# HG changeset patch
# User goetz
# Date 1377175700 -7200
# Node ID b8ed26962c6a94a78c255f31582e2a75a56aef5c
# Parent  9d6c9b0a8f15e650b1016c724d71489a6b3cbc86
opto: Optimize g1 pre_barrier in library_call.

diff --git a/src/share/vm/opto/library_call.cpp b/src/share/vm/opto/library_call.cpp
--- a/src/share/vm/opto/library_call.cpp
+++ b/src/share/vm/opto/library_call.cpp
@@ -2755,10 +2755,15 @@
       newval = _gvn.makecon(TypePtr::NULL_PTR);
 
     // Reference stores need a store barrier.
-    pre_barrier(true /* do_load*/,
-                control(), base, adr, alias_idx, newval, value_type->make_oopptr(),
-                NULL /* pre_val*/,
-                T_OBJECT);
+    if (kind == LS_cmpxchg) {
+      // The only known value which might get overwritten is oldval.
+      pre_barrier(false /* do_load */,
+                  control(), NULL, NULL, max_juint, NULL, NULL,
+                  oldval /* pre_val */,
+                  T_OBJECT);
+    }
+    // LS_xchg: see below.
+
 #ifdef _LP64
     if (adr->bottom_type()->is_ptr_to_narrowoop()) {
       Node *newval_enc = _gvn.transform(new (C) EncodePNode(newval, newval->bottom_type()->make_narrowoop()));
@@ -2794,16 +2799,26 @@
   Node* proj = _gvn.transform(new (C) SCMemProjNode(load_store));
   set_memory(proj, alias_idx);
 
-  // Add the trailing membar surrounding the access
-  insert_mem_bar(Op_MemBarCPUOrder);
-  insert_mem_bar(Op_MemBarAcquire);
-
 #ifdef _LP64
   if (type == T_OBJECT && adr->bottom_type()->is_ptr_to_narrowoop() && kind == LS_xchg) {
     load_store = _gvn.transform(new (C) DecodeNNode(load_store, load_store->get_ptr_type()));
   }
 #endif
 
+  // G1: Don't need to load pre_val. The old value is returned by load_store.
+  // The pre_barrier can execute after the xchg as long as no safepoint
+  // gets inserted between them.
+  if (type == T_OBJECT && kind == LS_xchg) {
+    pre_barrier(false /* do_load */,
+                control(), NULL, NULL, max_juint, NULL, NULL,
+                load_store /* pre_val */,
+                T_OBJECT);
+  }
+
+  // Add the trailing membar surrounding the access
+  insert_mem_bar(Op_MemBarCPUOrder);
+  insert_mem_bar(Op_MemBarAcquire);
+
   assert(type2size[load_store->bottom_type()->basic_type()] == type2size[rtype], "result type should match");
   set_result(load_store);
   return true;