Collects unused maps that are only kept alive by map transitions....

src/objects.h

 // Copyright 2006-2008 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 1023 matching lines @@
   inline bool IsOverflowed();
 
   // Mutate this object's map pointer to indicate that the object is
   // overflowed.
   inline void SetOverflow();
 
   // Mutate this object's map pointer to remove the indication that the
   // object is overflowed (ie, partially restore the map pointer).
   inline void ClearOverflow();
 
-  static inline Object* GetHeapObjectField(HeapObject* obj, int index);
+  // Returns the field at offset in obj, as a read/write Object* reference.
+  // Does no checking, and is safe to use during GC, while maps are invalid.
+  // Does not update remembered sets, so should only be assigned to
+  // during marking GC.
+  static inline Object* &RawField(HeapObject* obj, int offset);
 
   // Casting.
   static inline HeapObject* cast(Object* obj);
 
   // Return the write barrier mode for this.
   inline WriteBarrierMode GetWriteBarrierMode();
 
   // Dispatched behavior.
   void HeapObjectShortPrint(StringStream* accumulator);
 #ifdef DEBUG
@@ skipping 1359 matching lines @@
   // Returns the found code or undefined if absent.
   Object* FindInCodeCache(String* name, Code::Flags flags);
 
   // Returns the non-negative index of the code object if it is in the
   // cache and -1 otherwise.
   int IndexInCodeCache(Code* code);
 
   // Removes a code object from the code cache at the given index.
   void RemoveFromCodeCache(int index);
 
+  // For every transition in this map, makes the transition's
+  // target's prototype pointer point back to this map.
+  // This is undone in MarkCompactCollector::ClearNonLiveTransitions().
+  void CreateBackPointers();
+
+  // Set all map transitions from this map to dead maps to null.
+  // Also, restore the original prototype on the targets of these
+  // transitions, so that we do not process this map again while
+  // following back pointers.
+  void ClearNonLiveTransitions(Object* real_prototype);
+
   // Dispatched behavior.
   void MapIterateBody(ObjectVisitor* v);
 #ifdef DEBUG
   void MapPrint();
   void MapVerify();
 #endif
 
   // Layout description.
   static const int kInstanceSizesOffset = HeapObject::kHeaderSize;
   static const int kInstanceAttributesOffset = kInstanceSizesOffset + kIntSize;
@@ skipping 364 matching lines @@
 // Forward declaration.
 class JSBuiltinsObject;
 
 // Common super class for JavaScript global objects and the special
 // builtins global objects.
 class GlobalObject: public JSObject {
  public:
   // [builtins]: the object holding the runtime routines written in JS.
   DECL_ACCESSORS(builtins, JSBuiltinsObject)
 
-  // [global context]: the global context corresponding to this global objet.
+  // [global context]: the global context corresponding to this global object.
   DECL_ACCESSORS(global_context, Context)
 
   // [global receiver]: the global receiver object of the context
   DECL_ACCESSORS(global_receiver, JSObject)
 
   // Casting.
   static inline GlobalObject* cast(Object* obj);
 
   // Layout description.
   static const int kBuiltinsOffset = JSObject::kHeaderSize;
@@ skipping 1300 matching lines @@
     } else {
       value &= ~(1 << bit_position);
     }
     return value;
   }
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_H_