Re: [PATCH] Optimize nested permutation to single VEC_PERM_EXPR [PR54346]

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

On Thu, Oct 13, 2022 at 10:16 AM Lulu Cheng <chenglulu@loongson.cn> wrote:
>
>
> 在 2022/10/13 下午2:44, Xi Ruoyao 写道:
> > On Thu, 2022-10-13 at 14:15 +0800, Levy wrote:
> >> Hi RuoYao
> >>
> >> It’s probably because loongarch64 doesn’t support
> >> can_vec_perm_const_p(result_mode, op_mode, sel2, false)
> >>
> >> I’m not sure whether if loongarch will support it or should I just
> >> limit the test target for pr54346.c?
> > I'm not sure if we can add TARGET_VECTORIZE_VEC_PERM_CONST when we don't
> > actually support vector.  (LoongArch has SIMD instructions but the
> > support in GCC won't be added in a very recent future.)
> >
> If what I understand is correct, I think this might be a better solution.
>
>   /* { dg-do compile } */
>
> +/* { dg-require-effective-target vect_perm } */
>   /* { dg-options "-O -fdump-tree-dse1" } */

Btw, what forwprop does is check whether any of the original permutations are
not supported and then elide the supportability check for the result.
The reasoning
is that the original permute(s) would be lowered during vectlower so we can as
well do that for the result.  We should just never turn a supported permutation
sequence into a not supported one.

Richard.

>