Don't UnshiftGrey any black objects

src/heap/mark-compact.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_HEAP_MARK_COMPACT_H_
 #define V8_HEAP_MARK_COMPACT_H_
 
 #include "src/base/bits.h"
 #include "src/heap/spaces.h"
 
@@ skipping 226 matching lines @@
   INLINE(HeapObject* Pop()) {
     DCHECK(!IsEmpty());
     top_ = ((top_ - 1) & mask_);
     HeapObject* object = array_[top_];
     DCHECK(object->IsHeapObject());
     return object;
   }
 
   INLINE(void UnshiftGrey(HeapObject* object)) {
     DCHECK(object->IsHeapObject());
+    DCHECK(Marking::IsGrey(Marking::MarkBitFrom(object)));
     if (IsFull()) {
       SetOverflowed();
     } else {
       bottom_ = ((bottom_ - 1) & mask_);
       array_[bottom_] = object;
     }
   }
 
+  INLINE(void UnshiftBlack(HeapObject* object)) {
+    DCHECK(object->IsHeapObject());
+    DCHECK(Marking::IsBlack(Marking::MarkBitFrom(object)));
+    if (IsFull()) {
+      Marking::BlackToGrey(object);
+      MemoryChunk::IncrementLiveBytesFromGC(object->address(), -object->Size());
+      SetOverflowed();
+    } else {
+      bottom_ = ((bottom_ - 1) & mask_);
+      array_[bottom_] = object;
+    }
+  }
+
   HeapObject** array() { return array_; }
   int bottom() { return bottom_; }
   int top() { return top_; }
   int mask() { return mask_; }
   void set_top(int top) { top_ = top; }
 
  private:
   HeapObject** array_;
   // array_[(top - 1) & mask_] is the top element in the deque.  The Deque is
   // empty when top_ == bottom_.  It is full when top_ + 1 == bottom
@@ skipping 740 matching lines @@
  private:
   MarkCompactCollector* collector_;
 };
 
 
 const char* AllocationSpaceName(AllocationSpace space);
 }
 }  // namespace v8::internal
 
 #endif  // V8_HEAP_MARK_COMPACT_H_