dart2js cps: Compile some loops as 'for' loops.

pkg/compiler/lib/src/js_backend/codegen/codegen.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 code_generator;
 
 import 'glue.dart';
 
 import '../../closure.dart' show ClosureClassElement;
 import '../../common/codegen.dart' show CodegenRegistry;
@@ skipping 90 matching lines @@
       if (!declaredVariables.add(use.name)) break;
 
       js.VariableInitialization jsVariable = new js.VariableInitialization(
         new js.VariableDeclaration(use.name),
         assign.value);
       jsVariables.add(jsVariable);
 
       ++accumulatorIndex;
     }
 
+    // If the last statement is a for loop with an initializer expression, try
+    // to pull that expression into an initializer as well.
+    pullFromForLoop:
+    if (accumulatorIndex < accumulator.length &&
+        accumulator[accumulatorIndex] is js.For) {
+      js.For forLoop = accumulator[accumulatorIndex];
+      if (forLoop.init is! js.Assignment) break pullFromForLoop;
+      js.Assignment assign = forLoop.init;
+      if (assign.leftHandSide is! js.VariableUse) break pullFromForLoop;
+      if (assign.op != null) break pullFromForLoop; // Compound assignment.
+      js.VariableUse use = assign.leftHandSide;
+
+      // We cannot declare a variable more than once.
+      if (!declaredVariables.add(use.name)) break pullFromForLoop;
+
+      js.VariableInitialization jsVariable = new js.VariableInitialization(
+        new js.VariableDeclaration(use.name),
+        assign.value);
+      jsVariables.add(jsVariable);
+
+      // Remove the initializer from the for loop.
+      accumulator[accumulatorIndex] = 
+          new js.For(null, forLoop.condition, forLoop.update, forLoop.body);
+    }
+
     // Discard the statements that were pulled in the initializer.
     if (accumulatorIndex > 0) {
       accumulator = accumulator.sublist(accumulatorIndex);
     }
 
     // Declare remaining variables.
     for (tree_ir.Variable variable in variableNames.keys) {
       String name = getVariableName(variable);
       if (declaredVariables.contains(name)) continue;
       js.VariableInitialization jsVariable = new js.VariableInitialization(
@@ skipping 388 matching lines @@
 
   js.Block buildBodyBlock(tree_ir.Statement statement) {
     List<js.Statement> savedAccumulator = accumulator;
     accumulator = <js.Statement>[];
     visitStatement(statement);
     js.Statement result = new js.Block(accumulator);
     accumulator = savedAccumulator;
     return result;
   }
 
+  js.Expression makeSequence(List<tree_ir.Expression> list) {
+    return list.map(visitExpression).reduce((x,y) => new js.Binary(',', x, y));
+  }
+
   @override
   void visitWhileCondition(tree_ir.WhileCondition node) {
     js.Expression condition = visitExpression(node.condition);
     shortBreak.push(node.next);
     shortContinue.push(node);
     fallthrough.push(node);
-    js.Statement jsBody = buildBodyStatement(node.body);
+    js.Statement body = buildBodyStatement(node.body);
     fallthrough.pop();
     shortContinue.pop();
     shortBreak.pop();
-    accumulator.add(insertLabel(node.label, new js.While(condition, jsBody)));
+    js.Statement loopNode;
+    if (node.updates.isEmpty) {
+      loopNode = new js.While(condition, body);
+    } else { // Compile as a for loop.
+      js.Expression init;
+      if (accumulator.isNotEmpty && 
+          accumulator.last is js.ExpressionStatement) {
+        // Take the preceding expression from the accumulator and use
+        // it as the initializer expression.
+        js.ExpressionStatement initStmt = accumulator.removeLast();
+        init = initStmt.expression;
+      }
+      js.Expression update = makeSequence(node.updates);
+      loopNode = new js.For(init, condition, update, body);
+    }
+    accumulator.add(insertLabel(node.label, loopNode));
     visitStatement(node.next);
   }
 
   @override
   void visitWhileTrue(tree_ir.WhileTrue node) {
     js.Expression condition = new js.LiteralBool(true);
     // A short break in the while will jump to the current fallthrough target.
     shortBreak.push(fallthrough.target);
     shortContinue.push(node);
     fallthrough.push(node);
@@ skipping 312 matching lines @@
   js.Expression visitAwait(tree_ir.Await node) {
     return new js.Await(visitExpression(node.input));
   }
 
   visitFunctionExpression(tree_ir.FunctionExpression node) {
     // FunctionExpressions are currently unused.
     // We might need them if we want to emit raw JS nested functions.
     throw 'FunctionExpressions should not be used';
   }
 }