checkcast elimination

[Tom Rodriguez]

It's never ok to elide work that throws an exception unless you can  
prove that it won't occur.  The general rule for optimization in  
hotspot is that the JVM can do anything it wants as long as you can't  
write a program that can detect the difference, other than performance  
or limits.  In the case below a very clever JVM might able prove that  
only myClass'es were ever inserted into aList but because generics are  
based on type erasure we can't assume it's true.

tom

On Jan 14, 2009, at 12:25 PM, Deneau, Tom wrote:

> In the following method foo, an entry is being pulled from a  
> Collection
> class, in this case an ArrayList, and then being passed to another
> method which takes an Object.  The programmer stores the ArrayList
> return in a myClass reference but never really uses it as a myClass
> object.  The javac compiler generates a "checkcast" bytecode and the
> HotSpot JVM generates code for the checkcast.
>
> Question: is it legal for the JVM to eliminate this cast check or is  
> the
> semantics of this code such that the JVM is required to generate an
> exception if the cast fails (even though the reference is never really
> used as a myClass) ?
>
> -- Tom
>
> =================
> 	ArrayList<myClass> aList;
>
> 	void foo(int i) {
> 		myClass myc = aList.get(i);
> 		bar(myc);
> 	}
>
> 	void bar(Object x) {
>      ...
>      }
>