4965777: GC changes to support use of discovered field for pending references Summary: If and when the reference handler thread is able to use the discovered field to link reference objects in its pending list, so will GC. In that case, GC will scan through this field once a reference object has been placed on the pending list, but not scan that field before that stage, as the field is used by the concurrent GC thread to link discovered objects. When ReferenceHandleR thread does not use the discovered field for the purpose of linking the elements in the pending list, as would be the case in older JDKs, the JVM will fall back to the old behaviour of using the next field for that purpose. Reviewed-by: jcoomes, mchung, stefank

--- a/src/share/vm/prims/jvm.h	Wed Sep 07 15:00:13 2011 -0700
+++ b/src/share/vm/prims/jvm.h	Wed Sep 07 13:55:42 2011 -0700
@@ -1650,7 +1650,8 @@
      */
     unsigned int thread_park_blocker : 1;
     unsigned int post_vm_init_hook_enabled : 1;
-    unsigned int : 30;
+    unsigned int pending_list_uses_discovered_field : 1;
+    unsigned int : 29;
     unsigned int : 32;
     unsigned int : 32;
 } jdk_version_info;