dart2js cps: Let effective constant expressions propagate further.

pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart

 // Copyright (c) 2014, 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.
 
 library tree_ir.optimization.statement_rewriter;
 
 import 'optimization.dart' show Pass;
 import '../tree_ir_nodes.dart';
 import '../../io/source_information.dart';
 
@@ skipping 189 matching lines @@
   /// handler (i.e., if the code would be moved into a try from outside it).
   Set<Label> safeForInlining = new Set<Label>();
 
   /// Returns the redirect target of [jump] or [jump] itself if it should not
   /// be redirected.
   Jump redirect(Jump jump) {
     Jump newJump = labelRedirects[jump.target];
     return newJump != null ? newJump : jump;
   }
 
-  void inEmptyEnvironment(void action()) {
+  void inEmptyEnvironment(void action(), {bool keepConstants: true}) {
     List oldEnvironment = environment;
     Map oldConstantEnvironment = constantEnvironment;
     environment = <Expression>[];
-    constantEnvironment = <Variable, Expression>{};
+    if (!keepConstants) {
+      constantEnvironment = <Variable, Expression>{};
+    }
     action();
     assert(environment.isEmpty);
     environment = oldEnvironment;
-    constantEnvironment = oldConstantEnvironment;
+    if (!keepConstants) {
+      constantEnvironment = oldConstantEnvironment;
+    }
   }
 
   /// Left-hand side of the given assignment, or `null` if not an assignment.
   Variable getLeftHand(Expression e) {
     return e is Assign ? e.variable : null;
   }
 
   /// If the given expression always returns the value of one of its
   /// subexpressions, returns that subexpression, otherwise `null`.
   Expression getValueSubexpression(Expression e) {
@@ skipping 421 matching lines @@
     if (reduced != null) {
       return reduced;
     }
 
     return node;
   }
 
   Statement visitWhileTrue(WhileTrue node) {
     // Do not propagate assignments into loops.  Doing so is not safe for
     // variables modified in the loop (the initial value will be propagated).
+    // Do not propagate effective constant expressions into loops, since
+    // computing them is not free (e.g. interceptors are expensive).
     inEmptyEnvironment(() {
       node.body = visitStatement(node.body);
-    });
+    }, keepConstants: false);
     return node;
   }
 
   Statement visitFor(For node) {
     // Not introduced yet
     throw "Unexpected For in StatementRewriter";
   }
 
   Statement visitTry(Try node) {
     inEmptyEnvironment(() {
@@ skipping 546 matching lines @@
   VariableUseVisitor(this.callback);
 
   visitVariableUse(VariableUse use) => callback(use);
 
   visitInnerFunction(FunctionDefinition node) {}
 
   static void visit(Expression node, VariableUseCallback callback) {
     new VariableUseVisitor(callback).visitExpression(node);
   }
 }