--- old/src/share/vm/gc_interface/collectedHeap.cpp	Wed Aug 24 15:01:39 2011
+++ new/src/share/vm/gc_interface/collectedHeap.cpp	Wed Aug 24 15:01:39 2011
@@ -158,7 +158,13 @@
     Copy::zero_to_words(obj, new_tlab_size);
   } else {
     // ...and clear just the allocated object.
-    Copy::zero_to_words(obj, size);
+#ifdef ASSERT
+    // Skip mangling the space corresponding to the object header to
+    // ensure that the returned space is not considered parsable by
+    // any concurrent GC thread.
+    size_t hdr_size = oopDesc::header_size();
+    Copy::fill_to_words(obj + hdr_size, new_tlab_size - hdr_size, badHeapWordVal);
+#endif // ASSERT
   }
   thread->tlab().fill(obj, obj + size, new_tlab_size);
   return obj;