Re: [PATCH] PR tree-optimization/101895: Fold VEC_PERM to help recognize FMA.

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

On Sun, Mar 13, 2022 at 12:39 AM Marc Glisse via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> On Fri, 11 Mar 2022, Roger Sayle wrote:
>
> +(match vec_same_elem_p
> +  CONSTRUCTOR@0
> +  (if (uniform_vector_p (TREE_CODE (@0) == SSA_NAME
> +                        ? gimple_assign_rhs1 (SSA_NAME_DEF_STMT (@0)) : @0))))
>
> Ah, I didn't remember we needed that, we don't seem to be very consistent
> about it. Probably for this reason, the transformation "Prefer vector1 <<
> scalar to vector1 << vector2" does not match
>
> typedef int vec __attribute__((vector_size(16)));
> vec f(vec a, int b){
>    vec bb = { b, b, b, b };
>    return a << bb;
> }
>
> which is only optimized at vector lowering time.

Few more comments - since match.pd is matching in match.pd order the

(match vec_same_elem_p
  @0
  (...))

should come last.  Please use

+(match vec_same_elem_p
+  CONSTRUCTOR@0
    (if (TREE_CODE (@0) == SSA_NAME
         && uniform_vector_p (...

since otherwise we'll try uniform_vector_p twice on all CTORs (that
are not uniform).

> +/* Push VEC_PERM earlier if that may help FMA perception (PR101895).  */
> +(for plusminus (plus minus)
> +  (simplify
> +    (plusminus (vec_perm (mult@0 @1 vec_same_elem_p@2) @0 @3) @4)
> +    (plusminus (mult (vec_perm @1 @1 @3) @2) @4)))
>
> Don't you want :s on mult and vec_perm?

Yes.  Also for plus you want :c on it , likewise you want :c on the
mult.  The :c
on the plus will require splitting the plus and minus case :/

Otherwise looks reasonable.

Richard.

>
> --
> Marc Glisse