7032963: StoreCM shouldn't participate in store elimination

Reviewed-by: kvn

src/share/vm/opto/compile.cpp

@@ -2050,6 +2050,52 @@ static bool oop_offset_is_sane(const TypeInstPtr* tp) {
   // Note that OffsetBot and OffsetTop are very negative.
 }
 
+// Eliminate trivially redundant StoreCMs and accumulate their
+// precedence edges.
+static void eliminate_redundant_card_marks(Node* n) {
+  assert(n->Opcode() == Op_StoreCM, "expected StoreCM");
+  if (n->in(MemNode::Address)->outcnt() > 1) {
+    // There are multiple users of the same address so it might be
+    // possible to eliminate some of the StoreCMs
+    Node* mem = n->in(MemNode::Memory);
+    Node* adr = n->in(MemNode::Address);
+    Node* val = n->in(MemNode::ValueIn);
+    Node* prev = n;
+    bool done = false;
+    // Walk the chain of StoreCMs eliminating ones that match.  As
+    // long as it's a chain of single users then the optimization is
+    // safe.  Eliminating partially redundant StoreCMs would require
+    // cloning copies down the other paths.
+    while (mem->Opcode() == Op_StoreCM && mem->outcnt() == 1 && !done) {
+      if (adr == mem->in(MemNode::Address) &&
+          val == mem->in(MemNode::ValueIn)) {
+        // redundant StoreCM
+        if (mem->req() > MemNode::OopStore) {
+          // Hasn't been processed by this code yet.
+          n->add_prec(mem->in(MemNode::OopStore));
+        } else {
+          // Already converted to precedence edge
+          for (uint i = mem->req(); i < mem->len(); i++) {
+            // Accumulate any precedence edges
+            if (mem->in(i) != NULL) {
+              n->add_prec(mem->in(i));
+            }
+          }
+          // Everything above this point has been processed.
+          done = true;
+        }
+        // Eliminate the previous StoreCM
+        prev->set_req(MemNode::Memory, mem->in(MemNode::Memory));
+        assert(mem->outcnt() == 0, "should be dead");
+        mem->disconnect_inputs(NULL);
+      } else {
+        prev = mem;
+      }
+      mem = prev->in(MemNode::Memory);
+    }
+  }
+}
+
 //------------------------------final_graph_reshaping_impl----------------------
 // Implement items 1-5 from final_graph_reshaping below.
 static void final_graph_reshaping_impl( Node *n, Final_Reshape_Counts &frc ) {
@@ -2176,9 +2222,19 @@ static void final_graph_reshaping_impl( Node *n, Final_Reshape_Counts &frc ) {
     frc.inc_float_count();
     goto handle_mem;
 
+  case Op_StoreCM:
+    {
+      // Convert OopStore dependence into precedence edge
+      Node* prec = n->in(MemNode::OopStore);
+      n->del_req(MemNode::OopStore);
+      n->add_prec(prec);
+      eliminate_redundant_card_marks(n);
+    }
+
+    // fall through
+
   case Op_StoreB:
   case Op_StoreC:
-  case Op_StoreCM:
   case Op_StorePConditional:
   case Op_StoreI:
   case Op_StoreL:

src/share/vm/opto/lcm.cpp

@@ -685,20 +685,22 @@ bool Block::schedule_local(PhaseCFG *cfg, Matcher &matcher, int *ready_cnt, Vect
       }
       ready_cnt[n->_idx] = local; // Count em up
 
-      // A few node types require changing a required edge to a precedence edge
-      // before allocation.
+#ifdef ASSERT
       if( UseConcMarkSweepGC || UseG1GC ) {
         if( n->is_Mach() && n->as_Mach()->ideal_Opcode() == Op_StoreCM ) {
-          // Note: Required edges with an index greater than oper_input_base
-          // are not supported by the allocator.
-          // Note2: Can only depend on unmatched edge being last,
-          // can not depend on its absolute position.
-          Node *oop_store = n->in(n->req() - 1);
-          n->del_req(n->req() - 1);
-          n->add_prec(oop_store);
+          // Check the precedence edges
+          for (uint prec = n->req(); prec < n->len(); prec++) {
+            Node* oop_store = n->in(prec);
+            if (oop_store != NULL) {
               assert(cfg->_bbs[oop_store->_idx]->_dom_depth <= this->_dom_depth, "oop_store must dominate card-mark");
             }
           }
+        }
+      }
+#endif
+
+      // A few node types require changing a required edge to a precedence edge
+      // before allocation.
       if( n->is_Mach() && n->req() > TypeFunc::Parms &&
           (n->as_Mach()->ideal_Opcode() == Op_MemBarAcquire ||
            n->as_Mach()->ideal_Opcode() == Op_MemBarVolatile) ) {

src/share/vm/opto/memnode.cpp

@@ -2159,9 +2159,12 @@ Node *StoreNode::Ideal(PhaseGVN *phase, bool can_reshape) {
   Node* mem     = in(MemNode::Memory);
   Node* address = in(MemNode::Address);
 
-  // Back-to-back stores to same address?  Fold em up.
-  // Generally unsafe if I have intervening uses...
-  if (mem->is_Store() && phase->eqv_uncast(mem->in(MemNode::Address), address)) {
+  // Back-to-back stores to same address?  Fold em up.  Generally
+  // unsafe if I have intervening uses...  Also disallowed for StoreCM
+  // since they must follow each StoreP operation.  Redundant StoreCMs
+  // are eliminated just before matching in final_graph_reshape.
+  if (mem->is_Store() && phase->eqv_uncast(mem->in(MemNode::Address), address) &&
+      mem->Opcode() != Op_StoreCM) {
     // Looking at a dead closed cycle of memory?
     assert(mem != mem->in(MemNode::Memory), "dead loop in StoreNode::Ideal");
 

src/share/vm/opto/output.cpp

@@ -1354,8 +1354,11 @@ void Compile::Fill_buffer() {
         // Check that oop-store precedes the card-mark
         else if( mach->ideal_Opcode() == Op_StoreCM ) {
           uint storeCM_idx = j;
-          Node *oop_store = mach->in(mach->_cnt);  // First precedence edge
-          assert( oop_store != NULL, "storeCM expects a precedence edge");
+          int count = 0;
+          for (uint prec = mach->req(); prec < mach->len(); prec++) {
+            Node *oop_store = mach->in(prec);  // Precedence edge
+            if (oop_store == NULL) continue;
+            count++;
             uint i4;
             for( i4 = 0; i4 < last_inst; ++i4 ) {
               if( b->_nodes[i4] == oop_store ) break;
@@ -1364,6 +1367,8 @@ void Compile::Fill_buffer() {
             // edges have been added to the storeCMNode.
             assert( i4 == last_inst || i4 < storeCM_idx, "CM card-mark executes before oop-store");
           }
+          assert(count > 0, "storeCM expects at least one precedence edge");
+        }
 #endif
 
         else if( !n->is_Proj() ) {