Ensure black and gray objects are kept around by scavenger

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 <deque>
 
 #include "src/base/bits.h"
@@ skipping 139 matching lines @@
 
   static ObjectColor Color(MarkBit mark_bit) {
     if (IsBlack(mark_bit)) return BLACK_OBJECT;
     if (IsWhite(mark_bit)) return WHITE_OBJECT;
     if (IsGrey(mark_bit)) return GREY_OBJECT;
     UNREACHABLE();
     return IMPOSSIBLE_COLOR;
   }
 #endif
 
   // Returns true if the transferred color is black.

[Hannes Payer (out of office), 2016/05/20 07:34:41]

Mention that this function clears the color of the source object.

[Marcel Hlopko, 2016/05/20 14:31:41]

Done.

   INLINE(static bool TransferColor(HeapObject* from, HeapObject* to)) {
-    if (Page::FromAddress(to->address())->IsFlagSet(Page::BLACK_PAGE))
+    MarkBit from_mark_bit = MarkBitFrom(from);
+    if (Page::FromAddress(to->address())->IsFlagSet(Page::BLACK_PAGE)) {
+      if (!Marking::IsWhite(from_mark_bit)) {
+        Marking::MarkWhite(from_mark_bit);
+      }
       return true;
-    MarkBit from_mark_bit = MarkBitFrom(from);
+    }
     MarkBit to_mark_bit = MarkBitFrom(to);
     DCHECK(Marking::IsWhite(to_mark_bit));
     if (from_mark_bit.Get()) {
       to_mark_bit.Set();
+      from_mark_bit.Clear();
       if (from_mark_bit.Next().Get()) {
         to_mark_bit.Next().Set();
+        from_mark_bit.Next().Clear();
         return true;
       }
     }
     return false;
   }
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(Marking);
 };
 
@@ skipping 708 matching lines @@
  private:
   MarkCompactCollector* collector_;
 };
 
 
 const char* AllocationSpaceName(AllocationSpace space);
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_HEAP_MARK_COMPACT_H_