Avoid inserting new temporaries because of HTypeConversion nodes.

lib/compiler/implementation/ssa/nodes.dart

Index: lib/compiler/implementation/ssa/nodes.dart
===================================================================
--- lib/compiler/implementation/ssa/nodes.dart	(revision 7378)
+++ lib/compiler/implementation/ssa/nodes.dart	(working copy)
@@ -930,6 +930,13 @@
    * 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 {
@@ -953,7 +960,9 @@
   // 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 {
@@ -968,6 +977,7 @@
   }
 
   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;
@@ -976,6 +986,7 @@
   HType get guaranteedType() => isOn ? guardedType : HType.UNKNOWN;
 
   bool isControlFlow() => true;
+  bool returnsInput() => true;
 
   accept(HVisitor visitor) => visitor.visitTypeGuard(this);
   int typeCode() => 1;
@@ -996,8 +1007,8 @@
 
   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());
@@ -2158,6 +2169,10 @@
   HType get guaranteedType() => type;
 
   accept(HVisitor visitor) => visitor.visitTypeConversion(this);
+
+  HInstruction get input() => inputs[0];
+  bool returnsInput() => true;
+  bool hasSideEffects() => checked;
 }
 
 /**