On Tue, Aug 02, 2005 at 02:56:50PM +0200, Richard Guenther wrote:
> > > > > void nconc (unsigned x, unsigned y) {
> > > > > unsigned *ptr = &x;
> > > > > while(!(*ptr & 3))
> > > > > ptr = &((cons *)(*ptr))->cdr;
> > > > > *ptr = y;
> > > > > }
>
> 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;
> }
>
> which makes aliasing see that the store is not dead and in fact it never will
> be to the argument area.
Thank you. But while this does indeed save the store, I haven't made
sure nconc is never called with arguments where the while condition is
never true, and nothing should happen. (This version is a little
simplified, just to show the problem.) Another workaround seems to be to
declare ptr as volatile unsigned *. Or, just compile with
-fno-strict-aliasing.
