Decoupling MarkDescriptorArray as much as possible from the ContentArray.

src/objects.h

 // Copyright 2012 the V8 project authors. All rights reserved.
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@@ skipping 2456 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* GetValue(int descriptor_number);
+  inline Object** GetValueSlot(int descriptor_number);
   inline PropertyDetails GetDetails(int descriptor_number);
 
   inline Object* RawGetValue(int descriptor_number);
   inline PropertyDetails RawGetDetails(int descriptor_number);
 
   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 potentially unique heap objects can be freely assigned

[Michael Starzinger, 2012/05/24 14:28:54]

The reasoning is slightly off, I would replace it with something like the
following:

[...] descriptor array is white, 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. [...]

[Toon Verwaest, 2012/05/25 12:27:26]

Done.

+  // without potentially causing memory leaks. A witness is always
+  // stack-allocated right after creating a descriptor array. By allocating a witness,

[Michael Starzinger, 2012/05/24 14:28:54]

More than 80 characters.

[Toon Verwaest, 2012/05/25 12:27:26]

Done.

+  // 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 {
    public:
     inline explicit WhitenessWitness(DescriptorArray* array);
     inline ~WhitenessWitness();
 
    private:
     IncrementalMarking* marking_;
   };
 
   // Accessor for complete descriptor.
@@ skipping 6151 matching lines @@
     } else {
       value &= ~(1 << bit_position);
     }
     return value;
   }
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_H_