[Bug java/21855] array bounds checking elimination
rguenth at gcc dot gnu.org
gcc-bugzilla@gcc.gnu.org
Tue Jan 10 16:32:00 GMT 2012
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).
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