checkcast elimination

[Tom Rodriguez]

On Jan 14, 2009, at 5:18 PM, David Holmes - Sun Microsystems wrote:

> But in this case I think Tom D. is hoping for something the other  
> way around. As bar takes an Object and whatever comes back from  
> get(i) is an Object then there's no need to check if it is a  
> myClass. In short the code could be treated as:
>
> bar(aList.get(i));
>
> and the type of the actual object is immaterial. Now javac won't do  
> this itself, but could the JIT?

No, because that's not what the bytecodes say.  They say  
bar((myClass)aList.get(i)).  There's no way for the JIT to distinguish  
between javac inserted casts and user inserted casts, nor would we  
want to I think.  If it's legal to leave it out under those conditions  
out then javac should be doing it.

tom

>
>
> Cheers,
> David Holmes
>
> Tom Rodriguez said the following on 01/15/09 08:08:
>> 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) {
>>>     ...
>>>     }
>>>