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.

*** 2754,2767 ****
--- 2754,2785 ----
      // 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.
+     if (kind == LS_xchg) {
+       // If pre-barrier must execute before the oop store, old value will require do_load here.
+       if (!can_move_pre_barrier()) {
          pre_barrier(true /* do_load*/,
                      control(), base, adr, alias_idx, newval, value_type->make_oopptr(),
                      NULL /* pre_val*/,
                      T_OBJECT);
+       } // Else: see below.
+     } else if (kind == LS_cmpxchg) {
+       // Same as for newval above:
+       if (_gvn.type(oldval) == TypePtr::NULL_PTR) {
+         oldval = _gvn.makecon(TypePtr::NULL_PTR);
+       }
+       // 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);
+     } else {
+       ShouldNotReachHere();
+     }
+ 
  #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,
*** 2793,2811 ****
    // 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);
! 
  #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;
  }
--- 2811,2840 ----
    // 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);
  
!   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
+     if (can_move_pre_barrier()) {
+       // 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;
  }