xref: /external/v8/src/heap/mark-compact.cc

// Copyright 2012 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 "src/heap/mark-compact.h"

#include <unordered_map>

#include "src/base/utils/random-number-generator.h"
#include "src/cancelable-task.h"
#include "src/code-stubs.h"
#include "src/compilation-cache.h"
#include "src/deoptimizer.h"
#include "src/execution.h"
#include "src/frames-inl.h"
#include "src/global-handles.h"
#include "src/heap/array-buffer-collector.h"
#include "src/heap/array-buffer-tracker-inl.h"
#include "src/heap/gc-tracer.h"
#include "src/heap/incremental-marking.h"
#include "src/heap/invalidated-slots-inl.h"
#include "src/heap/item-parallel-job.h"
#include "src/heap/local-allocator-inl.h"
#include "src/heap/mark-compact-inl.h"
#include "src/heap/object-stats.h"
#include "src/heap/objects-visiting-inl.h"
#include "src/heap/spaces-inl.h"
#include "src/heap/sweeper.h"
#include "src/heap/worklist.h"
#include "src/ic/stub-cache.h"
#include "src/objects/hash-table-inl.h"
#include "src/transitions-inl.h"
#include "src/utils-inl.h"
#include "src/v8.h"
#include "src/vm-state-inl.h"

namespace v8 {
namespace internal {

const char* Marking::kWhiteBitPattern = "00";
const char* Marking::kBlackBitPattern = "11";
const char* Marking::kGreyBitPattern = "10";
const char* Marking::kImpossibleBitPattern = "01";

// The following has to hold in order for {MarkingState::MarkBitFrom} to not
// produce invalid {kImpossibleBitPattern} in the marking bitmap by overlapping.
STATIC_ASSERT(Heap::kMinObjectSizeInWords >= 2);

// =============================================================================
// Verifiers
// =============================================================================

#ifdef VERIFY_HEAP
namespace {

class MarkingVerifier : public ObjectVisitor, public RootVisitor {
 public:
  virtual void Run() = 0;

 protected:
  explicit MarkingVerifier(Heap* heap) : heap_(heap) {}

  virtual Bitmap* bitmap(const MemoryChunk* chunk) = 0;

  virtual void VerifyPointers(Object** start, Object** end) = 0;
  virtual void VerifyPointers(MaybeObject** start, MaybeObject** end) = 0;

  virtual bool IsMarked(HeapObject* object) = 0;

  virtual bool IsBlackOrGrey(HeapObject* object) = 0;

  void VisitPointers(HeapObject* host, Object** start, Object** end) override {
    VerifyPointers(start, end);
  }

  void VisitPointers(HeapObject* host, MaybeObject** start,
                     MaybeObject** end) override {
    VerifyPointers(start, end);
  }

  void VisitRootPointers(Root root, const char* description, Object** start,
                         Object** end) override {
    VerifyPointers(start, end);
  }

  void VerifyRoots(VisitMode mode);
  void VerifyMarkingOnPage(const Page* page, Address start, Address end);
  void VerifyMarking(NewSpace* new_space);
  void VerifyMarking(PagedSpace* paged_space);

  Heap* heap_;
};

void MarkingVerifier::VerifyRoots(VisitMode mode) {
  heap_->IterateStrongRoots(this, mode);
}

void MarkingVerifier::VerifyMarkingOnPage(const Page* page, Address start,
                                          Address end) {
  HeapObject* object;
  Address next_object_must_be_here_or_later = start;
  for (Address current = start; current < end;) {
    object = HeapObject::FromAddress(current);
    // One word fillers at the end of a black area can be grey.
    if (IsBlackOrGrey(object) &&
        object->map() != ReadOnlyRoots(heap_).one_pointer_filler_map()) {
      CHECK(IsMarked(object));
      CHECK(current >= next_object_must_be_here_or_later);
      object->Iterate(this);
      next_object_must_be_here_or_later = current + object->Size();
      // The object is either part of a black area of black allocation or a
      // regular black object
      CHECK(
          bitmap(page)->AllBitsSetInRange(
              page->AddressToMarkbitIndex(current),
              page->AddressToMarkbitIndex(next_object_must_be_here_or_later)) ||
          bitmap(page)->AllBitsClearInRange(
              page->AddressToMarkbitIndex(current + kPointerSize * 2),
              page->AddressToMarkbitIndex(next_object_must_be_here_or_later)));
      current = next_object_must_be_here_or_later;
    } else {
      current += kPointerSize;
    }
  }
}

void MarkingVerifier::VerifyMarking(NewSpace* space) {
  Address end = space->top();
  // The bottom position is at the start of its page. Allows us to use
  // page->area_start() as start of range on all pages.
  CHECK_EQ(space->first_allocatable_address(),
           space->first_page()->area_start());

  PageRange range(space->first_allocatable_address(), end);
  for (auto it = range.begin(); it != range.end();) {
    Page* page = *(it++);
    Address limit = it != range.end() ? page->area_end() : end;
    CHECK(limit == end || !page->Contains(end));
    VerifyMarkingOnPage(page, page->area_start(), limit);
  }
}

void MarkingVerifier::VerifyMarking(PagedSpace* space) {
  for (Page* p : *space) {
    VerifyMarkingOnPage(p, p->area_start(), p->area_end());
  }
}

class FullMarkingVerifier : public MarkingVerifier {
 public:
  explicit FullMarkingVerifier(Heap* heap)
      : MarkingVerifier(heap),
        marking_state_(
            heap->mark_compact_collector()->non_atomic_marking_state()) {}

  void Run() override {
    VerifyRoots(VISIT_ONLY_STRONG);
    VerifyMarking(heap_->new_space());
    VerifyMarking(heap_->old_space());
    VerifyMarking(heap_->code_space());
    VerifyMarking(heap_->map_space());

    LargeObjectIterator it(heap_->lo_space());
    for (HeapObject* obj = it.Next(); obj != nullptr; obj = it.Next()) {
      if (marking_state_->IsBlackOrGrey(obj)) {
        obj->Iterate(this);
      }
    }
  }

 protected:
  Bitmap* bitmap(const MemoryChunk* chunk) override {
    return marking_state_->bitmap(chunk);
  }

  bool IsMarked(HeapObject* object) override {
    return marking_state_->IsBlack(object);
  }

  bool IsBlackOrGrey(HeapObject* object) override {
    return marking_state_->IsBlackOrGrey(object);
  }

  void VerifyPointers(Object** start, Object** end) override {
    for (Object** current = start; current < end; current++) {
      if ((*current)->IsHeapObject()) {
        HeapObject* object = HeapObject::cast(*current);
        CHECK(marking_state_->IsBlackOrGrey(object));
      }
    }
  }

  void VerifyPointers(MaybeObject** start, MaybeObject** end) override {
    for (MaybeObject** current = start; current < end; current++) {
      HeapObject* object;
      if ((*current)->ToStrongHeapObject(&object)) {
        CHECK(marking_state_->IsBlackOrGrey(object));
      }
    }
  }

  void VisitEmbeddedPointer(Code* host, RelocInfo* rinfo) override {
    DCHECK(rinfo->rmode() == RelocInfo::EMBEDDED_OBJECT);
    if (!host->IsWeakObject(rinfo->target_object())) {
      Object* p = rinfo->target_object();
      VisitPointer(host, &p);
    }
  }

 private:
  MarkCompactCollector::NonAtomicMarkingState* marking_state_;
};

class EvacuationVerifier : public ObjectVisitor, public RootVisitor {
 public:
  virtual void Run() = 0;

  void VisitPointers(HeapObject* host, Object** start, Object** end) override {
    VerifyPointers(start, end);
  }

  void VisitPointers(HeapObject* host, MaybeObject** start,
                     MaybeObject** end) override {
    VerifyPointers(start, end);
  }

  void VisitRootPointers(Root root, const char* description, Object** start,
                         Object** end) override {
    VerifyPointers(start, end);
  }

 protected:
  explicit EvacuationVerifier(Heap* heap) : heap_(heap) {}

  inline Heap* heap() { return heap_; }

  virtual void VerifyPointers(Object** start, Object** end) = 0;
  virtual void VerifyPointers(MaybeObject** start, MaybeObject** end) = 0;

  void VerifyRoots(VisitMode mode);
  void VerifyEvacuationOnPage(Address start, Address end);
  void VerifyEvacuation(NewSpace* new_space);
  void VerifyEvacuation(PagedSpace* paged_space);

  Heap* heap_;
};

void EvacuationVerifier::VerifyRoots(VisitMode mode) {
  heap_->IterateStrongRoots(this, mode);
}

void EvacuationVerifier::VerifyEvacuationOnPage(Address start, Address end) {
  Address current = start;
  while (current < end) {
    HeapObject* object = HeapObject::FromAddress(current);
    if (!object->IsFiller()) object->Iterate(this);
    current += object->Size();
  }
}

void EvacuationVerifier::VerifyEvacuation(NewSpace* space) {
  PageRange range(space->first_allocatable_address(), space->top());
  for (auto it = range.begin(); it != range.end();) {
    Page* page = *(it++);
    Address current = page->area_start();
    Address limit = it != range.end() ? page->area_end() : space->top();
    CHECK(limit == space->top() || !page->Contains(space->top()));
    VerifyEvacuationOnPage(current, limit);
  }
}

void EvacuationVerifier::VerifyEvacuation(PagedSpace* space) {
  for (Page* p : *space) {
    if (p->IsEvacuationCandidate()) continue;
    if (p->Contains(space->top())) {
      CodePageMemoryModificationScope memory_modification_scope(p);
      heap_->CreateFillerObjectAt(
          space->top(), static_cast<int>(space->limit() - space->top()),
          ClearRecordedSlots::kNo);
    }
    VerifyEvacuationOnPage(p->area_start(), p->area_end());
  }
}

class FullEvacuationVerifier : public EvacuationVerifier {
 public:
  explicit FullEvacuationVerifier(Heap* heap) : EvacuationVerifier(heap) {}

  void Run() override {
    VerifyRoots(VISIT_ALL);
    VerifyEvacuation(heap_->new_space());
    VerifyEvacuation(heap_->old_space());
    VerifyEvacuation(heap_->code_space());
    VerifyEvacuation(heap_->map_space());
  }

 protected:
  void VerifyPointers(Object** start, Object** end) override {
    for (Object** current = start; current < end; current++) {
      if ((*current)->IsHeapObject()) {
        HeapObject* object = HeapObject::cast(*current);
        if (Heap::InNewSpace(object)) {
          CHECK(Heap::InToSpace(object));
        }
        CHECK(!MarkCompactCollector::IsOnEvacuationCandidate(object));
      }
    }
  }
  void VerifyPointers(MaybeObject** start, MaybeObject** end) override {
    for (MaybeObject** current = start; current < end; current++) {
      HeapObject* object;
      if ((*current)->ToStrongHeapObject(&object)) {
        if (Heap::InNewSpace(object)) {
          CHECK(Heap::InToSpace(object));
        }
        CHECK(!MarkCompactCollector::IsOnEvacuationCandidate(object));
      }
    }
  }
};

}  // namespace
#endif  // VERIFY_HEAP

// =============================================================================
// MarkCompactCollectorBase, MinorMarkCompactCollector, MarkCompactCollector
// =============================================================================

using MarkCompactMarkingVisitor =
    MarkingVisitor<FixedArrayVisitationMode::kRegular,
                   TraceRetainingPathMode::kEnabled,
                   MarkCompactCollector::MarkingState>;

namespace {

int NumberOfAvailableCores() {
  static int num_cores = V8::GetCurrentPlatform()->NumberOfWorkerThreads() + 1;
  // This number of cores should be greater than zero and never change.
  DCHECK_GE(num_cores, 1);
  DCHECK_EQ(num_cores, V8::GetCurrentPlatform()->NumberOfWorkerThreads() + 1);
  return num_cores;
}

}  // namespace

int MarkCompactCollectorBase::NumberOfParallelCompactionTasks(int pages) {
  DCHECK_GT(pages, 0);
  int tasks =
      FLAG_parallel_compaction ? Min(NumberOfAvailableCores(), pages) : 1;
  if (!heap_->CanExpandOldGeneration(
          static_cast<size_t>(tasks * Page::kPageSize))) {
    // Optimize for memory usage near the heap limit.
    tasks = 1;
  }
  return tasks;
}

int MarkCompactCollectorBase::NumberOfParallelPointerUpdateTasks(int pages,
                                                                 int slots) {
  DCHECK_GT(pages, 0);
  // Limit the number of update tasks as task creation often dominates the
  // actual work that is being done.
  const int kMaxPointerUpdateTasks = 8;
  const int kSlotsPerTask = 600;
  const int wanted_tasks =
      (slots >= 0) ? Max(1, Min(pages, slots / kSlotsPerTask)) : pages;
  return FLAG_parallel_pointer_update
             ? Min(kMaxPointerUpdateTasks,
                   Min(NumberOfAvailableCores(), wanted_tasks))
             : 1;
}

int MarkCompactCollectorBase::NumberOfParallelToSpacePointerUpdateTasks(
    int pages) {
  DCHECK_GT(pages, 0);
  // No cap needed because all pages we need to process are fully filled with
  // interesting objects.
  return FLAG_parallel_pointer_update ? Min(NumberOfAvailableCores(), pages)
                                      : 1;
}

MarkCompactCollector::MarkCompactCollector(Heap* heap)
    : MarkCompactCollectorBase(heap),
      page_parallel_job_semaphore_(0),
#ifdef DEBUG
      state_(IDLE),
#endif
      was_marked_incrementally_(false),
      evacuation_(false),
      compacting_(false),
      black_allocation_(false),
      have_code_to_deoptimize_(false),
      marking_worklist_(heap),
      sweeper_(new Sweeper(heap, non_atomic_marking_state())) {
  old_to_new_slots_ = -1;
}

MarkCompactCollector::~MarkCompactCollector() { delete sweeper_; }

void MarkCompactCollector::SetUp() {
  DCHECK_EQ(0, strcmp(Marking::kWhiteBitPattern, "00"));
  DCHECK_EQ(0, strcmp(Marking::kBlackBitPattern, "11"));
  DCHECK_EQ(0, strcmp(Marking::kGreyBitPattern, "10"));
  DCHECK_EQ(0, strcmp(Marking::kImpossibleBitPattern, "01"));
}

void MarkCompactCollector::TearDown() {
  AbortCompaction();
  AbortWeakObjects();
  if (heap()->incremental_marking()->IsMarking()) {
    marking_worklist()->Clear();
  }
}

void MarkCompactCollector::AddEvacuationCandidate(Page* p) {
  DCHECK(!p->NeverEvacuate());
  p->MarkEvacuationCandidate();
  evacuation_candidates_.push_back(p);
}


static void TraceFragmentation(PagedSpace* space) {
  int number_of_pages = space->CountTotalPages();
  intptr_t reserved = (number_of_pages * space->AreaSize());
  intptr_t free = reserved - space->SizeOfObjects();
  PrintF("[%s]: %d pages, %d (%.1f%%) free\n", space->name(), number_of_pages,
         static_cast<int>(free), static_cast<double>(free) * 100 / reserved);
}

bool MarkCompactCollector::StartCompaction() {
  if (!compacting_) {
    DCHECK(evacuation_candidates_.empty());

    CollectEvacuationCandidates(heap()->old_space());

    if (FLAG_compact_code_space) {
      CollectEvacuationCandidates(heap()->code_space());
    } else if (FLAG_trace_fragmentation) {
      TraceFragmentation(heap()->code_space());
    }

    if (FLAG_trace_fragmentation) {
      TraceFragmentation(heap()->map_space());
    }

    compacting_ = !evacuation_candidates_.empty();
  }

  return compacting_;
}

void MarkCompactCollector::CollectGarbage() {
  // Make sure that Prepare() has been called. The individual steps below will
  // update the state as they proceed.
  DCHECK(state_ == PREPARE_GC);

#ifdef ENABLE_MINOR_MC
  heap()->minor_mark_compact_collector()->CleanupSweepToIteratePages();
#endif  // ENABLE_MINOR_MC

  MarkLiveObjects();
  ClearNonLiveReferences();
  VerifyMarking();

  RecordObjectStats();

  StartSweepSpaces();

  Evacuate();

  Finish();
}

#ifdef VERIFY_HEAP
void MarkCompactCollector::VerifyMarkbitsAreDirty(PagedSpace* space) {
  HeapObjectIterator iterator(space);
  while (HeapObject* object = iterator.Next()) {
    CHECK(non_atomic_marking_state()->IsBlack(object));
  }
}

void MarkCompactCollector::VerifyMarkbitsAreClean(PagedSpace* space) {
  for (Page* p : *space) {
    CHECK(non_atomic_marking_state()->bitmap(p)->IsClean());
    CHECK_EQ(0, non_atomic_marking_state()->live_bytes(p));
  }
}


void MarkCompactCollector::VerifyMarkbitsAreClean(NewSpace* space) {
  for (Page* p : PageRange(space->first_allocatable_address(), space->top())) {
    CHECK(non_atomic_marking_state()->bitmap(p)->IsClean());
    CHECK_EQ(0, non_atomic_marking_state()->live_bytes(p));
  }
}


void MarkCompactCollector::VerifyMarkbitsAreClean() {
  VerifyMarkbitsAreClean(heap_->old_space());
  VerifyMarkbitsAreClean(heap_->code_space());
  VerifyMarkbitsAreClean(heap_->map_space());
  VerifyMarkbitsAreClean(heap_->new_space());
  // Read-only space should always be black since we never collect any objects
  // in it or linked from it.
  VerifyMarkbitsAreDirty(heap_->read_only_space());

  LargeObjectIterator it(heap_->lo_space());
  for (HeapObject* obj = it.Next(); obj != nullptr; obj = it.Next()) {
    CHECK(non_atomic_marking_state()->IsWhite(obj));
    CHECK_EQ(0, non_atomic_marking_state()->live_bytes(
                    MemoryChunk::FromAddress(obj->address())));
  }
}

#endif  // VERIFY_HEAP

void MarkCompactCollector::ClearMarkbitsInPagedSpace(PagedSpace* space) {
  for (Page* p : *space) {
    non_atomic_marking_state()->ClearLiveness(p);
  }
}

void MarkCompactCollector::ClearMarkbitsInNewSpace(NewSpace* space) {
  for (Page* p : *space) {
    non_atomic_marking_state()->ClearLiveness(p);
  }
}


void MarkCompactCollector::ClearMarkbits() {
  ClearMarkbitsInPagedSpace(heap_->code_space());
  ClearMarkbitsInPagedSpace(heap_->map_space());
  ClearMarkbitsInPagedSpace(heap_->old_space());
  ClearMarkbitsInNewSpace(heap_->new_space());
  heap_->lo_space()->ClearMarkingStateOfLiveObjects();
}

void MarkCompactCollector::EnsureSweepingCompleted() {
  if (!sweeper()->sweeping_in_progress()) return;

  sweeper()->EnsureCompleted();
  heap()->old_space()->RefillFreeList();
  heap()->code_space()->RefillFreeList();
  heap()->map_space()->RefillFreeList();

#ifdef VERIFY_HEAP
  if (FLAG_verify_heap && !evacuation()) {
    FullEvacuationVerifier verifier(heap());
    verifier.Run();
  }
#endif
}

void MarkCompactCollector::ComputeEvacuationHeuristics(
    size_t area_size, int* target_fragmentation_percent,
    size_t* max_evacuated_bytes) {
  // For memory reducing and optimize for memory mode we directly define both
  // constants.
  const int kTargetFragmentationPercentForReduceMemory = 20;
  const size_t kMaxEvacuatedBytesForReduceMemory = 12 * MB;
  const int kTargetFragmentationPercentForOptimizeMemory = 20;
  const size_t kMaxEvacuatedBytesForOptimizeMemory = 6 * MB;

  // For regular mode (which is latency critical) we define less aggressive
  // defaults to start and switch to a trace-based (using compaction speed)
  // approach as soon as we have enough samples.
  const int kTargetFragmentationPercent = 70;
  const size_t kMaxEvacuatedBytes = 4 * MB;
  // Time to take for a single area (=payload of page). Used as soon as there
  // exist enough compaction speed samples.
  const float kTargetMsPerArea = .5;

  if (heap()->ShouldReduceMemory()) {
    *target_fragmentation_percent = kTargetFragmentationPercentForReduceMemory;
    *max_evacuated_bytes = kMaxEvacuatedBytesForReduceMemory;
  } else if (heap()->ShouldOptimizeForMemoryUsage()) {
    *target_fragmentation_percent =
        kTargetFragmentationPercentForOptimizeMemory;
    *max_evacuated_bytes = kMaxEvacuatedBytesForOptimizeMemory;
  } else {
    const double estimated_compaction_speed =
        heap()->tracer()->CompactionSpeedInBytesPerMillisecond();
    if (estimated_compaction_speed != 0) {
      // Estimate the target fragmentation based on traced compaction speed
      // and a goal for a single page.
      const double estimated_ms_per_area =
          1 + area_size / estimated_compaction_speed;
      *target_fragmentation_percent = static_cast<int>(
          100 - 100 * kTargetMsPerArea / estimated_ms_per_area);
      if (*target_fragmentation_percent <
          kTargetFragmentationPercentForReduceMemory) {
        *target_fragmentation_percent =
            kTargetFragmentationPercentForReduceMemory;
      }
    } else {
      *target_fragmentation_percent = kTargetFragmentationPercent;
    }
    *max_evacuated_bytes = kMaxEvacuatedBytes;
  }
}

void MarkCompactCollector::CollectEvacuationCandidates(PagedSpace* space) {
  DCHECK(space->identity() == OLD_SPACE || space->identity() == CODE_SPACE);

  int number_of_pages = space->CountTotalPages();
  size_t area_size = space->AreaSize();

  // Pairs of (live_bytes_in_page, page).
  typedef std::pair<size_t, Page*> LiveBytesPagePair;
  std::vector<LiveBytesPagePair> pages;
  pages.reserve(number_of_pages);

  DCHECK(!sweeping_in_progress());
  Page* owner_of_linear_allocation_area =
      space->top() == space->limit()
          ? nullptr
          : Page::FromAllocationAreaAddress(space->top());
  for (Page* p : *space) {
    if (p->NeverEvacuate() || (p == owner_of_linear_allocation_area) ||
        !p->CanAllocate())
      continue;
    // Invariant: Evacuation candidates are just created when marking is
    // started. This means that sweeping has finished. Furthermore, at the end
    // of a GC all evacuation candidates are cleared and their slot buffers are
    // released.
    CHECK(!p->IsEvacuationCandidate());
    CHECK_NULL(p->slot_set<OLD_TO_OLD>());
    CHECK_NULL(p->typed_slot_set<OLD_TO_OLD>());
    CHECK(p->SweepingDone());
    DCHECK(p->area_size() == area_size);
    pages.push_back(std::make_pair(p->allocated_bytes(), p));
  }

  int candidate_count = 0;
  size_t total_live_bytes = 0;

  const bool reduce_memory = heap()->ShouldReduceMemory();
  if (FLAG_manual_evacuation_candidates_selection) {
    for (size_t i = 0; i < pages.size(); i++) {
      Page* p = pages[i].second;
      if (p->IsFlagSet(MemoryChunk::FORCE_EVACUATION_CANDIDATE_FOR_TESTING)) {
        candidate_count++;
        total_live_bytes += pages[i].first;
        p->ClearFlag(MemoryChunk::FORCE_EVACUATION_CANDIDATE_FOR_TESTING);
        AddEvacuationCandidate(p);
      }
    }
  } else if (FLAG_stress_compaction_random) {
    double fraction = isolate()->fuzzer_rng()->NextDouble();
    size_t pages_to_mark_count =
        static_cast<size_t>(fraction * (pages.size() + 1));
    for (uint64_t i : isolate()->fuzzer_rng()->NextSample(
             pages.size(), pages_to_mark_count)) {
      candidate_count++;
      total_live_bytes += pages[i].first;
      AddEvacuationCandidate(pages[i].second);
    }
  } else if (FLAG_stress_compaction) {
    for (size_t i = 0; i < pages.size(); i++) {
      Page* p = pages[i].second;
      if (i % 2 == 0) {
        candidate_count++;
        total_live_bytes += pages[i].first;
        AddEvacuationCandidate(p);
      }
    }
  } else {
    // The following approach determines the pages that should be evacuated.
    //
    // We use two conditions to decide whether a page qualifies as an evacuation
    // candidate, or not:
    // * Target fragmentation: How fragmented is a page, i.e., how is the ratio
    //   between live bytes and capacity of this page (= area).
    // * Evacuation quota: A global quota determining how much bytes should be
    //   compacted.
    //
    // The algorithm sorts all pages by live bytes and then iterates through
    // them starting with the page with the most free memory, adding them to the
    // set of evacuation candidates as long as both conditions (fragmentation
    // and quota) hold.
    size_t max_evacuated_bytes;
    int target_fragmentation_percent;
    ComputeEvacuationHeuristics(area_size, &target_fragmentation_percent,
                                &max_evacuated_bytes);

    const size_t free_bytes_threshold =
        target_fragmentation_percent * (area_size / 100);

    // Sort pages from the most free to the least free, then select
    // the first n pages for evacuation such that:
    // - the total size of evacuated objects does not exceed the specified
    // limit.
    // - fragmentation of (n+1)-th page does not exceed the specified limit.
    std::sort(pages.begin(), pages.end(),
              [](const LiveBytesPagePair& a, const LiveBytesPagePair& b) {
                return a.first < b.first;
              });
    for (size_t i = 0; i < pages.size(); i++) {
      size_t live_bytes = pages[i].first;
      DCHECK_GE(area_size, live_bytes);
      size_t free_bytes = area_size - live_bytes;
      if (FLAG_always_compact ||
          ((free_bytes >= free_bytes_threshold) &&
           ((total_live_bytes + live_bytes) <= max_evacuated_bytes))) {
        candidate_count++;
        total_live_bytes += live_bytes;
      }
      if (FLAG_trace_fragmentation_verbose) {
        PrintIsolate(isolate(),
                     "compaction-selection-page: space=%s free_bytes_page=%zu "
                     "fragmentation_limit_kb=%" PRIuS
                     " fragmentation_limit_percent=%d sum_compaction_kb=%zu "
                     "compaction_limit_kb=%zu\n",
                     space->name(), free_bytes / KB, free_bytes_threshold / KB,
                     target_fragmentation_percent, total_live_bytes / KB,
                     max_evacuated_bytes / KB);
      }
    }
    // How many pages we will allocated for the evacuated objects
    // in the worst case: ceil(total_live_bytes / area_size)
    int estimated_new_pages =
        static_cast<int>((total_live_bytes + area_size - 1) / area_size);
    DCHECK_LE(estimated_new_pages, candidate_count);
    int estimated_released_pages = candidate_count - estimated_new_pages;
    // Avoid (compact -> expand) cycles.
    if ((estimated_released_pages == 0) && !FLAG_always_compact) {
      candidate_count = 0;
    }
    for (int i = 0; i < candidate_count; i++) {
      AddEvacuationCandidate(pages[i].second);
    }
  }

  if (FLAG_trace_fragmentation) {
    PrintIsolate(isolate(),
                 "compaction-selection: space=%s reduce_memory=%d pages=%d "
                 "total_live_bytes=%zu\n",
                 space->name(), reduce_memory, candidate_count,
                 total_live_bytes / KB);
  }
}


void MarkCompactCollector::AbortCompaction() {
  if (compacting_) {
    RememberedSet<OLD_TO_OLD>::ClearAll(heap());
    for (Page* p : evacuation_candidates_) {
      p->ClearEvacuationCandidate();
    }
    compacting_ = false;
    evacuation_candidates_.clear();
  }
  DCHECK(evacuation_candidates_.empty());
}


void MarkCompactCollector::Prepare() {
  was_marked_incrementally_ = heap()->incremental_marking()->IsMarking();

#ifdef DEBUG
  DCHECK(state_ == IDLE);
  state_ = PREPARE_GC;
#endif

  DCHECK(!FLAG_never_compact || !FLAG_always_compact);

  // Instead of waiting we could also abort the sweeper threads here.
  EnsureSweepingCompleted();

  if (heap()->incremental_marking()->IsSweeping()) {
    heap()->incremental_marking()->Stop();
  }

  heap()->memory_allocator()->unmapper()->PrepareForMarkCompact();

  // Clear marking bits if incremental marking is aborted.
  if (was_marked_incrementally_ && heap_->ShouldAbortIncrementalMarking()) {
    heap()->incremental_marking()->Stop();
    heap()->incremental_marking()->AbortBlackAllocation();
    FinishConcurrentMarking(ConcurrentMarking::StopRequest::PREEMPT_TASKS);
    heap()->incremental_marking()->Deactivate();
    ClearMarkbits();
    AbortWeakObjects();
    AbortCompaction();
    heap_->local_embedder_heap_tracer()->AbortTracing();
    marking_worklist()->Clear();
    was_marked_incrementally_ = false;
  }

  if (!was_marked_incrementally_) {
    TRACE_GC(heap()->tracer(), GCTracer::Scope::MC_MARK_WRAPPER_PROLOGUE);
    heap_->local_embedder_heap_tracer()->TracePrologue();
  }

  // Don't start compaction if we are in the middle of incremental
  // marking cycle. We did not collect any slots.
  if (!FLAG_never_compact && !was_marked_incrementally_) {
    StartCompaction();
  }

  PagedSpaces spaces(heap());
  for (PagedSpace* space = spaces.next(); space != nullptr;
       space = spaces.next()) {
    space->PrepareForMarkCompact();
  }
  heap()->account_external_memory_concurrently_freed();

#ifdef VERIFY_HEAP
  if (!was_marked_incrementally_ && FLAG_verify_heap) {
    VerifyMarkbitsAreClean();
  }
#endif
}

void MarkCompactCollector::FinishConcurrentMarking(
    ConcurrentMarking::StopRequest stop_request) {
  if (FLAG_concurrent_marking) {
    heap()->concurrent_marking()->Stop(stop_request);
    heap()->concurrent_marking()->FlushLiveBytes(non_atomic_marking_state());
  }
}

void MarkCompactCollector::VerifyMarking() {
  CHECK(marking_worklist()->IsEmpty());
  DCHECK(heap_->incremental_marking()->IsStopped());
#ifdef VERIFY_HEAP
  if (FLAG_verify_heap) {
    FullMarkingVerifier verifier(heap());
    verifier.Run();
  }
#endif
#ifdef VERIFY_HEAP
  if (FLAG_verify_heap) {
    heap()->old_space()->VerifyLiveBytes();
    heap()->map_space()->VerifyLiveBytes();
    heap()->code_space()->VerifyLiveBytes();
  }
#endif
}

void MarkCompactCollector::Finish() {
  TRACE_GC(heap()->tracer(), GCTracer::Scope::MC_FINISH);

#ifdef DEBUG
  heap()->VerifyCountersBeforeConcurrentSweeping();
#endif

  CHECK(weak_objects_.current_ephemerons.IsEmpty());
  CHECK(weak_objects_.discovered_ephemerons.IsEmpty());
  weak_objects_.next_ephemerons.Clear();

  sweeper()->StartSweeperTasks();
  sweeper()->StartIterabilityTasks();

  // Clear the marking state of live large objects.
  heap_->lo_space()->ClearMarkingStateOfLiveObjects();

#ifdef DEBUG
  DCHECK(state_ == SWEEP_SPACES || state_ == RELOCATE_OBJECTS);
  state_ = IDLE;
#endif
  heap_->isolate()->inner_pointer_to_code_cache()->Flush();

  // The stub caches are not traversed during GC; clear them to force
  // their lazy re-initialization. This must be done after the
  // GC, because it relies on the new address of certain old space
  // objects (empty string, illegal builtin).
  isolate()->load_stub_cache()->Clear();
  isolate()->store_stub_cache()->Clear();

  if (have_code_to_deoptimize_) {
    // Some code objects were marked for deoptimization during the GC.
    Deoptimizer::DeoptimizeMarkedCode(isolate());
    have_code_to_deoptimize_ = false;
  }
}

class MarkCompactCollector::RootMarkingVisitor final : public RootVisitor {
 public:
  explicit RootMarkingVisitor(MarkCompactCollector* collector)
      : collector_(collector) {}

  void VisitRootPointer(Root root, const char* description, Object** p) final {
    MarkObjectByPointer(root, p);
  }

  void VisitRootPointers(Root root, const char* description, Object** start,
                         Object** end) final {
    for (Object** p = start; p < end; p++) MarkObjectByPointer(root, p);
  }

 private:
  V8_INLINE void MarkObjectByPointer(Root root, Object** p) {
    if (!(*p)->IsHeapObject()) return;

    collector_->MarkRootObject(root, HeapObject::cast(*p));
  }

  MarkCompactCollector* const collector_;
};

// This visitor is used to visit the body of special objects held alive by
// other roots.
//
// It is currently used for
// - Code held alive by the top optimized frame. This code cannot be deoptimized
// and thus have to be kept alive in an isolate way, i.e., it should not keep
// alive other code objects reachable through the weak list but they should
// keep alive its embedded pointers (which would otherwise be dropped).
// - Prefix of the string table.
class MarkCompactCollector::CustomRootBodyMarkingVisitor final
    : public ObjectVisitor {
 public:
  explicit CustomRootBodyMarkingVisitor(MarkCompactCollector* collector)
      : collector_(collector) {}

  void VisitPointer(HeapObject* host, Object** p) final {
    MarkObject(host, *p);
  }

  void VisitPointers(HeapObject* host, Object** start, Object** end) final {
    for (Object** p = start; p < end; p++) {
      DCHECK(!HasWeakHeapObjectTag(*p));
      MarkObject(host, *p);
    }
  }

  void VisitPointers(HeapObject* host, MaybeObject** start,
                     MaybeObject** end) final {
    // At the moment, custom roots cannot contain weak pointers.
    UNREACHABLE();
  }

  // VisitEmbedderPointer is defined by ObjectVisitor to call VisitPointers.

 private:
  void MarkObject(HeapObject* host, Object* object) {
    if (!object->IsHeapObject()) return;
    collector_->MarkObject(host, HeapObject::cast(object));
  }

  MarkCompactCollector* const collector_;
};

class InternalizedStringTableCleaner : public ObjectVisitor {
 public:
  InternalizedStringTableCleaner(Heap* heap, HeapObject* table)
      : heap_(heap), pointers_removed_(0), table_(table) {}

  void VisitPointers(HeapObject* host, Object** start, Object** end) override {
    // Visit all HeapObject pointers in [start, end).
    Object* the_hole = ReadOnlyRoots(heap_).the_hole_value();
    MarkCompactCollector::NonAtomicMarkingState* marking_state =
        heap_->mark_compact_collector()->non_atomic_marking_state();
    for (Object** p = start; p < end; p++) {
      Object* o = *p;
      if (o->IsHeapObject()) {
        HeapObject* heap_object = HeapObject::cast(o);
        if (marking_state->IsWhite(heap_object)) {
          pointers_removed_++;
          // Set the entry to the_hole_value (as deleted).
          *p = the_hole;
        } else {
          // StringTable contains only old space strings.
          DCHECK(!Heap::InNewSpace(o));
          MarkCompactCollector::RecordSlot(table_, p, heap_object);
        }
      }
    }
  }

  void VisitPointers(HeapObject* host, MaybeObject** start,
                     MaybeObject** end) final {
    UNREACHABLE();
  }

  int PointersRemoved() {
    return pointers_removed_;
  }

 private:
  Heap* heap_;
  int pointers_removed_;
  HeapObject* table_;
};

class ExternalStringTableCleaner : public RootVisitor {
 public:
  explicit ExternalStringTableCleaner(Heap* heap) : heap_(heap) {}

  void VisitRootPointers(Root root, const char* description, Object** start,
                         Object** end) override {
    // Visit all HeapObject pointers in [start, end).
    MarkCompactCollector::NonAtomicMarkingState* marking_state =
        heap_->mark_compact_collector()->non_atomic_marking_state();
    Object* the_hole = ReadOnlyRoots(heap_).the_hole_value();
    for (Object** p = start; p < end; p++) {
      Object* o = *p;
      if (o->IsHeapObject()) {
        HeapObject* heap_object = HeapObject::cast(o);
        if (marking_state->IsWhite(heap_object)) {
          if (o->IsExternalString()) {
            heap_->FinalizeExternalString(String::cast(*p));
          } else {
            // The original external string may have been internalized.
            DCHECK(o->IsThinString());
          }
          // Set the entry to the_hole_value (as deleted).
          *p = the_hole;
        }
      }
    }
  }

 private:
  Heap* heap_;
};

// Implementation of WeakObjectRetainer for mark compact GCs. All marked objects
// are retained.
class MarkCompactWeakObjectRetainer : public WeakObjectRetainer {
 public:
  explicit MarkCompactWeakObjectRetainer(
      MarkCompactCollector::NonAtomicMarkingState* marking_state)
      : marking_state_(marking_state) {}

  virtual Object* RetainAs(Object* object) {
    HeapObject* heap_object = HeapObject::cast(object);
    DCHECK(!marking_state_->IsGrey(heap_object));
    if (marking_state_->IsBlack(heap_object)) {
      return object;
    } else if (object->IsAllocationSite() &&
               !(AllocationSite::cast(object)->IsZombie())) {
      // "dead" AllocationSites need to live long enough for a traversal of new
      // space. These sites get a one-time reprieve.

      Object* nested = object;
      while (nested->IsAllocationSite()) {
        AllocationSite* current_site = AllocationSite::cast(nested);
        // MarkZombie will override the nested_site, read it first before
        // marking
        nested = current_site->nested_site();
        current_site->MarkZombie();
        marking_state_->WhiteToBlack(current_site);
      }

      return object;
    } else {
      return nullptr;
    }
  }

 private:
  MarkCompactCollector::NonAtomicMarkingState* marking_state_;
};

class RecordMigratedSlotVisitor : public ObjectVisitor {
 public:
  explicit RecordMigratedSlotVisitor(MarkCompactCollector* collector)
      : collector_(collector) {}

  inline void VisitPointer(HeapObject* host, Object** p) final {
    DCHECK(!HasWeakHeapObjectTag(*p));
    RecordMigratedSlot(host, reinterpret_cast<MaybeObject*>(*p),
                       reinterpret_cast<Address>(p));
  }

  inline void VisitPointer(HeapObject* host, MaybeObject** p) final {
    RecordMigratedSlot(host, *p, reinterpret_cast<Address>(p));
  }

  inline void VisitPointers(HeapObject* host, Object** start,
                            Object** end) final {
    while (start < end) {
      VisitPointer(host, start);
      ++start;
    }
  }

  inline void VisitPointers(HeapObject* host, MaybeObject** start,
                            MaybeObject** end) final {
    while (start < end) {
      VisitPointer(host, start);
      ++start;
    }
  }

  inline void VisitCodeTarget(Code* host, RelocInfo* rinfo) override {
    DCHECK_EQ(host, rinfo->host());
    DCHECK(RelocInfo::IsCodeTargetMode(rinfo->rmode()));
    Code* target = Code::GetCodeFromTargetAddress(rinfo->target_address());
    // The target is always in old space, we don't have to record the slot in
    // the old-to-new remembered set.
    DCHECK(!Heap::InNewSpace(target));
    collector_->RecordRelocSlot(host, rinfo, target);
  }

  inline void VisitEmbeddedPointer(Code* host, RelocInfo* rinfo) override {
    DCHECK_EQ(host, rinfo->host());
    DCHECK(rinfo->rmode() == RelocInfo::EMBEDDED_OBJECT);
    HeapObject* object = HeapObject::cast(rinfo->target_object());
    GenerationalBarrierForCode(host, rinfo, object);
    collector_->RecordRelocSlot(host, rinfo, object);
  }

  // Entries that are skipped for recording.
  inline void VisitExternalReference(Code* host, RelocInfo* rinfo) final {}
  inline void VisitExternalReference(Foreign* host, Address* p) final {}
  inline void VisitRuntimeEntry(Code* host, RelocInfo* rinfo) final {}
  inline void VisitInternalReference(Code* host, RelocInfo* rinfo) final {}

 protected:
  inline virtual void RecordMigratedSlot(HeapObject* host, MaybeObject* value,
                                         Address slot) {
    if (value->IsStrongOrWeakHeapObject()) {
      Page* p = Page::FromAddress(reinterpret_cast<Address>(value));
      if (p->InNewSpace()) {
        DCHECK_IMPLIES(p->InToSpace(),
                       p->IsFlagSet(Page::PAGE_NEW_NEW_PROMOTION));
        RememberedSet<OLD_TO_NEW>::Insert<AccessMode::NON_ATOMIC>(
            Page::FromAddress(slot), slot);
      } else if (p->IsEvacuationCandidate()) {
        RememberedSet<OLD_TO_OLD>::Insert<AccessMode::NON_ATOMIC>(
            Page::FromAddress(slot), slot);
      }
    }
  }

  MarkCompactCollector* collector_;
};

class MigrationObserver {
 public:
  explicit MigrationObserver(Heap* heap) : heap_(heap) {}

  virtual ~MigrationObserver() {}
  virtual void Move(AllocationSpace dest, HeapObject* src, HeapObject* dst,
                    int size) = 0;

 protected:
  Heap* heap_;
};

class ProfilingMigrationObserver final : public MigrationObserver {
 public:
  explicit ProfilingMigrationObserver(Heap* heap) : MigrationObserver(heap) {}

  inline void Move(AllocationSpace dest, HeapObject* src, HeapObject* dst,
                   int size) final {
    if (dest == CODE_SPACE || (dest == OLD_SPACE && dst->IsBytecodeArray())) {
      PROFILE(heap_->isolate(),
              CodeMoveEvent(AbstractCode::cast(src), AbstractCode::cast(dst)));
    }
    heap_->OnMoveEvent(dst, src, size);
  }
};

class HeapObjectVisitor {
 public:
  virtual ~HeapObjectVisitor() {}
  virtual bool Visit(HeapObject* object, int size) = 0;
};

class EvacuateVisitorBase : public HeapObjectVisitor {
 public:
  void AddObserver(MigrationObserver* observer) {
    migration_function_ = RawMigrateObject<MigrationMode::kObserved>;
    observers_.push_back(observer);
  }

 protected:
  enum MigrationMode { kFast, kObserved };

  typedef void (*MigrateFunction)(EvacuateVisitorBase* base, HeapObject* dst,
                                  HeapObject* src, int size,
                                  AllocationSpace dest);

  template <MigrationMode mode>
  static void RawMigrateObject(EvacuateVisitorBase* base, HeapObject* dst,
                               HeapObject* src, int size,
                               AllocationSpace dest) {
    Address dst_addr = dst->address();
    Address src_addr = src->address();
    DCHECK(base->heap_->AllowedToBeMigrated(src, dest));
    DCHECK(dest != LO_SPACE);
    if (dest == OLD_SPACE) {
      DCHECK_OBJECT_SIZE(size);
      DCHECK(IsAligned(size, kPointerSize));
      base->heap_->CopyBlock(dst_addr, src_addr, size);
      if (mode != MigrationMode::kFast)
        base->ExecuteMigrationObservers(dest, src, dst, size);
      dst->IterateBodyFast(dst->map(), size, base->record_visitor_);
    } else if (dest == CODE_SPACE) {
      DCHECK_CODEOBJECT_SIZE(size, base->heap_->code_space());
      base->heap_->CopyBlock(dst_addr, src_addr, size);
      Code::cast(dst)->Relocate(dst_addr - src_addr);
      if (mode != MigrationMode::kFast)
        base->ExecuteMigrationObservers(dest, src, dst, size);
      dst->IterateBodyFast(dst->map(), size, base->record_visitor_);
    } else {
      DCHECK_OBJECT_SIZE(size);
      DCHECK(dest == NEW_SPACE);
      base->heap_->CopyBlock(dst_addr, src_addr, size);
      if (mode != MigrationMode::kFast)
        base->ExecuteMigrationObservers(dest, src, dst, size);
    }
    base::Relaxed_Store(reinterpret_cast<base::AtomicWord*>(src_addr),
                        static_cast<base::AtomicWord>(dst_addr));
  }

  EvacuateVisitorBase(Heap* heap, LocalAllocator* local_allocator,
                      RecordMigratedSlotVisitor* record_visitor)
      : heap_(heap),
        local_allocator_(local_allocator),
        record_visitor_(record_visitor) {
    migration_function_ = RawMigrateObject<MigrationMode::kFast>;
  }

  inline bool TryEvacuateObject(AllocationSpace target_space,
                                HeapObject* object, int size,
                                HeapObject** target_object) {
#ifdef VERIFY_HEAP
    if (AbortCompactionForTesting(object)) return false;
#endif  // VERIFY_HEAP
    AllocationAlignment alignment =
        HeapObject::RequiredAlignment(object->map());
    AllocationResult allocation =
        local_allocator_->Allocate(target_space, size, alignment);
    if (allocation.To(target_object)) {
      MigrateObject(*target_object, object, size, target_space);
      return true;
    }
    return false;
  }

  inline void ExecuteMigrationObservers(AllocationSpace dest, HeapObject* src,
                                        HeapObject* dst, int size) {
    for (MigrationObserver* obs : observers_) {
      obs->Move(dest, src, dst, size);
    }
  }

  inline void MigrateObject(HeapObject* dst, HeapObject* src, int size,
                            AllocationSpace dest) {
    migration_function_(this, dst, src, size, dest);
  }

#ifdef VERIFY_HEAP
  bool AbortCompactionForTesting(HeapObject* object) {
    if (FLAG_stress_compaction) {
      const uintptr_t mask = static_cast<uintptr_t>(FLAG_random_seed) &
                             kPageAlignmentMask & ~kPointerAlignmentMask;
      if ((object->address() & kPageAlignmentMask) == mask) {
        Page* page = Page::FromAddress(object->address());
        if (page->IsFlagSet(Page::COMPACTION_WAS_ABORTED_FOR_TESTING)) {
          page->ClearFlag(Page::COMPACTION_WAS_ABORTED_FOR_TESTING);
        } else {
          page->SetFlag(Page::COMPACTION_WAS_ABORTED_FOR_TESTING);
          return true;
        }
      }
    }
    return false;
  }
#endif  // VERIFY_HEAP

  Heap* heap_;
  LocalAllocator* local_allocator_;
  RecordMigratedSlotVisitor* record_visitor_;
  std::vector<MigrationObserver*> observers_;
  MigrateFunction migration_function_;
};

class EvacuateNewSpaceVisitor final : public EvacuateVisitorBase {
 public:
  explicit EvacuateNewSpaceVisitor(
      Heap* heap, LocalAllocator* local_allocator,
      RecordMigratedSlotVisitor* record_visitor,
      Heap::PretenuringFeedbackMap* local_pretenuring_feedback)
      : EvacuateVisitorBase(heap, local_allocator, record_visitor),
        buffer_(LocalAllocationBuffer::InvalidBuffer()),
        promoted_size_(0),
        semispace_copied_size_(0),
        local_pretenuring_feedback_(local_pretenuring_feedback),
        is_incremental_marking_(heap->incremental_marking()->IsMarking()) {}

  inline bool Visit(HeapObject* object, int size) override {
    if (TryEvacuateWithoutCopy(object)) return true;
    HeapObject* target_object = nullptr;
    if (heap_->ShouldBePromoted(object->address()) &&
        TryEvacuateObject(OLD_SPACE, object, size, &target_object)) {
      promoted_size_ += size;
      return true;
    }
    heap_->UpdateAllocationSite(object->map(), object,
                                local_pretenuring_feedback_);
    HeapObject* target = nullptr;
    AllocationSpace space = AllocateTargetObject(object, size, &target);
    MigrateObject(HeapObject::cast(target), object, size, space);
    semispace_copied_size_ += size;
    return true;
  }

  intptr_t promoted_size() { return promoted_size_; }
  intptr_t semispace_copied_size() { return semispace_copied_size_; }

 private:
  inline bool TryEvacuateWithoutCopy(HeapObject* object) {
    if (is_incremental_marking_) return false;

    Map* map = object->map();

    // Some objects can be evacuated without creating a copy.
    if (map->visitor_id() == kVisitThinString) {
      HeapObject* actual = ThinString::cast(object)->unchecked_actual();
      if (MarkCompactCollector::IsOnEvacuationCandidate(actual)) return false;
      base::Relaxed_Store(
          reinterpret_cast<base::AtomicWord*>(object->address()),
          reinterpret_cast<base::AtomicWord>(
              MapWord::FromForwardingAddress(actual).ToMap()));
      return true;
    }
    // TODO(mlippautz): Handle ConsString.

    return false;
  }

  inline AllocationSpace AllocateTargetObject(HeapObject* old_object, int size,
                                              HeapObject** target_object) {
    AllocationAlignment alignment =
        HeapObject::RequiredAlignment(old_object->map());
    AllocationSpace space_allocated_in = NEW_SPACE;
    AllocationResult allocation =
        local_allocator_->Allocate(NEW_SPACE, size, alignment);
    if (allocation.IsRetry()) {
      allocation = AllocateInOldSpace(size, alignment);
      space_allocated_in = OLD_SPACE;
    }
    bool ok = allocation.To(target_object);
    DCHECK(ok);
    USE(ok);
    return space_allocated_in;
  }

  inline AllocationResult AllocateInOldSpace(int size_in_bytes,
                                             AllocationAlignment alignment) {
    AllocationResult allocation =
        local_allocator_->Allocate(OLD_SPACE, size_in_bytes, alignment);
    if (allocation.IsRetry()) {
      heap_->FatalProcessOutOfMemory(
          "MarkCompactCollector: semi-space copy, fallback in old gen");
    }
    return allocation;
  }

  LocalAllocationBuffer buffer_;
  intptr_t promoted_size_;
  intptr_t semispace_copied_size_;
  Heap::PretenuringFeedbackMap* local_pretenuring_feedback_;
  bool is_incremental_marking_;
};

template <PageEvacuationMode mode>
class EvacuateNewSpacePageVisitor final : public HeapObjectVisitor {
 public:
  explicit EvacuateNewSpacePageVisitor(
      Heap* heap, RecordMigratedSlotVisitor* record_visitor,
      Heap::PretenuringFeedbackMap* local_pretenuring_feedback)
      : heap_(heap),
        record_visitor_(record_visitor),
        moved_bytes_(0),
        local_pretenuring_feedback_(local_pretenuring_feedback) {}

  static void Move(Page* page) {
    switch (mode) {
      case NEW_TO_NEW:
        page->heap()->new_space()->MovePageFromSpaceToSpace(page);
        page->SetFlag(Page::PAGE_NEW_NEW_PROMOTION);
        break;
      case NEW_TO_OLD: {
        page->heap()->new_space()->from_space().RemovePage(page);
        Page* new_page = Page::ConvertNewToOld(page);
        DCHECK(!new_page->InNewSpace());
        new_page->SetFlag(Page::PAGE_NEW_OLD_PROMOTION);
        break;
      }
    }
  }

  inline bool Visit(HeapObject* object, int size) {
    if (mode == NEW_TO_NEW) {
      heap_->UpdateAllocationSite(object->map(), object,
                                  local_pretenuring_feedback_);
    } else if (mode == NEW_TO_OLD) {
      object->IterateBodyFast(record_visitor_);
    }
    return true;
  }

  intptr_t moved_bytes() { return moved_bytes_; }
  void account_moved_bytes(intptr_t bytes) { moved_bytes_ += bytes; }

 private:
  Heap* heap_;
  RecordMigratedSlotVisitor* record_visitor_;
  intptr_t moved_bytes_;
  Heap::PretenuringFeedbackMap* local_pretenuring_feedback_;
};

class EvacuateOldSpaceVisitor final : public EvacuateVisitorBase {
 public:
  EvacuateOldSpaceVisitor(Heap* heap, LocalAllocator* local_allocator,
                          RecordMigratedSlotVisitor* record_visitor)
      : EvacuateVisitorBase(heap, local_allocator, record_visitor) {}

  inline bool Visit(HeapObject* object, int size) override {
    HeapObject* target_object = nullptr;
    if (TryEvacuateObject(
            Page::FromAddress(object->address())->owner()->identity(), object,
            size, &target_object)) {
      DCHECK(object->map_word().IsForwardingAddress());
      return true;
    }
    return false;
  }
};

class EvacuateRecordOnlyVisitor final : public HeapObjectVisitor {
 public:
  explicit EvacuateRecordOnlyVisitor(Heap* heap) : heap_(heap) {}

  inline bool Visit(HeapObject* object, int size) {
    RecordMigratedSlotVisitor visitor(heap_->mark_compact_collector());
    object->IterateBodyFast(&visitor);
    return true;
  }

 private:
  Heap* heap_;
};

bool MarkCompactCollector::IsUnmarkedHeapObject(Heap* heap, Object** p) {
  Object* o = *p;
  if (!o->IsHeapObject()) return false;
  HeapObject* heap_object = HeapObject::cast(o);
  return heap->mark_compact_collector()->non_atomic_marking_state()->IsWhite(
      heap_object);
}

void MarkCompactCollector::MarkStringTable(
    ObjectVisitor* custom_root_body_visitor) {
  StringTable* string_table = heap()->string_table();
  // Mark the string table itself.
  if (marking_state()->WhiteToBlack(string_table)) {
    // Explicitly mark the prefix.
    string_table->IteratePrefix(custom_root_body_visitor);
  }
}

void MarkCompactCollector::MarkRoots(RootVisitor* root_visitor,
                                     ObjectVisitor* custom_root_body_visitor) {
  // Mark the heap roots including global variables, stack variables,
  // etc., and all objects reachable from them.
  heap()->IterateStrongRoots(root_visitor, VISIT_ONLY_STRONG);

  // Custom marking for string table and top optimized frame.
  MarkStringTable(custom_root_body_visitor);
  ProcessTopOptimizedFrame(custom_root_body_visitor);
}

void MarkCompactCollector::ProcessEphemeronsUntilFixpoint() {
  bool work_to_do = true;
  int iterations = 0;
  int max_iterations = FLAG_ephemeron_fixpoint_iterations;

  while (work_to_do) {
    PerformWrapperTracing();

    if (iterations >= max_iterations) {
      // Give up fixpoint iteration and switch to linear algorithm.
      ProcessEphemeronsLinear();
      break;
    }

    // Move ephemerons from next_ephemerons into current_ephemerons to
    // drain them in this iteration.
    weak_objects_.current_ephemerons.Swap(weak_objects_.next_ephemerons);
    heap()->concurrent_marking()->set_ephemeron_marked(false);

    {
      TRACE_GC(heap()->tracer(),
               GCTracer::Scope::MC_MARK_WEAK_CLOSURE_EPHEMERON_MARKING);

      if (FLAG_parallel_marking) {
        DCHECK(FLAG_concurrent_marking);
        heap_->concurrent_marking()->RescheduleTasksIfNeeded();
      }

      work_to_do = ProcessEphemerons();
      FinishConcurrentMarking(
          ConcurrentMarking::StopRequest::COMPLETE_ONGOING_TASKS);
    }

    CHECK(weak_objects_.current_ephemerons.IsEmpty());
    CHECK(weak_objects_.discovered_ephemerons.IsEmpty());

    work_to_do = work_to_do || !marking_worklist()->IsEmpty() ||
                 heap()->concurrent_marking()->ephemeron_marked() ||
                 !heap()->local_embedder_heap_tracer()->IsRemoteTracingDone();
    ++iterations;
  }

  CHECK(marking_worklist()->IsEmpty());
  CHECK(weak_objects_.current_ephemerons.IsEmpty());
  CHECK(weak_objects_.discovered_ephemerons.IsEmpty());
}

bool MarkCompactCollector::ProcessEphemerons() {
  Ephemeron ephemeron;
  bool ephemeron_marked = false;

  // Drain current_ephemerons and push ephemerons where key and value are still
  // unreachable into next_ephemerons.
  while (weak_objects_.current_ephemerons.Pop(kMainThread, &ephemeron)) {
    if (VisitEphemeron(ephemeron.key, ephemeron.value)) {
      ephemeron_marked = true;
    }
  }

  // Drain marking worklist and push discovered ephemerons into
  // discovered_ephemerons.
  ProcessMarkingWorklist();

  // Drain discovered_ephemerons (filled in the drain MarkingWorklist-phase
  // before) and push ephemerons where key and value are still unreachable into
  // next_ephemerons.
  while (weak_objects_.discovered_ephemerons.Pop(kMainThread, &ephemeron)) {
    if (VisitEphemeron(ephemeron.key, ephemeron.value)) {
      ephemeron_marked = true;
    }
  }

  // Flush local ephemerons for main task to global pool.
  weak_objects_.ephemeron_hash_tables.FlushToGlobal(kMainThread);
  weak_objects_.next_ephemerons.FlushToGlobal(kMainThread);

  return ephemeron_marked;
}

void MarkCompactCollector::ProcessEphemeronsLinear() {
  TRACE_GC(heap()->tracer(),
           GCTracer::Scope::MC_MARK_WEAK_CLOSURE_EPHEMERON_LINEAR);
  CHECK(heap()->concurrent_marking()->IsStopped());
  std::unordered_multimap<HeapObject*, HeapObject*> key_to_values;
  Ephemeron ephemeron;

  DCHECK(weak_objects_.current_ephemerons.IsEmpty());
  weak_objects_.current_ephemerons.Swap(weak_objects_.next_ephemerons);

  while (weak_objects_.current_ephemerons.Pop(kMainThread, &ephemeron)) {
    VisitEphemeron(ephemeron.key, ephemeron.value);

    if (non_atomic_marking_state()->IsWhite(ephemeron.value)) {
      key_to_values.insert(std::make_pair(ephemeron.key, ephemeron.value));
    }
  }

  ephemeron_marking_.newly_discovered_limit = key_to_values.size();
  bool work_to_do = true;

  while (work_to_do) {
    PerformWrapperTracing();

    ResetNewlyDiscovered();
    ephemeron_marking_.newly_discovered_limit = key_to_values.size();

    {
      TRACE_GC(heap()->tracer(),
               GCTracer::Scope::MC_MARK_WEAK_CLOSURE_EPHEMERON_MARKING);
      // Drain marking worklist and push all discovered objects into
      // newly_discovered.
      ProcessMarkingWorklistInternal<
          MarkCompactCollector::MarkingWorklistProcessingMode::
              kTrackNewlyDiscoveredObjects>();
    }

    while (weak_objects_.discovered_ephemerons.Pop(kMainThread, &ephemeron)) {
      VisitEphemeron(ephemeron.key, ephemeron.value);

      if (non_atomic_marking_state()->IsWhite(ephemeron.value)) {
        key_to_values.insert(std::make_pair(ephemeron.key, ephemeron.value));
      }
    }

    if (ephemeron_marking_.newly_discovered_overflowed) {
      // If newly_discovered was overflowed just visit all ephemerons in
      // next_ephemerons.
      weak_objects_.next_ephemerons.Iterate([&](Ephemeron ephemeron) {
        if (non_atomic_marking_state()->IsBlackOrGrey(ephemeron.key) &&
            non_atomic_marking_state()->WhiteToGrey(ephemeron.value)) {
          marking_worklist()->Push(ephemeron.value);
        }
      });

    } else {
      // This is the good case: newly_discovered stores all discovered
      // objects. Now use key_to_values to see if discovered objects keep more
      // objects alive due to ephemeron semantics.
      for (HeapObject* object : ephemeron_marking_.newly_discovered) {
        auto range = key_to_values.equal_range(object);
        for (auto it = range.first; it != range.second; ++it) {
          HeapObject* value = it->second;
          MarkObject(object, value);
        }
      }
    }

    // Do NOT drain marking worklist here, otherwise the current checks
    // for work_to_do are not sufficient for determining if another iteration
    // is necessary.

    work_to_do = !marking_worklist()->IsEmpty() ||
                 !heap()->local_embedder_heap_tracer()->IsRemoteTracingDone();
    CHECK(weak_objects_.discovered_ephemerons.IsEmpty());
  }

  ResetNewlyDiscovered();
  ephemeron_marking_.newly_discovered.shrink_to_fit();

  CHECK(marking_worklist()->IsEmpty());
}

void MarkCompactCollector::PerformWrapperTracing() {
  if (heap_->local_embedder_heap_tracer()->InUse()) {
    TRACE_GC(heap()->tracer(), GCTracer::Scope::MC_MARK_WRAPPER_TRACING);
    heap_->local_embedder_heap_tracer()->RegisterWrappersWithRemoteTracer();
    heap_->local_embedder_heap_tracer()->Trace(
        std::numeric_limits<double>::infinity());
  }
}

void MarkCompactCollector::ProcessMarkingWorklist() {
  ProcessMarkingWorklistInternal<
      MarkCompactCollector::MarkingWorklistProcessingMode::kDefault>();
}

template <MarkCompactCollector::MarkingWorklistProcessingMode mode>
void MarkCompactCollector::ProcessMarkingWorklistInternal() {
  HeapObject* object;
  MarkCompactMarkingVisitor visitor(this, marking_state());
  while ((object = marking_worklist()->Pop()) != nullptr) {
    DCHECK(!object->IsFiller());
    DCHECK(object->IsHeapObject());
    DCHECK(heap()->Contains(object));
    DCHECK(!(marking_state()->IsWhite(object)));
    marking_state()->GreyToBlack(object);
    if (mode == MarkCompactCollector::MarkingWorklistProcessingMode::
                    kTrackNewlyDiscoveredObjects) {
      AddNewlyDiscovered(object);
    }
    Map* map = object->map();
    MarkObject(object, map);
    visitor.Visit(map, object);
  }
  DCHECK(marking_worklist()->IsBailoutEmpty());
}

bool MarkCompactCollector::VisitEphemeron(HeapObject* key, HeapObject* value) {
  if (marking_state()->IsBlackOrGrey(key)) {
    if (marking_state()->WhiteToGrey(value)) {
      marking_worklist()->Push(value);
      return true;
    }

  } else if (marking_state()->IsWhite(value)) {
    weak_objects_.next_ephemerons.Push(kMainThread, Ephemeron{key, value});
  }

  return false;
}

void MarkCompactCollector::ProcessEphemeronMarking() {
  DCHECK(marking_worklist()->IsEmpty());

  // Incremental marking might leave ephemerons in main task's local
  // buffer, flush it into global pool.
  weak_objects_.next_ephemerons.FlushToGlobal(kMainThread);

  ProcessEphemeronsUntilFixpoint();

  CHECK(marking_worklist()->IsEmpty());
  CHECK(heap()->local_embedder_heap_tracer()->IsRemoteTracingDone());
}

void MarkCompactCollector::ProcessTopOptimizedFrame(ObjectVisitor* visitor) {
  for (StackFrameIterator it(isolate(), isolate()->thread_local_top());
       !it.done(); it.Advance()) {
    if (it.frame()->type() == StackFrame::INTERPRETED) {
      return;
    }
    if (it.frame()->type() == StackFrame::OPTIMIZED) {
      Code* code = it.frame()->LookupCode();
      if (!code->CanDeoptAt(it.frame()->pc())) {
        Code::BodyDescriptor::IterateBody(code->map(), code, visitor);
      }
      return;
    }
  }
}

void MarkCompactCollector::RecordObjectStats() {
  if (V8_UNLIKELY(FLAG_gc_stats)) {
    heap()->CreateObjectStats();
    ObjectStatsCollector collector(heap(), heap()->live_object_stats_,
                                   heap()->dead_object_stats_);
    collector.Collect();
    if (V8_UNLIKELY(FLAG_gc_stats &
                    v8::tracing::TracingCategoryObserver::ENABLED_BY_TRACING)) {
      std::stringstream live, dead;
      heap()->live_object_stats_->Dump(live);
      heap()->dead_object_stats_->Dump(dead);
      TRACE_EVENT_INSTANT2(TRACE_DISABLED_BY_DEFAULT("v8.gc_stats"),
                           "V8.GC_Objects_Stats", TRACE_EVENT_SCOPE_THREAD,
                           "live", TRACE_STR_COPY(live.str().c_str()), "dead",
                           TRACE_STR_COPY(dead.str().c_str()));
    }
    if (FLAG_trace_gc_object_stats) {
      heap()->live_object_stats_->PrintJSON("live");
      heap()->dead_object_stats_->PrintJSON("dead");
    }
    heap()->live_object_stats_->CheckpointObjectStats();
    heap()->dead_object_stats_->ClearObjectStats();
  }
}

void MarkCompactCollector::MarkLiveObjects() {
  TRACE_GC(heap()->tracer(), GCTracer::Scope::MC_MARK);
  // The recursive GC marker detects when it is nearing stack overflow,
  // and switches to a different marking system.  JS interrupts interfere
  // with the C stack limit check.
  PostponeInterruptsScope postpone(isolate());

  {
    TRACE_GC(heap()->tracer(), GCTracer::Scope::MC_MARK_FINISH_INCREMENTAL);
    IncrementalMarking* incremental_marking = heap_->incremental_marking();
    if (was_marked_incrementally_) {
      incremental_marking->Finalize();
    } else {
      CHECK(incremental_marking->IsStopped());
    }
  }

#ifdef DEBUG
  DCHECK(state_ == PREPARE_GC);
  state_ = MARK_LIVE_OBJECTS;
#endif

  heap_->local_embedder_heap_tracer()->EnterFinalPause();

  RootMarkingVisitor root_visitor(this);

  {
    TRACE_GC(heap()->tracer(), GCTracer::Scope::MC_MARK_ROOTS);
    CustomRootBodyMarkingVisitor custom_root_body_visitor(this);
    MarkRoots(&root_visitor, &custom_root_body_visitor);
  }

  {
    TRACE_GC(heap()->tracer(), GCTracer::Scope::MC_MARK_MAIN);
    if (FLAG_parallel_marking) {
      DCHECK(FLAG_concurrent_marking);
      heap_->concurrent_marking()->RescheduleTasksIfNeeded();
    }
    ProcessMarkingWorklist();

    FinishConcurrentMarking(
        ConcurrentMarking::StopRequest::COMPLETE_ONGOING_TASKS);
    ProcessMarkingWorklist();
  }

  {
    TRACE_GC(heap()->tracer(), GCTracer::Scope::MC_MARK_WEAK_CLOSURE);

    DCHECK(marking_worklist()->IsEmpty());

    // Mark objects reachable through the embedder heap. This phase is
    // opportunistic as it may not discover graphs that are only reachable
    // through ephemerons.
    {
      TRACE_GC(heap()->tracer(), GCTracer::Scope::MC_MARK_WRAPPERS);
      while (!heap_->local_embedder_heap_tracer()->IsRemoteTracingDone()) {
        PerformWrapperTracing();
        ProcessMarkingWorklist();
      }
      DCHECK(marking_worklist()->IsEmpty());
    }

    // The objects reachable from the roots are marked, yet unreachable objects
    // are unmarked. Mark objects reachable due to embedder heap tracing or
    // harmony weak maps.
    {
      TRACE_GC(heap()->tracer(),
               GCTracer::Scope::MC_MARK_WEAK_CLOSURE_EPHEMERON);
      ProcessEphemeronMarking();
      DCHECK(marking_worklist()->IsEmpty());
    }

    // The objects reachable from the roots, weak maps, and embedder heap
    // tracing are marked. Objects pointed to only by weak global handles cannot
    // be immediately reclaimed. Instead, we have to mark them as pending and
    // mark objects reachable from them.
    //
    // First we identify nonlive weak handles and mark them as pending
    // destruction.
    {
      TRACE_GC(heap()->tracer(),
               GCTracer::Scope::MC_MARK_WEAK_CLOSURE_WEAK_HANDLES);
      heap()->isolate()->global_handles()->IdentifyWeakHandles(
          &IsUnmarkedHeapObject);
      ProcessMarkingWorklist();
    }

    // Process finalizers, effectively keeping them alive until the next
    // garbage collection.
    {
      TRACE_GC(heap()->tracer(),
               GCTracer::Scope::MC_MARK_WEAK_CLOSURE_WEAK_ROOTS);
      heap()->isolate()->global_handles()->IterateWeakRootsForFinalizers(
          &root_visitor);
      ProcessMarkingWorklist();
    }

    // Repeat ephemeron processing from the newly marked objects.
    {
      TRACE_GC(heap()->tracer(), GCTracer::Scope::MC_MARK_WEAK_CLOSURE_HARMONY);
      ProcessEphemeronMarking();
      {
        TRACE_GC(heap()->tracer(), GCTracer::Scope::MC_MARK_WRAPPER_EPILOGUE);
        heap()->local_embedder_heap_tracer()->TraceEpilogue();
      }
      DCHECK(marking_worklist()->IsEmpty());
    }

    {
      heap()->isolate()->global_handles()->IterateWeakRootsForPhantomHandles(
          &IsUnmarkedHeapObject);
    }
  }

  if (was_marked_incrementally_) {
    heap()->incremental_marking()->Deactivate();
  }
}

void MarkCompactCollector::ClearNonLiveReferences() {
  TRACE_GC(heap()->tracer(), GCTracer::Scope::MC_CLEAR);

  {
    TRACE_GC(heap()->tracer(), GCTracer::Scope::MC_CLEAR_STRING_TABLE);

    // Prune the string table removing all strings only pointed to by the
    // string table.  Cannot use string_table() here because the string
    // table is marked.
    StringTable* string_table = heap()->string_table();
    InternalizedStringTableCleaner internalized_visitor(heap(), string_table);
    string_table->IterateElements(&internalized_visitor);
    string_table->ElementsRemoved(internalized_visitor.PointersRemoved());

    ExternalStringTableCleaner external_visitor(heap());
    heap()->external_string_table_.IterateAll(&external_visitor);
    heap()->external_string_table_.CleanUpAll();
  }

  {
    TRACE_GC(heap()->tracer(), GCTracer::Scope::MC_CLEAR_WEAK_LISTS);
    // Process the weak references.
    MarkCompactWeakObjectRetainer mark_compact_object_retainer(
        non_atomic_marking_state());
    heap()->ProcessAllWeakReferences(&mark_compact_object_retainer);
  }

  {
    TRACE_GC(heap()->tracer(), GCTracer::Scope::MC_CLEAR_MAPS);
    // ClearFullMapTransitions must be called before weak references are
    // cleared.
    ClearFullMapTransitions();
  }
  ClearWeakReferences();
  MarkDependentCodeForDeoptimization();

  ClearWeakCollections();

  DCHECK(weak_objects_.transition_arrays.IsEmpty());
  DCHECK(weak_objects_.weak_references.IsEmpty());
  DCHECK(weak_objects_.weak_objects_in_code.IsEmpty());
}

void MarkCompactCollector::MarkDependentCodeForDeoptimization() {
  std::pair<HeapObject*, Code*> weak_object_in_code;
  while (weak_objects_.weak_objects_in_code.Pop(kMainThread,
                                                &weak_object_in_code)) {
    HeapObject* object = weak_object_in_code.first;
    Code* code = weak_object_in_code.second;
    if (!non_atomic_marking_state()->IsBlackOrGrey(object) &&
        !code->marked_for_deoptimization()) {
      code->SetMarkedForDeoptimization("weak objects");
      code->InvalidateEmbeddedObjects(heap_);
      have_code_to_deoptimize_ = true;
    }
  }
}

void MarkCompactCollector::ClearPotentialSimpleMapTransition(Map* dead_target) {
  DCHECK(non_atomic_marking_state()->IsWhite(dead_target));
  Object* potential_parent = dead_target->constructor_or_backpointer();
  if (potential_parent->IsMap()) {
    Map* parent = Map::cast(potential_parent);
    DisallowHeapAllocation no_gc_obviously;
    if (non_atomic_marking_state()->IsBlackOrGrey(parent) &&
        TransitionsAccessor(isolate(), parent, &no_gc_obviously)
            .HasSimpleTransitionTo(dead_target)) {
      ClearPotentialSimpleMapTransition(parent, dead_target);
    }
  }
}

void MarkCompactCollector::ClearPotentialSimpleMapTransition(Map* map,
                                                             Map* dead_target) {
  DCHECK(!map->is_prototype_map());
  DCHECK(!dead_target->is_prototype_map());
  DCHECK_EQ(map->raw_transitions(), HeapObjectReference::Weak(dead_target));
  // Take ownership of the descriptor array.
  int number_of_own_descriptors = map->NumberOfOwnDescriptors();
  DescriptorArray* descriptors = map->instance_descriptors();
  if (descriptors == dead_target->instance_descriptors() &&
      number_of_own_descriptors > 0) {
    TrimDescriptorArray(map, descriptors);
    DCHECK(descriptors->number_of_descriptors() == number_of_own_descriptors);
  }
}

void MarkCompactCollector::ClearFullMapTransitions() {
  TransitionArray* array;
  while (weak_objects_.transition_arrays.Pop(kMainThread, &array)) {
    int num_transitions = array->number_of_entries();
    if (num_transitions > 0) {
      Map* map;
      // The array might contain "undefined" elements because it's not yet
      // filled. Allow it.
      if (array->GetTargetIfExists(0, isolate(), &map)) {
        DCHECK_NOT_NULL(map);  // Weak pointers aren't cleared yet.
        Map* parent = Map::cast(map->constructor_or_backpointer());
        bool parent_is_alive =
            non_atomic_marking_state()->IsBlackOrGrey(parent);
        DescriptorArray* descriptors =
            parent_is_alive ? parent->instance_descriptors() : nullptr;
        bool descriptors_owner_died =
            CompactTransitionArray(parent, array, descriptors);
        if (descriptors_owner_died) {
          TrimDescriptorArray(parent, descriptors);
        }
      }
    }
  }
}

bool MarkCompactCollector::CompactTransitionArray(
    Map* map, TransitionArray* transitions, DescriptorArray* descriptors) {
  DCHECK(!map->is_prototype_map());
  int num_transitions = transitions->number_of_entries();
  bool descriptors_owner_died = false;
  int transition_index = 0;
  // Compact all live transitions to the left.
  for (int i = 0; i < num_transitions; ++i) {
    Map* target = transitions->GetTarget(i);
    DCHECK_EQ(target->constructor_or_backpointer(), map);
    if (non_atomic_marking_state()->IsWhite(target)) {
      if (descriptors != nullptr &&
          target->instance_descriptors() == descriptors) {
        DCHECK(!target->is_prototype_map());
        descriptors_owner_died = true;
      }
    } else {
      if (i != transition_index) {
        Name* key = transitions->GetKey(i);
        transitions->SetKey(transition_index, key);
        HeapObjectReference** key_slot =
            transitions->GetKeySlot(transition_index);
        RecordSlot(transitions, key_slot, key);
        MaybeObject* raw_target = transitions->GetRawTarget(i);
        transitions->SetRawTarget(transition_index, raw_target);
        HeapObjectReference** target_slot =
            transitions->GetTargetSlot(transition_index);
        RecordSlot(transitions, target_slot, raw_target->GetHeapObject());
      }
      transition_index++;
    }
  }
  // If there are no transitions to be cleared, return.
  if (transition_index == num_transitions) {
    DCHECK(!descriptors_owner_died);
    return false;
  }
  // Note that we never eliminate a transition array, though we might right-trim
  // such that number_of_transitions() == 0. If this assumption changes,
  // TransitionArray::Insert() will need to deal with the case that a transition
  // array disappeared during GC.
  int trim = transitions->Capacity() - transition_index;
  if (trim > 0) {
    heap_->RightTrimWeakFixedArray(transitions,
                                   trim * TransitionArray::kEntrySize);
    transitions->SetNumberOfTransitions(transition_index);
  }
  return descriptors_owner_died;
}

void MarkCompactCollector::TrimDescriptorArray(Map* map,
                                               DescriptorArray* descriptors) {
  int number_of_own_descriptors = map->NumberOfOwnDescriptors();
  if (number_of_own_descriptors == 0) {
    DCHECK(descriptors == ReadOnlyRoots(heap_).empty_descriptor_array());
    return;
  }

  int number_of_descriptors = descriptors->number_of_descriptors_storage();
  int to_trim = number_of_descriptors - number_of_own_descriptors;
  if (to_trim > 0) {
    heap_->RightTrimWeakFixedArray(descriptors,
                                   to_trim * DescriptorArray::kEntrySize);
    descriptors->SetNumberOfDescriptors(number_of_own_descriptors);

    TrimEnumCache(map, descriptors);
    descriptors->Sort();

    if (FLAG_unbox_double_fields) {
      LayoutDescriptor* layout_descriptor = map->layout_descriptor();
      layout_descriptor = layout_descriptor->Trim(heap_, map, descriptors,
                                                  number_of_own_descriptors);
      SLOW_DCHECK(layout_descriptor->IsConsistentWithMap(map, true));
    }
  }
  DCHECK(descriptors->number_of_descriptors() == number_of_own_descriptors);
  map->set_owns_descriptors(true);
}

void MarkCompactCollector::TrimEnumCache(Map* map,
                                         DescriptorArray* descriptors) {
  int live_enum = map->EnumLength();
  if (live_enum == kInvalidEnumCacheSentinel) {
    live_enum = map->NumberOfEnumerableProperties();
  }
  if (live_enum == 0) return descriptors->ClearEnumCache();
  EnumCache* enum_cache = descriptors->GetEnumCache();

  FixedArray* keys = enum_cache->keys();
  int to_trim = keys->length() - live_enum;
  if (to_trim <= 0) return;
  heap_->RightTrimFixedArray(keys, to_trim);

  FixedArray* indices = enum_cache->indices();
  to_trim = indices->length() - live_enum;
  if (to_trim <= 0) return;
  heap_->RightTrimFixedArray(indices, to_trim);
}

void MarkCompactCollector::ClearWeakCollections() {
  TRACE_GC(heap()->tracer(), GCTracer::Scope::MC_CLEAR_WEAK_COLLECTIONS);
  EphemeronHashTable* table;

  while (weak_objects_.ephemeron_hash_tables.Pop(kMainThread, &table)) {
    for (int i = 0; i < table->Capacity(); i++) {
      HeapObject* key = HeapObject::cast(table->KeyAt(i));
#ifdef VERIFY_HEAP
      Object* value = table->ValueAt(i);

      if (value->IsHeapObject()) {
        CHECK_IMPLIES(
            non_atomic_marking_state()->IsBlackOrGrey(key),
            non_atomic_marking_state()->IsBlackOrGrey(HeapObject::cast(value)));
      }
#endif
      if (!non_atomic_marking_state()->IsBlackOrGrey(key)) {
        table->RemoveEntry(i);
      }
    }
  }
}

void MarkCompactCollector::ClearWeakReferences() {
  TRACE_GC(heap()->tracer(), GCTracer::Scope::MC_CLEAR_WEAK_REFERENCES);
  std::pair<HeapObject*, HeapObjectReference**> slot;
  while (weak_objects_.weak_references.Pop(kMainThread, &slot)) {
    HeapObject* value;
    HeapObjectReference** location = slot.second;
    if ((*location)->ToWeakHeapObject(&value)) {
      DCHECK(!value->IsCell());
      if (non_atomic_marking_state()->IsBlackOrGrey(value)) {
        // The value of the weak reference is alive.
        RecordSlot(slot.first, location, value);
      } else {
        if (value->IsMap()) {
          // The map is non-live.
          ClearPotentialSimpleMapTransition(Map::cast(value));
        }
        *location = HeapObjectReference::ClearedValue();
      }
    }
  }
}

void MarkCompactCollector::AbortWeakObjects() {
  weak_objects_.transition_arrays.Clear();
  weak_objects_.ephemeron_hash_tables.Clear();
  weak_objects_.current_ephemerons.Clear();
  weak_objects_.next_ephemerons.Clear();
  weak_objects_.discovered_ephemerons.Clear();
  weak_objects_.weak_references.Clear();
  weak_objects_.weak_objects_in_code.Clear();
}

void MarkCompactCollector::RecordRelocSlot(Code* host, RelocInfo* rinfo,
                                           Object* target) {
  Page* target_page = Page::FromAddress(reinterpret_cast<Address>(target));
  Page* source_page = Page::FromAddress(reinterpret_cast<Address>(host));
  if (target_page->IsEvacuationCandidate() &&
      (rinfo->host() == nullptr ||
       !source_page->ShouldSkipEvacuationSlotRecording())) {
    RelocInfo::Mode rmode = rinfo->rmode();
    Address addr = rinfo->pc();
    SlotType slot_type = SlotTypeForRelocInfoMode(rmode);
    if (rinfo->IsInConstantPool()) {
      addr = rinfo->constant_pool_entry_address();
      if (RelocInfo::IsCodeTargetMode(rmode)) {
        slot_type = CODE_ENTRY_SLOT;
      } else {
        DCHECK(RelocInfo::IsEmbeddedObject(rmode));
        slot_type = OBJECT_SLOT;
      }
    }
    RememberedSet<OLD_TO_OLD>::InsertTyped(
        source_page, reinterpret_cast<Address>(host), slot_type, addr);
  }
}

template <AccessMode access_mode>
static inline SlotCallbackResult UpdateSlot(
    MaybeObject** slot, MaybeObject* old, HeapObject* heap_obj,
    HeapObjectReferenceType reference_type) {
  MapWord map_word = heap_obj->map_word();
  if (map_word.IsForwardingAddress()) {
    DCHECK(Heap::InFromSpace(heap_obj) ||
           MarkCompactCollector::IsOnEvacuationCandidate(heap_obj) ||
           Page::FromAddress(heap_obj->address())
               ->IsFlagSet(Page::COMPACTION_WAS_ABORTED));
    MaybeObject* target =
        reference_type == HeapObjectReferenceType::WEAK
            ? HeapObjectReference::Weak(map_word.ToForwardingAddress())
            : HeapObjectReference::Strong(map_word.ToForwardingAddress());
    if (access_mode == AccessMode::NON_ATOMIC) {
      *slot = target;
    } else {
      base::AsAtomicPointer::Release_CompareAndSwap(slot, old, target);
    }
    DCHECK(!Heap::InFromSpace(target));
    DCHECK(!MarkCompactCollector::IsOnEvacuationCandidate(target));
  } else {
    DCHECK(heap_obj->map()->IsMap());
  }
  // OLD_TO_OLD slots are always removed after updating.
  return REMOVE_SLOT;
}

template <AccessMode access_mode>
static inline SlotCallbackResult UpdateSlot(MaybeObject** slot) {
  MaybeObject* obj = base::AsAtomicPointer::Relaxed_Load(slot);
  HeapObject* heap_obj;
  if (obj->ToWeakHeapObject(&heap_obj)) {
    UpdateSlot<access_mode>(slot, obj, heap_obj, HeapObjectReferenceType::WEAK);
  } else if (obj->ToStrongHeapObject(&heap_obj)) {
    return UpdateSlot<access_mode>(slot, obj, heap_obj,
                                   HeapObjectReferenceType::STRONG);
  }
  return REMOVE_SLOT;
}

template <AccessMode access_mode>
static inline SlotCallbackResult UpdateStrongSlot(MaybeObject** maybe_slot) {
  DCHECK((*maybe_slot)->IsSmi() || (*maybe_slot)->IsStrongHeapObject());
  Object** slot = reinterpret_cast<Object**>(maybe_slot);
  Object* obj = base::AsAtomicPointer::Relaxed_Load(slot);
  if (obj->IsHeapObject()) {
    HeapObject* heap_obj = HeapObject::cast(obj);
    return UpdateSlot<access_mode>(maybe_slot, MaybeObject::FromObject(obj),
                                   heap_obj, HeapObjectReferenceType::STRONG);
  }
  return REMOVE_SLOT;
}

// Visitor for updating root pointers and to-space pointers.
// It does not expect to encounter pointers to dead objects.
// TODO(ulan): Remove code object specific functions. This visitor
// nevers visits code objects.
class PointersUpdatingVisitor : public ObjectVisitor, public RootVisitor {
 public:
  explicit PointersUpdatingVisitor(Heap* heap) : heap_(heap) {}

  void VisitPointer(HeapObject* host, Object** p) override {
    UpdateStrongSlotInternal(p);
  }

  void VisitPointer(HeapObject* host, MaybeObject** p) override {
    UpdateSlotInternal(p);
  }

  void VisitPointers(HeapObject* host, Object** start, Object** end) override {
    for (Object** p = start; p < end; p++) {
      UpdateStrongSlotInternal(p);
    }
  }

  void VisitPointers(HeapObject* host, MaybeObject** start,
                     MaybeObject** end) final {
    for (MaybeObject** p = start; p < end; p++) {
      UpdateSlotInternal(p);
    }
  }

  void VisitRootPointer(Root root, const char* description,
                        Object** p) override {
    UpdateStrongSlotInternal(p);
  }

  void VisitRootPointers(Root root, const char* description, Object** start,
                         Object** end) override {
    for (Object** p = start; p < end; p++) UpdateStrongSlotInternal(p);
  }

  void VisitEmbeddedPointer(Code* host, RelocInfo* rinfo) override {
    UpdateTypedSlotHelper::UpdateEmbeddedPointer(
        heap_, rinfo, UpdateStrongMaybeObjectSlotInternal);
  }

  void VisitCodeTarget(Code* host, RelocInfo* rinfo) override {
    UpdateTypedSlotHelper::UpdateCodeTarget(
        rinfo, UpdateStrongMaybeObjectSlotInternal);
  }

 private:
  static inline SlotCallbackResult UpdateStrongMaybeObjectSlotInternal(
      MaybeObject** slot) {
    DCHECK(!(*slot)->IsWeakHeapObject());
    DCHECK(!(*slot)->IsClearedWeakHeapObject());
    return UpdateStrongSlotInternal(reinterpret_cast<Object**>(slot));
  }

  static inline SlotCallbackResult UpdateStrongSlotInternal(Object** slot) {
    DCHECK(!HasWeakHeapObjectTag(*slot));
    return UpdateStrongSlot<AccessMode::NON_ATOMIC>(
        reinterpret_cast<MaybeObject**>(slot));
  }

  static inline SlotCallbackResult UpdateSlotInternal(MaybeObject** slot) {
    return UpdateSlot<AccessMode::NON_ATOMIC>(slot);
  }

  Heap* heap_;
};

static String* UpdateReferenceInExternalStringTableEntry(Heap* heap,
                                                         Object** p) {
  MapWord map_word = HeapObject::cast(*p)->map_word();

  if (map_word.IsForwardingAddress()) {
    String* new_string = String::cast(map_word.ToForwardingAddress());

    if (new_string->IsExternalString()) {
      heap->ProcessMovedExternalString(
          Page::FromAddress(reinterpret_cast<Address>(*p)),
          Page::FromHeapObject(new_string), ExternalString::cast(new_string));
    }
    return new_string;
  }

  return String::cast(*p);
}

void MarkCompactCollector::EvacuatePrologue() {
  // New space.
  NewSpace* new_space = heap()->new_space();
  // Append the list of new space pages to be processed.
  for (Page* p :
       PageRange(new_space->first_allocatable_address(), new_space->top())) {
    new_space_evacuation_pages_.push_back(p);
  }
  new_space->Flip();
  new_space->ResetLinearAllocationArea();

  // Old space.
  DCHECK(old_space_evacuation_pages_.empty());
  old_space_evacuation_pages_ = std::move(evacuation_candidates_);
  evacuation_candidates_.clear();
  DCHECK(evacuation_candidates_.empty());
}

void MarkCompactCollector::EvacuateEpilogue() {
  aborted_evacuation_candidates_.clear();
  // New space.
  heap()->new_space()->set_age_mark(heap()->new_space()->top());
  // Deallocate unmarked large objects.
  heap()->lo_space()->FreeUnmarkedObjects();
  // Old space. Deallocate evacuated candidate pages.
  ReleaseEvacuationCandidates();
  // Give pages that are queued to be freed back to the OS.
  heap()->memory_allocator()->unmapper()->FreeQueuedChunks();
#ifdef DEBUG
  // Old-to-old slot sets must be empty after evacuation.
  for (Page* p : *heap()->old_space()) {
    DCHECK_NULL((p->slot_set<OLD_TO_OLD, AccessMode::ATOMIC>()));
    DCHECK_NULL((p->typed_slot_set<OLD_TO_OLD, AccessMode::ATOMIC>()));
    DCHECK_NULL(p->invalidated_slots());
  }
#endif
}

class Evacuator : public Malloced {
 public:
  enum EvacuationMode {
    kObjectsNewToOld,
    kPageNewToOld,
    kObjectsOldToOld,
    kPageNewToNew,
  };

  static inline EvacuationMode ComputeEvacuationMode(MemoryChunk* chunk) {
    // Note: The order of checks is important in this function.
    if (chunk->IsFlagSet(MemoryChunk::PAGE_NEW_OLD_PROMOTION))
      return kPageNewToOld;
    if (chunk->IsFlagSet(MemoryChunk::PAGE_NEW_NEW_PROMOTION))
      return kPageNewToNew;
    if (chunk->InNewSpace()) return kObjectsNewToOld;
    return kObjectsOldToOld;
  }

  // NewSpacePages with more live bytes than this threshold qualify for fast
  // evacuation.
  static int PageEvacuationThreshold() {
    if (FLAG_page_promotion)
      return FLAG_page_promotion_threshold * Page::kAllocatableMemory / 100;
    return Page::kAllocatableMemory + kPointerSize;
  }

  Evacuator(Heap* heap, RecordMigratedSlotVisitor* record_visitor)
      : heap_(heap),
        local_allocator_(heap_),
        local_pretenuring_feedback_(kInitialLocalPretenuringFeedbackCapacity),
        new_space_visitor_(heap_, &local_allocator_, record_visitor,
                           &local_pretenuring_feedback_),
        new_to_new_page_visitor_(heap_, record_visitor,
                                 &local_pretenuring_feedback_),
        new_to_old_page_visitor_(heap_, record_visitor,
                                 &local_pretenuring_feedback_),

        old_space_visitor_(heap_, &local_allocator_, record_visitor),
        duration_(0.0),
        bytes_compacted_(0) {}

  virtual ~Evacuator() {}

  void EvacuatePage(Page* page);

  void AddObserver(MigrationObserver* observer) {
    new_space_visitor_.AddObserver(observer);
    old_space_visitor_.AddObserver(observer);
  }

  // Merge back locally cached info sequentially. Note that this method needs
  // to be called from the main thread.
  inline void Finalize();

  virtual GCTracer::BackgroundScope::ScopeId GetBackgroundTracingScope() = 0;

 protected:
  static const int kInitialLocalPretenuringFeedbackCapacity = 256;

  // |saved_live_bytes| returns the live bytes of the page that was processed.
  virtual void RawEvacuatePage(Page* page, intptr_t* saved_live_bytes) = 0;

  inline Heap* heap() { return heap_; }

  void ReportCompactionProgress(double duration, intptr_t bytes_compacted) {
    duration_ += duration;
    bytes_compacted_ += bytes_compacted;
  }

  Heap* heap_;

  // Locally cached collector data.
  LocalAllocator local_allocator_;
  Heap::PretenuringFeedbackMap local_pretenuring_feedback_;

  // Visitors for the corresponding spaces.
  EvacuateNewSpaceVisitor new_space_visitor_;
  EvacuateNewSpacePageVisitor<PageEvacuationMode::NEW_TO_NEW>
      new_to_new_page_visitor_;
  EvacuateNewSpacePageVisitor<PageEvacuationMode::NEW_TO_OLD>
      new_to_old_page_visitor_;
  EvacuateOldSpaceVisitor old_space_visitor_;

  // Book keeping info.
  double duration_;
  intptr_t bytes_compacted_;
};

void Evacuator::EvacuatePage(Page* page) {
  TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("v8.gc"), "Evacuator::EvacuatePage");
  DCHECK(page->SweepingDone());
  intptr_t saved_live_bytes = 0;
  double evacuation_time = 0.0;
  {
    AlwaysAllocateScope always_allocate(heap()->isolate());
    TimedScope timed_scope(&evacuation_time);
    RawEvacuatePage(page, &saved_live_bytes);
  }
  ReportCompactionProgress(evacuation_time, saved_live_bytes);
  if (FLAG_trace_evacuation) {
    PrintIsolate(
        heap()->isolate(),
        "evacuation[%p]: page=%p new_space=%d "
        "page_evacuation=%d executable=%d contains_age_mark=%d "
        "live_bytes=%" V8PRIdPTR " time=%f success=%d\n",
        static_cast<void*>(this), static_cast<void*>(page), page->InNewSpace(),
        page->IsFlagSet(Page::PAGE_NEW_OLD_PROMOTION) ||
            page->IsFlagSet(Page::PAGE_NEW_NEW_PROMOTION),
        page->IsFlagSet(MemoryChunk::IS_EXECUTABLE),
        page->Contains(heap()->new_space()->age_mark()), saved_live_bytes,
        evacuation_time, page->IsFlagSet(Page::COMPACTION_WAS_ABORTED));
  }
}

void Evacuator::Finalize() {
  local_allocator_.Finalize();
  heap()->tracer()->AddCompactionEvent(duration_, bytes_compacted_);
  heap()->IncrementPromotedObjectsSize(new_space_visitor_.promoted_size() +
                                       new_to_old_page_visitor_.moved_bytes());
  heap()->IncrementSemiSpaceCopiedObjectSize(
      new_space_visitor_.semispace_copied_size() +
      new_to_new_page_visitor_.moved_bytes());
  heap()->IncrementYoungSurvivorsCounter(
      new_space_visitor_.promoted_size() +
      new_space_visitor_.semispace_copied_size() +
      new_to_old_page_visitor_.moved_bytes() +
      new_to_new_page_visitor_.moved_bytes());
  heap()->MergeAllocationSitePretenuringFeedback(local_pretenuring_feedback_);
}

class FullEvacuator : public Evacuator {
 public:
  FullEvacuator(MarkCompactCollector* collector,
                RecordMigratedSlotVisitor* record_visitor)
      : Evacuator(collector->heap(), record_visitor), collector_(collector) {}

  GCTracer::BackgroundScope::ScopeId GetBackgroundTracingScope() override {
    return GCTracer::BackgroundScope::MC_BACKGROUND_EVACUATE_COPY;
  }

 protected:
  void RawEvacuatePage(Page* page, intptr_t* live_bytes) override;

  MarkCompactCollector* collector_;
};

void FullEvacuator::RawEvacuatePage(Page* page, intptr_t* live_bytes) {
  const EvacuationMode evacuation_mode = ComputeEvacuationMode(page);
  TRACE_EVENT1(TRACE_DISABLED_BY_DEFAULT("v8.gc"),
               "FullEvacuator::RawEvacuatePage", "evacuation_mode",
               evacuation_mode);
  MarkCompactCollector::NonAtomicMarkingState* marking_state =
      collector_->non_atomic_marking_state();
  *live_bytes = marking_state->live_bytes(page);
  HeapObject* failed_object = nullptr;
  switch (evacuation_mode) {
    case kObjectsNewToOld:
      LiveObjectVisitor::VisitBlackObjectsNoFail(
          page, marking_state, &new_space_visitor_,
          LiveObjectVisitor::kClearMarkbits);
      // ArrayBufferTracker will be updated during pointers updating.
      break;
    case kPageNewToOld:
      LiveObjectVisitor::VisitBlackObjectsNoFail(
          page, marking_state, &new_to_old_page_visitor_,
          LiveObjectVisitor::kKeepMarking);
      new_to_old_page_visitor_.account_moved_bytes(
          marking_state->live_bytes(page));
      // ArrayBufferTracker will be updated during sweeping.
      break;
    case kPageNewToNew:
      LiveObjectVisitor::VisitBlackObjectsNoFail(
          page, marking_state, &new_to_new_page_visitor_,
          LiveObjectVisitor::kKeepMarking);
      new_to_new_page_visitor_.account_moved_bytes(
          marking_state->live_bytes(page));
      // ArrayBufferTracker will be updated during sweeping.
      break;
    case kObjectsOldToOld: {
      const bool success = LiveObjectVisitor::VisitBlackObjects(
          page, marking_state, &old_space_visitor_,
          LiveObjectVisitor::kClearMarkbits, &failed_object);
      if (!success) {
        // Aborted compaction page. Actual processing happens on the main
        // thread for simplicity reasons.
        collector_->ReportAbortedEvacuationCandidate(failed_object, page);
      } else {
        // ArrayBufferTracker will be updated during pointers updating.
      }
      break;
    }
  }
}

class PageEvacuationItem : public ItemParallelJob::Item {
 public:
  explicit PageEvacuationItem(Page* page) : page_(page) {}
  virtual ~PageEvacuationItem() {}
  Page* page() const { return page_; }

 private:
  Page* page_;
};

class PageEvacuationTask : public ItemParallelJob::Task {
 public:
  PageEvacuationTask(Isolate* isolate, Evacuator* evacuator)
      : ItemParallelJob::Task(isolate),
        evacuator_(evacuator),
        tracer_(isolate->heap()->tracer()) {}

  void RunInParallel() override {
    TRACE_BACKGROUND_GC(tracer_, evacuator_->GetBackgroundTracingScope());
    PageEvacuationItem* item = nullptr;
    while ((item = GetItem<PageEvacuationItem>()) != nullptr) {
      evacuator_->EvacuatePage(item->page());
      item->MarkFinished();
    }
  };

 private:
  Evacuator* evacuator_;
  GCTracer* tracer_;
};

template <class Evacuator, class Collector>
void MarkCompactCollectorBase::CreateAndExecuteEvacuationTasks(
    Collector* collector, ItemParallelJob* job,
    RecordMigratedSlotVisitor* record_visitor,
    MigrationObserver* migration_observer, const intptr_t live_bytes) {
  // Used for trace summary.
  double compaction_speed = 0;
  if (FLAG_trace_evacuation) {
    compaction_speed = heap()->tracer()->CompactionSpeedInBytesPerMillisecond();
  }

  const bool profiling =
      heap()->isolate()->is_profiling() ||
      heap()->isolate()->logger()->is_listening_to_code_events() ||
      heap()->isolate()->heap_profiler()->is_tracking_object_moves() ||
      heap()->has_heap_object_allocation_tracker();
  ProfilingMigrationObserver profiling_observer(heap());

  const int wanted_num_tasks =
      NumberOfParallelCompactionTasks(job->NumberOfItems());
  Evacuator** evacuators = new Evacuator*[wanted_num_tasks];
  for (int i = 0; i < wanted_num_tasks; i++) {
    evacuators[i] = new Evacuator(collector, record_visitor);
    if (profiling) evacuators[i]->AddObserver(&profiling_observer);
    if (migration_observer != nullptr)
      evacuators[i]->AddObserver(migration_observer);
    job->AddTask(new PageEvacuationTask(heap()->isolate(), evacuators[i]));
  }
  job->Run(isolate()->async_counters());
  for (int i = 0; i < wanted_num_tasks; i++) {
    evacuators[i]->Finalize();
    delete evacuators[i];
  }
  delete[] evacuators;

  if (FLAG_trace_evacuation) {
    PrintIsolate(isolate(),
                 "%8.0f ms: evacuation-summary: parallel=%s pages=%d "
                 "wanted_tasks=%d tasks=%d cores=%d live_bytes=%" V8PRIdPTR
                 " compaction_speed=%.f\n",
                 isolate()->time_millis_since_init(),
                 FLAG_parallel_compaction ? "yes" : "no", job->NumberOfItems(),
                 wanted_num_tasks, job->NumberOfTasks(),
                 V8::GetCurrentPlatform()->NumberOfWorkerThreads() + 1,
                 live_bytes, compaction_speed);
  }
}

bool MarkCompactCollectorBase::ShouldMovePage(Page* p, intptr_t live_bytes) {
  const bool reduce_memory = heap()->ShouldReduceMemory();
  const Address age_mark = heap()->new_space()->age_mark();
  return !reduce_memory && !p->NeverEvacuate() &&
         (live_bytes > Evacuator::PageEvacuationThreshold()) &&
         !p->Contains(age_mark) && heap()->CanExpandOldGeneration(live_bytes);
}

void MarkCompactCollector::EvacuatePagesInParallel() {
  ItemParallelJob evacuation_job(isolate()->cancelable_task_manager(),
                                 &page_parallel_job_semaphore_);
  intptr_t live_bytes = 0;

  for (Page* page : old_space_evacuation_pages_) {
    live_bytes += non_atomic_marking_state()->live_bytes(page);
    evacuation_job.AddItem(new PageEvacuationItem(page));
  }

  for (Page* page : new_space_evacuation_pages_) {
    intptr_t live_bytes_on_page = non_atomic_marking_state()->live_bytes(page);
    if (live_bytes_on_page == 0 && !page->contains_array_buffers()) continue;
    live_bytes += live_bytes_on_page;
    if (ShouldMovePage(page, live_bytes_on_page)) {
      if (page->IsFlagSet(MemoryChunk::NEW_SPACE_BELOW_AGE_MARK)) {
        EvacuateNewSpacePageVisitor<NEW_TO_OLD>::Move(page);
        DCHECK_EQ(heap()->old_space(), page->owner());
        // The move added page->allocated_bytes to the old space, but we are
        // going to sweep the page and add page->live_byte_count.
        heap()->old_space()->DecreaseAllocatedBytes(page->allocated_bytes(),
                                                    page);
      } else {
        EvacuateNewSpacePageVisitor<NEW_TO_NEW>::Move(page);
      }
    }
    evacuation_job.AddItem(new PageEvacuationItem(page));
  }
  if (evacuation_job.NumberOfItems() == 0) return;

  RecordMigratedSlotVisitor record_visitor(this);
  CreateAndExecuteEvacuationTasks<FullEvacuator>(
      this, &evacuation_job, &record_visitor, nullptr, live_bytes);
  PostProcessEvacuationCandidates();
}

class EvacuationWeakObjectRetainer : public WeakObjectRetainer {
 public:
  virtual Object* RetainAs(Object* object) {
    if (object->IsHeapObject()) {
      HeapObject* heap_object = HeapObject::cast(object);
      MapWord map_word = heap_object->map_word();
      if (map_word.IsForwardingAddress()) {
        return map_word.ToForwardingAddress();
      }
    }
    return object;
  }
};

// Return true if the given code is deoptimized or will be deoptimized.
bool MarkCompactCollector::WillBeDeoptimized(Code* code) {
  return code->is_optimized_code() && code->marked_for_deoptimization();
}

void MarkCompactCollector::RecordLiveSlotsOnPage(Page* page) {
  EvacuateRecordOnlyVisitor visitor(heap());
  LiveObjectVisitor::VisitBlackObjectsNoFail(page, non_atomic_marking_state(),
                                             &visitor,
                                             LiveObjectVisitor::kKeepMarking);
}

template <class Visitor, typename MarkingState>
bool LiveObjectVisitor::VisitBlackObjects(MemoryChunk* chunk,
                                          MarkingState* marking_state,
                                          Visitor* visitor,
                                          IterationMode iteration_mode,
                                          HeapObject** failed_object) {
  TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("v8.gc"),
               "LiveObjectVisitor::VisitBlackObjects");
  for (auto object_and_size :
       LiveObjectRange<kBlackObjects>(chunk, marking_state->bitmap(chunk))) {
    HeapObject* const object = object_and_size.first;
    if (!visitor->Visit(object, object_and_size.second)) {
      if (iteration_mode == kClearMarkbits) {
        marking_state->bitmap(chunk)->ClearRange(
            chunk->AddressToMarkbitIndex(chunk->area_start()),
            chunk->AddressToMarkbitIndex(object->address()));
        *failed_object = object;
      }
      return false;
    }
  }
  if (iteration_mode == kClearMarkbits) {
    marking_state->ClearLiveness(chunk);
  }
  return true;
}

template <class Visitor, typename MarkingState>
void LiveObjectVisitor::VisitBlackObjectsNoFail(MemoryChunk* chunk,
                                                MarkingState* marking_state,
                                                Visitor* visitor,
                                                IterationMode iteration_mode) {
  TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("v8.gc"),
               "LiveObjectVisitor::VisitBlackObjectsNoFail");
  for (auto object_and_size :
       LiveObjectRange<kBlackObjects>(chunk, marking_state->bitmap(chunk))) {
    HeapObject* const object = object_and_size.first;
    DCHECK(marking_state->IsBlack(object));
    const bool success = visitor->Visit(object, object_and_size.second);
    USE(success);
    DCHECK(success);
  }
  if (iteration_mode == kClearMarkbits) {
    marking_state->ClearLiveness(chunk);
  }
}

template <class Visitor, typename MarkingState>
void LiveObjectVisitor::VisitGreyObjectsNoFail(MemoryChunk* chunk,
                                               MarkingState* marking_state,
                                               Visitor* visitor,
                                               IterationMode iteration_mode) {
  TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("v8.gc"),
               "LiveObjectVisitor::VisitGreyObjectsNoFail");
  for (auto object_and_size :
       LiveObjectRange<kGreyObjects>(chunk, marking_state->bitmap(chunk))) {
    HeapObject* const object = object_and_size.first;
    DCHECK(marking_state->IsGrey(object));
    const bool success = visitor->Visit(object, object_and_size.second);
    USE(success);
    DCHECK(success);
  }
  if (iteration_mode == kClearMarkbits) {
    marking_state->ClearLiveness(chunk);
  }
}

template <typename MarkingState>
void LiveObjectVisitor::RecomputeLiveBytes(MemoryChunk* chunk,
                                           MarkingState* marking_state) {
  int new_live_size = 0;
  for (auto object_and_size :
       LiveObjectRange<kAllLiveObjects>(chunk, marking_state->bitmap(chunk))) {
    new_live_size += object_and_size.second;
  }
  marking_state->SetLiveBytes(chunk, new_live_size);
}

void MarkCompactCollector::Evacuate() {
  TRACE_GC(heap()->tracer(), GCTracer::Scope::MC_EVACUATE);
  base::LockGuard<base::Mutex> guard(heap()->relocation_mutex());

  {
    TRACE_GC(heap()->tracer(), GCTracer::Scope::MC_EVACUATE_PROLOGUE);
    EvacuatePrologue();
  }

  {
    TRACE_GC(heap()->tracer(), GCTracer::Scope::MC_EVACUATE_COPY);
    EvacuationScope evacuation_scope(this);
    EvacuatePagesInParallel();
  }

  UpdatePointersAfterEvacuation();

  {
    TRACE_GC(heap()->tracer(), GCTracer::Scope::MC_EVACUATE_REBALANCE);
    if (!heap()->new_space()->Rebalance()) {
      heap()->FatalProcessOutOfMemory("NewSpace::Rebalance");
    }
  }

  // Give pages that are queued to be freed back to the OS. Note that filtering
  // slots only handles old space (for unboxed doubles), and thus map space can
  // still contain stale pointers. We only free the chunks after pointer updates
  // to still have access to page headers.
  heap()->memory_allocator()->unmapper()->FreeQueuedChunks();

  {
    TRACE_GC(heap()->tracer(), GCTracer::Scope::MC_EVACUATE_CLEAN_UP);

    for (Page* p : new_space_evacuation_pages_) {
      if (p->IsFlagSet(Page::PAGE_NEW_NEW_PROMOTION)) {
        p->ClearFlag(Page::PAGE_NEW_NEW_PROMOTION);
        sweeper()->AddPageForIterability(p);
      } else if (p->IsFlagSet(Page::PAGE_NEW_OLD_PROMOTION)) {
        p->ClearFlag(Page::PAGE_NEW_OLD_PROMOTION);
        DCHECK_EQ(OLD_SPACE, p->owner()->identity());
        sweeper()->AddPage(OLD_SPACE, p, Sweeper::REGULAR);
      }
    }
    new_space_evacuation_pages_.clear();

    for (Page* p : old_space_evacuation_pages_) {
      // Important: skip list should be cleared only after roots were updated
      // because root iteration traverses the stack and might have to find
      // code objects from non-updated pc pointing into evacuation candidate.
      SkipList* list = p->skip_list();
      if (list != nullptr) list->Clear();
      if (p->IsFlagSet(Page::COMPACTION_WAS_ABORTED)) {
        sweeper()->AddPage(p->owner()->identity(), p, Sweeper::REGULAR);
        p->ClearFlag(Page::COMPACTION_WAS_ABORTED);
      }
    }
  }

  {
    TRACE_GC(heap()->tracer(), GCTracer::Scope::MC_EVACUATE_EPILOGUE);
    EvacuateEpilogue();
  }

#ifdef VERIFY_HEAP
  if (FLAG_verify_heap && !sweeper()->sweeping_in_progress()) {
    FullEvacuationVerifier verifier(heap());
    verifier.Run();
  }
#endif
}

class UpdatingItem : public ItemParallelJob::Item {
 public:
  virtual ~UpdatingItem() {}
  virtual void Process() = 0;
};

class PointersUpdatingTask : public ItemParallelJob::Task {
 public:
  explicit PointersUpdatingTask(Isolate* isolate,
                                GCTracer::BackgroundScope::ScopeId scope)
      : ItemParallelJob::Task(isolate),
        tracer_(isolate->heap()->tracer()),
        scope_(scope) {}

  void RunInParallel() override {
    TRACE_BACKGROUND_GC(tracer_, scope_);
    UpdatingItem* item = nullptr;
    while ((item = GetItem<UpdatingItem>()) != nullptr) {
      item->Process();
      item->MarkFinished();
    }
  };

 private:
  GCTracer* tracer_;
  GCTracer::BackgroundScope::ScopeId scope_;
};

template <typename MarkingState>
class ToSpaceUpdatingItem : public UpdatingItem {
 public:
  explicit ToSpaceUpdatingItem(MemoryChunk* chunk, Address start, Address end,
                               MarkingState* marking_state)
      : chunk_(chunk),
        start_(start),
        end_(end),
        marking_state_(marking_state) {}
  virtual ~ToSpaceUpdatingItem() {}

  void Process() override {
    if (chunk_->IsFlagSet(Page::PAGE_NEW_NEW_PROMOTION)) {
      // New->new promoted pages contain garbage so they require iteration using
      // markbits.
      ProcessVisitLive();
    } else {
      ProcessVisitAll();
    }
  }

 private:
  void ProcessVisitAll() {
    TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("v8.gc"),
                 "ToSpaceUpdatingItem::ProcessVisitAll");
    PointersUpdatingVisitor visitor(chunk_->heap());
    for (Address cur = start_; cur < end_;) {
      HeapObject* object = HeapObject::FromAddress(cur);
      Map* map = object->map();
      int size = object->SizeFromMap(map);
      object->IterateBodyFast(map, size, &visitor);
      cur += size;
    }
  }

  void ProcessVisitLive() {
    TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("v8.gc"),
                 "ToSpaceUpdatingItem::ProcessVisitLive");
    // For young generation evacuations we want to visit grey objects, for
    // full MC, we need to visit black objects.
    PointersUpdatingVisitor visitor(chunk_->heap());
    for (auto object_and_size : LiveObjectRange<kAllLiveObjects>(
             chunk_, marking_state_->bitmap(chunk_))) {
      object_and_size.first->IterateBodyFast(&visitor);
    }
  }

  MemoryChunk* chunk_;
  Address start_;
  Address end_;
  MarkingState* marking_state_;
};

template <typename MarkingState>
class RememberedSetUpdatingItem : public UpdatingItem {
 public:
  explicit RememberedSetUpdatingItem(Heap* heap, MarkingState* marking_state,
                                     MemoryChunk* chunk,
                                     RememberedSetUpdatingMode updating_mode)
      : heap_(heap),
        marking_state_(marking_state),
        chunk_(chunk),
        updating_mode_(updating_mode) {}
  virtual ~RememberedSetUpdatingItem() {}

  void Process() override {
    TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("v8.gc"),
                 "RememberedSetUpdatingItem::Process");
    base::LockGuard<base::Mutex> guard(chunk_->mutex());
    CodePageMemoryModificationScope memory_modification_scope(chunk_);
    UpdateUntypedPointers();
    UpdateTypedPointers();
  }

 private:
  inline SlotCallbackResult CheckAndUpdateOldToNewSlot(Address slot_address) {
    MaybeObject** slot = reinterpret_cast<MaybeObject**>(slot_address);
    HeapObject* heap_object;
    if (!(*slot)->ToStrongOrWeakHeapObject(&heap_object)) {
      return REMOVE_SLOT;
    }
    if (Heap::InFromSpace(heap_object)) {
      MapWord map_word = heap_object->map_word();
      if (map_word.IsForwardingAddress()) {
        HeapObjectReference::Update(
            reinterpret_cast<HeapObjectReference**>(slot),
            map_word.ToForwardingAddress());
      }
      bool success = (*slot)->ToStrongOrWeakHeapObject(&heap_object);
      USE(success);
      DCHECK(success);
      // If the object was in from space before and is after executing the
      // callback in to space, the object is still live.
      // Unfortunately, we do not know about the slot. It could be in a
      // just freed free space object.
      if (Heap::InToSpace(heap_object)) {
        return KEEP_SLOT;
      }
    } else if (Heap::InToSpace(heap_object)) {
      // Slots can point to "to" space if the page has been moved, or if the
      // slot has been recorded multiple times in the remembered set, or
      // if the slot was already updated during old->old updating.
      // In case the page has been moved, check markbits to determine liveness
      // of the slot. In the other case, the slot can just be kept.
      if (Page::FromAddress(heap_object->address())
              ->IsFlagSet(Page::PAGE_NEW_NEW_PROMOTION)) {
        // IsBlackOrGrey is required because objects are marked as grey for
        // the young generation collector while they are black for the full
        // MC.);
        if (marking_state_->IsBlackOrGrey(heap_object)) {
          return KEEP_SLOT;
        } else {
          return REMOVE_SLOT;
        }
      }
      return KEEP_SLOT;
    } else {
      DCHECK(!Heap::InNewSpace(heap_object));
    }
    return REMOVE_SLOT;
  }

  void UpdateUntypedPointers() {
    if (chunk_->slot_set<OLD_TO_NEW, AccessMode::NON_ATOMIC>() != nullptr) {
      RememberedSet<OLD_TO_NEW>::Iterate(
          chunk_,
          [this](Address slot) { return CheckAndUpdateOldToNewSlot(slot); },
          SlotSet::PREFREE_EMPTY_BUCKETS);
    }
    if ((updating_mode_ == RememberedSetUpdatingMode::ALL) &&
        (chunk_->slot_set<OLD_TO_OLD, AccessMode::NON_ATOMIC>() != nullptr)) {
      InvalidatedSlotsFilter filter(chunk_);
      RememberedSet<OLD_TO_OLD>::Iterate(
          chunk_,
          [&filter](Address slot) {
            if (!filter.IsValid(slot)) return REMOVE_SLOT;
            return UpdateSlot<AccessMode::NON_ATOMIC>(
                reinterpret_cast<MaybeObject**>(slot));
          },
          SlotSet::PREFREE_EMPTY_BUCKETS);
    }
    if ((updating_mode_ == RememberedSetUpdatingMode::ALL) &&
        chunk_->invalidated_slots() != nullptr) {
#ifdef DEBUG
      for (auto object_size : *chunk_->invalidated_slots()) {
        HeapObject* object = object_size.first;
        int size = object_size.second;
        DCHECK_LE(object