In-place shrinking of descriptor arrays with non-live transitions.

src/objects.h

 // Copyright 2012 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 2483 matching lines @@
 
   // Initialize or change the enum cache,
   // using the supplied storage for the small "bridge".
   void SetEnumCache(FixedArray* bridge_storage,
                     FixedArray* new_cache,
                     Object* new_index_cache);
 
   // Accessors for fetching instance descriptor at descriptor number.
   inline String* GetKey(int descriptor_number);
   inline Object** GetKeySlot(int descriptor_number);
+  inline void SetKeyUnchecked(Heap* heap, int descriptor_number, String* value);
   inline Object* GetValue(int descriptor_number);
   inline Object** GetValueSlot(int descriptor_number);
-  inline void SetNullValueUnchecked(int descriptor_number, Heap* heap);
+  inline void SetNullValueUnchecked(Heap* heap, int descriptor_number);
+  inline void SetValueUnchecked(Heap* heap,
+                                int descriptor_number,
+                                Object* value);
   inline PropertyDetails GetDetails(int descriptor_number);
   inline void SetDetailsUnchecked(int descriptor_number, Smi* value);
   inline PropertyType GetType(int descriptor_number);
   inline int GetFieldIndex(int descriptor_number);
   inline JSFunction* GetConstantFunction(int descriptor_number);
   inline Object* GetCallbacksObject(int descriptor_number);
   inline AccessorDescriptor* GetCallbacks(int descriptor_number);
   inline bool IsProperty(int descriptor_number);
   inline bool IsTransitionOnly(int descriptor_number);
-  inline bool IsNullDescriptor(int descriptor_number);
 
   // WhitenessWitness is used to prove that a specific descriptor array is white
   // (unmarked), so incremental write barriers can be skipped because the
   // marking invariant cannot be broken and slots pointing into evacuation
   // candidates will be discovered when the object is scanned. A witness is
   // always stack-allocated right after creating a descriptor array. By
   // allocating a witness, incremental marking is globally disabled. The witness
   // is then passed along wherever needed to statically prove that the
   // descriptor array is known to be white.
   class WhitenessWitness {
@@ skipping 2235 matching lines @@
 
   inline JSFunction* unchecked_constructor();
 
   // Should only be called by the code that initializes map to set initial valid
   // value of the instance descriptor member.
   inline void init_instance_descriptors();
 
   // [instance descriptors]: describes the object.
   DECL_ACCESSORS(instance_descriptors, DescriptorArray)
 
+  // Should only be called to clear a descriptor array that was only used to
+  // store transitions, which does not contain any live transitions anymore.

[Michael Starzinger, 2012/06/25 12:48:38]

"[...] store transitions and does not [...]"

+  inline void ClearDescriptorArray();
+
   // Sets the instance descriptor array for the map to be an empty descriptor
   // array.
   inline void clear_instance_descriptors();
 
   // [stub cache]: contains stubs compiled for this map.
   DECL_ACCESSORS(code_cache, Object)
 
   // [back pointer]: points back to the parent map from which a transition
   // leads to this map. The field overlaps with prototype transitions and the
   // back pointer will be moved into the prototype transitions array if
@@ skipping 4007 matching lines @@
     } else {
       value &= ~(1 << bit_position);
     }
     return value;
   }
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_H_