http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21855
--- Comment #18 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-01-10
16:35:46 UTC ---
(In reply to comment #17)
> (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).
Thus, similar to the C++ case with
struct Array { int length; void data[]; }
void foo (Array args)
{
...
}
foo cannot change the callers args.length (only its own copy) but it
can change the callers args.data[] contents. If the C++ frontend
decides to pass args by reference then it sets DECL_BY_REFERENCE
and uses a TYPE_RESTRICT qualified pointer. This way the optimization
will be the same as if it was passed "really" by value.
Not sure if the Java situation is similar enough.
