When replacing an instruction with another in GVN-like optimizations, try to find a better user of …

tests/compiler/dart2js/rewrite_better_user_test.dart

Index: tests/compiler/dart2js/rewrite_better_user_test.dart
===================================================================
--- tests/compiler/dart2js/rewrite_better_user_test.dart	(revision 0)
+++ tests/compiler/dart2js/rewrite_better_user_test.dart	(revision 0)
@@ -0,0 +1,35 @@
+// 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 that we get rid of duplicate type guards on a field when that
+// field is being gvn'ed.
+
+#import("compiler_helper.dart");
+
+final String TEST = @"""
+class A {
+  var field = 52;
+  foo() {
+    var a = this.field;
+    while (a + 42 == 42);
+    // This field get should be GVN'ed
+    a = this.field;
+    while (a + 87 == 87);
+    field = 'bar';
+  }
+}
+
+main() {
+  while (true) new A().foo();
+}
+""";
+
+main() {
+  String generated = compileAll(TEST);
+  RegExp regexp = const RegExp('foo\\\$0\\\$bailout');
+  Iterator matches = regexp.allMatches(generated).iterator();
+
+  // We check that there is only one call to the bailout method.
+  // One match for the call, one for the definition.
+  checkNumberOfMatches(matches, 2);
+}