xref: /third_party/node/deps/v8/src/handles/global-handles.cc

// Copyright 2009 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "src/handles/global-handles.h"

#include <algorithm>
#include <atomic>
#include <climits>
#include <cstdint>
#include <map>

#include "include/v8-traced-handle.h"
#include "src/api/api-inl.h"
#include "src/base/bits.h"
#include "src/base/compiler-specific.h"
#include "src/base/sanitizer/asan.h"
#include "src/common/allow-deprecated.h"
#include "src/common/globals.h"
#include "src/execution/vm-state-inl.h"
#include "src/heap/base/stack.h"
#include "src/heap/embedder-tracing.h"
#include "src/heap/heap-inl.h"
#include "src/heap/heap-write-barrier-inl.h"
#include "src/heap/heap-write-barrier.h"
#include "src/init/v8.h"
#include "src/logging/counters.h"
#include "src/objects/objects-inl.h"
#include "src/objects/slots.h"
#include "src/objects/visitors.h"
#include "src/tasks/cancelable-task.h"
#include "src/tasks/task-utils.h"
#include "src/utils/utils.h"

namespace v8 {
namespace internal {

namespace {

constexpr size_t kBlockSize = 256;

}  // namespace

template <class _NodeType>
class GlobalHandles::NodeBlock final {
 public:
  using BlockType = NodeBlock<_NodeType>;
  using NodeType = _NodeType;

  V8_INLINE static const NodeBlock* From(const NodeType* node);
  V8_INLINE static NodeBlock* From(NodeType* node);

  NodeBlock(GlobalHandles* global_handles,
            GlobalHandles::NodeSpace<NodeType>* space,
            NodeBlock* next) V8_NOEXCEPT : next_(next),
                                           global_handles_(global_handles),
                                           space_(space) {}

  NodeBlock(const NodeBlock&) = delete;
  NodeBlock& operator=(const NodeBlock&) = delete;

  NodeType* at(size_t index) { return &nodes_[index]; }
  const NodeType* at(size_t index) const { return &nodes_[index]; }
  GlobalHandles::NodeSpace<NodeType>* space() const { return space_; }
  GlobalHandles* global_handles() const { return global_handles_; }

  V8_INLINE bool IncreaseUsage();
  V8_INLINE bool DecreaseUsage();

  V8_INLINE void ListAdd(NodeBlock** top);
  V8_INLINE void ListRemove(NodeBlock** top);

  NodeBlock* next() const { return next_; }
  NodeBlock* next_used() const { return next_used_; }

  void set_markbit(size_t index) {
    const auto [cell, bit] = CellAndBit(index);
    reinterpret_cast<std::atomic<CellType>&>(mark_bits_[cell])
        .fetch_or(CellType{1} << bit, std::memory_order_relaxed);
  }

  void clear_markbit(size_t index) {
    const auto [cell, bit] = CellAndBit(index);
    mark_bits_[cell] &= ~(CellType{1} << bit);
  }

  bool markbit(size_t index) const {
    const auto [cell, bit] = CellAndBit(index);
    return mark_bits_[cell] & CellType{1} << bit;
  }

 private:
  using CellType = uint32_t;

  std::tuple<CellType, CellType> CellAndBit(size_t index) const {
    static constexpr CellType kMarkBitCellSizeLog2 = 5;
    static_assert(base::bits::IsPowerOfTwo(kBlockSize),
                  "Block size must be power of two.");
    static_assert(
        sizeof(CellType) * CHAR_BIT == (CellType{1} << kMarkBitCellSizeLog2),
        "Markbit CellType not matching defined log2 size.");
    static constexpr CellType kCellMask =
        (CellType{1} << kMarkBitCellSizeLog2) - 1;
    return {static_cast<CellType>(index >> kMarkBitCellSizeLog2),
            index & kCellMask};
  }

  NodeType nodes_[kBlockSize];
  NodeBlock* const next_;
  GlobalHandles* const global_handles_;
  GlobalHandles::NodeSpace<NodeType>* const space_;
  NodeBlock* next_used_ = nullptr;
  NodeBlock* prev_used_ = nullptr;
  uint32_t used_nodes_ = 0;
  CellType mark_bits_[kBlockSize / (sizeof(CellType) * CHAR_BIT)] = {0};
};

template <class NodeType>
const GlobalHandles::NodeBlock<NodeType>*
GlobalHandles::NodeBlock<NodeType>::From(const NodeType* node) {
  const NodeType* firstNode = node - node->index();
  const BlockType* block = reinterpret_cast<const BlockType*>(firstNode);
  DCHECK_EQ(node, block->at(node->index()));
  return block;
}

template <class NodeType>
GlobalHandles::NodeBlock<NodeType>* GlobalHandles::NodeBlock<NodeType>::From(
    NodeType* node) {
  NodeType* firstNode = node - node->index();
  BlockType* block = reinterpret_cast<BlockType*>(firstNode);
  DCHECK_EQ(node, block->at(node->index()));
  return block;
}

template <class NodeType>
bool GlobalHandles::NodeBlock<NodeType>::IncreaseUsage() {
  DCHECK_LT(used_nodes_, kBlockSize);
  return used_nodes_++ == 0;
}

template <class NodeType>
void GlobalHandles::NodeBlock<NodeType>::ListAdd(BlockType** top) {
  BlockType* old_top = *top;
  *top = this;
  next_used_ = old_top;
  prev_used_ = nullptr;
  if (old_top != nullptr) {
    old_top->prev_used_ = this;
  }
}

template <class NodeType>
bool GlobalHandles::NodeBlock<NodeType>::DecreaseUsage() {
  DCHECK_GT(used_nodes_, 0);
  return --used_nodes_ == 0;
}

template <class NodeType>
void GlobalHandles::NodeBlock<NodeType>::ListRemove(BlockType** top) {
  if (next_used_ != nullptr) next_used_->prev_used_ = prev_used_;
  if (prev_used_ != nullptr) prev_used_->next_used_ = next_used_;
  if (this == *top) {
    *top = next_used_;
  }
}

template <class BlockType>
class GlobalHandles::NodeIterator final {
 public:
  using NodeType = typename BlockType::NodeType;

  // Iterator traits.
  using iterator_category = std::forward_iterator_tag;
  using difference_type = std::ptrdiff_t;
  using value_type = NodeType*;
  using reference = value_type;
  using pointer = value_type*;

  explicit NodeIterator(BlockType* block) V8_NOEXCEPT : block_(block) {}
  NodeIterator(NodeIterator&& other) V8_NOEXCEPT : block_(other.block_),
                                                   index_(other.index_) {}

  NodeIterator(const NodeIterator&) = delete;
  NodeIterator& operator=(const NodeIterator&) = delete;

  bool operator==(const NodeIterator& other) const {
    return block_ == other.block_;
  }
  bool operator!=(const NodeIterator& other) const {
    return block_ != other.block_;
  }

  NodeIterator& operator++() {
    if (++index_ < kBlockSize) return *this;
    index_ = 0;
    block_ = block_->next_used();
    return *this;
  }

  NodeType* operator*() { return block_->at(index_); }
  NodeType* operator->() { return block_->at(index_); }

 private:
  BlockType* block_ = nullptr;
  size_t index_ = 0;
};

template <class NodeType>
class GlobalHandles::NodeSpace final {
 public:
  using BlockType = NodeBlock<NodeType>;
  using iterator = NodeIterator<BlockType>;

  static NodeSpace* From(NodeType* node);
  static void Release(NodeType* node);

  explicit NodeSpace(GlobalHandles* global_handles) V8_NOEXCEPT
      : global_handles_(global_handles) {}
  ~NodeSpace();

  V8_INLINE NodeType* Acquire(Object object);

  iterator begin() { return iterator(first_used_block_); }
  iterator end() { return iterator(nullptr); }

  size_t TotalSize() const { return blocks_ * sizeof(NodeType) * kBlockSize; }
  size_t handles_count() const { return handles_count_; }

 private:
  void PutNodesOnFreeList(BlockType* block);
  V8_INLINE void Free(NodeType* node);

  GlobalHandles* const global_handles_;
  BlockType* first_block_ = nullptr;
  BlockType* first_used_block_ = nullptr;
  NodeType* first_free_ = nullptr;
  size_t blocks_ = 0;
  size_t handles_count_ = 0;
};

template <class NodeType>
GlobalHandles::NodeSpace<NodeType>::~NodeSpace() {
  auto* block = first_block_;
  while (block != nullptr) {
    auto* tmp = block->next();
    delete block;
    block = tmp;
  }
}

template <class NodeType>
NodeType* GlobalHandles::NodeSpace<NodeType>::Acquire(Object object) {
  if (first_free_ == nullptr) {
    first_block_ = new BlockType(global_handles_, this, first_block_);
    blocks_++;
    PutNodesOnFreeList(first_block_);
  }
  DCHECK_NOT_NULL(first_free_);
  NodeType* node = first_free_;
  first_free_ = first_free_->next_free();
  node->Acquire(object);
  BlockType* block = BlockType::From(node);
  if (block->IncreaseUsage()) {
    block->ListAdd(&first_used_block_);
  }
  global_handles_->isolate()->counters()->global_handles()->Increment();
  handles_count_++;
  DCHECK(node->IsInUse());
  return node;
}

template <class NodeType>
void GlobalHandles::NodeSpace<NodeType>::PutNodesOnFreeList(BlockType* block) {
  for (int32_t i = kBlockSize - 1; i >= 0; --i) {
    NodeType* node = block->at(i);
    const uint8_t index = static_cast<uint8_t>(i);
    DCHECK_EQ(i, index);
    node->set_index(index);
    node->Free(first_free_);
    first_free_ = node;
  }
}

template <class NodeType>
void GlobalHandles::NodeSpace<NodeType>::Release(NodeType* node) {
  BlockType* block = BlockType::From(node);
  block->space()->Free(node);
}

template <class NodeType>
void GlobalHandles::NodeSpace<NodeType>::Free(NodeType* node) {
  node->Release(first_free_);
  first_free_ = node;
  BlockType* block = BlockType::From(node);
  if (block->DecreaseUsage()) {
    block->ListRemove(&first_used_block_);
  }
  global_handles_->isolate()->counters()->global_handles()->Decrement();
  handles_count_--;
}

template <class Child>
class NodeBase {
 public:
  static const Child* FromLocation(const Address* location) {
    return reinterpret_cast<const Child*>(location);
  }

  static Child* FromLocation(Address* location) {
    return reinterpret_cast<Child*>(location);
  }

  NodeBase() {
    DCHECK_EQ(offsetof(NodeBase, object_), 0);
    DCHECK_EQ(offsetof(NodeBase, class_id_), Internals::kNodeClassIdOffset);
    DCHECK_EQ(offsetof(NodeBase, flags_), Internals::kNodeFlagsOffset);
  }

#ifdef ENABLE_HANDLE_ZAPPING
  ~NodeBase() {
    ClearFields();
    data_.next_free = nullptr;
    index_ = 0;
  }
#endif

  void Free(Child* free_list) {
    ClearFields();
    AsChild()->MarkAsFree();
    data_.next_free = free_list;
  }

  void Acquire(Object object) {
    DCHECK(!AsChild()->IsInUse());
    CheckFieldsAreCleared();
    reinterpret_cast<std::atomic<Address>*>(&object_)->store(
        object.ptr(), std::memory_order_relaxed);
    AsChild()->MarkAsUsed();
    data_.parameter = nullptr;
    DCHECK(AsChild()->IsInUse());
  }

  void Release(Child* free_list) {
    DCHECK(AsChild()->IsInUse());
    Free(free_list);
    DCHECK(!AsChild()->IsInUse());
  }

  Object object() const { return Object(object_); }
  FullObjectSlot location() { return FullObjectSlot(&object_); }
  Handle<Object> handle() { return Handle<Object>(&object_); }

  uint8_t index() const { return index_; }
  void set_index(uint8_t value) { index_ = value; }

  uint16_t wrapper_class_id() const { return class_id_; }
  bool has_wrapper_class_id() const {
    return class_id_ != v8::HeapProfiler::kPersistentHandleNoClassId;
  }

  // Accessors for next free node in the free list.
  Child* next_free() {
    DCHECK(!AsChild()->IsInUse());
    return data_.next_free;
  }

  void set_parameter(void* parameter) {
    DCHECK(AsChild()->IsInUse());
    data_.parameter = parameter;
  }
  void* parameter() const {
    DCHECK(AsChild()->IsInUse());
    return data_.parameter;
  }

 protected:
  Child* AsChild() { return reinterpret_cast<Child*>(this); }
  const Child* AsChild() const { return reinterpret_cast<const Child*>(this); }

  void ClearFields() {
    // Zap the values for eager trapping.
    object_ = kGlobalHandleZapValue;
    class_id_ = v8::HeapProfiler::kPersistentHandleNoClassId;
    AsChild()->ClearImplFields();
  }

  void CheckFieldsAreCleared() {
    DCHECK_EQ(kGlobalHandleZapValue, object_);
    DCHECK_EQ(v8::HeapProfiler::kPersistentHandleNoClassId, class_id_);
    AsChild()->CheckImplFieldsAreCleared();
  }

  // Storage for object pointer.
  //
  // Placed first to avoid offset computation. The stored data is equivalent to
  // an Object. It is stored as a plain Address for convenience (smallest number
  // of casts), and because it is a private implementation detail: the public
  // interface provides type safety.
  Address object_;

  // Class id set by the embedder.
  uint16_t class_id_;

  // Index in the containing handle block.
  uint8_t index_;

  uint8_t flags_;

  // The meaning of this field depends on node state:
  // - Node in free list: Stores next free node pointer.
  // - Otherwise, specific to the node implementation.
  union {
    Child* next_free;
    void* parameter;
  } data_;
};

namespace {

void ExtractInternalFields(JSObject jsobject, void** embedder_fields, int len) {
  int field_count = jsobject.GetEmbedderFieldCount();
  Isolate* isolate = GetIsolateForSandbox(jsobject);
  for (int i = 0; i < len; ++i) {
    if (field_count == i) break;
    void* pointer;
    if (EmbedderDataSlot(jsobject, i).ToAlignedPointer(isolate, &pointer)) {
      embedder_fields[i] = pointer;
    }
  }
}

}  // namespace

class GlobalHandles::Node final : public NodeBase<GlobalHandles::Node> {
 public:
  // State transition diagram:
  // FREE -> NORMAL <-> WEAK -> PENDING -> NEAR_DEATH -> { NORMAL, WEAK, FREE }
  enum State {
    FREE = 0,
    NORMAL,      // Normal global handle.
    WEAK,        // Flagged as weak but not yet finalized.
    PENDING,     // Has been recognized as only reachable by weak handles.
    NEAR_DEATH,  // Callback has informed the handle is near death.
    NUMBER_OF_NODE_STATES
  };

  Node() {
    STATIC_ASSERT(static_cast<int>(NodeState::kMask) ==
                  Internals::kNodeStateMask);
    STATIC_ASSERT(WEAK == Internals::kNodeStateIsWeakValue);
    STATIC_ASSERT(PENDING == Internals::kNodeStateIsPendingValue);
    set_in_young_list(false);
  }

  Node(const Node&) = delete;
  Node& operator=(const Node&) = delete;

  const char* label() const {
    return state() == NORMAL ? reinterpret_cast<char*>(data_.parameter)
                             : nullptr;
  }

  // State and flag accessors.

  State state() const { return NodeState::decode(flags_); }
  void set_state(State state) { flags_ = NodeState::update(flags_, state); }

  bool is_in_young_list() const { return IsInYoungList::decode(flags_); }
  void set_in_young_list(bool v) { flags_ = IsInYoungList::update(flags_, v); }

  WeaknessType weakness_type() const {
    return NodeWeaknessType::decode(flags_);
  }
  void set_weakness_type(WeaknessType weakness_type) {
    flags_ = NodeWeaknessType::update(flags_, weakness_type);
  }

  bool IsWeak() const { return state() == WEAK; }

  bool IsInUse() const { return state() != FREE; }

  bool IsPhantomCallback() const {
    return weakness_type() == PHANTOM_WEAK ||
           weakness_type() == PHANTOM_WEAK_2_EMBEDDER_FIELDS;
  }

  bool IsPhantomResetHandle() const {
    return weakness_type() == PHANTOM_WEAK_RESET_HANDLE;
  }

  bool IsFinalizerHandle() const { return weakness_type() == FINALIZER_WEAK; }

  bool IsPendingPhantomCallback() const {
    return state() == PENDING && IsPhantomCallback();
  }

  bool IsPendingPhantomResetHandle() const {
    return state() == PENDING && IsPhantomResetHandle();
  }

  bool IsPendingFinalizer() const {
    return state() == PENDING && weakness_type() == FINALIZER_WEAK;
  }

  bool IsPending() const { return state() == PENDING; }

  bool IsRetainer() const {
    return state() != FREE &&
           !(state() == NEAR_DEATH && weakness_type() != FINALIZER_WEAK);
  }

  bool IsStrongRetainer() const { return state() == NORMAL; }

  bool IsWeakRetainer() const {
    return state() == WEAK || state() == PENDING ||
           (state() == NEAR_DEATH && weakness_type() == FINALIZER_WEAK);
  }

  void MarkPending() {
    DCHECK(state() == WEAK);
    set_state(PENDING);
  }

  bool has_callback() const { return weak_callback_ != nullptr; }

  // Accessors for next free node in the free list.
  Node* next_free() {
    DCHECK_EQ(FREE, state());
    return data_.next_free;
  }

  void MakeWeak(void* parameter,
                WeakCallbackInfo<void>::Callback phantom_callback,
                v8::WeakCallbackType type) {
    DCHECK_NOT_NULL(phantom_callback);
    DCHECK(IsInUse());
    CHECK_NE(object_, kGlobalHandleZapValue);
    set_state(WEAK);
    switch (type) {
      case v8::WeakCallbackType::kParameter:
        set_weakness_type(PHANTOM_WEAK);
        break;
      case v8::WeakCallbackType::kInternalFields:
        set_weakness_type(PHANTOM_WEAK_2_EMBEDDER_FIELDS);
        break;
        START_ALLOW_USE_DEPRECATED()
      case v8::WeakCallbackType::kFinalizer:
        set_weakness_type(FINALIZER_WEAK);
        break;
        END_ALLOW_USE_DEPRECATED()
    }
    set_parameter(parameter);
    weak_callback_ = phantom_callback;
  }

  void MakeWeak(Address** location_addr) {
    DCHECK(IsInUse());
    CHECK_NE(object_, kGlobalHandleZapValue);
    set_state(WEAK);
    set_weakness_type(PHANTOM_WEAK_RESET_HANDLE);
    set_parameter(location_addr);
    weak_callback_ = nullptr;
  }

  void* ClearWeakness() {
    DCHECK(IsInUse());
    void* p = parameter();
    set_state(NORMAL);
    set_parameter(nullptr);
    return p;
  }

  void AnnotateStrongRetainer(const char* label) {
    DCHECK_EQ(state(), NORMAL);
    data_.parameter = const_cast<char*>(label);
  }

  void CollectPhantomCallbackData(
      std::vector<std::pair<Node*, PendingPhantomCallback>>*
          pending_phantom_callbacks) {
    DCHECK(weakness_type() == PHANTOM_WEAK ||
           weakness_type() == PHANTOM_WEAK_2_EMBEDDER_FIELDS);
    DCHECK(state() == PENDING);
    DCHECK_NOT_NULL(weak_callback_);

    void* embedder_fields[v8::kEmbedderFieldsInWeakCallback] = {nullptr,
                                                                nullptr};
    if (weakness_type() != PHANTOM_WEAK && object().IsJSObject()) {
      ExtractInternalFields(JSObject::cast(object()), embedder_fields,
                            v8::kEmbedderFieldsInWeakCallback);
    }

    // Zap with something dangerous.
    location().store(Object(0xCA11));

    pending_phantom_callbacks->push_back(std::make_pair(
        this,
        PendingPhantomCallback(weak_callback_, parameter(), embedder_fields)));
    DCHECK(IsInUse());
    set_state(NEAR_DEATH);
  }

  void ResetPhantomHandle() {
    DCHECK_EQ(PHANTOM_WEAK_RESET_HANDLE, weakness_type());
    DCHECK_EQ(PENDING, state());
    DCHECK_NULL(weak_callback_);
    Address** handle = reinterpret_cast<Address**>(parameter());
    *handle = nullptr;
    NodeSpace<Node>::Release(this);
  }

  void PostGarbageCollectionProcessing(Isolate* isolate) {
    // This method invokes a finalizer. Updating the method name would require
    // adjusting CFI blocklist as weak_callback_ is invoked on the wrong type.
    CHECK(IsPendingFinalizer());
    set_state(NEAR_DEATH);
    // Check that we are not passing a finalized external string to
    // the callback.
    DCHECK(!object().IsExternalOneByteString() ||
           ExternalOneByteString::cast(object()).resource() != nullptr);
    DCHECK(!object().IsExternalTwoByteString() ||
           ExternalTwoByteString::cast(object()).resource() != nullptr);
    // Leaving V8.
    VMState<EXTERNAL> vmstate(isolate);
    HandleScope handle_scope(isolate);
    void* embedder_fields[v8::kEmbedderFieldsInWeakCallback] = {nullptr,
                                                                nullptr};
    v8::WeakCallbackInfo<void> data(reinterpret_cast<v8::Isolate*>(isolate),
                                    parameter(), embedder_fields, nullptr);
    weak_callback_(data);
    // For finalizers the handle must have either been reset or made strong.
    // Both cases reset the state.
    CHECK_NE(NEAR_DEATH, state());
  }

  void MarkAsFree() { set_state(FREE); }
  void MarkAsUsed() { set_state(NORMAL); }

  GlobalHandles* global_handles() {
    return NodeBlock<Node>::From(this)->global_handles();
  }

 private:
  // Fields that are not used for managing node memory.
  void ClearImplFields() { weak_callback_ = nullptr; }

  void CheckImplFieldsAreCleared() { DCHECK_EQ(nullptr, weak_callback_); }

  // This stores three flags (independent, partially_dependent and
  // in_young_list) and a State.
  using NodeState = base::BitField8<State, 0, 3>;
  using IsInYoungList = NodeState::Next<bool, 1>;
  using NodeWeaknessType = IsInYoungList::Next<WeaknessType, 2>;

  // Handle specific callback - might be a weak reference in disguise.
  WeakCallbackInfo<void>::Callback weak_callback_;

  friend class NodeBase<Node>;
};

class GlobalHandles::TracedNode final
    : public NodeBase<GlobalHandles::TracedNode> {
 public:
  TracedNode() { set_in_young_list(false); }

  // Copy and move ctors are used when constructing a TracedNode when recording
  // a node for on-stack data structures. (Older compilers may refer to copy
  // instead of move ctor.)
  TracedNode(TracedNode&& other) V8_NOEXCEPT = default;
  TracedNode(const TracedNode& other) V8_NOEXCEPT = default;

  enum State { FREE = 0, NORMAL, NEAR_DEATH };

  State state() const { return NodeState::decode(flags_); }
  void set_state(State state) { flags_ = NodeState::update(flags_, state); }

  void MarkAsFree() { set_state(FREE); }
  void MarkAsUsed() { set_state(NORMAL); }
  bool IsInUse() const { return state() != FREE; }
  bool IsRetainer() const { return state() == NORMAL; }

  bool is_in_young_list() const { return IsInYoungList::decode(flags_); }
  void set_in_young_list(bool v) { flags_ = IsInYoungList::update(flags_, v); }

  bool is_root() const { return IsRoot::decode(flags_); }
  void set_root(bool v) { flags_ = IsRoot::update(flags_, v); }

  void set_markbit() {
    NodeBlock<TracedNode>::From(this)->set_markbit(index());
  }

  bool markbit() const {
    return NodeBlock<TracedNode>::From(this)->markbit(index());
  }
  void clear_markbit() {
    NodeBlock<TracedNode>::From(this)->clear_markbit(index());
  }

  bool is_on_stack() const { return IsOnStack::decode(flags_); }
  void set_is_on_stack(bool v) { flags_ = IsOnStack::update(flags_, v); }

  void clear_object() {
    reinterpret_cast<std::atomic<Address>*>(&object_)->store(
        kNullAddress, std::memory_order_relaxed);
  }

  void CopyObjectReference(const TracedNode& other) {
    reinterpret_cast<std::atomic<Address>*>(&object_)->store(
        other.object_, std::memory_order_relaxed);
  }

  void ResetPhantomHandle() {
    DCHECK(IsInUse());
    NodeSpace<TracedNode>::Release(this);
    DCHECK(!IsInUse());
  }

  static void Verify(GlobalHandles* global_handles, const Address* const* slot);

 protected:
  // Various state is managed in a bit field. Mark bits are used concurrently
  // and held externally in a NodeBlock.
  using NodeState = base::BitField8<State, 0, 2>;
  using IsInYoungList = NodeState::Next<bool, 1>;
  using IsRoot = IsInYoungList::Next<bool, 1>;
  using IsOnStack = IsRoot::Next<bool, 1>;
  void ClearImplFields() {
    set_root(true);
    set_is_on_stack(false);
  }

  void CheckImplFieldsAreCleared() const { DCHECK(is_root()); }

  friend class NodeBase<GlobalHandles::TracedNode>;
};

// Space to keep track of on-stack handles (e.g. TracedReference). Such
// references are treated as root for any V8 garbage collection. The data
// structure is self healing and pessimistally filters outdated entries on
// insertion and iteration.
//
// Design doc: http://bit.ly/on-stack-traced-reference
class GlobalHandles::OnStackTracedNodeSpace final {
 public:
  static GlobalHandles* GetGlobalHandles(const TracedNode* on_stack_node) {
    DCHECK(on_stack_node->is_on_stack());
    return reinterpret_cast<const NodeEntry*>(on_stack_node)->global_handles;
  }

  explicit OnStackTracedNodeSpace(GlobalHandles* global_handles)
      : global_handles_(global_handles) {}

  void SetStackStart(void* stack_start) {
    CHECK(on_stack_nodes_.empty());
    stack_.SetStackStart(base::Stack::GetRealStackAddressForSlot(stack_start));
  }

  V8_INLINE bool IsOnStack(uintptr_t slot) const;

  void Iterate(RootVisitor* v);
  TracedNode* Acquire(Object value, uintptr_t address);
  void CleanupBelowCurrentStackPosition();
  void NotifyEmptyEmbedderStack();

  size_t NumberOfHandlesForTesting() const { return on_stack_nodes_.size(); }

 private:
  struct NodeEntry {
    TracedNode node;
    // Used to find back to GlobalHandles from a Node on copy. Needs to follow
    // node.
    GlobalHandles* global_handles;
  };

  // Keeps track of registered handles. The data structure is cleaned on
  // iteration and when adding new references using the current stack address.
  // Cleaning is based on current stack address and the key of the map which is
  // slightly different for ASAN configs -- see below.
#ifdef V8_USE_ADDRESS_SANITIZER
  // Mapping from stack slots or real stack frames to the corresponding nodes.
  // In case a reference is part of a fake frame, we map it to the real stack
  // frame base instead of the actual stack slot. The list keeps all nodes for
  // a particular real frame.
  std::map<uintptr_t, std::list<NodeEntry>> on_stack_nodes_;
#else   // !V8_USE_ADDRESS_SANITIZER
  // Mapping from stack slots to the corresponding nodes. We don't expect
  // aliasing with overlapping lifetimes of nodes.
  std::map<uintptr_t, NodeEntry> on_stack_nodes_;
#endif  // !V8_USE_ADDRESS_SANITIZER

  ::heap::base::Stack stack_;
  GlobalHandles* global_handles_ = nullptr;
  size_t acquire_count_ = 0;
};

bool GlobalHandles::OnStackTracedNodeSpace::IsOnStack(uintptr_t slot) const {
  // By the time this function is called, the stack start may not be set (i.e.
  // SetStackStart() was not called). In that case, assume the slot is not on
  // stack.
  if (!stack_.stack_start()) return false;
  return stack_.IsOnStack(reinterpret_cast<void*>(slot));
}

void GlobalHandles::OnStackTracedNodeSpace::NotifyEmptyEmbedderStack() {
  on_stack_nodes_.clear();
}

void GlobalHandles::OnStackTracedNodeSpace::Iterate(RootVisitor* v) {
#ifdef V8_USE_ADDRESS_SANITIZER
  for (auto& pair : on_stack_nodes_) {
    for (auto& node_entry : pair.second) {
      TracedNode& node = node_entry.node;
      if (node.IsRetainer()) {
        v->VisitRootPointer(Root::kGlobalHandles, "on-stack TracedReference",
                            node.location());
      }
    }
  }
#else   // !V8_USE_ADDRESS_SANITIZER
  // Handles have been cleaned from the GC entry point which is higher up the
  // stack.
  for (auto& pair : on_stack_nodes_) {
    TracedNode& node = pair.second.node;
    if (node.IsRetainer()) {
      v->VisitRootPointer(Root::kGlobalHandles, "on-stack TracedReference",
                          node.location());
    }
  }
#endif  // !V8_USE_ADDRESS_SANITIZER
}

GlobalHandles::TracedNode* GlobalHandles::OnStackTracedNodeSpace::Acquire(
    Object value, uintptr_t slot) {
  constexpr size_t kAcquireCleanupThresholdLog2 = 8;
  constexpr size_t kAcquireCleanupThresholdMask =
      (size_t{1} << kAcquireCleanupThresholdLog2) - 1;
  DCHECK(IsOnStack(slot));
  if (((acquire_count_++) & kAcquireCleanupThresholdMask) == 0) {
    CleanupBelowCurrentStackPosition();
  }
  NodeEntry entry;
  entry.node.Free(nullptr);
  entry.global_handles = global_handles_;
#ifdef V8_USE_ADDRESS_SANITIZER
  auto pair = on_stack_nodes_.insert(
      {base::Stack::GetRealStackAddressForSlot(slot), {}});
  pair.first->second.push_back(std::move(entry));
  TracedNode* result = &(pair.first->second.back().node);
#else   // !V8_USE_ADDRESS_SANITIZER
  auto pair = on_stack_nodes_.insert(
      {base::Stack::GetRealStackAddressForSlot(slot), std::move(entry)});
  if (!pair.second) {
    // Insertion failed because there already was an entry present for that
    // stack address. This can happen because cleanup is conservative in which
    // stack limits it used. Reusing the entry is fine as there's no aliasing of
    // different references with the same stack slot.
    pair.first->second.node.Free(nullptr);
  }
  TracedNode* result = &(pair.first->second.node);
#endif  // !V8_USE_ADDRESS_SANITIZER
  result->Acquire(value);
  result->set_is_on_stack(true);
  return result;
}

void GlobalHandles::OnStackTracedNodeSpace::CleanupBelowCurrentStackPosition() {
  if (on_stack_nodes_.empty()) return;
  const uintptr_t stack_ptr = reinterpret_cast<uintptr_t>(
      ::heap::base::Stack::GetCurrentStackPointerForLocalVariables());
  const auto it = on_stack_nodes_.upper_bound(stack_ptr);
  on_stack_nodes_.erase(on_stack_nodes_.begin(), it);
}

// static
void GlobalHandles::EnableMarkingBarrier(Isolate* isolate) {
  auto* global_handles = isolate->global_handles();
  DCHECK(!global_handles->is_marking_);
  global_handles->is_marking_ = true;
}

// static
void GlobalHandles::DisableMarkingBarrier(Isolate* isolate) {
  auto* global_handles = isolate->global_handles();
  DCHECK(global_handles->is_marking_);
  global_handles->is_marking_ = false;
}

// static
void GlobalHandles::TracedNode::Verify(GlobalHandles* global_handles,
                                       const Address* const* slot) {
#ifdef DEBUG
  const TracedNode* node = FromLocation(*slot);
  DCHECK(node->IsInUse());
  bool slot_on_stack = global_handles->on_stack_nodes_->IsOnStack(
      reinterpret_cast<uintptr_t>(slot));
  DCHECK_EQ(slot_on_stack, node->is_on_stack());
  if (!node->is_on_stack()) {
    // On-heap nodes have seprate lists for young generation processing.
    bool is_young_gen_object = ObjectInYoungGeneration(node->object());
    DCHECK_IMPLIES(is_young_gen_object, node->is_in_young_list());
  }
  bool in_young_list =
      std::find(global_handles->traced_young_nodes_.begin(),
                global_handles->traced_young_nodes_.end(),
                node) != global_handles->traced_young_nodes_.end();
  DCHECK_EQ(in_young_list, node->is_in_young_list());
#endif  // DEBUG
}

void GlobalHandles::CleanupOnStackReferencesBelowCurrentStackPosition() {
  on_stack_nodes_->CleanupBelowCurrentStackPosition();
}

size_t GlobalHandles::NumberOfOnStackHandlesForTesting() {
  return on_stack_nodes_->NumberOfHandlesForTesting();
}

size_t GlobalHandles::TotalSize() const {
  return regular_nodes_->TotalSize() + traced_nodes_->TotalSize();
}

size_t GlobalHandles::UsedSize() const {
  return regular_nodes_->handles_count() * sizeof(Node) +
         traced_nodes_->handles_count() * sizeof(TracedNode);
}

size_t GlobalHandles::handles_count() const {
  return regular_nodes_->handles_count() + traced_nodes_->handles_count();
}

void GlobalHandles::SetStackStart(void* stack_start) {
  on_stack_nodes_->SetStackStart(stack_start);
}

void GlobalHandles::NotifyEmptyEmbedderStack() {
  on_stack_nodes_->NotifyEmptyEmbedderStack();
}

GlobalHandles::GlobalHandles(Isolate* isolate)
    : isolate_(isolate),
      regular_nodes_(new NodeSpace<GlobalHandles::Node>(this)),
      traced_nodes_(new NodeSpace<GlobalHandles::TracedNode>(this)),
      on_stack_nodes_(new OnStackTracedNodeSpace(this)) {}

GlobalHandles::~GlobalHandles() { regular_nodes_.reset(nullptr); }

Handle<Object> GlobalHandles::Create(Object value) {
  GlobalHandles::Node* result = regular_nodes_->Acquire(value);
  if (ObjectInYoungGeneration(value) && !result->is_in_young_list()) {
    young_nodes_.push_back(result);
    result->set_in_young_list(true);
  }
  return result->handle();
}

Handle<Object> GlobalHandles::Create(Address value) {
  return Create(Object(value));
}

Handle<Object> GlobalHandles::CreateTraced(Object value, Address* slot,
                                           GlobalHandleStoreMode store_mode) {
  return CreateTraced(
      value, slot, store_mode,
      on_stack_nodes_->IsOnStack(reinterpret_cast<uintptr_t>(slot)));
}

Handle<Object> GlobalHandles::CreateTraced(Object value, Address* slot,
                                           GlobalHandleStoreMode store_mode,
                                           bool is_on_stack) {
  GlobalHandles::TracedNode* result;
  if (is_on_stack) {
    result = on_stack_nodes_->Acquire(value, reinterpret_cast<uintptr_t>(slot));
  } else {
    result = traced_nodes_->Acquire(value);
    if (ObjectInYoungGeneration(value) && !result->is_in_young_list()) {
      traced_young_nodes_.push_back(result);
      result->set_in_young_list(true);
    }
    // Nodes are black allocated for simplicity.
    result->set_markbit();
    if (store_mode != GlobalHandleStoreMode::kInitializingStore) {
      WriteBarrier::MarkingFromGlobalHandle(value);
    }
  }
  result->set_parameter(nullptr);
  return result->handle();
}

Handle<Object> GlobalHandles::CreateTraced(Address value, Address* slot,
                                           GlobalHandleStoreMode store_mode) {
  return CreateTraced(Object(value), slot, store_mode);
}

Handle<Object> GlobalHandles::CopyGlobal(Address* location) {
  DCHECK_NOT_NULL(location);
  GlobalHandles* global_handles =
      Node::FromLocation(location)->global_handles();
#ifdef VERIFY_HEAP
  if (i::FLAG_verify_heap) {
    Object(*location).ObjectVerify(global_handles->isolate());
  }
#endif  // VERIFY_HEAP
  return global_handles->Create(*location);
}

namespace {
void SetSlotThreadSafe(Address** slot, Address* val) {
  reinterpret_cast<std::atomic<Address*>*>(slot)->store(
      val, std::memory_order_relaxed);
}
}  // namespace

// static
void GlobalHandles::CopyTracedReference(const Address* const* from,
                                        Address** to) {
  DCHECK_NOT_NULL(*from);
  DCHECK_NULL(*to);
  const TracedNode* node = TracedNode::FromLocation(*from);
  GlobalHandles* global_handles =
      GlobalHandles::From(const_cast<TracedNode*>(node));
  Handle<Object> o = global_handles->CreateTraced(
      node->object(), reinterpret_cast<Address*>(to),
      GlobalHandleStoreMode::kAssigningStore);
  SetSlotThreadSafe(to, o.location());
  TracedNode::Verify(global_handles, from);
  TracedNode::Verify(global_handles, to);
#ifdef VERIFY_HEAP
  if (i::FLAG_verify_heap) {
    Object(**to).ObjectVerify(global_handles->isolate());
  }
#endif  // VERIFY_HEAP
}

void GlobalHandles::MoveGlobal(Address** from, Address** to) {
  DCHECK_NOT_NULL(*from);
  DCHECK_NOT_NULL(*to);
  DCHECK_EQ(*from, *to);
  Node* node = Node::FromLocation(*from);
  if (node->IsWeak() && node->IsPhantomResetHandle()) {
    node->set_parameter(to);
  }

  // - Strong handles do not require fixups.
  // - Weak handles with finalizers and callbacks are too general to fix up. For
  //   those the callers need to ensure consistency.
}

void GlobalHandles::MoveTracedReference(Address** from, Address** to) {
  // Fast path for moving from an empty reference.
  if (!*from) {
    DestroyTracedReference(*to);
    SetSlotThreadSafe(to, nullptr);
    return;
  }

  // Determining whether from or to are on stack.
  TracedNode* from_node = TracedNode::FromLocation(*from);
  DCHECK(from_node->IsInUse());
  TracedNode* to_node = TracedNode::FromLocation(*to);
  GlobalHandles* global_handles = nullptr;
#ifdef DEBUG
  global_handles = GlobalHandles::From(from_node);
#endif  // DEBUG
  bool from_on_stack = from_node->is_on_stack();
  bool to_on_stack = false;
  if (!to_node) {
    // Figure out whether stack or heap to allow fast path for heap->heap move.
    global_handles = GlobalHandles::From(from_node);
    to_on_stack = global_handles->on_stack_nodes_->IsOnStack(
        reinterpret_cast<uintptr_t>(to));
  } else {
    to_on_stack = to_node->is_on_stack();
  }

  // Moving.
  if (from_on_stack || to_on_stack) {
    // Move involving a stack slot.
    if (!to_node) {
      DCHECK(global_handles);
      Handle<Object> o = global_handles->CreateTraced(
          from_node->object(), reinterpret_cast<Address*>(to),
          GlobalHandleStoreMode::kAssigningStore, to_on_stack);
      SetSlotThreadSafe(to, o.location());
      to_node = TracedNode::FromLocation(*to);
      DCHECK_IMPLIES(!to_node->is_on_stack(), to_node->markbit());
    } else {
      DCHECK(to_node->IsInUse());
      to_node->CopyObjectReference(*from_node);
      if (!to_node->is_on_stack() && !to_node->is_in_young_list() &&
          ObjectInYoungGeneration(to_node->object())) {
        global_handles = GlobalHandles::From(from_node);
        global_handles->traced_young_nodes_.push_back(to_node);
        to_node->set_in_young_list(true);
      }
      if (!to_on_stack) {
        WriteBarrier::MarkingFromGlobalHandle(to_node->object());
      }
    }
    DestroyTracedReference(*from);
    SetSlotThreadSafe(from, nullptr);
  } else {
    // Pure heap move.
    DestroyTracedReference(*to);
    SetSlotThreadSafe(to, *from);
    to_node = from_node;
    DCHECK_NOT_NULL(*from);
    DCHECK_NOT_NULL(*to);
    DCHECK_EQ(*from, *to);
    WriteBarrier::MarkingFromGlobalHandle(to_node->object());
    SetSlotThreadSafe(from, nullptr);
  }
  TracedNode::Verify(global_handles, to);
}

// static
GlobalHandles* GlobalHandles::From(const TracedNode* node) {
  return node->is_on_stack()
             ? OnStackTracedNodeSpace::GetGlobalHandles(node)
             : NodeBlock<TracedNode>::From(node)->global_handles();
}

void GlobalHandles::MarkTraced(Address* location) {
  TracedNode* node = TracedNode::FromLocation(location);
  DCHECK(node->IsInUse());
  if (node->is_on_stack()) return;
  node->set_markbit();
}

void GlobalHandles::Destroy(Address* location) {
  if (location != nullptr) {
    NodeSpace<Node>::Release(Node::FromLocation(location));
  }
}

// static
void GlobalHandles::DestroyTracedReference(Address* location) {
  if (location != nullptr) {
    TracedNode* node = TracedNode::FromLocation(location);
    if (node->is_on_stack()) {
      node->Release(nullptr);
      return;
    }
    DCHECK(!node->is_on_stack());

    auto* global_handles = GlobalHandles::From(node);
    // When marking is off the handle may be freed immediately. Note that this
    // includes also the case when invoking the first pass callbacks during the
    // atomic pause which requires releasing a node fully.
    if (!global_handles->is_marking_) {
      NodeSpace<TracedNode>::Release(node);
      return;
    }

    // Incremental marking is on. This also covers the scavenge case which
    // prohibits eagerly reclaiming nodes when marking is on during a scavenge.
    //
    // On-heap traced nodes are released in the atomic pause in
    // `IterateWeakRootsForPhantomHandles()` when they are discovered as not
    // marked.
    //
    // Eagerly clear out the object here to avoid needlessly marking it from
    // this point on. Also clear out callback and backreference for the version
    // with callbacks to avoid calling into possibly dead memory later.
    //
    // In the case this happens during incremental marking, the node may
    // still be spuriously marked as live and is then only reclaimed on the
    // next cycle.
    node->clear_object();
    node->set_parameter(nullptr);
  }
}

using GenericCallback = v8::WeakCallbackInfo<void>::Callback;

void GlobalHandles::MakeWeak(Address* location, void* parameter,
                             GenericCallback phantom_callback,
                             v8::WeakCallbackType type) {
  Node::FromLocation(location)->MakeWeak(parameter, phantom_callback, type);
}

void GlobalHandles::MakeWeak(Address** location_addr) {
  Node::FromLocation(*location_addr)->MakeWeak(location_addr);
}

void* GlobalHandles::ClearWeakness(Address* location) {
  return Node::FromLocation(location)->ClearWeakness();
}

void GlobalHandles::AnnotateStrongRetainer(Address* location,
                                           const char* label) {
  Node::FromLocation(location)->AnnotateStrongRetainer(label);
}

bool GlobalHandles::IsWeak(Address* location) {
  return Node::FromLocation(location)->IsWeak();
}

DISABLE_CFI_PERF
void GlobalHandles::IterateWeakRootsForFinalizers(RootVisitor* v) {
  for (Node* node : *regular_nodes_) {
    if (node->IsWeakRetainer() && node->state() == Node::PENDING) {
      DCHECK(!node->IsPhantomCallback());
      DCHECK(!node->IsPhantomResetHandle());
      // Finalizers need to survive.
      v->VisitRootPointer(Root::kGlobalHandles, node->label(),
                          node->location());
    }
  }
}

DISABLE_CFI_PERF
void GlobalHandles::IterateWeakRootsForPhantomHandles(
    WeakSlotCallbackWithHeap should_reset_handle) {
  for (Node* node : *regular_nodes_) {
    if (node->IsWeakRetainer() &&
        should_reset_handle(isolate()->heap(), node->location())) {
      if (node->IsPhantomResetHandle()) {
        node->MarkPending();
        node->ResetPhantomHandle();
        ++number_of_phantom_handle_resets_;
      } else if (node->IsPhantomCallback()) {
        node->MarkPending();
        node->CollectPhantomCallbackData(&regular_pending_phantom_callbacks_);
      }
    }
  }
  for (TracedNode* node : *traced_nodes_) {
    if (!node->IsInUse()) continue;
    // Detect unreachable nodes first.
    if (!node->markbit()) {
      // The handle itself is unreachable. We can clear it even if the target V8
      // object is alive.
      node->ResetPhantomHandle();
      ++number_of_phantom_handle_resets_;
      continue;
    }
    // Clear the markbit for the next GC.
    node->clear_markbit();
    DCHECK(node->IsInUse());
    // Detect nodes with unreachable target objects.
    if (should_reset_handle(isolate()->heap(), node->location())) {
      node->ResetPhantomHandle();
      ++number_of_phantom_handle_resets_;
    }
  }
}

void GlobalHandles::IterateWeakRootsIdentifyFinalizers(
    WeakSlotCallbackWithHeap should_reset_handle) {
  for (Node* node : *regular_nodes_) {
    if (node->IsWeak() &&
        should_reset_handle(isolate()->heap(), node->location())) {
      if (node->IsFinalizerHandle()) {
        node->MarkPending();
      }
    }
  }
}

void GlobalHandles::IdentifyWeakUnmodifiedObjects(
    WeakSlotCallback is_unmodified) {
  if (!FLAG_reclaim_unmodified_wrappers) return;

  // Treat all objects as roots during incremental marking to avoid corrupting
  // marking worklists.
  if (isolate()->heap()->incremental_marking()->IsMarking()) return;

  auto* const handler = isolate()->heap()->GetEmbedderRootsHandler();
  for (TracedNode* node : traced_young_nodes_) {
    if (node->IsInUse()) {
      DCHECK(node->is_root());
      if (is_unmodified(node->location())) {
        v8::Value* value = ToApi<v8::Value>(node->handle());
        node->set_root(handler->IsRoot(
            *reinterpret_cast<v8::TracedReference<v8::Value>*>(&value)));
      }
    }
  }
}

void GlobalHandles::IterateYoungStrongAndDependentRoots(RootVisitor* v) {
  for (Node* node : young_nodes_) {
    if (node->IsStrongRetainer()) {
      v->VisitRootPointer(Root::kGlobalHandles, node->label(),
                          node->location());
    }
  }
  for (TracedNode* node : traced_young_nodes_) {
    if (node->IsInUse() && node->is_root()) {
      v->VisitRootPointer(Root::kGlobalHandles, nullptr, node->location());
    }
  }
}

void GlobalHandles::MarkYoungWeakDeadObjectsPending(
    WeakSlotCallbackWithHeap is_dead) {
  for (Node* node : young_nodes_) {
    DCHECK(node->is_in_young_list());
    if (node->IsWeak() && is_dead(isolate_->heap(), node->location())) {
      if (!node->IsPhantomCallback() && !node->IsPhantomResetHandle()) {
        node->MarkPending();
      }
    }
  }
}

void GlobalHandles::IterateYoungWeakDeadObjectsForFinalizers(RootVisitor* v) {
  for (Node* node : young_nodes_) {
    DCHECK(node->is_in_young_list());
    if (node->IsWeakRetainer() && (node->state() == Node::PENDING)) {
      DCHECK(!node->IsPhantomCallback());
      DCHECK(!node->IsPhantomResetHandle());
      // Finalizers need to survive.
      v->VisitRootPointer(Root::kGlobalHandles, node->label(),
                          node->location());
    }
  }
}

void GlobalHandles::IterateYoungWeakObjectsForPhantomHandles(
    RootVisitor* v, WeakSlotCallbackWithHeap should_reset_handle) {
  for (Node* node : young_nodes_) {
    DCHECK(node->is_in_young_list());
    if (node->IsWeakRetainer() && (node->state() != Node::PENDING)) {
      if (should_reset_handle(isolate_->heap(), node->location())) {
        DCHECK(node->IsPhantomResetHandle() || node->IsPhantomCallback());
        if (node->IsPhantomResetHandle()) {
          node->MarkPending();
          node->ResetPhantomHandle();
          ++number_of_phantom_handle_resets_;
        } else if (node->IsPhantomCallback()) {
          node->MarkPending();
          node->CollectPhantomCallbackData(&regular_pending_phantom_callbacks_);
        } else {
          UNREACHABLE();
        }
      } else {
        // Node survived and needs to be visited.
        v->VisitRootPointer(Root::kGlobalHandles, node->label(),
                            node->location());
      }
    }
  }

  if (!FLAG_reclaim_unmodified_wrappers) return;

  auto* const handler = isolate()->heap()->GetEmbedderRootsHandler();
  for (TracedNode* node : traced_young_nodes_) {
    if (!node->IsInUse()) continue;

    DCHECK_IMPLIES(node->is_root(),
                   !should_reset_handle(isolate_->heap(), node->location()));
    if (should_reset_handle(isolate_->heap(), node->location())) {
      v8::Value* value = ToApi<v8::Value>(node->handle());
      handler->ResetRoot(
          *reinterpret_cast<v8::TracedReference<v8::Value>*>(&value));
      // We cannot check whether a node is in use here as the reset behavior
      // depends on whether incremental marking is running when reclaiming
      // young objects.
      ++number_of_phantom_handle_resets_;
    } else {
      if (!node->is_root()) {
        node->set_root(true);
        v->VisitRootPointer(Root::kGlobalHandles, nullptr, node->location());
      }
    }
  }
}

void GlobalHandles::InvokeSecondPassPhantomCallbacksFromTask() {
  DCHECK(second_pass_callbacks_task_posted_);
  second_pass_callbacks_task_posted_ = false;
  Heap::DevToolsTraceEventScope devtools_trace_event_scope(
      isolate()->heap(), "MajorGC", "invoke weak phantom callbacks");
  TRACE_EVENT0("v8", "V8.GCPhantomHandleProcessingCallback");
  isolate()->heap()->CallGCPrologueCallbacks(
      GCType::kGCTypeProcessWeakCallbacks, kNoGCCallbackFlags);
  InvokeSecondPassPhantomCallbacks();
  isolate()->heap()->CallGCEpilogueCallbacks(
      GCType::kGCTypeProcessWeakCallbacks, kNoGCCallbackFlags);
}

void GlobalHandles::InvokeSecondPassPhantomCallbacks() {
  // The callbacks may execute JS, which in turn may lead to another GC run.
  // If we are already processing the callbacks, we do not want to start over
  // from within the inner GC. Newly added callbacks will always be run by the
  // outermost GC run only.
  if (running_second_pass_callbacks_) return;
  running_second_pass_callbacks_ = true;

  AllowJavascriptExecution allow_js(isolate());
  while (!second_pass_callbacks_.empty()) {
    auto callback = second_pass_callbacks_.back();
    second_pass_callbacks_.pop_back();
    callback.Invoke(isolate(), PendingPhantomCallback::kSecondPass);
  }
  running_second_pass_callbacks_ = false;
}

size_t GlobalHandles::PostScavengeProcessing(unsigned post_processing_count) {
  size_t freed_nodes = 0;
  for (Node* node : young_nodes_) {
    // Filter free nodes.
    if (!node->IsRetainer()) continue;

    if (node->IsPending()) {
      DCHECK(node->has_callback());
      DCHECK(node->IsPendingFinalizer());
      node->PostGarbageCollectionProcessing(isolate_);
    }
    if (InRecursiveGC(post_processing_count)) return freed_nodes;

    if (!node->IsRetainer()) freed_nodes++;
  }
  return freed_nodes;
}

size_t GlobalHandles::PostMarkSweepProcessing(unsigned post_processing_count) {
  size_t freed_nodes = 0;
  for (Node* node : *regular_nodes_) {
    // Filter free nodes.
    if (!node->IsRetainer()) continue;

    if (node->IsPending()) {
      DCHECK(node->has_callback());
      DCHECK(node->IsPendingFinalizer());
      node->PostGarbageCollectionProcessing(isolate_);
    }
    if (InRecursiveGC(post_processing_count)) return freed_nodes;

    if (!node->IsRetainer()) freed_nodes++;
  }
  return freed_nodes;
}

template <typename T>
void GlobalHandles::UpdateAndCompactListOfYoungNode(
    std::vector<T*>* node_list) {
  size_t last = 0;
  for (T* node : *node_list) {
    DCHECK(node->is_in_young_list());
    if (node->IsInUse()) {
      if (ObjectInYoungGeneration(node->object())) {
        (*node_list)[last++] = node;
        isolate_->heap()->IncrementNodesCopiedInNewSpace();
      } else {
        node->set_in_young_list(false);
        isolate_->heap()->IncrementNodesPromoted();
      }
    } else {
      node->set_in_young_list(false);
      isolate_->heap()->IncrementNodesDiedInNewSpace();
    }
  }
  DCHECK_LE(last, node_list->size());
  node_list->resize(last);
  node_list->shrink_to_fit();
}

void GlobalHandles::UpdateListOfYoungNodes() {
  UpdateAndCompactListOfYoungNode(&young_nodes_);
  UpdateAndCompactListOfYoungNode(&traced_young_nodes_);
}

template <typename T>
size_t GlobalHandles::InvokeFirstPassWeakCallbacks(
    std::vector<std::pair<T*, PendingPhantomCallback>>* pending) {
  size_t freed_nodes = 0;
  std::vector<std::pair<T*, PendingPhantomCallback>> pending_phantom_callbacks;
  pending_phantom_callbacks.swap(*pending);
  {
    // The initial pass callbacks must simply clear the nodes.
    for (auto& pair : pending_phantom_callbacks) {
      T* node = pair.first;
      DCHECK_EQ(T::NEAR_DEATH, node->state());
      pair.second.Invoke(isolate(), PendingPhantomCallback::kFirstPass);

      // Transition to second pass. It is required that the first pass callback
      // resets the handle using |v8::PersistentBase::Reset|. Also see comments
      // on |v8::WeakCallbackInfo|.
      CHECK_WITH_MSG(T::FREE == node->state(),
                     "Handle not reset in first callback. See comments on "
                     "|v8::WeakCallbackInfo|.");

      if (pair.second.callback()) second_pass_callbacks_.push_back(pair.second);
      freed_nodes++;
    }
  }
  return freed_nodes;
}

size_t GlobalHandles::InvokeFirstPassWeakCallbacks() {
  return InvokeFirstPassWeakCallbacks(&regular_pending_phantom_callbacks_) +
         InvokeFirstPassWeakCallbacks(&traced_pending_phantom_callbacks_);
}

void GlobalHandles::InvokeOrScheduleSecondPassPhantomCallbacks(
    bool synchronous_second_pass) {
  if (!second_pass_callbacks_.empty()) {
    if (FLAG_optimize_for_size || FLAG_predictable || synchronous_second_pass) {
      Heap::DevToolsTraceEventScope devtools_trace_event_scope(
          isolate()->heap(), "MajorGC", "invoke weak phantom callbacks");
      isolate()->heap()->CallGCPrologueCallbacks(
          GCType::kGCTypeProcessWeakCallbacks, kNoGCCallbackFlags);
      InvokeSecondPassPhantomCallbacks();
      isolate()->heap()->CallGCEpilogueCallbacks(
          GCType::kGCTypeProcessWeakCallbacks, kNoGCCallbackFlags);
    } else if (!second_pass_callbacks_task_posted_) {
      second_pass_callbacks_task_posted_ = true;
      auto taskrunner = V8::GetCurrentPlatform()->GetForegroundTaskRunner(
          reinterpret_cast<v8::Isolate*>(isolate()));
      taskrunner->PostTask(MakeCancelableTask(
          isolate(), [this] { InvokeSecondPassPhantomCallbacksFromTask(); }));
    }
  }
}

void GlobalHandles::PendingPhantomCallback::Invoke(Isolate* isolate,
                                                   InvocationType type) {
  Data::Callback* callback_addr = nullptr;
  if (type == kFirstPass) {
    callback_addr = &callback_;
  }
  Data data(reinterpret_cast<v8::Isolate*>(isolate), parameter_,
            embedder_fields_, callback_addr);
  Data::Callback callback = callback_;
  callback_ = nullptr;
  callback(data);
}

bool GlobalHandles::InRecursiveGC(unsigned gc_processing_counter) {
  return gc_processing_counter != post_gc_processing_count_;
}

size_t GlobalHandles::PostGarbageCollectionProcessing(
    GarbageCollector collector, const v8::GCCallbackFlags gc_callback_flags) {
  // Process weak global handle callbacks. This must be done after the
  // GC is completely done, because the callbacks may invoke arbitrary
  // API functions.
  DCHECK_EQ(Heap::NOT_IN_GC, isolate_->heap()->gc_state());
  const unsigned post_processing_count = ++post_gc_processing_count_;
  size_t freed_nodes = 0;
  bool synchronous_second_pass =
      isolate_->heap()->IsTearingDown() ||
      (gc_callback_flags &
       (kGCCallbackFlagForced | kGCCallbackFlagCollectAllAvailableGarbage |
        kGCCallbackFlagSynchronousPhantomCallbackProcessing)) != 0;
  InvokeOrScheduleSecondPassPhantomCallbacks(synchronous_second_pass);
  if (InRecursiveGC(post_processing_count)) return freed_nodes;

  freed_nodes += Heap::IsYoungGenerationCollector(collector)
                     ? PostScavengeProcessing(post_processing_count)
                     : PostMarkSweepProcessing(post_processing_count);
  if (InRecursiveGC(post_processing_count)) return freed_nodes;

  UpdateListOfYoungNodes();
  return freed_nodes;
}

void GlobalHandles::IterateStrongRoots(RootVisitor* v) {
  for (Node* node : *regular_nodes_) {
    if (node->IsStrongRetainer()) {
      v->VisitRootPointer(Root::kGlobalHandles, node->label(),
                          node->location());
    }
  }
}

void GlobalHandles::IterateStrongStackRoots(RootVisitor* v) {
  on_stack_nodes_->Iterate(v);
}

void GlobalHandles::IterateWeakRoots(RootVisitor* v) {
  for (Node* node : *regular_nodes_) {
    if (node->IsWeak()) {
      v->VisitRootPointer(Root::kGlobalHandles, node->label(),
                          node->location());
    }
  }
  for (TracedNode* node : *traced_nodes_) {
    if (node->IsInUse()) {
      v->VisitRootPointer(Root::kGlobalHandles, nullptr, node->location());
    }
  }
}

DISABLE_CFI_PERF
void GlobalHandles::IterateAllRoots(RootVisitor* v) {
  for (Node* node : *regular_nodes_) {
    if (node->IsRetainer()) {
      v->VisitRootPointer(Root::kGlobalHandles, node->label(),
                          node->location());
    }
  }
  for (TracedNode* node : *traced_nodes_) {
    if (node->IsRetainer()) {
      v->VisitRootPointer(Root::kGlobalHandles, nullptr, node->location());
    }
  }
  on_stack_nodes_->Iterate(v);
}

DISABLE_CFI_PERF
void GlobalHandles::IterateAllYoungRoots(RootVisitor* v) {
  for (Node* node : young_nodes_) {
    if (node->IsRetainer()) {
      v->VisitRootPointer(Root::kGlobalHandles, node->label(),
                          node->location());
    }
  }
  for (TracedNode* node : traced_young_nodes_) {
    if (node->IsRetainer()) {
      v->VisitRootPointer(Root::kGlobalHandles, nullptr, node->location());
    }
  }
  on_stack_nodes_->Iterate(v);
}

DISABLE_CFI_PERF
void GlobalHandles::ApplyPersistentHandleVisitor(
    v8::PersistentHandleVisitor* visitor, GlobalHandles::Node* node) {
  v8::Value* value = ToApi<v8::Value>(node->handle());
  visitor->VisitPersistentHandle(
      reinterpret_cast<v8::Persistent<v8::Value>*>(&value),
      node->wrapper_class_id());
}

DISABLE_CFI_PERF
void GlobalHandles::IterateAllRootsWithClassIds(
    v8::PersistentHandleVisitor* visitor) {
  for (Node* node : *regular_nodes_) {
    if (node->IsRetainer() && node->has_wrapper_class_id()) {
      ApplyPersistentHandleVisitor(visitor, node);
    }
  }
}

DISABLE_CFI_PERF
void GlobalHandles::IterateTracedNodes(
    v8::EmbedderHeapTracer::TracedGlobalHandleVisitor* visitor) {
  for (TracedNode* node : *traced_nodes_) {
    if (node->IsInUse()) {
      v8::Value* value = ToApi<v8::Value>(node->handle());
      visitor->VisitTracedReference(
          *reinterpret_cast<v8::TracedReference<v8::Value>*>(&value));
    }
  }
}

DISABLE_CFI_PERF
void GlobalHandles::IterateAllYoungRootsWithClassIds(
    v8::PersistentHandleVisitor* visitor) {
  for (Node* node : young_nodes_) {
    if (node->IsRetainer() && node->has_wrapper_class_id()) {
      ApplyPersistentHandleVisitor(visitor, node);
    }
  }
}

DISABLE_CFI_PERF
void GlobalHandles::IterateYoungWeakRootsWithClassIds(
    v8::PersistentHandleVisitor* visitor) {
  for (Node* node : young_nodes_) {
    if (node->has_wrapper_class_id() && node->IsWeak()) {
      ApplyPersistentHandleVisitor(visitor, node);
    }
  }
}

void GlobalHandles::RecordStats(HeapStats* stats) {
  *stats->global_handle_count = 0;
  *stats->weak_global_handle_count = 0;
  *stats->pending_global_handle_count = 0;
  *stats->near_death_global_handle_count = 0;
  *stats->free_global_handle_count = 0;
  for (Node* node : *regular_nodes_) {
    *stats->global_handle_count += 1;
    if (node->state() == Node::WEAK) {
      *stats->weak_global_handle_count += 1;
    } else if (node->state() == Node::PENDING) {
      *stats->pending_global_handle_count += 1;
    } else if (node->state() == Node::NEAR_DEATH) {
      *stats->near_death_global_handle_count += 1;
    } else if (node->state() == Node::FREE) {
      *stats->free_global_handle_count += 1;
    }
  }
}

#ifdef DEBUG

void GlobalHandles::PrintStats() {
  int total = 0;
  int weak = 0;
  int pending = 0;
  int near_death = 0;
  int destroyed = 0;

  for (Node* node : *regular_nodes_) {
    total++;
    if (node->state() == Node::WEAK) weak++;
    if (node->state() == Node::PENDING) pending++;
    if (node->state() == Node::NEAR_DEATH) near_death++;
    if (node->state() == Node::FREE) destroyed++;
  }

  PrintF("Global Handle Statistics:\n");
  PrintF("  allocated memory = %zuB\n", total * sizeof(Node));
  PrintF("  # weak       = %d\n", weak);
  PrintF("  # pending    = %d\n", pending);
  PrintF("  # near_death = %d\n", near_death);
  PrintF("  # free       = %d\n", destroyed);
  PrintF("  # total      = %d\n", total);
}

void GlobalHandles::Print() {
  PrintF("Global handles:\n");
  for (Node* node : *regular_nodes_) {
    PrintF("  handle %p to %p%s\n", node->location().ToVoidPtr(),
           reinterpret_cast<void*>(node->object().ptr()),
           node->IsWeak() ? " (weak)" : "");
  }
}

#endif

EternalHandles::~EternalHandles() {
  for (Address* block : blocks_) delete[] block;
}

void EternalHandles::IterateAllRoots(RootVisitor* visitor) {
  int limit = size_;
  for (Address* block : blocks_) {
    DCHECK_GT(limit, 0);
    visitor->VisitRootPointers(
        Root::kEternalHandles, nullptr, FullObjectSlot(block),
        FullObjectSlot(block + std::min({limit, kSize})));
    limit -= kSize;
  }
}

void EternalHandles::IterateYoungRoots(RootVisitor* visitor) {
  for (int index : young_node_indices_) {
    visitor->VisitRootPointer(Root::kEternalHandles, nullptr,
                              FullObjectSlot(GetLocation(index)));
  }
}

void EternalHandles::PostGarbageCollectionProcessing() {
  size_t last = 0;
  for (int index : young_node_indices_) {
    if (ObjectInYoungGeneration(Object(*GetLocation(index)))) {
      young_node_indices_[last++] = index;
    }
  }
  DCHECK_LE(last, young_node_indices_.size());
  young_node_indices_.resize(last);
}

void EternalHandles::Create(Isolate* isolate, Object object, int* index) {
  DCHECK_EQ(kInvalidIndex, *index);
  if (object == Object()) return;
  Object the_hole = ReadOnlyRoots(isolate).the_hole_value();
  DCHECK_NE(the_hole, object);
  int block = size_ >> kShift;
  int offset = size_ & kMask;
  // Need to resize.
  if (offset == 0) {
    Address* next_block = new Address[kSize];
    MemsetPointer(FullObjectSlot(next_block), the_hole, kSize);
    blocks_.push_back(next_block);
  }
  DCHECK_EQ(the_hole.ptr(), blocks_[block][offset]);
  blocks_[block][offset] = object.ptr();
  if (ObjectInYoungGeneration(object)) {
    young_node_indices_.push_back(size_);
  }
  *index = size_++;
}

}  // namespace internal
}  // namespace v8