Make GlobalHandle::NodeBlock deletable

src/global-handles.cc

 // Copyright 2009 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 38 matching lines @@
 
 class 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.
+    NEAR_DEATH,  // Callback has informed the handle is near death.
+
+    NUMBER_OF_STATES
   };
 
   // Maps handle location (slot) to the containing node.
   static Node* FromLocation(Object** location) {
     ASSERT(OFFSET_OF(Node, object_) == 0);
     return reinterpret_cast<Node*>(location);
   }
 
   Node() {
     ASSERT(OFFSET_OF(Node, class_id_) == Internals::kNodeClassIdOffset);
@@ skipping 16 matching lines @@
     class_id_ = v8::HeapProfiler::kPersistentHandleNoClassId;
     index_ = 0;
     set_independent(false);
     set_partially_dependent(false);
     set_in_new_space_list(false);
     parameter_or_next_free_.next_free = NULL;
     weak_reference_callback_ = NULL;
   }
 #endif
 
-  void Initialize(int index, Node** first_free) {
+  void Initialize(int index, Node* first_free) {
     index_ = static_cast<uint8_t>(index);
     ASSERT(static_cast<int>(index_) == index);
     set_state(FREE);
     set_in_new_space_list(false);
-    parameter_or_next_free_.next_free = *first_free;
-    *first_free = this;
+    parameter_or_next_free_.next_free = first_free;
   }
 
   void Acquire(Object* object) {
     ASSERT(state() == FREE);
     object_ = object;
     class_id_ = v8::HeapProfiler::kPersistentHandleNoClassId;
     set_independent(false);
     set_partially_dependent(false);
     set_state(NORMAL);
     parameter_or_next_free_.parameter = NULL;
     weak_reference_callback_ = NULL;
-    IncreaseBlockUses();
   }
 
   void Release() {
     ASSERT(state() != FREE);
     set_state(FREE);
 #ifdef ENABLE_EXTRA_CHECKS
     // Zap the values for eager trapping.
     object_ = reinterpret_cast<Object*>(kGlobalHandleZapValue);
     class_id_ = v8::HeapProfiler::kPersistentHandleNoClassId;
     set_independent(false);
     set_partially_dependent(false);
     weak_reference_callback_ = NULL;
 #endif
-    DecreaseBlockUses();
+    ReleaseFromBlock();
   }
 
   // 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;
@@ skipping 58 matching lines @@
   }
 
   void MarkPartiallyDependent() {
     ASSERT(state() != FREE);
     if (GetGlobalHandles()->isolate()->heap()->InNewSpace(object_)) {
       set_partially_dependent(true);
     }
   }
   void clear_partially_dependent() { set_partially_dependent(false); }
 
-  // Callback accessor.
-  // TODO(svenpanne) Re-enable or nuke later.
-  // WeakReferenceCallback callback() { return callback_; }
-
   // Callback parameter accessors.
   void set_parameter(void* parameter) {
     ASSERT(state() != FREE);
     parameter_or_next_free_.parameter = parameter;
   }
   void* parameter() const {
     ASSERT(state() != FREE);
     return parameter_or_next_free_.parameter;
   }
 
@@ skipping 48 matching lines @@
     }
     // Absence of explicit cleanup or revival of weak handle
     // in most of the cases would lead to memory leak.
     ASSERT(state() != NEAR_DEATH);
     return true;
   }
 
  private:
   inline NodeBlock* FindBlock();
   inline GlobalHandles* GetGlobalHandles();
-  inline void IncreaseBlockUses();
-  inline void DecreaseBlockUses();
+  inline void ReleaseFromBlock();
 
   // 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_;
@@ skipping 17 matching lines @@
   // the free list link.
   union {
     void* parameter;
     Node* next_free;
   } parameter_or_next_free_;
 
   DISALLOW_COPY_AND_ASSIGN(Node);
 };
 
 
-class GlobalHandles::NodeBlock {
+class GlobalHandles::BlockListIterator {
+ public:
+  explicit inline BlockListIterator(BlockList* anchor)
+    : anchor_(anchor), current_(anchor->next()) {

[Michael Starzinger, 2013/08/02 11:59:51]

nit: Indentation is off.

+    ASSERT(anchor->IsAnchor());
+  }
+  inline BlockList* block() const {
+    ASSERT(!done());
+    return current_;
+  }
+  inline bool done() const {
+    ASSERT_EQ(anchor_ == current_, current_->IsAnchor());
+    return anchor_ == current_;
+  }
+  inline void Advance() {
+    ASSERT(!done());
+    current_ = current_->next();
+  }
+
+ private:
+  BlockList* const anchor_;
+  BlockList* current_;
+  DISALLOW_COPY_AND_ASSIGN(BlockListIterator);
+};
+
+
+GlobalHandles::BlockList::BlockList()
+    : prev_block_(this),
+      next_block_(this),
+      first_free_(NULL),
+      used_nodes_(0) {}
+
+
+void GlobalHandles::BlockList::InsertAsNext(BlockList* const block) {
+  ASSERT(block != this);
+  ASSERT(!block->IsAnchor());
+  ASSERT(block->IsDetached());
+  block->prev_block_ = this;
+  block->next_block_ = next_block_;
+  next_block_->prev_block_ = block;
+  next_block_ = block;
+  ASSERT(!IsDetached());
+  ASSERT(!block->IsDetached());
+}
+
+
+void GlobalHandles::BlockList::Detach() {
+  ASSERT(!IsAnchor());
+  ASSERT(!IsDetached());
+  prev_block_->next_block_ = next_block_;
+  next_block_->prev_block_ = prev_block_;
+  prev_block_ = this;
+  next_block_ = this;
+  ASSERT(IsDetached());
+}
+
+
+bool GlobalHandles::BlockList::HasAtLeastLength(int length) {
+  ASSERT(IsAnchor());
+  ASSERT(length > 0);
+  for (BlockListIterator it(this); !it.done(); it.Advance()) {
+    if (--length <= 0) return true;
+  }
+  return false;
+}
+
+
+#ifdef DEBUG
+int GlobalHandles::BlockList::LengthOfFreeList() {
+  int count = 0;
+  Node* node = first_free_;
+  while (node != NULL) {
+    count++;
+    node = node->next_free();
+  }
+  return count;
+}
+#endif
+
+
+int GlobalHandles::BlockList::CompareBlocks(const void* a, const void* b) {
+  const BlockList* block_a =
+      *reinterpret_cast<const BlockList**>(reinterpret_cast<uintptr_t>(a));
+  const BlockList* block_b =
+      *reinterpret_cast<const BlockList**>(reinterpret_cast<uintptr_t>(b));
+  if (block_a->used_nodes() > block_b->used_nodes()) return -1;
+  if (block_a->used_nodes() == block_b->used_nodes()) return 0;
+  return 1;
+}
+
+
+class GlobalHandles::NodeBlock : public BlockList {
  public:
   static const int kSize = 256;
 
-  explicit NodeBlock(GlobalHandles* global_handles, NodeBlock* next)
-      : next_(next),
-        used_nodes_(0),
-        next_used_(NULL),
-        prev_used_(NULL),
-        global_handles_(global_handles) {}
-
-  void PutNodesOnFreeList(Node** first_free) {
+  explicit NodeBlock(GlobalHandles* global_handles)
+      : global_handles_(global_handles) {
+    // Initialize nodes
+    Node* first_free = first_free_;
     for (int i = kSize - 1; i >= 0; --i) {
       nodes_[i].Initialize(i, first_free);
+      first_free = &nodes_[i];
+    }
+    first_free_ = first_free;
+    ASSERT(!IsAnchor());
+    // Link into global_handles
+    ASSERT(global_handles->non_full_blocks_.IsDetached());
+    global_handles->non_full_blocks_.InsertAsHead(this);
+    global_handles->number_of_blocks_++;
+  }
+
+  Node* Acquire(Object* o) {
+    ASSERT(used_nodes_ < kSize);
+    ASSERT(first_free_ != NULL);
+    ASSERT(global_handles_->non_full_blocks_.next() == this);
+    // Remove from free list
+    Node* node = first_free_;
+    first_free_ = node->next_free();
+    // Increment counters
+    global_handles_->isolate()->counters()->global_handles()->Increment();
+    global_handles_->number_of_global_handles_++;
+    // Initialize node with value
+    node->Acquire(o);
+    bool now_full = ++used_nodes_ == kSize;
+    ASSERT_EQ(now_full, first_free_ == NULL);
+    if (now_full) {
+      // Move block to tail of non_full_blocks_
+      Detach();
+      global_handles_->full_blocks_.InsertAsTail(this);
+    }
+    return node;
+  }
+
+  void Release(Node* node) {
+    ASSERT(used_nodes_ > 0);
+    // Add to free list
+    node->set_next_free(first_free_);
+    first_free_ = node;
+    // Decrement counters
+    global_handles_->isolate()->counters()->global_handles()->Decrement();
+    global_handles_->number_of_global_handles_--;
+    bool was_full = used_nodes_-- == kSize;
+    ASSERT_EQ(was_full, first_free_->next_free() == NULL);
+    if (was_full) {
+      // Move this block to head of non_full_blocks_
+      Detach();
+      global_handles_->non_full_blocks_.InsertAsHead(this);
     }
   }
 
   Node* node_at(int index) {
     ASSERT(0 <= index && index < kSize);
     return &nodes_[index];
   }
 
-  void IncreaseUses() {
-    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;
-    }
-  }
-
-  void DecreaseUses() {
-    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_;
-      }
-    }
-  }
-
   GlobalHandles* global_handles() { return global_handles_; }
 
-  // Next block in the list of all blocks.
-  NodeBlock* next() const { return next_; }
-
-  // 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_; }
+  static NodeBlock* Cast(BlockList* block_list) {
+    ASSERT(!block_list->IsAnchor());
+    return static_cast<NodeBlock*>(block_list);
+  }
+
+  static NodeBlock* From(Node* node, uint8_t index) {
+    uintptr_t ptr = reinterpret_cast<uintptr_t>(node - index);
+    ptr -= OFFSET_OF(NodeBlock, nodes_);
+    NodeBlock* block = reinterpret_cast<NodeBlock*>(ptr);
+    ASSERT(block->node_at(index) == node);
+    return block;
+  }
 
  private:
   Node nodes_[kSize];
-  NodeBlock* const next_;
-  int used_nodes_;
-  NodeBlock* next_used_;
-  NodeBlock* prev_used_;
   GlobalHandles* global_handles_;
 };
 
 
+void GlobalHandles::BlockList::SortBlocks(GlobalHandles* global_handles,
+                                          bool prune) {
+  // Always sort at least 2 blocks
+  if (!global_handles->non_full_blocks_.HasAtLeastLength(2)) return;
+  // build a vector that could contain the upper bound of the block count
+  int number_of_blocks = global_handles->block_count();
+  // Build array of blocks and update number_of_blocks to actual count
+  ScopedVector<BlockList*> blocks(number_of_blocks);
+  {
+    int i = 0;
+    BlockList* anchor = &global_handles->non_full_blocks_;
+    for (BlockListIterator it(anchor); !it.done(); it.Advance()) {
+      blocks[i++] = it.block();
+    }
+    number_of_blocks = i;
+  }
+  // Nothing to do.
+  if (number_of_blocks <= 1) return;
+  // Sort blocks
+  qsort(blocks.start(), number_of_blocks, sizeof(blocks[0]), CompareBlocks);
+  // Prune empties
+  if (prune) {
+    static const double kUnusedPercentage = 0.30;
+    static const double kUsedPercentage = 1.30;
+    int total_slots = global_handles->number_of_blocks_ * NodeBlock::kSize;
+    const int total_used = global_handles->number_of_global_handles_;
+    const int target_unused = static_cast<int>(Max(
+        total_used * kUsedPercentage,
+        total_slots * kUnusedPercentage));
+    // Reverse through empty blocks. Note: always leave one block free.
+    int blocks_deleted = 0;
+    for (int i = number_of_blocks - 1; i > 0 && blocks[i]->IsUnused(); i--) {
+      // Not worth deleting
+      if (total_slots - total_used < target_unused) break;
+      blocks[i]->Detach();
+      delete blocks[i];
+      blocks_deleted++;
+      total_slots -= NodeBlock::kSize;
+    }
+    global_handles->number_of_blocks_ -= blocks_deleted;
+    number_of_blocks -= blocks_deleted;
+  }
+  // Relink all blocks
+  for (int i = 0; i < number_of_blocks; i++) {
+    blocks[i]->Detach();
+    global_handles->non_full_blocks_.InsertAsTail(blocks[i]);
+  }
+#ifdef DEBUG
+  // Check sorting
+  BlockList* anchor = &global_handles->non_full_blocks_;
+  int last_size = NodeBlock::kSize;
+  for (BlockListIterator it(anchor); !it.done(); it.Advance()) {
+    ASSERT(it.block()->used_nodes() <= last_size);
+    last_size = it.block()->used_nodes();
+  }
+#endif
+}
+
+
+#ifdef DEBUG
+void GlobalHandles::VerifyBlockInvariants() {
+  int number_of_blocks = 0;
+  int number_of_handles = 0;
+  BlockList* anchors[2] = {&full_blocks_, &non_full_blocks_};
+  for (unsigned i = 0; i < ARRAY_SIZE(anchors); i++) {
+    for (BlockListIterator it(anchors[i]); !it.done(); it.Advance()) {
+      BlockList* block = it.block();
+      number_of_blocks++;
+      int used_nodes = block->used_nodes();
+      number_of_handles += used_nodes;
+      int unused_nodes = block->LengthOfFreeList();
+      ASSERT_EQ(used_nodes + unused_nodes, NodeBlock::kSize);
+      if (anchors[i] == &full_blocks_) {
+        ASSERT_EQ(NodeBlock::kSize, used_nodes);
+      } else {
+        ASSERT_NE(NodeBlock::kSize, used_nodes);
+      }
+    }
+  }
+  ASSERT_EQ(number_of_handles, number_of_global_handles_);
+  ASSERT_EQ(number_of_blocks, number_of_blocks_);
+}
+#endif
+
+
+void GlobalHandles::SortBlocks(bool shouldPrune) {
+#ifdef DEBUG
+  VerifyBlockInvariants();
+#endif
+  BlockList::SortBlocks(this, shouldPrune);
+#ifdef DEBUG
+  VerifyBlockInvariants();
+#endif
+}
+
+
 GlobalHandles* GlobalHandles::Node::GetGlobalHandles() {
   return FindBlock()->global_handles();
 }
 
 
 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() {
-  NodeBlock* node_block = FindBlock();
-  node_block->IncreaseUses();
-  GlobalHandles* global_handles = node_block->global_handles();
-  global_handles->isolate()->counters()->global_handles()->Increment();
-  global_handles->number_of_global_handles_++;
-}
-
-
-void GlobalHandles::Node::DecreaseBlockUses() {
-  NodeBlock* node_block = FindBlock();
-  GlobalHandles* global_handles = node_block->global_handles();
-  parameter_or_next_free_.next_free = global_handles->first_free_;
-  global_handles->first_free_ = this;
-  node_block->DecreaseUses();
-  global_handles->isolate()->counters()->global_handles()->Decrement();
-  global_handles->number_of_global_handles_--;
+  return NodeBlock::From(this, index_);
+}
+
+
+void GlobalHandles::Node::ReleaseFromBlock() {
+  FindBlock()->Release(this);
 }
 
 
 class GlobalHandles::NodeIterator {
  public:
   explicit NodeIterator(GlobalHandles* global_handles)
-      : block_(global_handles->first_used_block_),
-        index_(0) {}
-
-  bool done() const { return block_ == NULL; }
+      : current_anchor_(&global_handles->full_blocks_),
+        next_anchor_(&global_handles->non_full_blocks_),
+        block_(current_anchor_),
+        index_(0) {
+    AdvanceBlock();
+  }
+
+  bool done() const {
+    ASSERT_EQ(block_ == NULL, current_anchor_ == NULL && next_anchor_ == NULL);
+    return block_ == NULL;
+  }
 
   Node* node() const {
     ASSERT(!done());
-    return block_->node_at(index_);
+    return NodeBlock::Cast(block_)->node_at(index_);
   }
 
   void Advance() {
     ASSERT(!done());
     if (++index_ < NodeBlock::kSize) return;
     index_ = 0;
-    block_ = block_->next_used();
-  }
+    AdvanceBlock();
+  }
+
+  typedef int CountArray[Node::NUMBER_OF_STATES];
+  static int CollectStats(GlobalHandles* global_handles, CountArray counts);
 
  private:
-  NodeBlock* block_;
+  void AdvanceBlock() {
+    while (current_anchor_ != NULL) {
+      block_ = block_->next();
+      // block is valid
+      if (block_ != current_anchor_) {
+        ASSERT(!done());
+        ASSERT(!block_->IsAnchor());
+        // skip empty blocks
+        if (block_->IsUnused()) continue;
+        return;
+      }
+      // jump lists
+      current_anchor_ = next_anchor_;
+      next_anchor_ = NULL;
+      block_ = current_anchor_;
+    }
+    ASSERT(done());
+  }
+
+  BlockList* current_anchor_;
+  BlockList* next_anchor_;
+  BlockList* block_;
   int index_;
 
   DISALLOW_COPY_AND_ASSIGN(NodeIterator);
 };
 
 
+int GlobalHandles::NodeIterator::CollectStats(GlobalHandles* global_handles,
+                                              CountArray counts) {
+  static const int kSize = Node::NUMBER_OF_STATES;
+  for (int i = 0; i < kSize; i++) {
+    counts[i] = 0;
+  }
+  int total = 0;
+  for (NodeIterator it(global_handles); !it.done(); it.Advance()) {
+    total++;
+    Node::State state = it.node()->state();
+    ASSERT(state >= 0 && state < kSize);
+    counts[state]++;
+  }
+  // NodeIterator skips empty blocks
+  int skipped = global_handles->number_of_blocks_ * NodeBlock::kSize - total;
+  total += skipped;
+  counts[Node::FREE] += total;
+  return total;
+}
+
+
 GlobalHandles::GlobalHandles(Isolate* isolate)
     : isolate_(isolate),
+      number_of_blocks_(0),
       number_of_global_handles_(0),
-      first_block_(NULL),
-      first_used_block_(NULL),
-      first_free_(NULL),
       post_gc_processing_count_(0),
       object_group_connections_(kObjectGroupConnectionsCapacity) {}
 
 
 GlobalHandles::~GlobalHandles() {
-  NodeBlock* block = first_block_;
-  while (block != NULL) {
-    NodeBlock* tmp = block->next();
-    delete block;
-    block = tmp;
-  }
-  first_block_ = NULL;
+  BlockList* anchors[2] = { &full_blocks_, &non_full_blocks_ };

[Michael Starzinger, 2013/08/02 11:59:51]

Can we hoist out this list of anchors to be a field on GlobalHandles so that we
have a single place where all anchors are defined?

+  for (size_t i = 0; i < ARRAY_SIZE(anchors); i++) {
+    BlockList* block = anchors[i]->next();
+    while (block != anchors[i]) {
+      BlockList* tmp = block->next();
+      block->Detach();
+      delete NodeBlock::Cast(block);
+      block = tmp;
+    }
+  }
 }
 
 
 Handle<Object> GlobalHandles::Create(Object* value) {
-  if (first_free_ == NULL) {
-    first_block_ = new NodeBlock(this, first_block_);
-    first_block_->PutNodesOnFreeList(&first_free_);
-  }
-  ASSERT(first_free_ != NULL);
-  // Take the first node in the free list.
-  Node* result = first_free_;
-  first_free_ = result->next_free();
-  result->Acquire(value);
+  if (non_full_blocks_.IsDetached()) {
+    new NodeBlock(this);
+    ASSERT(!non_full_blocks_.IsDetached());
+  }
+  ASSERT(non_full_blocks_.IsAnchor());
+  ASSERT(!non_full_blocks_.next()->IsAnchor());
+  Node* result = NodeBlock::Cast(non_full_blocks_.next())->Acquire(value);
   if (isolate_->heap()->InNewSpace(value) &&
       !result->is_in_new_space_list()) {
     new_space_nodes_.Add(result);
     result->set_in_new_space_list(true);
   }
   return result->handle();
 }
 
 
 void GlobalHandles::Destroy(Object** location) {
@@ skipping 169 matching lines @@
           // have been deleted in that round, so we need to bail out (or
           // restart the processing).
           return next_gc_likely_to_collect_more;
         }
       }
       if (!node->IsRetainer()) {
         next_gc_likely_to_collect_more = true;
       }
     }
   } else {
-    for (NodeIterator it(this); !it.done(); it.Advance()) {
-      if (!it.node()->IsRetainer()) {
-        // Free nodes do not have weak callbacks. Do not use them to compute
-        // the next_gc_likely_to_collect_more.
-        continue;
-      }
-      it.node()->clear_partially_dependent();
-      if (it.node()->PostGarbageCollectionProcessing(isolate_)) {
-        if (initial_post_gc_processing_count != post_gc_processing_count_) {
-          // See the comment above.
-          return next_gc_likely_to_collect_more;
+    // Must cache all blocks, as NodeIterator can't survive mutation.
+    List<NodeBlock*> blocks(number_of_blocks_);
+    {
+      BlockList* anchors[2] = {&full_blocks_, &non_full_blocks_};

[Michael Starzinger, 2013/08/02 11:59:51]

See comment about.

+      for (unsigned i = 0; i < ARRAY_SIZE(anchors); i++) {
+        for (BlockListIterator it(anchors[i]); !it.done(); it.Advance()) {
+          blocks.Add(NodeBlock::Cast(it.block()));
         }
       }
-      if (!it.node()->IsRetainer()) {
-        next_gc_likely_to_collect_more = true;
+    }
+    for (int block_index = 0; block_index < blocks.length(); block_index++) {
+      NodeBlock* block = blocks[block_index];
+      for (int node_index = 0; node_index < NodeBlock::kSize; node_index++) {
+        Node* node = block->node_at(node_index);
+        if (!node->IsRetainer()) {
+          // Free nodes do not have weak callbacks. Do not use them to compute
+          // the next_gc_likely_to_collect_more.
+          continue;
+        }
+        node->clear_partially_dependent();
+        if (node->PostGarbageCollectionProcessing(isolate_)) {
+          if (initial_post_gc_processing_count != post_gc_processing_count_) {
+            // See the comment above.
+            return next_gc_likely_to_collect_more;
+          }
+        }
+        if (!node->IsRetainer()) {
+          next_gc_likely_to_collect_more = true;
+        }
       }
     }
   }
   // Update the list of new space nodes.
   int last = 0;
   for (int i = 0; i < new_space_nodes_.length(); ++i) {
     Node* node = new_space_nodes_[i];
     ASSERT(node->is_in_new_space_list());
     if (node->IsRetainer()) {
       if (isolate_->heap()->InNewSpace(node->object())) {
         new_space_nodes_[last++] = node;
         tracer->increment_nodes_copied_in_new_space();
       } else {
         node->set_in_new_space_list(false);
         tracer->increment_nodes_promoted();
       }
     } else {
       node->set_in_new_space_list(false);
       tracer->increment_nodes_died_in_new_space();
     }
   }
   new_space_nodes_.Rewind(last);
+  bool shouldPruneBlocks = collector != SCAVENGER;
+  SortBlocks(shouldPruneBlocks);
   return next_gc_likely_to_collect_more;
 }
 
 
 void GlobalHandles::IterateStrongRoots(ObjectVisitor* v) {
   for (NodeIterator it(this); !it.done(); it.Advance()) {
     if (it.node()->IsStrongRetainer()) {
       v->VisitPointer(it.node()->location());
     }
   }
@@ skipping 47 matching lines @@
     if (it.node()->IsWeakRetainer() &&
         it.node()->object()->IsJSGlobalObject()) {
       count++;
     }
   }
   return count;
 }
 
 
 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 (NodeIterator it(this); !it.done(); it.Advance()) {
-    *stats->global_handle_count += 1;
-    if (it.node()->state() == Node::WEAK) {
-      *stats->weak_global_handle_count += 1;
-    } else if (it.node()->state() == Node::PENDING) {
-      *stats->pending_global_handle_count += 1;
-    } else if (it.node()->state() == Node::NEAR_DEATH) {
-      *stats->near_death_global_handle_count += 1;
-    } else if (it.node()->state() == Node::FREE) {
-      *stats->free_global_handle_count += 1;
-    }
-  }
+  NodeIterator::CountArray counts;
+  int total = NodeIterator::CollectStats(this, counts);
+  *stats->global_handle_count = total;
+  *stats->weak_global_handle_count = counts[Node::WEAK];
+  *stats->pending_global_handle_count = counts[Node::PENDING];
+  *stats->near_death_global_handle_count = counts[Node::NEAR_DEATH];
+  *stats->free_global_handle_count = counts[Node::FREE];
 }
 
+
 #ifdef DEBUG
 
 void GlobalHandles::PrintStats() {
-  int total = 0;
-  int weak = 0;
-  int pending = 0;
-  int near_death = 0;
-  int destroyed = 0;
-
-  for (NodeIterator it(this); !it.done(); it.Advance()) {
-    total++;
-    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::FREE) destroyed++;
-  }
-
+  NodeIterator::CountArray counts;
+  int total = NodeIterator::CollectStats(this, counts);
+  size_t total_consumed = sizeof(NodeBlock) * number_of_blocks_;
   PrintF("Global Handle Statistics:\n");
-  PrintF("  allocated memory = %" V8_PTR_PREFIX "dB\n", sizeof(Node) * total);
-  PrintF("  # weak       = %d\n", weak);
-  PrintF("  # pending    = %d\n", pending);
-  PrintF("  # near_death = %d\n", near_death);
-  PrintF("  # free       = %d\n", destroyed);
+  PrintF("  allocated blocks = %d\n", number_of_blocks_);
+  PrintF("  allocated memory = %" V8_PTR_PREFIX "dB\n", total_consumed);
+  PrintF("  # normal     = %d\n", counts[Node::NORMAL]);
+  PrintF("  # weak       = %d\n", counts[Node::WEAK]);
+  PrintF("  # pending    = %d\n", counts[Node::PENDING]);
+  PrintF("  # near_death = %d\n", counts[Node::NEAR_DEATH]);
+  PrintF("  # free       = %d\n", counts[Node::FREE]);
   PrintF("  # total      = %d\n", total);
 }
 
 
 void GlobalHandles::Print() {
   PrintF("Global handles:\n");
   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()),
@@ skipping 192 matching lines @@
     }
   }
   object_group_connections_.Clear();
   object_group_connections_.Initialize(kObjectGroupConnectionsCapacity);
   retainer_infos_.Clear();
   implicit_ref_connections_.Clear();
 }
 
 
 } }  // namespace v8::internal