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.

*** 2753,2767 ****
      // 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*/,
                  T_OBJECT);
  #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,
--- 2753,2775 ----
      // 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);
  
!     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,
*** 2793,2811 ****
    // 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;
  }
--- 2801,2828 ----
    // 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
+     // 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;
  }