WIP: Blocks for global handles.

src/global-handles.cc

Index: src/global-handles.cc
diff --git a/src/global-handles.cc b/src/global-handles.cc
index e4bbc9582e6d33e755e67a352c1d9679241ad394..cf552b6bb8c6b91bf545baf02755ba9809d524ba 100644
--- a/src/global-handles.cc
+++ b/src/global-handles.cc
@@ -41,53 +41,125 @@ ObjectGroup::~ObjectGroup() {
 }
 
 
-class GlobalHandles::Node : public Malloced {
+class GlobalHandles::Node {
  public:
+  // State transition diagram:
+  // NORMAL <-> WEAK -> PENDING -> NEAR_DEATH -> { NORMAL, WEAK, DESTROYED }
+  enum State {
+    DESTROYED,
+    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.
+  };
 
-  void Initialize(Object* object) {
-    // Set the initial value of the handle.
-    object_ = object;
-    class_id_ = v8::HeapProfiler::kPersistentHandleNoClassId;
-    independent_ = false;
-    state_  = NORMAL;
-    parameter_or_next_free_.parameter = NULL;
-    callback_ = NULL;
-  }
-
-  Node() {
-    state_ = DESTROYED;
+  // Maps handle location (slot) to the containing node.
+  static Node* FromLocation(Object** location) {
+    ASSERT(OFFSET_OF(Node, object_) == 0);
+    return reinterpret_cast<Node*>(location);
   }
 
-  explicit Node(Object* object) {
-    Initialize(object);
-    // Initialize link structure.
-    next_ = NULL;
-  }
+  Node() {}
 
-  ~Node() {
-    if (state_ != DESTROYED) Destroy(Isolate::Current()->global_handles());
 #ifdef DEBUG
+  ~Node() {
+    // ASSERT(state_ == DESTROYED);
     // Zap the values for eager trapping.
     object_ = NULL;
-    next_ = NULL;
+    class_id_ = v8::HeapProfiler::kPersistentHandleNoClassId;
+    index_ = 0;
+    independent_ = false;
     parameter_or_next_free_.next_free = NULL;
+    callback_ = NULL;
+  }
 #endif
+
+  void PreInitialize(int index, GlobalHandles* global_handles) {
+    index_ = index;
+    state_ = DESTROYED;
+    parameter_or_next_free_.next_free = global_handles->first_free_;
+    global_handles->first_free_ = this;
+  }
+
+  void Initialize(Object* object, GlobalHandles* global_handles) {
+    // Set the initial value of the handle.
+    object_ = object;
+    class_id_ = v8::HeapProfiler::kPersistentHandleNoClassId;
+    independent_ = false;
+    state_  = NORMAL;
+    parameter_or_next_free_.parameter = NULL;
+    callback_ = NULL;
+    IncreaseBlockUses(global_handles);
   }
 
   void Destroy(GlobalHandles* global_handles) {
-    if (state_ == WEAK || IsNearDeath()) {
+    if (IsWeakRetainer()) {
       global_handles->number_of_weak_handles_--;
       if (object_->IsJSGlobalObject()) {
         global_handles->number_of_global_object_weak_handles_--;
       }
     }
     state_ = DESTROYED;
+    parameter_or_next_free_.next_free = global_handles->first_free_;
+    global_handles->first_free_ = this;
+    DecreaseBlockUses(global_handles);
+  }
+
+  // Object slot accessors.
+  Object* object() const { return object_; }
+  Object** location() { return &object_; }
+  Handle<Object> handle() { return Handle<Object>(location()); }
+
+  // Wrapper class ID accessors.
+  bool has_wrapper_class_id() const {
+    return class_id_ != v8::HeapProfiler::kPersistentHandleNoClassId;
+  }
+  uint16_t wrapper_class_id() const { return class_id_; }
+  void set_wrapper_class_id(uint16_t class_id) {
+    class_id_ = class_id;
+  }
+
+  // State accessors.
+
+  State state() const { return state_; }
+
+  bool IsNearDeath() const {
+    // Check for PENDING to ensure correct answer when processing callbacks.
+    return state_ == PENDING || state_ == NEAR_DEATH;
   }
 
-  // Accessors for next_.
-  Node* next() { return next_; }
-  void set_next(Node* value) { next_ = value; }
-  Node** next_addr() { return &next_; }
+  bool IsWeak() const { return state_ == WEAK; }
+
+  bool IsRetainer() const { return state_ != DESTROYED; }
+
+  bool IsWeakRetainer() const {
+    return state_ == WEAK || state_ == PENDING || state_ == NEAR_DEATH;
+  }
+
+  void MarkPending() {
+    ASSERT(state_ == WEAK);
+    state_ = PENDING;
+  }
+
+  // Independent flag accessors.
+  void MarkIndependent() {
+    ASSERT(state_ != DESTROYED);
+    independent_ = true;
+  }
+  bool is_independent() const { return independent_; }
+
+  // Callback accessor.
+  WeakReferenceCallback callback() { return callback_; }
+
+  // Callback parameter accessors.
+  void set_parameter(void* parameter) {
+    ASSERT(state_ != DESTROYED);
+    parameter_or_next_free_.parameter = parameter;
+  }
+  void* parameter() const {
+    ASSERT(state_ != DESTROYED);
+    return parameter_or_next_free_.parameter;
+  }
 
   // Accessors for next free node in the free list.
   Node* next_free() {
@@ -99,22 +171,11 @@ class GlobalHandles::Node : public Malloced {
     parameter_or_next_free_.next_free = value;
   }
 
-  // Returns a link from the handle.
-  static Node* FromLocation(Object** location) {
-    ASSERT(OFFSET_OF(Node, object_) == 0);
-    return reinterpret_cast<Node*>(location);
-  }
-
-  // Returns the handle.
-  Handle<Object> handle() { return Handle<Object>(&object_); }
-
-  // Make this handle weak.
-  void MakeWeak(GlobalHandles* global_handles, void* parameter,
+  void MakeWeak(GlobalHandles* global_handles,
+                void* parameter,
                 WeakReferenceCallback callback) {
-    LOG(global_handles->isolate(),
-        HandleEvent("GlobalHandle::MakeWeak", handle().location()));
     ASSERT(state_ != DESTROYED);
-    if (state_ != WEAK && !IsNearDeath()) {
+    if (!IsWeakRetainer()) {
       global_handles->number_of_weak_handles_++;
       if (object_->IsJSGlobalObject()) {
         global_handles->number_of_global_object_weak_handles_++;
@@ -126,10 +187,8 @@ class GlobalHandles::Node : public Malloced {
   }
 
   void ClearWeakness(GlobalHandles* global_handles) {
-    LOG(global_handles->isolate(),
-        HandleEvent("GlobalHandle::ClearWeakness", handle().location()));
     ASSERT(state_ != DESTROYED);
-    if (state_ == WEAK || IsNearDeath()) {
+    if (IsWeakRetainer()) {
       global_handles->number_of_weak_handles_--;
       if (object_->IsJSGlobalObject()) {
         global_handles->number_of_global_object_weak_handles_--;
@@ -139,47 +198,9 @@ class GlobalHandles::Node : public Malloced {
     set_parameter(NULL);
   }
 
-  void MarkIndependent(GlobalHandles* global_handles) {
-    LOG(global_handles->isolate(),
-        HandleEvent("GlobalHandle::MarkIndependent", handle().location()));
-    ASSERT(state_ != DESTROYED);
-    independent_ = true;
-  }
-
-  bool IsNearDeath() {
-    // Check for PENDING to ensure correct answer when processing callbacks.
-    return state_ == PENDING || state_ == NEAR_DEATH;
-  }
-
-  bool IsWeak() {
-    return state_ == WEAK;
-  }
-
-  bool CanBeRetainer() {
-    return state_ != DESTROYED && state_ != NEAR_DEATH;
-  }
-
-  void SetWrapperClassId(uint16_t class_id) {
-    class_id_ = class_id;
-  }
-
-  // Returns the id for this weak handle.
-  void set_parameter(void* parameter) {
-    ASSERT(state_ != DESTROYED);
-    parameter_or_next_free_.parameter = parameter;
-  }
-  void* parameter() {
-    ASSERT(state_ != DESTROYED);
-    return parameter_or_next_free_.parameter;
-  }
-
-  // Returns the callback for this weak handle.
-  WeakReferenceCallback callback() { return callback_; }
-
   bool PostGarbageCollectionProcessing(Isolate* isolate,
                                        GlobalHandles* global_handles) {
     if (state_ != Node::PENDING) return false;
-    LOG(isolate, HandleEvent("GlobalHandle::Processing", handle().location()));
     WeakReferenceCallback func = callback();
     if (func == NULL) {
       Destroy(global_handles);
@@ -191,13 +212,6 @@ class GlobalHandles::Node : public Malloced {
 
     v8::Persistent<v8::Object> object = ToApi<v8::Object>(handle());
     {
-      // Forbid reuse of destroyed nodes as they might be already deallocated.
-      // It's fine though to reuse nodes that were destroyed in weak callback
-      // as those cannot be deallocated until we are back from the callback.
-      global_handles->set_first_free(NULL);
-      if (global_handles->first_deallocated()) {
-        global_handles->first_deallocated()->set_next(global_handles->head());
-      }
       // Check that we are not passing a finalized external string to
       // the callback.
       ASSERT(!object_->IsExternalAsciiString() ||
@@ -214,27 +228,32 @@ class GlobalHandles::Node : public Malloced {
     return true;
   }
 
-  // Place the handle address first to avoid offset computation.
-  Object* object_;  // Storage for object pointer.
+ private:
+  inline NodeBlock* FindBlock();
+  inline void IncreaseBlockUses(GlobalHandles* global_handles);
+  inline void DecreaseBlockUses(GlobalHandles* global_handles);
+
+  // Storage for object pointer.
+  // Placed first to avoid offset computation.
+  Object* object_;
 
+  // Next word stores class_id, index, state, and independent.
+  // Note: the most aligned fields should go first.
+
+  // Wrapper class ID.
   uint16_t class_id_;
 
-  // Transition diagram:
-  // NORMAL <-> WEAK -> PENDING -> NEAR_DEATH -> { NORMAL, WEAK, DESTROYED }
-  enum State {
-    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.
-    DESTROYED
-  };
-  State state_ : 4;  // Need one more bit for MSVC as it treats enums as signed.
+  // Index in the containing handle block.
+  uint8_t index_;
+
+  // Need one more bit for MSVC as it treats enums as signed.
+  State state_ : 4;
 
   bool independent_ : 1;
 
- private:
   // Handle specific callback.
   WeakReferenceCallback callback_;
+
   // Provided data for callback.  In DESTROYED state, this is used for
   // the free list link.
   union {
@@ -242,69 +261,112 @@ class GlobalHandles::Node : public Malloced {
     Node* next_free;
   } parameter_or_next_free_;
 
-  // Linkage for the list.
-  Node* next_;
-
- public:
-  TRACK_MEMORY("GlobalHandles::Node")
+  DISALLOW_COPY_AND_ASSIGN(Node);
 };
 
 
-class GlobalHandles::Pool {
-  public:
-    Pool() {
-      current_ = new Chunk();
-      current_->previous = NULL;
-      next_ = current_->nodes;
-      limit_ = current_->nodes + kNodesPerChunk;
+class GlobalHandles::NodeBlock {
+ public:
+  static const int kSize = 256;
+
+  explicit NodeBlock(GlobalHandles* global_handles) {
+    next_ = global_handles->first_block_;
+    global_handles->first_block_ = this;
+    next_used_ = prev_used_ = NULL;
+    used_nodes_ = 0;
+    for (int i = kSize - 1; i >= 0; --i) {
+      nodes_[i].PreInitialize(i, global_handles);
     }
+  }
 
-    ~Pool() {
-      if (current_ != NULL) {
-        Release();
-      }
+  Node* node_at(int index) {
+    ASSERT(0 <= index && index < kSize);
+    return &nodes_[index];
+  }
+
+  void IncreaseUses(GlobalHandles* global_handles) {
+    ASSERT(used_nodes_ < kSize);
+    if (used_nodes_++ == 0) {
+      NodeBlock* old_first = global_handles->first_used_block_;
+      global_handles->first_used_block_ = this;
+      next_used_ = old_first;
+      prev_used_ = NULL;
+      if (old_first == NULL) return;
+      old_first->prev_used_ = this;
     }
+  }
 
-    Node* Allocate() {
-      if (next_ < limit_) {
-        return next_++;
+  void DecreaseUses(GlobalHandles* global_handles) {
+    ASSERT(used_nodes_ > 0);
+    if (--used_nodes_ == 0) {
+      if (next_used_ != NULL) next_used_->prev_used_ = prev_used_;
+      if (prev_used_ != NULL) prev_used_->next_used_ = next_used_;
+      if (this == global_handles->first_used_block_) {
+        global_handles->first_used_block_ = next_used_;
       }
-      return SlowAllocate();
     }
+  }
 
-    void Release() {
-      Chunk* current = current_;
-      ASSERT(current != NULL);  // At least a single block must by allocated
-      do {
-        Chunk* previous = current->previous;
-        delete current;
-        current = previous;
-      } while (current != NULL);
-      current_ = NULL;
-      next_ = limit_ = NULL;
-    }
+  // Next block in the list of all blocks.
+  NodeBlock* next() const { return next_; }
 
-  private:
-    static const int kNodesPerChunk = (1 << 12) - 1;
-    struct Chunk : public Malloced {
-      Chunk* previous;
-      Node nodes[kNodesPerChunk];
-    };
-
-    Node* SlowAllocate() {
-      Chunk* chunk = new Chunk();
-      chunk->previous = current_;
-      current_ = chunk;
-
-      Node* new_nodes = current_->nodes;
-      next_ = new_nodes + 1;
-      limit_ = new_nodes + kNodesPerChunk;
-      return new_nodes;
-    }
+  // Next/previous block in the list of blocks with used nodes.
+  NodeBlock* next_used() const { return next_used_; }
+  NodeBlock* prev_used() const { return prev_used_; }
 
-    Chunk* current_;
-    Node* next_;
-    Node* limit_;
+ private:
+  Node nodes_[kSize];
+  NodeBlock* next_;
+  int used_nodes_;
+  NodeBlock* next_used_;
+  NodeBlock* prev_used_;
+};
+
+
+GlobalHandles::NodeBlock* GlobalHandles::Node::FindBlock() {
+  intptr_t ptr = reinterpret_cast<intptr_t>(this);
+  ptr = ptr - index_ * sizeof(Node);
+  NodeBlock* block = reinterpret_cast<NodeBlock*>(ptr);
+  ASSERT(block->node_at(index_) == this);
+  return block;
+}
+
+
+void GlobalHandles::Node::IncreaseBlockUses(GlobalHandles* global_handles) {
+  FindBlock()->IncreaseUses(global_handles);
+}
+
+
+void GlobalHandles::Node::DecreaseBlockUses(GlobalHandles* global_handles) {
+  FindBlock()->DecreaseUses(global_handles);
+}
+
+
+class GlobalHandles::NodeIterator {
+ public:
+  explicit NodeIterator(GlobalHandles* global_handles)
+      : block_(global_handles->first_used_block_),
+        index_(0) {}
+
+  bool done() const { return block_ == NULL; }
+
+  Node* node() const {
+    ASSERT(!done());
+    return block_->node_at(index_);
+  }
+
+  void Advance() {
+    ASSERT(!done());
+    if (++index_ < NodeBlock::kSize) return;
+    index_ = 0;
+    block_ = block_->next_used();
+  }
+
+ private:
+  NodeBlock* block_;
+  int index_;
+
+  DISALLOW_COPY_AND_ASSIGN(NodeIterator);
 };
 
 
@@ -312,41 +374,33 @@ GlobalHandles::GlobalHandles(Isolate* isolate)
     : isolate_(isolate),
       number_of_weak_handles_(0),
       number_of_global_object_weak_handles_(0),
-      head_(NULL),
+      first_block_(NULL),
+      first_used_block_(NULL),
       first_free_(NULL),
-      first_deallocated_(NULL),
-      pool_(new Pool()),
-      post_gc_processing_count_(0),
-      object_groups_(4) {
-}
+      post_gc_processing_count_(0) {}
 
 
 GlobalHandles::~GlobalHandles() {
-  delete pool_;
-  pool_ = 0;
+  NodeBlock* block = first_block_;
+  while (block != NULL) {
+    NodeBlock* tmp = block->next();
+    delete block;
+    block = tmp;
+  }
+  first_block_ = NULL;
 }
 
 
 Handle<Object> GlobalHandles::Create(Object* value) {
   isolate_->counters()->global_handles()->Increment();
-  Node* result;
-  if (first_free()) {
-    // Take the first node in the free list.
-    result = first_free();
-    set_first_free(result->next_free());
-  } else if (first_deallocated()) {
-    // Next try deallocated list
-    result = first_deallocated();
-    set_first_deallocated(result->next_free());
-    ASSERT(result->next() == head());
-    set_head(result);
-  } else {
-    // Allocate a new node.
-    result = pool_->Allocate();
-    result->set_next(head());
-    set_head(result);
-  }
-  result->Initialize(value);
+  if (first_free_ == NULL) {
+    new NodeBlock(this);

[Vyacheslav Egorov (Chromium), 2011/05/24 16:45:12]

Personally I don't like constructors with deep implicit side effects.

+  }
+  ASSERT(first_free_ != NULL);
+  // Take the first node in the free list.
+  Node* result = first_free_;
+  first_free_ = result->next_free();
+  result->Initialize(value, this);
   return result->handle();
 }
 
@@ -354,11 +408,7 @@ Handle<Object> GlobalHandles::Create(Object* value) {
 void GlobalHandles::Destroy(Object** location) {
   isolate_->counters()->global_handles()->Decrement();
   if (location == NULL) return;
-  Node* node = Node::FromLocation(location);
-  node->Destroy(this);
-  // Link the destroyed.
-  node->set_next_free(first_free());
-  set_first_free(node);
+  Node::FromLocation(location)->Destroy(this);
 }
 
 
@@ -375,7 +425,12 @@ void GlobalHandles::ClearWeakness(Object** location) {
 
 
 void GlobalHandles::MarkIndependent(Object** location) {
-  Node::FromLocation(location)->MarkIndependent(this);
+  Node* node = Node::FromLocation(location);
+  if (node->is_independent()) return;
+  node->MarkIndependent();
+  if (isolate_->heap()->InNewSpace(*location)) {
+    new_space_independent_.Add(node);
+  }
 }
 
 
@@ -390,68 +445,51 @@ bool GlobalHandles::IsWeak(Object** location) {
 
 
 void GlobalHandles::SetWrapperClassId(Object** location, uint16_t class_id) {
-  Node::FromLocation(location)->SetWrapperClassId(class_id);
+  Node::FromLocation(location)->set_wrapper_class_id(class_id);
 }
 
 
 void GlobalHandles::IterateWeakRoots(ObjectVisitor* v) {
-  // Traversal of GC roots in the global handle list that are marked as
-  // WEAK, PENDING or NEAR_DEATH.
-  for (Node* current = head_; current != NULL; current = current->next()) {
-    if (current->state_ == Node::WEAK
-      || current->state_ == Node::PENDING
-      || current->state_ == Node::NEAR_DEATH) {
-      v->VisitPointer(&current->object_);
-    }
+  for (NodeIterator it(this); !it.done(); it.Advance()) {
+    if (it.node()->IsWeakRetainer()) v->VisitPointer(it.node()->location());
   }
 }
 
 
 void GlobalHandles::IterateWeakIndependentRoots(ObjectVisitor* v) {
-  // Traversal of GC roots in the global handle list that are independent
-  // and marked as WEAK, PENDING or NEAR_DEATH.
-  for (Node* current = head_; current != NULL; current = current->next()) {
-    if (!current->independent_) continue;
-    if (current->state_ == Node::WEAK
-      || current->state_ == Node::PENDING
-      || current->state_ == Node::NEAR_DEATH) {
-      v->VisitPointer(&current->object_);
-    }
+  for (int i = 0; i < new_space_independent_.length(); ++i) {
+    Node* node = new_space_independent_[i];
+    ASSERT(node->is_independent());
+    if (node->IsWeakRetainer()) v->VisitPointer(node->location());
   }
 }
 
 
 void GlobalHandles::IterateWeakRoots(WeakReferenceGuest f,
                                      WeakReferenceCallback callback) {
-  for (Node* current = head_; current != NULL; current = current->next()) {
-    if (current->IsWeak() && current->callback() == callback) {
-      f(current->object_, current->parameter());
+  for (NodeIterator it(this); !it.done(); it.Advance()) {
+    if (it.node()->IsWeak() && it.node()->callback() == callback) {
+      f(it.node()->object(), it.node()->parameter());
     }
   }
 }
 
 
 void GlobalHandles::IdentifyWeakHandles(WeakSlotCallback f) {
-  for (Node* current = head_; current != NULL; current = current->next()) {
-    if (current->state_ == Node::WEAK) {
-      if (f(&current->object_)) {
-        current->state_ = Node::PENDING;
-        LOG(isolate_,
-            HandleEvent("GlobalHandle::Pending", current->handle().location()));
-      }
+  for (NodeIterator it(this); !it.done(); it.Advance()) {
+    if (it.node()->IsWeak() && f(it.node()->location())) {
+      it.node()->MarkPending();
     }
   }
 }
 
 
 void GlobalHandles::IdentifyWeakIndependentHandles(WeakSlotCallbackWithHeap f) {
-  for (Node* current = head_; current != NULL; current = current->next()) {
-    if (current->state_ == Node::WEAK && current->independent_) {
-      if (f(isolate_->heap(), &current->object_)) {
-        current->state_ = Node::PENDING;
-        LOG(isolate_,
-            HandleEvent("GlobalHandle::Pending", current->handle().location()));
-      }
+  for (int i = 0; i < new_space_independent_.length(); ++i) {
+    Node* node = new_space_independent_[i];
+    ASSERT(node->is_independent());
+    if (node->IsWeak() && f(isolate_->heap(), node->location())) {
+      node->MarkPending();
     }
   }
 }
@@ -462,97 +500,81 @@ bool GlobalHandles::PostGarbageCollectionProcessing(
   // Process weak global handle callbacks. This must be done after the
   // GC is completely done, because the callbacks may invoke arbitrary
   // API functions.
-  // At the same time deallocate all DESTROYED nodes.
   ASSERT(isolate_->heap()->gc_state() == Heap::NOT_IN_GC);
   const int initial_post_gc_processing_count = ++post_gc_processing_count_;
   bool next_gc_likely_to_collect_more = false;
-  Node** p = &head_;
-  while (*p != NULL) {
+  for (NodeIterator it(this); !it.done(); it.Advance()) {
     // Skip dependent handles. Their weak callbacks might expect to be
     // called between two global garbage collection callbacks which
     // are not called for minor collections.
-    if (collector == SCAVENGER && !(*p)->independent_) {
-      p = (*p)->next_addr();
-      continue;
-    }
+    if (collector == SCAVENGER && !it.node()->is_independent()) continue;

[Vyacheslav Egorov (Chromium), 2011/05/24 16:45:12]

Interesting: so skipping blocks with no young independent handles here does not
win us anything, does it?

 
-    if ((*p)->PostGarbageCollectionProcessing(isolate_, this)) {
+    if (it.node()->PostGarbageCollectionProcessing(isolate_, this)) {
       if (initial_post_gc_processing_count != post_gc_processing_count_) {
         // Weak callback triggered another GC and another round of
         // PostGarbageCollection processing.  The current node might
         // have been deleted in that round, so we need to bail out (or
         // restart the processing).
-        break;
+        return next_gc_likely_to_collect_more;
       }
     }
-    if ((*p)->state_ == Node::DESTROYED) {
-      // Delete the link.
-      Node* node = *p;
-      *p = node->next();  // Update the link.
-      if (first_deallocated()) {
-        first_deallocated()->set_next(node);
-      }
-      node->set_next_free(first_deallocated());
-      set_first_deallocated(node);
+    if (!it.node()->IsRetainer()) {
       next_gc_likely_to_collect_more = true;
-    } else {
-      p = (*p)->next_addr();
     }
   }
-  set_first_free(NULL);
-  if (first_deallocated()) {
-    first_deallocated()->set_next(head());
+  int last = 0;
+  for (int i = 0; i < new_space_independent_.length(); ++i) {
+    Node* node = new_space_independent_[i];
+    if (node->IsRetainer() && isolate_->heap()->InNewSpace(node->object())) {
+      new_space_independent_[last++] = node;
+    }
   }
-
+  new_space_independent_.Rewind(last);
   return next_gc_likely_to_collect_more;
 }
 
 
 void GlobalHandles::IterateStrongRoots(ObjectVisitor* v) {
   // Traversal of global handles marked as NORMAL.
-  for (Node* current = head_; current != NULL; current = current->next()) {
-    if (current->state_ == Node::NORMAL) {
-      v->VisitPointer(&current->object_);
+  for (NodeIterator it(this); !it.done(); it.Advance()) {
+    if (it.node()->state() == Node::NORMAL) {
+      v->VisitPointer(it.node()->location());
     }
   }
 }
 
 
 void GlobalHandles::IterateAllRoots(ObjectVisitor* v) {
-  for (Node* current = head_; current != NULL; current = current->next()) {
-    if (current->state_ != Node::DESTROYED) {
-      v->VisitPointer(&current->object_);
+  for (NodeIterator it(this); !it.done(); it.Advance()) {
+    if (it.node()->IsRetainer()) {
+      v->VisitPointer(it.node()->location());
     }
   }
 }
 
 
 void GlobalHandles::IterateStrongAndDependentRoots(ObjectVisitor* v) {
-  for (Node* current = head_; current != NULL; current = current->next()) {
-    if ((current->independent_ && current->state_ == Node::NORMAL) ||
-        (!current->independent_ && current->state_ != Node::DESTROYED)) {
-      v->VisitPointer(&current->object_);
+  for (NodeIterator it(this); !it.done(); it.Advance()) {
+    if ((it.node()->is_independent() && it.node()->state() == Node::NORMAL) ||
+        (!it.node()->is_independent() && it.node()->IsRetainer())) {
+      v->VisitPointer(it.node()->location());
     }
   }
 }
 
 
 void GlobalHandles::IterateAllRootsWithClassIds(ObjectVisitor* v) {
-  for (Node* current = head_; current != NULL; current = current->next()) {
-    if (current->class_id_ != v8::HeapProfiler::kPersistentHandleNoClassId &&
-        current->CanBeRetainer()) {
-      v->VisitEmbedderReference(&current->object_, current->class_id_);
+  for (NodeIterator it(this); !it.done(); it.Advance()) {
+    if (it.node()->has_wrapper_class_id() && it.node()->IsRetainer()) {
+      v->VisitEmbedderReference(it.node()->location(),
+                                it.node()->wrapper_class_id());
     }
   }
 }
 
 
 void GlobalHandles::TearDown() {
-  // Reset all the lists.
-  set_head(NULL);
-  set_first_free(NULL);
-  set_first_deallocated(NULL);
-  pool_->Release();
+  // TODO(vitalyr): dispose stuff?
 }
 
 
@@ -562,15 +584,15 @@ void GlobalHandles::RecordStats(HeapStats* stats) {
   *stats->pending_global_handle_count = 0;
   *stats->near_death_global_handle_count = 0;
   *stats->destroyed_global_handle_count = 0;
-  for (Node* current = head_; current != NULL; current = current->next()) {
+  for (NodeIterator it(this); !it.done(); it.Advance()) {
     *stats->global_handle_count += 1;
-    if (current->state_ == Node::WEAK) {
+    if (it.node()->state() == Node::WEAK) {
       *stats->weak_global_handle_count += 1;
-    } else if (current->state_ == Node::PENDING) {
+    } else if (it.node()->state() == Node::PENDING) {
       *stats->pending_global_handle_count += 1;
-    } else if (current->state_ == Node::NEAR_DEATH) {
+    } else if (it.node()->state() == Node::NEAR_DEATH) {
       *stats->near_death_global_handle_count += 1;
-    } else if (current->state_ == Node::DESTROYED) {
+    } else if (it.node()->state() == Node::DESTROYED) {
       *stats->destroyed_global_handle_count += 1;
     }
   }
@@ -585,12 +607,12 @@ void GlobalHandles::PrintStats() {
   int near_death = 0;
   int destroyed = 0;
 
-  for (Node* current = head_; current != NULL; current = current->next()) {
+  for (NodeIterator it(this); !it.done(); it.Advance()) {
     total++;
-    if (current->state_ == Node::WEAK) weak++;
-    if (current->state_ == Node::PENDING) pending++;
-    if (current->state_ == Node::NEAR_DEATH) near_death++;
-    if (current->state_ == Node::DESTROYED) destroyed++;
+    if (it.node()->state() == Node::WEAK) weak++;
+    if (it.node()->state() == Node::PENDING) pending++;
+    if (it.node()->state() == Node::NEAR_DEATH) near_death++;
+    if (it.node()->state() == Node::DESTROYED) destroyed++;
   }
 
   PrintF("Global Handle Statistics:\n");
@@ -604,11 +626,11 @@ void GlobalHandles::PrintStats() {
 
 void GlobalHandles::Print() {
   PrintF("Global handles:\n");
-  for (Node* current = head_; current != NULL; current = current->next()) {
-    PrintF("  handle %p to %p (weak=%d)\n",
-           reinterpret_cast<void*>(current->handle().location()),
-           reinterpret_cast<void*>(*current->handle()),
-           current->state_ == Node::WEAK);
+  for (NodeIterator it(this); !it.done(); it.Advance()) {
+    PrintF("  handle %p to %p%s\n",
+           reinterpret_cast<void*>(it.node()->location()),
+           reinterpret_cast<void*>(it.node()->object()),
+           it.node()->IsWeak() ? " (weak)" : "");
   }
 }
 
@@ -621,7 +643,7 @@ void GlobalHandles::AddObjectGroup(Object*** handles,
                                    v8::RetainedObjectInfo* info) {
 #ifdef DEBUG
   for (size_t i = 0; i < length; ++i) {
-    ASSERT(!Node::FromLocation(handles[i])->independent_);
+    ASSERT(!Node::FromLocation(handles[i])->is_independent());
   }
 #endif
   if (length == 0) {
@@ -636,9 +658,9 @@ void GlobalHandles::AddImplicitReferences(HeapObject** parent,
                                           Object*** children,
                                           size_t length) {
 #ifdef DEBUG
-  ASSERT(!Node::FromLocation(BitCast<Object**>(parent))->independent_);
+  ASSERT(!Node::FromLocation(BitCast<Object**>(parent))->is_independent());
   for (size_t i = 0; i < length; ++i) {
-    ASSERT(!Node::FromLocation(children[i])->independent_);
+    ASSERT(!Node::FromLocation(children[i])->is_independent());
   }
 #endif
   if (length == 0) return;