On Tue, Aug 02, 2005 at 09:08:51AM -0400, Diego Novillo wrote:
> On Tue, Aug 02, 2005 at 02:56:50PM +0200, Richard Guenther wrote:
> > Oh, and a workaround and slight correction would be to write
> >
> > void nconc (unsigned x, unsigned y) {
> > unsigned *ptr = &((cons *)x)->cdr;
> > while(!(*ptr & 3))
> > ptr = &((cons *)(*ptr))->cdr;
> > *ptr = y;
> > }
> >
> No. Same problem. The aliaser would say "yes, ptr points
> anywhere, but it cannot escape the local frame". The final store
> is dead just the same.
>
> We only "get it right" because we do not distinguish between
> different degrees of points-anywhere.
Then the alias analyzer's broken. This isn't pointer arithmetic in the
sense that you mean. It would be if the line were:
ptr = &((cons *)(ptr))->cdr;
which is equivalent to some offset plus ptr. But there's an extra
dereference:
ptr = &((cons *)(*ptr))->cdr;
^
As far as I can tell, this code doesn't actually violate any of the
aliasing rules. It just looks funny.
--
Daniel Jacobowitz
CodeSourcery, LLC
