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

src/compiler/machine-operator-reducer.cc

Index: src/compiler/machine-operator-reducer.cc
diff --git a/src/compiler/machine-operator-reducer.cc b/src/compiler/machine-operator-reducer.cc
index 5a5c8902dde364cfa57773179a42b01c5a11c47b..57de1acda94dfbd1e2f6462fdd7c8014cdb84b80 100644
--- a/src/compiler/machine-operator-reducer.cc
+++ b/src/compiler/machine-operator-reducer.cc
@@ -473,6 +473,21 @@ Reduction MachineOperatorReducer::Reduce(Node* node) {
       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;
   }