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

src/incremental-marking-inl.h

 // Copyright 2012 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
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@@ skipping 26 matching lines @@
 bool IncrementalMarking::BaseRecordWrite(HeapObject* obj,
                                          Object** slot,
                                          Object* value) {
   HeapObject* value_heap_obj = HeapObject::cast(value);
   MarkBit value_bit = Marking::MarkBitFrom(value_heap_obj);
   if (Marking::IsWhite(value_bit)) {
     MarkBit obj_bit = Marking::MarkBitFrom(obj);
     if (Marking::IsBlack(obj_bit)) {
       MemoryChunk* chunk = MemoryChunk::FromAddress(obj->address());
       if (chunk->IsFlagSet(MemoryChunk::HAS_PROGRESS_BAR)) {
-        WhiteToGreyAndPush(value_heap_obj, value_bit);
-        RestartIfNotMarking();
+        if (reinterpret_cast<Address>(slot) - reinterpret_cast<Address>(obj)

[Michael Starzinger, 2012/11/21 17:11:38]

We don't even need to record relocation slots when the slot is right of the bar,
so we can early return with false in this case.

Also, can we have the address computation trickery as a helper function in
MemoryChunk. All the assertions could also go in there. For example with the
following signature.

bool MemoryChunk::IsAheadOfProgressBar(Object** slot);

[Hannes Payer (out of office), 2012/11/21 18:03:57]

Done.

+            < chunk->progress_bar()) {
+          ASSERT(reinterpret_cast<Address>(slot)
+                 > reinterpret_cast<Address>(obj));
+          WhiteToGreyAndPush(value_heap_obj, value_bit);
+          RestartIfNotMarking();
+        }
       } else {
         BlackToGreyAndUnshift(obj, obj_bit);
         RestartIfNotMarking();
         return false;
       }
     } else {
       return false;
     }
   }
   if (!is_compacting_) return false;
@@ skipping 69 matching lines @@
 
 void IncrementalMarking::WhiteToGreyAndPush(HeapObject* obj, MarkBit mark_bit) {
   Marking::WhiteToGrey(mark_bit);
   marking_deque_.PushGrey(obj);
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_INCREMENTAL_MARKING_INL_H_