[Bug java/21855] array bounds checking elimination

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

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

--- Comment #17 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-01-10 16:30:30 UTC ---
(In reply to comment #16)
> (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.

You mean that System.out.println could change the elements of the array
(well, it doesn't, but theoretically it could)?

> > 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?

No.  What you can do is, via the method I outlined, tell GCC that
args is to be treated similar to a local automatic variable - thus
it cannot be refered to from other functions (unless you pass them
its address of course).