASSERT(MemoryChunk::kSizeOffset == offsetof(MemoryChunk, size_));
STATIC_ASSERT(MemoryChunk::kLiveBytesOffset ==
offsetof(MemoryChunk, live_byte_count_));
STATIC_ASSERT(MemoryChunk::kOldToNewSlotsOffset ==
offsetof(MemoryChunk, old_to_new_slots_));
STATIC_ASSERT(MemoryChunk::kWriteBarrierCounterOffset ==
offsetof(MemoryChunk, write_barrier_counter_));
// Validate our estimates on the header size.
STATIC_ASSERT(sizeof(MemoryChunk) <= MemoryChunk::kHeaderSize);
STATIC_ASSERT(sizeof(LargePage) <= MemoryChunk::kHeaderSize);
STATIC_ASSERT(sizeof(Page) <= MemoryChunk::kHeaderSize);
// ----------------------------------------------------------------------------
// All heap objects containing executable code (code objects) must be allocated
// from a 2 GB range of memory, so that they can call each other using 32-bit
// displacements. This happens automatically on 32-bit platforms, where 32-bit
// displacements cover the entire 4GB virtual address space. On 64-bit
// platforms, we support this using the CodeRange object, which reserves and
// manages a range of virtual memory.
class CodeRange {
explicit CodeRange(Isolate* isolate);
~CodeRange() { TearDown(); }
// Reserves a range of virtual memory, but does not commit any of it.
// Can only be called once, at heap initialization time.
// Returns false on failure.
bool SetUp(size_t requested_size);
bool valid() { return code_range_ != NULL; }
Address start() {
return static_cast<Address>(code_range_->address());
size_t size() {
return code_range_->size();
bool contains(Address address) {
if (!valid()) return false;
Address start = static_cast<Address>(code_range_->address());
return start <= address && address < start + code_range_->size();
// Allocates a chunk of memory from the large-object portion of
// the code range. On platforms with no separate code range, should
// not be called.
MUST_USE_RESULT Address AllocateRawMemory(const size_t requested_size,
const size_t commit_size,
size_t* allocated);
bool CommitRawMemory(Address start, size_t length);
bool UncommitRawMemory(Address start, size_t length);
void FreeRawMemory(Address buf, size_t length);
// Frees the range of virtual memory, and frees the data structures used to
// manage it.
void TearDown();
Isolate* isolate_;
// The reserved range of virtual memory that all code objects are put in.
base::VirtualMemory* code_range_;
// Plain old data class, just a struct plus a constructor.
class FreeBlock {
FreeBlock() : start(0), size(0) {}
FreeBlock(Address start_arg, size_t size_arg)
: start(start_arg), size(size_arg) {
DCHECK(IsAddressAligned(start, MemoryChunk::kAlignment));
DCHECK(size >= static_cast<size_t>(Page::kPageSize));
FreeBlock(void* start_arg, size_t size_arg)
: start(static_cast<Address>(start_arg)), size(size_arg) {
DCHECK(IsAddressAligned(start, MemoryChunk::kAlignment));
DCHECK(size >= static_cast<size_t>(Page::kPageSize));
Address start;
size_t size;
// The global mutex guards free_list_ and allocation_list_ as GC threads may
// access both lists concurrently to the main thread.
base::Mutex code_range_mutex_;
// Freed blocks of memory are added to the free list. When the allocation
// list is exhausted, the free list is sorted and merged to make the new
// allocation list.
List<FreeBlock> free_list_;
// Memory is allocated from the free blocks on the allocation list.
// The block at current_allocation_block_index_ is the current block.
List<FreeBlock> allocation_list_;
int current_allocation_block_index_;
// Finds a block on the allocation list that contains at least the
// requested amount of memory. If none is found, sorts and merges
// the existing free memory blocks, and searches again.
// If none can be found, returns false.
bool GetNextAllocationBlock(size_t requested);
// Compares the start addresses of two free blocks.
static int CompareFreeBlockAddress(const FreeBlock* left,
const FreeBlock* right);
bool ReserveBlock(const size_t requested_size, FreeBlock* block);
void ReleaseBlock(const FreeBlock* block);
class SkipList {
SkipList() { Clear(); }
void Clear() {
for (int idx = 0; idx < kSize; idx++) {
starts_[idx] = reinterpret_cast<Address>(-1);
Address StartFor(Address addr) { return starts_[RegionNumber(addr)]; }
void AddObject(Address addr, int size) {
int start_region = RegionNumber(addr);
int end_region = RegionNumber(addr + size - kPointerSize);
for (int idx = start_region; idx <= end_region; idx++) {
if (starts_[idx] > addr) {
starts_[idx] = addr;
} else {
// In the first region, there may already be an object closer to the
// start of the region. Do not change the start in that case. If this
// is not the first region, you probably added overlapping objects.
DCHECK_EQ(start_region, idx);
static inline int RegionNumber(Address addr) {
return (OffsetFrom(addr) & Page::kPageAlignmentMask) >> kRegionSizeLog2;
static void Update(Address addr, int size) {
Page* page = Page::FromAddress(addr);
SkipList* list = page->skip_list();
if (list == NULL) {
list = new SkipList();
list->AddObject(addr, size);
static const int kRegionSizeLog2 = 13;
static const int kRegionSize = 1 << kRegionSizeLog2;
static const int kSize = Page::kPageSize / kRegionSize;
STATIC_ASSERT(Page::kPageSize % kRegionSize == 0);
Address starts_[kSize];
// ----------------------------------------------------------------------------
// A space acquires chunks of memory from the operating system. The memory
// allocator allocated and deallocates pages for the paged heap spaces and large
// pages for large object space.
// Each space has to manage it's own pages.
class MemoryAllocator {
enum AllocationMode {
explicit MemoryAllocator(Isolate* isolate);
// Initializes its internal bookkeeping structures.
// Max capacity of the total space and executable memory limit.
bool SetUp(intptr_t max_capacity, intptr_t capacity_executable,
intptr_t code_range_size);
void TearDown();
// Allocates a Page from the allocator. AllocationMode is used to indicate
// whether pooled allocation, which only works for MemoryChunk::kPageSize,
// should be tried first.
template <MemoryAllocator::AllocationMode alloc_mode = kRegular,
typename SpaceType>
Page* AllocatePage(intptr_t size, SpaceType* owner, Executability executable);
LargePage* AllocateLargePage(intptr_t size, LargeObjectSpace* owner,
Executability executable);
// PreFree logically frees the object, i.e., it takes care of the size
// bookkeeping and calls the allocation callback.
void PreFreeMemory(MemoryChunk* chunk);
// FreeMemory can be called concurrently when PreFree was executed before.
void PerformFreeMemory(MemoryChunk* chunk);
// Free is a wrapper method. For kRegular AllocationMode it calls PreFree and
// PerformFreeMemory together. For kPooled it will dispatch to pooled free.
template <MemoryAllocator::AllocationMode mode = kRegular>
void Free(MemoryChunk* chunk);
// Returns allocated spaces in bytes.
intptr_t Size() { return size_.Value(); }
// Returns allocated executable spaces in bytes.
intptr_t SizeExecutable() { return size_executable_.Value(); }
// Returns the maximum available bytes of heaps.
intptr_t Available() {
intptr_t size = Size();
return capacity_ < size ? 0 : capacity_ - size;
// Returns the maximum available executable bytes of heaps.
intptr_t AvailableExecutable() {
intptr_t executable_size = SizeExecutable();
if (capacity_executable_ < executable_size) return 0;
return capacity_executable_ - executable_size;
// Returns maximum available bytes that the old space can have.
intptr_t MaxAvailable() {
return (Available() / Page::kPageSize) * Page::kAllocatableMemory;
// Returns an indication of whether a pointer is in a space that has
// been allocated by this MemoryAllocator.
V8_INLINE bool IsOutsideAllocatedSpace(const void* address) {
return address < lowest_ever_allocated_.Value() ||
address >= highest_ever_allocated_.Value();
#ifdef DEBUG
// Reports statistic info of the space.
void ReportStatistics();
// Returns a MemoryChunk in which the memory region from commit_area_size to
// reserve_area_size of the chunk area is reserved but not committed, it
// could be committed later by calling MemoryChunk::CommitArea.
MemoryChunk* AllocateChunk(intptr_t reserve_area_size,
intptr_t commit_area_size,
Executability executable, Space* space);
Address ReserveAlignedMemory(size_t requested, size_t alignment,
base::VirtualMemory* controller);
Address AllocateAlignedMemory(size_t reserve_size, size_t commit_size,
size_t alignment, Executability executable,
base::VirtualMemory* controller);
bool CommitMemory(Address addr, size_t size, Executability executable);
void FreeMemory(base::VirtualMemory* reservation, Executability executable);
void FreeMemory(Address addr, size_t size, Executability executable);
// Commit a contiguous block of memory from the initial chunk. Assumes that
// the address is not NULL, the size is greater than zero, and that the
// block is contained in the initial chunk. Returns true if it succeeded
// and false otherwise.
bool CommitBlock(Address start, size_t size, Executability executable);
// Uncommit a contiguous block of memory [start..(start+size)[.
// start is not NULL, the size is greater than zero, and the
// block is contained in the initial chunk. Returns true if it succeeded
// and false otherwise.
bool UncommitBlock(Address start, size_t size);
// Zaps a contiguous block of memory [start..(start+size)[ thus
// filling it up with a recognizable non-NULL bit pattern.
void ZapBlock(Address start, size_t size);
void PerformAllocationCallback(ObjectSpace space, AllocationAction action,
size_t size);
void AddMemoryAllocationCallback(MemoryAllocationCallback callback,
ObjectSpace space, AllocationAction action);
void RemoveMemoryAllocationCallback(MemoryAllocationCallback callback);
bool MemoryAllocationCallbackRegistered(MemoryAllocationCallback callback);
static int CodePageGuardStartOffset();
static int CodePageGuardSize();
static int CodePageAreaStartOffset();
static int CodePageAreaEndOffset();
static int CodePageAreaSize() {
return CodePageAreaEndOffset() - CodePageAreaStartOffset();
static int PageAreaSize(AllocationSpace space) {
return (space == CODE_SPACE) ? CodePageAreaSize()
: Page::kAllocatableMemory;
MUST_USE_RESULT bool CommitExecutableMemory(base::VirtualMemory* vm,
Address start, size_t commit_size,
size_t reserved_size);
CodeRange* code_range() { return code_range_; }
// See AllocatePage for public interface. Note that currently we only support
// pools for NOT_EXECUTABLE pages of size MemoryChunk::kPageSize.
template <typename SpaceType>
MemoryChunk* AllocatePagePooled(SpaceType* owner);
// Free that chunk into the pool.
void FreePooled(MemoryChunk* chunk);
Isolate* isolate_;
CodeRange* code_range_;
// Maximum space size in bytes.
intptr_t capacity_;
// Maximum subset of capacity_ that can be executable
intptr_t capacity_executable_;
// Allocated space size in bytes.
AtomicNumber<intptr_t> size_;
// Allocated executable space size in bytes.
AtomicNumber<intptr_t> size_executable_;
// We keep the lowest and highest addresses allocated as a quick way
// of determining that pointers are outside the heap. The estimate is
// conservative, i.e. not all addrsses in 'allocated' space are allocated
// to our heap. The range is [lowest, highest[, inclusive on the low end
// and exclusive on the high end.
AtomicValue<void*> lowest_ever_allocated_;
AtomicValue<void*> highest_ever_allocated_;
struct MemoryAllocationCallbackRegistration {
MemoryAllocationCallbackRegistration(MemoryAllocationCallback callback,
ObjectSpace space,
AllocationAction action)
: callback(callback), space(space), action(action) {}
MemoryAllocationCallback callback;
ObjectSpace space;
AllocationAction action;
// A List of callback that are triggered when memory is allocated or free'd
List<MemoryAllocationCallbackRegistration> memory_allocation_callbacks_;
// Initializes pages in a chunk. Returns the first page address.
// This function and GetChunkId() are provided for the mark-compact
// collector to rebuild page headers in the from space, which is
// used as a marking stack and its page headers are destroyed.
Page* InitializePagesInChunk(int chunk_id, int pages_in_chunk,
PagedSpace* owner);
void UpdateAllocatedSpaceLimits(void* low, void* high) {
// The use of atomic primitives does not guarantee correctness (wrt.
// desired semantics) by default. The loop here ensures that we update the
// values only if they did not change in between.
void* ptr = nullptr;
do {
ptr = lowest_ever_allocated_.Value();
} while ((low < ptr) && !lowest_ever_allocated_.TrySetValue(ptr, low));
do {
ptr = highest_ever_allocated_.Value();
} while ((high > ptr) && !highest_ever_allocated_.TrySetValue(ptr, high));
List<MemoryChunk*> chunk_pool_;
base::VirtualMemory last_chunk_;
friend class TestCodeRangeScope;
// -----------------------------------------------------------------------------
// Interface for heap object iterator to be implemented by all object space
// object iterators.
// NOTE: The space specific object iterators also implements the own next()
// method which is used to avoid using virtual functions
// iterating a specific space.
class ObjectIterator : public Malloced {
virtual ~ObjectIterator() {}
virtual HeapObject* next_object() = 0;
// -----------------------------------------------------------------------------
// Heap object iterator in new/old/map spaces.
// A HeapObjectIterator iterates objects from the bottom of the given space
// to its top or from the bottom of the given page to its top.
// If objects are allocated in the page during iteration the iterator may
// or may not iterate over those objects. The caller must create a new
// iterator in order to be sure to visit these new objects.
class HeapObjectIterator : public ObjectIterator {
// Creates a new object iterator in a given space.
explicit HeapObjectIterator(PagedSpace* space);
explicit HeapObjectIterator(Page* page);
// Advance to the next object, skipping free spaces and other fillers and
// skipping the special garbage section of which there is one per space.
// Returns NULL when the iteration has ended.
inline HeapObject* Next();
inline HeapObject* next_object() override;
enum PageMode { kOnePageOnly, kAllPagesInSpace };
Address cur_addr_; // Current iteration point.
Address cur_end_; // End iteration point.
PagedSpace* space_;
PageMode page_mode_;
// Fast (inlined) path of next().
inline HeapObject* FromCurrentPage();
// Slow path of next(), goes into the next page. Returns false if the
// iteration has ended.
bool AdvanceToNextPage();
// Initializes fields.
inline void Initialize(PagedSpace* owner, Address start, Address end,
PageMode mode);
// -----------------------------------------------------------------------------
// A PageIterator iterates the pages in a paged space.
class PageIterator BASE_EMBEDDED {
explicit inline PageIterator(PagedSpace* space);
inline bool has_next();
inline Page* next();
PagedSpace* space_;
Page* prev_page_; // Previous page returned.
// Next page that will be returned. Cached here so that we can use this
// iterator for operations that deallocate pages.
Page* next_page_;
// -----------------------------------------------------------------------------
// A space has a circular list of pages. The next page can be accessed via
// Page::next_page() call.
// An abstraction of allocation and relocation pointers in a page-structured
// space.
class AllocationInfo {
AllocationInfo() : top_(nullptr), limit_(nullptr) {}
AllocationInfo(Address top, Address limit) : top_(top), limit_(limit) {}
void Reset(Address top, Address limit) {
INLINE(void set_top(Address top)) {
(reinterpret_cast<intptr_t>(top) & kHeapObjectTagMask) == 0);
top_ = top;
INLINE(Address top()) const {
(reinterpret_cast<intptr_t>(top_) & kHeapObjectTagMask) == 0);
return top_;
Address* top_address() { return &top_; }
INLINE(void set_limit(Address limit)) {
limit_ = limit;
INLINE(Address limit()) const {
return limit_;
Address* limit_address() { return &limit_; }
#ifdef DEBUG
bool VerifyPagedAllocation() {
return (Page::FromAllocationAreaAddress(top_) ==
Page::FromAllocationAreaAddress(limit_)) &&
(top_ <= limit_);
// Current allocation top.
Address top_;
// Current allocation limit.
Address limit_;
// An abstraction of the accounting statistics of a page-structured space.
// The stats are only set by functions that ensure they stay balanced. These
// functions increase or decrease one of the non-capacity stats in conjunction
// with capacity, or else they always balance increases and decreases to the
// non-capacity stats.
class AllocationStats BASE_EMBEDDED {
AllocationStats() { Clear(); }
// Zero out all the allocation statistics (i.e., no capacity).
void Clear() {
capacity_ = 0;
max_capacity_ = 0;
size_ = 0;
void ClearSize() { size_ = capacity_; }
// Accessors for the allocation statistics.
intptr_t Capacity() { return capacity_; }
intptr_t MaxCapacity() { return max_capacity_; }
intptr_t Size() {
CHECK_GE(size_, 0);
return size_;
// Grow the space by adding available bytes. They are initially marked as
// being in use (part of the size), but will normally be immediately freed,
// putting them on the free list and removing them from size_.
void ExpandSpace(int size_in_bytes) {
capacity_ += size_in_bytes;
size_ += size_in_bytes;
if (capacity_ > max_capacity_) {
max_capacity_ = capacity_;
CHECK(size_ >= 0);
// Shrink the space by removing available bytes. Since shrinking is done
// during sweeping, bytes have been marked as being in use (part of the size)
// and are hereby freed.
void ShrinkSpace(int size_in_bytes) {
capacity_ -= size_in_bytes;
size_ -= size_in_bytes;
CHECK_GE(size_, 0);
// Allocate from available bytes (available -> size).
void AllocateBytes(intptr_t size_in_bytes) {
size_ += size_in_bytes;
CHECK_GE(size_, 0);
// Free allocated bytes, making them available (size -> available).
void DeallocateBytes(intptr_t size_in_bytes) {
size_ -= size_in_bytes;
CHECK_GE(size_, 0);
// Merge {other} into {this}.
void Merge(const AllocationStats& other) {
capacity_ += other.capacity_;
size_ += other.size_;
if (other.max_capacity_ > max_capacity_) {
max_capacity_ = other.max_capacity_;
CHECK_GE(size_, 0);
void DecreaseCapacity(intptr_t size_in_bytes) {
capacity_ -= size_in_bytes;
CHECK_GE(capacity_, 0);
CHECK_GE(capacity_, size_);
void IncreaseCapacity(intptr_t size_in_bytes) { capacity_ += size_in_bytes; }
// |capacity_|: The number of object-area bytes (i.e., not including page
// bookkeeping structures) currently in the space.
intptr_t capacity_;
// |max_capacity_|: The maximum capacity ever observed.
intptr_t max_capacity_;
// |size_|: The number of allocated bytes.
intptr_t size_;
// A free list maintaining free blocks of memory. The free list is organized in
// a way to encourage objects allocated around the same time to be near each
// other. The normal way to allocate is intended to be by bumping a 'top'
// pointer until it hits a 'limit' pointer. When the limit is hit we need to
// find a new space to allocate from. This is done with the free list, which is
// divided up into rough categories to cut down on waste. Having finer
// categories would scatter allocation more.
// The free list is organized in categories as follows:
// kMinBlockSize-10 words (tiniest): The tiniest blocks are only used for
// allocation, when categories >= small do not have entries anymore.
// 11-31 words (tiny): The tiny blocks are only used for allocation, when
// categories >= small do not have entries anymore.
// 32-255 words (small): Used for allocating free space between 1-31 words in
// size.
// 256-2047 words (medium): Used for allocating free space between 32-255 words
// in size.
// 1048-16383 words (large): Used for allocating free space between 256-2047
// words in size.
// At least 16384 words (huge): This list is for objects of 2048 words or
// larger. Empty pages are also added to this list.
class FreeList {
// This method returns how much memory can be allocated after freeing
// maximum_freed memory.
static inline int GuaranteedAllocatable(int maximum_freed) {
if (maximum_freed <= kTiniestListMax) {
// Since we are not iterating over all list entries, we cannot guarantee
// that we can find the maximum freed block in that free list.
return 0;
} else if (maximum_freed <= kTinyListMax) {
return kTinyAllocationMax;
} else if (maximum_freed <= kSmallListMax) {
return kSmallAllocationMax;
} else if (maximum_freed <= kMediumListMax) {
return kMediumAllocationMax;
} else if (maximum_freed <= kLargeListMax) {
return kLargeAllocationMax;
return maximum_freed;
explicit FreeList(PagedSpace* owner);
// Adds a node on the free list. The block of size {size_in_bytes} starting
// at {start} is placed on the free list. The return value is the number of
// bytes that were not added to the free list, because they freed memory block
// was too small. Bookkeeping information will be written to the block, i.e.,
// its contents will be destroyed. The start address should be word aligned,
// and the size should be a non-zero multiple of the word size.
int Free(Address start, int size_in_bytes, FreeMode mode);
// Allocate a block of size {size_in_bytes} from the free list. The block is
// unitialized. A failure is returned if no block is available. The size
// should be a non-zero multiple of the word size.
MUST_USE_RESULT HeapObject* Allocate(int size_in_bytes);
// Clear the free list.
void Reset();
void ResetStats() {
[](FreeListCategory* category) { category->ResetStats(); });
// Return the number of bytes available on the free list.
intptr_t Available() {
intptr_t available = 0;
ForAllFreeListCategories([&available](FreeListCategory* category) {
available += category->available();
return available;
bool IsEmpty() {
bool empty = true;
ForAllFreeListCategories([&empty](FreeListCategory* category) {
if (!category->is_empty()) empty = false;
return empty;
// Used after booting the VM.
void RepairLists(Heap* heap);
intptr_t EvictFreeListItems(Page* page);
bool ContainsPageFreeListItems(Page* page);
PagedSpace* owner() { return owner_; }
intptr_t wasted_bytes() { return wasted_bytes_.Value(); }
template <typename Callback>
void ForAllFreeListCategories(FreeListCategoryType type, Callback callback) {
FreeListCategory* current = categories_[type];
while (current != nullptr) {
FreeListCategory* next = current->next();
current = next;
template <typename Callback>
void ForAllFreeListCategories(Callback callback) {
for (int i = kFirstCategory; i < kNumberOfCategories; i++) {
ForAllFreeListCategories(static_cast<FreeListCategoryType>(i), callback);
bool AddCategory(FreeListCategory* category);
void RemoveCategory(FreeListCategory* category);
void PrintCategories(FreeListCategoryType type);
#ifdef DEBUG
intptr_t SumFreeLists();
bool IsVeryLong();
class FreeListCategoryIterator {
FreeListCategoryIterator(FreeList* free_list, FreeListCategoryType type)
: current_(free_list->categories_[type]) {}
bool HasNext() { return current_ != nullptr; }
FreeListCategory* Next() {
FreeListCategory* tmp = current_;
current_ = current_->next();
return tmp;
FreeListCategory* current_;
// The size range of blocks, in bytes.
static const int kMinBlockSize = 3 * kPointerSize;
static const int kMaxBlockSize = Page::kAllocatableMemory;
static const int kTiniestListMax = 0xa * kPointerSize;
static const int kTinyListMax = 0x1f * kPointerSize;
static const int kSmallListMax = 0xff * kPointerSize;
static const int kMediumListMax = 0x7ff * kPointerSize;
static const int kLargeListMax = 0x3fff * kPointerSize;
static const int kTinyAllocationMax = kTiniestListMax;
static const int kSmallAllocationMax = kTinyListMax;
static const int kMediumAllocationMax = kSmallListMax;
static const int kLargeAllocationMax = kMediumListMax;
FreeSpace* FindNodeFor(int size_in_bytes, int* node_size);
// Walks all available categories for a given |type| and tries to retrieve
// a node. Returns nullptr if the category is empty.
FreeSpace* FindNodeIn(FreeListCategoryType type, int* node_size);
// Tries to retrieve a node from the first category in a given |type|.
// Returns nullptr if the category is empty.
FreeSpace* TryFindNodeIn(FreeListCategoryType type, int* node_size,
int minimum_size);
// Searches a given |type| for a node of at least |minimum_size|.
FreeSpace* SearchForNodeInList(FreeListCategoryType type, int* node_size,
int minimum_size);
FreeListCategoryType SelectFreeListCategoryType(size_t size_in_bytes) {
if (size_in_bytes <= kTiniestListMax) {
return kTiniest;
} else if (size_in_bytes <= kTinyListMax) {
return kTiny;
} else if (size_in_bytes <= kSmallListMax) {
return kSmall;
} else if (size_in_bytes <= kMediumListMax) {
return kMedium;
} else if (size_in_bytes <= kLargeListMax) {
return kLarge;
return kHuge;
// The tiny categories are not used for fast allocation.
FreeListCategoryType SelectFastAllocationFreeListCategoryType(
size_t size_in_bytes) {
if (size_in_bytes <= kSmallAllocationMax) {
return kSmall;
} else if (size_in_bytes <= kMediumAllocationMax) {
return kMedium;
} else if (size_in_bytes <= kLargeAllocationMax) {
return kLarge;
return kHuge;
FreeListCategory* top(FreeListCategoryType type) { return categories_[type]; }
PagedSpace* owner_;
AtomicNumber<intptr_t> wasted_bytes_;
FreeListCategory* categories_[kNumberOfCategories];
friend class FreeListCategory;
class AllocationResult {
// Implicit constructor from Object*.
AllocationResult(Object* object) // NOLINT
: object_(object) {
// AllocationResults can't return Smis, which are used to represent
// failure and the space to retry in.
AllocationResult() : object_(Smi::FromInt(NEW_SPACE)) {}
static inline AllocationResult Retry(AllocationSpace space = NEW_SPACE) {
return AllocationResult(space);
inline bool IsRetry() { return object_->IsSmi(); }
template <typename T>
bool To(T** obj) {
if (IsRetry()) return false;
*obj = T::cast(object_);
return true;
Object* ToObjectChecked() {
return object_;
inline AllocationSpace RetrySpace();
explicit AllocationResult(AllocationSpace space)
: object_(Smi::FromInt(static_cast<int>(space))) {}
Object* object_;
STATIC_ASSERT(sizeof(AllocationResult) == kPointerSize);
// LocalAllocationBuffer represents a linear allocation area that is created
// from a given {AllocationResult} and can be used to allocate memory without
// synchronization.
// The buffer is properly closed upon destruction and reassignment.
// Example:
// {
// AllocationResult result = ...;
// LocalAllocationBuffer a(heap, result, size);
// LocalAllocationBuffer b = a;
// CHECK(!a.IsValid());
// CHECK(b.IsValid());
// // {a} is invalid now and cannot be used for further allocations.
// }
// // Since {b} went out of scope, the LAB is closed, resulting in creating a
// // filler object for the remaining area.
class LocalAllocationBuffer {
// Indicates that a buffer cannot be used for allocations anymore. Can result
// from either reassigning a buffer, or trying to construct it from an
// invalid {AllocationResult}.
static inline LocalAllocationBuffer InvalidBuffer();
// Creates a new LAB from a given {AllocationResult}. Results in
// InvalidBuffer if the result indicates a retry.
static inline LocalAllocationBuffer FromResult(Heap* heap,
AllocationResult result,
intptr_t size);
~LocalAllocationBuffer() { Close(); }
// Convert to C++11 move-semantics once allowed by the style guide.
LocalAllocationBuffer(const LocalAllocationBuffer& other);
LocalAllocationBuffer& operator=(const LocalAllocationBuffer& other);
MUST_USE_RESULT inline AllocationResult AllocateRawAligned(
int size_in_bytes, AllocationAlignment alignment);
inline bool IsValid() { return != nullptr; }
// Try to merge LABs, which is only possible when they are adjacent in memory.
// Returns true if the merge was successful, false otherwise.
inline bool TryMerge(LocalAllocationBuffer* other);
LocalAllocationBuffer(Heap* heap, AllocationInfo allocation_info);
void Close();
Heap* heap_;
AllocationInfo allocation_info_;
class PagedSpace : public Space {
static const intptr_t kCompactionMemoryWanted = 500 * KB;
// Creates a space with an id.
PagedSpace(Heap* heap, AllocationSpace id, Executability executable);
~PagedSpace() override { TearDown(); }
// Set up the space using the given address range of virtual memory (from
// the memory allocator's initial chunk) if possible. If the block of
// addresses is not big enough to contain a single page-aligned page, a
// fresh chunk will be allocated.
bool SetUp();
// Returns true if the space has been successfully set up and not
// subsequently torn down.
bool HasBeenSetUp();
// Checks whether an object/address is in this space.
inline bool Contains(Address a);
inline bool Contains(Object* o);
bool ContainsSlow(Address addr);
// Given an address occupied by a live object, return that object if it is
// in this space, or a Smi if it is not. The implementation iterates over
// objects in the page containing the address, the cost is linear in the
// number of objects in the page. It may be slow.
Object* FindObject(Address addr);
// During boot the free_space_map is created, and afterwards we may need
// to write it into the free list nodes that were already created.
void RepairFreeListsAfterDeserialization();
// Prepares for a mark-compact GC.
void PrepareForMarkCompact();
// Current capacity without growing (Size() + Available()).
intptr_t Capacity() { return accounting_stats_.Capacity(); }
// Approximate amount of physical memory committed for this space.
size_t CommittedPhysicalMemory() override;
void ResetFreeListStatistics();
// Sets the capacity, the available space and the wasted space to zero.
// The stats are rebuilt during sweeping by adding each page to the
// capacity and the size when it is encountered. As free spaces are
// discovered during the sweeping they are subtracted from the size and added
// to the available and wasted totals.
void ClearStats() {
// Available bytes without growing. These are the bytes on the free list.
// The bytes in the linear allocation area are not included in this total
// because updating the stats would slow down allocation. New pages are
// immediately added to the free list so they show up here.
intptr_t Available() override { return free_list_.Available(); }
// Allocated bytes in this space. Garbage bytes that were not found due to
// concurrent sweeping are counted as being allocated! The bytes in the
// current linear allocation area (between top and limit) are also counted
// here.
intptr_t Size() override { return accounting_stats_.Size(); }
// As size, but the bytes in lazily swept pages are estimated and the bytes
// in the current linear allocation area are not included.
intptr_t SizeOfObjects() override;
// Wasted bytes in this space. These are just the bytes that were thrown away
// due to being too small to use for allocation.
virtual intptr_t Waste() { return free_list_.wasted_bytes(); }
// Returns the allocation pointer in this space.
Address top() { return; }
Address limit() { return allocation_info_.limit(); }
// The allocation top address.
Address* allocation_top_address() { return allocation_info_.top_address(); }
// The allocation limit address.
Address* allocation_limit_address() {
return allocation_info_.limit_address();
// Allocate the requested number of bytes in the space if possible, return a
// failure object if not. Only use IGNORE_SKIP_LIST if the skip list is going
// to be manually updated later.
MUST_USE_RESULT inline AllocationResult AllocateRawUnaligned(
int size_in_bytes, UpdateSkipList update_skip_list = UPDATE_SKIP_LIST);
MUST_USE_RESULT inline AllocationResult AllocateRawUnalignedSynchronized(
int size_in_bytes);
// Allocate the requested number of bytes in the space double aligned if
// possible, return a failure object if not.
MUST_USE_RESULT inline AllocationResult AllocateRawAligned(
int size_in_bytes, AllocationAlignment alignment);
// Allocate the requested number of bytes in the space and consider allocation
// alignment if needed.
MUST_USE_RESULT inline AllocationResult AllocateRaw(
int size_in_bytes, AllocationAlignment alignment);
// Give a block of memory to the space's free list. It might be added to
// the free list or accounted as waste.
// If add_to_freelist is false then just accounting stats are updated and
// no attempt to add area to free list is made.
int Free(Address start, int size_in_bytes) {
int wasted = free_list_.Free(start, size_in_bytes, kLinkCategory);
return size_in_bytes - wasted;
int UnaccountedFree(Address start, int size_in_bytes) {
int wasted = free_list_.Free(start, size_in_bytes, kDoNotLinkCategory);
return size_in_bytes - wasted;
void ResetFreeList() { free_list_.Reset(); }
// Set space allocation info.
void SetTopAndLimit(Address top, Address limit) {
DCHECK(top == limit ||
Page::FromAddress(top) == Page::FromAddress(limit - 1));
allocation_info_.Reset(top, limit);
// Empty space allocation info, returning unused area to free list.
void EmptyAllocationInfo() {
// Mark the old linear allocation area with a free space map so it can be
// skipped when scanning the heap.
int old_linear_size = static_cast<int>(limit() - top());
Free(top(), old_linear_size);
SetTopAndLimit(NULL, NULL);
void Allocate(int bytes) { accounting_stats_.AllocateBytes(bytes); }
void IncreaseCapacity(int size);
// Releases an unused page and shrinks the space.
void ReleasePage(Page* page);
// The dummy page that anchors the linked list of pages.
Page* anchor() { return &anchor_; }
// Verify integrity of this space.
virtual void Verify(ObjectVisitor* visitor);
// Overridden by subclasses to verify space-specific object
// properties (e.g., only maps or free-list nodes are in map space).
virtual void VerifyObject(HeapObject* obj) {}
#ifdef DEBUG
// Print meta info and objects in this space.
void Print() override;
// Reports statistics for the space
void ReportStatistics();
// Report code object related statistics
void CollectCodeStatistics();
static void ReportCodeStatistics(Isolate* isolate);
static void ResetCodeStatistics(Isolate* isolate);
// This function tries to steal size_in_bytes memory from the sweeper threads
// free-lists. If it does not succeed stealing enough memory, it will wait
// for the sweeper threads to finish sweeping.
// It returns true when sweeping is completed and false otherwise.
bool EnsureSweeperProgress(intptr_t size_in_bytes);
Page* FirstPage() { return anchor_.next_page(); }
Page* LastPage() { return anchor_.prev_page(); }
void EvictEvacuationCandidatesFromLinearAllocationArea();
bool CanExpand(size_t size);
// Returns the number of total pages in this space.
int CountTotalPages();
// Return size of allocatable area on a page in this space.
inline int AreaSize() { return area_size_; }
virtual bool is_local() { return false; }
// Merges {other} into the current space. Note that this modifies {other},
// e.g., removes its bump pointer area and resets statistics.
void MergeCompactionSpace(CompactionSpace* other);
// Refills the free list from the corresponding free list filled by the
// sweeper.
virtual void RefillFreeList();
FreeList* free_list() { return &free_list_; }
base::Mutex* mutex() { return &space_mutex_; }
inline void UnlinkFreeListCategories(Page* page);
inline intptr_t RelinkFreeListCategories(Page* page);
// PagedSpaces that should be included in snapshots have different, i.e.,
// smaller, initial pages.
virtual bool snapshotable() { return true; }
bool HasPages() { return anchor_.next_page() != &anchor_; }
// Cleans up the space, frees all pages in this space except those belonging
// to the initial chunk, uncommits addresses in the initial chunk.
void TearDown();
// Expands the space by allocating a fixed number of pages. Returns false if
// it cannot allocate requested number of pages from OS, or if the hard heap
// size limit has been hit.
bool Expand();
// Generic fast case allocation function that tries linear allocation at the
// address denoted by top in allocation_info_.
inline HeapObject* AllocateLinearly(int size_in_bytes);
// Generic fast case allocation function that tries aligned linear allocation
// at the address denoted by top in allocation_info_. Writes the aligned
// allocation size, which includes the filler size, to size_in_bytes.
inline HeapObject* AllocateLinearlyAligned(int* size_in_bytes,
AllocationAlignment alignment);
// If sweeping is still in progress try to sweep unswept pages. If that is
// not successful, wait for the sweeper threads and re-try free-list
// allocation.
MUST_USE_RESULT virtual HeapObject* SweepAndRetryAllocation(
int size_in_bytes);
// Slow path of AllocateRaw. This function is space-dependent.
MUST_USE_RESULT HeapObject* SlowAllocateRaw(int size_in_bytes);
int area_size_;
// Accounting information for this space.
AllocationStats accounting_stats_;
// The dummy page that anchors the double linked list of pages.
Page anchor_;
// The space's free list.
FreeList free_list_;
// Normal allocation information.
AllocationInfo allocation_info_;
// Mutex guarding any concurrent access to the space.
base::Mutex space_mutex_;
friend class IncrementalMarking;
friend class MarkCompactCollector;
friend class PageIterator;
// Used in cctest.
friend class HeapTester;
class NumberAndSizeInfo BASE_EMBEDDED {
NumberAndSizeInfo() : number_(0), bytes_(0) {}
int number() const { return number_; }
void increment_number(int num) { number_ += num; }
int bytes() const { return bytes_; }
void increment_bytes(int size) { bytes_ += size; }
void clear() {
number_ = 0;
bytes_ = 0;
int number_;
int bytes_;
// HistogramInfo class for recording a single "bar" of a histogram. This
// class is used for collecting statistics to print to the log file.
class HistogramInfo : public NumberAndSizeInfo {
HistogramInfo() : NumberAndSizeInfo() {}
const char* name() { return name_; }
void set_name(const char* name) { name_ = name; }
const char* name_;
enum SemiSpaceId { kFromSpace = 0, kToSpace = 1 };
// -----------------------------------------------------------------------------
// SemiSpace in young generation
// A SemiSpace is a contiguous chunk of memory holding page-like memory chunks.
// The mark-compact collector uses the memory of the first page in the from
// space as a marking stack when tracing live objects.
class SemiSpace : public Space {
static void Swap(SemiSpace* from, SemiSpace* to);
SemiSpace(Heap* heap, SemiSpaceId semispace)
current_page_(nullptr) {}
inline bool Contains(HeapObject* o);
inline bool Contains(Object* o);
inline bool ContainsSlow(Address a);
void SetUp(int initial_capacity, int maximum_capacity);
void TearDown();
bool HasBeenSetUp() { return maximum_capacity_ != 0; }
bool Commit();
bool Uncommit();
bool is_committed() { return committed_; }
// Grow the semispace to the new capacity. The new capacity requested must
// be larger than the current capacity and less than the maximum capacity.
bool GrowTo(int new_capacity);
// Shrinks the semispace to the new capacity. The new capacity requested
// must be more than the amount of used memory in the semispace and less
// than the current capacity.
bool ShrinkTo(int new_capacity);
// Returns the start address of the first page of the space.
Address space_start() {
DCHECK_NE(anchor_.next_page(), anchor());
return anchor_.next_page()->area_start();
Page* first_page() { return anchor_.next_page(); }
Page* current_page() { return current_page_; }
// Returns one past the end address of the space.
Address space_end() { return anchor_.prev_page()->area_end(); }
// Returns the start address of the current page of the space.
Address page_low() { return current_page_->area_start(); }
// Returns one past the end address of the current page of the space.
Address page_high() { return current_page_->area_end(); }
bool AdvancePage() {
Page* next_page = current_page_->next_page();
if (next_page == anchor()) return false;
current_page_ = next_page;
return true;
// Resets the space to using the first page.
void Reset();
void ReplaceWithEmptyPage(Page* page);
// Age mark accessors.
Address age_mark() { return age_mark_; }
void set_age_mark(Address mark);
// Returns the current capacity of the semispace.
int current_capacity() { return current_capacity_; }
// Returns the maximum capacity of the semispace.
int maximum_capacity() { return maximum_capacity_; }
// Returns the initial capacity of the semispace.
int minimum_capacity() { return minimum_capacity_; }
SemiSpaceId id() { return id_; }
// Approximate amount of physical memory committed for this space.
size_t CommittedPhysicalMemory() override;
// If we don't have these here then SemiSpace will be abstract. However
// they should never be called:
intptr_t Size() override {
return 0;
intptr_t SizeOfObjects() override { return Size(); }
intptr_t Available() override {
return 0;
#ifdef DEBUG
void Print() override;
// Validate a range of of addresses in a SemiSpace.
// The "from" address must be on a page prior to the "to" address,
// in the linked page order, or it must be earlier on the same page.
static void AssertValidRange(Address from, Address to);
// Do nothing.
inline static void AssertValidRange(Address from, Address to) {}
virtual void Verify();
void RewindPages(Page* start, int num_pages);
inline Page* anchor() { return &anchor_; }
// Copies the flags into the masked positions on all pages in the space.
void FixPagesFlags(intptr_t flags, intptr_t flag_mask);
// The currently committed space capacity.
int current_capacity_;
// The maximum capacity that can be used by this space.
int maximum_capacity_;
// The minimum capacity for the space. A space cannot shrink below this size.
int minimum_capacity_;
// Used to govern object promotion during mark-compact collection.
Address age_mark_;
bool committed_;
SemiSpaceId id_;
Page anchor_;
Page* current_page_;
friend class SemiSpaceIterator;
friend class NewSpacePageIterator;
// A SemiSpaceIterator is an ObjectIterator that iterates over the active
// semispace of the heap's new space. It iterates over the objects in the
// semispace from a given start address (defaulting to the bottom of the
// semispace) to the top of the semispace. New objects allocated after the
// iterator is created are not iterated.
class SemiSpaceIterator : public ObjectIterator {
// Create an iterator over the allocated objects in the given to-space.
explicit SemiSpaceIterator(NewSpace* space);
inline HeapObject* Next();
// Implementation of the ObjectIterator functions.
inline HeapObject* next_object() override;
void Initialize(Address start, Address end);
// The current iteration point.
Address current_;
// The end of iteration.
Address limit_;
// -----------------------------------------------------------------------------
// A PageIterator iterates the pages in a semi-space.
class NewSpacePageIterator BASE_EMBEDDED {
// Make an iterator that runs over all pages in to-space.
explicit inline NewSpacePageIterator(NewSpace* space);
// Make an iterator that runs over all pages in the given semispace,
// even those not used in allocation.
explicit inline NewSpacePageIterator(SemiSpace* space);
// Make iterator that iterates from the page containing start
// to the page that contains limit in the same semispace.
inline NewSpacePageIterator(Address start, Address limit);
inline bool has_next();
inline Page* next();
Page* prev_page_; // Previous page returned.
// Next page that will be returned. Cached here so that we can use this
// iterator for operations that deallocate pages.
Page* next_page_;
// Last page returned.
Page* last_page_;
// -----------------------------------------------------------------------------
// The young generation space.
// The new space consists of a contiguous pair of semispaces. It simply
// forwards most functions to the appropriate semispace.
class NewSpace : public Space {
explicit NewSpace(Heap* heap)
to_space_(heap, kToSpace),
from_space_(heap, kFromSpace),
promoted_histogram_(nullptr) {}
inline bool Contains(HeapObject* o);
inline bool ContainsSlow(Address a);
inline bool Contains(Object* o);
bool SetUp(int initial_semispace_capacity, int max_semispace_capacity);
// Tears down the space. Heap memory was not allocated by the space, so it
// is not deallocated here.
void TearDown();
// True if the space has been set up but not torn down.
bool HasBeenSetUp() {
return to_space_.HasBeenSetUp() && from_space_.HasBeenSetUp();
// Flip the pair of spaces.
void Flip();
// Grow the capacity of the semispaces. Assumes that they are not at
// their maximum capacity.
void Grow();
// Shrink the capacity of the semispaces.
void Shrink();
// Return the allocated bytes in the active semispace.
intptr_t Size() override {
return pages_used_ * Page::kAllocatableMemory +
static_cast<int>(top() - to_space_.page_low());
// The same, but returning an int. We have to have the one that returns
// intptr_t because it is inherited, but if we know we are dealing with the
// new space, which can't get as big as the other spaces then this is useful:
int SizeAsInt() { return static_cast<int>(Size()); }
// Return the allocatable capacity of a semispace.
intptr_t Capacity() {
SLOW_DCHECK(to_space_.current_capacity() == from_space_.current_capacity());
return (to_space_.current_capacity() / Page::kPageSize) *
// Return the current size of a semispace, allocatable and non-allocatable
// memory.
intptr_t TotalCapacity() {
DCHECK(to_space_.current_capacity() == from_space_.current_capacity());
return to_space_.current_capacity();
// Committed memory for NewSpace is the committed memory of both semi-spaces
// combined.
intptr_t CommittedMemory() override {
return from_space_.CommittedMemory() + to_space_.CommittedMemory();
intptr_t MaximumCommittedMemory() override {
return from_space_.MaximumCommittedMemory() +
// Approximate amount of physical memory committed for this space.
size_t CommittedPhysicalMemory() override;
// Return the available bytes without growing.
intptr_t Available() override { return Capacity() - Size(); }
inline size_t AllocatedSinceLastGC();
void ReplaceWithEmptyPage(Page* page) {
// This method is called after flipping the semispace.
// Return the maximum capacity of a semispace.
int MaximumCapacity() {
DCHECK(to_space_.maximum_capacity() == from_space_.maximum_capacity());
return to_space_.maximum_capacity();
bool IsAtMaximumCapacity() { return TotalCapacity() == MaximumCapacity(); }
// Returns the initial capacity of a semispace.
int InitialTotalCapacity() {
DCHECK(to_space_.minimum_capacity() == from_space_.minimum_capacity());
return to_space_.minimum_capacity();
// Return the address of the allocation pointer in the active semispace.
Address top() {
// Return the address of the allocation pointer limit in the active semispace.
Address limit() {
return allocation_info_.limit();
// Return the address of the first object in the active semispace.
Address bottom() { return to_space_.space_start(); }
// Get the age mark of the inactive semispace.
Address age_mark() { return from_space_.age_mark(); }
// Set the age mark in the active semispace.
void set_age_mark(Address mark) {
allocated_since_last_gc_ = 0;
// The allocation top and limit address.
Address* allocation_top_address() { return allocation_info_.top_address(); }
// The allocation limit address.
Address* allocation_limit_address() {
return allocation_info_.limit_address();
MUST_USE_RESULT INLINE(AllocationResult AllocateRawAligned(
int size_in_bytes, AllocationAlignment alignment));
AllocationResult AllocateRawUnaligned(int size_in_bytes));
MUST_USE_RESULT INLINE(AllocationResult AllocateRaw(
int size_in_bytes, AllocationAlignment alignment));
MUST_USE_RESULT inline AllocationResult AllocateRawSynchronized(
int size_in_bytes, AllocationAlignment alignment);
// Reset the allocation pointer to the beginning of the active semispace.
void ResetAllocationInfo();
// When inline allocation stepping is active, either because of incremental
// marking, idle scavenge, or allocation statistics gathering, we 'interrupt'
// inline allocation every once in a while. This is done by setting
// allocation_info_.limit to be lower than the actual limit and and increasing
// it in steps to guarantee that the observers are notified periodically.
void UpdateInlineAllocationLimit(int size_in_bytes);
void DisableInlineAllocationSteps() {
top_on_previous_step_ = 0;
// Allows observation of inline allocation. The observer->Step() method gets
// called after every step_size bytes have been allocated (approximately).
// This works by adjusting the allocation limit to a lower value and adjusting
// it after each step.
void AddAllocationObserver(AllocationObserver* observer) override;
void RemoveAllocationObserver(AllocationObserver* observer) override;
// Get the extent of the inactive semispace (for use as a marking stack,
// or to zap it). Notice: space-addresses are not necessarily on the
// same page, so FromSpaceStart() might be above FromSpaceEnd().
Address FromSpacePageLow() { return from_space_.page_low(); }
Address FromSpacePageHigh() { return from_space_.page_high(); }
Address FromSpaceStart() { return from_space_.space_start(); }
Address FromSpaceEnd() { return from_space_.space_end(); }
// Get the extent of the active semispace's pages' memory.
Address ToSpaceStart() { return to_space_.space_start(); }
Address ToSpaceEnd() { return to_space_.space_end(); }
inline bool ToSpaceContainsSlow(Address a);
inline bool FromSpaceContainsSlow(Address a);
inline bool ToSpaceContains(Object* o);
inline bool FromSpaceContains(Object* o);
// Try to switch the active semispace to a new, empty, page.
// Returns false if this isn't possible or reasonable (i.e., there
// are no pages, or the current page is already empty), or true
// if successful.
bool AddFreshPage();
bool AddFreshPageSynchronized();
// Verify the active semispace.
virtual void Verify();
#ifdef DEBUG
// Print the active semispace.
void Print() override { to_space_.Print(); }
// Iterates the active semispace to collect statistics.
void CollectStatistics();
// Reports previously collected statistics of the active semispace.
void ReportStatistics();
// Clears previously collected statistics.
void ClearHistograms();
// Record the allocation or promotion of a heap object. Note that we don't
// record every single allocation, but only those that happen in the
// to space during a scavenge GC.
void RecordAllocation(HeapObject* obj);
void RecordPromotion(HeapObject* obj);
// Return whether the operation succeded.
bool CommitFromSpaceIfNeeded() {
if (from_space_.is_committed()) return true;
return from_space_.Commit();
bool UncommitFromSpace() {
if (!from_space_.is_committed()) return true;
return from_space_.Uncommit();
bool IsFromSpaceCommitted() { return from_space_.is_committed(); }
SemiSpace* active_space() { return &to_space_; }
void PauseAllocationObservers() override;