HChange is only deletable under certain circumstances.

src/hydrogen-instructions.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1176 matching lines @@
 class HUnaryOperation: public HTemplateInstruction<1> {
  public:
   explicit HUnaryOperation(HValue* value) {
     SetOperandAt(0, value);
   }
 
   static HUnaryOperation* cast(HValue* value) {
     return reinterpret_cast<HUnaryOperation*>(value);
   }
 
-  HValue* value() { return OperandAt(0); }
+  HValue* value() const { return OperandAt(0); }
   virtual void PrintDataTo(StringStream* stream);
 };
 
 
 class HThrow: public HTemplateInstruction<2> {
  public:
   HThrow(HValue* context, HValue* value) {
     SetOperandAt(0, context);
     SetOperandAt(1, value);
     SetAllSideEffects();
@@ skipping 55 matching lines @@
     SetFlag(kUseGVN);
     if (deoptimize_on_undefined) SetFlag(kDeoptimizeOnUndefined);
     if (is_truncating) SetFlag(kTruncatingToInt32);
     if (to.IsTagged()) SetGVNFlag(kChangesNewSpacePromotion);
   }
 
   virtual HValue* EnsureAndPropagateNotMinusZero(BitVector* visited);
   virtual HType CalculateInferredType();
   virtual HValue* Canonicalize();
 
-  Representation from() { return value()->representation(); }
-  Representation to() { return representation(); }
+  Representation from() const { return value()->representation(); }
+  Representation to() const { return representation(); }
   bool deoptimize_on_undefined() const {
     return CheckFlag(kDeoptimizeOnUndefined);
   }
   bool deoptimize_on_minus_zero() const {
     return CheckFlag(kBailoutOnMinusZero);
   }
   virtual Representation RequiredInputRepresentation(int index) {
     return from();
   }
 
   virtual Range* InferRange(Zone* zone);
 
   virtual void PrintDataTo(StringStream* stream);
 
   DECLARE_CONCRETE_INSTRUCTION(Change)
 
  protected:
   virtual bool DataEquals(HValue* other) { return true; }
 
  private:
-  virtual bool IsDeletable() const { return true; }
+  virtual bool IsDeletable() const {
+    return !from().IsTagged() || (!to().IsDouble() && value()->type().IsSmi());

[Michael Starzinger, 2012/10/11 13:48:43]

Why do we check that Smi->Double is not deleted? Shouldn't that be possible
without side-effects?

[Sven Panne, 2012/10/11 14:11:50]

Yep, it seems that the !to().IsDouble() part can be removed...

+  }
 };
 
 
 class HClampToUint8: public HUnaryOperation {
  public:
   explicit HClampToUint8(HValue* value)
       : HUnaryOperation(value) {
     set_representation(Representation::Integer32());
     SetFlag(kUseGVN);
   }
@@ skipping 4101 matching lines @@
   virtual bool IsDeletable() const { return true; }
 };
 
 
 #undef DECLARE_INSTRUCTION
 #undef DECLARE_CONCRETE_INSTRUCTION
 
 } }  // namespace v8::internal
 
 #endif  // V8_HYDROGEN_INSTRUCTIONS_H_