text: // 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.
  
  #ifndef V8_HEAP_HEAP_H_
  #define V8_HEAP_HEAP_H_
  
  #include <cmath>
  #include <map>
  #include <unordered_map>
  #include <unordered_set>
  #include <vector>
  
  // Clients of this interface shouldn't depend on lots of heap internals.
  // Do not include anything from src/heap here!
  #include "include/v8-internal.h"
  #include "include/v8.h"
  #include "src/accessors.h"
  #include "src/allocation.h"
  #include "src/assert-scope.h"
  #include "src/base/atomic-utils.h"
  #include "src/globals.h"
  #include "src/heap-symbols.h"
  #include "src/objects.h"
  #include "src/objects/fixed-array.h"
  #include "src/objects/heap-object.h"
  #include "src/objects/smi.h"
  #include "src/objects/string-table.h"
  #include "src/visitors.h"
  
  namespace v8 {
  
  namespace debug {
  typedef void (*OutOfMemoryCallback)(void* data);
  }  // namespace debug
  
  namespace internal {
  
  namespace heap {
  class HeapTester;
  class TestMemoryAllocatorScope;
  }  // namespace heap
  
  class AllocationMemento;
  class ObjectBoilerplateDescription;
  class BytecodeArray;
  class CodeDataContainer;
  class DeoptimizationData;
  class HandlerTable;
  class IncrementalMarking;
  class JSArrayBuffer;
  class ExternalString;
  using v8::MemoryPressureLevel;
  
  class AllocationObserver;
  class ArrayBufferCollector;
  class ArrayBufferTracker;
  class ConcurrentMarking;
  class GCIdleTimeAction;
  class GCIdleTimeHandler;
  class GCIdleTimeHeapState;
  class GCTracer;
  class HeapController;
  class HeapObjectAllocationTracker;
  class HeapObjectPtr;
  class HeapObjectsFilter;
  class HeapStats;
  class HistogramTimer;
  class Isolate;
  class JSWeakFactory;
  class LocalEmbedderHeapTracer;
  class MemoryAllocator;
  class MemoryReducer;
  class MinorMarkCompactCollector;
  class ObjectIterator;
  class ObjectStats;
  class Page;
  class PagedSpace;
  class RootVisitor;
  class ScavengeJob;
  class Scavenger;
  class ScavengerCollector;
  class Space;
  class StoreBuffer;
  class StressScavengeObserver;
  class TimedHistogram;
  class TracePossibleWrapperReporter;
  class WeakObjectRetainer;
  
  enum ArrayStorageAllocationMode {
    DONT_INITIALIZE_ARRAY_ELEMENTS,
    INITIALIZE_ARRAY_ELEMENTS_WITH_HOLE
  };
  
  enum class ClearRecordedSlots { kYes, kNo };
  
  enum class ClearFreedMemoryMode { kClearFreedMemory, kDontClearFreedMemory };
  
  enum ExternalBackingStoreType { kArrayBuffer, kExternalString, kNumTypes };
  
  enum class FixedArrayVisitationMode { kRegular, kIncremental };
  
  enum class TraceRetainingPathMode { kEnabled, kDisabled };
  
  enum class RetainingPathOption { kDefault, kTrackEphemeronPath };
  
  enum class GarbageCollectionReason {
    kUnknown = 0,
    kAllocationFailure = 1,
    kAllocationLimit = 2,
    kContextDisposal = 3,
    kCountersExtension = 4,
    kDebugger = 5,
    kDeserializer = 6,
    kExternalMemoryPressure = 7,
    kFinalizeMarkingViaStackGuard = 8,
    kFinalizeMarkingViaTask = 9,
    kFullHashtable = 10,
    kHeapProfiler = 11,
    kIdleTask = 12,
    kLastResort = 13,
    kLowMemoryNotification = 14,
    kMakeHeapIterable = 15,
    kMemoryPressure = 16,
    kMemoryReducer = 17,
    kRuntime = 18,
    kSamplingProfiler = 19,
    kSnapshotCreator = 20,
    kTesting = 21,
    kExternalFinalize = 22
    // If you add new items here, then update the incremental_marking_reason,
    // mark_compact_reason, and scavenge_reason counters in counters.h.
    // Also update src/tools/metrics/histograms/histograms.xml in chromium.
  };
  
  enum class YoungGenerationHandling {
    kRegularScavenge = 0,
    kFastPromotionDuringScavenge = 1,
    // Histogram::InspectConstructionArguments in chromium requires us to have at
    // least three buckets.
    kUnusedBucket = 2,
    // If you add new items here, then update the young_generation_handling in
    // counters.h.
    // Also update src/tools/metrics/histograms/histograms.xml in chromium.
  };
  
  class AllocationResult {
   public:
    static inline AllocationResult Retry(AllocationSpace space = NEW_SPACE) {
      return AllocationResult(space);
    }
  
    // Implicit constructor from Object*.
    // TODO(3770): This constructor should go away eventually, replaced by
    // the ObjectPtr alternative below.
    AllocationResult(Object* object)  // NOLINT
        : object_(ObjectPtr(object->ptr())) {
      // AllocationResults can't return Smis, which are used to represent
      // failure and the space to retry in.
      CHECK(!object->IsSmi());
    }
  
    AllocationResult(ObjectPtr object)  // NOLINT
        : object_(object) {
      // AllocationResults can't return Smis, which are used to represent
      // failure and the space to retry in.
      CHECK(!object->IsSmi());
    }
  
    AllocationResult() : object_(Smi::FromInt(NEW_SPACE)) {}
  
    inline bool IsRetry() { return object_->IsSmi(); }
    inline HeapObject* ToObjectChecked();
    inline AllocationSpace RetrySpace();
  
    template <typename T, typename = typename std::enable_if<
                              std::is_base_of<Object, T>::value>::type>
    bool To(T** obj) {
      if (IsRetry()) return false;
      *obj = T::cast(object_);
      return true;
    }
  
    template <typename T, typename = typename std::enable_if<
                              std::is_base_of<ObjectPtr, T>::value>::type>
    bool To(T* obj) {
      if (IsRetry()) return false;
      *obj = T::cast(object_);
      return true;
    }
  
   private:
    explicit AllocationResult(AllocationSpace space)
        : object_(Smi::FromInt(static_cast<int>(space))) {}
  
    ObjectPtr object_;
  };
  
  STATIC_ASSERT(sizeof(AllocationResult) == kPointerSize);
  
  #ifdef DEBUG
  struct CommentStatistic {
    const char* comment;
    int size;
    int count;
    void Clear() {
      comment = nullptr;
      size = 0;
      count = 0;
    }
    // Must be small, since an iteration is used for lookup.
    static const int kMaxComments = 64;
  };
  #endif
  
  class Heap {
   public:
    enum FindMementoMode { kForRuntime, kForGC };
  
    enum HeapState {
      NOT_IN_GC,
      SCAVENGE,
      MARK_COMPACT,
      MINOR_MARK_COMPACT,
      TEAR_DOWN
    };
  
    using PretenuringFeedbackMap = std::unordered_map<AllocationSite*, size_t>;
  
    // Taking this mutex prevents the GC from entering a phase that relocates
    // object references.
    base::Mutex* relocation_mutex() { return &relocation_mutex_; }
  
    // Support for partial snapshots.  After calling this we have a linear
    // space to write objects in each space.
    struct Chunk {
      uint32_t size;
      Address start;
      Address end;
    };
    typedef std::vector<Chunk> Reservation;
  
    static const int kInitalOldGenerationLimitFactor = 2;
  
  #if V8_OS_ANDROID
    // Don't apply pointer multiplier on Android since it has no swap space and
    // should instead adapt it's heap size based on available physical memory.
    static const int kPointerMultiplier = 1;
  #else
    static const int kPointerMultiplier = i::kPointerSize / 4;
  #endif
  
    // Semi-space size needs to be a multiple of page size.
    static const size_t kMinSemiSpaceSizeInKB =
        1 * kPointerMultiplier * ((1 << kPageSizeBits) / KB);
    static const size_t kMaxSemiSpaceSizeInKB =
        16 * kPointerMultiplier * ((1 << kPageSizeBits) / KB);
  
    static const int kTraceRingBufferSize = 512;
    static const int kStacktraceBufferSize = 512;
  
    static const int kNoGCFlags = 0;
    static const int kReduceMemoryFootprintMask = 1;
  
    // The minimum size of a HeapObject on the heap.
    static const int kMinObjectSizeInWords = 2;
  
    static const int kMinPromotedPercentForFastPromotionMode = 90;
  
    STATIC_ASSERT(static_cast<int>(RootIndex::kUndefinedValue) ==
                  Internals::kUndefinedValueRootIndex);
    STATIC_ASSERT(static_cast<int>(RootIndex::kTheHoleValue) ==
                  Internals::kTheHoleValueRootIndex);
    STATIC_ASSERT(static_cast<int>(RootIndex::kNullValue) ==
                  Internals::kNullValueRootIndex);
    STATIC_ASSERT(static_cast<int>(RootIndex::kTrueValue) ==
                  Internals::kTrueValueRootIndex);
    STATIC_ASSERT(static_cast<int>(RootIndex::kFalseValue) ==
                  Internals::kFalseValueRootIndex);
    STATIC_ASSERT(static_cast<int>(RootIndex::kempty_string) ==
                  Internals::kEmptyStringRootIndex);
  
    // Calculates the maximum amount of filler that could be required by the
    // given alignment.
    static int GetMaximumFillToAlign(AllocationAlignment alignment);
    // Calculates the actual amount of filler required for a given address at the
    // given alignment.
    static int GetFillToAlign(Address address, AllocationAlignment alignment);
  
    void FatalProcessOutOfMemory(const char* location);
  
    // Checks whether the space is valid.
    static bool IsValidAllocationSpace(AllocationSpace space);
  
    // Zapping is needed for verify heap, and always done in debug builds.
    static inline bool ShouldZapGarbage() {
  #ifdef DEBUG
      return true;
  #else
  #ifdef VERIFY_HEAP
      return FLAG_verify_heap;
  #else
      return false;
  #endif
  #endif
    }
  
    static uintptr_t ZapValue() {
      return FLAG_clear_free_memory ? kClearedFreeMemoryValue : kZapValue;
    }
  
    static inline bool IsYoungGenerationCollector(GarbageCollector collector) {
      return collector == SCAVENGER || collector == MINOR_MARK_COMPACTOR;
    }
  
    static inline GarbageCollector YoungGenerationCollector() {
  #if ENABLE_MINOR_MC
      return (FLAG_minor_mc) ? MINOR_MARK_COMPACTOR : SCAVENGER;
  #else
      return SCAVENGER;
  #endif  // ENABLE_MINOR_MC
    }
  
    static inline const char* CollectorName(GarbageCollector collector) {
      switch (collector) {
        case SCAVENGER:
          return "Scavenger";
        case MARK_COMPACTOR:
          return "Mark-Compact";
        case MINOR_MARK_COMPACTOR:
          return "Minor Mark-Compact";
      }
      return "Unknown collector";
    }
  
    // Copy block of memory from src to dst. Size of block should be aligned
    // by pointer size.
    static inline void CopyBlock(Address dst, Address src, int byte_size);
  
    V8_EXPORT_PRIVATE static void WriteBarrierForCodeSlow(Code host);
    V8_EXPORT_PRIVATE static void GenerationalBarrierSlow(HeapObject* object,
                                                          Address slot,
                                                          HeapObject* value);
    V8_EXPORT_PRIVATE static void GenerationalBarrierForElementsSlow(
        Heap* heap, FixedArray* array, int offset, int length);
    V8_EXPORT_PRIVATE static void GenerationalBarrierForCodeSlow(
        Code host, RelocInfo* rinfo, HeapObject* value);
    V8_EXPORT_PRIVATE static void MarkingBarrierSlow(HeapObject* object,
                                                     Address slot,
                                                     HeapObject* value);
    V8_EXPORT_PRIVATE static void MarkingBarrierForElementsSlow(
        Heap* heap, HeapObject* object);
    V8_EXPORT_PRIVATE static void MarkingBarrierForCodeSlow(Code host,
                                                            RelocInfo* rinfo,
                                                            HeapObject* value);
    V8_EXPORT_PRIVATE static bool PageFlagsAreConsistent(HeapObject* object);
  
    // Notifies the heap that is ok to start marking or other activities that
    // should not happen during deserialization.
    void NotifyDeserializationComplete();
  
    inline Address* NewSpaceAllocationTopAddress();
    inline Address* NewSpaceAllocationLimitAddress();
    inline Address* OldSpaceAllocationTopAddress();
    inline Address* OldSpaceAllocationLimitAddress();
  
    // Move len elements within a given array from src_index index to dst_index
    // index.
    void MoveElements(FixedArray* array, int dst_index, int src_index, int len,
                      WriteBarrierMode mode = UPDATE_WRITE_BARRIER);
  
    // Initialize a filler object to keep the ability to iterate over the heap
    // when introducing gaps within pages. If slots could have been recorded in
    // the freed area, then pass ClearRecordedSlots::kYes as the mode. Otherwise,
    // pass ClearRecordedSlots::kNo. If the memory after the object header of
    // the filler should be cleared, pass in kClearFreedMemory. The default is
    // kDontClearFreedMemory.
    V8_EXPORT_PRIVATE HeapObject* CreateFillerObjectAt(
        Address addr, int size, ClearRecordedSlots clear_slots_mode,
        ClearFreedMemoryMode clear_memory_mode =
            ClearFreedMemoryMode::kDontClearFreedMemory);
  
    template <typename T>
    void CreateFillerForArray(T* object, int elements_to_trim, int bytes_to_trim);
  
    bool CanMoveObjectStart(HeapObject* object);
  
    static bool IsImmovable(HeapObject* object);
  
    // Trim the given array from the left. Note that this relocates the object
    // start and hence is only valid if there is only a single reference to it.
    FixedArrayBase* LeftTrimFixedArray(FixedArrayBase* obj, int elements_to_trim);
  
    // Trim the given array from the right.
    void RightTrimFixedArray(FixedArrayBase* obj, int elements_to_trim);
    void RightTrimWeakFixedArray(WeakFixedArray* obj, int elements_to_trim);
  
    // Converts the given boolean condition to JavaScript boolean value.
    inline Oddball* ToBoolean(bool condition);
  
    // Notify the heap that a context has been disposed.
    int NotifyContextDisposed(bool dependant_context);
  
    void set_native_contexts_list(Object* object) {
      native_contexts_list_ = object;
    }
    Object* native_contexts_list() const { return native_contexts_list_; }
  
    void set_allocation_sites_list(Object* object) {
      allocation_sites_list_ = object;
    }
    Object* allocation_sites_list() { return allocation_sites_list_; }
  
    // Used in CreateAllocationSiteStub and the (de)serializer.
    Address allocation_sites_list_address() {
      return reinterpret_cast<Address>(&allocation_sites_list_);
    }
  
    // Traverse all the allocaions_sites [nested_site and weak_next] in the list
    // and foreach call the visitor
    void ForeachAllocationSite(
        Object* list, const std::function<void(AllocationSite*)>& visitor);
  
    // Number of mark-sweeps.
    int ms_count() const { return ms_count_; }
  
    // Checks whether the given object is allowed to be migrated from it's
    // current space into the given destination space. Used for debugging.
    bool AllowedToBeMigrated(HeapObject* object, AllocationSpace dest);
  
    void CheckHandleCount();
  
    // Number of "runtime allocations" done so far.
    uint32_t allocations_count() { return allocations_count_; }
  
    // Print short heap statistics.
    void PrintShortHeapStatistics();
  
    bool write_protect_code_memory() const { return write_protect_code_memory_; }
  
    uintptr_t code_space_memory_modification_scope_depth() {
      return code_space_memory_modification_scope_depth_;
    }
  
    void increment_code_space_memory_modification_scope_depth() {
      code_space_memory_modification_scope_depth_++;
    }
  
    void decrement_code_space_memory_modification_scope_depth() {
      code_space_memory_modification_scope_depth_--;
    }
  
    void UnprotectAndRegisterMemoryChunk(MemoryChunk* chunk);
    void UnprotectAndRegisterMemoryChunk(HeapObject* object);
    void UnregisterUnprotectedMemoryChunk(MemoryChunk* chunk);
    V8_EXPORT_PRIVATE void ProtectUnprotectedMemoryChunks();
  
    void EnableUnprotectedMemoryChunksRegistry() {
      unprotected_memory_chunks_registry_enabled_ = true;
    }
  
    void DisableUnprotectedMemoryChunksRegistry() {
      unprotected_memory_chunks_registry_enabled_ = false;
    }
  
    bool unprotected_memory_chunks_registry_enabled() {
      return unprotected_memory_chunks_registry_enabled_;
    }
  
    inline HeapState gc_state() { return gc_state_; }
    void SetGCState(HeapState state);
    bool IsTearingDown() const { return gc_state_ == TEAR_DOWN; }
  
    inline bool IsInGCPostProcessing() { return gc_post_processing_depth_ > 0; }
  
    // If an object has an AllocationMemento trailing it, return it, otherwise
    // return nullptr;
    template <FindMementoMode mode>
    inline AllocationMemento* FindAllocationMemento(Map map, HeapObject* object);
  
    // Returns false if not able to reserve.
    bool ReserveSpace(Reservation* reservations, std::vector<Address>* maps);
  
    //
    // Support for the API.
    //
  
    void CreateApiObjects();
  
    // Implements the corresponding V8 API function.
    bool IdleNotification(double deadline_in_seconds);
    bool IdleNotification(int idle_time_in_ms);
  
    void MemoryPressureNotification(MemoryPressureLevel level,
                                    bool is_isolate_locked);
    void CheckMemoryPressure();
  
    void AddNearHeapLimitCallback(v8::NearHeapLimitCallback, void* data);
    void RemoveNearHeapLimitCallback(v8::NearHeapLimitCallback callback,
                                     size_t heap_limit);
  
    double MonotonicallyIncreasingTimeInMs();
  
    void RecordStats(HeapStats* stats, bool take_snapshot = false);
  
    // Check new space expansion criteria and expand semispaces if it was hit.
    void CheckNewSpaceExpansionCriteria();
  
    void VisitExternalResources(v8::ExternalResourceVisitor* visitor);
  
    // An object should be promoted if the object has survived a
    // scavenge operation.
    inline bool ShouldBePromoted(Address old_address);
  
    void IncrementDeferredCount(v8::Isolate::UseCounterFeature feature);
  
    inline uint64_t HashSeed();
  
    inline int NextScriptId();
    inline int NextDebuggingId();
    inline int GetNextTemplateSerialNumber();
  
    void SetSerializedObjects(FixedArray* objects);
    void SetSerializedGlobalProxySizes(FixedArray* sizes);
  
    // For post mortem debugging.
    void RememberUnmappedPage(Address page, bool compacted);
  
    int64_t external_memory_hard_limit() { return MaxOldGenerationSize() / 2; }
  
    V8_INLINE int64_t external_memory();
    V8_INLINE void update_external_memory(int64_t delta);
    V8_INLINE void update_external_memory_concurrently_freed(intptr_t freed);
    V8_INLINE void account_external_memory_concurrently_freed();
  
    size_t backing_store_bytes() const { return backing_store_bytes_; }
  
    void CompactWeakArrayLists(PretenureFlag pretenure);
  
    void AddRetainedMap(Handle<Map> map);
  
    // This event is triggered after successful allocation of a new object made
    // by runtime. Allocations of target space for object evacuation do not
    // trigger the event. In order to track ALL allocations one must turn off
    // FLAG_inline_new.
    inline void OnAllocationEvent(HeapObject* object, int size_in_bytes);
  
    // This event is triggered after object is moved to a new place.
    inline void OnMoveEvent(HeapObject* target, HeapObject* source,
                            int size_in_bytes);
  
    inline bool CanAllocateInReadOnlySpace();
    bool deserialization_complete() const { return deserialization_complete_; }
  
    bool HasLowAllocationRate();
    bool HasHighFragmentation();
    bool HasHighFragmentation(size_t used, size_t committed);
  
    void ActivateMemoryReducerIfNeeded();
  
    bool ShouldOptimizeForMemoryUsage();
  
    bool HighMemoryPressure() {
      return memory_pressure_level_ != MemoryPressureLevel::kNone;
    }
  
    void RestoreHeapLimit(size_t heap_limit) {
      // Do not set the limit lower than the live size + some slack.
      size_t min_limit = SizeOfObjects() + SizeOfObjects() / 4;
      max_old_generation_size_ =
          Min(max_old_generation_size_, Max(heap_limit, min_limit));
    }
  
    // ===========================================================================
    // Initialization. ===========================================================
    // ===========================================================================
  
    // Configure heap sizes
    // max_semi_space_size_in_kb: maximum semi-space size in KB
    // max_old_generation_size_in_mb: maximum old generation size in MB
    // code_range_size_in_mb: code range size in MB
    void ConfigureHeap(size_t max_semi_space_size_in_kb,
                       size_t max_old_generation_size_in_mb,
                       size_t code_range_size_in_mb);
    void ConfigureHeapDefault();
  
    // Prepares the heap, setting up memory areas that are needed in the isolate
    // without actually creating any objects.
    void SetUp();
  
    // (Re-)Initialize hash seed from flag or RNG.
    void InitializeHashSeed();
  
    // Bootstraps the object heap with the core set of objects required to run.
    // Returns whether it succeeded.
    bool CreateHeapObjects();
  
    // Create ObjectStats if live_object_stats_ or dead_object_stats_ are nullptr.
    void CreateObjectStats();
  
    // Sets the TearDown state, so no new GC tasks get posted.
    void StartTearDown();
  
    // Destroys all memory allocated by the heap.
    void TearDown();
  
    // Returns whether SetUp has been called.
    bool HasBeenSetUp();
  
    // ===========================================================================
    // Getters for spaces. =======================================================
    // ===========================================================================
  
    inline Address NewSpaceTop();
  
    NewSpace* new_space() { return new_space_; }
    OldSpace* old_space() { return old_space_; }
    CodeSpace* code_space() { return code_space_; }
    MapSpace* map_space() { return map_space_; }
    LargeObjectSpace* lo_space() { return lo_space_; }
    NewLargeObjectSpace* new_lo_space() { return new_lo_space_; }
    ReadOnlySpace* read_only_space() { return read_only_space_; }
  
    inline PagedSpace* paged_space(int idx);
    inline Space* space(int idx);
  
    // Returns name of the space.
    const char* GetSpaceName(int idx);
  
    // ===========================================================================
    // Getters to other components. ==============================================
    // ===========================================================================
  
    GCTracer* tracer() { return tracer_; }
  
    MemoryAllocator* memory_allocator() { return memory_allocator_; }
  
    inline Isolate* isolate();
  
    MarkCompactCollector* mark_compact_collector() {
      return mark_compact_collector_;
    }
  
    MinorMarkCompactCollector* minor_mark_compact_collector() {
      return minor_mark_compact_collector_;
    }
  
    ArrayBufferCollector* array_buffer_collector() {
      return array_buffer_collector_;
    }
  
    // ===========================================================================
    // Root set access. ==========================================================
    // ===========================================================================
  
    // Shortcut to the roots table stored in the Isolate.
    V8_INLINE RootsTable& roots_table();
  
  // Heap root getters.
  #define ROOT_ACCESSOR(type, name, CamelName) inline type name();
    MUTABLE_ROOT_LIST(ROOT_ACCESSOR)
  #undef ROOT_ACCESSOR
  
    // Sets the stub_cache_ (only used when expanding the dictionary).
    V8_INLINE void SetRootCodeStubs(SimpleNumberDictionary* value);
    V8_INLINE void SetRootMaterializedObjects(FixedArray* objects);
    V8_INLINE void SetRootScriptList(Object* value);
    V8_INLINE void SetRootStringTable(StringTable* value);
    V8_INLINE void SetRootNoScriptSharedFunctionInfos(Object* value);
    V8_INLINE void SetMessageListeners(TemplateList* value);
  
    // Set the stack limit in the roots table.  Some architectures generate
    // code that looks here, because it is faster than loading from the static
    // jslimit_/real_jslimit_ variable in the StackGuard.
    void SetStackLimits();
  
    // The stack limit is thread-dependent. To be able to reproduce the same
    // snapshot blob, we need to reset it before serializing.
    void ClearStackLimits();
  
    void RegisterStrongRoots(ObjectSlot start, ObjectSlot end);
    void UnregisterStrongRoots(ObjectSlot start);
  
    void SetBuiltinsConstantsTable(FixedArray* cache);
  
    // A full copy of the interpreter entry trampoline, used as a template to
    // create copies of the builtin at runtime. The copies are used to create
    // better profiling information for ticks in bytecode execution. Note that
    // this is always a copy of the full builtin, i.e. not the off-heap
    // trampoline.
    // See also: FLAG_interpreted_frames_native_stack.
    void SetInterpreterEntryTrampolineForProfiling(Code code);
  
    // Add weak_factory into the dirty_js_weak_factories list.
    void AddDirtyJSWeakFactory(
        JSWeakFactory* weak_factory,
        std::function<void(HeapObject* object, ObjectSlot slot, Object* target)>
            gc_notify_updated_slot);
  
    void AddKeepDuringJobTarget(Handle<JSReceiver> target);
    void ClearKeepDuringJobSet();
  
    // ===========================================================================
    // Inline allocation. ========================================================
    // ===========================================================================
  
    // Indicates whether inline bump-pointer allocation has been disabled.
    bool inline_allocation_disabled() { return inline_allocation_disabled_; }
  
    // Switch whether inline bump-pointer allocation should be used.
    void EnableInlineAllocation();
    void DisableInlineAllocation();
  
    // ===========================================================================
    // Methods triggering GCs. ===================================================
    // ===========================================================================
  
    // Performs garbage collection operation.
    // Returns whether there is a chance that another major GC could
    // collect more garbage.
    V8_EXPORT_PRIVATE bool CollectGarbage(
        AllocationSpace space, GarbageCollectionReason gc_reason,
        const GCCallbackFlags gc_callback_flags = kNoGCCallbackFlags);
  
    // Performs a full garbage collection.
    V8_EXPORT_PRIVATE void CollectAllGarbage(
        int flags, GarbageCollectionReason gc_reason,
        const GCCallbackFlags gc_callback_flags = kNoGCCallbackFlags);
  
    // Last hope GC, should try to squeeze as much as possible.
    void CollectAllAvailableGarbage(GarbageCollectionReason gc_reason);
  
    // Precise garbage collection that potentially finalizes already running
    // incremental marking before performing an atomic garbage collection.
    // Only use if absolutely necessary or in tests to avoid floating garbage!
    void PreciseCollectAllGarbage(
        int flags, GarbageCollectionReason gc_reason,
        const GCCallbackFlags gc_callback_flags = kNoGCCallbackFlags);
  
    // Reports and external memory pressure event, either performs a major GC or
    // completes incremental marking in order to free external resources.
    void ReportExternalMemoryPressure();
  
    typedef v8::Isolate::GetExternallyAllocatedMemoryInBytesCallback
        GetExternallyAllocatedMemoryInBytesCallback;
  
    void SetGetExternallyAllocatedMemoryInBytesCallback(
        GetExternallyAllocatedMemoryInBytesCallback callback) {
      external_memory_callback_ = callback;
    }
  
    // Invoked when GC was requested via the stack guard.
    void HandleGCRequest();
  
    // ===========================================================================
    // Builtins. =================================================================
    // ===========================================================================
  
    Code builtin(int index);
    Address builtin_address(int index);
    void set_builtin(int index, Code builtin);
  
    // ===========================================================================
    // Iterators. ================================================================
    // ===========================================================================
  
    // None of these methods iterate over the read-only roots. To do this use
    // ReadOnlyRoots::Iterate. Read-only root iteration is not necessary for
    // garbage collection and is usually only performed as part of
    // (de)serialization or heap verification.
  
    // Iterates over the strong roots and the weak roots.
    void IterateRoots(RootVisitor* v, VisitMode mode);
    // Iterates over the strong roots.
    void IterateStrongRoots(RootVisitor* v, VisitMode mode);
    // Iterates over entries in the smi roots list.  Only interesting to the
    // serializer/deserializer, since GC does not care about smis.
    void IterateSmiRoots(RootVisitor* v);
    // Iterates over weak string tables.
    void IterateWeakRoots(RootVisitor* v, VisitMode mode);
    // Iterates over weak global handles.
    void IterateWeakGlobalHandles(RootVisitor* v);
    // Iterates over builtins.
    void IterateBuiltins(RootVisitor* v);
  
    // ===========================================================================
    // Store buffer API. =========================================================
    // ===========================================================================
  
    // Used for query incremental marking status in generated code.
    Address* IsMarkingFlagAddress() {
      return reinterpret_cast<Address*>(&is_marking_flag_);
    }
  
    void SetIsMarkingFlag(uint8_t flag) { is_marking_flag_ = flag; }
  
    Address* store_buffer_top_address();
    static intptr_t store_buffer_mask_constant();
    static Address store_buffer_overflow_function_address();
  
    void ClearRecordedSlot(HeapObject* object, ObjectSlot slot);
    void ClearRecordedSlotRange(Address start, Address end);
  
  #ifdef DEBUG
    void VerifyClearedSlot(HeapObject* object, ObjectSlot slot);
  #endif
  
    // ===========================================================================
    // Incremental marking API. ==================================================
    // ===========================================================================
  
    int GCFlagsForIncrementalMarking() {
      return ShouldOptimizeForMemoryUsage() ? kReduceMemoryFootprintMask
                                            : kNoGCFlags;
    }
  
    // Start incremental marking and ensure that idle time handler can perform
    // incremental steps.
    void StartIdleIncrementalMarking(
        GarbageCollectionReason gc_reason,
        GCCallbackFlags gc_callback_flags = GCCallbackFlags::kNoGCCallbackFlags);
  
    // Starts incremental marking assuming incremental marking is currently
    // stopped.
    void StartIncrementalMarking(
        int gc_flags, GarbageCollectionReason gc_reason,
        GCCallbackFlags gc_callback_flags = GCCallbackFlags::kNoGCCallbackFlags);
  
    void StartIncrementalMarkingIfAllocationLimitIsReached(
        int gc_flags,
        GCCallbackFlags gc_callback_flags = GCCallbackFlags::kNoGCCallbackFlags);
  
    void FinalizeIncrementalMarkingIfComplete(GarbageCollectionReason gc_reason);
    // Synchronously finalizes incremental marking.
    void FinalizeIncrementalMarkingAtomically(GarbageCollectionReason gc_reason);
  
    void RegisterDeserializedObjectsForBlackAllocation(
        Reservation* reservations, const std::vector<HeapObject*>& large_objects,
        const std::vector<Address>& maps);
  
    IncrementalMarking* incremental_marking() { return incremental_marking_; }
  
    // ===========================================================================
    // Concurrent marking API. ===================================================
    // ===========================================================================
  
    ConcurrentMarking* concurrent_marking() { return concurrent_marking_; }
  
    // The runtime uses this function to notify potentially unsafe object layout
    // changes that require special synchronization with the concurrent marker.
    // The old size is the size of the object before layout change.
    void NotifyObjectLayoutChange(HeapObject* object, int old_size,
                                  const DisallowHeapAllocation&);
  
  #ifdef VERIFY_HEAP
    // This function checks that either
    // - the map transition is safe,
    // - or it was communicated to GC using NotifyObjectLayoutChange.
    void VerifyObjectLayoutChange(HeapObject* object, Map new_map);
  #endif
  
    // ===========================================================================
    // Deoptimization support API. ===============================================
    // ===========================================================================
  
    // Setters for code offsets of well-known deoptimization targets.
    void SetArgumentsAdaptorDeoptPCOffset(int pc_offset);
    void SetConstructStubCreateDeoptPCOffset(int pc_offset);
    void SetConstructStubInvokeDeoptPCOffset(int pc_offset);
    void SetInterpreterEntryReturnPCOffset(int pc_offset);
  
    // Invalidates references in the given {code} object that are referenced
    // transitively from the deoptimization data. Mutates write-protected code.
    void InvalidateCodeDeoptimizationData(Code code);
  
    void DeoptMarkedAllocationSites();
  
    bool DeoptMaybeTenuredAllocationSites();
  
    // ===========================================================================
    // Embedder heap tracer support. =============================================
    // ===========================================================================
  
    LocalEmbedderHeapTracer* local_embedder_heap_tracer() const {
      return local_embedder_heap_tracer_;
    }
  
    void SetEmbedderHeapTracer(EmbedderHeapTracer* tracer);
    EmbedderHeapTracer* GetEmbedderHeapTracer() const;
  
    void TracePossibleWrapper(JSObject* js_object);
    void RegisterExternallyReferencedObject(Address* location);
    void SetEmbedderStackStateForNextFinalizaton(
        EmbedderHeapTracer::EmbedderStackState stack_state);
  
    // ===========================================================================
    // External string table API. ================================================
    // ===========================================================================
  
    // Registers an external string.
    inline void RegisterExternalString(String* string);
  
    // Called when a string's resource is changed. The size of the payload is sent
    // as argument of the method.
    inline void UpdateExternalString(String* string, size_t old_payload,
                                     size_t new_payload);
  
    // Finalizes an external string by deleting the associated external
    // data and clearing the resource pointer.
    inline void FinalizeExternalString(String* string);
  
    static String* UpdateNewSpaceReferenceInExternalStringTableEntry(
        Heap* heap, ObjectSlot pointer);
  
    // ===========================================================================
    // Methods checking/returning the space of a given object/address. ===========
    // ===========================================================================
  
    // Returns whether the object resides in new space.
    static inline bool InNewSpace(Object* object);
    static inline bool InNewSpace(MaybeObject object);
    static inline bool InNewSpace(HeapObject* heap_object);
    static inline bool InNewSpace(HeapObjectPtr heap_object);
    static inline bool InFromSpace(Object* object);
    static inline bool InFromSpace(MaybeObject object);
    static inline bool InFromSpace(HeapObject* heap_object);
    static inline bool InToSpace(Object* object);
    static inline bool InToSpace(MaybeObject object);
    static inline bool InToSpace(HeapObject* heap_object);
    static inline bool InToSpace(HeapObjectPtr heap_object);
  
    // Returns whether the object resides in old space.
    inline bool InOldSpace(Object* object);
  
    // Returns whether the object resides in read-only space.
    inline bool InReadOnlySpace(Object* object);
  
    // Checks whether an address/object in the heap (including auxiliary
    // area and unused area).
    bool Contains(HeapObject* value);
  
    // Checks whether an address/object in a space.
    // Currently used by tests, serialization and heap verification only.
    bool InSpace(HeapObject* value, AllocationSpace space);
  
    // Slow methods that can be used for verification as they can also be used
    // with off-heap Addresses.
    bool InSpaceSlow(Address addr, AllocationSpace space);
  
    // Find the heap which owns this HeapObject. Should never be called for
    // objects in RO space.
    static inline Heap* FromWritableHeapObject(const HeapObject* obj);
    // This takes a HeapObjectPtr* (as opposed to a plain HeapObjectPtr)
    // to keep the WRITE_BARRIER macro syntax-compatible to the HeapObject*
    // version above.
    // TODO(3770): This should probably take a HeapObjectPtr eventually.
    static inline Heap* FromWritableHeapObject(const HeapObjectPtr* obj);
  
    // ===========================================================================
    // Object statistics tracking. ===============================================
    // ===========================================================================
  
    // Returns the number of buckets used by object statistics tracking during a
    // major GC. Note that the following methods fail gracefully when the bounds
    // are exceeded though.
    size_t NumberOfTrackedHeapObjectTypes();
  
    // Returns object statistics about count and size at the last major GC.
    // Objects are being grouped into buckets that roughly resemble existing
    // instance types.
    size_t ObjectCountAtLastGC(size_t index);
    size_t ObjectSizeAtLastGC(size_t index);
  
    // Retrieves names of buckets used by object statistics tracking.
    bool GetObjectTypeName(size_t index, const char** object_type,
                           const char** object_sub_type);
  
    // The total number of native contexts object on the heap.
    size_t NumberOfNativeContexts();
    // The total number of native contexts that were detached but were not
    // garbage collected yet.
    size_t NumberOfDetachedContexts();
  
    // ===========================================================================
    // Code statistics. ==========================================================
    // ===========================================================================
  
    // Collect code (Code and BytecodeArray objects) statistics.
    void CollectCodeStatistics();
  
    // ===========================================================================
    // GC statistics. ============================================================
    // ===========================================================================
  
    // Returns the maximum amount of memory reserved for the heap.
    size_t MaxReserved();
    size_t MaxSemiSpaceSize() { return max_semi_space_size_; }
    size_t InitialSemiSpaceSize() { return initial_semispace_size_; }
    size_t MaxOldGenerationSize() { return max_old_generation_size_; }
  
    V8_EXPORT_PRIVATE static size_t ComputeMaxOldGenerationSize(
        uint64_t physical_memory);
  
    static size_t ComputeMaxSemiSpaceSize(uint64_t physical_memory) {
      const uint64_t min_physical_memory = 512 * MB;
      const uint64_t max_physical_memory = 3 * static_cast<uint64_t>(GB);
  
      uint64_t capped_physical_memory =
          Max(Min(physical_memory, max_physical_memory), min_physical_memory);
      // linearly scale max semi-space size: (X-A)/(B-A)*(D-C)+C
      size_t semi_space_size_in_kb =
          static_cast<size_t>(((capped_physical_memory - min_physical_memory) *
                               (kMaxSemiSpaceSizeInKB - kMinSemiSpaceSizeInKB)) /
                                  (max_physical_memory - min_physical_memory) +
                              kMinSemiSpaceSizeInKB);
      return RoundUp(semi_space_size_in_kb, (1 << kPageSizeBits) / KB);
    }
  
    // Returns the capacity of the heap in bytes w/o growing. Heap grows when
    // more spaces are needed until it reaches the limit.
    size_t Capacity();
  
    // Returns the capacity of the old generation.
    size_t OldGenerationCapacity();
  
    // Returns the amount of memory currently held alive by the unmapper.
    size_t CommittedMemoryOfUnmapper();
  
    // Returns the amount of memory currently committed for the heap.
    size_t CommittedMemory();
  
    // Returns the amount of memory currently committed for the old space.
    size_t CommittedOldGenerationMemory();
  
    // Returns the amount of executable memory currently committed for the heap.
    size_t CommittedMemoryExecutable();
  
    // Returns the amount of phyical memory currently committed for the heap.
    size_t CommittedPhysicalMemory();
  
    // Returns the maximum amount of memory ever committed for the heap.
    size_t MaximumCommittedMemory() { return maximum_committed_; }
  
    // Updates the maximum committed memory for the heap. Should be called
    // whenever a space grows.
    void UpdateMaximumCommitted();
  
    // Returns the available bytes in space w/o growing.
    // Heap doesn't guarantee that it can allocate an object that requires
    // all available bytes. Check MaxHeapObjectSize() instead.
    size_t Available();
  
    // Returns of size of all objects residing in the heap.
    size_t SizeOfObjects();
  
    void UpdateSurvivalStatistics(int start_new_space_size);
  
    inline void IncrementPromotedObjectsSize(size_t object_size) {
      promoted_objects_size_ += object_size;
    }
    inline size_t promoted_objects_size() { return promoted_objects_size_; }
  
    inline void IncrementSemiSpaceCopiedObjectSize(size_t object_size) {
      semi_space_copied_object_size_ += object_size;
    }
    inline size_t semi_space_copied_object_size() {
      return semi_space_copied_object_size_;
    }
  
    inline size_t SurvivedNewSpaceObjectSize() {
      return promoted_objects_size_ + semi_space_copied_object_size_;
    }
  
    inline void IncrementNodesDiedInNewSpace() { nodes_died_in_new_space_++; }
  
    inline void IncrementNodesCopiedInNewSpace() { nodes_copied_in_new_space_++; }
  
    inline void IncrementNodesPromoted() { nodes_promoted_++; }
  
    inline void IncrementYoungSurvivorsCounter(size_t survived) {
      survived_last_scavenge_ = survived;
      survived_since_last_expansion_ += survived;
    }
  
    inline uint64_t OldGenerationObjectsAndPromotedExternalMemorySize() {
      return OldGenerationSizeOfObjects() + PromotedExternalMemorySize();
    }
  
    inline void UpdateNewSpaceAllocationCounter();
  
    inline size_t NewSpaceAllocationCounter();
  
    // This should be used only for testing.
    void set_new_space_allocation_counter(size_t new_value) {
      new_space_allocation_counter_ = new_value;
    }
  
    void UpdateOldGenerationAllocationCounter() {
      old_generation_allocation_counter_at_last_gc_ =
          OldGenerationAllocationCounter();
      old_generation_size_at_last_gc_ = 0;
    }
  
    size_t OldGenerationAllocationCounter() {
      return old_generation_allocation_counter_at_last_gc_ +
             PromotedSinceLastGC();
    }
  
    // This should be used only for testing.
    void set_old_generation_allocation_counter_at_last_gc(size_t new_value) {
      old_generation_allocation_counter_at_last_gc_ = new_value;
    }
  
    size_t PromotedSinceLastGC() {
      size_t old_generation_size = OldGenerationSizeOfObjects();
      DCHECK_GE(old_generation_size, old_generation_size_at_last_gc_);
      return old_generation_size - old_generation_size_at_last_gc_;
    }
  
    // This is called by the sweeper when it discovers more free space
    // than expected at the end of the preceding GC.
    void NotifyRefinedOldGenerationSize(size_t decreased_bytes) {
      if (old_generation_size_at_last_gc_ != 0) {
        // OldGenerationSizeOfObjects() is now smaller by |decreased_bytes|.
        // Adjust old_generation_size_at_last_gc_ too, so that PromotedSinceLastGC
        // continues to increase monotonically, rather than decreasing here.
        DCHECK_GE(old_generation_size_at_last_gc_, decreased_bytes);
        old_generation_size_at_last_gc_ -= decreased_bytes;
      }
    }
  
    int gc_count() const { return gc_count_; }
  
    // Returns the size of objects residing in non-new spaces.
    // Excludes external memory held by those objects.
    size_t OldGenerationSizeOfObjects();
  
    // ===========================================================================
    // Prologue/epilogue callback methods.========================================
    // ===========================================================================
  
    void AddGCPrologueCallback(v8::Isolate::GCCallbackWithData callback,
                               GCType gc_type_filter, void* data);
    void RemoveGCPrologueCallback(v8::Isolate::GCCallbackWithData callback,
                                  void* data);
  
    void AddGCEpilogueCallback(v8::Isolate::GCCallbackWithData callback,
                               GCType gc_type_filter, void* data);
    void RemoveGCEpilogueCallback(v8::Isolate::GCCallbackWithData callback,
                                  void* data);
  
    void CallGCPrologueCallbacks(GCType gc_type, GCCallbackFlags flags);
    void CallGCEpilogueCallbacks(GCType gc_type, GCCallbackFlags flags);
  
    // ===========================================================================
    // Allocation methods. =======================================================
    // ===========================================================================
  
    // Creates a filler object and returns a heap object immediately after it.
    V8_WARN_UNUSED_RESULT HeapObject* PrecedeWithFiller(HeapObject* object,
                                                        int filler_size);
  
    // Creates a filler object if needed for alignment and returns a heap object
    // immediately after it. If any space is left after the returned object,
    // another filler object is created so the over allocated memory is iterable.
    V8_WARN_UNUSED_RESULT HeapObject* AlignWithFiller(
        HeapObject* object, int object_size, int allocation_size,
        AllocationAlignment alignment);
  
    // ===========================================================================
    // ArrayBuffer tracking. =====================================================
    // ===========================================================================
  
    // TODO(gc): API usability: encapsulate mutation of JSArrayBuffer::is_external
    // in the registration/unregistration APIs. Consider dropping the "New" from
    // "RegisterNewArrayBuffer" because one can re-register a previously
    // unregistered buffer, too, and the name is confusing.
    void RegisterNewArrayBuffer(JSArrayBuffer* buffer);
    void UnregisterArrayBuffer(JSArrayBuffer* buffer);
  
    // ===========================================================================
    // Allocation site tracking. =================================================
    // ===========================================================================
  
    // Updates the AllocationSite of a given {object}. The entry (including the
    // count) is cached on the local pretenuring feedback.
    inline void UpdateAllocationSite(
        Map map, HeapObject* object,
        PretenuringFeedbackMap* pretenuring_feedback);
  
    // Merges local pretenuring feedback into the global one. Note that this
    // method needs to be called after evacuation, as allocation sites may be
    // evacuated and this method resolves forward pointers accordingly.
    void MergeAllocationSitePretenuringFeedback(
        const PretenuringFeedbackMap& local_pretenuring_feedback);
  
    // ===========================================================================
    // Allocation tracking. ======================================================
    // ===========================================================================
  
    // Adds {new_space_observer} to new space and {observer} to any other space.
    void AddAllocationObserversToAllSpaces(
        AllocationObserver* observer, AllocationObserver* new_space_observer);
  
    // Removes {new_space_observer} from new space and {observer} from any other
    // space.
    void RemoveAllocationObserversFromAllSpaces(
        AllocationObserver* observer, AllocationObserver* new_space_observer);
  
    bool allocation_step_in_progress() { return allocation_step_in_progress_; }
    void set_allocation_step_in_progress(bool val) {
      allocation_step_in_progress_ = val;
    }
  
    // ===========================================================================
    // Heap object allocation tracking. ==========================================
    // ===========================================================================
  
    void AddHeapObjectAllocationTracker(HeapObjectAllocationTracker* tracker);
    void RemoveHeapObjectAllocationTracker(HeapObjectAllocationTracker* tracker);
    bool has_heap_object_allocation_tracker() const {
      return !allocation_trackers_.empty();
    }
  
    // ===========================================================================
    // Retaining path tracking. ==================================================
    // ===========================================================================
  
    // Adds the given object to the weak table of retaining path targets.
    // On each GC if the marker discovers the object, it will print the retaining
    // path. This requires --track-retaining-path flag.
    void AddRetainingPathTarget(Handle<HeapObject> object,
                                RetainingPathOption option);
  
    // ===========================================================================
    // Stack frame support. ======================================================
    // ===========================================================================
  
    // Returns the Code object for a given interior pointer.
    Code GcSafeFindCodeForInnerPointer(Address inner_pointer);
  
    // Returns true if {addr} is contained within {code} and false otherwise.
    // Mostly useful for debugging.
    bool GcSafeCodeContains(Code code, Address addr);
  
  // =============================================================================
  #ifdef VERIFY_HEAP
    // Verify the heap is in its normal state before or after a GC.
    void Verify();
    void VerifyRememberedSetFor(HeapObject* object);
  #endif
  
  #ifdef V8_ENABLE_ALLOCATION_TIMEOUT
    void set_allocation_timeout(int timeout) { allocation_timeout_ = timeout; }
  #endif
  
  #ifdef DEBUG
    void VerifyCountersAfterSweeping();
    void VerifyCountersBeforeConcurrentSweeping();
  
    void Print();
    void PrintHandles();
  
    // Report code statistics.
    void ReportCodeStatistics(const char* title);
  #endif
    void* GetRandomMmapAddr() {
      void* result = v8::internal::GetRandomMmapAddr();
  #if V8_TARGET_ARCH_X64
  #if V8_OS_MACOSX
      // The Darwin kernel [as of macOS 10.12.5] does not clean up page
      // directory entries [PDE] created from mmap or mach_vm_allocate, even
      // after the region is destroyed. Using a virtual address space that is
      // too large causes a leak of about 1 wired [can never be paged out] page
      // per call to mmap(). The page is only reclaimed when the process is
      // killed. Confine the hint to a 32-bit section of the virtual address
      // space. See crbug.com/700928.
      uintptr_t offset =
          reinterpret_cast<uintptr_t>(v8::internal::GetRandomMmapAddr()) &
          kMmapRegionMask;
      result = reinterpret_cast<void*>(mmap_region_base_ + offset);
  #endif  // V8_OS_MACOSX
  #endif  // V8_TARGET_ARCH_X64
      return result;
    }
  
    static const char* GarbageCollectionReasonToString(
        GarbageCollectionReason gc_reason);
  
    // Calculates the nof entries for the full sized number to string cache.
    inline int MaxNumberToStringCacheSize() const;
  
   private:
    class SkipStoreBufferScope;
  
    typedef String* (*ExternalStringTableUpdaterCallback)(Heap* heap,
                                                          ObjectSlot pointer);
  
    // External strings table is a place where all external strings are
    // registered.  We need to keep track of such strings to properly
    // finalize them.
    class ExternalStringTable {
     public:
      explicit ExternalStringTable(Heap* heap) : heap_(heap) {}
  
      // Registers an external string.
      inline void AddString(String* string);
      bool Contains(HeapObject* obj);
  
      void IterateAll(RootVisitor* v);
      void IterateNewSpaceStrings(RootVisitor* v);
      void PromoteAllNewSpaceStrings();
  
      // Restores internal invariant and gets rid of collected strings. Must be
      // called after each Iterate*() that modified the strings.
      void CleanUpAll();
      void CleanUpNewSpaceStrings();
  
      // Finalize all registered external strings and clear tables.
      void TearDown();
  
      void UpdateNewSpaceReferences(
          Heap::ExternalStringTableUpdaterCallback updater_func);
      void UpdateReferences(
          Heap::ExternalStringTableUpdaterCallback updater_func);
  
     private:
      void Verify();
      void VerifyNewSpace();
  
      Heap* const heap_;
  
      // To speed up scavenge collections new space string are kept
      // separate from old space strings.
      std::vector<Object*> new_space_strings_;
      std::vector<Object*> old_space_strings_;
  
      DISALLOW_COPY_AND_ASSIGN(ExternalStringTable);
    };
  
    struct StrongRootsList;
  
    struct StringTypeTable {
      InstanceType type;
      int size;
      RootIndex index;
    };
  
    struct ConstantStringTable {
      const char* contents;
      RootIndex index;
    };
  
    struct StructTable {
      InstanceType type;
      int size;
      RootIndex index;
    };
  
    struct GCCallbackTuple {
      GCCallbackTuple(v8::Isolate::GCCallbackWithData callback, GCType gc_type,
                      void* data)
          : callback(callback), gc_type(gc_type), data(data) {}
  
      bool operator==(const GCCallbackTuple& other) const;
      GCCallbackTuple& operator=(const GCCallbackTuple& other);
  
      v8::Isolate::GCCallbackWithData callback;
      GCType gc_type;
      void* data;
    };
  
    static const int kInitialStringTableSize = StringTable::kMinCapacity;
    static const int kInitialEvalCacheSize = 64;
    static const int kInitialNumberStringCacheSize = 256;
  
    static const int kRememberedUnmappedPages = 128;
  
    static const StringTypeTable string_type_table[];
    static const ConstantStringTable constant_string_table[];
    static const StructTable struct_table[];
  
    static const int kYoungSurvivalRateHighThreshold = 90;
    static const int kYoungSurvivalRateAllowedDeviation = 15;
    static const int kOldSurvivalRateLowThreshold = 10;
  
    static const int kMaxMarkCompactsInIdleRound = 7;
    static const int kIdleScavengeThreshold = 5;
  
    static const int kInitialFeedbackCapacity = 256;
  
    Heap();
  
    // Selects the proper allocation space based on the pretenuring decision.
    static AllocationSpace SelectSpace(PretenureFlag pretenure) {
      switch (pretenure) {
        case TENURED_READ_ONLY:
          return RO_SPACE;
        case TENURED:
          return OLD_SPACE;
        case NOT_TENURED:
          return NEW_SPACE;
        default:
          UNREACHABLE();
      }
    }
  
    static size_t DefaultGetExternallyAllocatedMemoryInBytesCallback() {
      return 0;
    }
  
  #define ROOT_ACCESSOR(type, name, CamelName) inline void set_##name(type value);
    ROOT_LIST(ROOT_ACCESSOR)
  #undef ROOT_ACCESSOR
  
    StoreBuffer* store_buffer() { return store_buffer_; }
  
    void set_current_gc_flags(int flags) {
      current_gc_flags_ = flags;
    }
  
    inline bool ShouldReduceMemory() const {
      return (current_gc_flags_ & kReduceMemoryFootprintMask) != 0;
    }
  
    int NumberOfScavengeTasks();
  
    // Checks whether a global GC is necessary
    GarbageCollector SelectGarbageCollector(AllocationSpace space,
                                            const char** reason);
  
    // Make sure there is a filler value behind the top of the new space
    // so that the GC does not confuse some unintialized/stale memory
    // with the allocation memento of the object at the top
    void EnsureFillerObjectAtTop();
  
    // Ensure that we have swept all spaces in such a way that we can iterate
    // over all objects.  May cause a GC.
    void MakeHeapIterable();
  
    // Performs garbage collection
    // Returns whether there is a chance another major GC could
    // collect more garbage.
    bool PerformGarbageCollection(
        GarbageCollector collector,
        const GCCallbackFlags gc_callback_flags = kNoGCCallbackFlags);
  
    inline void UpdateOldSpaceLimits();
  
    bool CreateInitialMaps();
    void CreateInternalAccessorInfoObjects();
    void CreateInitialObjects();
  
    // These five Create*EntryStub functions are here and forced to not be inlined
    // because of a gcc-4.4 bug that assigns wrong vtable entries.
    V8_NOINLINE void CreateJSEntryStub();
    V8_NOINLINE void CreateJSConstructEntryStub();
    V8_NOINLINE void CreateJSRunMicrotasksEntryStub();
  
    void CreateFixedStubs();
  
    // Commits from space if it is uncommitted.
    void EnsureFromSpaceIsCommitted();
  
    // Uncommit unused semi space.
    bool UncommitFromSpace();
  
    // Fill in bogus values in from space
    void ZapFromSpace();
  
    // Zaps the memory of a code object.
    void ZapCodeObject(Address start_address, int size_in_bytes);
  
    // Deopts all code that contains allocation instruction which are tenured or
    // not tenured. Moreover it clears the pretenuring allocation site statistics.
    void ResetAllAllocationSitesDependentCode(PretenureFlag flag);
  
    // Evaluates local pretenuring for the old space and calls
    // ResetAllTenuredAllocationSitesDependentCode if too many objects died in
    // the old space.
    void EvaluateOldSpaceLocalPretenuring(uint64_t size_of_objects_before_gc);
  
    // Record statistics after garbage collection.
    void ReportStatisticsAfterGC();
  
    // Flush the number to string cache.
    void FlushNumberStringCache();
  
    void ConfigureInitialOldGenerationSize();
  
    bool HasLowYoungGenerationAllocationRate();
    bool HasLowOldGenerationAllocationRate();
    double YoungGenerationMutatorUtilization();
    double OldGenerationMutatorUtilization();
  
    void ReduceNewSpaceSize();
  
    GCIdleTimeHeapState ComputeHeapState();
  
    bool PerformIdleTimeAction(GCIdleTimeAction action,
                               GCIdleTimeHeapState heap_state,
                               double deadline_in_ms);
  
    void IdleNotificationEpilogue(GCIdleTimeAction action,
                                  GCIdleTimeHeapState heap_state, double start_ms,
                                  double deadline_in_ms);
  
    int NextAllocationTimeout(int current_timeout = 0);
    inline void UpdateAllocationsHash(HeapObject* object);
    inline void UpdateAllocationsHash(uint32_t value);
    void PrintAllocationsHash();
  
    void PrintMaxMarkingLimitReached();
    void PrintMaxNewSpaceSizeReached();
  
    int NextStressMarkingLimit();
  
    void AddToRingBuffer(const char* string);
    void GetFromRingBuffer(char* buffer);
  
    void CompactRetainedMaps(WeakArrayList* retained_maps);
  
    void CollectGarbageOnMemoryPressure();
  
    void EagerlyFreeExternalMemory();
  
    bool InvokeNearHeapLimitCallback();
  
    void ComputeFastPromotionMode();
  
    // Attempt to over-approximate the weak closure by marking object groups and
    // implicit references from global handles, but don't atomically complete
    // marking. If we continue to mark incrementally, we might have marked
    // objects that die later.
    void FinalizeIncrementalMarkingIncrementally(
        GarbageCollectionReason gc_reason);
  
    // Returns the timer used for a given GC type.
    // - GCScavenger: young generation GC
    // - GCCompactor: full GC
    // - GCFinalzeMC: finalization of incremental full GC
    // - GCFinalizeMCReduceMemory: finalization of incremental full GC with
    // memory reduction
    TimedHistogram* GCTypeTimer(GarbageCollector collector);
    TimedHistogram* GCTypePriorityTimer(GarbageCollector collector);
  
    // ===========================================================================
    // Pretenuring. ==============================================================
    // ===========================================================================
  
    // Pretenuring decisions are made based on feedback collected during new space
    // evacuation. Note that between feedback collection and calling this method
    // object in old space must not move.
    void ProcessPretenuringFeedback();
  
    // Removes an entry from the global pretenuring storage.
    void RemoveAllocationSitePretenuringFeedback(AllocationSite* site);
  
    // ===========================================================================
    // Actual GC. ================================================================
    // ===========================================================================
  
    // Code that should be run before and after each GC.  Includes some
    // reporting/verification activities when compiled with DEBUG set.
    void GarbageCollectionPrologue();
    void GarbageCollectionEpilogue();
  
    // Performs a major collection in the whole heap.
    void MarkCompact();
    // Performs a minor collection of just the young generation.
    void MinorMarkCompact();
  
    // Code to be run before and after mark-compact.
    void MarkCompactPrologue();
    void MarkCompactEpilogue();
  
    // Performs a minor collection in new generation.
    void Scavenge();
    void EvacuateYoungGeneration();
  
    void UpdateNewSpaceReferencesInExternalStringTable(
        ExternalStringTableUpdaterCallback updater_func);
  
    void UpdateReferencesInExternalStringTable(
        ExternalStringTableUpdaterCallback updater_func);
  
    void ProcessAllWeakReferences(WeakObjectRetainer* retainer);
    void ProcessYoungWeakReferences(WeakObjectRetainer* retainer);
    void ProcessNativeContexts(WeakObjectRetainer* retainer);
    void ProcessAllocationSites(WeakObjectRetainer* retainer);
    void ProcessWeakListRoots(WeakObjectRetainer* retainer);
  
    // ===========================================================================
    // GC statistics. ============================================================
    // ===========================================================================
  
    inline size_t OldGenerationSpaceAvailable() {
      if (old_generation_allocation_limit_ <=
          OldGenerationObjectsAndPromotedExternalMemorySize())
        return 0;
      return old_generation_allocation_limit_ -
             static_cast<size_t>(
                 OldGenerationObjectsAndPromotedExternalMemorySize());
    }
  
    // We allow incremental marking to overshoot the allocation limit for
    // performace reasons. If the overshoot is too large then we are more
    // eager to finalize incremental marking.
    inline bool AllocationLimitOvershotByLargeMargin() {
      // This guards against too eager finalization in small heaps.
      // The number is chosen based on v8.browsing_mobile on Nexus 7v2.
      size_t kMarginForSmallHeaps = 32u * MB;
      if (old_generation_allocation_limit_ >=
          OldGenerationObjectsAndPromotedExternalMemorySize())
        return false;
      uint64_t overshoot = OldGenerationObjectsAndPromotedExternalMemorySize() -
                           old_generation_allocation_limit_;
      // Overshoot margin is 50% of allocation limit or half-way to the max heap
      // with special handling of small heaps.
      uint64_t margin =
          Min(Max(old_generation_allocation_limit_ / 2, kMarginForSmallHeaps),
              (max_old_generation_size_ - old_generation_allocation_limit_) / 2);
      return overshoot >= margin;
    }
  
    void UpdateTotalGCTime(double duration);
  
    bool MaximumSizeScavenge() { return maximum_size_scavenges_ > 0; }
  
    bool IsIneffectiveMarkCompact(size_t old_generation_size,
                                  double mutator_utilization);
    void CheckIneffectiveMarkCompact(size_t old_generation_size,
                                     double mutator_utilization);
  
    inline void IncrementExternalBackingStoreBytes(ExternalBackingStoreType type,
                                                   size_t amount);
  
    inline void DecrementExternalBackingStoreBytes(ExternalBackingStoreType type,
                                                   size_t amount);
  
    // ===========================================================================
    // Growing strategy. =========================================================
    // ===========================================================================
  
    HeapController* heap_controller() { return heap_controller_; }
    MemoryReducer* memory_reducer() { return memory_reducer_; }
  
    // For some webpages RAIL mode does not switch from PERFORMANCE_LOAD.
    // This constant limits the effect of load RAIL mode on GC.
    // The value is arbitrary and chosen as the largest load time observed in
    // v8 browsing benchmarks.
    static const int kMaxLoadTimeMs = 7000;
  
    bool ShouldOptimizeForLoadTime();
  
    size_t old_generation_allocation_limit() const {
      return old_generation_allocation_limit_;
    }
  
    bool always_allocate() { return always_allocate_scope_count_ != 0; }
  
    bool CanExpandOldGeneration(size_t size);
  
    bool ShouldExpandOldGenerationOnSlowAllocation();
  
    enum class HeapGrowingMode { kSlow, kConservative, kMinimal, kDefault };
  
    HeapGrowingMode CurrentHeapGrowingMode();
  
    enum class IncrementalMarkingLimit { kNoLimit, kSoftLimit, kHardLimit };
    IncrementalMarkingLimit IncrementalMarkingLimitReached();
  
    // ===========================================================================
    // Idle notification. ========================================================
    // ===========================================================================
  
    bool RecentIdleNotificationHappened();
    void ScheduleIdleScavengeIfNeeded(int bytes_allocated);
  
    // ===========================================================================
    // HeapIterator helpers. =====================================================
    // ===========================================================================
  
    void heap_iterator_start() { heap_iterator_depth_++; }
  
    void heap_iterator_end() { heap_iterator_depth_--; }
  
    bool in_heap_iterator() { return heap_iterator_depth_ > 0; }
  
    // ===========================================================================
    // Allocation methods. =======================================================
    // ===========================================================================
  
    // Allocates a JS Map in the heap.
    V8_WARN_UNUSED_RESULT AllocationResult
    AllocateMap(InstanceType instance_type, int instance_size,
                ElementsKind elements_kind = TERMINAL_FAST_ELEMENTS_KIND,
                int inobject_properties = 0);
  
    // Allocate an uninitialized object.  The memory is non-executable if the
    // hardware and OS allow.  This is the single choke-point for allocations
    // performed by the runtime and should not be bypassed (to extend this to
    // inlined allocations, use the Heap::DisableInlineAllocation() support).
    V8_WARN_UNUSED_RESULT inline AllocationResult AllocateRaw(
        int size_in_bytes, AllocationSpace space,
        AllocationAlignment aligment = kWordAligned);
  
    // This method will try to perform an allocation of a given size in a given
    // space. If the allocation fails, a regular full garbage collection is
    // triggered and the allocation is retried. This is performed multiple times.
    // If after that retry procedure the allocation still fails nullptr is
    // returned.
    HeapObject* AllocateRawWithLightRetry(
        int size, AllocationSpace space,
        AllocationAlignment alignment = kWordAligned);
  
    // This method will try to perform an allocation of a given size in a given
    // space. If the allocation fails, a regular full garbage collection is
    // triggered and the allocation is retried. This is performed multiple times.
    // If after that retry procedure the allocation still fails a "hammer"
    // garbage collection is triggered which tries to significantly reduce memory.
    // If the allocation still fails after that a fatal error is thrown.
    HeapObject* AllocateRawWithRetryOrFail(
        int size, AllocationSpace space,
        AllocationAlignment alignment = kWordAligned);
    HeapObject* AllocateRawCodeInLargeObjectSpace(int size);
  
    // Allocates a heap object based on the map.
    V8_WARN_UNUSED_RESULT AllocationResult Allocate(Map map,
                                                    AllocationSpace space);
  
    // Takes a code object and checks if it is on memory which is not subject to
    // compaction. This method will return a new code object on an immovable
    // memory location if the original code object was movable.
    HeapObject* EnsureImmovableCode(HeapObject* heap_object, int object_size);
  
    // Allocates a partial map for bootstrapping.
    V8_WARN_UNUSED_RESULT AllocationResult
    AllocatePartialMap(InstanceType instance_type, int instance_size);
  
    void FinalizePartialMap(Map map);
  
    // Allocate empty fixed typed array of given type.
    V8_WARN_UNUSED_RESULT AllocationResult
    AllocateEmptyFixedTypedArray(ExternalArrayType array_type);
  
    void set_force_oom(bool value) { force_oom_ = value; }
  
    // ===========================================================================
    // Retaining path tracing ====================================================
    // ===========================================================================
  
    void AddRetainer(HeapObject* retainer, HeapObject* object);
    void AddEphemeronRetainer(HeapObject* retainer, HeapObject* object);
    void AddRetainingRoot(Root root, HeapObject* object);
    // Returns true if the given object is a target of retaining path tracking.
    // Stores the option corresponding to the object in the provided *option.
    bool IsRetainingPathTarget(HeapObject* object, RetainingPathOption* option);
    void PrintRetainingPath(HeapObject* object, RetainingPathOption option);
  
  #ifdef DEBUG
    void IncrementObjectCounters();
  #endif  // DEBUG
  
    // The amount of memory that has been freed concurrently.
    std::atomic<intptr_t> external_memory_concurrently_freed_{0};
  
    // This can be calculated directly from a pointer to the heap; however, it is
    // more expedient to get at the isolate directly from within Heap methods.
    Isolate* isolate_ = nullptr;
  
    size_t code_range_size_ = 0;
    size_t max_semi_space_size_ = 8 * (kPointerSize / 4) * MB;
    size_t initial_semispace_size_ = kMinSemiSpaceSizeInKB * KB;
    size_t max_old_generation_size_ = 700ul * (kPointerSize / 4) * MB;
    size_t initial_max_old_generation_size_;
    size_t initial_old_generation_size_;
    bool old_generation_size_configured_ = false;
    size_t maximum_committed_ = 0;
  
    // Backing store bytes (array buffers and external strings).
    std::atomic<size_t> backing_store_bytes_{0};
  
    // For keeping track of how much data has survived
    // scavenge since last new space expansion.
    size_t survived_since_last_expansion_ = 0;
  
    // ... and since the last scavenge.
    size_t survived_last_scavenge_ = 0;
  
    // This is not the depth of nested AlwaysAllocateScope's but rather a single
    // count, as scopes can be acquired from multiple tasks (read: threads).
    std::atomic<size_t> always_allocate_scope_count_{0};
  
    // Stores the memory pressure level that set by MemoryPressureNotification
    // and reset by a mark-compact garbage collection.
    std::atomic<MemoryPressureLevel> memory_pressure_level_;
  
    std::vector<std::pair<v8::NearHeapLimitCallback, void*> >
        near_heap_limit_callbacks_;
  
    // For keeping track of context disposals.
    int contexts_disposed_ = 0;
  
    // The length of the retained_maps array at the time of context disposal.
    // This separates maps in the retained_maps array that were created before
    // and after context disposal.
    int number_of_disposed_maps_ = 0;
  
    NewSpace* new_space_ = nullptr;
    OldSpace* old_space_ = nullptr;
    CodeSpace* code_space_ = nullptr;
    MapSpace* map_space_ = nullptr;
    LargeObjectSpace* lo_space_ = nullptr;
    NewLargeObjectSpace* new_lo_space_ = nullptr;
    ReadOnlySpace* read_only_space_ = nullptr;
    // Map from the space id to the space.
    Space* space_[LAST_SPACE + 1];
  
    // Determines whether code space is write-protected. This is essentially a
    // race-free copy of the {FLAG_write_protect_code_memory} flag.
    bool write_protect_code_memory_ = false;
  
    // Holds the number of open CodeSpaceMemoryModificationScopes.
    uintptr_t code_space_memory_modification_scope_depth_ = 0;
  
    HeapState gc_state_ = NOT_IN_GC;
  
    int gc_post_processing_depth_ = 0;
  
    // Returns the amount of external memory registered since last global gc.
    uint64_t PromotedExternalMemorySize();
  
    // How many "runtime allocations" happened.
    uint32_t allocations_count_ = 0;
  
    // Running hash over allocations performed.
    uint32_t raw_allocations_hash_ = 0;
  
    // Starts marking when stress_marking_percentage_% of the marking start limit
    // is reached.
    int stress_marking_percentage_ = 0;
  
    // Observer that causes more frequent checks for reached incremental marking
    // limit.
    AllocationObserver* stress_marking_observer_ = nullptr;
  
    // Observer that can cause early scavenge start.
    StressScavengeObserver* stress_scavenge_observer_ = nullptr;
  
    bool allocation_step_in_progress_ = false;
  
    // The maximum percent of the marking limit reached wihout causing marking.
    // This is tracked when specyfing --fuzzer-gc-analysis.
    double max_marking_limit_reached_ = 0.0;
  
    // How many mark-sweep collections happened.
    unsigned int ms_count_ = 0;
  
    // How many gc happened.
    unsigned int gc_count_ = 0;
  
    // The number of Mark-Compact garbage collections that are considered as
    // ineffective. See IsIneffectiveMarkCompact() predicate.
    int consecutive_ineffective_mark_compacts_ = 0;
  
    static const uintptr_t kMmapRegionMask = 0xFFFFFFFFu;
    uintptr_t mmap_region_base_ = 0;
  
    // For post mortem debugging.
    int remembered_unmapped_pages_index_ = 0;
    Address remembered_unmapped_pages_[kRememberedUnmappedPages];
  
    // Limit that triggers a global GC on the next (normally caused) GC.  This
    // is checked when we have already decided to do a GC to help determine
    // which collector to invoke, before expanding a paged space in the old
    // generation and on every allocation in large object space.
    size_t old_generation_allocation_limit_;
  
    // Indicates that inline bump-pointer allocation has been globally disabled
    // for all spaces. This is used to disable allocations in generated code.
    bool inline_allocation_disabled_ = false;
  
    // Weak list heads, threaded through the objects.
    // List heads are initialized lazily and contain the undefined_value at start.
    Object* native_contexts_list_;
    Object* allocation_sites_list_;
  
    std::vector<GCCallbackTuple> gc_epilogue_callbacks_;
    std::vector<GCCallbackTuple> gc_prologue_callbacks_;
  
    GetExternallyAllocatedMemoryInBytesCallback external_memory_callback_;
  
    int deferred_counters_[v8::Isolate::kUseCounterFeatureCount];
  
    size_t promoted_objects_size_ = 0;
    double promotion_ratio_ = 0.0;
    double promotion_rate_ = 0.0;
    size_t semi_space_copied_object_size_ = 0;
    size_t previous_semi_space_copied_object_size_ = 0;
    double semi_space_copied_rate_ = 0.0;
    int nodes_died_in_new_space_ = 0;
    int nodes_copied_in_new_space_ = 0;
    int nodes_promoted_ = 0;
  
    // This is the pretenuring trigger for allocation sites that are in maybe
    // tenure state. When we switched to the maximum new space size we deoptimize
    // the code that belongs to the allocation site and derive the lifetime
    // of the allocation site.
    unsigned int maximum_size_scavenges_ = 0;
  
    // Total time spent in GC.
    double total_gc_time_ms_;
  
    // Last time an idle notification happened.
    double last_idle_notification_time_ = 0.0;
  
    // Last time a garbage collection happened.
    double last_gc_time_ = 0.0;
  
    GCTracer* tracer_ = nullptr;
    MarkCompactCollector* mark_compact_collector_ = nullptr;
    MinorMarkCompactCollector* minor_mark_compact_collector_ = nullptr;
    ScavengerCollector* scavenger_collector_ = nullptr;
    ArrayBufferCollector* array_buffer_collector_ = nullptr;
    MemoryAllocator* memory_allocator_ = nullptr;
    StoreBuffer* store_buffer_ = nullptr;
    HeapController* heap_controller_ = nullptr;
    IncrementalMarking* incremental_marking_ = nullptr;
    ConcurrentMarking* concurrent_marking_ = nullptr;
    GCIdleTimeHandler* gc_idle_time_handler_ = nullptr;
    MemoryReducer* memory_reducer_ = nullptr;
    ObjectStats* live_object_stats_ = nullptr;
    ObjectStats* dead_object_stats_ = nullptr;
    ScavengeJob* scavenge_job_ = nullptr;
    AllocationObserver* idle_scavenge_observer_ = nullptr;
    LocalEmbedderHeapTracer* local_embedder_heap_tracer_ = nullptr;
    StrongRootsList* strong_roots_list_ = nullptr;
  
    // This counter is increased before each GC and never reset.
    // To account for the bytes allocated since the last GC, use the
    // NewSpaceAllocationCounter() function.
    size_t new_space_allocation_counter_ = 0;
  
    // This counter is increased before each GC and never reset. To
    // account for the bytes allocated since the last GC, use the
    // OldGenerationAllocationCounter() function.
    size_t old_generation_allocation_counter_at_last_gc_ = 0;
  
    // The size of objects in old generation after the last MarkCompact GC.
    size_t old_generation_size_at_last_gc_ = 0;
  
    // The feedback storage is used to store allocation sites (keys) and how often
    // they have been visited (values) by finding a memento behind an object. The
    // storage is only alive temporary during a GC. The invariant is that all
    // pointers in this map are already fixed, i.e., they do not point to
    // forwarding pointers.
    PretenuringFeedbackMap global_pretenuring_feedback_;
  
    char trace_ring_buffer_[kTraceRingBufferSize];
  
    // Used as boolean.
    uint8_t is_marking_flag_ = 0;
  
    // If it's not full then the data is from 0 to ring_buffer_end_.  If it's
    // full then the data is from ring_buffer_end_ to the end of the buffer and
    // from 0 to ring_buffer_end_.
    bool ring_buffer_full_ = false;
    size_t ring_buffer_end_ = 0;
  
    // Flag is set when the heap has been configured.  The heap can be repeatedly
    // configured through the API until it is set up.
    bool configured_ = false;
  
    // Currently set GC flags that are respected by all GC components.
    int current_gc_flags_ = Heap::kNoGCFlags;
  
    // Currently set GC callback flags that are used to pass information between
    // the embedder and V8's GC.
    GCCallbackFlags current_gc_callback_flags_;
  
    ExternalStringTable external_string_table_;
  
    base::Mutex relocation_mutex_;
  
    int gc_callbacks_depth_ = 0;
  
    bool deserialization_complete_ = false;
  
    // The depth of HeapIterator nestings.
    int heap_iterator_depth_ = 0;
  
    bool fast_promotion_mode_ = false;
  
    // Used for testing purposes.
    bool force_oom_ = false;
    bool delay_sweeper_tasks_for_testing_ = false;
  
    HeapObject* pending_layout_change_object_ = nullptr;
  
    base::Mutex unprotected_memory_chunks_mutex_;
    std::unordered_set<MemoryChunk*> unprotected_memory_chunks_;
    bool unprotected_memory_chunks_registry_enabled_ = false;
  
  #ifdef V8_ENABLE_ALLOCATION_TIMEOUT
    // If the --gc-interval flag is set to a positive value, this
    // variable holds the value indicating the number of allocations
    // remain until the next failure and garbage collection.
    int allocation_timeout_ = 0;
  #endif  // V8_ENABLE_ALLOCATION_TIMEOUT
  
    std::map<HeapObject*, HeapObject*> retainer_;
    std::map<HeapObject*, Root> retaining_root_;
    // If an object is retained by an ephemeron, then the retaining key of the
    // ephemeron is stored in this map.
    std::map<HeapObject*, HeapObject*> ephemeron_retainer_;
    // For each index inthe retaining_path_targets_ array this map
    // stores the option of the corresponding target.
    std::map<int, RetainingPathOption> retaining_path_target_option_;
  
    std::vector<HeapObjectAllocationTracker*> allocation_trackers_;
  
    // Classes in "heap" can be friends.
    friend class AlwaysAllocateScope;
    friend class ArrayBufferCollector;
    friend class ConcurrentMarking;
    friend class EphemeronHashTableMarkingTask;
    friend class GCCallbacksScope;
    friend class GCTracer;
    friend class MemoryController;
    friend class HeapIterator;
    friend class IdleScavengeObserver;
    friend class IncrementalMarking;
    friend class IncrementalMarkingJob;
    friend class LargeObjectSpace;
    template <FixedArrayVisitationMode fixed_array_mode,
              TraceRetainingPathMode retaining_path_mode, typename MarkingState>
    friend class MarkingVisitor;
    friend class MarkCompactCollector;
    friend class MarkCompactCollectorBase;
    friend class MinorMarkCompactCollector;
    friend class NewSpace;
    friend class ObjectStatsCollector;
    friend class Page;
    friend class PagedSpace;
    friend class ReadOnlyRoots;
    friend class Scavenger;
    friend class ScavengerCollector;
    friend class Space;
    friend class StoreBuffer;
    friend class Sweeper;
    friend class heap::TestMemoryAllocatorScope;
  
    // The allocator interface.
    friend class Factory;
  
    // The Isolate constructs us.
    friend class Isolate;
  
    // Used in cctest.
    friend class heap::HeapTester;
  
    FRIEND_TEST(HeapControllerTest, OldGenerationAllocationLimit);
    FRIEND_TEST(HeapTest, ExternalLimitDefault);
    FRIEND_TEST(HeapTest, ExternalLimitStaysAboveDefaultForExplicitHandling);
    DISALLOW_COPY_AND_ASSIGN(Heap);
  };
  
  
  class HeapStats {
   public:
    static const int kStartMarker = 0xDECADE00;
    static const int kEndMarker = 0xDECADE01;
  
    intptr_t* start_marker;                  //  0
    size_t* ro_space_size;                   //  1
    size_t* ro_space_capacity;               //  2
    size_t* new_space_size;                  //  3
    size_t* new_space_capacity;              //  4
    size_t* old_space_size;                  //  5
    size_t* old_space_capacity;              //  6
    size_t* code_space_size;                 //  7
    size_t* code_space_capacity;             //  8
    size_t* map_space_size;                  //  9
    size_t* map_space_capacity;              // 10
    size_t* lo_space_size;                   // 11
    size_t* global_handle_count;             // 12
    size_t* weak_global_handle_count;        // 13
    size_t* pending_global_handle_count;     // 14
    size_t* near_death_global_handle_count;  // 15
    size_t* free_global_handle_count;        // 16
    size_t* memory_allocator_size;           // 17
    size_t* memory_allocator_capacity;       // 18
    size_t* malloced_memory;                 // 19
    size_t* malloced_peak_memory;            // 20
    size_t* objects_per_type;                // 21
    size_t* size_per_type;                   // 22
    int* os_error;                           // 23
    char* last_few_messages;                 // 24
    char* js_stacktrace;                     // 25
    intptr_t* end_marker;                    // 26
  };
  
  
  class AlwaysAllocateScope {
   public:
    explicit inline AlwaysAllocateScope(Isolate* isolate);
    inline ~AlwaysAllocateScope();
  
   private:
    Heap* heap_;
  };
  
  // The CodeSpaceMemoryModificationScope can only be used by the main thread.
  class CodeSpaceMemoryModificationScope {
   public:
    explicit inline CodeSpaceMemoryModificationScope(Heap* heap);
    inline ~CodeSpaceMemoryModificationScope();
  
   private:
    Heap* heap_;
  };
  
  // The CodePageCollectionMemoryModificationScope can only be used by the main
  // thread. It will not be enabled if a CodeSpaceMemoryModificationScope is
  // already active.
  class CodePageCollectionMemoryModificationScope {
   public:
    explicit inline CodePageCollectionMemoryModificationScope(Heap* heap);
    inline ~CodePageCollectionMemoryModificationScope();
  
   private:
    Heap* heap_;
  };
  
  // The CodePageMemoryModificationScope does not check if tansitions to
  // writeable and back to executable are actually allowed, i.e. the MemoryChunk
  // was registered to be executable. It can be used by concurrent threads.
  class CodePageMemoryModificationScope {
   public:
    explicit inline CodePageMemoryModificationScope(MemoryChunk* chunk);
    inline ~CodePageMemoryModificationScope();
  
   private:
    MemoryChunk* chunk_;
    bool scope_active_;
  
    // Disallow any GCs inside this scope, as a relocation of the underlying
    // object would change the {MemoryChunk} that this scope targets.
    DISALLOW_HEAP_ALLOCATION(no_heap_allocation_);
  };
  
  // Visitor class to verify interior pointers in spaces that do not contain
  // or care about intergenerational references. All heap object pointers have to
  // point into the heap to a location that has a map pointer at its first word.
  // Caveat: Heap::Contains is an approximation because it can return true for
  // objects in a heap space but above the allocation pointer.
  class VerifyPointersVisitor : public ObjectVisitor, public RootVisitor {
   public:
    explicit VerifyPointersVisitor(Heap* heap) : heap_(heap) {}
    void VisitPointers(HeapObject* host, ObjectSlot start,
                       ObjectSlot end) override;
    void VisitPointers(HeapObject* host, MaybeObjectSlot start,
                       MaybeObjectSlot end) override;
    void VisitRootPointers(Root root, const char* description, ObjectSlot start,
                           ObjectSlot end) override;
  
   protected:
    virtual void VerifyPointers(HeapObject* host, MaybeObjectSlot start,
                                MaybeObjectSlot end);
  
    Heap* heap_;
  };
  
  
  // Verify that all objects are Smis.
  class VerifySmisVisitor : public RootVisitor {
   public:
    void VisitRootPointers(Root root, const char* description, ObjectSlot start,
                           ObjectSlot end) override;
  };
  
  // Space iterator for iterating over all the paged spaces of the heap: Map
  // space, old space, code space and optionally read only space. Returns each
  // space in turn, and null when it is done.
  class V8_EXPORT_PRIVATE PagedSpaces {
   public:
    enum class SpacesSpecifier { kSweepablePagedSpaces, kAllPagedSpaces };
  
    explicit PagedSpaces(Heap* heap, SpacesSpecifier specifier =
                                         SpacesSpecifier::kSweepablePagedSpaces)
        : heap_(heap),
          counter_(specifier == SpacesSpecifier::kAllPagedSpaces ? RO_SPACE
                                                                 : OLD_SPACE) {}
    PagedSpace* next();
  
   private:
    Heap* heap_;
    int counter_;
  };
  
  
  class SpaceIterator : public Malloced {
   public:
    explicit SpaceIterator(Heap* heap);
    virtual ~SpaceIterator();
  
    bool has_next();
    Space* next();
  
   private:
    Heap* heap_;
    int current_space_;         // from enum AllocationSpace.
  };
  
  
  // A HeapIterator provides iteration over the whole heap. It
  // aggregates the specific iterators for the different spaces as
  // these can only iterate over one space only.
  //
  // HeapIterator ensures there is no allocation during its lifetime
  // (using an embedded DisallowHeapAllocation instance).
  //
  // HeapIterator can skip free list nodes (that is, de-allocated heap
  // objects that still remain in the heap). As implementation of free
  // nodes filtering uses GC marks, it can't be used during MS/MC GC
  // phases. Also, it is forbidden to interrupt iteration in this mode,
  // as this will leave heap objects marked (and thus, unusable).
  class HeapIterator {
   public:
    enum HeapObjectsFiltering { kNoFiltering, kFilterUnreachable };
  
    explicit HeapIterator(Heap* heap,
                          HeapObjectsFiltering filtering = kNoFiltering);
    ~HeapIterator();
  
    HeapObject* next();
  
   private:
    HeapObject* NextObject();
  
    DISALLOW_HEAP_ALLOCATION(no_heap_allocation_);
  
    Heap* heap_;
    HeapObjectsFiltering filtering_;
    HeapObjectsFilter* filter_;
    // Space iterator for iterating all the spaces.
    SpaceIterator* space_iterator_;
    // Object iterator for the space currently being iterated.
    std::unique_ptr<ObjectIterator> object_iterator_;
  };
  
  // Abstract base class for checking whether a weak object should be retained.
  class WeakObjectRetainer {
   public:
    virtual ~WeakObjectRetainer() = default;
  
    // Return whether this object should be retained. If nullptr is returned the
    // object has no references. Otherwise the address of the retained object
    // should be returned as in some GC situations the object has been moved.
    virtual Object* RetainAs(Object* object) = 0;
  };
  
  // -----------------------------------------------------------------------------
  // Allows observation of allocations.
  class AllocationObserver {
   public:
    explicit AllocationObserver(intptr_t step_size)
        : step_size_(step_size), bytes_to_next_step_(step_size) {
      DCHECK_LE(kPointerSize, step_size);
    }
    virtual ~AllocationObserver() = default;
  
    // Called each time the observed space does an allocation step. This may be
    // more frequently than the step_size we are monitoring (e.g. when there are
    // multiple observers, or when page or space boundary is encountered.)
    void AllocationStep(int bytes_allocated, Address soon_object, size_t size);
  
   protected:
    intptr_t step_size() const { return step_size_; }
    intptr_t bytes_to_next_step() const { return bytes_to_next_step_; }
  
    // Pure virtual method provided by the subclasses that gets called when at
    // least step_size bytes have been allocated. soon_object is the address just
    // allocated (but not yet initialized.) size is the size of the object as
    // requested (i.e. w/o the alignment fillers). Some complexities to be aware
    // of:
    // 1) soon_object will be nullptr in cases where we end up observing an
    //    allocation that happens to be a filler space (e.g. page boundaries.)
    // 2) size is the requested size at the time of allocation. Right-trimming
    //    may change the object size dynamically.
    // 3) soon_object may actually be the first object in an allocation-folding
    //    group. In such a case size is the size of the group rather than the
    //    first object.
    virtual void Step(int bytes_allocated, Address soon_object, size_t size) = 0;
  
    // Subclasses can override this method to make step size dynamic.
    virtual intptr_t GetNextStepSize() { return step_size_; }
  
    intptr_t step_size_;
    intptr_t bytes_to_next_step_;
  
   private:
    friend class Space;
    DISALLOW_COPY_AND_ASSIGN(AllocationObserver);
  };
  
  V8_EXPORT_PRIVATE const char* AllocationSpaceName(AllocationSpace space);
  
  // -----------------------------------------------------------------------------
  // Allows observation of heap object allocations.
  class HeapObjectAllocationTracker {
   public:
    virtual void AllocationEvent(Address addr, int size) = 0;
    virtual void MoveEvent(Address from, Address to, int size) {}
    virtual void UpdateObjectSizeEvent(Address addr, int size) {}
    virtual ~HeapObjectAllocationTracker() = default;
  };
  
  }  // namespace internal
  }  // namespace v8
  
  #endif  // V8_HEAP_HEAP_H_
  