[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 #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.