Implement simple dead store elimination.

tests/language/vm/store_elimination_vm_test.dart

+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+// Test correctness of side effects tracking used by load to load forwarding.
+
+// VMOptions=--optimization-counter-threshold=10
+
+import "package:expect/expect.dart";
+
+class C {

[Vyacheslav Egorov (Google), 2014/05/08 12:53:20]

Might be good to have this test cover other types of stores as well, i.e.
indexed, context and unboxed instance fields.

[Florian Schneider, 2014/05/08 14:21:43]

Done.

+  var x;
+  var y;
+  final z = 123;
+}
+
+s1(a) {
+  a.x = 42;
+  a.x = 43;
+  return a.x;
+}
+
+void foo(a) {
+  Expect.equals(42, a.x);
+}
+
+s1a(a) {
+  a.x = 42;
+  foo(a);
+  a.x = 43;
+  return a.x;
+}
+
+s2() {
+  var t = new C();
+  return t;
+}
+
+s3(a, b) {

[Vyacheslav Egorov (Google), 2014/05/08 12:53:20]

You should also have a test where redundant store itself is inside the control
flow.

a.x = 10
if (b % 2 == 0) { a.x = 20; }
a.x = 30;

also add tests where x is not a simple value but e.g. boxed double to see that
the code is cleaned up.

[Florian Schneider, 2014/05/08 14:21:43]

Done.

+  a.x = b + 1;
+  if (b % 2 == 0) {
+    a.x = 0;
+  } else {
+    a.x = 0;
+  }
+  return a.x;
+}
+
+
+main() {
+  for (var i = 0; i < 20; i++) Expect.equals(43, s1(new C()));
+  for (var i = 0; i < 20; i++) Expect.equals(43, s1a(new C()));
+  for (var i = 0; i < 20; i++) Expect.equals(123, s2().z);
+  for (var i = 0; i < 20; i++) Expect.equals(0, s3(new C(), i));
+}