[Bug java/21855] array bounds checking elimination

["aph at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21855

--- Comment #16 from Andrew Haley <aph at gcc dot gnu.org> 2012-01-10 16:26:51 UTC ---
(In reply to comment #15)
> (In reply to comment #14)
> > (In reply to comment #13)
> > > We can't optimize this because System.out.println can change args[].
> > 
> > That's the whole point: System.out.println cannot change args[], which is a
> > java array, and the length of a Java array is constant.  It is not an invalid
> > test case.
> 
> I suppose
> 
>   public static void main(String[] args)
> 
> is passing args by value (but the implementation detail uses reference
> passing for efficiency?).

args is indeed a reference to a Java array.  The length field of a Java
array is immutable.  The elements of an array are not immutable.

> In this case the Java frontend should do
> like the C++ frontend and tell this to the middle-end by properly
> marking args as 1) DECL_BY_REFERENCE, 2) use a TYPE_RESTRICT qualified
> pointer for the reference.  Then we would optimize this case.

If we could mark the length field as immutable that would fix it.  Is there any
way to do that?