Implement shrinking reductions in CPS IR

sdk/lib/_internal/compiler/implementation/cps_ir/cps_ir_nodes.dart

 // Copyright (c) 2013, 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.
 
 // IrNodes are kept in a separate library to have precise control over their
 // dependencies on other parts of the system.
 library dart2js.ir_nodes;
 
 import '../dart2jslib.dart' as dart2js show Constant, ConstructedConstant,
   StringConstant, ListConstant, MapConstant;
 import '../elements/elements.dart';
 import '../universe/universe.dart' show Selector, SelectorKind;
 import '../dart_types.dart' show DartType, GenericType;
 import 'const_expression.dart';
 import '../helpers/helpers.dart';
 
 abstract class Node {
   static int hashCount = 0;
   final int hashCode = hashCount = (hashCount + 1) & 0x3fffffff;
 
+  /// A pointer to the parent node. Is null until set by optimization passes.
+  Node parent;
+
   accept(Visitor visitor);
 }
 
 abstract class Expression extends Node {
   Expression plug(Expression expr) => throw 'impossible';
 }
 
 /// The base class of things that variables can refer to: primitives,
 /// continuations, function and continuation parameters, etc.
 abstract class Definition extends Node {
@@ skipping 45 matching lines @@
   }
 }
 
 /// Operands to invocations and primitives are always variables.  They point to
 /// their definition and are doubly-linked into a list of occurrences.
 class Reference {
   Definition definition;
   Reference previous = null;
   Reference next = null;
 
+  /// A pointer to the parent node. Is null until set by optimization passes.
+  Node parent;
+
   Reference(this.definition) {
     next = definition.firstRef;
     if (next != null) next.previous = this;
     definition.firstRef = this;
   }
 
   /// Unlinks this reference from the list of occurrences.
   void unlink() {
     if (previous == null) {
       assert(definition.firstRef == this);
@@ skipping 801 matching lines @@
       release(node.parameters[i]);
     }
   }
 
   void visitIsTrue(IsTrue node) {
     visitReference(node.value);
   }
 
 }