--- old/src/share/vm/opto/graphKit.cpp	Wed Mar 30 17:59:14 2011
+++ new/src/share/vm/opto/graphKit.cpp	Wed Mar 30 17:59:14 2011
@@ -3447,12 +3447,24 @@
 
   // Combine card table base and card offset
   Node* card_adr = __ AddP(__ top(), byte_map_base_node(), card_offset );
-
   // Get the alias_index for raw card-mark memory
   int adr_type = Compile::AliasIdxRaw;
-  // Smash zero into card
-  Node*   zero = __ ConI(0);
+  Node*   zero = __ ConI(0); // Dirty card value
   BasicType bt = T_BYTE;
+
+  if (UseCondCardMark) {
+    // The classic GC reference write barrier is typically implemented
+    // as a store into the global card mark table.  Unfortunately
+    // unconditional stores can result in false sharing and excessive
+    // coherence traffic as well as false transactional aborts.
+    // UseCondCardMark enables MP "polite" conditional card mark
+    // stores.  In theory we could relax the load from ctrl() to
+    // no_ctrl, but that doesn't buy much latitude.
+    Node* card_val = __ load( __ ctrl(), card_adr, TypeInt::BYTE, bt, adr_type);
+    __ if_then(card_val, BoolTest::ne, zero);
+  }
+
+  // Smash zero into card
   if( !UseConcMarkSweepGC ) {
     __ store(__ ctrl(), card_adr, zero, bt, adr_type);
   } else {
@@ -3460,6 +3472,10 @@
     __ storeCM(__ ctrl(), card_adr, zero, oop_store, adr_idx, bt, adr_type);
   }
 
+  if (UseCondCardMark) {
+    __ end_if();
+  }
+
   // Final sync IdealKit and GraphKit.
   sync_kit(ideal);
 }