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

part of tree_ir.optimization;

/// Rewrites [WhileTrue] statements with an [If] body into a [WhileCondition],
/// in situations where only one of the branches contains a [Continue] to the
/// loop. Schematically:
///
///   L:
///   while (true) {
///     if (E) {
///       S1  (has references to L)
///     } else {
///       S2  (has no references to L)
///     }
///   }
///     ==>
///   L:
///   while (E) {
///     S1
///   };
///   S2
///
/// A similar transformation is used when S2 occurs in the 'then' position.
///
/// Note that the above pattern needs no iteration since nested ifs
/// have been collapsed previously in the [StatementRewriter] phase.
class LoopRewriter extends RecursiveTransformer
                   implements Pass {
  String get passName => 'Loop rewriter';

  Set<Label> usedContinueLabels = new Set<Label>();

  void rewrite(RootNode root) {
    root.replaceEachBody(visitStatement);
  }

  @override
  void visitInnerFunction(FunctionDefinition node) {
    node.body = new LoopRewriter().visitStatement(node.body);
  }

  Statement visitContinue(Continue node) {
    usedContinueLabels.add(node.target);
    return node;
  }

  Statement visitWhileTrue(WhileTrue node) {
    assert(!usedContinueLabels.contains(node.label));
    if (node.body is If) {
      If body = node.body;
      body.thenStatement = visitStatement(body.thenStatement);
      bool thenHasContinue = usedContinueLabels.remove(node.label);
      body.elseStatement = visitStatement(body.elseStatement);
      bool elseHasContinue = usedContinueLabels.remove(node.label);
      if (thenHasContinue && !elseHasContinue) {
        node.label.binding = null; // Prepare to rebind the label.
        return new WhileCondition(
            node.label,
            body.condition,
            body.thenStatement,
            body.elseStatement);
      } else if (!thenHasContinue && elseHasContinue) {
        node.label.binding = null;
        return new WhileCondition(
            node.label,
            new Not(body.condition),
            body.elseStatement,
            body.thenStatement);
      }
    } else {
      node.body = visitStatement(node.body);
      usedContinueLabels.remove(node.label);
    }
    return node;
  }
}