src/global-handles.h

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// Copyright 2007-2008 the V8 project authors. All rights reserved.
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#ifndef V8_GLOBAL_HANDLES_H_
#define V8_GLOBAL_HANDLES_H_

#include "list-inl.h"

namespace v8 { namespace internal {

// Structure for tracking global handles.
// A single list keeps all the allocated global handles.
// Destroyed handles stay in the list but is added to the free list.
// At GC the destroyed global handles are removed from the free list
// and deallocated.

// Callback function on handling weak global handles.
// typedef bool (*WeakSlotCallback)(Object** pointer);

// An object group is indexed by an id. An object group is treated like
// a single JS object: if one of object in the group is alive,
// all objects in the same group are considered alive.
// An object group is used to simulate object relationship in a DOM tree.
class ObjectGroup : public Malloced {
 public:
  explicit ObjectGroup(void* id) : id_(id), objects_(4) {}

  void* id_;
  List<Object**> objects_;
};


class GlobalHandles : public AllStatic {
 public:
  // Creates a new global handle that is alive until Destroy is called.
  static Handle<Object> Create(Object* value);

  // Destroy a global handle.
  static void Destroy(Object** location);

  // Make the global handle weak and set the callback parameter for the
  // handle.  When the garbage collector recognizes that only weak global
  // handles point to an object the handles are cleared and the callback
  // function is invoked (for each handle) with the handle and corresponding
  // parameter as arguments.  Note: cleared means set to Smi::FromInt(0). The
  // reason is that Smi::FromInt(0) does not change during garage collection.
  static void MakeWeak(Object** location,
                       void* parameter,
                       WeakReferenceCallback callback);

  // Returns the current number of weak handles.
  static int NumberOfWeakHandles() { return number_of_weak_handles_; }

  // Returns the current number of weak handles to global objects.
  // These handles are also included in NumberOfWeakHandles().
  static int NumberOfGlobalObjectWeakHandles() {
    return number_of_global_object_weak_handles_;
  }

  // Clear the weakness of a global handle.
  static void ClearWeakness(Object** location);

  // Tells whether global handle is near death.
  static bool IsNearDeath(Object** location);

  // Tells whether global handle is weak.
  static bool IsWeak(Object** location);

  // Process pending weak handles.
  static void PostGarbageCollectionProcessing();

  // Iterates over all handles.
  static void IterateRoots(ObjectVisitor* v);

  // Iterates over all weak roots in heap.
  static void IterateWeakRoots(ObjectVisitor* v);

  // Mark the weak pointers based on the callback.
  static void MarkWeakRoots(WeakSlotCallback f);

  // Add an object to a group indexed by an id.
  // Should only used in GC callback function before a collection.
  // All groups are destroyed after a mark-compact collection.
  static void AddToGroup(void* id, Object** location);

  // Returns the object groups.
  static List<ObjectGroup*>& ObjectGroups() {
    return object_groups_;
  }

  // Remove bags, this should only happen after GC.
  static void RemoveObjectGroups();

  // Tear down the global handle structure.
  static void TearDown();

#ifdef DEBUG
  static void PrintStats();
  static void Print();
#endif
 private:
  // Internal node structure, one for each global handle.
  class Node;

  // Field always containing the number of weak and near-death handles.
  static int number_of_weak_handles_;

  // Field always containing the number of weak and near-death handles
  // to global objects.  These objects are also included in
  // number_of_weak_handles_.
  static int number_of_global_object_weak_handles_;

  // Global handles are kept in a single linked list pointed to by head_.
  static Node* head_;
  static Node* head() { return head_; }
  static void set_head(Node* value) { head_ = value; }

  // Free list for DESTROYED global handles not yet deallocated.
  static Node* first_free_;
  static Node* first_free() { return first_free_; }
  static void set_first_free(Node* value) { first_free_ = value; }

  // A list of object groups.
  static List<ObjectGroup*> object_groups_;
};


} }  // namespace v8::internal

#endif  // V8_GLOBAL_HANDLES_H_