[turbofan] Eliminate redundant masking operations for word8/word16 stores.

src/compiler/machine-operator-reducer.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler/machine-operator-reducer.h"
 
 #include "src/base/bits.h"
 #include "src/codegen.h"
 #include "src/compiler/generic-node-inl.h"
 #include "src/compiler/graph.h"
@@ skipping 455 matching lines @@
       if (m.HasValue()) return ReplaceInt32(static_cast<int32_t>(m.Value()));
       if (m.IsChangeInt32ToInt64()) return Replace(m.node()->InputAt(0));
       break;
     }
     case IrOpcode::kTruncateFloat64ToFloat32: {
       Float64Matcher m(node->InputAt(0));
       if (m.HasValue()) return ReplaceFloat32(DoubleToFloat32(m.Value()));
       if (m.IsChangeFloat32ToFloat64()) return Replace(m.node()->InputAt(0));
       break;
     }
+    case IrOpcode::kStore: {
+      Node* const value = node->InputAt(2);
+      if (value->opcode() == IrOpcode::kWord32And) {
+        MachineType const rep = static_cast<MachineType>(
+            StoreRepresentationOf(node->op()).machine_type() & kRepMask);
+        Uint32BinopMatcher m(value);
+        if (m.right().HasValue() &&
+            ((rep == kRepWord8 && (m.right().Value() & 0xff) == 0xff) ||
+             (rep == kRepWord16 && (m.right().Value() & 0xffff) == 0xffff))) {

[titzer, 2014/10/09 10:54:47]

We could generalize this to 64-bits as well, but probably not worth it. Can you
drop a TODO(turbofan) here?

+          node->ReplaceInput(2, m.left().node());
+          return Changed(node);
+        }
+      }
+      break;
+    }
     default:
       break;
   }
   return NoChange();
 }
 
 
 Reduction MachineOperatorReducer::ReduceProjection(size_t index, Node* node) {
   switch (node->opcode()) {
     case IrOpcode::kInt32AddWithOverflow: {
@@ skipping 39 matching lines @@
 MachineOperatorBuilder* MachineOperatorReducer::machine() const {
   return jsgraph()->machine();
 }
 
 
 Graph* MachineOperatorReducer::graph() const { return jsgraph()->graph(); }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8