Forward white to grey transition only if slot is before the progress bar of a given object.

src/spaces.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 485 matching lines @@
     progress_bar_ = progress_bar;
   }
 
   void ResetProgressBar() {
     if (IsFlagSet(MemoryChunk::HAS_PROGRESS_BAR)) {
       set_progress_bar(0);
       ClearFlag(MemoryChunk::HAS_PROGRESS_BAR);
     }
   }
 
+  bool IsAheadOfProgressBar(Object** slot) {

[Michael Starzinger, 2012/11/22 21:13:22]

Apparently we have a different understanding of the semantics of "ahead of"
because for me it means exactly the opposite of what this functions is checking.
So this proves two things:

a) We need a better name. How about IsLeftOfProgressBar?
b) We need a short comment explaining this method.

[Hannes Payer (out of office), 2012/11/23 10:12:49]

Done.

+    ASSERT(reinterpret_cast<Address>(slot) > reinterpret_cast<Address>(this));
+    if (reinterpret_cast<Address>(slot) - reinterpret_cast<Address>(this) <

[Michael Starzinger, 2012/11/22 21:13:22]

The pointer arithmetic is off. The progress bar actually specifies the offset
into the object, not the offset into the memory chunk. So you need to take
kObjectStartOffset into account. That would be an awfully hard to find bug.

Also can we just return the result of the comparison instead of doing an if and
the returning boolean constants?

[Hannes Payer (out of office), 2012/11/23 10:12:49]

Done.

+        progress_bar()) {
+      return true;
+    }
+    return false;          
+  }
 
   static void IncrementLiveBytesFromGC(Address address, int by) {
     MemoryChunk::FromAddress(address)->IncrementLiveBytes(by);
   }
 
   static void IncrementLiveBytesFromMutator(Address address, int by);
 
   static const intptr_t kAlignment =
       (static_cast<uintptr_t>(1) << kPageSizeBits);
 
@@ skipping 2206 matching lines @@
   }
   // Must be small, since an iteration is used for lookup.
   static const int kMaxComments = 64;
 };
 #endif
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_SPACES_H_