HChange is only deletable under certain circumstances.

src/hydrogen-instructions.h

Index: src/hydrogen-instructions.h
diff --git a/src/hydrogen-instructions.h b/src/hydrogen-instructions.h
index ae769fea0aefbbf8b0621a0967222e126dbf5c9a..828b9ed6be71bb20e24920d177d8a5e7a7dba808 100644
--- a/src/hydrogen-instructions.h
+++ b/src/hydrogen-instructions.h
@@ -1194,7 +1194,7 @@ class HUnaryOperation: public HTemplateInstruction<1> {
     return reinterpret_cast<HUnaryOperation*>(value);
   }
 
-  HValue* value() { return OperandAt(0); }
+  HValue* value() const { return OperandAt(0); }
   virtual void PrintDataTo(StringStream* stream);
 };
 
@@ -1270,8 +1270,8 @@ class HChange: public HUnaryOperation {
   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);
   }
@@ -1292,7 +1292,9 @@ class HChange: public HUnaryOperation {
   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...

+  }
 };