src/share/vm/opto/library_call.cpp

*** 2982,2992 ****
--- 2982,3007 ----
        } else {
          assert(kind == LS_cmpxchg, "wrong LoadStore operation");
          load_store = _gvn.transform(new CompareAndSwapPNode(control(), mem, adr, newval, oldval));
        }
      }
+     if (kind == LS_cmpxchg) {
+       // Emit the post barrier only when the actual store happened.
+       // This makes sense to check only for compareAndSet that can fail to set the value.
+       // CAS success path is marked more likely since we anticipate this is a performance
+       // critical path, while CAS failure path can use the penalty for going through unlikely
+       // path as backoff. Which is still better than doing a store barrier there.
+       IdealKit ideal(this);
+       ideal.if_then(load_store, BoolTest::ne, ideal.ConI(0), PROB_STATIC_FREQUENT); {
+         sync_kit(ideal);
+         post_barrier(ideal.ctrl(), load_store, base, adr, alias_idx, newval, T_OBJECT, true);
+         ideal.sync_kit(this);
+       } ideal.end_if();
+       final_sync(ideal);
+     } else {
        post_barrier(control(), load_store, base, adr, alias_idx, newval, T_OBJECT, true);
+     }
      break;
    default:
      fatal(err_msg_res("unexpected type %d: %s", type, type2name(type)));
      break;
    }