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

src/spaces.h

Index: src/spaces.h
diff --git a/src/spaces.h b/src/spaces.h
index 4fbabd6349d43364e9fe387c3ad0352e6ed42462..16e535348fba1fbc827a2c9e1e5219629749c286 100644
--- a/src/spaces.h
+++ b/src/spaces.h
@@ -503,6 +503,14 @@ class MemoryChunk {
     }
   }
 
+  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);