Decoupling MarkDescriptorArray as much as possible from the ContentArray.

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 2435 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 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 {
    public:
     inline explicit WhitenessWitness(DescriptorArray* array);
     inline ~WhitenessWitness();
 
    private:
     IncrementalMarking* marking_;
   };
 
   // Accessor for complete descriptor.
@@ skipping 108 matching lines @@
 
   // Are two DescriptorArrays equal?
   bool IsEqualTo(DescriptorArray* other);
 #endif
 
   // The maximum number of descriptors we want in a descriptor array (should
   // fit in a page).
   static const int kMaxNumberOfDescriptors = 1024 + 512;
 
  private:
+  friend class IntrusiveMapTransitionIterator;
+
   // An entry in a DescriptorArray, represented as an (array, index) pair.
   class Entry {
    public:
     inline explicit Entry(DescriptorArray* descs, int index) :
         descs_(descs), index_(index) { }
 
     inline PropertyType type() { return descs_->GetType(index_); }
     inline Object* GetCallbackObject() { return descs_->GetValue(index_); }
 
    private:
@@ skipping 18 matching lines @@
   static inline void NoIncrementalWriteBarrierSwap(
       FixedArray* array, int first, int second);
 
   // Swap descriptor first and second.
   inline void NoIncrementalWriteBarrierSwapDescriptors(
       int first, int second);
 
   FixedArray* GetContentArray() {
     return FixedArray::cast(get(kContentArrayIndex));
   }
+
   DISALLOW_IMPLICIT_CONSTRUCTORS(DescriptorArray);
 };
 
 
 // HashTable is a subclass of FixedArray that implements a hash table
 // that uses open addressing and quadratic probing.
 //
 // In order for the quadratic probing to work, elements that have not
 // yet been used and elements that have been deleted are
 // distinguished.  Probing continues when deleted elements are
@@ skipping 6030 matching lines @@
     } else {
       value &= ~(1 << bit_position);
     }
     return value;
   }
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_H_