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// Copyright 2020 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.
#include <atomic>
#include <memory>
#include "src/base/macros.h"
#include "src/base/platform/mutex.h"
#include "src/heap/heap.h"
#include "src/heap/spaces.h"
#include "src/logging/log.h"
#include "src/objects/heap-object.h"
namespace v8 {
namespace internal {
class Heap;
class MemoryChunk;
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 {
using iterator = PageIterator;
using const_iterator = ConstPageIterator;
static void Swap(SemiSpace* from, SemiSpace* to);
SemiSpace(Heap* heap, SemiSpaceId semispace)
: Space(heap, NEW_SPACE, new NoFreeList()),
pages_used_(0) {}
inline bool Contains(HeapObject o) const;
inline bool Contains(Object o) const;
inline bool ContainsSlow(Address a) const;
void SetUp(size_t initial_capacity, size_t maximum_capacity);
void TearDown();
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(size_t 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(size_t new_capacity);
bool EnsureCurrentCapacity();
Address space_end() { return memory_chunk_list_.back()->area_end(); }
// Returns the start address of the first page of the space.
Address space_start() {
DCHECK_NE(memory_chunk_list_.front(), nullptr);
return memory_chunk_list_.front()->area_start();
Page* current_page() { return current_page_; }
int pages_used() { return pages_used_; }
// 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();
// We cannot expand if we reached the maximum number of pages already. Note
// that we need to account for the next page already for this check as we
// could potentially fill the whole page after advancing.
const bool reached_max_pages = (pages_used_ + 1) == max_pages();
if (next_page == nullptr || reached_max_pages) {
return false;
current_page_ = next_page;
return true;
// Resets the space to using the first page.
void Reset();
void RemovePage(Page* page);
void PrependPage(Page* page);
Page* InitializePage(MemoryChunk* chunk);
// Age mark accessors.
Address age_mark() { return age_mark_; }
void set_age_mark(Address mark);
// Returns the current capacity of the semispace.
size_t current_capacity() { return current_capacity_; }
// Returns the maximum capacity of the semispace.
size_t maximum_capacity() { return maximum_capacity_; }
// Returns the initial capacity of the semispace.
size_t 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:
size_t Size() override { UNREACHABLE(); }
size_t SizeOfObjects() override { return Size(); }
size_t Available() override { UNREACHABLE(); }
Page* first_page() { return reinterpret_cast<Page*>(Space::first_page()); }
Page* last_page() { return reinterpret_cast<Page*>(Space::last_page()); }
const Page* first_page() const {
return reinterpret_cast<const Page*>(Space::first_page());
const Page* last_page() const {
return reinterpret_cast<const Page*>(Space::last_page());
iterator begin() { return iterator(first_page()); }
iterator end() { return iterator(nullptr); }
const_iterator begin() const { return const_iterator(first_page()); }
const_iterator end() const { return const_iterator(nullptr); }
std::unique_ptr<ObjectIterator> GetObjectIterator(Heap* heap) override;
#ifdef DEBUG
V8_EXPORT_PRIVATE 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(int num_pages);
inline int max_pages() {
return static_cast<int>(current_capacity_ / Page::kPageSize);
// 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.
size_t current_capacity_;
// The maximum capacity that can be used by this space. A space cannot grow
// beyond that size.
size_t maximum_capacity_;
// The minimum capacity for the space. A space cannot shrink below this size.
size_t minimum_capacity_;
// Used to govern object promotion during mark-compact collection.
Address age_mark_;
bool committed_;
SemiSpaceId id_;
Page* current_page_;
int pages_used_;
friend class NewSpace;
friend class SemiSpaceObjectIterator;
// A SemiSpaceObjectIterator 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 SemiSpaceObjectIterator : public ObjectIterator {
// Create an iterator over the allocated objects in the given to-space.
explicit SemiSpaceObjectIterator(NewSpace* space);
inline HeapObject Next() override;
void Initialize(Address start, Address end);
// The current iteration point.
Address current_;
// The end of iteration.
Address limit_;
// -----------------------------------------------------------------------------
// The young generation space.
// The new space consists of a contiguous pair of semispaces. It simply
// forwards most functions to the appropriate semispace.
class V8_EXPORT_PRIVATE NewSpace
: NON_EXPORTED_BASE(public SpaceWithLinearArea) {
using iterator = PageIterator;
using const_iterator = ConstPageIterator;
NewSpace(Heap* heap, v8::PageAllocator* page_allocator,
size_t initial_semispace_capacity, size_t max_semispace_capacity);
~NewSpace() override { TearDown(); }
inline bool ContainsSlow(Address a) const;
inline bool Contains(Object o) const;
inline bool Contains(HeapObject o) const;
// Tears down the space. Heap memory was not allocated by the space, so it
// is not deallocated here.
void TearDown();
// 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.
size_t Size() final {
DCHECK_GE(top(), to_space_.page_low());
return to_space_.pages_used() *
MemoryChunkLayout::AllocatableMemoryInDataPage() +
static_cast<size_t>(top() - to_space_.page_low());
size_t SizeOfObjects() final { return Size(); }
// Return the allocatable capacity of a semispace.
size_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.
size_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.
size_t CommittedMemory() final {
return from_space_.CommittedMemory() + to_space_.CommittedMemory();
size_t MaximumCommittedMemory() final {
return from_space_.MaximumCommittedMemory() +
// Approximate amount of physical memory committed for this space.
size_t CommittedPhysicalMemory() final;
// Return the available bytes without growing.
size_t Available() final {
DCHECK_GE(Capacity(), Size());
return Capacity() - Size();
size_t ExternalBackingStoreBytes(ExternalBackingStoreType type) const final {
type == ExternalBackingStoreType::kArrayBuffer)
return heap()->YoungArrayBufferBytes();
DCHECK_EQ(0, from_space_.ExternalBackingStoreBytes(type));
return to_space_.ExternalBackingStoreBytes(type);
size_t ExternalBackingStoreBytes() {
size_t result = 0;
for (int i = 0; i < ExternalBackingStoreType::kNumTypes; i++) {
result +=
return result;
size_t AllocatedSinceLastGC() {
const Address age_mark = to_space_.age_mark();
DCHECK_NE(age_mark, kNullAddress);
DCHECK_NE(top(), kNullAddress);
Page* const age_mark_page = Page::FromAllocationAreaAddress(age_mark);
Page* const last_page = Page::FromAllocationAreaAddress(top());
Page* current_page = age_mark_page;
size_t allocated = 0;
if (current_page != last_page) {
DCHECK_EQ(current_page, age_mark_page);
DCHECK_GE(age_mark_page->area_end(), age_mark);
allocated += age_mark_page->area_end() - age_mark;
current_page = current_page->next_page();
} else {
DCHECK_GE(top(), age_mark);
return top() - age_mark;
while (current_page != last_page) {
DCHECK_NE(current_page, age_mark_page);
allocated += MemoryChunkLayout::AllocatableMemoryInDataPage();
current_page = current_page->next_page();
DCHECK_GE(top(), current_page->area_start());
allocated += top() - current_page->area_start();
DCHECK_LE(allocated, Size());
return allocated;
void MovePageFromSpaceToSpace(Page* page) {
bool Rebalance();
// Return the maximum capacity of a semispace.
size_t 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.
size_t InitialTotalCapacity() {
DCHECK(to_space_.minimum_capacity() == from_space_.minimum_capacity());
return to_space_.minimum_capacity();
void ResetOriginalTop() {
DCHECK_GE(top(), original_top_);
DCHECK_LE(top(), original_limit_);, std::memory_order_release);
Address original_top_acquire() {
return original_top_.load(std::memory_order_acquire);
Address original_limit_relaxed() {
return original_limit_.load(std::memory_order_relaxed);
// Return the address of the first allocatable address in the active
// semispace. This may be the address where the first object resides.
Address first_allocatable_address() { 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) { to_space_.set_age_mark(mark); }
AllocateRaw(int size_in_bytes, AllocationAlignment alignment,
AllocationOrigin origin = AllocationOrigin::kRuntime);
V8_WARN_UNUSED_RESULT inline AllocationResult AllocateRawSynchronized(
int size_in_bytes, AllocationAlignment alignment,
AllocationOrigin origin = AllocationOrigin::kRuntime);
// Reset the allocation pointer to the beginning of the active semispace.
void ResetLinearAllocationArea();
// 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(size_t size_in_bytes) override;
inline bool ToSpaceContainsSlow(Address a) const;
inline bool ToSpaceContains(Object o) const;
inline bool FromSpaceContains(Object o) const;
// 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(Isolate* isolate);
#ifdef DEBUG
// Print the active semispace.
void Print() override { to_space_.Print(); }
// Return whether the operation succeeded.
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_; }
Page* first_page() { return to_space_.first_page(); }
Page* last_page() { return to_space_.last_page(); }
iterator begin() { return to_space_.begin(); }
iterator end() { return to_space_.end(); }
const_iterator begin() const { return to_space_.begin(); }
const_iterator end() const { return to_space_.end(); }
std::unique_ptr<ObjectIterator> GetObjectIterator(Heap* heap) override;
SemiSpace& from_space() { return from_space_; }
SemiSpace& to_space() { return to_space_; }
// Update linear allocation area to match the current to-space page.
void UpdateLinearAllocationArea();
base::Mutex mutex_;
// The top and the limit at the time of setting the linear allocation area.
// These values can be accessed by background tasks.
std::atomic<Address> original_top_;
std::atomic<Address> original_limit_;
// The semispaces.
SemiSpace to_space_;
SemiSpace from_space_;
VirtualMemory reservation_;
// Internal allocation methods.
AllocateFastAligned(int size_in_bytes, int* aligned_size_in_bytes,
AllocationAlignment alignment, AllocationOrigin origin);
AllocateFastUnaligned(int size_in_bytes, AllocationOrigin origin);
V8_WARN_UNUSED_RESULT AllocationResult
AllocateRawSlow(int size_in_bytes, AllocationAlignment alignment,
AllocationOrigin origin);
V8_WARN_UNUSED_RESULT AllocationResult
AllocateRawAligned(int size_in_bytes, AllocationAlignment alignment,
AllocationOrigin origin = AllocationOrigin::kRuntime);
V8_WARN_UNUSED_RESULT AllocationResult AllocateRawUnaligned(
int size_in_bytes, AllocationOrigin origin = AllocationOrigin::kRuntime);
bool EnsureAllocation(int size_in_bytes, AllocationAlignment alignment);
bool SupportsAllocationObserver() override { return true; }
friend class SemiSpaceObjectIterator;
// For contiguous spaces, top should be in the space (or at the end) and limit
// should be the end of the space.
SLOW_DCHECK((space).page_low() <= (info).top() && \
(info).top() <= (space).page_high() && \
(info).limit() <= (space).page_high())
} // namespace internal
} // namespace v8
#endif // V8_HEAP_NEW_SPACES_H_