# HG changeset patch
# User Martin Doerr
# Date 1377525197 -7200
# Node ID 4f1b0f5ab21c2c5499f7d897f340d10426d31444
# Parent  73921c720b943ee44be1e883c8383baf512a5c20
8023597: Optimize G1 barriers code for unsafe load_store
Summary: Avoid loading old values in G1 pre-barriers for inlined unsafe load_store nodes.

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
@@ -2755,11 +2755,19 @@
     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*/,
-                T_OBJECT);
+    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()));
@@ -2795,16 +2803,25 @@
   Node* proj = _gvn.transform(new (C) SCMemProjNode(load_store));
   set_memory(proj, alias_idx);
 
+  if (type == T_OBJECT && kind == LS_xchg) {
+#ifdef _LP64
+    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);
 
-#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
-
   assert(type2size[load_store->bottom_type()->basic_type()] == type2size[rtype], "result type should match");
   set_result(load_store);
   return true;