MarkBit cleanup: They have to be accessed through Marking accessors.

src/heap/incremental-marking.cc

Index: src/heap/incremental-marking.cc
diff --git a/src/heap/incremental-marking.cc b/src/heap/incremental-marking.cc
index b3609e4adcf7db1baa16dde096f9ee48d5290045..c1a7a09b833ab2fa2a3083312cd067635ebbf09a 100644
--- a/src/heap/incremental-marking.cc
+++ b/src/heap/incremental-marking.cc
@@ -138,10 +138,9 @@ static void MarkObjectGreyDoNotEnqueue(Object* obj) {
 static inline void MarkBlackOrKeepGrey(HeapObject* heap_object,
                                        MarkBit mark_bit, int size) {
   DCHECK(!Marking::IsImpossible(mark_bit));
-  if (mark_bit.Get()) return;
-  mark_bit.Set();
+  if (Marking::IsMarked(mark_bit)) return;

[ulan, 2015/03/27 08:36:55]

I like IsWhite(), IsBlack() predicates because they don't expose internal
marking patterns. IsMarked() however is at a lower level and assumes that the
reader knows about the bit shared between black and grey colors.

Maybe rename IsMarked to IsBlackOrGrey or IsNotWhite?

[Hannes Payer (out of office), 2015/03/30 15:09:25]

Done.

+  Marking::MarkBlack(mark_bit);
   MemoryChunk::IncrementLiveBytesFromGC(heap_object->address(), size);
-  DCHECK(Marking::IsBlack(mark_bit));
 }
 
 
@@ -151,7 +150,6 @@ static inline void MarkBlackOrKeepBlack(HeapObject* heap_object,
   if (Marking::IsBlack(mark_bit)) return;
   Marking::MarkBlack(mark_bit);
   MemoryChunk::IncrementLiveBytesFromGC(heap_object->address(), size);
-  DCHECK(Marking::IsBlack(mark_bit));
 }
 
 
@@ -261,7 +259,7 @@ class IncrementalMarkingMarkingVisitor
     HeapObject* heap_object = HeapObject::cast(obj);
     MarkBit mark_bit = Marking::MarkBitFrom(heap_object);
     if (Marking::IsWhite(mark_bit)) {
-      mark_bit.Set();
+      Marking::MarkBlack(mark_bit);
       MemoryChunk::IncrementLiveBytesFromGC(heap_object->address(),
                                             heap_object->Size());
       return true;