// 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/heap.h"

#include "src/accessors.h"
#include "src/api.h"
#include "src/assembler-inl.h"
#include "src/ast/context-slot-cache.h"
#include "src/base/bits.h"
#include "src/base/once.h"
#include "src/base/utils/random-number-generator.h"
#include "src/bootstrapper.h"
#include "src/codegen.h"
#include "src/compilation-cache.h"
#include "src/compiler-dispatcher/optimizing-compile-dispatcher.h"
#include "src/conversions.h"
#include "src/debug/debug.h"
#include "src/deoptimizer.h"
#include "src/feedback-vector.h"
#include "src/global-handles.h"
#include "src/heap/array-buffer-tracker-inl.h"
#include "src/heap/code-stats.h"
#include "src/heap/concurrent-marking.h"
#include "src/heap/embedder-tracing.h"
#include "src/heap/gc-idle-time-handler.h"
#include "src/heap/gc-tracer.h"
#include "src/heap/incremental-marking.h"
#include "src/heap/mark-compact-inl.h"
#include "src/heap/mark-compact.h"
#include "src/heap/memory-reducer.h"
#include "src/heap/object-stats.h"
#include "src/heap/objects-visiting-inl.h"
#include "src/heap/objects-visiting.h"
#include "src/heap/remembered-set.h"
#include "src/heap/scavenge-job.h"
#include "src/heap/scavenger-inl.h"
#include "src/heap/store-buffer.h"
#include "src/interpreter/interpreter.h"
#include "src/regexp/jsregexp.h"
#include "src/runtime-profiler.h"
#include "src/snapshot/natives.h"
#include "src/snapshot/serializer-common.h"
#include "src/snapshot/snapshot.h"
#include "src/tracing/trace-event.h"
#include "src/utils.h"
#include "src/v8.h"
#include "src/v8threads.h"
#include "src/vm-state-inl.h"

namespace v8 {
namespace internal {


struct Heap::StrongRootsList {
  Object** start;
  Object** end;
  StrongRootsList* next;
};

class IdleScavengeObserver : public AllocationObserver {
 public:
  IdleScavengeObserver(Heap& heap, intptr_t step_size)
      : AllocationObserver(step_size), heap_(heap) {}

  void Step(int bytes_allocated, Address, size_t) override {
    heap_.ScheduleIdleScavengeIfNeeded(bytes_allocated);
  }

 private:
  Heap& heap_;
};

Heap::Heap()
    : external_memory_(0),
      external_memory_limit_(kExternalAllocationSoftLimit),
      external_memory_at_last_mark_compact_(0),
      isolate_(nullptr),
      code_range_size_(0),
      // semispace_size_ should be a power of 2 and old_generation_size_ should
      // be a multiple of Page::kPageSize.
      max_semi_space_size_(8 * (kPointerSize / 4) * MB),
      initial_semispace_size_(MB),
      max_old_generation_size_(700ul * (kPointerSize / 4) * MB),
      initial_max_old_generation_size_(max_old_generation_size_),
      initial_old_generation_size_(max_old_generation_size_ /
                                   kInitalOldGenerationLimitFactor),
      old_generation_size_configured_(false),
      max_executable_size_(256ul * (kPointerSize / 4) * MB),
      // Variables set based on semispace_size_ and old_generation_size_ in
      // ConfigureHeap.
      // Will be 4 * reserved_semispace_size_ to ensure that young
      // generation can be aligned to its size.
      maximum_committed_(0),
      survived_since_last_expansion_(0),
      survived_last_scavenge_(0),
      always_allocate_scope_count_(0),
      memory_pressure_level_(MemoryPressureLevel::kNone),
      out_of_memory_callback_(nullptr),
      out_of_memory_callback_data_(nullptr),
      contexts_disposed_(0),
      number_of_disposed_maps_(0),
      global_ic_age_(0),
      new_space_(nullptr),
      old_space_(NULL),
      code_space_(NULL),
      map_space_(NULL),
      lo_space_(NULL),
      gc_state_(NOT_IN_GC),
      gc_post_processing_depth_(0),
      allocations_count_(0),
      raw_allocations_hash_(0),
      ms_count_(0),
      gc_count_(0),
      remembered_unmapped_pages_index_(0),
#ifdef DEBUG
      allocation_timeout_(0),
#endif  // DEBUG
      old_generation_allocation_limit_(initial_old_generation_size_),
      inline_allocation_disabled_(false),
      tracer_(nullptr),
      promoted_objects_size_(0),
      promotion_ratio_(0),
      semi_space_copied_object_size_(0),
      previous_semi_space_copied_object_size_(0),
      semi_space_copied_rate_(0),
      nodes_died_in_new_space_(0),
      nodes_copied_in_new_space_(0),
      nodes_promoted_(0),
      maximum_size_scavenges_(0),
      last_idle_notification_time_(0.0),
      last_gc_time_(0.0),
      scavenge_collector_(nullptr),
      mark_compact_collector_(nullptr),
      memory_allocator_(nullptr),
      store_buffer_(nullptr),
      incremental_marking_(nullptr),
      concurrent_marking_(nullptr),
      gc_idle_time_handler_(nullptr),
      memory_reducer_(nullptr),
      live_object_stats_(nullptr),
      dead_object_stats_(nullptr),
      scavenge_job_(nullptr),
      idle_scavenge_observer_(nullptr),
      new_space_allocation_counter_(0),
      old_generation_allocation_counter_at_last_gc_(0),
      old_generation_size_at_last_gc_(0),
      gcs_since_last_deopt_(0),
      global_pretenuring_feedback_(nullptr),
      ring_buffer_full_(false),
      ring_buffer_end_(0),
      promotion_queue_(this),
      configured_(false),
      current_gc_flags_(Heap::kNoGCFlags),
      current_gc_callback_flags_(GCCallbackFlags::kNoGCCallbackFlags),
      external_string_table_(this),
      gc_callbacks_depth_(0),
      deserialization_complete_(false),
      strong_roots_list_(NULL),
      heap_iterator_depth_(0),
      local_embedder_heap_tracer_(nullptr),
      fast_promotion_mode_(false),
      force_oom_(false),
      delay_sweeper_tasks_for_testing_(false),
      pending_layout_change_object_(nullptr) {
// Allow build-time customization of the max semispace size. Building
// V8 with snapshots and a non-default max semispace size is much
// easier if you can define it as part of the build environment.
#if defined(V8_MAX_SEMISPACE_SIZE)
  max_semi_space_size_ = reserved_semispace_size_ = V8_MAX_SEMISPACE_SIZE;
#endif

  // Ensure old_generation_size_ is a multiple of kPageSize.
  DCHECK((max_old_generation_size_ & (Page::kPageSize - 1)) == 0);

  memset(roots_, 0, sizeof(roots_[0]) * kRootListLength);
  set_native_contexts_list(NULL);
  set_allocation_sites_list(Smi::kZero);
  set_encountered_weak_collections(Smi::kZero);
  set_encountered_weak_cells(Smi::kZero);
  set_encountered_transition_arrays(Smi::kZero);
  // Put a dummy entry in the remembered pages so we can find the list the
  // minidump even if there are no real unmapped pages.
  RememberUnmappedPage(NULL, false);
}

size_t Heap::Capacity() {
  if (!HasBeenSetUp()) return 0;

  return new_space_->Capacity() + OldGenerationCapacity();
}

size_t Heap::OldGenerationCapacity() {
  if (!HasBeenSetUp()) return 0;

  return old_space_->Capacity() + code_space_->Capacity() +
         map_space_->Capacity() + lo_space_->SizeOfObjects();
}

size_t Heap::CommittedOldGenerationMemory() {
  if (!HasBeenSetUp()) return 0;

  return old_space_->CommittedMemory() + code_space_->CommittedMemory() +
         map_space_->CommittedMemory() + lo_space_->Size();
}

size_t Heap::CommittedMemory() {
  if (!HasBeenSetUp()) return 0;

  return new_space_->CommittedMemory() + CommittedOldGenerationMemory();
}


size_t Heap::CommittedPhysicalMemory() {
  if (!HasBeenSetUp()) return 0;

  return new_space_->CommittedPhysicalMemory() +
         old_space_->CommittedPhysicalMemory() +
         code_space_->CommittedPhysicalMemory() +
         map_space_->CommittedPhysicalMemory() +
         lo_space_->CommittedPhysicalMemory();
}

size_t Heap::CommittedMemoryExecutable() {
  if (!HasBeenSetUp()) return 0;

  return static_cast<size_t>(memory_allocator()->SizeExecutable());
}


void Heap::UpdateMaximumCommitted() {
  if (!HasBeenSetUp()) return;

  const size_t current_committed_memory = CommittedMemory();
  if (current_committed_memory > maximum_committed_) {
    maximum_committed_ = current_committed_memory;
  }
}

size_t Heap::Available() {
  if (!HasBeenSetUp()) return 0;

  size_t total = 0;
  AllSpaces spaces(this);
  for (Space* space = spaces.next(); space != NULL; space = spaces.next()) {
    total += space->Available();
  }
  return total;
}


bool Heap::HasBeenSetUp() {
  return old_space_ != NULL && code_space_ != NULL && map_space_ != NULL &&
         lo_space_ != NULL;
}


GarbageCollector Heap::SelectGarbageCollector(AllocationSpace space,
                                              const char** reason) {
  // Is global GC requested?
  if (space != NEW_SPACE) {
    isolate_->counters()->gc_compactor_caused_by_request()->Increment();
    *reason = "GC in old space requested";
    return MARK_COMPACTOR;
  }

  if (FLAG_gc_global || (FLAG_stress_compaction && (gc_count_ & 1) != 0)) {
    *reason = "GC in old space forced by flags";
    return MARK_COMPACTOR;
  }

  if (incremental_marking()->NeedsFinalization() &&
      AllocationLimitOvershotByLargeMargin()) {
    *reason = "Incremental marking needs finalization";
    return MARK_COMPACTOR;
  }

  // Is there enough space left in OLD to guarantee that a scavenge can
  // succeed?
  //
  // Note that MemoryAllocator->MaxAvailable() undercounts the memory available
  // for object promotion. It counts only the bytes that the memory
  // allocator has not yet allocated from the OS and assigned to any space,
  // and does not count available bytes already in the old space or code
  // space.  Undercounting is safe---we may get an unrequested full GC when
  // a scavenge would have succeeded.
  if (memory_allocator()->MaxAvailable() <= new_space_->Size()) {
    isolate_->counters()
        ->gc_compactor_caused_by_oldspace_exhaustion()
        ->Increment();
    *reason = "scavenge might not succeed";
    return MARK_COMPACTOR;
  }

  // Default
  *reason = NULL;
  return YoungGenerationCollector();
}

void Heap::SetGCState(HeapState state) {
  gc_state_ = state;
}

// TODO(1238405): Combine the infrastructure for --heap-stats and
// --log-gc to avoid the complicated preprocessor and flag testing.
void Heap::ReportStatisticsBeforeGC() {
// Heap::ReportHeapStatistics will also log NewSpace statistics when
// compiled --log-gc is set.  The following logic is used to avoid
// double logging.
#ifdef DEBUG
  if (FLAG_heap_stats || FLAG_log_gc) new_space_->CollectStatistics();
  if (FLAG_heap_stats) {
    ReportHeapStatistics("Before GC");
  } else if (FLAG_log_gc) {
    new_space_->ReportStatistics();
  }
  if (FLAG_heap_stats || FLAG_log_gc) new_space_->ClearHistograms();
#else
  if (FLAG_log_gc) {
    new_space_->CollectStatistics();
    new_space_->ReportStatistics();
    new_space_->ClearHistograms();
  }
#endif  // DEBUG
}


void Heap::PrintShortHeapStatistics() {
  if (!FLAG_trace_gc_verbose) return;
  PrintIsolate(isolate_, "Memory allocator,   used: %6" PRIuS
                         " KB,"
                         " available: %6" PRIuS " KB\n",
               memory_allocator()->Size() / KB,
               memory_allocator()->Available() / KB);
  PrintIsolate(isolate_, "New space,          used: %6" PRIuS
                         " KB"
                         ", available: %6" PRIuS
                         " KB"
                         ", committed: %6" PRIuS " KB\n",
               new_space_->Size() / KB, new_space_->Available() / KB,
               new_space_->CommittedMemory() / KB);
  PrintIsolate(isolate_, "Old space,          used: %6" PRIuS
                         " KB"
                         ", available: %6" PRIuS
                         " KB"
                         ", committed: %6" PRIuS " KB\n",
               old_space_->SizeOfObjects() / KB, old_space_->Available() / KB,
               old_space_->CommittedMemory() / KB);
  PrintIsolate(isolate_, "Code space,         used: %6" PRIuS
                         " KB"
                         ", available: %6" PRIuS
                         " KB"
                         ", committed: %6" PRIuS "KB\n",
               code_space_->SizeOfObjects() / KB, code_space_->Available() / KB,
               code_space_->CommittedMemory() / KB);
  PrintIsolate(isolate_, "Map space,          used: %6" PRIuS
                         " KB"
                         ", available: %6" PRIuS
                         " KB"
                         ", committed: %6" PRIuS " KB\n",
               map_space_->SizeOfObjects() / KB, map_space_->Available() / KB,
               map_space_->CommittedMemory() / KB);
  PrintIsolate(isolate_, "Large object space, used: %6" PRIuS
                         " KB"
                         ", available: %6" PRIuS
                         " KB"
                         ", committed: %6" PRIuS " KB\n",
               lo_space_->SizeOfObjects() / KB, lo_space_->Available() / KB,
               lo_space_->CommittedMemory() / KB);
  PrintIsolate(isolate_, "All spaces,         used: %6" PRIuS
                         " KB"
                         ", available: %6" PRIuS
                         " KB"
                         ", committed: %6" PRIuS "KB\n",
               this->SizeOfObjects() / KB, this->Available() / KB,
               this->CommittedMemory() / KB);
  PrintIsolate(isolate_, "External memory reported: %6" PRId64 " KB\n",
               external_memory_ / KB);
  PrintIsolate(isolate_, "Total time spent in GC  : %.1f ms\n",
               total_gc_time_ms_);
}

// TODO(1238405): Combine the infrastructure for --heap-stats and
// --log-gc to avoid the complicated preprocessor and flag testing.
void Heap::ReportStatisticsAfterGC() {
// Similar to the before GC, we use some complicated logic to ensure that
// NewSpace statistics are logged exactly once when --log-gc is turned on.
#if defined(DEBUG)
  if (FLAG_heap_stats) {
    new_space_->CollectStatistics();
    ReportHeapStatistics("After GC");
  } else if (FLAG_log_gc) {
    new_space_->ReportStatistics();
  }
#else
  if (FLAG_log_gc) new_space_->ReportStatistics();
#endif  // DEBUG
  for (int i = 0; i < static_cast<int>(v8::Isolate::kUseCounterFeatureCount);
       ++i) {
    int count = deferred_counters_[i];
    deferred_counters_[i] = 0;
    while (count > 0) {
      count--;
      isolate()->CountUsage(static_cast<v8::Isolate::UseCounterFeature>(i));
    }
  }
}


void Heap::IncrementDeferredCount(v8::Isolate::UseCounterFeature feature) {
  deferred_counters_[feature]++;
}

bool Heap::UncommitFromSpace() { return new_space_->UncommitFromSpace(); }

void Heap::GarbageCollectionPrologue() {
  TRACE_GC(tracer(), GCTracer::Scope::HEAP_PROLOGUE);
  {
    AllowHeapAllocation for_the_first_part_of_prologue;
    gc_count_++;

#ifdef VERIFY_HEAP
    if (FLAG_verify_heap) {
      Verify();
    }
#endif
  }

  // Reset GC statistics.
  promoted_objects_size_ = 0;
  previous_semi_space_copied_object_size_ = semi_space_copied_object_size_;
  semi_space_copied_object_size_ = 0;
  nodes_died_in_new_space_ = 0;
  nodes_copied_in_new_space_ = 0;
  nodes_promoted_ = 0;

  UpdateMaximumCommitted();

#ifdef DEBUG
  DCHECK(!AllowHeapAllocation::IsAllowed() && gc_state_ == NOT_IN_GC);

  if (FLAG_gc_verbose) Print();

  ReportStatisticsBeforeGC();
#endif  // DEBUG

  if (new_space_->IsAtMaximumCapacity()) {
    maximum_size_scavenges_++;
  } else {
    maximum_size_scavenges_ = 0;
  }
  CheckNewSpaceExpansionCriteria();
  UpdateNewSpaceAllocationCounter();
}

size_t Heap::SizeOfObjects() {
  size_t total = 0;
  AllSpaces spaces(this);
  for (Space* space = spaces.next(); space != NULL; space = spaces.next()) {
    total += space->SizeOfObjects();
  }
  return total;
}


const char* Heap::GetSpaceName(int idx) {
  switch (idx) {
    case NEW_SPACE:
      return "new_space";
    case OLD_SPACE:
      return "old_space";
    case MAP_SPACE:
      return "map_space";
    case CODE_SPACE:
      return "code_space";
    case LO_SPACE:
      return "large_object_space";
    default:
      UNREACHABLE();
  }
  return nullptr;
}


void Heap::RepairFreeListsAfterDeserialization() {
  PagedSpaces spaces(this);
  for (PagedSpace* space = spaces.next(); space != NULL;
       space = spaces.next()) {
    space->RepairFreeListsAfterDeserialization();
  }
}

void Heap::MergeAllocationSitePretenuringFeedback(
    const base::HashMap& local_pretenuring_feedback) {
  AllocationSite* site = nullptr;
  for (base::HashMap::Entry* local_entry = local_pretenuring_feedback.Start();
       local_entry != nullptr;
       local_entry = local_pretenuring_feedback.Next(local_entry)) {
    site = reinterpret_cast<AllocationSite*>(local_entry->key);
    MapWord map_word = site->map_word();
    if (map_word.IsForwardingAddress()) {
      site = AllocationSite::cast(map_word.ToForwardingAddress());
    }

    // We have not validated the allocation site yet, since we have not
    // dereferenced the site during collecting information.
    // This is an inlined check of AllocationMemento::IsValid.
    if (!site->IsAllocationSite() || site->IsZombie()) continue;

    int value =
        static_cast<int>(reinterpret_cast<intptr_t>(local_entry->value));
    DCHECK_GT(value, 0);

    if (site->IncrementMementoFoundCount(value)) {
      global_pretenuring_feedback_->LookupOrInsert(site,
                                                   ObjectHash(site->address()));
    }
  }
}

class Heap::SkipStoreBufferScope {
 public:
  explicit SkipStoreBufferScope(StoreBuffer* store_buffer)
      : store_buffer_(store_buffer) {
    store_buffer_->MoveAllEntriesToRememberedSet();
    store_buffer_->SetMode(StoreBuffer::IN_GC);
  }

  ~SkipStoreBufferScope() {
    DCHECK(store_buffer_->Empty());
    store_buffer_->SetMode(StoreBuffer::NOT_IN_GC);
  }

 private:
  StoreBuffer* store_buffer_;
};

class Heap::PretenuringScope {
 public:
  explicit PretenuringScope(Heap* heap) : heap_(heap) {
    heap_->global_pretenuring_feedback_ =
        new base::HashMap(kInitialFeedbackCapacity);
  }

  ~PretenuringScope() {
    delete heap_->global_pretenuring_feedback_;
    heap_->global_pretenuring_feedback_ = nullptr;
  }

 private:
  Heap* heap_;
};


void Heap::ProcessPretenuringFeedback() {
  bool trigger_deoptimization = false;
  if (FLAG_allocation_site_pretenuring) {
    int tenure_decisions = 0;
    int dont_tenure_decisions = 0;
    int allocation_mementos_found = 0;
    int allocation_sites = 0;
    int active_allocation_sites = 0;

    AllocationSite* site = nullptr;

    // Step 1: Digest feedback for recorded allocation sites.
    bool maximum_size_scavenge = MaximumSizeScavenge();
    for (base::HashMap::Entry* e = global_pretenuring_feedback_->Start();
         e != nullptr; e = global_pretenuring_feedback_->Next(e)) {
      allocation_sites++;
      site = reinterpret_cast<AllocationSite*>(e->key);
      int found_count = site->memento_found_count();
      // An entry in the storage does not imply that the count is > 0 because
      // allocation sites might have been reset due to too many objects dying
      // in old space.
      if (found_count > 0) {
        DCHECK(site->IsAllocationSite());
        active_allocation_sites++;
        allocation_mementos_found += found_count;
        if (site->DigestPretenuringFeedback(maximum_size_scavenge)) {
          trigger_deoptimization = true;
        }
        if (site->GetPretenureMode() == TENURED) {
          tenure_decisions++;
        } else {
          dont_tenure_decisions++;
        }
      }
    }

    // Step 2: Deopt maybe tenured allocation sites if necessary.
    bool deopt_maybe_tenured = DeoptMaybeTenuredAllocationSites();
    if (deopt_maybe_tenured) {
      Object* list_element = allocation_sites_list();
      while (list_element->IsAllocationSite()) {
        site = AllocationSite::cast(list_element);
        DCHECK(site->IsAllocationSite());
        allocation_sites++;
        if (site->IsMaybeTenure()) {
          site->set_deopt_dependent_code(true);
          trigger_deoptimization = true;
        }
        list_element = site->weak_next();
      }
    }

    if (trigger_deoptimization) {
      isolate_->stack_guard()->RequestDeoptMarkedAllocationSites();
    }

    if (FLAG_trace_pretenuring_statistics &&
        (allocation_mementos_found > 0 || tenure_decisions > 0 ||
         dont_tenure_decisions > 0)) {
      PrintIsolate(isolate(),
                   "pretenuring: deopt_maybe_tenured=%d visited_sites=%d "
                   "active_sites=%d "
                   "mementos=%d tenured=%d not_tenured=%d\n",
                   deopt_maybe_tenured ? 1 : 0, allocation_sites,
                   active_allocation_sites, allocation_mementos_found,
                   tenure_decisions, dont_tenure_decisions);
    }
  }
}


void Heap::DeoptMarkedAllocationSites() {
  // TODO(hpayer): If iterating over the allocation sites list becomes a
  // performance issue, use a cache data structure in heap instead.
  Object* list_element = allocation_sites_list();
  while (list_element->IsAllocationSite()) {
    AllocationSite* site = AllocationSite::cast(list_element);
    if (site->deopt_dependent_code()) {
      site->dependent_code()->MarkCodeForDeoptimization(
          isolate_, DependentCode::kAllocationSiteTenuringChangedGroup);
      site->set_deopt_dependent_code(false);
    }
    list_element = site->weak_next();
  }
  Deoptimizer::DeoptimizeMarkedCode(isolate_);
}


void Heap::GarbageCollectionEpilogue() {
  TRACE_GC(tracer(), GCTracer::Scope::HEAP_EPILOGUE);
  // In release mode, we only zap the from space under heap verification.
  if (Heap::ShouldZapGarbage()) {
    ZapFromSpace();
  }

#ifdef VERIFY_HEAP
  if (FLAG_verify_heap) {
    Verify();
  }
#endif

  AllowHeapAllocation for_the_rest_of_the_epilogue;

#ifdef DEBUG
  if (FLAG_print_global_handles) isolate_->global_handles()->Print();
  if (FLAG_print_handles) PrintHandles();
  if (FLAG_gc_verbose) Print();
  if (FLAG_code_stats) ReportCodeStatistics("After GC");
  if (FLAG_check_handle_count) CheckHandleCount();
#endif
  if (FLAG_deopt_every_n_garbage_collections > 0) {
    // TODO(jkummerow/ulan/jarin): This is not safe! We can't assume that
    // the topmost optimized frame can be deoptimized safely, because it
    // might not have a lazy bailout point right after its current PC.
    if (++gcs_since_last_deopt_ == FLAG_deopt_every_n_garbage_collections) {
      Deoptimizer::DeoptimizeAll(isolate());
      gcs_since_last_deopt_ = 0;
    }
  }

  UpdateMaximumCommitted();

  isolate_->counters()->alive_after_last_gc()->Set(
      static_cast<int>(SizeOfObjects()));

  isolate_->counters()->string_table_capacity()->Set(
      string_table()->Capacity());
  isolate_->counters()->number_of_symbols()->Set(
      string_table()->NumberOfElements());

  if (CommittedMemory() > 0) {
    isolate_->counters()->external_fragmentation_total()->AddSample(
        static_cast<int>(100 - (SizeOfObjects() * 100.0) / CommittedMemory()));

    isolate_->counters()->heap_fraction_new_space()->AddSample(static_cast<int>(
        (new_space()->CommittedMemory() * 100.0) / CommittedMemory()));
    isolate_->counters()->heap_fraction_old_space()->AddSample(static_cast<int>(
        (old_space()->CommittedMemory() * 100.0) / CommittedMemory()));
    isolate_->counters()->heap_fraction_code_space()->AddSample(
        static_cast<int>((code_space()->CommittedMemory() * 100.0) /
                         CommittedMemory()));
    isolate_->counters()->heap_fraction_map_space()->AddSample(static_cast<int>(
        (map_space()->CommittedMemory() * 100.0) / CommittedMemory()));
    isolate_->counters()->heap_fraction_lo_space()->AddSample(static_cast<int>(
        (lo_space()->CommittedMemory() * 100.0) / CommittedMemory()));

    isolate_->counters()->heap_sample_total_committed()->AddSample(
        static_cast<int>(CommittedMemory() / KB));
    isolate_->counters()->heap_sample_total_used()->AddSample(
        static_cast<int>(SizeOfObjects() / KB));
    isolate_->counters()->heap_sample_map_space_committed()->AddSample(
        static_cast<int>(map_space()->CommittedMemory() / KB));
    isolate_->counters()->heap_sample_code_space_committed()->AddSample(
        static_cast<int>(code_space()->CommittedMemory() / KB));

    isolate_->counters()->heap_sample_maximum_committed()->AddSample(
        static_cast<int>(MaximumCommittedMemory() / KB));
  }

#define UPDATE_COUNTERS_FOR_SPACE(space)                \
  isolate_->counters()->space##_bytes_available()->Set( \
      static_cast<int>(space()->Available()));          \
  isolate_->counters()->space##_bytes_committed()->Set( \
      static_cast<int>(space()->CommittedMemory()));    \
  isolate_->counters()->space##_bytes_used()->Set(      \
      static_cast<int>(space()->SizeOfObjects()));
#define UPDATE_FRAGMENTATION_FOR_SPACE(space)                          \
  if (space()->CommittedMemory() > 0) {                                \
    isolate_->counters()->external_fragmentation_##space()->AddSample( \
        static_cast<int>(100 -                                         \
                         (space()->SizeOfObjects() * 100.0) /          \
                             space()->CommittedMemory()));             \
  }
#define UPDATE_COUNTERS_AND_FRAGMENTATION_FOR_SPACE(space) \
  UPDATE_COUNTERS_FOR_SPACE(space)                         \
  UPDATE_FRAGMENTATION_FOR_SPACE(space)

  UPDATE_COUNTERS_FOR_SPACE(new_space)
  UPDATE_COUNTERS_AND_FRAGMENTATION_FOR_SPACE(old_space)
  UPDATE_COUNTERS_AND_FRAGMENTATION_FOR_SPACE(code_space)
  UPDATE_COUNTERS_AND_FRAGMENTATION_FOR_SPACE(map_space)
  UPDATE_COUNTERS_AND_FRAGMENTATION_FOR_SPACE(lo_space)
#undef UPDATE_COUNTERS_FOR_SPACE
#undef UPDATE_FRAGMENTATION_FOR_SPACE
#undef UPDATE_COUNTERS_AND_FRAGMENTATION_FOR_SPACE

#ifdef DEBUG
  ReportStatisticsAfterGC();
#endif  // DEBUG

  // Remember the last top pointer so that we can later find out
  // whether we allocated in new space since the last GC.
  new_space_top_after_last_gc_ = new_space()->top();
  last_gc_time_ = MonotonicallyIncreasingTimeInMs();

  {
    TRACE_GC(tracer(), GCTracer::Scope::HEAP_EPILOGUE_REDUCE_NEW_SPACE);
    ReduceNewSpaceSize();
  }
}


void Heap::PreprocessStackTraces() {
  WeakFixedArray::Iterator iterator(weak_stack_trace_list());
  FixedArray* elements;
  while ((elements = iterator.Next<FixedArray>())) {
    for (int j = 1; j < elements->length(); j += 4) {
      Object* maybe_code = elements->get(j + 2);
      // If GC happens while adding a stack trace to the weak fixed array,
      // which has been copied into a larger backing store, we may run into
      // a stack trace that has already been preprocessed. Guard against this.
      if (!maybe_code->IsAbstractCode()) break;
      AbstractCode* abstract_code = AbstractCode::cast(maybe_code);
      int offset = Smi::cast(elements->get(j + 3))->value();
      int pos = abstract_code->SourcePosition(offset);
      elements->set(j + 2, Smi::FromInt(pos));
    }
  }
  // We must not compact the weak fixed list here, as we may be in the middle
  // of writing to it, when the GC triggered. Instead, we reset the root value.
  set_weak_stack_trace_list(Smi::kZero);
}


class GCCallbacksScope {
 public:
  explicit GCCallbacksScope(Heap* heap) : heap_(heap) {
    heap_->gc_callbacks_depth_++;
  }
  ~GCCallbacksScope() { heap_->gc_callbacks_depth_--; }

  bool CheckReenter() { return heap_->gc_callbacks_depth_ == 1; }

 private:
  Heap* heap_;
};


void Heap::HandleGCRequest() {
  if (HighMemoryPressure()) {
    incremental_marking()->reset_request_type();
    CheckMemoryPressure();
  } else if (incremental_marking()->request_type() ==
             IncrementalMarking::COMPLETE_MARKING) {
    incremental_marking()->reset_request_type();
    CollectAllGarbage(current_gc_flags_,
                      GarbageCollectionReason::kFinalizeMarkingViaStackGuard,
                      current_gc_callback_flags_);
  } else if (incremental_marking()->request_type() ==
                 IncrementalMarking::FINALIZATION &&
             incremental_marking()->IsMarking() &&
             !incremental_marking()->finalize_marking_completed()) {
    incremental_marking()->reset_request_type();
    FinalizeIncrementalMarking(
        GarbageCollectionReason::kFinalizeMarkingViaStackGuard);
  }
}


void Heap::ScheduleIdleScavengeIfNeeded(int bytes_allocated) {
  scavenge_job_->ScheduleIdleTaskIfNeeded(this, bytes_allocated);
}

void Heap::FinalizeIncrementalMarking(GarbageCollectionReason gc_reason) {
  if (FLAG_trace_incremental_marking) {
    isolate()->PrintWithTimestamp(
        "[IncrementalMarking] (%s).\n",
        Heap::GarbageCollectionReasonToString(gc_reason));
  }

  HistogramTimerScope incremental_marking_scope(
      isolate()->counters()->gc_incremental_marking_finalize());
  TRACE_EVENT0("v8", "V8.GCIncrementalMarkingFinalize");
  TRACE_GC(tracer(), GCTracer::Scope::MC_INCREMENTAL_FINALIZE);

  {
    GCCallbacksScope scope(this);
    if (scope.CheckReenter()) {
      AllowHeapAllocation allow_allocation;
      TRACE_GC(tracer(), GCTracer::Scope::MC_INCREMENTAL_EXTERNAL_PROLOGUE);
      VMState<EXTERNAL> state(isolate_);
      HandleScope handle_scope(isolate_);
      CallGCPrologueCallbacks(kGCTypeIncrementalMarking, kNoGCCallbackFlags);
    }
  }
  incremental_marking()->FinalizeIncrementally();
  {
    GCCallbacksScope scope(this);
    if (scope.CheckReenter()) {
      AllowHeapAllocation allow_allocation;
      TRACE_GC(tracer(), GCTracer::Scope::MC_INCREMENTAL_EXTERNAL_EPILOGUE);
      VMState<EXTERNAL> state(isolate_);
      HandleScope handle_scope(isolate_);
      CallGCEpilogueCallbacks(kGCTypeIncrementalMarking, kNoGCCallbackFlags);
    }
  }
}


HistogramTimer* Heap::GCTypeTimer(GarbageCollector collector) {
  if (IsYoungGenerationCollector(collector)) {
    return isolate_->counters()->gc_scavenger();
  } else {
    if (!incremental_marking()->IsStopped()) {
      if (ShouldReduceMemory()) {
        return isolate_->counters()->gc_finalize_reduce_memory();
      } else {
        return isolate_->counters()->gc_finalize();
      }
    } else {
      return isolate_->counters()->gc_compactor();
    }
  }
}

void Heap::CollectAllGarbage(int flags, GarbageCollectionReason gc_reason,
                             const v8::GCCallbackFlags gc_callback_flags) {
  // Since we are ignoring the return value, the exact choice of space does
  // not matter, so long as we do not specify NEW_SPACE, which would not
  // cause a full GC.
  set_current_gc_flags(flags);
  CollectGarbage(OLD_SPACE, gc_reason, gc_callback_flags);
  set_current_gc_flags(kNoGCFlags);
}

void Heap::CollectAllAvailableGarbage(GarbageCollectionReason gc_reason) {
  // Since we are ignoring the return value, the exact choice of space does
  // not matter, so long as we do not specify NEW_SPACE, which would not
  // cause a full GC.
  // Major GC would invoke weak handle callbacks on weakly reachable
  // handles, but won't collect weakly reachable objects until next
  // major GC.  Therefore if we collect aggressively and weak handle callback
  // has been invoked, we rerun major GC to release objects which become
  // garbage.
  // Note: as weak callbacks can execute arbitrary code, we cannot
  // hope that eventually there will be no weak callbacks invocations.
  // Therefore stop recollecting after several attempts.
  if (gc_reason == GarbageCollectionReason::kLastResort) {
    InvokeOutOfMemoryCallback();
  }
  RuntimeCallTimerScope(isolate(), &RuntimeCallStats::GC_AllAvailableGarbage);
  if (isolate()->concurrent_recompilation_enabled()) {
    // The optimizing compiler may be unnecessarily holding on to memory.
    DisallowHeapAllocation no_recursive_gc;
    isolate()->optimizing_compile_dispatcher()->Flush(
        OptimizingCompileDispatcher::BlockingBehavior::kDontBlock);
  }
  isolate()->ClearSerializerData();
  set_current_gc_flags(kMakeHeapIterableMask | kReduceMemoryFootprintMask);
  isolate_->compilation_cache()->Clear();
  const int kMaxNumberOfAttempts = 7;
  const int kMinNumberOfAttempts = 2;
  for (int attempt = 0; attempt < kMaxNumberOfAttempts; attempt++) {
    if (!CollectGarbage(MARK_COMPACTOR, gc_reason, NULL,
                        v8::kGCCallbackFlagCollectAllAvailableGarbage) &&
        attempt + 1 >= kMinNumberOfAttempts) {
      break;
    }
  }
  set_current_gc_flags(kNoGCFlags);
  new_space_->Shrink();
  UncommitFromSpace();
}

void Heap::ReportExternalMemoryPressure() {
  if (external_memory_ >
      (external_memory_at_last_mark_compact_ + external_memory_hard_limit())) {
    CollectAllGarbage(
        kReduceMemoryFootprintMask | kFinalizeIncrementalMarkingMask,
        GarbageCollectionReason::kExternalMemoryPressure,
        static_cast<GCCallbackFlags>(kGCCallbackFlagCollectAllAvailableGarbage |
                                     kGCCallbackFlagCollectAllExternalMemory));
    return;
  }
  if (incremental_marking()->IsStopped()) {
    if (incremental_marking()->CanBeActivated()) {
      StartIncrementalMarking(
          i::Heap::kNoGCFlags, GarbageCollectionReason::kExternalMemoryPressure,
          static_cast<GCCallbackFlags>(
              kGCCallbackFlagSynchronousPhantomCallbackProcessing |
              kGCCallbackFlagCollectAllExternalMemory));
    } else {
      CollectAllGarbage(i::Heap::kNoGCFlags,
                        GarbageCollectionReason::kExternalMemoryPressure,
                        kGCCallbackFlagSynchronousPhantomCallbackProcessing);
    }
  } else {
    // Incremental marking is turned on an has already been started.
    const double pressure =
        static_cast<double>(external_memory_ -
                            external_memory_at_last_mark_compact_ -
                            kExternalAllocationSoftLimit) /
        external_memory_hard_limit();
    DCHECK_GE(1, pressure);
    const double kMaxStepSizeOnExternalLimit = 25;
    const double deadline = MonotonicallyIncreasingTimeInMs() +
                            pressure * kMaxStepSizeOnExternalLimit;
    incremental_marking()->AdvanceIncrementalMarking(
        deadline, IncrementalMarking::GC_VIA_STACK_GUARD,
        IncrementalMarking::FORCE_COMPLETION, StepOrigin::kV8);
  }
}


void Heap::EnsureFillerObjectAtTop() {
  // There may be an allocation memento behind objects in new space. Upon
  // evacuation of a non-full new space (or if we are on the last page) there
  // may be uninitialized memory behind top. We fill the remainder of the page
  // with a filler.
  Address to_top = new_space_->top();
  Page* page = Page::FromAddress(to_top - kPointerSize);
  if (page->Contains(to_top)) {
    int remaining_in_page = static_cast<int>(page->area_end() - to_top);
    CreateFillerObjectAt(to_top, remaining_in_page, ClearRecordedSlots::kNo);
  }
}

bool Heap::CollectGarbage(GarbageCollector collector,
                          GarbageCollectionReason gc_reason,
                          const char* collector_reason,
                          const v8::GCCallbackFlags gc_callback_flags) {
  // The VM is in the GC state until exiting this function.
  VMState<GC> state(isolate_);
  RuntimeCallTimerScope(isolate(), &RuntimeCallStats::GC);

#ifdef DEBUG
  // Reset the allocation timeout to the GC interval, but make sure to
  // allow at least a few allocations after a collection. The reason
  // for this is that we have a lot of allocation sequences and we
  // assume that a garbage collection will allow the subsequent
  // allocation attempts to go through.
  allocation_timeout_ = Max(6, FLAG_gc_interval);
#endif

  EnsureFillerObjectAtTop();

  if (IsYoungGenerationCollector(collector) &&
      !incremental_marking()->IsStopped()) {
    if (FLAG_trace_incremental_marking) {
      isolate()->PrintWithTimestamp(
          "[IncrementalMarking] Scavenge during marking.\n");
    }
  }

  bool next_gc_likely_to_collect_more = false;
  size_t committed_memory_before = 0;

  if (collector == MARK_COMPACTOR) {
    committed_memory_before = CommittedOldGenerationMemory();
  }

  {
    tracer()->Start(collector, gc_reason, collector_reason);
    DCHECK(AllowHeapAllocation::IsAllowed());
    DisallowHeapAllocation no_allocation_during_gc;
    GarbageCollectionPrologue();

    {
      HistogramTimer* gc_type_timer = GCTypeTimer(collector);
      HistogramTimerScope histogram_timer_scope(gc_type_timer);
      TRACE_EVENT0("v8", gc_type_timer->name());

      next_gc_likely_to_collect_more =
          PerformGarbageCollection(collector, gc_callback_flags);
    }

    GarbageCollectionEpilogue();
    if (collector == MARK_COMPACTOR && FLAG_track_detached_contexts) {
      isolate()->CheckDetachedContextsAfterGC();
    }

    if (collector == MARK_COMPACTOR) {
      size_t committed_memory_after = CommittedOldGenerationMemory();
      size_t used_memory_after = PromotedSpaceSizeOfObjects();
      MemoryReducer::Event event;
      event.type = MemoryReducer::kMarkCompact;
      event.time_ms = MonotonicallyIncreasingTimeInMs();
      // Trigger one more GC if
      // - this GC decreased committed memory,
      // - there is high fragmentation,
      // - there are live detached contexts.
      event.next_gc_likely_to_collect_more =
          (committed_memory_before > committed_memory_after + MB) ||
          HasHighFragmentation(used_memory_after, committed_memory_after) ||
          (detached_contexts()->length() > 0);
      event.committed_memory = committed_memory_after;
      if (deserialization_complete_) {
        memory_reducer_->NotifyMarkCompact(event);
      }
      memory_pressure_level_.SetValue(MemoryPressureLevel::kNone);
    }

    tracer()->Stop(collector);
  }

  if (collector == MARK_COMPACTOR &&
      (gc_callback_flags & (kGCCallbackFlagForced |
                            kGCCallbackFlagCollectAllAvailableGarbage)) != 0) {
    isolate()->CountUsage(v8::Isolate::kForcedGC);
  }

  // Start incremental marking for the next cycle. The heap snapshot
  // generator needs incremental marking to stay off after it aborted.
  // We do this only for scavenger to avoid a loop where mark-compact
  // causes another mark-compact.
  if (IsYoungGenerationCollector(collector) &&
      !ShouldAbortIncrementalMarking()) {
    StartIncrementalMarkingIfAllocationLimitIsReached(kNoGCFlags,
                                                      kNoGCCallbackFlags);
  }

  return next_gc_likely_to_collect_more;
}


int Heap::NotifyContextDisposed(bool dependant_context) {
  if (!dependant_context) {
    tracer()->ResetSurvivalEvents();
    old_generation_size_configured_ = false;
    MemoryReducer::Event event;
    event.type = MemoryReducer::kPossibleGarbage;
    event.time_ms = MonotonicallyIncreasingTimeInMs();
    memory_reducer_->NotifyPossibleGarbage(event);
  }
  if (isolate()->concurrent_recompilation_enabled()) {
    // Flush the queued recompilation tasks.
    isolate()->optimizing_compile_dispatcher()->Flush(
        OptimizingCompileDispatcher::BlockingBehavior::kDontBlock);
  }
  AgeInlineCaches();
  number_of_disposed_maps_ = retained_maps()->Length();
  tracer()->AddContextDisposalTime(MonotonicallyIncreasingTimeInMs());
  return ++contexts_disposed_;
}

void Heap::StartIncrementalMarking(int gc_flags,
                                   GarbageCollectionReason gc_reason,
                                   GCCallbackFlags gc_callback_flags) {
  DCHECK(incremental_marking()->IsStopped());
  set_current_gc_flags(gc_flags);
  current_gc_callback_flags_ = gc_callback_flags;
  incremental_marking()->Start(gc_reason);
}

void Heap::StartIncrementalMarkingIfAllocationLimitIsReached(
    int gc_flags, const GCCallbackFlags gc_callback_flags) {
  if (incremental_marking()->IsStopped()) {
    IncrementalMarkingLimit reached_limit = IncrementalMarkingLimitReached();
    if (reached_limit == IncrementalMarkingLimit::kSoftLimit) {
      incremental_marking()->incremental_marking_job()->ScheduleTask(this);
    } else if (reached_limit == IncrementalMarkingLimit::kHardLimit) {
      StartIncrementalMarking(gc_flags,
                              GarbageCollectionReason::kAllocationLimit,
                              gc_callback_flags);
    }
  }
}

void Heap::StartIdleIncrementalMarking(GarbageCollectionReason gc_reason) {
  gc_idle_time_handler_->ResetNoProgressCounter();
  StartIncrementalMarking(kReduceMemoryFootprintMask, gc_reason,
                          kNoGCCallbackFlags);
}


void Heap::MoveElements(FixedArray* array, int dst_index, int src_index,
                        int len) {
  if (len == 0) return;

  DCHECK(array->map() != fixed_cow_array_map());
  Object** dst_objects = array->data_start() + dst_index;
  MemMove(dst_objects, array->data_start() + src_index, len * kPointerSize);
  FIXED_ARRAY_ELEMENTS_WRITE_BARRIER(this, array, dst_index, len);
}


#ifdef VERIFY_HEAP
// Helper class for verifying the string table.
class StringTableVerifier : public ObjectVisitor {
 public:
  void VisitPointers(Object** start, Object** end) override {
    // Visit all HeapObject pointers in [start, end).
    for (Object** p = start; p < end; p++) {
      if ((*p)->IsHeapObject()) {
        HeapObject* object = HeapObject::cast(*p);
        Isolate* isolate = object->GetIsolate();
        // Check that the string is actually internalized.
        CHECK(object->IsTheHole(isolate) || object->IsUndefined(isolate) ||
              object->IsInternalizedString());
      }
    }
  }
};


static void VerifyStringTable(Heap* heap) {
  StringTableVerifier verifier;
  heap->string_table()->IterateElements(&verifier);
}
#endif  // VERIFY_HEAP

bool Heap::ReserveSpace(Reservation* reservations, List<Address>* maps) {
  bool gc_performed = true;
  int counter = 0;
  static const int kThreshold = 20;
  while (gc_performed && counter++ < kThreshold) {
    gc_performed = false;
    for (int space = NEW_SPACE; space < SerializerDeserializer::kNumberOfSpaces;
         space++) {
      Reservation* reservation = &reservations[space];
      DCHECK_LE(1, reservation->length());
      if (reservation->at(0).size == 0) continue;
      bool perform_gc = false;
      if (space == MAP_SPACE) {
        // We allocate each map individually to avoid fragmentation.
        maps->Clear();
        DCHECK_EQ(1, reservation->length());
        int num_maps = reservation->at(0).size / Map::kSize;
        for (int i = 0; i < num_maps; i++) {
          // The deserializer will update the skip list.
          AllocationResult allocation = map_space()->AllocateRawUnaligned(
              Map::kSize, PagedSpace::IGNORE_SKIP_LIST);
          HeapObject* free_space = nullptr;
          if (allocation.To(&free_space)) {
            // Mark with a free list node, in case we have a GC before
            // deserializing.
            Address free_space_address = free_space->address();
            CreateFillerObjectAt(free_space_address, Map::kSize,
                                 ClearRecordedSlots::kNo);
            maps->Add(free_space_address);
          } else {
            perform_gc = true;
            break;
          }
        }
      } else if (space == LO_SPACE) {
        // Just check that we can allocate during deserialization.
        DCHECK_EQ(1, reservation->length());
        perform_gc = !CanExpandOldGeneration(reservation->at(0).size);
      } else {
        for (auto& chunk : *reservation) {
          AllocationResult allocation;
          int size = chunk.size;
          DCHECK_LE(static_cast<size_t>(size),
                    MemoryAllocator::PageAreaSize(
                        static_cast<AllocationSpace>(space)));
          if (space == NEW_SPACE) {
            allocation = new_space()->AllocateRawUnaligned(size);
          } else {
            // The deserializer will update the skip list.
            allocation = paged_space(space)->AllocateRawUnaligned(
                size, PagedSpace::IGNORE_SKIP_LIST);
          }
          HeapObject* free_space = nullptr;
          if (allocation.To(&free_space)) {
            // Mark with a free list node, in case we have a GC before
            // deserializing.
            Address free_space_address = free_space->address();
            CreateFillerObjectAt(free_space_address, size,
                                 ClearRecordedSlots::kNo);
            DCHECK(space < SerializerDeserializer::kNumberOfPreallocatedSpaces);
            chunk.start = free_space_address;
            chunk.end = free_space_address + size;
          } else {
            perform_gc = true;
            break;
          }
        }
      }
      if (perform_gc) {
        if (space == NEW_SPACE) {
          CollectGarbage(NEW_SPACE, GarbageCollectionReason::kDeserializer);
        } else {
          if (counter > 1) {
            CollectAllGarbage(
                kReduceMemoryFootprintMask | kAbortIncrementalMarkingMask,
                GarbageCollectionReason::kDeserializer);
          } else {
            CollectAllGarbage(kAbortIncrementalMarkingMask,
                              GarbageCollectionReason::kDeserializer);
          }
        }
        gc_performed = true;
        break;  // Abort for-loop over spaces and retry.
      }
    }
  }

  return !gc_performed;
}


void Heap::EnsureFromSpaceIsCommitted() {
  if (new_space_->CommitFromSpaceIfNeeded()) return;

  // Committing memory to from space failed.
  // Memory is exhausted and we will die.
  V8::FatalProcessOutOfMemory("Committing semi space failed.");
}


void Heap::ClearNormalizedMapCaches() {
  if (isolate_->bootstrapper()->IsActive() &&
      !incremental_marking()->IsMarking()) {
    return;
  }

  Object* context = native_contexts_list();
  while (!context->IsUndefined(isolate())) {
    // GC can happen when the context is not fully initialized,
    // so the cache can be undefined.
    Object* cache =
        Context::cast(context)->get(Context::NORMALIZED_MAP_CACHE_INDEX);
    if (!cache->IsUndefined(isolate())) {
      NormalizedMapCache::cast(cache)->Clear();
    }
    context = Context::cast(context)->next_context_link();
  }
}


void Heap::UpdateSurvivalStatistics(int start_new_space_size) {
  if (start_new_space_size == 0) return;

  promotion_ratio_ = (static_cast<double>(promoted_objects_size_) /
                      static_cast<double>(start_new_space_size) * 100);

  if (previous_semi_space_copied_object_size_ > 0) {
    promotion_rate_ =
        (static_cast<double>(promoted_objects_size_) /
         static_cast<double>(previous_semi_space_copied_object_size_) * 100);
  } else {
    promotion_rate_ = 0;
  }

  semi_space_copied_rate_ =
      (static_cast<double>(semi_space_copied_object_size_) /
       static_cast<double>(start_new_space_size) * 100);

  double survival_rate = promotion_ratio_ + semi_space_copied_rate_;
  tracer()->AddSurvivalRatio(survival_rate);
}

bool Heap::PerformGarbageCollection(
    GarbageCollector collector, const v8::GCCallbackFlags gc_callback_flags) {
  int freed_global_handles = 0;

  if (!IsYoungGenerationCollector(collector)) {
    PROFILE(isolate_, CodeMovingGCEvent());
  }

#ifdef VERIFY_HEAP
  if (FLAG_verify_heap) {
    VerifyStringTable(this);
  }
#endif

  GCType gc_type =
      collector == MARK_COMPACTOR ? kGCTypeMarkSweepCompact : kGCTypeScavenge;

  {
    GCCallbacksScope scope(this);
    if (scope.CheckReenter()) {
      AllowHeapAllocation allow_allocation;
      TRACE_GC(tracer(), GCTracer::Scope::HEAP_EXTERNAL_PROLOGUE);
      VMState<EXTERNAL> state(isolate_);
      HandleScope handle_scope(isolate_);
      CallGCPrologueCallbacks(gc_type, kNoGCCallbackFlags);
    }
  }

  EnsureFromSpaceIsCommitted();

  int start_new_space_size = static_cast<int>(Heap::new_space()->Size());

  {
    Heap::PretenuringScope pretenuring_scope(this);
    Heap::SkipStoreBufferScope skip_store_buffer_scope(store_buffer_);

    switch (collector) {
      case MARK_COMPACTOR:
        UpdateOldGenerationAllocationCounter();
        // Perform mark-sweep with optional compaction.
        MarkCompact();
        old_generation_size_configured_ = true;
        // This should be updated before PostGarbageCollectionProcessing, which
        // can cause another GC. Take into account the objects promoted during
        // GC.
        old_generation_allocation_counter_at_last_gc_ +=
            static_cast<size_t>(promoted_objects_size_);
        old_generation_size_at_last_gc_ = PromotedSpaceSizeOfObjects();
        break;
      case MINOR_MARK_COMPACTOR:
        MinorMarkCompact();
        break;
      case SCAVENGER:
        if (fast_promotion_mode_ &&
            CanExpandOldGeneration(new_space()->Size())) {
          tracer()->NotifyYoungGenerationHandling(
              YoungGenerationHandling::kFastPromotionDuringScavenge);
          EvacuateYoungGeneration();
        } else {
          tracer()->NotifyYoungGenerationHandling(
              YoungGenerationHandling::kRegularScavenge);

          Scavenge();
        }
        break;
    }

    ProcessPretenuringFeedback();
  }

  UpdateSurvivalStatistics(start_new_space_size);
  ConfigureInitialOldGenerationSize();

  if (!fast_promotion_mode_ || collector == MARK_COMPACTOR) {
    ComputeFastPromotionMode(promotion_ratio_ + semi_space_copied_rate_);
  }

  isolate_->counters()->objs_since_last_young()->Set(0);

  gc_post_processing_depth_++;
  {
    AllowHeapAllocation allow_allocation;
    TRACE_GC(tracer(), GCTracer::Scope::HEAP_EXTERNAL_WEAK_GLOBAL_HANDLES);
    freed_global_handles =
        isolate_->global_handles()->PostGarbageCollectionProcessing(
            collector, gc_callback_flags);
  }
  gc_post_processing_depth_--;

  isolate_->eternal_handles()->PostGarbageCollectionProcessing(this);

  // Update relocatables.
  Relocatable::PostGarbageCollectionProcessing(isolate_);

  double gc_speed = tracer()->CombinedMarkCompactSpeedInBytesPerMillisecond();
  double mutator_speed =
      tracer()->CurrentOldGenerationAllocationThroughputInBytesPerMillisecond();
  size_t old_gen_size = PromotedSpaceSizeOfObjects();
  if (collector == MARK_COMPACTOR) {
    // Register the amount of external allocated memory.
    external_memory_at_last_mark_compact_ = external_memory_;
    external_memory_limit_ = external_memory_ + kExternalAllocationSoftLimit;
    SetOldGenerationAllocationLimit(old_gen_size, gc_speed, mutator_speed);
  } else if (HasLowYoungGenerationAllocationRate() &&
             old_generation_size_configured_) {
    DampenOldGenerationAllocationLimit(old_gen_size, gc_speed, mutator_speed);
  }

  {
    GCCallbacksScope scope(this);
    if (scope.CheckReenter()) {
      AllowHeapAllocation allow_allocation;
      TRACE_GC(tracer(), GCTracer::Scope::HEAP_EXTERNAL_EPILOGUE);
      VMState<EXTERNAL> state(isolate_);
      HandleScope handle_scope(isolate_);
      CallGCEpilogueCallbacks(gc_type, gc_callback_flags);
    }
  }

#ifdef VERIFY_HEAP
  if (FLAG_verify_heap) {
    VerifyStringTable(this);
  }
#endif

  return freed_global_handles > 0;
}


void Heap::CallGCPrologueCallbacks(GCType gc_type, GCCallbackFlags flags) {
  RuntimeCallTimerScope(isolate(), &RuntimeCallStats::GCPrologueCallback);
  for (int i = 0; i < gc_prologue_callbacks_.length(); ++i) {
    if (gc_type & gc_prologue_callbacks_[i].gc_type) {
      if (!gc_prologue_callbacks_[i].pass_isolate) {
        v8::GCCallback callback = reinterpret_cast<v8::GCCallback>(
            gc_prologue_callbacks_[i].callback);
        callback(gc_type, flags);
      } else {
        v8::Isolate* isolate = reinterpret_cast<v8::Isolate*>(this->isolate());
        gc_prologue_callbacks_[i].callback(isolate, gc_type, flags);
      }
    }
  }
}


void Heap::CallGCEpilogueCallbacks(GCType gc_type,
                                   GCCallbackFlags gc_callback_flags) {
  RuntimeCallTimerScope(isolate(), &RuntimeCallStats::GCEpilogueCallback);
  for (int i = 0; i < gc_epilogue_callbacks_.length(); ++i) {
    if (gc_type & gc_epilogue_callbacks_[i].gc_type) {
      if (!gc_epilogue_callbacks_[i].pass_isolate) {
        v8::GCCallback callback = reinterpret_cast<v8::GCCallback>(
            gc_epilogue_callbacks_[i].callback);
        callback(gc_type, gc_callback_flags);
      } else {
        v8::Isolate* isolate = reinterpret_cast<v8::Isolate*>(this->isolate());
        gc_epilogue_callbacks_[i].callback(isolate, gc_type, gc_callback_flags);
      }
    }
  }
}


void Heap::MarkCompact() {
  PauseAllocationObserversScope pause_observers(this);

  SetGCState(MARK_COMPACT);

  LOG(isolate_, ResourceEvent("markcompact", "begin"));

  uint64_t size_of_objects_before_gc = SizeOfObjects();

  mark_compact_collector()->Prepare();

  ms_count_++;

  MarkCompactPrologue();

  mark_compact_collector()->CollectGarbage();

  LOG(isolate_, ResourceEvent("markcompact", "end"));

  MarkCompactEpilogue();

  if (FLAG_allocation_site_pretenuring) {
    EvaluateOldSpaceLocalPretenuring(size_of_objects_before_gc);
  }
}

void Heap::MinorMarkCompact() { UNREACHABLE(); }

void Heap::MarkCompactEpilogue() {
  TRACE_GC(tracer(), GCTracer::Scope::MC_EPILOGUE);
  SetGCState(NOT_IN_GC);

  isolate_->counters()->objs_since_last_full()->Set(0);

  incremental_marking()->Epilogue();

  PreprocessStackTraces();
  DCHECK(incremental_marking()->IsStopped());

  mark_compact_collector()->marking_deque()->StopUsing();
}


void Heap::MarkCompactPrologue() {
  TRACE_GC(tracer(), GCTracer::Scope::MC_PROLOGUE);
  isolate_->context_slot_cache()->Clear();
  isolate_->descriptor_lookup_cache()->Clear();
  RegExpResultsCache::Clear(string_split_cache());
  RegExpResultsCache::Clear(regexp_multiple_cache());

  isolate_->compilation_cache()->MarkCompactPrologue();

  CompletelyClearInstanceofCache();

  FlushNumberStringCache();
  ClearNormalizedMapCaches();
}


void Heap::CheckNewSpaceExpansionCriteria() {
  if (FLAG_experimental_new_space_growth_heuristic) {
    if (new_space_->TotalCapacity() < new_space_->MaximumCapacity() &&
        survived_last_scavenge_ * 100 / new_space_->TotalCapacity() >= 10) {
      // Grow the size of new space if there is room to grow, and more than 10%
      // have survived the last scavenge.
      new_space_->Grow();
      survived_since_last_expansion_ = 0;
    }
  } else if (new_space_->TotalCapacity() < new_space_->MaximumCapacity() &&
             survived_since_last_expansion_ > new_space_->TotalCapacity()) {
    // Grow the size of new space if there is room to grow, and enough data
    // has survived scavenge since the last expansion.
    new_space_->Grow();
    survived_since_last_expansion_ = 0;
  }
}


static bool IsUnscavengedHeapObject(Heap* heap, Object** p) {
  return heap->InNewSpace(*p) &&
         !HeapObject::cast(*p)->map_word().IsForwardingAddress();
}

void PromotionQueue::Initialize() {
  // The last to-space page may be used for promotion queue. On promotion
  // conflict, we use the emergency stack.
  DCHECK((Page::kPageSize - MemoryChunk::kBodyOffset) % (2 * kPointerSize) ==
         0);
  front_ = rear_ =
      reinterpret_cast<struct Entry*>(heap_->new_space()->ToSpaceEnd());
  limit_ = reinterpret_cast<struct Entry*>(
      Page::FromAllocationAreaAddress(reinterpret_cast<Address>(rear_))
          ->area_start());
  emergency_stack_ = NULL;
}

void PromotionQueue::Destroy() {
  DCHECK(is_empty());
  delete emergency_stack_;
  emergency_stack_ = NULL;
}

void PromotionQueue::RelocateQueueHead() {
  DCHECK(emergency_stack_ == NULL);

  Page* p = Page::FromAllocationAreaAddress(reinterpret_cast<Address>(rear_));
  struct Entry* head_start = rear_;
  struct Entry* head_end =
      Min(front_, reinterpret_cast<struct Entry*>(p->area_end()));

  int entries_count =
      static_cast<int>(head_end - head_start) / sizeof(struct Entry);

  emergency_stack_ = new List<Entry>(2 * entries_count);

  while (head_start != head_end) {
    struct Entry* entry = head_start++;
    // New space allocation in SemiSpaceCopyObject marked the region
    // overlapping with promotion queue as uninitialized.
    MSAN_MEMORY_IS_INITIALIZED(entry, sizeof(struct Entry));
    emergency_stack_->Add(*entry);
  }
  rear_ = head_end;
}


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

  virtual Object* RetainAs(Object* object) {
    if (!heap_->InFromSpace(object)) {
      return object;
    }

    MapWord map_word = HeapObject::cast(object)->map_word();
    if (map_word.IsForwardingAddress()) {
      return map_word.ToForwardingAddress();
    }
    return NULL;
  }

 private:
  Heap* heap_;
};

void Heap::EvacuateYoungGeneration() {
  TRACE_GC(tracer(), GCTracer::Scope::SCAVENGER_EVACUATE);
  DCHECK(fast_promotion_mode_);
  DCHECK(CanExpandOldGeneration(new_space()->Size()));

  mark_compact_collector()->sweeper().EnsureNewSpaceCompleted();

  SetGCState(SCAVENGE);
  LOG(isolate_, ResourceEvent("scavenge", "begin"));

  // Move pages from new->old generation.
  PageRange range(new_space()->bottom(), new_space()->top());
  for (auto it = range.begin(); it != range.end();) {
    Page* p = (*++it)->prev_page();
    p->Unlink();
    Page::ConvertNewToOld(p);
    if (incremental_marking()->IsMarking())
      mark_compact_collector()->RecordLiveSlotsOnPage(p);
  }

  // Reset new space.
  if (!new_space()->Rebalance()) {
    FatalProcessOutOfMemory("NewSpace::Rebalance");
  }
  new_space()->ResetAllocationInfo();
  new_space()->set_age_mark(new_space()->top());

  // Fix up special trackers.
  external_string_table_.PromoteAllNewSpaceStrings();
  // GlobalHandles are updated in PostGarbageCollectonProcessing

  IncrementYoungSurvivorsCounter(new_space()->Size());
  IncrementPromotedObjectsSize(new_space()->Size());
  IncrementSemiSpaceCopiedObjectSize(0);

  LOG(isolate_, ResourceEvent("scavenge", "end"));
  SetGCState(NOT_IN_GC);
}

void Heap::Scavenge() {
  TRACE_GC(tracer(), GCTracer::Scope::SCAVENGER_SCAVENGE);
  RelocationLock relocation_lock(this);
  // There are soft limits in the allocation code, designed to trigger a mark
  // sweep collection by failing allocations. There is no sense in trying to
  // trigger one during scavenge: scavenges allocation should always succeed.
  AlwaysAllocateScope scope(isolate());

  // Bump-pointer allocations done during scavenge are not real allocations.
  // Pause the inline allocation steps.
  PauseAllocationObserversScope pause_observers(this);

  mark_compact_collector()->sweeper().EnsureNewSpaceCompleted();

  SetGCState(SCAVENGE);

  // Implements Cheney's copying algorithm
  LOG(isolate_, ResourceEvent("scavenge", "begin"));

  // Used for updating survived_since_last_expansion_ at function end.
  size_t survived_watermark = PromotedSpaceSizeOfObjects();

  scavenge_collector_->SelectScavengingVisitorsTable();

  // Flip the semispaces.  After flipping, to space is empty, from space has
  // live objects.
  new_space_->Flip();
  new_space_->ResetAllocationInfo();

  // We need to sweep newly copied objects which can be either in the
  // to space or promoted to the old generation.  For to-space
  // objects, we treat the bottom of the to space as a queue.  Newly
  // copied and unswept objects lie between a 'front' mark and the
  // allocation pointer.
  //
  // Promoted objects can go into various old-generation spaces, and
  // can be allocated internally in the spaces (from the free list).
  // We treat the top of the to space as a queue of addresses of
  // promoted objects.  The addresses of newly promoted and unswept
  // objects lie between a 'front' mark and a 'rear' mark that is
  // updated as a side effect of promoting an object.
  //
  // There is guaranteed to be enough room at the top of the to space
  // for the addresses of promoted objects: every object promoted
  // frees up its size in bytes from the top of the new space, and
  // objects are at least one pointer in size.
  Address new_space_front = new_space_->ToSpaceStart();
  promotion_queue_.Initialize();

  ScavengeVisitor scavenge_visitor(this);

  isolate()->global_handles()->IdentifyWeakUnmodifiedObjects(
      &IsUnmodifiedHeapObject);

  {
    // Copy roots.
    TRACE_GC(tracer(), GCTracer::Scope::SCAVENGER_ROOTS);
    IterateRoots(&scavenge_visitor, VISIT_ALL_IN_SCAVENGE);
  }

  {
    // Copy objects reachable from the old generation.
    TRACE_GC(tracer(), GCTracer::Scope::SCAVENGER_OLD_TO_NEW_POINTERS);
    RememberedSet<OLD_TO_NEW>::Iterate(this, [this](Address addr) {
      return Scavenger::CheckAndScavengeObject(this, addr);
    });

    RememberedSet<OLD_TO_NEW>::IterateTyped(
        this, [this](SlotType type, Address host_addr, Address addr) {
          return UpdateTypedSlotHelper::UpdateTypedSlot(
              isolate(), type, addr, [this](Object** addr) {
                // We expect that objects referenced by code are long living.
                // If we do not force promotion, then we need to clear
                // old_to_new slots in dead code objects after mark-compact.
                return Scavenger::CheckAndScavengeObject(
                    this, reinterpret_cast<Address>(addr));
              });
        });
  }

  {
    TRACE_GC(tracer(), GCTracer::Scope::SCAVENGER_WEAK);
    // Copy objects reachable from the encountered weak collections list.
    scavenge_visitor.VisitPointer(&encountered_weak_collections_);
  }

  {
    // Copy objects reachable from the code flushing candidates list.
    TRACE_GC(tracer(), GCTracer::Scope::SCAVENGER_CODE_FLUSH_CANDIDATES);
    MarkCompactCollector* collector = mark_compact_collector();
    if (collector->is_code_flushing_enabled()) {
      collector->code_flusher()->IteratePointersToFromSpace(&scavenge_visitor);
    }
  }

  {
    TRACE_GC(tracer(), GCTracer::Scope::SCAVENGER_SEMISPACE);
    new_space_front = DoScavenge(&scavenge_visitor, new_space_front);
  }

  isolate()->global_handles()->MarkNewSpaceWeakUnmodifiedObjectsPending(
      &IsUnscavengedHeapObject);

  isolate()
      ->global_handles()
      ->IterateNewSpaceWeakUnmodifiedRoots<
          GlobalHandles::HANDLE_PHANTOM_NODES_VISIT_OTHERS>(&scavenge_visitor);
  new_space_front = DoScavenge(&scavenge_visitor, new_space_front);

  UpdateNewSpaceReferencesInExternalStringTable(
      &UpdateNewSpaceReferenceInExternalStringTableEntry);

  promotion_queue_.Destroy();

  incremental_marking()->UpdateMarkingDequeAfterScavenge();

  ScavengeWeakObjectRetainer weak_object_retainer(this);
  ProcessYoungWeakReferences(&weak_object_retainer);

  DCHECK(new_space_front == new_space_->top());

  // Set age mark.
  new_space_->set_age_mark(new_space_->top());

  ArrayBufferTracker::FreeDeadInNewSpace(this);

  // Update how much has survived scavenge.
  DCHECK_GE(PromotedSpaceSizeOfObjects(), survived_watermark);
  IncrementYoungSurvivorsCounter(PromotedSpaceSizeOfObjects() +
                                 new_space_->Size() - survived_watermark);

  // Scavenger may find new wrappers by iterating objects promoted onto a black
  // page.
  local_embedder_heap_tracer()->RegisterWrappersWithRemoteTracer();

  LOG(isolate_, ResourceEvent("scavenge", "end"));

  SetGCState(NOT_IN_GC);
}

void Heap::ComputeFastPromotionMode(double survival_rate) {
  const size_t survived_in_new_space =
      survived_last_scavenge_ * 100 / new_space_->Capacity();
  fast_promotion_mode_ =
      !FLAG_optimize_for_size && FLAG_fast_promotion_new_space &&
      !ShouldReduceMemory() && new_space_->IsAtMaximumCapacity() &&
      survived_in_new_space >= kMinPromotedPercentForFastPromotionMode;
  if (FLAG_trace_gc_verbose) {
    PrintIsolate(
        isolate(), "Fast promotion mode: %s survival rate: %" PRIuS "%%\n",
        fast_promotion_mode_ ? "true" : "false", survived_in_new_space);
  }
}

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

  if (!first_word.IsForwardingAddress()) {
    // Unreachable external string can be finalized.
    String* string = String::cast(*p);
    if (!string->IsExternalString()) {
      // Original external string has been internalized.
      DCHECK(string->IsThinString());
      return NULL;
    }
    heap->FinalizeExternalString(string);
    return NULL;
  }

  // String is still reachable.
  String* string = String::cast(first_word.ToForwardingAddress());
  if (string->IsThinString()) string = ThinString::cast(string)->actual();
  // Internalization can replace external strings with non-external strings.
  return string->IsExternalString() ? string : nullptr;
}


void Heap::UpdateNewSpaceReferencesInExternalStringTable(
    ExternalStringTableUpdaterCallback updater_func) {
  if (external_string_table_.new_space_strings_.is_empty()) return;

  Object** start = &external_string_table_.new_space_strings_[0];
  Object** end = start + external_string_table_.new_space_strings_.length();
  Object** last = start;

  for (Object** p = start; p < end; ++p) {
    String* target = updater_func(this, p);

    if (target == NULL) continue;

    DCHECK(target->IsExternalString());

    if (InNewSpace(target)) {
      // String is still in new space.  Update the table entry.
      *last = target;
      ++last;
    } else {
      // String got promoted.  Move it to the old string list.
      external_string_table_.AddOldString(target);
    }
  }

  DCHECK(last <= end);
  external_string_table_.ShrinkNewStrings(static_cast<int>(last - start));
}


void Heap::UpdateReferencesInExternalStringTable(
    ExternalStringTableUpdaterCallback updater_func) {
  // Update old space string references.
  if (external_string_table_.old_space_strings_.length() > 0) {
    Object** start = &external_string_table_.old_space_strings_[0];
    Object** end = start + external_string_table_.old_space_strings_.length();
    for (Object** p = start; p < end; ++p) *p = updater_func(this, p);
  }

  UpdateNewSpaceReferencesInExternalStringTable(updater_func);
}


void Heap::ProcessAllWeakReferences(WeakObjectRetainer* retainer) {
  ProcessNativeContexts(retainer);
  ProcessAllocationSites(retainer);
}


void Heap::ProcessYoungWeakReferences(WeakObjectRetainer* retainer) {
  ProcessNativeContexts(retainer);
}


void Heap::ProcessNativeContexts(WeakObjectRetainer* retainer) {
  Object* head = VisitWeakList<Context>(this, native_contexts_list(), retainer);
  // Update the head of the list of contexts.
  set_native_contexts_list(head);
}


void Heap::ProcessAllocationSites(WeakObjectRetainer* retainer) {
  Object* allocation_site_obj =
      VisitWeakList<AllocationSite>(this, allocation_sites_list(), retainer);
  set_allocation_sites_list(allocation_site_obj);
}

void Heap::ProcessWeakListRoots(WeakObjectRetainer* retainer) {
  set_native_contexts_list(retainer->RetainAs(native_contexts_list()));
  set_allocation_sites_list(retainer->RetainAs(allocation_sites_list()));
}

void Heap::ResetAllAllocationSitesDependentCode(PretenureFlag flag) {
  DisallowHeapAllocation no_allocation_scope;
  Object* cur = allocation_sites_list();
  bool marked = false;
  while (cur->IsAllocationSite()) {
    AllocationSite* casted = AllocationSite::cast(cur);
    if (casted->GetPretenureMode() == flag) {
      casted->ResetPretenureDecision();
      casted->set_deopt_dependent_code(true);
      marked = true;
      RemoveAllocationSitePretenuringFeedback(casted);
    }
    cur = casted->weak_next();
  }
  if (marked) isolate_->stack_guard()->RequestDeoptMarkedAllocationSites();
}


void Heap::EvaluateOldSpaceLocalPretenuring(
    uint64_t size_of_objects_before_gc) {
  uint64_t size_of_objects_after_gc = SizeOfObjects();
  double old_generation_survival_rate =
      (static_cast<double>(size_of_objects_after_gc) * 100) /
      static_cast<double>(size_of_objects_before_gc);

  if (old_generation_survival_rate < kOldSurvivalRateLowThreshold) {
    // Too many objects died in the old generation, pretenuring of wrong
    // allocation sites may be the cause for that. We have to deopt all
    // dependent code registered in the allocation sites to re-evaluate
    // our pretenuring decisions.
    ResetAllAllocationSitesDependentCode(TENURED);
    if (FLAG_trace_pretenuring) {
      PrintF(
          "Deopt all allocation sites dependent code due to low survival "
          "rate in the old generation %f\n",
          old_generation_survival_rate);
    }
  }
}


void Heap::VisitExternalResources(v8::ExternalResourceVisitor* visitor) {
  DisallowHeapAllocation no_allocation;
  // All external strings are listed in the external string table.

  class ExternalStringTableVisitorAdapter : public ObjectVisitor {
   public:
    explicit ExternalStringTableVisitorAdapter(
        v8::ExternalResourceVisitor* visitor)
        : visitor_(visitor) {}
    virtual void VisitPointers(Object** start, Object** end) {
      for (Object** p = start; p < end; p++) {
        DCHECK((*p)->IsExternalString());
        visitor_->VisitExternalString(
            Utils::ToLocal(Handle<String>(String::cast(*p))));
      }
    }

   private:
    v8::ExternalResourceVisitor* visitor_;
  } external_string_table_visitor(visitor);

  external_string_table_.IterateAll(&external_string_table_visitor);
}

Address Heap::DoScavenge(ObjectVisitor* scavenge_visitor,
                         Address new_space_front) {
  do {
    SemiSpace::AssertValidRange(new_space_front, new_space_->top());
    // The addresses new_space_front and new_space_.top() define a
    // queue of unprocessed copied objects.  Process them until the
    // queue is empty.
    while (new_space_front != new_space_->top()) {
      if (!Page::IsAlignedToPageSize(new_space_front)) {
        HeapObject* object = HeapObject::FromAddress(new_space_front);
        new_space_front +=
            StaticScavengeVisitor::IterateBody(object->map(), object);
      } else {
        new_space_front = Page::FromAllocationAreaAddress(new_space_front)
                              ->next_page()
                              ->area_start();
      }
    }

    // Promote and process all the to-be-promoted objects.
    {
      while (!promotion_queue()->is_empty()) {
        HeapObject* target;
        int32_t size;
        bool was_marked_black;
        promotion_queue()->remove(&target, &size, &was_marked_black);

        // Promoted object might be already partially visited
        // during old space pointer iteration. Thus we search specifically
        // for pointers to from semispace instead of looking for pointers
        // to new space.
        DCHECK(!target->IsMap());

        IterateAndScavengePromotedObject(target, static_cast<int>(size),
                                         was_marked_black);
      }
    }

    // Take another spin if there are now unswept objects in new space
    // (there are currently no more unswept promoted objects).
  } while (new_space_front != new_space_->top());

  return new_space_front;
}


STATIC_ASSERT((FixedDoubleArray::kHeaderSize & kDoubleAlignmentMask) ==
              0);  // NOLINT
STATIC_ASSERT((FixedTypedArrayBase::kDataOffset & kDoubleAlignmentMask) ==
              0);  // NOLINT
#ifdef V8_HOST_ARCH_32_BIT
STATIC_ASSERT((HeapNumber::kValueOffset & kDoubleAlignmentMask) !=
              0);  // NOLINT
#endif


int Heap::GetMaximumFillToAlign(AllocationAlignment alignment) {
  switch (alignment) {
    case kWordAligned:
      return 0;
    case kDoubleAligned:
    case kDoubleUnaligned:
      return kDoubleSize - kPointerSize;
    default:
      UNREACHABLE();
  }
  return 0;
}


int Heap::GetFillToAlign(Address address, AllocationAlignment alignment) {
  intptr_t offset = OffsetFrom(address);
  if (alignment == kDoubleAligned && (offset & kDoubleAlignmentMask) != 0)
    return kPointerSize;
  if (alignment == kDoubleUnaligned && (offset & kDoubleAlignmentMask) == 0)
    return kDoubleSize - kPointerSize;  // No fill if double is always aligned.
  return 0;
}


HeapObject* Heap::PrecedeWithFiller(HeapObject* object, int filler_size) {
  CreateFillerObjectAt(object->address(), filler_size, ClearRecordedSlots::kNo);
  return HeapObject::FromAddress(object->address() + filler_size);
}


HeapObject* Heap::AlignWithFiller(HeapObject* object, int object_size,
                                  int allocation_size,
                                  AllocationAlignment alignment) {
  int filler_size = allocation_size - object_size;
  DCHECK(filler_size > 0);
  int pre_filler = GetFillToAlign(object->address(), alignment);
  if (pre_filler) {
    object = PrecedeWithFiller(object, pre_filler);
    filler_size -= pre_filler;
  }
  if (filler_size)
    CreateFillerObjectAt(object->address() + object_size, filler_size,
                         ClearRecordedSlots::kNo);
  return object;
}


HeapObject* Heap::DoubleAlignForDeserialization(HeapObject* object, int size) {
  return AlignWithFiller(object, size - kPointerSize, size, kDoubleAligned);
}


void Heap::RegisterNewArrayBuffer(JSArrayBuffer* buffer) {
  ArrayBufferTracker::RegisterNew(this, buffer);
}


void Heap::UnregisterArrayBuffer(JSArrayBuffer* buffer) {
  ArrayBufferTracker::Unregister(this, buffer);
}

void Heap::ConfigureInitialOldGenerationSize() {
  if (!old_generation_size_configured_ && tracer()->SurvivalEventsRecorded()) {
    old_generation_allocation_limit_ =
        Max(MinimumAllocationLimitGrowingStep(),
            static_cast<size_t>(
                static_cast<double>(old_generation_allocation_limit_) *
                (tracer()->AverageSurvivalRatio() / 100)));
  }
}

AllocationResult Heap::AllocatePartialMap(InstanceType instance_type,
                                          int instance_size) {
  Object* result = nullptr;
  AllocationResult allocation = AllocateRaw(Map::kSize, MAP_SPACE);
  if (!allocation.To(&result)) return allocation;

  // Map::cast cannot be used due to uninitialized map field.
  reinterpret_cast<Map*>(result)->set_map(
      reinterpret_cast<Map*>(root(kMetaMapRootIndex)));
  reinterpret_cast<Map*>(result)->set_instance_type(instance_type);
  reinterpret_cast<Map*>(result)->set_instance_size(instance_size);
  // Initialize to only containing tagged fields.
  reinterpret_cast<Map*>(result)->set_visitor_id(
      StaticVisitorBase::GetVisitorId(instance_type, instance_size, false));
  if (FLAG_unbox_double_fields) {
    reinterpret_cast<Map*>(result)
        ->set_layout_descriptor(LayoutDescriptor::FastPointerLayout());
  }
  reinterpret_cast<Map*>(result)->clear_unused();
  reinterpret_cast<Map*>(result)
      ->set_inobject_properties_or_constructor_function_index(0);
  reinterpret_cast<Map*>(result)->set_unused_property_fields(0);
  reinterpret_cast<Map*>(result)->set_bit_field(0);
  reinterpret_cast<Map*>(result)->set_bit_field2(0);
  int bit_field3 = Map::EnumLengthBits::encode(kInvalidEnumCacheSentinel) |
                   Map::OwnsDescriptors::encode(true) |
                   Map::ConstructionCounter::encode(Map::kNoSlackTracking);
  reinterpret_cast<Map*>(result)->set_bit_field3(bit_field3);
  reinterpret_cast<Map*>(result)->set_weak_cell_cache(Smi::kZero);
  return result;
}


AllocationResult Heap::AllocateMap(InstanceType instance_type,
                                   int instance_size,
                                   ElementsKind elements_kind) {
  HeapObject* result = nullptr;
  AllocationResult allocation = AllocateRaw(Map::kSize, MAP_SPACE);
  if (!allocation.To(&result)) return allocation;

  isolate()->counters()->maps_created()->Increment();
  result->set_map_no_write_barrier(meta_map());
  Map* map = Map::cast(result);
  map->set_instance_type(instance_type);
  map->set_prototype(null_value(), SKIP_WRITE_BARRIER);
  map->set_constructor_or_backpointer(null_value(), SKIP_WRITE_BARRIER);
  map->set_instance_size(instance_size);
  map->clear_unused();
  map->set_inobject_properties_or_constructor_function_index(0);
  map->set_code_cache(empty_fixed_array(), SKIP_WRITE_BARRIER);
  map->set_dependent_code(DependentCode::cast(empty_fixed_array()),
                          SKIP_WRITE_BARRIER);
  map->set_weak_cell_cache(Smi::kZero);
  map->set_raw_transitions(Smi::kZero);
  map->set_unused_property_fields(0);
  map->set_instance_descriptors(empty_descriptor_array());
  if (FLAG_unbox_double_fields) {
    map->set_layout_descriptor(LayoutDescriptor::FastPointerLayout());
  }
  // Must be called only after |instance_type|, |instance_size| and
  // |layout_descriptor| are set.
  map->set_visitor_id(Heap::GetStaticVisitorIdForMap(map));
  map->set_bit_field(0);
  map->set_bit_field2(1 << Map::kIsExtensible);
  int bit_field3 = Map::EnumLengthBits::encode(kInvalidEnumCacheSentinel) |
                   Map::OwnsDescriptors::encode(true) |
                   Map::ConstructionCounter::encode(Map::kNoSlackTracking);
  map->set_bit_field3(bit_field3);
  map->set_elements_kind(elements_kind);
  map->set_new_target_is_base(true);

  return map;
}


AllocationResult Heap::AllocateFillerObject(int size, bool double_align,
                                            AllocationSpace space) {
  HeapObject* obj = nullptr;
  {
    AllocationAlignment align = double_align ? kDoubleAligned : kWordAligned;
    AllocationResult allocation = AllocateRaw(size, space, align);
    if (!allocation.To(&obj)) return allocation;
  }
#ifdef DEBUG
  MemoryChunk* chunk = MemoryChunk::FromAddress(obj->address());
  DCHECK(chunk->owner()->identity() == space);
#endif
  CreateFillerObjectAt(obj->address(), size, ClearRecordedSlots::kNo);
  return obj;
}


const Heap::StringTypeTable Heap::string_type_table[] = {
#define STRING_TYPE_ELEMENT(type, size, name, camel_name) \
  { type, size, k##camel_name##MapRootIndex }             \
  ,
    STRING_TYPE_LIST(STRING_TYPE_ELEMENT)
#undef STRING_TYPE_ELEMENT
};


const Heap::ConstantStringTable Heap::constant_string_table[] = {
    {"", kempty_stringRootIndex},
#define CONSTANT_STRING_ELEMENT(name, contents) \
  { contents, k##name##RootIndex }              \
  ,
    INTERNALIZED_STRING_LIST(CONSTANT_STRING_ELEMENT)
#undef CONSTANT_STRING_ELEMENT
};


const Heap::StructTable Heap::struct_table[] = {
#define STRUCT_TABLE_ELEMENT(NAME, Name, name)        \
  { NAME##_TYPE, Name::kSize, k##Name##MapRootIndex } \
  ,
    STRUCT_LIST(STRUCT_TABLE_ELEMENT)
#undef STRUCT_TABLE_ELEMENT
};

namespace {

void FinalizePartialMap(Heap* heap, Map* map) {
  map->set_code_cache(heap->empty_fixed_array());
  map->set_dependent_code(DependentCode::cast(heap->empty_fixed_array()));
  map->set_raw_transitions(Smi::kZero);
  map->set_instance_descriptors(heap->empty_descriptor_array());
  if (FLAG_unbox_double_fields) {
    map->set_layout_descriptor(LayoutDescriptor::FastPointerLayout());
  }
  map->set_prototype(heap->null_value());
  map->set_constructor_or_backpointer(heap->null_value());
}

}  // namespace

bool Heap::CreateInitialMaps() {
  HeapObject* obj = nullptr;
  {
    AllocationResult allocation = AllocatePartialMap(MAP_TYPE, Map::kSize);
    if (!allocation.To(&obj)) return false;
  }
  // Map::cast cannot be used due to uninitialized map field.
  Map* new_meta_map = reinterpret_cast<Map*>(obj);
  set_meta_map(new_meta_map);
  new_meta_map->set_map(new_meta_map);

  {  // Partial map allocation
#define ALLOCATE_PARTIAL_MAP(instance_type, size, field_name)                \
  {                                                                          \
    Map* map;                                                                \
    if (!AllocatePartialMap((instance_type), (size)).To(&map)) return false; \
    set_##field_name##_map(map);                                             \
  }

    ALLOCATE_PARTIAL_MAP(FIXED_ARRAY_TYPE, kVariableSizeSentinel, fixed_array);
    fixed_array_map()->set_elements_kind(FAST_HOLEY_ELEMENTS);
    ALLOCATE_PARTIAL_MAP(ODDBALL_TYPE, Oddball::kSize, undefined);
    ALLOCATE_PARTIAL_MAP(ODDBALL_TYPE, Oddball::kSize, null);
    ALLOCATE_PARTIAL_MAP(ODDBALL_TYPE, Oddball::kSize, the_hole);

#undef ALLOCATE_PARTIAL_MAP
  }

  // Allocate the empty array.
  {
    AllocationResult allocation = AllocateEmptyFixedArray();
    if (!allocation.To(&obj)) return false;
  }
  set_empty_fixed_array(FixedArray::cast(obj));

  {
    AllocationResult allocation = Allocate(null_map(), OLD_SPACE);
    if (!allocation.To(&obj)) return false;
  }
  set_null_value(Oddball::cast(obj));
  Oddball::cast(obj)->set_kind(Oddball::kNull);

  {
    AllocationResult allocation = Allocate(undefined_map(), OLD_SPACE);
    if (!allocation.To(&obj)) return false;
  }
  set_undefined_value(Oddball::cast(obj));
  Oddball::cast(obj)->set_kind(Oddball::kUndefined);
  DCHECK(!InNewSpace(undefined_value()));
  {
    AllocationResult allocation = Allocate(the_hole_map(), OLD_SPACE);
    if (!allocation.To(&obj)) return false;
  }
  set_the_hole_value(Oddball::cast(obj));
  Oddball::cast(obj)->set_kind(Oddball::kTheHole);

  // Set preliminary exception sentinel value before actually initializing it.
  set_exception(null_value());

  // Allocate the empty descriptor array.
  {
    AllocationResult allocation = AllocateEmptyFixedArray();
    if (!allocation.To(&obj)) return false;
  }
  set_empty_descriptor_array(DescriptorArray::cast(obj));

  // Fix the instance_descriptors for the existing maps.
  FinalizePartialMap(this, meta_map());
  FinalizePartialMap(this, fixed_array_map());
  FinalizePartialMap(this, undefined_map());
  undefined_map()->set_is_undetectable();
  FinalizePartialMap(this, null_map());
  null_map()->set_is_undetectable();
  FinalizePartialMap(this, the_hole_map());

  {  // Map allocation
#define ALLOCATE_MAP(instance_type, size, field_name)               \
  {                                                                 \
    Map* map;                                                       \
    if (!AllocateMap((instance_type), size).To(&map)) return false; \
    set_##field_name##_map(map);                                    \
  }

#define ALLOCATE_VARSIZE_MAP(instance_type, field_name) \
  ALLOCATE_MAP(instance_type, kVariableSizeSentinel, field_name)

#define ALLOCATE_PRIMITIVE_MAP(instance_type, size, field_name, \
                               constructor_function_index)      \
  {                                                             \
    ALLOCATE_MAP((instance_type), (size), field_name);          \
    field_name##_map()->SetConstructorFunctionIndex(            \
        (constructor_function_index));                          \
  }

    ALLOCATE_VARSIZE_MAP(FIXED_ARRAY_TYPE, fixed_cow_array)
    fixed_cow_array_map()->set_elements_kind(FAST_HOLEY_ELEMENTS);
    DCHECK_NE(fixed_array_map(), fixed_cow_array_map());

    ALLOCATE_VARSIZE_MAP(FIXED_ARRAY_TYPE, scope_info)
    ALLOCATE_VARSIZE_MAP(FIXED_ARRAY_TYPE, module_info)
    ALLOCATE_VARSIZE_MAP(FIXED_ARRAY_TYPE, feedback_vector)
    ALLOCATE_PRIMITIVE_MAP(HEAP_NUMBER_TYPE, HeapNumber::kSize, heap_number,
                           Context::NUMBER_FUNCTION_INDEX)
    ALLOCATE_MAP(MUTABLE_HEAP_NUMBER_TYPE, HeapNumber::kSize,
                 mutable_heap_number)
    ALLOCATE_PRIMITIVE_MAP(SYMBOL_TYPE, Symbol::kSize, symbol,
                           Context::SYMBOL_FUNCTION_INDEX)
    ALLOCATE_MAP(FOREIGN_TYPE, Foreign::kSize, foreign)

    ALLOCATE_PRIMITIVE_MAP(ODDBALL_TYPE, Oddball::kSize, boolean,
                           Context::BOOLEAN_FUNCTION_INDEX);
    ALLOCATE_MAP(ODDBALL_TYPE, Oddball::kSize, uninitialized);
    ALLOCATE_MAP(ODDBALL_TYPE, Oddball::kSize, arguments_marker);
    ALLOCATE_MAP(ODDBALL_TYPE, Oddball::kSize, exception);
    ALLOCATE_MAP(ODDBALL_TYPE, Oddball::kSize, termination_exception);
    ALLOCATE_MAP(ODDBALL_TYPE, Oddball::kSize, optimized_out);
    ALLOCATE_MAP(ODDBALL_TYPE, Oddball::kSize, stale_register);

    ALLOCATE_MAP(JS_PROMISE_CAPABILITY_TYPE, JSPromiseCapability::kSize,
                 js_promise_capability);

    for (unsigned i = 0; i < arraysize(string_type_table); i++) {
      const StringTypeTable& entry = string_type_table[i];
      {
        AllocationResult allocation = AllocateMap(entry.type, entry.size);
        if (!allocation.To(&obj)) return false;
      }
      Map* map = Map::cast(obj);
      map->SetConstructorFunctionIndex(Context::STRING_FUNCTION_INDEX);
      // Mark cons string maps as unstable, because their objects can change
      // maps during GC.
      if (StringShape(entry.type).IsCons()) map->mark_unstable();
      roots_[entry.index] = map;
    }

    {  // Create a separate external one byte string map for native sources.
      AllocationResult allocation =
          AllocateMap(SHORT_EXTERNAL_ONE_BYTE_STRING_TYPE,
                      ExternalOneByteString::kShortSize);
      if (!allocation.To(&obj)) return false;
      Map* map = Map::cast(obj);
      map->SetConstructorFunctionIndex(Context::STRING_FUNCTION_INDEX);
      set_native_source_string_map(map);
    }

    ALLOCATE_VARSIZE_MAP(FIXED_DOUBLE_ARRAY_TYPE, fixed_double_array)
    fixed_double_array_map()->set_elements_kind(FAST_HOLEY_DOUBLE_ELEMENTS);
    ALLOCATE_VARSIZE_MAP(BYTE_ARRAY_TYPE, byte_array)
    ALLOCATE_VARSIZE_MAP(BYTECODE_ARRAY_TYPE, bytecode_array)
    ALLOCATE_VARSIZE_MAP(FREE_SPACE_TYPE, free_space)

#define ALLOCATE_FIXED_TYPED_ARRAY_MAP(Type, type, TYPE, ctype, size) \
  ALLOCATE_VARSIZE_MAP(FIXED_##TYPE##_ARRAY_TYPE, fixed_##type##_array)

    TYPED_ARRAYS(ALLOCATE_FIXED_TYPED_ARRAY_MAP)
#undef ALLOCATE_FIXED_TYPED_ARRAY_MAP

    ALLOCATE_VARSIZE_MAP(FIXED_ARRAY_TYPE, sloppy_arguments_elements)

    ALLOCATE_VARSIZE_MAP(CODE_TYPE, code)

    ALLOCATE_MAP(CELL_TYPE, Cell::kSize, cell)
    ALLOCATE_MAP(PROPERTY_CELL_TYPE, PropertyCell::kSize, global_property_cell)
    ALLOCATE_MAP(WEAK_CELL_TYPE, WeakCell::kSize, weak_cell)
    ALLOCATE_MAP(CELL_TYPE, Cell::kSize, no_closures_cell)
    ALLOCATE_MAP(CELL_TYPE, Cell::kSize, one_closure_cell)
    ALLOCATE_MAP(CELL_TYPE, Cell::kSize, many_closures_cell)
    ALLOCATE_MAP(FILLER_TYPE, kPointerSize, one_pointer_filler)
    ALLOCATE_MAP(FILLER_TYPE, 2 * kPointerSize, two_pointer_filler)

    ALLOCATE_VARSIZE_MAP(TRANSITION_ARRAY_TYPE, transition_array)

    for (unsigned i = 0; i < arraysize(struct_table); i++) {
      const StructTable& entry = struct_table[i];
      Map* map;
      if (!AllocateMap(entry.type, entry.size).To(&map)) return false;
      roots_[entry.index] = map;
    }

    ALLOCATE_VARSIZE_MAP(FIXED_ARRAY_TYPE, hash_table)
    ALLOCATE_VARSIZE_MAP(FIXED_ARRAY_TYPE, ordered_hash_table)
    ALLOCATE_VARSIZE_MAP(FIXED_ARRAY_TYPE, unseeded_number_dictionary)

    ALLOCATE_VARSIZE_MAP(FIXED_ARRAY_TYPE, function_context)
    ALLOCATE_VARSIZE_MAP(FIXED_ARRAY_TYPE, catch_context)
    ALLOCATE_VARSIZE_MAP(FIXED_ARRAY_TYPE, with_context)
    ALLOCATE_VARSIZE_MAP(FIXED_ARRAY_TYPE, debug_evaluate_context)
    ALLOCATE_VARSIZE_MAP(FIXED_ARRAY_TYPE, block_context)
    ALLOCATE_VARSIZE_MAP(FIXED_ARRAY_TYPE, module_context)
    ALLOCATE_VARSIZE_MAP(FIXED_ARRAY_TYPE, eval_context)
    ALLOCATE_VARSIZE_MAP(FIXED_ARRAY_TYPE, script_context)
    ALLOCATE_VARSIZE_MAP(FIXED_ARRAY_TYPE, script_context_table)

    ALLOCATE_VARSIZE_MAP(FIXED_ARRAY_TYPE, native_context)
    native_context_map()->set_dictionary_map(true);
    native_context_map()->set_visitor_id(
        StaticVisitorBase::kVisitNativeContext);

    ALLOCATE_MAP(SHARED_FUNCTION_INFO_TYPE, SharedFunctionInfo::kAlignedSize,
                 shared_function_info)

    ALLOCATE_MAP(JS_MESSAGE_OBJECT_TYPE, JSMessageObject::kSize, message_object)
    ALLOCATE_MAP(JS_OBJECT_TYPE, JSObject::kHeaderSize + kPointerSize, external)
    external_map()->set_is_extensible(false);
#undef ALLOCATE_PRIMITIVE_MAP
#undef ALLOCATE_VARSIZE_MAP
#undef ALLOCATE_MAP
  }

  {
    AllocationResult allocation = AllocateEmptyScopeInfo();
    if (!allocation.To(&obj)) return false;
  }

  set_empty_scope_info(ScopeInfo::cast(obj));
  {
    AllocationResult allocation = Allocate(boolean_map(), OLD_SPACE);
    if (!allocation.To(&obj)) return false;
  }
  set_true_value(Oddball::cast(obj));
  Oddball::cast(obj)->set_kind(Oddball::kTrue);

  {
    AllocationResult allocation = Allocate(boolean_map(), OLD_SPACE);
    if (!allocation.To(&obj)) return false;
  }
  set_false_value(Oddball::cast(obj));
  Oddball::cast(obj)->set_kind(Oddball::kFalse);

  {  // Empty arrays
    {
      ByteArray* byte_array;
      if (!AllocateByteArray(0, TENURED).To(&byte_array)) return false;
      set_empty_byte_array(byte_array);
    }

#define ALLOCATE_EMPTY_FIXED_TYPED_ARRAY(Type, type, TYPE, ctype, size) \
  {                                                                     \
    FixedTypedArrayBase* obj;                                           \
    if (!AllocateEmptyFixedTypedArray(kExternal##Type##Array).To(&obj)) \
      return false;                                                     \
    set_empty_fixed_##type##_array(obj);                                \
  }

    TYPED_ARRAYS(ALLOCATE_EMPTY_FIXED_TYPED_ARRAY)
#undef ALLOCATE_EMPTY_FIXED_TYPED_ARRAY
  }
  DCHECK(!InNewSpace(empty_fixed_array()));
  return true;
}

AllocationResult Heap::AllocateHeapNumber(MutableMode mode,
                                          PretenureFlag pretenure) {
  // Statically ensure that it is safe to allocate heap numbers in paged
  // spaces.
  int size = HeapNumber::kSize;
  STATIC_ASSERT(HeapNumber::kSize <= kMaxRegularHeapObjectSize);

  AllocationSpace space = SelectSpace(pretenure);

  HeapObject* result = nullptr;
  {
    AllocationResult allocation = AllocateRaw(size, space, kDoubleUnaligned);
    if (!allocation.To(&result)) return allocation;
  }

  Map* map = mode == MUTABLE ? mutable_heap_number_map() : heap_number_map();
  HeapObject::cast(result)->set_map_no_write_barrier(map);
  return result;
}

AllocationResult Heap::AllocateCell(Object* value) {
  int size = Cell::kSize;
  STATIC_ASSERT(Cell::kSize <= kMaxRegularHeapObjectSize);

  HeapObject* result = nullptr;
  {
    AllocationResult allocation = AllocateRaw(size, OLD_SPACE);
    if (!allocation.To(&result)) return allocation;
  }
  result->set_map_no_write_barrier(cell_map());
  Cell::cast(result)->set_value(value);
  return result;
}

AllocationResult Heap::AllocatePropertyCell() {
  int size = PropertyCell::kSize;
  STATIC_ASSERT(PropertyCell::kSize <= kMaxRegularHeapObjectSize);

  HeapObject* result = nullptr;
  AllocationResult allocation = AllocateRaw(size, OLD_SPACE);
  if (!allocation.To(&result)) return allocation;

  result->set_map_no_write_barrier(global_property_cell_map());
  PropertyCell* cell = PropertyCell::cast(result);
  cell->set_dependent_code(DependentCode::cast(empty_fixed_array()),
                           SKIP_WRITE_BARRIER);
  cell->set_property_details(PropertyDetails(Smi::kZero));
  cell->set_value(the_hole_value());
  return result;
}


AllocationResult Heap::AllocateWeakCell(HeapObject* value) {
  int size = WeakCell::kSize;
  STATIC_ASSERT(WeakCell::kSize <= kMaxRegularHeapObjectSize);
  HeapObject* result = nullptr;
  {
    AllocationResult allocation = AllocateRaw(size, OLD_SPACE);
    if (!allocation.To(&result)) return allocation;
  }
  result->set_map_no_write_barrier(weak_cell_map());
  WeakCell::cast(result)->initialize(value);
  WeakCell::cast(result)->clear_next(the_hole_value());
  return result;
}


AllocationResult Heap::AllocateTransitionArray(int capacity) {
  DCHECK(capacity > 0);
  HeapObject* raw_array = nullptr;
  {
    AllocationResult allocation = AllocateRawFixedArray(capacity, TENURED);
    if (!allocation.To(&raw_array)) return allocation;
  }
  raw_array->set_map_no_write_barrier(transition_array_map());
  TransitionArray* array = TransitionArray::cast(raw_array);
  array->set_length(capacity);
  MemsetPointer(array->data_start(), undefined_value(), capacity);
  // Transition arrays are tenured. When black allocation is on we have to
  // add the transition array to the list of encountered_transition_arrays.
  if (incremental_marking()->black_allocation()) {
    array->set_next_link(encountered_transition_arrays(),
                         UPDATE_WEAK_WRITE_BARRIER);
    set_encountered_transition_arrays(array);
  } else {
    array->set_next_link(undefined_value(), SKIP_WRITE_BARRIER);
  }
  return array;
}

bool Heap::CreateApiObjects() {
  HandleScope scope(isolate());
  set_message_listeners(*TemplateList::New(isolate(), 2));
  HeapObject* obj = nullptr;
  {
    AllocationResult allocation = AllocateStruct(INTERCEPTOR_INFO_TYPE);
    if (!allocation.To(&obj)) return false;
  }
  InterceptorInfo* info = InterceptorInfo::cast(obj);
  info->set_flags(0);
  set_noop_interceptor_info(info);
  return true;
}


void Heap::CreateJSEntryStub() {
  JSEntryStub stub(isolate(), StackFrame::ENTRY);
  set_js_entry_code(*stub.GetCode());
}


void Heap::CreateJSConstructEntryStub() {
  JSEntryStub stub(isolate(), StackFrame::ENTRY_CONSTRUCT);
  set_js_construct_entry_code(*stub.GetCode());
}


void Heap::CreateFixedStubs() {
  // Here we create roots for fixed stubs. They are needed at GC
  // for cooking and uncooking (check out frames.cc).
  // The eliminates the need for doing dictionary lookup in the
  // stub cache for these stubs.
  HandleScope scope(isolate());

  // Create stubs that should be there, so we don't unexpectedly have to
  // create them if we need them during the creation of another stub.
  // Stub creation mixes raw pointers and handles in an unsafe manner so
  // we cannot create stubs while we are creating stubs.
  CodeStub::GenerateStubsAheadOfTime(isolate());

  // MacroAssembler::Abort calls (usually enabled with --debug-code) depend on
  // CEntryStub, so we need to call GenerateStubsAheadOfTime before JSEntryStub
  // is created.

  // gcc-4.4 has problem generating correct code of following snippet:
  // {  JSEntryStub stub;
  //    js_entry_code_ = *stub.GetCode();
  // }
  // {  JSConstructEntryStub stub;
  //    js_construct_entry_code_ = *stub.GetCode();
  // }
  // To workaround the problem, make separate functions without inlining.
  Heap::CreateJSEntryStub();
  Heap::CreateJSConstructEntryStub();
}


void Heap::CreateInitialObjects() {
  HandleScope scope(isolate());
  Factory* factory = isolate()->factory();

  // The -0 value must be set before NewNumber works.
  set_minus_zero_value(*factory->NewHeapNumber(-0.0, IMMUTABLE, TENURED));
  DCHECK(std::signbit(minus_zero_value()->Number()) != 0);

  set_nan_value(*factory->NewHeapNumber(
      std::numeric_limits<double>::quiet_NaN(), IMMUTABLE, TENURED));
  set_hole_nan_value(
      *factory->NewHeapNumberFromBits(kHoleNanInt64, IMMUTABLE, TENURED));
  set_infinity_value(*factory->NewHeapNumber(V8_INFINITY, IMMUTABLE, TENURED));
  set_minus_infinity_value(
      *factory->NewHeapNumber(-V8_INFINITY, IMMUTABLE, TENURED));

  // Allocate initial string table.
  set_string_table(*StringTable::New(isolate(), kInitialStringTableSize));

  // Allocate

  // Finish initializing oddballs after creating the string table.
  Oddball::Initialize(isolate(), factory->undefined_value(), "undefined",
                      factory->nan_value(), "undefined", Oddball::kUndefined);

  // Initialize the null_value.
  Oddball::Initialize(isolate(), factory->null_value(), "null",
                      handle(Smi::kZero, isolate()), "object", Oddball::kNull);

  // Initialize the_hole_value.
  Oddball::Initialize(isolate(), factory->the_hole_value(), "hole",
                      factory->hole_nan_value(), "undefined",
                      Oddball::kTheHole);

  // Initialize the true_value.
  Oddball::Initialize(isolate(), factory->true_value(), "true",
                      handle(Smi::FromInt(1), isolate()), "boolean",
                      Oddball::kTrue);

  // Initialize the false_value.
  Oddball::Initialize(isolate(), factory->false_value(), "false",
                      handle(Smi::kZero, isolate()), "boolean",
                      Oddball::kFalse);

  set_uninitialized_value(
      *factory->NewOddball(factory->uninitialized_map(), "uninitialized",
                           handle(Smi::FromInt(-1), isolate()), "undefined",
                           Oddball::kUninitialized));

  set_arguments_marker(
      *factory->NewOddball(factory->arguments_marker_map(), "arguments_marker",
                           handle(Smi::FromInt(-4), isolate()), "undefined",
                           Oddball::kArgumentsMarker));

  set_termination_exception(*factory->NewOddball(
      factory->termination_exception_map(), "termination_exception",
      handle(Smi::FromInt(-3), isolate()), "undefined", Oddball::kOther));

  set_exception(*factory->NewOddball(factory->exception_map(), "exception",
                                     handle(Smi::FromInt(-5), isolate()),
                                     "undefined", Oddball::kException));

  set_optimized_out(*factory->NewOddball(factory->optimized_out_map(),
                                         "optimized_out",
                                         handle(Smi::FromInt(-6), isolate()),
                                         "undefined", Oddball::kOptimizedOut));

  set_stale_register(
      *factory->NewOddball(factory->stale_register_map(), "stale_register",
                           handle(Smi::FromInt(-7), isolate()), "undefined",
                           Oddball::kStaleRegister));

  for (unsigned i = 0; i < arraysize(constant_string_table); i++) {
    Handle<String> str =
        factory->InternalizeUtf8String(constant_string_table[i].contents);
    roots_[constant_string_table[i].index] = *str;
  }

  // Create the code_stubs dictionary. The initial size is set to avoid
  // expanding the dictionary during bootstrapping.
  set_code_stubs(*UnseededNumberDictionary::New(isolate(), 128));

  set_instanceof_cache_function(Smi::kZero);
  set_instanceof_cache_map(Smi::kZero);
  set_instanceof_cache_answer(Smi::kZero);

  {
    HandleScope scope(isolate());
#define SYMBOL_INIT(name)                                              \
  {                                                                    \
    Handle<String> name##d = factory->NewStringFromStaticChars(#name); \
    Handle<Symbol> symbol(isolate()->factory()->NewPrivateSymbol());   \
    symbol->set_name(*name##d);                                        \
    roots_[k##name##RootIndex] = *symbol;                              \
  }
    PRIVATE_SYMBOL_LIST(SYMBOL_INIT)
#undef SYMBOL_INIT
  }

  {
    HandleScope scope(isolate());
#define SYMBOL_INIT(name, description)                                      \
  Handle<Symbol> name = factory->NewSymbol();                               \
  Handle<String> name##d = factory->NewStringFromStaticChars(#description); \
  name->set_name(*name##d);                                                 \
  roots_[k##name##RootIndex] = *name;
    PUBLIC_SYMBOL_LIST(SYMBOL_INIT)
#undef SYMBOL_INIT

#define SYMBOL_INIT(name, description)                                      \
  Handle<Symbol> name = factory->NewSymbol();                               \
  Handle<String> name##d = factory->NewStringFromStaticChars(#description); \
  name->set_is_well_known_symbol(true);                                     \
  name->set_name(*name##d);                                                 \
  roots_[k##name##RootIndex] = *name;
    WELL_KNOWN_SYMBOL_LIST(SYMBOL_INIT)
#undef SYMBOL_INIT
  }

  Handle<NameDictionary> empty_properties_dictionary =
      NameDictionary::NewEmpty(isolate(), TENURED);
  empty_properties_dictionary->SetRequiresCopyOnCapacityChange();
  set_empty_properties_dictionary(*empty_properties_dictionary);

  set_public_symbol_table(*empty_properties_dictionary);
  set_api_symbol_table(*empty_properties_dictionary);
  set_api_private_symbol_table(*empty_properties_dictionary);

  set_number_string_cache(
      *factory->NewFixedArray(kInitialNumberStringCacheSize * 2, TENURED));

  // Allocate cache for single character one byte strings.
  set_single_character_string_cache(
      *factory->NewFixedArray(String::kMaxOneByteCharCode + 1, TENURED));

  // Allocate cache for string split and regexp-multiple.
  set_string_split_cache(*factory->NewFixedArray(
      RegExpResultsCache::kRegExpResultsCacheSize, TENURED));
  set_regexp_multiple_cache(*factory->NewFixedArray(
      RegExpResultsCache::kRegExpResultsCacheSize, TENURED));

  // Allocate cache for external strings pointing to native source code.
  set_natives_source_cache(
      *factory->NewFixedArray(Natives::GetBuiltinsCount()));

  set_extra_natives_source_cache(
      *factory->NewFixedArray(ExtraNatives::GetBuiltinsCount()));

  set_experimental_extra_natives_source_cache(
      *factory->NewFixedArray(ExperimentalExtraNatives::GetBuiltinsCount()));

  set_undefined_cell(*factory->NewCell(factory->undefined_value()));

  // Microtask queue uses the empty fixed array as a sentinel for "empty".
  // Number of queued microtasks stored in Isolate::pending_microtask_count().
  set_microtask_queue(empty_fixed_array());

  {
    Handle<FixedArray> empty_sloppy_arguments_elements =
        factory->NewFixedArray(2, TENURED);
    empty_sloppy_arguments_elements->set_map(sloppy_arguments_elements_map());
    set_empty_sloppy_arguments_elements(*empty_sloppy_arguments_elements);
  }

  {
    Handle<WeakCell> cell = factory->NewWeakCell(factory->undefined_value());
    set_empty_weak_cell(*cell);
    cell->clear();
  }

  set_detached_contexts(empty_fixed_array());
  set_retained_maps(ArrayList::cast(empty_fixed_array()));

  set_weak_object_to_code_table(
      *WeakHashTable::New(isolate(), 16, USE_DEFAULT_MINIMUM_CAPACITY,
                          TENURED));

  set_weak_new_space_object_to_code_list(
      ArrayList::cast(*(factory->NewFixedArray(16, TENURED))));
  weak_new_space_object_to_code_list()->SetLength(0);

  set_code_coverage_list(undefined_value());

  set_script_list(Smi::kZero);

  Handle<SeededNumberDictionary> slow_element_dictionary =
      SeededNumberDictionary::NewEmpty(isolate(), TENURED);
  slow_element_dictionary->set_requires_slow_elements();
  set_empty_slow_element_dictionary(*slow_element_dictionary);

  set_materialized_objects(*factory->NewFixedArray(0, TENURED));

  // Handling of script id generation is in Heap::NextScriptId().
  set_last_script_id(Smi::FromInt(v8::UnboundScript::kNoScriptId));
  set_next_template_serial_number(Smi::kZero);

  // Allocate the empty script.
  Handle<Script> script = factory->NewScript(factory->empty_string());
  script->set_type(Script::TYPE_NATIVE);
  set_empty_script(*script);

  Handle<PropertyCell> cell = factory->NewPropertyCell();
  cell->set_value(Smi::FromInt(Isolate::kProtectorValid));
  set_array_protector(*cell);

  cell = factory->NewPropertyCell();
  cell->set_value(the_hole_value());
  set_empty_property_cell(*cell);

  cell = factory->NewPropertyCell();
  cell->set_value(Smi::FromInt(Isolate::kProtectorValid));
  set_array_iterator_protector(*cell);

  Handle<Cell> is_concat_spreadable_cell = factory->NewCell(
      handle(Smi::FromInt(Isolate::kProtectorValid), isolate()));
  set_is_concat_spreadable_protector(*is_concat_spreadable_cell);

  Handle<Cell> species_cell = factory->NewCell(
      handle(Smi::FromInt(Isolate::kProtectorValid), isolate()));
  set_species_protector(*species_cell);

  cell = factory->NewPropertyCell();
  cell->set_value(Smi::FromInt(Isolate::kProtectorValid));
  set_string_length_protector(*cell);

  Handle<Cell> fast_array_iteration_cell = factory->NewCell(
      handle(Smi::FromInt(Isolate::kProtectorValid), isolate()));
  set_fast_array_iteration_protector(*fast_array_iteration_cell);

  cell = factory->NewPropertyCell();
  cell->set_value(Smi::FromInt(Isolate::kProtectorValid));
  set_array_buffer_neutering_protector(*cell);

  set_serialized_templates(empty_fixed_array());
  set_serialized_global_proxy_sizes(empty_fixed_array());

  set_weak_stack_trace_list(Smi::kZero);

  set_noscript_shared_function_infos(Smi::kZero);

  // Initialize context slot cache.
  isolate_->context_slot_cache()->Clear();

  // Initialize descriptor cache.
  isolate_->descriptor_lookup_cache()->Clear();

  // Initialize compilation cache.
  isolate_->compilation_cache()->Clear();

  // Finish creating JSPromiseCapabilityMap
  {
    // TODO(caitp): This initialization can be removed once PromiseCapability
    // object is no longer used by builtins implemented in javascript.
    Handle<Map> map = factory->js_promise_capability_map();
    map->set_inobject_properties_or_constructor_function_index(3);

    Map::EnsureDescriptorSlack(map, 3);

    PropertyAttributes attrs =
        static_cast<PropertyAttributes>(READ_ONLY | DONT_DELETE);
    {  // promise
      Descriptor d = Descriptor::DataField(factory->promise_string(),
                                           JSPromiseCapability::kPromiseIndex,
                                           attrs, Representation::Tagged());
      map->AppendDescriptor(&d);
    }

    {  // resolve
      Descriptor d = Descriptor::DataField(factory->resolve_string(),
                                           JSPromiseCapability::kResolveIndex,
                                           attrs, Representation::Tagged());
      map->AppendDescriptor(&d);
    }

    {  // reject
      Descriptor d = Descriptor::DataField(factory->reject_string(),
                                           JSPromiseCapability::kRejectIndex,
                                           attrs, Representation::Tagged());
      map->AppendDescriptor(&d);
    }

    map->set_is_extensible(false);
    set_js_promise_capability_map(*map);
  }
}

bool Heap::RootCanBeWrittenAfterInitialization(Heap::RootListIndex root_index) {
  switch (root_index) {
    case kNumberStringCacheRootIndex:
    case kInstanceofCacheFunctionRootIndex:
    case kInstanceofCacheMapRootIndex:
    case kInstanceofCacheAnswerRootIndex:
    case kCodeStubsRootIndex:
    case kEmptyScriptRootIndex:
    case kScriptListRootIndex:
    case kMaterializedObjectsRootIndex:
    case kMicrotaskQueueRootIndex:
    case kDetachedContextsRootIndex:
    case kWeakObjectToCodeTableRootIndex:
    case kWeakNewSpaceObjectToCodeListRootIndex:
    case kRetainedMapsRootIndex:
    case kCodeCoverageListRootIndex:
    case kNoScriptSharedFunctionInfosRootIndex:
    case kWeakStackTraceListRootIndex:
    case kSerializedTemplatesRootIndex:
    case kSerializedGlobalProxySizesRootIndex:
    case kPublicSymbolTableRootIndex:
    case kApiSymbolTableRootIndex:
    case kApiPrivateSymbolTableRootIndex:
// Smi values
#define SMI_ENTRY(type, name, Name) case k##Name##RootIndex:
      SMI_ROOT_LIST(SMI_ENTRY)
#undef SMI_ENTRY
    // String table
    case kStringTableRootIndex:
      return true;

    default:
      return false;
  }
}

bool Heap::RootCanBeTreatedAsConstant(RootListIndex root_index) {
  return !RootCanBeWrittenAfterInitialization(root_index) &&
         !InNewSpace(root(root_index));
}

bool Heap::IsUnmodifiedHeapObject(Object** p) {
  Object* object = *p;
  if (object->IsSmi()) return false;
  HeapObject* heap_object = HeapObject::cast(object);
  if (!object->IsJSObject()) return false;
  JSObject* js_object = JSObject::cast(object);
  if (!js_object->WasConstructedFromApiFunction()) return false;
  Object* maybe_constructor = js_object->map()->GetConstructor();
  if (!maybe_constructor->IsJSFunction()) return false;
  JSFunction* constructor = JSFunction::cast(maybe_constructor);

  return constructor->initial_map() == heap_object->map();
}

int Heap::FullSizeNumberStringCacheLength() {
  // Compute the size of the number string cache based on the max newspace size.
  // The number string cache has a minimum size based on twice the initial cache
  // size to ensure that it is bigger after being made 'full size'.
  size_t number_string_cache_size = max_semi_space_size_ / 512;
  number_string_cache_size =
      Max(static_cast<size_t>(kInitialNumberStringCacheSize * 2),
          Min<size_t>(0x4000u, number_string_cache_size));
  // There is a string and a number per entry so the length is twice the number
  // of entries.
  return static_cast<int>(number_string_cache_size * 2);
}


void Heap::FlushNumberStringCache() {
  // Flush the number to string cache.
  int len = number_string_cache()->length();
  for (int i = 0; i < len; i++) {
    number_string_cache()->set_undefined(i);
  }
}


Map* Heap::MapForFixedTypedArray(ExternalArrayType array_type) {
  return Map::cast(roots_[RootIndexForFixedTypedArray(array_type)]);
}


Heap::RootListIndex Heap::RootIndexForFixedTypedArray(
    ExternalArrayType array_type) {
  switch (array_type) {
#define ARRAY_TYPE_TO_ROOT_INDEX(Type, type, TYPE, ctype, size) \
  case kExternal##Type##Array:                                  \
    return kFixed##Type##ArrayMapRootIndex;

    TYPED_ARRAYS(ARRAY_TYPE_TO_ROOT_INDEX)
#undef ARRAY_TYPE_TO_ROOT_INDEX

    default:
      UNREACHABLE();
      return kUndefinedValueRootIndex;
  }
}


Heap::RootListIndex Heap::RootIndexForEmptyFixedTypedArray(
    ElementsKind elementsKind) {
  switch (elementsKind) {
#define ELEMENT_KIND_TO_ROOT_INDEX(Type, type, TYPE, ctype, size) \
  case TYPE##_ELEMENTS:                                           \
    return kEmptyFixed##Type##ArrayRootIndex;

    TYPED_ARRAYS(ELEMENT_KIND_TO_ROOT_INDEX)
#undef ELEMENT_KIND_TO_ROOT_INDEX
    default:
      UNREACHABLE();
      return kUndefinedValueRootIndex;
  }
}


FixedTypedArrayBase* Heap::EmptyFixedTypedArrayForMap(Map* map) {
  return FixedTypedArrayBase::cast(
      roots_[RootIndexForEmptyFixedTypedArray(map->elements_kind())]);
}


AllocationResult Heap::AllocateForeign(Address address,
                                       PretenureFlag pretenure) {
  // Statically ensure that it is safe to allocate foreigns in paged spaces.
  STATIC_ASSERT(Foreign::kSize <= kMaxRegularHeapObjectSize);
  AllocationSpace space = (pretenure == TENURED) ? OLD_SPACE : NEW_SPACE;
  Foreign* result = nullptr;
  AllocationResult allocation = Allocate(foreign_map(), space);
  if (!allocation.To(&result)) return allocation;
  result->set_foreign_address(address);
  return result;
}


AllocationResult Heap::AllocateByteArray(int length, PretenureFlag pretenure) {
  if (length < 0 || length > ByteArray::kMaxLength) {
    v8::internal::Heap::FatalProcessOutOfMemory("invalid array length", true);
  }
  int size = ByteArray::SizeFor(length);
  AllocationSpace space = SelectSpace(pretenure);
  HeapObject* result = nullptr;
  {
    AllocationResult allocation = AllocateRaw(size, space);
    if (!allocation.To(&result)) return allocation;
  }

  result->set_map_no_write_barrier(byte_array_map());
  ByteArray::cast(result)->set_length(length);
  return result;
}


AllocationResult Heap::AllocateBytecodeArray(int length,
                                             const byte* const raw_bytecodes,
                                             int frame_size,
                                             int parameter_count,
                                             FixedArray* constant_pool) {
  if (length < 0 || length > BytecodeArray::kMaxLength) {
    v8::internal::Heap::FatalProcessOutOfMemory("invalid array length", true);
  }
  // Bytecode array is pretenured, so constant pool array should be to.
  DCHECK(!InNewSpace(constant_pool));

  int size = BytecodeArray::SizeFor(length);
  HeapObject* result = nullptr;
  {
    AllocationResult allocation = AllocateRaw(size, OLD_SPACE);
    if (!allocation.To(&result)) return allocation;
  }

  result->set_map_no_write_barrier(bytecode_array_map());
  BytecodeArray* instance = BytecodeArray::cast(result);
  instance->set_length(length);
  instance->set_frame_size(frame_size);
  instance->set_parameter_count(parameter_count);
  instance->set_interrupt_budget(interpreter::Interpreter::InterruptBudget());
  instance->set_osr_loop_nesting_level(0);
  instance->set_bytecode_age(BytecodeArray::kNoAgeBytecodeAge);
  instance->set_constant_pool(constant_pool);
  instance->set_handler_table(empty_fixed_array());
  instance->set_source_position_table(empty_byte_array());
  CopyBytes(instance->GetFirstBytecodeAddress(), raw_bytecodes, length);

  return result;
}

HeapObject* Heap::CreateFillerObjectAt(Address addr, int size,
                                       ClearRecordedSlots mode) {
  if (size == 0) return nullptr;
  HeapObject* filler = HeapObject::FromAddress(addr);
  if (size == kPointerSize) {
    filler->set_map_no_write_barrier(
        reinterpret_cast<Map*>(root(kOnePointerFillerMapRootIndex)));
  } else if (size == 2 * kPointerSize) {
    filler->set_map_no_write_barrier(
        reinterpret_cast<Map*>(root(kTwoPointerFillerMapRootIndex)));
  } else {
    DCHECK_GT(size, 2 * kPointerSize);
    filler->set_map_no_write_barrier(
        reinterpret_cast<Map*>(root(kFreeSpaceMapRootIndex)));
    FreeSpace::cast(filler)->nobarrier_set_size(size);
  }
  if (mode == ClearRecordedSlots::kYes) {
    ClearRecordedSlotRange(addr, addr + size);
  }

  // At this point, we may be deserializing the heap from a snapshot, and
  // none of the maps have been created yet and are NULL.
  DCHECK((filler->map() == NULL && !deserialization_complete_) ||
         filler->map()->IsMap());
  return filler;
}


bool Heap::CanMoveObjectStart(HeapObject* object) {
  if (!FLAG_move_object_start) return false;

  // Sampling heap profiler may have a reference to the object.
  if (isolate()->heap_profiler()->is_sampling_allocations()) return false;

  Address address = object->address();

  if (lo_space()->Contains(object)) return false;

  // We can move the object start if the page was already swept.
  return Page::FromAddress(address)->SweepingDone();
}

bool Heap::IsImmovable(HeapObject* object) {
  MemoryChunk* chunk = MemoryChunk::FromAddress(object->address());
  return chunk->NeverEvacuate() || chunk->owner()->identity() == LO_SPACE;
}

void Heap::AdjustLiveBytes(HeapObject* object, int by) {
  // As long as the inspected object is black and we are currently not iterating
  // the heap using HeapIterator, we can update the live byte count. We cannot
  // update while using HeapIterator because the iterator is temporarily
  // marking the whole object graph, without updating live bytes.
  if (lo_space()->Contains(object)) {
    lo_space()->AdjustLiveBytes(by);
  } else if (!in_heap_iterator() &&
             !mark_compact_collector()->sweeping_in_progress() &&
             ObjectMarking::IsBlack(object)) {
    DCHECK(MemoryChunk::FromAddress(object->address())->SweepingDone());
    MemoryChunk::IncrementLiveBytes(object, by);
  }
}


FixedArrayBase* Heap::LeftTrimFixedArray(FixedArrayBase* object,
                                         int elements_to_trim) {
  CHECK_NOT_NULL(object);
  DCHECK(CanMoveObjectStart(object));
  DCHECK(!object->IsFixedTypedArrayBase());
  DCHECK(!object->IsByteArray());
  const int element_size = object->IsFixedArray() ? kPointerSize : kDoubleSize;
  const int bytes_to_trim = elements_to_trim * element_size;
  Map* map = object->map();

  // For now this trick is only applied to objects in new and paged space.
  // In large object space the object's start must coincide with chunk
  // and thus the trick is just not applicable.
  DCHECK(!lo_space()->Contains(object));
  DCHECK(object->map() != fixed_cow_array_map());

  STATIC_ASSERT(FixedArrayBase::kMapOffset == 0);
  STATIC_ASSERT(FixedArrayBase::kLengthOffset == kPointerSize);
  STATIC_ASSERT(FixedArrayBase::kHeaderSize == 2 * kPointerSize);

  const int len = object->length();
  DCHECK(elements_to_trim <= len);

  // Calculate location of new array start.
  Address old_start = object->address();
  Address new_start = old_start + bytes_to_trim;

  // Transfer the mark bits to their new location if the object is not within
  // a black area.
  if (!incremental_marking()->black_allocation() ||
      !Marking::IsBlack(
          ObjectMarking::MarkBitFrom(HeapObject::FromAddress(new_start)))) {
    IncrementalMarking::TransferMark(this, object,
                                     HeapObject::FromAddress(new_start));
  }

  // Technically in new space this write might be omitted (except for
  // debug mode which iterates through the heap), but to play safer
  // we still do it.
  CreateFillerObjectAt(old_start, bytes_to_trim, ClearRecordedSlots::kYes);

  // Initialize header of the trimmed array. Since left trimming is only
  // performed on pages which are not concurrently swept creating a filler
  // object does not require synchronization.
  Object** former_start = HeapObject::RawField(object, 0);
  int new_start_index = elements_to_trim * (element_size / kPointerSize);
  former_start[new_start_index] = map;
  former_start[new_start_index + 1] = Smi::FromInt(len - elements_to_trim);

  FixedArrayBase* new_object =
      FixedArrayBase::cast(HeapObject::FromAddress(new_start));

  // Maintain consistency of live bytes during incremental marking
  AdjustLiveBytes(new_object, -bytes_to_trim);

  // Remove recorded slots for the new map and length offset.
  ClearRecordedSlot(new_object, HeapObject::RawField(new_object, 0));
  ClearRecordedSlot(new_object, HeapObject::RawField(
                                    new_object, FixedArrayBase::kLengthOffset));

  // Notify the heap profiler of change in object layout.
  OnMoveEvent(new_object, object, new_object->Size());
  return new_object;
}

void Heap::RightTrimFixedArray(FixedArrayBase* object, int elements_to_trim) {
  const int len = object->length();
  DCHECK_LE(elements_to_trim, len);
  DCHECK_GE(elements_to_trim, 0);

  int bytes_to_trim;
  if (object->IsFixedTypedArrayBase()) {
    InstanceType type = object->map()->instance_type();
    bytes_to_trim =
        FixedTypedArrayBase::TypedArraySize(type, len) -
        FixedTypedArrayBase::TypedArraySize(type, len - elements_to_trim);
  } else if (object->IsByteArray()) {
    int new_size = ByteArray::SizeFor(len - elements_to_trim);
    bytes_to_trim = ByteArray::SizeFor(len) - new_size;
    DCHECK_GE(bytes_to_trim, 0);
  } else {
    const int element_size =
        object->IsFixedArray() ? kPointerSize : kDoubleSize;
    bytes_to_trim = elements_to_trim * element_size;
  }


  // For now this trick is only applied to objects in new and paged space.
  DCHECK(object->map() != fixed_cow_array_map());

  if (bytes_to_trim == 0) {
    // No need to create filler and update live bytes counters, just initialize
    // header of the trimmed array.
    object->synchronized_set_length(len - elements_to_trim);
    return;
  }

  // Calculate location of new array end.
  Address old_end = object->address() + object->Size();
  Address new_end = old_end - bytes_to_trim;

  // Technically in new space this write might be omitted (except for
  // debug mode which iterates through the heap), but to play safer
  // we still do it.
  // We do not create a filler for objects in large object space.
  // TODO(hpayer): We should shrink the large object page if the size
  // of the object changed significantly.
  if (!lo_space()->Contains(object)) {
    HeapObject* filler =
        CreateFillerObjectAt(new_end, bytes_to_trim, ClearRecordedSlots::kYes);
    DCHECK_NOT_NULL(filler);
    // Clear the mark bits of the black area that belongs now to the filler.
    // This is an optimization. The sweeper will release black fillers anyway.
    if (incremental_marking()->black_allocation() &&
        ObjectMarking::IsBlackOrGrey(filler)) {
      Page* page = Page::FromAddress(new_end);
      page->markbits()->ClearRange(
          page->AddressToMarkbitIndex(new_end),
          page->AddressToMarkbitIndex(new_end + bytes_to_trim));
    }
  }

  // Initialize header of the trimmed array. We are storing the new length
  // using release store after creating a filler for the left-over space to
  // avoid races with the sweeper thread.
  object->synchronized_set_length(len - elements_to_trim);

  // Maintain consistency of live bytes during incremental marking
  AdjustLiveBytes(object, -bytes_to_trim);

  // Notify the heap profiler of change in object layout. The array may not be
  // moved during GC, and size has to be adjusted nevertheless.
  HeapProfiler* profiler = isolate()->heap_profiler();
  if (profiler->is_tracking_allocations()) {
    profiler->UpdateObjectSizeEvent(object->address(), object->Size());
  }
}


AllocationResult Heap::AllocateFixedTypedArrayWithExternalPointer(
    int length, ExternalArrayType array_type, void* external_pointer,
    PretenureFlag pretenure) {
  int size = FixedTypedArrayBase::kHeaderSize;
  AllocationSpace space = SelectSpace(pretenure);
  HeapObject* result = nullptr;
  {
    AllocationResult allocation = AllocateRaw(size, space);
    if (!allocation.To(&result)) return allocation;
  }

  result->set_map_no_write_barrier(MapForFixedTypedArray(array_type));
  FixedTypedArrayBase* elements = FixedTypedArrayBase::cast(result);
  elements->set_base_pointer(Smi::kZero, SKIP_WRITE_BARRIER);
  elements->set_external_pointer(external_pointer, SKIP_WRITE_BARRIER);
  elements->set_length(length);
  return elements;
}

static void ForFixedTypedArray(ExternalArrayType array_type, int* element_size,
                               ElementsKind* element_kind) {
  switch (array_type) {
#define TYPED_ARRAY_CASE(Type, type, TYPE, ctype, size) \
  case kExternal##Type##Array:                          \
    *element_size = size;                               \
    *element_kind = TYPE##_ELEMENTS;                    \
    return;

    TYPED_ARRAYS(TYPED_ARRAY_CASE)
#undef TYPED_ARRAY_CASE

    default:
      *element_size = 0;               // Bogus
      *element_kind = UINT8_ELEMENTS;  // Bogus
      UNREACHABLE();
  }
}


AllocationResult Heap::AllocateFixedTypedArray(int length,
                                               ExternalArrayType array_type,
                                               bool initialize,
                                               PretenureFlag pretenure) {
  int element_size;
  ElementsKind elements_kind;
  ForFixedTypedArray(array_type, &element_size, &elements_kind);
  int size = OBJECT_POINTER_ALIGN(length * element_size +
                                  FixedTypedArrayBase::kDataOffset);
  AllocationSpace space = SelectSpace(pretenure);

  HeapObject* object = nullptr;
  AllocationResult allocation = AllocateRaw(
      size, space,
      array_type == kExternalFloat64Array ? kDoubleAligned : kWordAligned);
  if (!allocation.To(&object)) return allocation;

  object->set_map_no_write_barrier(MapForFixedTypedArray(array_type));
  FixedTypedArrayBase* elements = FixedTypedArrayBase::cast(object);
  elements->set_base_pointer(elements, SKIP_WRITE_BARRIER);
  elements->set_external_pointer(
      ExternalReference::fixed_typed_array_base_data_offset().address(),
      SKIP_WRITE_BARRIER);
  elements->set_length(length);
  if (initialize) memset(elements->DataPtr(), 0, elements->DataSize());
  return elements;
}


AllocationResult Heap::AllocateCode(int object_size, bool immovable) {
  DCHECK(IsAligned(static_cast<intptr_t>(object_size), kCodeAlignment));
  AllocationResult allocation = AllocateRaw(object_size, CODE_SPACE);

  HeapObject* result = nullptr;
  if (!allocation.To(&result)) return allocation;
  if (immovable) {
    Address address = result->address();
    MemoryChunk* chunk = MemoryChunk::FromAddress(address);
    // Code objects which should stay at a fixed address are allocated either
    // in the first page of code space (objects on the first page of each space
    // are never moved), in large object space, or (during snapshot creation)
    // the containing page is marked as immovable.
    if (!Heap::IsImmovable(result) &&
        !code_space_->FirstPage()->Contains(address)) {
      if (isolate()->serializer_enabled()) {
        chunk->MarkNeverEvacuate();
      } else {
        // Discard the first code allocation, which was on a page where it could
        // be moved.
        CreateFillerObjectAt(result->address(), object_size,
                             ClearRecordedSlots::kNo);
        allocation = lo_space_->AllocateRaw(object_size, EXECUTABLE);
        if (!allocation.To(&result)) return allocation;
        OnAllocationEvent(result, object_size);
      }
    }
  }

  result->set_map_no_write_barrier(code_map());
  Code* code = Code::cast(result);
  DCHECK(IsAligned(bit_cast<intptr_t>(code->address()), kCodeAlignment));
  DCHECK(!memory_allocator()->code_range()->valid() ||
         memory_allocator()->code_range()->contains(code->address()) ||
         object_size <= code_space()->AreaSize());
  code->set_gc_metadata(Smi::kZero);
  code->set_ic_age(global_ic_age_);
  return code;
}


AllocationResult Heap::CopyCode(Code* code) {
  AllocationResult allocation;

  HeapObject* result = nullptr;
  // Allocate an object the same size as the code object.
  int obj_size = code->Size();
  allocation = AllocateRaw(obj_size, CODE_SPACE);
  if (!allocation.To(&result)) return allocation;

  // Copy code object.
  Address old_addr = code->address();
  Address new_addr = result->address();
  CopyBlock(new_addr, old_addr, obj_size);
  Code* new_code = Code::cast(result);

  // Relocate the copy.
  DCHECK(IsAligned(bit_cast<intptr_t>(new_code->address()), kCodeAlignment));
  DCHECK(!memory_allocator()->code_range()->valid() ||
         memory_allocator()->code_range()->contains(code->address()) ||
         obj_size <= code_space()->AreaSize());
  new_code->Relocate(new_addr - old_addr);
  // We have to iterate over the object and process its pointers when black
  // allocation is on.
  incremental_marking()->IterateBlackObject(new_code);
  // Record all references to embedded objects in the new code object.
  RecordWritesIntoCode(new_code);
  return new_code;
}

AllocationResult Heap::CopyBytecodeArray(BytecodeArray* bytecode_array) {
  int size = BytecodeArray::SizeFor(bytecode_array->length());
  HeapObject* result = nullptr;
  {
    AllocationResult allocation = AllocateRaw(size, OLD_SPACE);
    if (!allocation.To(&result)) return allocation;
  }

  result->set_map_no_write_barrier(bytecode_array_map());
  BytecodeArray* copy = BytecodeArray::cast(result);
  copy->set_length(bytecode_array->length());
  copy->set_frame_size(bytecode_array->frame_size());
  copy->set_parameter_count(bytecode_array->parameter_count());
  copy->set_constant_pool(bytecode_array->constant_pool());
  copy->set_handler_table(bytecode_array->handler_table());
  copy->set_source_position_table(bytecode_array->source_position_table());
  copy->set_interrupt_budget(bytecode_array->interrupt_budget());
  copy->set_osr_loop_nesting_level(bytecode_array->osr_loop_nesting_level());
  copy->set_bytecode_age(bytecode_array->bytecode_age());
  bytecode_array->CopyBytecodesTo(copy);
  return copy;
}

void Heap::InitializeAllocationMemento(AllocationMemento* memento,
                                       AllocationSite* allocation_site) {
  memento->set_map_no_write_barrier(allocation_memento_map());
  DCHECK(allocation_site->map() == allocation_site_map());
  memento->set_allocation_site(allocation_site, SKIP_WRITE_BARRIER);
  if (FLAG_allocation_site_pretenuring) {
    allocation_site->IncrementMementoCreateCount();
  }
}


AllocationResult Heap::Allocate(Map* map, AllocationSpace space,
                                AllocationSite* allocation_site) {
  DCHECK(gc_state_ == NOT_IN_GC);
  DCHECK(map->instance_type() != MAP_TYPE);
  int size = map->instance_size();
  if (allocation_site != NULL) {
    size += AllocationMemento::kSize;
  }
  HeapObject* result = nullptr;
  AllocationResult allocation = AllocateRaw(size, space);
  if (!allocation.To(&result)) return allocation;
  // No need for write barrier since object is white and map is in old space.
  result->set_map_no_write_barrier(map);
  if (allocation_site != NULL) {
    AllocationMemento* alloc_memento = reinterpret_cast<AllocationMemento*>(
        reinterpret_cast<Address>(result) + map->instance_size());
    InitializeAllocationMemento(alloc_memento, allocation_site);
  }
  return result;
}


void Heap::InitializeJSObjectFromMap(JSObject* obj, FixedArray* properties,
                                     Map* map) {
  obj->set_properties(properties);
  obj->initialize_elements();
  // TODO(1240798): Initialize the object's body using valid initial values
  // according to the object's initial map.  For example, if the map's
  // instance type is JS_ARRAY_TYPE, the length field should be initialized
  // to a number (e.g. Smi::kZero) and the elements initialized to a
  // fixed array (e.g. Heap::empty_fixed_array()).  Currently, the object
  // verification code has to cope with (temporarily) invalid objects.  See
  // for example, JSArray::JSArrayVerify).
  InitializeJSObjectBody(obj, map, JSObject::kHeaderSize);
}


void Heap::InitializeJSObjectBody(JSObject* obj, Map* map, int start_offset) {
  if (start_offset == map->instance_size()) return;
  DCHECK_LT(start_offset, map->instance_size());

  // We cannot always fill with one_pointer_filler_map because objects
  // created from API functions expect their internal fields to be initialized
  // with undefined_value.
  // Pre-allocated fields need to be initialized with undefined_value as well
  // so that object accesses before the constructor completes (e.g. in the
  // debugger) will not cause a crash.

  // In case of Array subclassing the |map| could already be transitioned
  // to different elements kind from the initial map on which we track slack.
  bool in_progress = map->IsInobjectSlackTrackingInProgress();
  Object* filler;
  if (in_progress) {
    filler = one_pointer_filler_map();
  } else {
    filler = undefined_value();
  }
  obj->InitializeBody(map, start_offset, Heap::undefined_value(), filler);
  if (in_progress) {
    map->FindRootMap()->InobjectSlackTrackingStep();
  }
}


AllocationResult Heap::AllocateJSObjectFromMap(
    Map* map, PretenureFlag pretenure, AllocationSite* allocation_site) {
  // JSFunctions should be allocated using AllocateFunction to be
  // properly initialized.
  DCHECK(map->instance_type() != JS_FUNCTION_TYPE);

  // Both types of global objects should be allocated using
  // AllocateGlobalObject to be properly initialized.
  DCHECK(map->instance_type() != JS_GLOBAL_OBJECT_TYPE);

  // Allocate the backing storage for the properties.
  FixedArray* properties = empty_fixed_array();

  // Allocate the JSObject.
  AllocationSpace space = SelectSpace(pretenure);
  JSObject* js_obj = nullptr;
  AllocationResult allocation = Allocate(map, space, allocation_site);
  if (!allocation.To(&js_obj)) return allocation;

  // Initialize the JSObject.
  InitializeJSObjectFromMap(js_obj, properties, map);
  DCHECK(js_obj->HasFastElements() || js_obj->HasFixedTypedArrayElements() ||
         js_obj->HasFastStringWrapperElements() ||
         js_obj->HasFastArgumentsElements());
  return js_obj;
}


AllocationResult Heap::AllocateJSObject(JSFunction* constructor,
                                        PretenureFlag pretenure,
                                        AllocationSite* allocation_site) {
  DCHECK(constructor->has_initial_map());

  // Allocate the object based on the constructors initial map.
  AllocationResult allocation = AllocateJSObjectFromMap(
      constructor->initial_map(), pretenure, allocation_site);
#ifdef DEBUG
  // Make sure result is NOT a global object if valid.
  HeapObject* obj = nullptr;
  DCHECK(!allocation.To(&obj) || !obj->IsJSGlobalObject());
#endif
  return allocation;
}


AllocationResult Heap::CopyJSObject(JSObject* source, AllocationSite* site) {
  // Make the clone.
  Map* map = source->map();

  // We can only clone regexps, normal objects, api objects, errors or arrays.
  // Copying anything else will break invariants.
  CHECK(map->instance_type() == JS_REGEXP_TYPE ||
        map->instance_type() == JS_OBJECT_TYPE ||
        map->instance_type() == JS_ERROR_TYPE ||
        map->instance_type() == JS_ARRAY_TYPE ||
        map->instance_type() == JS_API_OBJECT_TYPE ||
        map->instance_type() == JS_SPECIAL_API_OBJECT_TYPE);

  int object_size = map->instance_size();
  HeapObject* clone = nullptr;

  DCHECK(site == NULL || AllocationSite::CanTrack(map->instance_type()));

  int adjusted_object_size =
      site != NULL ? object_size + AllocationMemento::kSize : object_size;
  AllocationResult allocation = AllocateRaw(adjusted_object_size, NEW_SPACE);
  if (!allocation.To(&clone)) return allocation;

  SLOW_DCHECK(InNewSpace(clone));
  // Since we know the clone is allocated in new space, we can copy
  // the contents without worrying about updating the write barrier.
  CopyBlock(clone->address(), source->address(), object_size);

  if (site != NULL) {
    AllocationMemento* alloc_memento = reinterpret_cast<AllocationMemento*>(
        reinterpret_cast<Address>(clone) + object_size);
    InitializeAllocationMemento(alloc_memento, site);
  }

  SLOW_DCHECK(JSObject::cast(clone)->GetElementsKind() ==
              source->GetElementsKind());
  FixedArrayBase* elements = FixedArrayBase::cast(source->elements());
  FixedArray* properties = FixedArray::cast(source->properties());
  // Update elements if necessary.
  if (elements->length() > 0) {
    FixedArrayBase* elem = nullptr;
    {
      AllocationResult allocation;
      if (elements->map() == fixed_cow_array_map()) {
        allocation = FixedArray::cast(elements);
      } else if (source->HasFastDoubleElements()) {
        allocation = CopyFixedDoubleArray(FixedDoubleArray::cast(elements));
      } else {
        allocation = CopyFixedArray(FixedArray::cast(elements));
      }
      if (!allocation.To(&elem)) return allocation;
    }
    JSObject::cast(clone)->set_elements(elem, SKIP_WRITE_BARRIER);
  }
  // Update properties if necessary.
  if (properties->length() > 0) {
    FixedArray* prop = nullptr;
    {
      AllocationResult allocation = CopyFixedArray(properties);
      if (!allocation.To(&prop)) return allocation;
    }
    JSObject::cast(clone)->set_properties(prop, SKIP_WRITE_BARRIER);
  }
  // Return the new clone.
  return clone;
}


static inline void WriteOneByteData(Vector<const char> vector, uint8_t* chars,
                                    int len) {
  // Only works for one byte strings.
  DCHECK(vector.length() == len);
  MemCopy(chars, vector.start(), len);
}

static inline void WriteTwoByteData(Vector<const char> vector, uint16_t* chars,
                                    int len) {
  const uint8_t* stream = reinterpret_cast<const uint8_t*>(vector.start());
  size_t stream_length = vector.length();
  while (stream_length != 0) {
    size_t consumed = 0;
    uint32_t c = unibrow::Utf8::ValueOf(stream, stream_length, &consumed);
    DCHECK(c != unibrow::Utf8::kBadChar);
    DCHECK(consumed <= stream_length);
    stream_length -= consumed;
    stream += consumed;
    if (c > unibrow::Utf16::kMaxNonSurrogateCharCode) {
      len -= 2;
      if (len < 0) break;
      *chars++ = unibrow::Utf16::LeadSurrogate(c);
      *chars++ = unibrow::Utf16::TrailSurrogate(c);
    } else {
      len -= 1;
      if (len < 0) break;
      *chars++ = c;
    }
  }
  DCHECK(stream_length == 0);
  DCHECK(len == 0);
}


static inline void WriteOneByteData(String* s, uint8_t* chars, int len) {
  DCHECK(s->length() == len);
  String::WriteToFlat(s, chars, 0, len);
}


static inline void WriteTwoByteData(String* s, uint16_t* chars, int len) {
  DCHECK(s->length() == len);
  String::WriteToFlat(s, chars, 0, len);
}


template <bool is_one_byte, typename T>
AllocationResult Heap::AllocateInternalizedStringImpl(T t, int chars,
                                                      uint32_t hash_field) {
  DCHECK(chars >= 0);
  // Compute map and object size.
  int size;
  Map* map;

  DCHECK_LE(0, chars);
  DCHECK_GE(String::kMaxLength, chars);
  if (is_one_byte) {
    map = one_byte_internalized_string_map();
    size = SeqOneByteString::SizeFor(chars);
  } else {
    map = internalized_string_map();
    size = SeqTwoByteString::SizeFor(chars);
  }

  // Allocate string.
  HeapObject* result = nullptr;
  {
    AllocationResult allocation = AllocateRaw(size, OLD_SPACE);
    if (!allocation.To(&result)) return allocation;
  }

  result->set_map_no_write_barrier(map);
  // Set length and hash fields of the allocated string.
  String* answer = String::cast(result);
  answer->set_length(chars);
  answer->set_hash_field(hash_field);

  DCHECK_EQ(size, answer->Size());

  if (is_one_byte) {
    WriteOneByteData(t, SeqOneByteString::cast(answer)->GetChars(), chars);
  } else {
    WriteTwoByteData(t, SeqTwoByteString::cast(answer)->GetChars(), chars);
  }
  return answer;
}


// Need explicit instantiations.
template AllocationResult Heap::AllocateInternalizedStringImpl<true>(String*,
                                                                     int,
                                                                     uint32_t);
template AllocationResult Heap::AllocateInternalizedStringImpl<false>(String*,
                                                                      int,
                                                                      uint32_t);
template AllocationResult Heap::AllocateInternalizedStringImpl<false>(
    Vector<const char>, int, uint32_t);


AllocationResult Heap::AllocateRawOneByteString(int length,
                                                PretenureFlag pretenure) {
  DCHECK_LE(0, length);
  DCHECK_GE(String::kMaxLength, length);
  int size = SeqOneByteString::SizeFor(length);
  DCHECK(size <= SeqOneByteString::kMaxSize);
  AllocationSpace space = SelectSpace(pretenure);

  HeapObject* result = nullptr;
  {
    AllocationResult allocation = AllocateRaw(size, space);
    if (!allocation.To(&result)) return allocation;
  }

  // Partially initialize the object.
  result->set_map_no_write_barrier(one_byte_string_map());
  String::cast(result)->set_length(length);
  String::cast(result)->set_hash_field(String::kEmptyHashField);
  DCHECK_EQ(size, HeapObject::cast(result)->Size());

  return result;
}


AllocationResult Heap::AllocateRawTwoByteString(int length,
                                                PretenureFlag pretenure) {
  DCHECK_LE(0, length);
  DCHECK_GE(String::kMaxLength, length);
  int size = SeqTwoByteString::SizeFor(length);
  DCHECK(size <= SeqTwoByteString::kMaxSize);
  AllocationSpace space = SelectSpace(pretenure);

  HeapObject* result = nullptr;
  {
    AllocationResult allocation = AllocateRaw(size, space);
    if (!allocation.To(&result)) return allocation;
  }

  // Partially initialize the object.
  result->set_map_no_write_barrier(string_map());
  String::cast(result)->set_length(length);
  String::cast(result)->set_hash_field(String::kEmptyHashField);
  DCHECK_EQ(size, HeapObject::cast(result)->Size());
  return result;
}


AllocationResult Heap::AllocateEmptyFixedArray() {
  int size = FixedArray::SizeFor(0);
  HeapObject* result = nullptr;
  {
    AllocationResult allocation = AllocateRaw(size, OLD_SPACE);
    if (!allocation.To(&result)) return allocation;
  }
  // Initialize the object.
  result->set_map_no_write_barrier(fixed_array_map());
  FixedArray::cast(result)->set_length(0);
  return result;
}

AllocationResult Heap::AllocateEmptyScopeInfo() {
  int size = FixedArray::SizeFor(0);
  HeapObject* result = nullptr;
  {
    AllocationResult allocation = AllocateRaw(size, OLD_SPACE);
    if (!allocation.To(&result)) return allocation;
  }
  // Initialize the object.
  result->set_map_no_write_barrier(scope_info_map());
  FixedArray::cast(result)->set_length(0);
  return result;
}

AllocationResult Heap::CopyAndTenureFixedCOWArray(FixedArray* src) {
  if (!InNewSpace(src)) {
    return src;
  }

  int len = src->length();
  HeapObject* obj = nullptr;
  {
    AllocationResult allocation = AllocateRawFixedArray(len, TENURED);
    if (!allocation.To(&obj)) return allocation;
  }
  obj->set_map_no_write_barrier(fixed_array_map());
  FixedArray* result = FixedArray::cast(obj);
  result->set_length(len);

  // Copy the content.
  DisallowHeapAllocation no_gc;
  WriteBarrierMode mode = result->GetWriteBarrierMode(no_gc);
  for (int i = 0; i < len; i++) result->set(i, src->get(i), mode);

  // TODO(mvstanton): The map is set twice because of protection against calling
  // set() on a COW FixedArray. Issue v8:3221 created to track this, and
  // we might then be able to remove this whole method.
  HeapObject::cast(obj)->set_map_no_write_barrier(fixed_cow_array_map());
  return result;
}


AllocationResult Heap::AllocateEmptyFixedTypedArray(
    ExternalArrayType array_type) {
  return AllocateFixedTypedArray(0, array_type, false, TENURED);
}


AllocationResult Heap::CopyFixedArrayAndGrow(FixedArray* src, int grow_by,
                                             PretenureFlag pretenure) {
  int old_len = src->length();
  int new_len = old_len + grow_by;
  DCHECK(new_len >= old_len);
  HeapObject* obj = nullptr;
  {
    AllocationResult allocation = AllocateRawFixedArray(new_len, pretenure);
    if (!allocation.To(&obj)) return allocation;
  }

  obj->set_map_no_write_barrier(fixed_array_map());
  FixedArray* result = FixedArray::cast(obj);
  result->set_length(new_len);

  // Copy the content.
  DisallowHeapAllocation no_gc;
  WriteBarrierMode mode = obj->GetWriteBarrierMode(no_gc);
  for (int i = 0; i < old_len; i++) result->set(i, src->get(i), mode);
  MemsetPointer(result->data_start() + old_len, undefined_value(), grow_by);
  return result;
}

AllocationResult Heap::CopyFixedArrayUpTo(FixedArray* src, int new_len,
                                          PretenureFlag pretenure) {
  if (new_len == 0) return empty_fixed_array();

  DCHECK_LE(new_len, src->length());

  HeapObject* obj = nullptr;
  {
    AllocationResult allocation = AllocateRawFixedArray(new_len, pretenure);
    if (!allocation.To(&obj)) return allocation;
  }
  obj->set_map_no_write_barrier(fixed_array_map());

  FixedArray* result = FixedArray::cast(obj);
  result->set_length(new_len);

  // Copy the content.
  DisallowHeapAllocation no_gc;
  WriteBarrierMode mode = result->GetWriteBarrierMode(no_gc);
  for (int i = 0; i < new_len; i++) result->set(i, src->get(i), mode);
  return result;
}

AllocationResult Heap::CopyFixedArrayWithMap(FixedArray* src, Map* map) {
  int len = src->length();
  HeapObject* obj = nullptr;
  {
    AllocationResult allocation = AllocateRawFixedArray(len, NOT_TENURED);
    if (!allocation.To(&obj)) return allocation;
  }
  obj->set_map_no_write_barrier(map);

  FixedArray* result = FixedArray::cast(obj);
  DisallowHeapAllocation no_gc;
  WriteBarrierMode mode = result->GetWriteBarrierMode(no_gc);

  // Eliminate the write barrier if possible.
  if (mode == SKIP_WRITE_BARRIER) {
    CopyBlock(obj->address() + kPointerSize, src->address() + kPointerSize,
              FixedArray::SizeFor(len) - kPointerSize);
    return obj;
  }

  // Slow case: Just copy the content one-by-one.
  result->set_length(len);
  for (int i = 0; i < len; i++) result->set(i, src->get(i), mode);
  return result;
}


AllocationResult Heap::CopyFixedDoubleArrayWithMap(FixedDoubleArray* src,
                                                   Map* map) {
  int len = src->length();
  HeapObject* obj = nullptr;
  {
    AllocationResult allocation = AllocateRawFixedDoubleArray(len, NOT_TENURED);
    if (!allocation.To(&obj)) return allocation;
  }
  obj->set_map_no_write_barrier(map);
  CopyBlock(obj->address() + FixedDoubleArray::kLengthOffset,
            src->address() + FixedDoubleArray::kLengthOffset,
            FixedDoubleArray::SizeFor(len) - FixedDoubleArray::kLengthOffset);
  return obj;
}


AllocationResult Heap::AllocateRawFixedArray(int length,
                                             PretenureFlag pretenure) {
  if (length < 0 || length > FixedArray::kMaxLength) {
    v8::internal::Heap::FatalProcessOutOfMemory("invalid array length", true);
  }
  int size = FixedArray::SizeFor(length);
  AllocationSpace space = SelectSpace(pretenure);

  AllocationResult result = AllocateRaw(size, space);
  if (!result.IsRetry() && size > kMaxRegularHeapObjectSize &&
      FLAG_use_marking_progress_bar) {
    MemoryChunk* chunk =
        MemoryChunk::FromAddress(result.ToObjectChecked()->address());
    chunk->SetFlag(MemoryChunk::HAS_PROGRESS_BAR);
  }
  return result;
}


AllocationResult Heap::AllocateFixedArrayWithFiller(int length,
                                                    PretenureFlag pretenure,
                                                    Object* filler) {
  DCHECK(length >= 0);
  DCHECK(empty_fixed_array()->IsFixedArray());
  if (length == 0) return empty_fixed_array();

  DCHECK(!InNewSpace(filler));
  HeapObject* result = nullptr;
  {
    AllocationResult allocation = AllocateRawFixedArray(length, pretenure);
    if (!allocation.To(&result)) return allocation;
  }

  result->set_map_no_write_barrier(fixed_array_map());
  FixedArray* array = FixedArray::cast(result);
  array->set_length(length);
  MemsetPointer(array->data_start(), filler, length);
  return array;
}


AllocationResult Heap::AllocateFixedArray(int length, PretenureFlag pretenure) {
  return AllocateFixedArrayWithFiller(length, pretenure, undefined_value());
}


AllocationResult Heap::AllocateUninitializedFixedArray(int length) {
  if (length == 0) return empty_fixed_array();

  HeapObject* obj = nullptr;
  {
    AllocationResult allocation = AllocateRawFixedArray(length, NOT_TENURED);
    if (!allocation.To(&obj)) return allocation;
  }

  obj->set_map_no_write_barrier(fixed_array_map());
  FixedArray::cast(obj)->set_length(length);
  return obj;
}


AllocationResult Heap::AllocateUninitializedFixedDoubleArray(
    int length, PretenureFlag pretenure) {
  if (length == 0) return empty_fixed_array();

  HeapObject* elements = nullptr;
  AllocationResult allocation = AllocateRawFixedDoubleArray(length, pretenure);
  if (!allocation.To(&elements)) return allocation;

  elements->set_map_no_write_barrier(fixed_double_array_map());
  FixedDoubleArray::cast(elements)->set_length(length);
  return elements;
}


AllocationResult Heap::AllocateRawFixedDoubleArray(int length,
                                                   PretenureFlag pretenure) {
  if (length < 0 || length > FixedDoubleArray::kMaxLength) {
    v8::internal::Heap::FatalProcessOutOfMemory("invalid array length", true);
  }
  int size = FixedDoubleArray::SizeFor(length);
  AllocationSpace space = SelectSpace(pretenure);

  HeapObject* object = nullptr;
  {
    AllocationResult allocation = AllocateRaw(size, space, kDoubleAligned);
    if (!allocation.To(&object)) return allocation;
  }

  return object;
}


AllocationResult Heap::AllocateSymbol() {
  // Statically ensure that it is safe to allocate symbols in paged spaces.
  STATIC_ASSERT(Symbol::kSize <= kMaxRegularHeapObjectSize);

  HeapObject* result = nullptr;
  AllocationResult allocation = AllocateRaw(Symbol::kSize, OLD_SPACE);
  if (!allocation.To(&result)) return allocation;

  result->set_map_no_write_barrier(symbol_map());

  // Generate a random hash value.
  int hash = isolate()->GenerateIdentityHash(Name::kHashBitMask);

  Symbol::cast(result)
      ->set_hash_field(Name::kIsNotArrayIndexMask | (hash << Name::kHashShift));
  Symbol::cast(result)->set_name(undefined_value());
  Symbol::cast(result)->set_flags(0);

  DCHECK(!Symbol::cast(result)->is_private());
  return result;
}


AllocationResult Heap::AllocateStruct(InstanceType type) {
  Map* map;
  switch (type) {
#define MAKE_CASE(NAME, Name, name) \
  case NAME##_TYPE:                 \
    map = name##_map();             \
    break;
    STRUCT_LIST(MAKE_CASE)
#undef MAKE_CASE
    default:
      UNREACHABLE();
      return exception();
  }
  int size = map->instance_size();
  Struct* result = nullptr;
  {
    AllocationResult allocation = Allocate(map, OLD_SPACE);
    if (!allocation.To(&result)) return allocation;
  }
  result->InitializeBody(size);
  return result;
}


void Heap::MakeHeapIterable() {
  mark_compact_collector()->EnsureSweepingCompleted();
}


static double ComputeMutatorUtilization(double mutator_speed, double gc_speed) {
  const double kMinMutatorUtilization = 0.0;
  const double kConservativeGcSpeedInBytesPerMillisecond = 200000;
  if (mutator_speed == 0) return kMinMutatorUtilization;
  if (gc_speed == 0) gc_speed = kConservativeGcSpeedInBytesPerMillisecond;
  // Derivation:
  // mutator_utilization = mutator_time / (mutator_time + gc_time)
  // mutator_time = 1 / mutator_speed
  // gc_time = 1 / gc_speed
  // mutator_utilization = (1 / mutator_speed) /
  //                       (1 / mutator_speed + 1 / gc_speed)
  // mutator_utilization = gc_speed / (mutator_speed + gc_speed)
  return gc_speed / (mutator_speed + gc_speed);
}


double Heap::YoungGenerationMutatorUtilization() {
  double mutator_speed = static_cast<double>(
      tracer()->NewSpaceAllocationThroughputInBytesPerMillisecond());
  double gc_speed =
      tracer()->ScavengeSpeedInBytesPerMillisecond(kForSurvivedObjects);
  double result = ComputeMutatorUtilization(mutator_speed, gc_speed);
  if (FLAG_trace_mutator_utilization) {
    isolate()->PrintWithTimestamp(
        "Young generation mutator utilization = %.3f ("
        "mutator_speed=%.f, gc_speed=%.f)\n",
        result, mutator_speed, gc_speed);
  }
  return result;
}


double Heap::OldGenerationMutatorUtilization() {
  double mutator_speed = static_cast<double>(
      tracer()->OldGenerationAllocationThroughputInBytesPerMillisecond());
  double gc_speed = static_cast<double>(
      tracer()->CombinedMarkCompactSpeedInBytesPerMillisecond());
  double result = ComputeMutatorUtilization(mutator_speed, gc_speed);
  if (FLAG_trace_mutator_utilization) {
    isolate()->PrintWithTimestamp(
        "Old generation mutator utilization = %.3f ("
        "mutator_speed=%.f, gc_speed=%.f)\n",
        result, mutator_speed, gc_speed);
  }
  return result;
}


bool Heap::HasLowYoungGenerationAllocationRate() {
  const double high_mutator_utilization = 0.993;
  return YoungGenerationMutatorUtilization() > high_mutator_utilization;
}


bool Heap::HasLowOldGenerationAllocationRate() {
  const double high_mutator_utilization = 0.993;
  return OldGenerationMutatorUtilization() > high_mutator_utilization;
}


bool Heap::HasLowAllocationRate() {
  return HasLowYoungGenerationAllocationRate() &&
         HasLowOldGenerationAllocationRate();
}


bool Heap::HasHighFragmentation() {
  size_t used = PromotedSpaceSizeOfObjects();
  size_t committed = CommittedOldGenerationMemory();
  return HasHighFragmentation(used, committed);
}

bool Heap::HasHighFragmentation(size_t used, size_t committed) {
  const size_t kSlack = 16 * MB;
  // Fragmentation is high if committed > 2 * used + kSlack.
  // Rewrite the exression to avoid overflow.
  DCHECK_GE(committed, used);
  return committed - used > used + kSlack;
}

bool Heap::ShouldOptimizeForMemoryUsage() {
  return FLAG_optimize_for_size || isolate()->IsIsolateInBackground() ||
         HighMemoryPressure() || IsLowMemoryDevice();
}

void Heap::ActivateMemoryReducerIfNeeded() {
  // Activate memory reducer when switching to background if
  // - there was no mark compact since the start.
  // - the committed memory can be potentially reduced.
  // 2 pages for the old, code, and map space + 1 page for new space.
  const int kMinCommittedMemory = 7 * Page::kPageSize;
  if (ms_count_ == 0 && CommittedMemory() > kMinCommittedMemory &&
      isolate()->IsIsolateInBackground()) {
    MemoryReducer::Event event;
    event.type = MemoryReducer::kPossibleGarbage;
    event.time_ms = MonotonicallyIncreasingTimeInMs();
    memory_reducer_->NotifyPossibleGarbage(event);
  }
}

void Heap::ReduceNewSpaceSize() {
  // TODO(ulan): Unify this constant with the similar constant in
  // GCIdleTimeHandler once the change is merged to 4.5.
  static const size_t kLowAllocationThroughput = 1000;
  const double allocation_throughput =
      tracer()->CurrentAllocationThroughputInBytesPerMillisecond();

  if (FLAG_predictable) return;

  if (ShouldReduceMemory() ||
      ((allocation_throughput != 0) &&
       (allocation_throughput < kLowAllocationThroughput))) {
    new_space_->Shrink();
    UncommitFromSpace();
  }
}

void Heap::FinalizeIncrementalMarkingIfComplete(
    GarbageCollectionReason gc_reason) {
  if (incremental_marking()->IsMarking() &&
      (incremental_marking()->IsReadyToOverApproximateWeakClosure() ||
       (!incremental_marking()->finalize_marking_completed() &&
        mark_compact_collector()->marking_deque()->IsEmpty() &&
        local_embedder_heap_tracer()->ShouldFinalizeIncrementalMarking()))) {
    FinalizeIncrementalMarking(gc_reason);
  } else if (incremental_marking()->IsComplete() ||
             (mark_compact_collector()->marking_deque()->IsEmpty() &&
              local_embedder_heap_tracer()
                  ->ShouldFinalizeIncrementalMarking())) {
    CollectAllGarbage(current_gc_flags_, gc_reason);
  }
}

bool Heap::TryFinalizeIdleIncrementalMarking(
    double idle_time_in_ms, GarbageCollectionReason gc_reason) {
  size_t size_of_objects = static_cast<size_t>(SizeOfObjects());
  double final_incremental_mark_compact_speed_in_bytes_per_ms =
      tracer()->FinalIncrementalMarkCompactSpeedInBytesPerMillisecond();
  if (incremental_marking()->IsReadyToOverApproximateWeakClosure() ||
      (!incremental_marking()->finalize_marking_completed() &&
       mark_compact_collector()->marking_deque()->IsEmpty() &&
       local_embedder_heap_tracer()->ShouldFinalizeIncrementalMarking() &&
       gc_idle_time_handler_->ShouldDoOverApproximateWeakClosure(
           idle_time_in_ms))) {
    FinalizeIncrementalMarking(gc_reason);
    return true;
  } else if (incremental_marking()->IsComplete() ||
             (mark_compact_collector()->marking_deque()->IsEmpty() &&
              local_embedder_heap_tracer()
                  ->ShouldFinalizeIncrementalMarking() &&
              gc_idle_time_handler_->ShouldDoFinalIncrementalMarkCompact(
                  idle_time_in_ms, size_of_objects,
                  final_incremental_mark_compact_speed_in_bytes_per_ms))) {
    CollectAllGarbage(current_gc_flags_, gc_reason);
    return true;
  }
  return false;
}

void Heap::RegisterReservationsForBlackAllocation(Reservation* reservations) {
  // TODO(hpayer): We do not have to iterate reservations on black objects
  // for marking. We just have to execute the special visiting side effect
  // code that adds objects to global data structures, e.g. for array buffers.

  if (incremental_marking()->black_allocation()) {
    // Iterate black objects in old space, code space, map space, and large
    // object space for side effects.
    for (int i = OLD_SPACE; i < Serializer::kNumberOfSpaces; i++) {
      const Heap::Reservation& res = reservations[i];
      for (auto& chunk : res) {
        Address addr = chunk.start;
        while (addr < chunk.end) {
          HeapObject* obj = HeapObject::FromAddress(addr);
          // There might be grey objects due to black to grey transitions in
          // incremental marking. E.g. see VisitNativeContextIncremental.
          DCHECK(ObjectMarking::IsBlackOrGrey(obj));
          if (ObjectMarking::IsBlack(obj)) {
            incremental_marking()->IterateBlackObject(obj);
          }
          addr += obj->Size();
        }
      }
    }
  }
}

void Heap::NotifyObjectLayoutChange(HeapObject* object,
                                    const DisallowHeapAllocation&) {
  if (FLAG_incremental_marking && incremental_marking()->IsMarking()) {
    incremental_marking()->MarkGrey(this, object);
  }
#ifdef VERIFY_HEAP
  DCHECK(pending_layout_change_object_ == nullptr);
  pending_layout_change_object_ = object;
#endif
}

#ifdef VERIFY_HEAP
void Heap::VerifyObjectLayoutChange(HeapObject* object, Map* new_map) {
  if (pending_layout_change_object_ == nullptr) {
    DCHECK(!object->IsJSObject() ||
           !object->map()->TransitionRequiresSynchronizationWithGC(new_map));
  } else {
    DCHECK_EQ(pending_layout_change_object_, object);
    pending_layout_change_object_ = nullptr;
  }
}
#endif

GCIdleTimeHeapState Heap::ComputeHeapState() {
  GCIdleTimeHeapState heap_state;
  heap_state.contexts_disposed = contexts_disposed_;
  heap_state.contexts_disposal_rate =
      tracer()->ContextDisposalRateInMilliseconds();
  heap_state.size_of_objects = static_cast<size_t>(SizeOfObjects());
  heap_state.incremental_marking_stopped = incremental_marking()->IsStopped();
  return heap_state;
}


bool Heap::PerformIdleTimeAction(GCIdleTimeAction action,
                                 GCIdleTimeHeapState heap_state,
                                 double deadline_in_ms) {
  bool result = false;
  switch (action.type) {
    case DONE:
      result = true;
      break;
    case DO_INCREMENTAL_STEP: {
      const double remaining_idle_time_in_ms =
          incremental_marking()->AdvanceIncrementalMarking(
              deadline_in_ms, IncrementalMarking::NO_GC_VIA_STACK_GUARD,
              IncrementalMarking::FORCE_COMPLETION, StepOrigin::kTask);
      if (remaining_idle_time_in_ms > 0.0) {
        TryFinalizeIdleIncrementalMarking(
            remaining_idle_time_in_ms,
            GarbageCollectionReason::kFinalizeMarkingViaTask);
      }
      result = incremental_marking()->IsStopped();
      break;
    }
    case DO_FULL_GC: {
      DCHECK(contexts_disposed_ > 0);
      HistogramTimerScope scope(isolate_->counters()->gc_context());
      TRACE_EVENT0("v8", "V8.GCContext");
      CollectAllGarbage(kNoGCFlags, GarbageCollectionReason::kContextDisposal);
      break;
    }
    case DO_NOTHING:
      break;
  }

  return result;
}


void Heap::IdleNotificationEpilogue(GCIdleTimeAction action,
                                    GCIdleTimeHeapState heap_state,
                                    double start_ms, double deadline_in_ms) {
  double idle_time_in_ms = deadline_in_ms - start_ms;
  double current_time = MonotonicallyIncreasingTimeInMs();
  last_idle_notification_time_ = current_time;
  double deadline_difference = deadline_in_ms - current_time;

  contexts_disposed_ = 0;

  isolate()->counters()->gc_idle_time_allotted_in_ms()->AddSample(
      static_cast<int>(idle_time_in_ms));

  if (deadline_in_ms - start_ms >
      GCIdleTimeHandler::kMaxFrameRenderingIdleTime) {
    int committed_memory = static_cast<int>(CommittedMemory() / KB);
    int used_memory = static_cast<int>(heap_state.size_of_objects / KB);
    isolate()->counters()->aggregated_memory_heap_committed()->AddSample(
        start_ms, committed_memory);
    isolate()->counters()->aggregated_memory_heap_used()->AddSample(
        start_ms, used_memory);
  }

  if (deadline_difference >= 0) {
    if (action.type != DONE && action.type != DO_NOTHING) {
      isolate()->counters()->gc_idle_time_limit_undershot()->AddSample(
          static_cast<int>(deadline_difference));
    }
  } else {
    isolate()->counters()->gc_idle_time_limit_overshot()->AddSample(
        static_cast<int>(-deadline_difference));
  }

  if ((FLAG_trace_idle_notification && action.type > DO_NOTHING) ||
      FLAG_trace_idle_notification_verbose) {
    isolate_->PrintWithTimestamp(
        "Idle notification: requested idle time %.2f ms, used idle time %.2f "
        "ms, deadline usage %.2f ms [",
        idle_time_in_ms, idle_time_in_ms - deadline_difference,
        deadline_difference);
    action.Print();
    PrintF("]");
    if (FLAG_trace_idle_notification_verbose) {
      PrintF("[");
      heap_state.Print();
      PrintF("]");
    }
    PrintF("\n");
  }
}


double Heap::MonotonicallyIncreasingTimeInMs() {
  return V8::GetCurrentPlatform()->MonotonicallyIncreasingTime() *
         static_cast<double>(base::Time::kMillisecondsPerSecond);
}


bool Heap::IdleNotification(int idle_time_in_ms) {
  return IdleNotification(
      V8::GetCurrentPlatform()->MonotonicallyIncreasingTime() +
      (static_cast<double>(idle_time_in_ms) /
       static_cast<double>(base::Time::kMillisecondsPerSecond)));
}


bool Heap::IdleNotification(double deadline_in_seconds) {
  CHECK(HasBeenSetUp());
  double deadline_in_ms =
      deadline_in_seconds *
      static_cast<double>(base::Time::kMillisecondsPerSecond);
  HistogramTimerScope idle_notification_scope(
      isolate_->counters()->gc_idle_notification());
  TRACE_EVENT0("v8", "V8.GCIdleNotification");
  double start_ms = MonotonicallyIncreasingTimeInMs();
  double idle_time_in_ms = deadline_in_ms - start_ms;

  tracer()->SampleAllocation(start_ms, NewSpaceAllocationCounter(),
                             OldGenerationAllocationCounter());

  GCIdleTimeHeapState heap_state = ComputeHeapState();

  GCIdleTimeAction action =
      gc_idle_time_handler_->Compute(idle_time_in_ms, heap_state);

  bool result = PerformIdleTimeAction(action, heap_state, deadline_in_ms);

  IdleNotificationEpilogue(action, heap_state, start_ms, deadline_in_ms);
  return result;
}


bool Heap::RecentIdleNotificationHappened() {
  return (last_idle_notification_time_ +
          GCIdleTimeHandler::kMaxScheduledIdleTime) >
         MonotonicallyIncreasingTimeInMs();
}

class MemoryPressureInterruptTask : public CancelableTask {
 public:
  explicit MemoryPressureInterruptTask(Heap* heap)
      : CancelableTask(heap->isolate()), heap_(heap) {}

  virtual ~MemoryPressureInterruptTask() {}

 private:
  // v8::internal::CancelableTask overrides.
  void RunInternal() override { heap_->CheckMemoryPressure(); }

  Heap* heap_;
  DISALLOW_COPY_AND_ASSIGN(MemoryPressureInterruptTask);
};

void Heap::CheckMemoryPressure() {
  if (HighMemoryPressure()) {
    if (isolate()->concurrent_recompilation_enabled()) {
      // The optimizing compiler may be unnecessarily holding on to memory.
      DisallowHeapAllocation no_recursive_gc;
      isolate()->optimizing_compile_dispatcher()->Flush(
          OptimizingCompileDispatcher::BlockingBehavior::kDontBlock);
    }
  }
  if (memory_pressure_level_.Value() == MemoryPressureLevel::kCritical) {
    CollectGarbageOnMemoryPressure();
  } else if (memory_pressure_level_.Value() == MemoryPressureLevel::kModerate) {
    if (FLAG_incremental_marking && incremental_marking()->IsStopped()) {
      StartIncrementalMarking(kReduceMemoryFootprintMask,
                              GarbageCollectionReason::kMemoryPressure);
    }
  }
  MemoryReducer::Event event;
  event.type = MemoryReducer::kPossibleGarbage;
  event.time_ms = MonotonicallyIncreasingTimeInMs();
  memory_reducer_->NotifyPossibleGarbage(event);
}

void Heap::CollectGarbageOnMemoryPressure() {
  const int kGarbageThresholdInBytes = 8 * MB;
  const double kGarbageThresholdAsFractionOfTotalMemory = 0.1;
  // This constant is the maximum response time in RAIL performance model.
  const double kMaxMemoryPressurePauseMs = 100;

  double start = MonotonicallyIncreasingTimeInMs();
  CollectAllGarbage(kReduceMemoryFootprintMask | kAbortIncrementalMarkingMask,
                    GarbageCollectionReason::kMemoryPressure,
                    kGCCallbackFlagCollectAllAvailableGarbage);
  double end = MonotonicallyIncreasingTimeInMs();

  // Estimate how much memory we can free.
  int64_t potential_garbage =
      (CommittedMemory() - SizeOfObjects()) + external_memory_;
  // If we can potentially free large amount of memory, then start GC right
  // away instead of waiting for memory reducer.
  if (potential_garbage >= kGarbageThresholdInBytes &&
      potential_garbage >=
          CommittedMemory() * kGarbageThresholdAsFractionOfTotalMemory) {
    // If we spent less than half of the time budget, then perform full GC
    // Otherwise, start incremental marking.
    if (end - start < kMaxMemoryPressurePauseMs / 2) {
      CollectAllGarbage(
          kReduceMemoryFootprintMask | kAbortIncrementalMarkingMask,
          GarbageCollectionReason::kMemoryPressure,
          kGCCallbackFlagCollectAllAvailableGarbage);
    } else {
      if (FLAG_incremental_marking && incremental_marking()->IsStopped()) {
        StartIncrementalMarking(kReduceMemoryFootprintMask,
                                GarbageCollectionReason::kMemoryPressure);
      }
    }
  }
}

void Heap::MemoryPressureNotification(MemoryPressureLevel level,
                                      bool is_isolate_locked) {
  MemoryPressureLevel previous = memory_pressure_level_.Value();
  memory_pressure_level_.SetValue(level);
  if ((previous != MemoryPressureLevel::kCritical &&
       level == MemoryPressureLevel::kCritical) ||
      (previous == MemoryPressureLevel::kNone &&
       level == MemoryPressureLevel::kModerate)) {
    if (is_isolate_locked) {
      CheckMemoryPressure();
    } else {
      ExecutionAccess access(isolate());
      isolate()->stack_guard()->RequestGC();
      V8::GetCurrentPlatform()->CallOnForegroundThread(
          reinterpret_cast<v8::Isolate*>(isolate()),
          new MemoryPressureInterruptTask(this));
    }
  }
}

void Heap::SetOutOfMemoryCallback(v8::debug::OutOfMemoryCallback callback,
                                  void* data) {
  out_of_memory_callback_ = callback;
  out_of_memory_callback_data_ = data;
}

void Heap::InvokeOutOfMemoryCallback() {
  if (out_of_memory_callback_) {
    out_of_memory_callback_(out_of_memory_callback_data_);
  }
}

void Heap::CollectCodeStatistics() {
  CodeStatistics::ResetCodeAndMetadataStatistics(isolate());
  // We do not look for code in new space, or map space.  If code
  // somehow ends up in those spaces, we would miss it here.
  CodeStatistics::CollectCodeStatistics(code_space_, isolate());
  CodeStatistics::CollectCodeStatistics(old_space_, isolate());
  CodeStatistics::CollectCodeStatistics(lo_space_, isolate());
}

#ifdef DEBUG

void Heap::Print() {
  if (!HasBeenSetUp()) return;
  isolate()->PrintStack(stdout);
  AllSpaces spaces(this);
  for (Space* space = spaces.next(); space != NULL; space = spaces.next()) {
    space->Print();
  }
}


void Heap::ReportCodeStatistics(const char* title) {
  PrintF(">>>>>> Code Stats (%s) >>>>>>\n", title);
  CollectCodeStatistics();
  CodeStatistics::ReportCodeStatistics(isolate());
}


// This function expects that NewSpace's allocated objects histogram is
// populated (via a call to CollectStatistics or else as a side effect of a
// just-completed scavenge collection).
void Heap::ReportHeapStatistics(const char* title) {
  USE(title);
  PrintF(">>>>>> =============== %s (%d) =============== >>>>>>\n", title,
         gc_count_);
  PrintF("old_generation_allocation_limit_ %" V8PRIdPTR "\n",
         old_generation_allocation_limit_);

  PrintF("\n");
  PrintF("Number of handles : %d\n", HandleScope::NumberOfHandles(isolate_));
  isolate_->global_handles()->PrintStats();
  PrintF("\n");

  PrintF("Heap statistics : ");
  memory_allocator()->ReportStatistics();
  PrintF("To space : ");
  new_space_->ReportStatistics();
  PrintF("Old space : ");
  old_space_->ReportStatistics();
  PrintF("Code space : ");
  code_space_->ReportStatistics();
  PrintF("Map space : ");
  map_space_->ReportStatistics();
  PrintF("Large object space : ");
  lo_space_->ReportStatistics();
  PrintF(">>>>>> ========================================= >>>>>>\n");
}

#endif  // DEBUG

const char* Heap::GarbageCollectionReasonToString(
    GarbageCollectionReason gc_reason) {
  switch (gc_reason) {
    case GarbageCollectionReason::kAllocationFailure:
      return "allocation failure";
    case GarbageCollectionReason::kAllocationLimit:
      return "allocation limit";
    case GarbageCollectionReason::kContextDisposal:
      return "context disposal";
    case GarbageCollectionReason::kCountersExtension:
      return "counters extension";
    case GarbageCollectionReason::kDebugger:
      return "debugger";
    case GarbageCollectionReason::kDeserializer:
      return "deserialize";
    case GarbageCollectionReason::kExternalMemoryPressure:
      return "external memory pressure";
    case GarbageCollectionReason::kFinalizeMarkingViaStackGuard:
      return "finalize incremental marking via stack guard";
    case GarbageCollectionReason::kFinalizeMarkingViaTask:
      return "finalize incremental marking via task";
    case GarbageCollectionReason::kFullHashtable:
      return "full hash-table";
    case GarbageCollectionReason::kHeapProfiler:
      return "heap profiler";
    case GarbageCollectionReason::kIdleTask:
      return "idle task";
    case GarbageCollectionReason::kLastResort:
      return "last resort";
    case GarbageCollectionReason::kLowMemoryNotification:
      return "low memory notification";
    case GarbageCollectionReason::kMakeHeapIterable:
      return "make heap iterable";
    case GarbageCollectionReason::kMemoryPressure:
      return "memory pressure";
    case GarbageCollectionReason::kMemoryReducer:
      return "memory reducer";
    case GarbageCollectionReason::kRuntime:
      return "runtime";
    case GarbageCollectionReason::kSamplingProfiler:
      return "sampling profiler";
    case GarbageCollectionReason::kSnapshotCreator:
      return "snapshot creator";
    case GarbageCollectionReason::kTesting:
      return "testing";
    case GarbageCollectionReason::kUnknown:
      return "unknown";
  }
  UNREACHABLE();
  return "";
}

bool Heap::Contains(HeapObject* value) {
  if (memory_allocator()->IsOutsideAllocatedSpace(value->address())) {
    return false;
  }
  return HasBeenSetUp() &&
         (new_space_->ToSpaceContains(value) || old_space_->Contains(value) ||
          code_space_->Contains(value) || map_space_->Contains(value) ||
          lo_space_->Contains(value));
}

bool Heap::ContainsSlow(Address addr) {
  if (memory_allocator()->IsOutsideAllocatedSpace(addr)) {
    return false;
  }
  return HasBeenSetUp() &&
         (new_space_->ToSpaceContainsSlow(addr) ||
          old_space_->ContainsSlow(addr) || code_space_->ContainsSlow(addr) ||
          map_space_->ContainsSlow(addr) || lo_space_->ContainsSlow(addr));
}

bool Heap::InSpace(HeapObject* value, AllocationSpace space) {
  if (memory_allocator()->IsOutsideAllocatedSpace(value->address())) {
    return false;
  }
  if (!HasBeenSetUp()) return false;

  switch (space) {
    case NEW_SPACE:
      return new_space_->ToSpaceContains(value);
    case OLD_SPACE:
      return old_space_->Contains(value);
    case CODE_SPACE:
      return code_space_->Contains(value);
    case MAP_SPACE:
      return map_space_->Contains(value);
    case LO_SPACE:
      return lo_space_->Contains(value);
  }
  UNREACHABLE();
  return false;
}

bool Heap::InSpaceSlow(Address addr, AllocationSpace space) {
  if (memory_allocator()->IsOutsideAllocatedSpace(addr)) {
    return false;
  }
  if (!HasBeenSetUp()) return false;

  switch (space) {
    case NEW_SPACE:
      return new_space_->ToSpaceContainsSlow(addr);
    case OLD_SPACE:
      return old_space_->ContainsSlow(addr);
    case CODE_SPACE:
      return code_space_->ContainsSlow(addr);
    case MAP_SPACE:
      return map_space_->ContainsSlow(addr);
    case LO_SPACE:
      return lo_space_->ContainsSlow(addr);
  }
  UNREACHABLE();
  return false;
}


bool Heap::IsValidAllocationSpace(AllocationSpace space) {
  switch (space) {
    case NEW_SPACE:
    case OLD_SPACE:
    case CODE_SPACE:
    case MAP_SPACE:
    case LO_SPACE:
      return true;
    default:
      return false;
  }
}


bool Heap::RootIsImmortalImmovable(int root_index) {
  switch (root_index) {
#define IMMORTAL_IMMOVABLE_ROOT(name) case Heap::k##name##RootIndex:
    IMMORTAL_IMMOVABLE_ROOT_LIST(IMMORTAL_IMMOVABLE_ROOT)
#undef IMMORTAL_IMMOVABLE_ROOT
#define INTERNALIZED_STRING(name, value) case Heap::k##name##RootIndex:
    INTERNALIZED_STRING_LIST(INTERNALIZED_STRING)
#undef INTERNALIZED_STRING
#define STRING_TYPE(NAME, size, name, Name) case Heap::k##Name##MapRootIndex:
    STRING_TYPE_LIST(STRING_TYPE)
#undef STRING_TYPE
    return true;
    default:
      return false;
  }
}


#ifdef VERIFY_HEAP
void Heap::Verify() {
  CHECK(HasBeenSetUp());
  HandleScope scope(isolate());

  // We have to wait here for the sweeper threads to have an iterable heap.
  mark_compact_collector()->EnsureSweepingCompleted();

  VerifyPointersVisitor visitor;
  IterateRoots(&visitor, VISIT_ONLY_STRONG);

  VerifySmisVisitor smis_visitor;
  IterateSmiRoots(&smis_visitor);

  new_space_->Verify();

  old_space_->Verify(&visitor);
  map_space_->Verify(&visitor);

  VerifyPointersVisitor no_dirty_regions_visitor;
  code_space_->Verify(&no_dirty_regions_visitor);

  lo_space_->Verify();

  mark_compact_collector()->VerifyWeakEmbeddedObjectsInCode();
  if (FLAG_omit_map_checks_for_leaf_maps) {
    mark_compact_collector()->VerifyOmittedMapChecks();
  }
}
#endif


void Heap::ZapFromSpace() {
  if (!new_space_->IsFromSpaceCommitted()) return;
  for (Page* page :
       PageRange(new_space_->FromSpaceStart(), new_space_->FromSpaceEnd())) {
    for (Address cursor = page->area_start(), limit = page->area_end();
         cursor < limit; cursor += kPointerSize) {
      Memory::Address_at(cursor) = kFromSpaceZapValue;
    }
  }
}

class IterateAndScavengePromotedObjectsVisitor final : public ObjectVisitor {
 public:
  IterateAndScavengePromotedObjectsVisitor(Heap* heap, HeapObject* target,
                                           bool record_slots)
      : heap_(heap), target_(target), record_slots_(record_slots) {}

  inline void VisitPointers(Object** start, Object** end) override {
    Address slot_address = reinterpret_cast<Address>(start);
    Page* page = Page::FromAddress(slot_address);

    while (slot_address < reinterpret_cast<Address>(end)) {
      Object** slot = reinterpret_cast<Object**>(slot_address);
      Object* target = *slot;

      if (target->IsHeapObject()) {
        if (heap_->InFromSpace(target)) {
          Scavenger::ScavengeObject(reinterpret_cast<HeapObject**>(slot),
                                    HeapObject::cast(target));
          target = *slot;
          if (heap_->InNewSpace(target)) {
            SLOW_DCHECK(heap_->InToSpace(target));
            SLOW_DCHECK(target->IsHeapObject());
            RememberedSet<OLD_TO_NEW>::Insert(page, slot_address);
          }
          SLOW_DCHECK(!MarkCompactCollector::IsOnEvacuationCandidate(
              HeapObject::cast(target)));
        } else if (record_slots_ &&
                   MarkCompactCollector::IsOnEvacuationCandidate(
                       HeapObject::cast(target))) {
          heap_->mark_compact_collector()->RecordSlot(target_, slot, target);
        }
      }

      slot_address += kPointerSize;
    }
  }

  inline void VisitCodeEntry(Address code_entry_slot) override {
    // Black allocation requires us to process objects referenced by
    // promoted objects.
    if (heap_->incremental_marking()->black_allocation()) {
      Code* code = Code::cast(Code::GetObjectFromEntryAddress(code_entry_slot));
      IncrementalMarking::MarkGrey(heap_, code);
    }
  }

 private:
  Heap* heap_;
  HeapObject* target_;
  bool record_slots_;
};

void Heap::IterateAndScavengePromotedObject(HeapObject* target, int size,
                                            bool was_marked_black) {
  // We are not collecting slots on new space objects during mutation
  // thus we have to scan for pointers to evacuation candidates when we
  // promote objects. But we should not record any slots in non-black
  // objects. Grey object's slots would be rescanned.
  // White object might not survive until the end of collection
  // it would be a violation of the invariant to record it's slots.
  bool record_slots = false;
  if (incremental_marking()->IsCompacting()) {
    record_slots = ObjectMarking::IsBlack(target);
  }

  IterateAndScavengePromotedObjectsVisitor visitor(this, target, record_slots);
  if (target->IsJSFunction()) {
    // JSFunctions reachable through kNextFunctionLinkOffset are weak. Slots for
    // this links are recorded during processing of weak lists.
    JSFunction::BodyDescriptorWeakCode::IterateBody(target, size, &visitor);
  } else {
    target->IterateBody(target->map()->instance_type(), size, &visitor);
  }

  // When black allocations is on, we have to visit not already marked black
  // objects (in new space) promoted to black pages to keep their references
  // alive.
  // TODO(hpayer): Implement a special promotion visitor that incorporates
  // regular visiting and IteratePromotedObjectPointers.
  if (!was_marked_black) {
    if (incremental_marking()->black_allocation()) {
      IncrementalMarking::MarkGrey(this, target->map());
      incremental_marking()->IterateBlackObject(target);
    }
  }
}


void Heap::IterateRoots(ObjectVisitor* v, VisitMode mode) {
  IterateStrongRoots(v, mode);
  IterateWeakRoots(v, mode);
}


void Heap::IterateWeakRoots(ObjectVisitor* v, VisitMode mode) {
  v->VisitPointer(reinterpret_cast<Object**>(&roots_[kStringTableRootIndex]));
  v->Synchronize(VisitorSynchronization::kStringTable);
  if (mode != VISIT_ALL_IN_SCAVENGE && mode != VISIT_ALL_IN_SWEEP_NEWSPACE) {
    // Scavenge collections have special processing for this.
    external_string_table_.IterateAll(v);
  }
  v->Synchronize(VisitorSynchronization::kExternalStringsTable);
}


void Heap::IterateSmiRoots(ObjectVisitor* v) {
  // Acquire execution access since we are going to read stack limit values.
  ExecutionAccess access(isolate());
  v->VisitPointers(&roots_[kSmiRootsStart], &roots_[kRootListLength]);
  v->Synchronize(VisitorSynchronization::kSmiRootList);
}

// We cannot avoid stale handles to left-trimmed objects, but can only make
// sure all handles still needed are updated. Filter out a stale pointer
// and clear the slot to allow post processing of handles (needed because
// the sweeper might actually free the underlying page).
class FixStaleLeftTrimmedHandlesVisitor : public ObjectVisitor {
 public:
  explicit FixStaleLeftTrimmedHandlesVisitor(Heap* heap) : heap_(heap) {
    USE(heap_);
  }

  void VisitPointer(Object** p) override { FixHandle(p); }

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

 private:
  inline void FixHandle(Object** p) {
    HeapObject* current = reinterpret_cast<HeapObject*>(*p);
    if (!current->IsHeapObject()) return;
    const MapWord map_word = current->map_word();
    if (!map_word.IsForwardingAddress() && current->IsFiller()) {
#ifdef DEBUG
      // We need to find a FixedArrayBase map after walking the fillers.
      while (current->IsFiller()) {
        Address next = reinterpret_cast<Address>(current);
        if (current->map() == heap_->one_pointer_filler_map()) {
          next += kPointerSize;
        } else if (current->map() == heap_->two_pointer_filler_map()) {
          next += 2 * kPointerSize;
        } else {
          next += current->Size();
        }
        current = reinterpret_cast<HeapObject*>(next);
      }
      DCHECK(current->IsFixedArrayBase());
#endif  // DEBUG
      *p = nullptr;
    }
  }

  Heap* heap_;
};

void Heap::IterateStrongRoots(ObjectVisitor* v, VisitMode mode) {
  v->VisitPointers(&roots_[0], &roots_[kStrongRootListLength]);
  v->Synchronize(VisitorSynchronization::kStrongRootList);
  // The serializer/deserializer iterates the root list twice, first to pick
  // off immortal immovable roots to make sure they end up on the first page,
  // and then again for the rest.
  if (mode == VISIT_ONLY_STRONG_ROOT_LIST) return;

  isolate_->bootstrapper()->Iterate(v);
  v->Synchronize(VisitorSynchronization::kBootstrapper);
  isolate_->Iterate(v);
  v->Synchronize(VisitorSynchronization::kTop);
  Relocatable::Iterate(isolate_, v);
  v->Synchronize(VisitorSynchronization::kRelocatable);
  isolate_->debug()->Iterate(v);
  v->Synchronize(VisitorSynchronization::kDebug);

  isolate_->compilation_cache()->Iterate(v);
  v->Synchronize(VisitorSynchronization::kCompilationCache);

  // Iterate over local handles in handle scopes.
  FixStaleLeftTrimmedHandlesVisitor left_trim_visitor(this);
  isolate_->handle_scope_implementer()->Iterate(&left_trim_visitor);
  isolate_->handle_scope_implementer()->Iterate(v);
  isolate_->IterateDeferredHandles(v);
  v->Synchronize(VisitorSynchronization::kHandleScope);

  // Iterate over the builtin code objects and code stubs in the
  // heap. Note that it is not necessary to iterate over code objects
  // on scavenge collections.
  if (mode != VISIT_ALL_IN_SCAVENGE) {
    isolate_->builtins()->IterateBuiltins(v);
    v->Synchronize(VisitorSynchronization::kBuiltins);
    isolate_->interpreter()->IterateDispatchTable(v);
    v->Synchronize(VisitorSynchronization::kDispatchTable);
  }

  // Iterate over global handles.
  switch (mode) {
    case VISIT_ONLY_STRONG_ROOT_LIST:
      UNREACHABLE();
      break;
    case VISIT_ONLY_STRONG_FOR_SERIALIZATION:
      break;
    case VISIT_ONLY_STRONG:
      isolate_->global_handles()->IterateStrongRoots(v);
      break;
    case VISIT_ALL_IN_SCAVENGE:
      isolate_->global_handles()->IterateNewSpaceStrongAndDependentRoots(v);
      break;
    case VISIT_ALL_IN_SWEEP_NEWSPACE:
    case VISIT_ALL:
      isolate_->global_handles()->IterateAllRoots(v);
      break;
  }
  v->Synchronize(VisitorSynchronization::kGlobalHandles);

  // Iterate over eternal handles.
  if (mode == VISIT_ALL_IN_SCAVENGE) {
    isolate_->eternal_handles()->IterateNewSpaceRoots(v);
  } else {
    isolate_->eternal_handles()->IterateAllRoots(v);
  }
  v->Synchronize(VisitorSynchronization::kEternalHandles);

  // Iterate over pointers being held by inactive threads.
  isolate_->thread_manager()->Iterate(v);
  v->Synchronize(VisitorSynchronization::kThreadManager);

  // Iterate over other strong roots (currently only identity maps).
  for (StrongRootsList* list = strong_roots_list_; list; list = list->next) {
    v->VisitPointers(list->start, list->end);
  }
  v->Synchronize(VisitorSynchronization::kStrongRoots);

  // Iterate over the partial snapshot cache unless serializing.
  if (mode != VISIT_ONLY_STRONG_FOR_SERIALIZATION) {
    SerializerDeserializer::Iterate(isolate_, v);
  }
  // We don't do a v->Synchronize call here, because in debug mode that will
  // output a flag to the snapshot.  However at this point the serializer and
  // deserializer are deliberately a little unsynchronized (see above) so the
  // checking of the sync flag in the snapshot would fail.
}


// TODO(1236194): Since the heap size is configurable on the command line
// and through the API, we should gracefully handle the case that the heap
// size is not big enough to fit all the initial objects.
bool Heap::ConfigureHeap(size_t max_semi_space_size, size_t max_old_space_size,
                         size_t max_executable_size, size_t code_range_size) {
  if (HasBeenSetUp()) return false;

  // Overwrite default configuration.
  if (max_semi_space_size != 0) {
    max_semi_space_size_ = max_semi_space_size * MB;
  }
  if (max_old_space_size != 0) {
    max_old_generation_size_ = max_old_space_size * MB;
  }
  if (max_executable_size != 0) {
    max_executable_size_ = max_executable_size * MB;
  }

  // If max space size flags are specified overwrite the configuration.
  if (FLAG_max_semi_space_size > 0) {
    max_semi_space_size_ = static_cast<size_t>(FLAG_max_semi_space_size) * MB;
  }
  if (FLAG_max_old_space_size > 0) {
    max_old_generation_size_ =
        static_cast<size_t>(FLAG_max_old_space_size) * MB;
  }
  if (FLAG_max_executable_size > 0) {
    max_executable_size_ = static_cast<size_t>(FLAG_max_executable_size) * MB;
  }

  if (Page::kPageSize > MB) {
    max_semi_space_size_ = ROUND_UP(max_semi_space_size_, Page::kPageSize);
    max_old_generation_size_ =
        ROUND_UP(max_old_generation_size_, Page::kPageSize);
    max_executable_size_ = ROUND_UP(max_executable_size_, Page::kPageSize);
  }

  if (FLAG_stress_compaction) {
    // This will cause more frequent GCs when stressing.
    max_semi_space_size_ = MB;
  }

  // The new space size must be a power of two to support single-bit testing
  // for containment.
  max_semi_space_size_ = base::bits::RoundUpToPowerOfTwo32(
      static_cast<uint32_t>(max_semi_space_size_));

  if (FLAG_min_semi_space_size > 0) {
    size_t initial_semispace_size =
        static_cast<size_t>(FLAG_min_semi_space_size) * MB;
    if (initial_semispace_size > max_semi_space_size_) {
      initial_semispace_size_ = max_semi_space_size_;
      if (FLAG_trace_gc) {
        PrintIsolate(isolate_,
                     "Min semi-space size cannot be more than the maximum "
                     "semi-space size of %" PRIuS " MB\n",
                     max_semi_space_size_ / MB);
      }
    } else {
      initial_semispace_size_ =
          ROUND_UP(initial_semispace_size, Page::kPageSize);
    }
  }

  initial_semispace_size_ = Min(initial_semispace_size_, max_semi_space_size_);

  if (FLAG_semi_space_growth_factor < 2) {
    FLAG_semi_space_growth_factor = 2;
  }

  // The old generation is paged and needs at least one page for each space.
  int paged_space_count = LAST_PAGED_SPACE - FIRST_PAGED_SPACE + 1;
  initial_max_old_generation_size_ = max_old_generation_size_ =
      Max(static_cast<size_t>(paged_space_count * Page::kPageSize),
          max_old_generation_size_);

  // The max executable size must be less than or equal to the max old
  // generation size.
  if (max_executable_size_ > max_old_generation_size_) {
    max_executable_size_ = max_old_generation_size_;
  }

  if (FLAG_initial_old_space_size > 0) {
    initial_old_generation_size_ = FLAG_initial_old_space_size * MB;
  } else {
    initial_old_generation_size_ =
        max_old_generation_size_ / kInitalOldGenerationLimitFactor;
  }
  old_generation_allocation_limit_ = initial_old_generation_size_;

  // We rely on being able to allocate new arrays in paged spaces.
  DCHECK(kMaxRegularHeapObjectSize >=
         (JSArray::kSize +
          FixedArray::SizeFor(JSArray::kInitialMaxFastElementArray) +
          AllocationMemento::kSize));

  code_range_size_ = code_range_size * MB;

  configured_ = true;
  return true;
}


void Heap::AddToRingBuffer(const char* string) {
  size_t first_part =
      Min(strlen(string), kTraceRingBufferSize - ring_buffer_end_);
  memcpy(trace_ring_buffer_ + ring_buffer_end_, string, first_part);
  ring_buffer_end_ += first_part;
  if (first_part < strlen(string)) {
    ring_buffer_full_ = true;
    size_t second_part = strlen(string) - first_part;
    memcpy(trace_ring_buffer_, string + first_part, second_part);
    ring_buffer_end_ = second_part;
  }
}


void Heap::GetFromRingBuffer(char* buffer) {
  size_t copied = 0;
  if (ring_buffer_full_) {
    copied = kTraceRingBufferSize - ring_buffer_end_;
    memcpy(buffer, trace_ring_buffer_ + ring_buffer_end_, copied);
  }
  memcpy(buffer + copied, trace_ring_buffer_, ring_buffer_end_);
}


bool Heap::ConfigureHeapDefault() { return ConfigureHeap(0, 0, 0, 0); }


void Heap::RecordStats(HeapStats* stats, bool take_snapshot) {
  *stats->start_marker = HeapStats::kStartMarker;
  *stats->end_marker = HeapStats::kEndMarker;
  *stats->new_space_size = new_space_->Size();
  *stats->new_space_capacity = new_space_->Capacity();
  *stats->old_space_size = old_space_->SizeOfObjects();
  *stats->old_space_capacity = old_space_->Capacity();
  *stats->code_space_size = code_space_->SizeOfObjects();
  *stats->code_space_capacity = code_space_->Capacity();
  *stats->map_space_size = map_space_->SizeOfObjects();
  *stats->map_space_capacity = map_space_->Capacity();
  *stats->lo_space_size = lo_space_->Size();
  isolate_->global_handles()->RecordStats(stats);
  *stats->memory_allocator_size = memory_allocator()->Size();
  *stats->memory_allocator_capacity =
      memory_allocator()->Size() + memory_allocator()->Available();
  *stats->os_error = base::OS::GetLastError();
  *stats->malloced_memory = isolate_->allocator()->GetCurrentMemoryUsage();
  *stats->malloced_peak_memory = isolate_->allocator()->GetMaxMemoryUsage();
  if (take_snapshot) {
    HeapIterator iterator(this);
    for (HeapObject* obj = iterator.next(); obj != NULL;
         obj = iterator.next()) {
      InstanceType type = obj->map()->instance_type();
      DCHECK(0 <= type && type <= LAST_TYPE);
      stats->objects_per_type[type]++;
      stats->size_per_type[type] += obj->Size();
    }
  }
  if (stats->last_few_messages != NULL)
    GetFromRingBuffer(stats->last_few_messages);
  if (stats->js_stacktrace != NULL) {
    FixedStringAllocator fixed(stats->js_stacktrace, kStacktraceBufferSize - 1);
    StringStream accumulator(&fixed, StringStream::kPrintObjectConcise);
    if (gc_state() == Heap::NOT_IN_GC) {
      isolate()->PrintStack(&accumulator, Isolate::kPrintStackVerbose);
    } else {
      accumulator.Add("Cannot get stack trace in GC.");
    }
  }
}

size_t Heap::PromotedSpaceSizeOfObjects() {
  return old_space_->SizeOfObjects() + code_space_->SizeOfObjects() +
         map_space_->SizeOfObjects() + lo_space_->SizeOfObjects();
}

uint64_t Heap::PromotedExternalMemorySize() {
  if (external_memory_ <= external_memory_at_last_mark_compact_) return 0;
  return static_cast<uint64_t>(external_memory_ -
                               external_memory_at_last_mark_compact_);
}


const double Heap::kMinHeapGrowingFactor = 1.1;
const double Heap::kMaxHeapGrowingFactor = 4.0;
const double Heap::kMaxHeapGrowingFactorMemoryConstrained = 2.0;
const double Heap::kMaxHeapGrowingFactorIdle = 1.5;
const double Heap::kConservativeHeapGrowingFactor = 1.3;
const double Heap::kTargetMutatorUtilization = 0.97;

// Given GC speed in bytes per ms, the allocation throughput in bytes per ms
// (mutator speed), this function returns the heap growing factor that will
// achieve the kTargetMutatorUtilisation if the GC speed and the mutator speed
// remain the same until the next GC.
//
// For a fixed time-frame T = TM + TG, the mutator utilization is the ratio
// TM / (TM + TG), where TM is the time spent in the mutator and TG is the
// time spent in the garbage collector.
//
// Let MU be kTargetMutatorUtilisation, the desired mutator utilization for the
// time-frame from the end of the current GC to the end of the next GC. Based
// on the MU we can compute the heap growing factor F as
//
// F = R * (1 - MU) / (R * (1 - MU) - MU), where R = gc_speed / mutator_speed.
//
// This formula can be derived as follows.
//
// F = Limit / Live by definition, where the Limit is the allocation limit,
// and the Live is size of live objects.
// Let’s assume that we already know the Limit. Then:
//   TG = Limit / gc_speed
//   TM = (TM + TG) * MU, by definition of MU.
//   TM = TG * MU / (1 - MU)
//   TM = Limit *  MU / (gc_speed * (1 - MU))
// On the other hand, if the allocation throughput remains constant:
//   Limit = Live + TM * allocation_throughput = Live + TM * mutator_speed
// Solving it for TM, we get
//   TM = (Limit - Live) / mutator_speed
// Combining the two equation for TM:
//   (Limit - Live) / mutator_speed = Limit * MU / (gc_speed * (1 - MU))
//   (Limit - Live) = Limit * MU * mutator_speed / (gc_speed * (1 - MU))
// substitute R = gc_speed / mutator_speed
//   (Limit - Live) = Limit * MU  / (R * (1 - MU))
// substitute F = Limit / Live
//   F - 1 = F * MU  / (R * (1 - MU))
//   F - F * MU / (R * (1 - MU)) = 1
//   F * (1 - MU / (R * (1 - MU))) = 1
//   F * (R * (1 - MU) - MU) / (R * (1 - MU)) = 1
//   F = R * (1 - MU) / (R * (1 - MU) - MU)
double Heap::HeapGrowingFactor(double gc_speed, double mutator_speed) {
  if (gc_speed == 0 || mutator_speed == 0) return kMaxHeapGrowingFactor;

  const double speed_ratio = gc_speed / mutator_speed;
  const double mu = kTargetMutatorUtilization;

  const double a = speed_ratio * (1 - mu);
  const double b = speed_ratio * (1 - mu) - mu;

  // The factor is a / b, but we need to check for small b first.
  double factor =
      (a < b * kMaxHeapGrowingFactor) ? a / b : kMaxHeapGrowingFactor;
  factor = Min(factor, kMaxHeapGrowingFactor);
  factor = Max(factor, kMinHeapGrowingFactor);
  return factor;
}

size_t Heap::CalculateOldGenerationAllocationLimit(double factor,
                                                   size_t old_gen_size) {
  CHECK(factor > 1.0);
  CHECK(old_gen_size > 0);
  uint64_t limit = static_cast<uint64_t>(old_gen_size * factor);
  limit = Max(limit, static_cast<uint64_t>(old_gen_size) +
                         MinimumAllocationLimitGrowingStep());
  limit += new_space_->Capacity();
  uint64_t halfway_to_the_max =
      (static_cast<uint64_t>(old_gen_size) + max_old_generation_size_) / 2;
  return static_cast<size_t>(Min(limit, halfway_to_the_max));
}

size_t Heap::MinimumAllocationLimitGrowingStep() {
  const size_t kRegularAllocationLimitGrowingStep = 8;
  const size_t kLowMemoryAllocationLimitGrowingStep = 2;
  size_t limit = (Page::kPageSize > MB ? Page::kPageSize : MB);
  return limit * (ShouldOptimizeForMemoryUsage()
                      ? kLowMemoryAllocationLimitGrowingStep
                      : kRegularAllocationLimitGrowingStep);
}

void Heap::SetOldGenerationAllocationLimit(size_t old_gen_size, double gc_speed,
                                           double mutator_speed) {
  double factor = HeapGrowingFactor(gc_speed, mutator_speed);

  if (FLAG_trace_gc_verbose) {
    isolate_->PrintWithTimestamp(
        "Heap growing factor %.1f based on mu=%.3f, speed_ratio=%.f "
        "(gc=%.f, mutator=%.f)\n",
        factor, kTargetMutatorUtilization, gc_speed / mutator_speed, gc_speed,
        mutator_speed);
  }

  if (IsMemoryConstrainedDevice()) {
    factor = Min(factor, kMaxHeapGrowingFactorMemoryConstrained);
  }

  if (memory_reducer_->ShouldGrowHeapSlowly() ||
      ShouldOptimizeForMemoryUsage()) {
    factor = Min(factor, kConservativeHeapGrowingFactor);
  }

  if (FLAG_stress_compaction || ShouldReduceMemory()) {
    factor = kMinHeapGrowingFactor;
  }

  if (FLAG_heap_growing_percent > 0) {
    factor = 1.0 + FLAG_heap_growing_percent / 100.0;
  }

  old_generation_allocation_limit_ =
      CalculateOldGenerationAllocationLimit(factor, old_gen_size);

  if (FLAG_trace_gc_verbose) {
    isolate_->PrintWithTimestamp(
        "Grow: old size: %" PRIuS " KB, new limit: %" PRIuS " KB (%.1f)\n",
        old_gen_size / KB, old_generation_allocation_limit_ / KB, factor);
  }
}

void Heap::DampenOldGenerationAllocationLimit(size_t old_gen_size,
                                              double gc_speed,
                                              double mutator_speed) {
  double factor = HeapGrowingFactor(gc_speed, mutator_speed);
  size_t limit = CalculateOldGenerationAllocationLimit(factor, old_gen_size);
  if (limit < old_generation_allocation_limit_) {
    if (FLAG_trace_gc_verbose) {
      isolate_->PrintWithTimestamp(
          "Dampen: old size: %" PRIuS " KB, old limit: %" PRIuS
          " KB, "
          "new limit: %" PRIuS " KB (%.1f)\n",
          old_gen_size / KB, old_generation_allocation_limit_ / KB, limit / KB,
          factor);
    }
    old_generation_allocation_limit_ = limit;
  }
}

bool Heap::ShouldOptimizeForLoadTime() {
  return isolate()->rail_mode() == PERFORMANCE_LOAD &&
         !AllocationLimitOvershotByLargeMargin() &&
         MonotonicallyIncreasingTimeInMs() <
             isolate()->LoadStartTimeMs() + kMaxLoadTimeMs;
}

// This predicate is called when an old generation space cannot allocated from
// the free list and is about to add a new page. Returning false will cause a
// major GC. It happens when the old generation allocation limit is reached and
// - either we need to optimize for memory usage,
// - or the incremental marking is not in progress and we cannot start it.
bool Heap::ShouldExpandOldGenerationOnSlowAllocation() {
  if (always_allocate() || OldGenerationSpaceAvailable() > 0) return true;
  // We reached the old generation allocation limit.

  if (ShouldOptimizeForMemoryUsage()) return false;

  if (ShouldOptimizeForLoadTime()) return true;

  if (incremental_marking()->NeedsFinalization()) {
    return !AllocationLimitOvershotByLargeMargin();
  }

  if (incremental_marking()->IsStopped() &&
      IncrementalMarkingLimitReached() == IncrementalMarkingLimit::kNoLimit) {
    // We cannot start incremental marking.
    return false;
  }
  return true;
}

// This function returns either kNoLimit, kSoftLimit, or kHardLimit.
// The kNoLimit means that either incremental marking is disabled or it is too
// early to start incremental marking.
// The kSoftLimit means that incremental marking should be started soon.
// The kHardLimit means that incremental marking should be started immediately.
Heap::IncrementalMarkingLimit Heap::IncrementalMarkingLimitReached() {
  if (!incremental_marking()->CanBeActivated() ||
      PromotedSpaceSizeOfObjects() <=
          IncrementalMarking::kActivationThreshold) {
    // Incremental marking is disabled or it is too early to start.
    return IncrementalMarkingLimit::kNoLimit;
  }
  if ((FLAG_stress_compaction && (gc_count_ & 1) != 0) ||
      HighMemoryPressure()) {
    // If there is high memory pressure or stress testing is enabled, then
    // start marking immediately.
    return IncrementalMarkingLimit::kHardLimit;
  }
  size_t old_generation_space_available = OldGenerationSpaceAvailable();
  if (old_generation_space_available > new_space_->Capacity()) {
    return IncrementalMarkingLimit::kNoLimit;
  }
  if (ShouldOptimizeForMemoryUsage()) {
    return IncrementalMarkingLimit::kHardLimit;
  }
  if (ShouldOptimizeForLoadTime()) {
    return IncrementalMarkingLimit::kNoLimit;
  }
  if (old_generation_space_available == 0) {
    return IncrementalMarkingLimit::kHardLimit;
  }
  return IncrementalMarkingLimit::kSoftLimit;
}

void Heap::EnableInlineAllocation() {
  if (!inline_allocation_disabled_) return;
  inline_allocation_disabled_ = false;

  // Update inline allocation limit for new space.
  new_space()->UpdateInlineAllocationLimit(0);
}


void Heap::DisableInlineAllocation() {
  if (inline_allocation_disabled_) return;
  inline_allocation_disabled_ = true;

  // Update inline allocation limit for new space.
  new_space()->UpdateInlineAllocationLimit(0);

  // Update inline allocation limit for old spaces.
  PagedSpaces spaces(this);
  for (PagedSpace* space = spaces.next(); space != NULL;
       space = spaces.next()) {
    space->EmptyAllocationInfo();
  }
}


V8_DECLARE_ONCE(initialize_gc_once);

static void InitializeGCOnce() {
  Scavenger::Initialize();
  StaticScavengeVisitor::Initialize();
  MarkCompactCollector::Initialize();
}


bool Heap::SetUp() {
#ifdef DEBUG
  allocation_timeout_ = FLAG_gc_interval;
#endif

  // Initialize heap spaces and initial maps and objects. Whenever something
  // goes wrong, just return false. The caller should check the results and
  // call Heap::TearDown() to release allocated memory.
  //
  // If the heap is not yet configured (e.g. through the API), configure it.
  // Configuration is based on the flags new-space-size (really the semispace
  // size) and old-space-size if set or the initial values of semispace_size_
  // and old_generation_size_ otherwise.
  if (!configured_) {
    if (!ConfigureHeapDefault()) return false;
  }

  base::CallOnce(&initialize_gc_once, &InitializeGCOnce);

  // Set up memory allocator.
  memory_allocator_ = new MemoryAllocator(isolate_);
  if (!memory_allocator_->SetUp(MaxReserved(), MaxExecutableSize(),
                                code_range_size_))
    return false;

  store_buffer_ = new StoreBuffer(this);

  incremental_marking_ = new IncrementalMarking(this);

  concurrent_marking_ = new ConcurrentMarking(this);

  for (int i = 0; i <= LAST_SPACE; i++) {
    space_[i] = nullptr;
  }

  space_[NEW_SPACE] = new_space_ = new NewSpace(this);
  if (!new_space_->SetUp(initial_semispace_size_, max_semi_space_size_)) {
    return false;
  }
  new_space_top_after_last_gc_ = new_space()->top();

  space_[OLD_SPACE] = old_space_ =
      new OldSpace(this, OLD_SPACE, NOT_EXECUTABLE);
  if (!old_space_->SetUp()) return false;

  space_[CODE_SPACE] = code_space_ = new OldSpace(this, CODE_SPACE, EXECUTABLE);
  if (!code_space_->SetUp()) return false;

  space_[MAP_SPACE] = map_space_ = new MapSpace(this, MAP_SPACE);
  if (!map_space_->SetUp()) return false;

  // The large object code space may contain code or data.  We set the memory
  // to be non-executable here for safety, but this means we need to enable it
  // explicitly when allocating large code objects.
  space_[LO_SPACE] = lo_space_ = new LargeObjectSpace(this, LO_SPACE);
  if (!lo_space_->SetUp()) return false;

  // Set up the seed that is used to randomize the string hash function.
  DCHECK(hash_seed() == 0);
  if (FLAG_randomize_hashes) {
    if (FLAG_hash_seed == 0) {
      int rnd = isolate()->random_number_generator()->NextInt();
      set_hash_seed(Smi::FromInt(rnd & Name::kHashBitMask));
    } else {
      set_hash_seed(Smi::FromInt(FLAG_hash_seed));
    }
  }

  for (int i = 0; i < static_cast<int>(v8::Isolate::kUseCounterFeatureCount);
       i++) {
    deferred_counters_[i] = 0;
  }

  tracer_ = new GCTracer(this);
  scavenge_collector_ = new Scavenger(this);
  mark_compact_collector_ = new MarkCompactCollector(this);
  gc_idle_time_handler_ = new GCIdleTimeHandler();
  memory_reducer_ = new MemoryReducer(this);
  if (V8_UNLIKELY(FLAG_gc_stats)) {
    live_object_stats_ = new ObjectStats(this);
    dead_object_stats_ = new ObjectStats(this);
  }
  scavenge_job_ = new ScavengeJob();
  local_embedder_heap_tracer_ = new LocalEmbedderHeapTracer();

  LOG(isolate_, IntPtrTEvent("heap-capacity", Capacity()));
  LOG(isolate_, IntPtrTEvent("heap-available", Available()));

  store_buffer()->SetUp();

  mark_compact_collector()->SetUp();

  idle_scavenge_observer_ = new IdleScavengeObserver(
      *this, ScavengeJob::kBytesAllocatedBeforeNextIdleTask);
  new_space()->AddAllocationObserver(idle_scavenge_observer_);

  return true;
}


bool Heap::CreateHeapObjects() {
  // Create initial maps.
  if (!CreateInitialMaps()) return false;
  if (!CreateApiObjects()) return false;

  // Create initial objects
  CreateInitialObjects();
  CHECK_EQ(0u, gc_count_);

  set_native_contexts_list(undefined_value());
  set_allocation_sites_list(undefined_value());

  return true;
}


void Heap::SetStackLimits() {
  DCHECK(isolate_ != NULL);
  DCHECK(isolate_ == isolate());
  // On 64 bit machines, pointers are generally out of range of Smis.  We write
  // something that looks like an out of range Smi to the GC.

  // Set up the special root array entries containing the stack limits.
  // These are actually addresses, but the tag makes the GC ignore it.
  roots_[kStackLimitRootIndex] = reinterpret_cast<Object*>(
      (isolate_->stack_guard()->jslimit() & ~kSmiTagMask) | kSmiTag);
  roots_[kRealStackLimitRootIndex] = reinterpret_cast<Object*>(
      (isolate_->stack_guard()->real_jslimit() & ~kSmiTagMask) | kSmiTag);
}

void Heap::ClearStackLimits() {
  roots_[kStackLimitRootIndex] = Smi::kZero;
  roots_[kRealStackLimitRootIndex] = Smi::kZero;
}

void Heap::PrintAlloctionsHash() {
  uint32_t hash = StringHasher::GetHashCore(raw_allocations_hash_);
  PrintF("\n### Allocations = %u, hash = 0x%08x\n", allocations_count(), hash);
}


void Heap::NotifyDeserializationComplete() {
  DCHECK_EQ(0, gc_count());
  PagedSpaces spaces(this);
  for (PagedSpace* s = spaces.next(); s != NULL; s = spaces.next()) {
    if (isolate()->snapshot_available()) s->ShrinkImmortalImmovablePages();
#ifdef DEBUG
    // All pages right after bootstrapping must be marked as never-evacuate.
    for (Page* p : *s) {
      CHECK(p->NeverEvacuate());
    }
#endif  // DEBUG
  }

  deserialization_complete_ = true;
}

void Heap::SetEmbedderHeapTracer(EmbedderHeapTracer* tracer) {
  DCHECK_EQ(gc_state_, HeapState::NOT_IN_GC);
  local_embedder_heap_tracer()->SetRemoteTracer(tracer);
}

void Heap::TracePossibleWrapper(JSObject* js_object) {
  DCHECK(js_object->WasConstructedFromApiFunction());
  if (js_object->GetInternalFieldCount() >= 2 &&
      js_object->GetInternalField(0) &&
      js_object->GetInternalField(0) != undefined_value() &&
      js_object->GetInternalField(1) != undefined_value()) {
    DCHECK(reinterpret_cast<intptr_t>(js_object->GetInternalField(0)) % 2 == 0);
    local_embedder_heap_tracer()->AddWrapperToTrace(std::pair<void*, void*>(
        reinterpret_cast<void*>(js_object->GetInternalField(0)),
        reinterpret_cast<void*>(js_object->GetInternalField(1))));
  }
}

void Heap::RegisterExternallyReferencedObject(Object** object) {
  HeapObject* heap_object = HeapObject::cast(*object);
  DCHECK(Contains(heap_object));
  if (FLAG_incremental_marking_wrappers && incremental_marking()->IsMarking()) {
    IncrementalMarking::MarkGrey(this, heap_object);
  } else {
    DCHECK(mark_compact_collector()->in_use());
    mark_compact_collector()->MarkObject(heap_object);
  }
}

void Heap::TearDown() {
#ifdef VERIFY_HEAP
  if (FLAG_verify_heap) {
    Verify();
  }
#endif

  UpdateMaximumCommitted();

  if (FLAG_verify_predictable) {
    PrintAlloctionsHash();
  }

  new_space()->RemoveAllocationObserver(idle_scavenge_observer_);
  delete idle_scavenge_observer_;
  idle_scavenge_observer_ = nullptr;

  delete scavenge_collector_;
  scavenge_collector_ = nullptr;

  if (mark_compact_collector_ != nullptr) {
    mark_compact_collector_->TearDown();
    delete mark_compact_collector_;
    mark_compact_collector_ = nullptr;
  }

  delete incremental_marking_;
  incremental_marking_ = nullptr;

  delete concurrent_marking_;
  concurrent_marking_ = nullptr;

  delete gc_idle_time_handler_;
  gc_idle_time_handler_ = nullptr;

  if (memory_reducer_ != nullptr) {
    memory_reducer_->TearDown();
    delete memory_reducer_;
    memory_reducer_ = nullptr;
  }

  if (live_object_stats_ != nullptr) {
    delete live_object_stats_;
    live_object_stats_ = nullptr;
  }

  if (dead_object_stats_ != nullptr) {
    delete dead_object_stats_;
    dead_object_stats_ = nullptr;
  }

  delete local_embedder_heap_tracer_;
  local_embedder_heap_tracer_ = nullptr;

  delete scavenge_job_;
  scavenge_job_ = nullptr;

  isolate_->global_handles()->TearDown();

  external_string_table_.TearDown();

  delete tracer_;
  tracer_ = nullptr;

  new_space_->TearDown();
  delete new_space_;
  new_space_ = nullptr;

  if (old_space_ != NULL) {
    delete old_space_;
    old_space_ = NULL;
  }

  if (code_space_ != NULL) {
    delete code_space_;
    code_space_ = NULL;
  }

  if (map_space_ != NULL) {
    delete map_space_;
    map_space_ = NULL;
  }

  if (lo_space_ != NULL) {
    lo_space_->TearDown();
    delete lo_space_;
    lo_space_ = NULL;
  }

  store_buffer()->TearDown();

  memory_allocator()->TearDown();

  StrongRootsList* next = NULL;
  for (StrongRootsList* list = strong_roots_list_; list; list = next) {
    next = list->next;
    delete list;
  }
  strong_roots_list_ = NULL;

  delete store_buffer_;
  store_buffer_ = nullptr;

  delete memory_allocator_;
  memory_allocator_ = nullptr;
}


void Heap::AddGCPrologueCallback(v8::Isolate::GCCallback callback,
                                 GCType gc_type, bool pass_isolate) {
  DCHECK(callback != NULL);
  GCCallbackPair pair(callback, gc_type, pass_isolate);
  DCHECK(!gc_prologue_callbacks_.Contains(pair));
  return gc_prologue_callbacks_.Add(pair);
}


void Heap::RemoveGCPrologueCallback(v8::Isolate::GCCallback callback) {
  DCHECK(callback != NULL);
  for (int i = 0; i < gc_prologue_callbacks_.length(); ++i) {
    if (gc_prologue_callbacks_[i].callback == callback) {
      gc_prologue_callbacks_.Remove(i);
      return;
    }
  }
  UNREACHABLE();
}


void Heap::AddGCEpilogueCallback(v8::Isolate::GCCallback callback,
                                 GCType gc_type, bool pass_isolate) {
  DCHECK(callback != NULL);
  GCCallbackPair pair(callback, gc_type, pass_isolate);
  DCHECK(!gc_epilogue_callbacks_.Contains(pair));
  return gc_epilogue_callbacks_.Add(pair);
}


void Heap::RemoveGCEpilogueCallback(v8::Isolate::GCCallback callback) {
  DCHECK(callback != NULL);
  for (int i = 0; i < gc_epilogue_callbacks_.length(); ++i) {
    if (gc_epilogue_callbacks_[i].callback == callback) {
      gc_epilogue_callbacks_.Remove(i);
      return;
    }
  }
  UNREACHABLE();
}

// TODO(ishell): Find a better place for this.
void Heap::AddWeakNewSpaceObjectToCodeDependency(Handle<HeapObject> obj,
                                                 Handle<WeakCell> code) {
  DCHECK(InNewSpace(*obj));
  DCHECK(!InNewSpace(*code));
  Handle<ArrayList> list(weak_new_space_object_to_code_list(), isolate());
  list = ArrayList::Add(list, isolate()->factory()->NewWeakCell(obj), code);
  if (*list != weak_new_space_object_to_code_list()) {
    set_weak_new_space_object_to_code_list(*list);
  }
}

// TODO(ishell): Find a better place for this.
void Heap::AddWeakObjectToCodeDependency(Handle<HeapObject> obj,
                                         Handle<DependentCode> dep) {
  DCHECK(!InNewSpace(*obj));
  DCHECK(!InNewSpace(*dep));
  Handle<WeakHashTable> table(weak_object_to_code_table(), isolate());
  table = WeakHashTable::Put(table, obj, dep);
  if (*table != weak_object_to_code_table())
    set_weak_object_to_code_table(*table);
  DCHECK_EQ(*dep, LookupWeakObjectToCodeDependency(obj));
}


DependentCode* Heap::LookupWeakObjectToCodeDependency(Handle<HeapObject> obj) {
  Object* dep = weak_object_to_code_table()->Lookup(obj);
  if (dep->IsDependentCode()) return DependentCode::cast(dep);
  return DependentCode::cast(empty_fixed_array());
}

namespace {
void CompactWeakFixedArray(Object* object) {
  if (object->IsWeakFixedArray()) {
    WeakFixedArray* array = WeakFixedArray::cast(object);
    array->Compact<WeakFixedArray::NullCallback>();
  }
}
}  // anonymous namespace

void Heap::CompactWeakFixedArrays() {
  // Find known WeakFixedArrays and compact them.
  HeapIterator iterator(this);
  for (HeapObject* o = iterator.next(); o != NULL; o = iterator.next()) {
    if (o->IsPrototypeInfo()) {
      Object* prototype_users = PrototypeInfo::cast(o)->prototype_users();
      if (prototype_users->IsWeakFixedArray()) {
        WeakFixedArray* array = WeakFixedArray::cast(prototype_users);
        array->Compact<JSObject::PrototypeRegistryCompactionCallback>();
      }
    }
  }
  CompactWeakFixedArray(noscript_shared_function_infos());
  CompactWeakFixedArray(script_list());
  CompactWeakFixedArray(weak_stack_trace_list());
}

void Heap::AddRetainedMap(Handle<Map> map) {
  Handle<WeakCell> cell = Map::WeakCellForMap(map);
  Handle<ArrayList> array(retained_maps(), isolate());
  if (array->IsFull()) {
    CompactRetainedMaps(*array);
  }
  array = ArrayList::Add(
      array, cell, handle(Smi::FromInt(FLAG_retain_maps_for_n_gc), isolate()),
      ArrayList::kReloadLengthAfterAllocation);
  if (*array != retained_maps()) {
    set_retained_maps(*array);
  }
}


void Heap::CompactRetainedMaps(ArrayList* retained_maps) {
  DCHECK_EQ(retained_maps, this->retained_maps());
  int length = retained_maps->Length();
  int new_length = 0;
  int new_number_of_disposed_maps = 0;
  // This loop compacts the array by removing cleared weak cells.
  for (int i = 0; i < length; i += 2) {
    DCHECK(retained_maps->Get(i)->IsWeakCell());
    WeakCell* cell = WeakCell::cast(retained_maps->Get(i));
    Object* age = retained_maps->Get(i + 1);
    if (cell->cleared()) continue;
    if (i != new_length) {
      retained_maps->Set(new_length, cell);
      retained_maps->Set(new_length + 1, age);
    }
    if (i < number_of_disposed_maps_) {
      new_number_of_disposed_maps += 2;
    }
    new_length += 2;
  }
  number_of_disposed_maps_ = new_number_of_disposed_maps;
  Object* undefined = undefined_value();
  for (int i = new_length; i < length; i++) {
    retained_maps->Clear(i, undefined);
  }
  if (new_length != length) retained_maps->SetLength(new_length);
}

void Heap::FatalProcessOutOfMemory(const char* location, bool is_heap_oom) {
  v8::internal::V8::FatalProcessOutOfMemory(location, is_heap_oom);
}

#ifdef DEBUG

class PrintHandleVisitor : public ObjectVisitor {
 public:
  void VisitPointers(Object** start, Object** end) override {
    for (Object** p = start; p < end; p++)
      PrintF("  handle %p to %p\n", reinterpret_cast<void*>(p),
             reinterpret_cast<void*>(*p));
  }
};


void Heap::PrintHandles() {
  PrintF("Handles:\n");
  PrintHandleVisitor v;
  isolate_->handle_scope_implementer()->Iterate(&v);
}

#endif

class CheckHandleCountVisitor : public ObjectVisitor {
 public:
  CheckHandleCountVisitor() : handle_count_(0) {}
  ~CheckHandleCountVisitor() override {
    CHECK(handle_count_ < HandleScope::kCheckHandleThreshold);
  }
  void VisitPointers(Object** start, Object** end) override {
    handle_count_ += end - start;
  }

 private:
  ptrdiff_t handle_count_;
};


void Heap::CheckHandleCount() {
  CheckHandleCountVisitor v;
  isolate_->handle_scope_implementer()->Iterate(&v);
}

void Heap::ClearRecordedSlot(HeapObject* object, Object** slot) {
  if (!InNewSpace(object)) {
    Address slot_addr = reinterpret_cast<Address>(slot);
    Page* page = Page::FromAddress(slot_addr);
    DCHECK_EQ(page->owner()->identity(), OLD_SPACE);
    store_buffer()->DeleteEntry(slot_addr);
    RememberedSet<OLD_TO_OLD>::Remove(page, slot_addr);
  }
}

bool Heap::HasRecordedSlot(HeapObject* object, Object** slot) {
  if (InNewSpace(object)) {
    return false;
  }
  Address slot_addr = reinterpret_cast<Address>(slot);
  Page* page = Page::FromAddress(slot_addr);
  DCHECK_EQ(page->owner()->identity(), OLD_SPACE);
  store_buffer()->MoveAllEntriesToRememberedSet();
  return RememberedSet<OLD_TO_NEW>::Contains(page, slot_addr) ||
         RememberedSet<OLD_TO_OLD>::Contains(page, slot_addr);
}

void Heap::ClearRecordedSlotRange(Address start, Address end) {
  Page* page = Page::FromAddress(start);
  if (!page->InNewSpace()) {
    DCHECK_EQ(page->owner()->identity(), OLD_SPACE);
    store_buffer()->DeleteEntry(start, end);
    RememberedSet<OLD_TO_OLD>::RemoveRange(page, start, end,
                                           SlotSet::FREE_EMPTY_BUCKETS);
  }
}

void Heap::RecordWriteIntoCodeSlow(Code* host, RelocInfo* rinfo,
                                   Object* value) {
  DCHECK(InNewSpace(value));
  Page* source_page = Page::FromAddress(reinterpret_cast<Address>(host));
  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::IsCodeTarget(rmode)) {
      slot_type = CODE_ENTRY_SLOT;
    } else {
      DCHECK(RelocInfo::IsEmbeddedObject(rmode));
      slot_type = OBJECT_SLOT;
    }
  }
  RememberedSet<OLD_TO_NEW>::InsertTyped(
      source_page, reinterpret_cast<Address>(host), slot_type, addr);
}

void Heap::RecordWritesIntoCode(Code* code) {
  for (RelocIterator it(code, RelocInfo::ModeMask(RelocInfo::EMBEDDED_OBJECT));
       !it.done(); it.next()) {
    RecordWriteIntoCode(code, it.rinfo(), it.rinfo()->target_object());
  }
}

Space* AllSpaces::next() {
  switch (counter_++) {
    case NEW_SPACE:
      return heap_->new_space();
    case OLD_SPACE:
      return heap_->old_space();
    case CODE_SPACE:
      return heap_->code_space();
    case MAP_SPACE:
      return heap_->map_space();
    case LO_SPACE:
      return heap_->lo_space();
    default:
      return NULL;
  }
}

PagedSpace* PagedSpaces::next() {
  switch (counter_++) {
    case OLD_SPACE:
      return heap_->old_space();
    case CODE_SPACE:
      return heap_->code_space();
    case MAP_SPACE:
      return heap_->map_space();
    default:
      return NULL;
  }
}


OldSpace* OldSpaces::next() {
  switch (counter_++) {
    case OLD_SPACE:
      return heap_->old_space();
    case CODE_SPACE:
      return heap_->code_space();
    default:
      return NULL;
  }
}

SpaceIterator::SpaceIterator(Heap* heap)
    : heap_(heap), current_space_(FIRST_SPACE - 1) {}

SpaceIterator::~SpaceIterator() {
}


bool SpaceIterator::has_next() {
  // Iterate until no more spaces.
  return current_space_ != LAST_SPACE;
}

Space* SpaceIterator::next() {
  DCHECK(has_next());
  return heap_->space(++current_space_);
}


class HeapObjectsFilter {
 public:
  virtual ~HeapObjectsFilter() {}
  virtual bool SkipObject(HeapObject* object) = 0;
};


class UnreachableObjectsFilter : public HeapObjectsFilter {
 public:
  explicit UnreachableObjectsFilter(Heap* heap) : heap_(heap) {
    MarkReachableObjects();
  }

  ~UnreachableObjectsFilter() {
    heap_->mark_compact_collector()->ClearMarkbits();
  }

  bool SkipObject(HeapObject* object) {
    if (object->IsFiller()) return true;
    return ObjectMarking::IsWhite(object);
  }

 private:
  class MarkingVisitor : public ObjectVisitor {
   public:
    MarkingVisitor() : marking_stack_(10) {}

    void VisitPointers(Object** start, Object** end) override {
      for (Object** p = start; p < end; p++) {
        if (!(*p)->IsHeapObject()) continue;
        HeapObject* obj = HeapObject::cast(*p);
        // Use Marking instead of ObjectMarking to avoid adjusting live bytes
        // counter.
        MarkBit mark_bit = ObjectMarking::MarkBitFrom(obj);
        if (Marking::IsWhite(mark_bit)) {
          Marking::WhiteToBlack(mark_bit);
          marking_stack_.Add(obj);
        }
      }
    }

    void TransitiveClosure() {
      while (!marking_stack_.is_empty()) {
        HeapObject* obj = marking_stack_.RemoveLast();
        obj->Iterate(this);
      }
    }

   private:
    List<HeapObject*> marking_stack_;
  };

  void MarkReachableObjects() {
    MarkingVisitor visitor;
    heap_->IterateRoots(&visitor, VISIT_ALL);
    visitor.TransitiveClosure();
  }

  Heap* heap_;
  DisallowHeapAllocation no_allocation_;
};

HeapIterator::HeapIterator(Heap* heap,
                           HeapIterator::HeapObjectsFiltering filtering)
    : no_heap_allocation_(),
      heap_(heap),
      filtering_(filtering),
      filter_(nullptr),
      space_iterator_(nullptr),
      object_iterator_(nullptr) {
  heap_->MakeHeapIterable();
  heap_->heap_iterator_start();
  // Start the iteration.
  space_iterator_ = new SpaceIterator(heap_);
  switch (filtering_) {
    case kFilterUnreachable:
      filter_ = new UnreachableObjectsFilter(heap_);
      break;
    default:
      break;
  }
  object_iterator_ = space_iterator_->next()->GetObjectIterator();
}


HeapIterator::~HeapIterator() {
  heap_->heap_iterator_end();
#ifdef DEBUG
  // Assert that in filtering mode we have iterated through all
  // objects. Otherwise, heap will be left in an inconsistent state.
  if (filtering_ != kNoFiltering) {
    DCHECK(object_iterator_ == nullptr);
  }
#endif
  delete space_iterator_;
  delete filter_;
}


HeapObject* HeapIterator::next() {
  if (filter_ == nullptr) return NextObject();

  HeapObject* obj = NextObject();
  while ((obj != nullptr) && (filter_->SkipObject(obj))) obj = NextObject();
  return obj;
}


HeapObject* HeapIterator::NextObject() {
  // No iterator means we are done.
  if (object_iterator_.get() == nullptr) return nullptr;

  if (HeapObject* obj = object_iterator_.get()->Next()) {
    // If the current iterator has more objects we are fine.
    return obj;
  } else {
    // Go though the spaces looking for one that has objects.
    while (space_iterator_->has_next()) {
      object_iterator_ = space_iterator_->next()->GetObjectIterator();
      if (HeapObject* obj = object_iterator_.get()->Next()) {
        return obj;
      }
    }
  }
  // Done with the last space.
  object_iterator_.reset(nullptr);
  return nullptr;
}


void Heap::UpdateTotalGCTime(double duration) {
  if (FLAG_trace_gc_verbose) {
    total_gc_time_ms_ += duration;
  }
}

void Heap::ExternalStringTable::CleanUpNewSpaceStrings() {
  int last = 0;
  Isolate* isolate = heap_->isolate();
  for (int i = 0; i < new_space_strings_.length(); ++i) {
    Object* o = new_space_strings_[i];
    if (o->IsTheHole(isolate)) {
      continue;
    }
    if (o->IsThinString()) {
      o = ThinString::cast(o)->actual();
      if (!o->IsExternalString()) continue;
    }
    DCHECK(o->IsExternalString());
    if (heap_->InNewSpace(o)) {
      new_space_strings_[last++] = o;
    } else {
      old_space_strings_.Add(o);
    }
  }
  new_space_strings_.Rewind(last);
  new_space_strings_.Trim();
}

void Heap::ExternalStringTable::CleanUpAll() {
  CleanUpNewSpaceStrings();
  int last = 0;
  Isolate* isolate = heap_->isolate();
  for (int i = 0; i < old_space_strings_.length(); ++i) {
    Object* o = old_space_strings_[i];
    if (o->IsTheHole(isolate)) {
      continue;
    }
    if (o->IsThinString()) {
      o = ThinString::cast(o)->actual();
      if (!o->IsExternalString()) continue;
    }
    DCHECK(o->IsExternalString());
    DCHECK(!heap_->InNewSpace(o));
    old_space_strings_[last++] = o;
  }
  old_space_strings_.Rewind(last);
  old_space_strings_.Trim();
#ifdef VERIFY_HEAP
  if (FLAG_verify_heap) {
    Verify();
  }
#endif
}

void Heap::ExternalStringTable::TearDown() {
  for (int i = 0; i < new_space_strings_.length(); ++i) {
    Object* o = new_space_strings_[i];
    if (o->IsThinString()) {
      o = ThinString::cast(o)->actual();
      if (!o->IsExternalString()) continue;
    }
    heap_->FinalizeExternalString(ExternalString::cast(o));
  }
  new_space_strings_.Free();
  for (int i = 0; i < old_space_strings_.length(); ++i) {
    Object* o = old_space_strings_[i];
    if (o->IsThinString()) {
      o = ThinString::cast(o)->actual();
      if (!o->IsExternalString()) continue;
    }
    heap_->FinalizeExternalString(ExternalString::cast(o));
  }
  old_space_strings_.Free();
}


void Heap::RememberUnmappedPage(Address page, bool compacted) {
  uintptr_t p = reinterpret_cast<uintptr_t>(page);
  // Tag the page pointer to make it findable in the dump file.
  if (compacted) {
    p ^= 0xc1ead & (Page::kPageSize - 1);  // Cleared.
  } else {
    p ^= 0x1d1ed & (Page::kPageSize - 1);  // I died.
  }
  remembered_unmapped_pages_[remembered_unmapped_pages_index_] =
      reinterpret_cast<Address>(p);
  remembered_unmapped_pages_index_++;
  remembered_unmapped_pages_index_ %= kRememberedUnmappedPages;
}


void Heap::RegisterStrongRoots(Object** start, Object** end) {
  StrongRootsList* list = new StrongRootsList();
  list->next = strong_roots_list_;
  list->start = start;
  list->end = end;
  strong_roots_list_ = list;
}


void Heap::UnregisterStrongRoots(Object** start) {
  StrongRootsList* prev = NULL;
  StrongRootsList* list = strong_roots_list_;
  while (list != nullptr) {
    StrongRootsList* next = list->next;
    if (list->start == start) {
      if (prev) {
        prev->next = next;
      } else {
        strong_roots_list_ = next;
      }
      delete list;
    } else {
      prev = list;
    }
    list = next;
  }
}


size_t Heap::NumberOfTrackedHeapObjectTypes() {
  return ObjectStats::OBJECT_STATS_COUNT;
}


size_t Heap::ObjectCountAtLastGC(size_t index) {
  if (live_object_stats_ == nullptr || index >= ObjectStats::OBJECT_STATS_COUNT)
    return 0;
  return live_object_stats_->object_count_last_gc(index);
}


size_t Heap::ObjectSizeAtLastGC(size_t index) {
  if (live_object_stats_ == nullptr || index >= ObjectStats::OBJECT_STATS_COUNT)
    return 0;
  return live_object_stats_->object_size_last_gc(index);
}


bool Heap::GetObjectTypeName(size_t index, const char** object_type,
                             const char** object_sub_type) {
  if (index >= ObjectStats::OBJECT_STATS_COUNT) return false;

  switch (static_cast<int>(index)) {
#define COMPARE_AND_RETURN_NAME(name) \
  case name:                          \
    *object_type = #name;             \
    *object_sub_type = "";            \
    return true;
    INSTANCE_TYPE_LIST(COMPARE_AND_RETURN_NAME)
#undef COMPARE_AND_RETURN_NAME
#define COMPARE_AND_RETURN_NAME(name)                      \
  case ObjectStats::FIRST_CODE_KIND_SUB_TYPE + Code::name: \
    *object_type = "CODE_TYPE";                            \
    *object_sub_type = "CODE_KIND/" #name;                 \
    return true;
    CODE_KIND_LIST(COMPARE_AND_RETURN_NAME)
#undef COMPARE_AND_RETURN_NAME
#define COMPARE_AND_RETURN_NAME(name)                  \
  case ObjectStats::FIRST_FIXED_ARRAY_SUB_TYPE + name: \
    *object_type = "FIXED_ARRAY_TYPE";                 \
    *object_sub_type = #name;                          \
    return true;
    FIXED_ARRAY_SUB_INSTANCE_TYPE_LIST(COMPARE_AND_RETURN_NAME)
#undef COMPARE_AND_RETURN_NAME
#define COMPARE_AND_RETURN_NAME(name)                                  \
  case ObjectStats::FIRST_CODE_AGE_SUB_TYPE + Code::k##name##CodeAge - \
      Code::kFirstCodeAge:                                             \
    *object_type = "CODE_TYPE";                                        \
    *object_sub_type = "CODE_AGE/" #name;                              \
    return true;
    CODE_AGE_LIST_COMPLETE(COMPARE_AND_RETURN_NAME)
#undef COMPARE_AND_RETURN_NAME
  }
  return false;
}


// static
int Heap::GetStaticVisitorIdForMap(Map* map) {
  return StaticVisitorBase::GetVisitorId(map);
}

}  // namespace internal
}  // namespace v8