# HG changeset patch
# User shade
# Date 1439362719 -10800
#      Wed Aug 12 09:58:39 2015 +0300
# Node ID fbb0d7ae88a097acc97c1abe50ee537d6968327b
# Parent  e66c3813789debfc06f206afde1bf7a84cb08451
8019968: Reference CAS induces GC store barrier even on failure
Reviewed-by: kvn, adinn

diff -r e66c3813789d -r fbb0d7ae88a0 src/share/vm/opto/library_call.cpp
--- a/src/share/vm/opto/library_call.cpp	Thu Aug 06 11:18:59 2015 -0700
+++ b/src/share/vm/opto/library_call.cpp	Wed Aug 12 09:58:39 2015 +0300
@@ -2730,7 +2730,22 @@
         load_store = _gvn.transform(new CompareAndSwapPNode(control(), mem, adr, newval, oldval));
       }
     }
-    post_barrier(control(), load_store, base, adr, alias_idx, newval, T_OBJECT, true);
+    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)));