Avoid inserting new temporaries because of HTypeConversion nodes.

lib/compiler/implementation/ssa/nodes.dart

 // Copyright (c) 2011, 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.
 
 interface HVisitor<R> {
   R visitAdd(HAdd node);
   R visitBitAnd(HBitAnd node);
   R visitBitNot(HBitNot node);
   R visitBitOr(HBitOr node);
   R visitBitXor(HBitXor node);
@@ skipping 912 matching lines @@
     validator.visitInstruction(this);
     return validator.isValid;
   }
 
   /**
    * The code for computing a bailout environment, and the code
    * generation must agree on what does not need to be captured,
    * so should always be generated at use site.
    */
   bool isCodeMotionInvariant() => false;
+
+  /**
+   * Returns whether this instruction produces the same value as its
+   * input.
+   */
+  bool returnsInput() => false;

[Lasse Reichstein Nielsen, 2012/05/08 12:39:08]

returnsSingleInput?

Isn't our behavior here a little odd? We insert instructions that are guaranteed
to return their input - so we could just as well have them return no input
immediately, and reuse the original input later.
For some reason we don't do that here, but have a later step that does the
transformation. Why?

[ngeoffray, 2012/05/08 16:09:12]

As discussed, to get the dependencies right. The checks need to be before the
uses.

+  HInstruction get input() => null;

[ngeoffray, 2012/05/08 11:17:51]

I could instead add a common super class for instructions that return their
input. Your call.

[Lasse Reichstein Nielsen, 2012/05/08 12:39:08]

What's the "input" getter for? Add comment, especially when adding an,
effectively, abstract method where the user can't even look at the code.

[floitsch, 2012/05/08 13:21:42]

I would prefer a common superclass or alternatively a common interface.

[ngeoffray, 2012/05/08 16:09:12]

I'm now using HCheck as the superclass of all these classes. So I removed
returnsInput and the input getter.

 }
 
 class HBoolify extends HInstruction {
   HBoolify(HInstruction value) : super(<HInstruction>[value]);
   void prepareGvn() {
     assert(!hasSideEffects());
     setUseGvn();
   }
 
   HType get guaranteedType() => HType.BOOLEAN;
 
   accept(HVisitor visitor) => visitor.visitBoolify(this);
   int typeCode() => 0;
   bool typeEquals(other) => other is HBoolify;
   bool dataEquals(HInstruction other) => true;
 }
 
 class HCheck extends HInstruction {
   HCheck(inputs) : super(inputs);
 
   // TODO(floitsch): make class abstract instead of adding an abstract method.
   abstract accept(HVisitor visitor);
 
+  HInstruction get input() => inputs[0];
   bool isControlFlow() => true;
+  bool returnsInput() => true;
 }
 
 class HTypeGuard extends HInstruction {
   final int state;
   final HType guardedType;
   bool isOn = false;
   HTypeGuard(this.guardedType, this.state, List<HInstruction> env) : super(env);
 
   void prepareGvn() {
     assert(!hasSideEffects());
     setUseGvn();
   }
 
   HInstruction get guarded() => inputs.last();
+  HInstruction get input() => guarded;
 

[Lasse Reichstein Nielsen, 2012/05/08 12:39:08]

Could you put the new methods in a somewhat consistent position inside the
classes. At least adjacent, as they were in the superclass.

   HType computeTypeFromInputTypes() {
     return isOn ? guardedType : guarded.propagatedType;
   }
 
   HType get guaranteedType() => isOn ? guardedType : HType.UNKNOWN;
 
   bool isControlFlow() => true;
+  bool returnsInput() => true;
 
   accept(HVisitor visitor) => visitor.visitTypeGuard(this);
   int typeCode() => 1;
   bool typeEquals(other) => other is HTypeGuard;
   bool dataEquals(HTypeGuard other) => guardedType == other.guardedType;
 }
 
 class HBoundsCheck extends HCheck {
   static final int ALWAYS_FALSE = 0;
   static final int FULL_CHECK = 1;
   static final int ALWAYS_ABOVE_ZERO = 2;
   static final int ALWAYS_TRUE = 3;
   /**
    * Details which tests have been done statically during compilation.
    * Default is that all checks must be performed dynamically.
    */
   int staticChecks = FULL_CHECK;
 
   HBoundsCheck(length, index) : super(<HInstruction>[length, index]);
 
-  HInstruction get length() => inputs[0];
-  HInstruction get index() => inputs[1];
+  HInstruction get length() => inputs[1];
+  HInstruction get index() => inputs[0];
 
   void prepareGvn() {
     assert(!hasSideEffects());
     setUseGvn();
   }
 
   HType get guaranteedType() => HType.INTEGER;
 
   accept(HVisitor visitor) => visitor.visitBoundsCheck(this);
   int typeCode() => 2;
@@ skipping 1140 matching lines @@
   HTypeConversion(HType this.type,
                   HInstruction input,
                   [bool this.checked = false])
     : super(<HInstruction>[input]) {
       sourceElement = input.sourceElement;
   }
 
   HType get guaranteedType() => type;
 
   accept(HVisitor visitor) => visitor.visitTypeConversion(this);
+
+  HInstruction get input() => inputs[0];
+  bool returnsInput() => true;
+  bool hasSideEffects() => checked;
 }
 
 /**
  * Information about a syntactic-like structure that can be attached
  * to a [HBasicBlock].
  */
 interface HBlockInformation {
   HBasicBlock get start();
   HBasicBlock get end();
   bool accept(HBlockInformationVisitor visitor);
@@ skipping 234 matching lines @@
                        this.finallyBlock,
                        this.joinBlock);
 
   HBasicBlock get start() => body.start;
   HBasicBlock get end() =>
       finallyBlock === null ? catchBlock.end : finallyBlock.end;
 
   bool accept(HStatementInformationVisitor visitor) =>
     visitor.visitTryInfo(this);
 }