Estimate a (close) upper bound on the size of black-marked objects on each page.

src/mark-compact.h

 // Copyright 2011 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 101 matching lines @@
     ASSERT(IsGrey(markbit));
     markbit.Next().Clear();
     ASSERT(IsBlack(markbit));
   }
 
   static inline void BlackToGrey(HeapObject* obj) {
     ASSERT(obj->Size() >= 2 * kPointerSize);
     BlackToGrey(MarkBitFrom(obj));
   }
 
-  void TransferMark(Address old_start, Address new_start);
+  // Returns true if the the object whose mark is transferred is marked black.
+  bool TransferMark(Address old_start, Address new_start);
 
 #ifdef DEBUG
   enum ObjectColor {
     BLACK_OBJECT,
     WHITE_OBJECT,
     GREY_OBJECT,
     IMPOSSIBLE_COLOR
   };
 
   static const char* ColorName(ObjectColor color) {
@@ skipping 12 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
 
-  INLINE(static void TransferColor(HeapObject* from,
+  // Returns true if the transferred color is black.
+  INLINE(static bool TransferColor(HeapObject* from,
                                    HeapObject* to)) {
     MarkBit from_mark_bit = MarkBitFrom(from);
     MarkBit to_mark_bit = MarkBitFrom(to);
-    if (from_mark_bit.Get()) to_mark_bit.Set();
-    if (from_mark_bit.Next().Get()) to_mark_bit.Next().Set();
+    bool is_black = false;
+    if (from_mark_bit.Get()) {
+      to_mark_bit.Set();
+      is_black = true;  // Looks black so far.
+    }
+    if (from_mark_bit.Next().Get()) {
+      to_mark_bit.Next().Set();
+      is_black = false;  // Was actually gray.
+    }
     ASSERT(Color(from) == Color(to));
+    ASSERT(is_black == (Color(to) == BLACK_OBJECT));
+    return is_black;
   }
 
  private:
   Heap* heap_;
 };
 
 // ----------------------------------------------------------------------------
 // Marking deque for tracing live objects.
 
 class MarkingDeque {
@@ skipping 421 matching lines @@
 
   List<Page*> evacuation_candidates_;
 
   friend class Heap;
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_MARK_COMPACT_H_