src/share/vm/opto/library_call.cpp

rev 5127 : 8023597: Optimize G1 barriers code for unsafe load_store
Summary: Avoid loading old values in G1 pre-barriers for inlined unsafe load_store nodes.

@@ -2753,15 +2753,23 @@
     // could be delayed during Parse (for example, in adjust_map_after_if()).
     // Execute transformation here to avoid barrier generation in such case.
     if (_gvn.type(newval) == TypePtr::NULL_PTR)
       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*/,
+    if (kind == LS_cmpxchg) {
+      if (_gvn.type(oldval) == TypePtr::NULL_PTR) {
+        oldval = _gvn.makecon(TypePtr::NULL_PTR);
+      } else {
+        // 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()));
       if (kind == LS_xchg) {
         load_store = _gvn.transform(new (C) GetAndSetNNode(control(), mem, adr,

@@ -2793,19 +2801,28 @@
   // main role is to prevent LoadStore nodes from being optimized away
   // when their results aren't used.
   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);
-
+  if (type == T_OBJECT && kind == LS_xchg) {
 #ifdef _LP64
-  if (type == T_OBJECT && adr->bottom_type()->is_ptr_to_narrowoop() && kind == LS_xchg) {
+    if (adr->bottom_type()->is_ptr_to_narrowoop()) {
     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.
+    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;
 }