Re: [PATCH] [RFC] vect: Fix infinite loop while determining peeling amount

[Richard Biener <richard.guenther@gmail.com>]

On Wed, Jul 22, 2020 at 5:18 PM Stefan Schulze Frielinghaus via
Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
>
> This is a follow up to commit 5c9669a0e6c respectively discussion
> https://gcc.gnu.org/pipermail/gcc-patches/2020-June/549132.html
>
> In case that an alignment constraint is less than the size of a
> corresponding scalar type, ensure that we advance at least by one
> iteration.  For example, on s390x we have for a long double an alignment
> constraint of 8 bytes whereas the size is 16 bytes.  Therefore,
> TARGET_ALIGN / DR_SIZE equals zero resulting in an infinite loop which
> can be reproduced by the following MWE:

But we guard this case with vector_alignment_reachable_p, so we shouldn't
have ended up here and the patch looks bogus.

Richard.

> extern long double *a;
> extern double *b;
> void fun(void) {
>   for (int i = 0; i < 42; i++)
>     a[i] = b[i];
> }
>
> Increasing the number of peelings in each iteration at least by one
> fixes the issue for me.  Any comments?
>
> Bootstrapped and regtested on s390x.
>
> gcc/ChangeLog:
>
>         * tree-vect-data-refs.c (vect_enhance_data_refs_alignment):
>         Ensure that loop variable npeel_tmp advances in each iteration.
> ---
>  gcc/tree-vect-data-refs.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/gcc/tree-vect-data-refs.c b/gcc/tree-vect-data-refs.c
> index e35a215e042..a78ae61d1b0 100644
> --- a/gcc/tree-vect-data-refs.c
> +++ b/gcc/tree-vect-data-refs.c
> @@ -1779,7 +1779,7 @@ vect_enhance_data_refs_alignment (loop_vec_info loop_vinfo)
>                  {
>                    vect_peeling_hash_insert (&peeling_htab, loop_vinfo,
>                                             dr_info, npeel_tmp);
> -                 npeel_tmp += target_align / dr_size;
> +                 npeel_tmp += MAX (1, target_align / dr_size);
>                  }
>
>               one_misalignment_known = true;
> --
> 2.25.3
>