https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120972
--- Comment #6 from Feng Xue <fxue at os dot amperecomputing.com> ---
(In reply to rguent...@suse.de from comment #5)
> I see. I see annotating loops with pragmas as a more user-friendly way
> of getting there. What we have at our disposal in terms of pragmas
> is quite limiting though, and if the application is a benchmark where
> changes are not allowed this doesn't help, of course.
>
> Handling outermost scope variables is reasonable I think. I'll note
> another problem though, which is
>
> void foo (int *a, int *b)
> {
> int * __restrict p = a;
> *p = *b;
> p = b;
> *p = *a;
> }
>
> I'd have to re-read the standard wording but since on GIMPLE we can't
> restrict ourselves to variables with initializer (that distinction
> is lost), we have to deal with possibly multiple assignments to
> the restrict qualified pointer (and the "first" one possibly being
> DCEd already).
>
> Another issue is that of propagation. We are happily replacing
>
> int * __restrict p = a;
> *p = *b;
>
> with
>
> *a = *b;
>
> but you can't reply on all uses of 'p' being replaced, so you
> might get an invalid mix of accesses via the restrict qualified
> and the not restrict qualified pointer from the initializer.
A simple and safe way is to forget __restrict on "p" if it is replaced by "a".
Or in some limited sceinaros, there is no memory access or any address
computation on "a" before assignment of "p = a" and after assignment of "a", we
also set "a" as __restrict.
