Decoupling MarkDescriptorArray as much as possible from the ContentArray.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 116c92b54025c5ec938a81c04c0c4d87085d5460..c9a47c599c6ece3bade6c02acd0c50c39feebbf9 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -2474,6 +2474,7 @@ class DescriptorArray: public FixedArray {
   // 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);
@@ -2488,6 +2489,13 @@ class DescriptorArray: public FixedArray {
   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);