Prefer open-coding vector integer division

[Richard Biener]

On Thu, May 24, 2018 at 2:52 PM Richard Sandiford <
richard.sandiford@linaro.org> wrote:

> Richard Biener <richard.guenther@gmail.com> writes:
> > On Thu, May 24, 2018 at 10:31 AM Jakub Jelinek <jakub@redhat.com> wrote:
> >> On Thu, May 24, 2018 at 09:21:35AM +0100, Richard Sandiford wrote:
> >> > vect_recog_divmod_pattern currently bails out if the target has
> >> > native support for integer division, but I think in practice
> >> > it's always going to be better to open-code it anyway, just as
> >> > we usually open-code scalar divisions by constants.
> >> >
> >> > I think the only currently affected target is MIPS MSA, where for:
> >
> >> Isn't powerpcspe affected too?  It has a divv2si3 pattern.

> Ah, yeah.  I forgot to update the description after you mentioned
> that on IRC, sorry.

> >> > --- gcc/tree-vect-patterns.c  2018-05-16 12:48:59.115202362 +0100
> >> > +++ gcc/tree-vect-patterns.c  2018-05-24 09:18:10.445466941 +0100
> >> > @@ -2639,7 +2639,6 @@ vect_recog_divmod_pattern (vec<gimple *>
> >> >    enum tree_code rhs_code;
> >> >    stmt_vec_info stmt_vinfo = vinfo_for_stmt (last_stmt);
> >> >    vec_info *vinfo = stmt_vinfo->vinfo;
> >> > -  optab optab;
> >> >    tree q;
> >> >    int dummy_int, prec;
> >> >    stmt_vec_info def_stmt_vinfo;
> >> > @@ -2674,17 +2673,6 @@ vect_recog_divmod_pattern (vec<gimple *>
> >> >    if (vectype == NULL_TREE)
> >> >      return NULL;
> >> >
> >> > -  /* If the target can handle vectorized division or modulo
natively,
> >> > -     don't attempt to optimize this.  */
> >> > -  optab = optab_for_tree_code (rhs_code, vectype, optab_default);
> >> > -  if (optab != unknown_optab)
> >> > -    {
> >> > -      machine_mode vec_mode = TYPE_MODE (vectype);
> >> > -      int icode = (int) optab_handler (optab, vec_mode);
> >> > -      if (icode != CODE_FOR_nothing)
> >> > -     return NULL;
> >> > -    }
> >> > -
> >
> >> Shouldn't we instead keep it, but only do it if
> >> optimize_bb_for_size_p (gimple_bb (last_stmt)) ?
> >> I mean, a hw division is very likely shorter than what we replace it
> > with...
> >
> > Good idea.  While loops are checked for optimize_loop_nest_for_speed if
> > not forced vect BB vectorization has no such predicate.

> Like this, if it passes testing?

Looks good.

Thanks,
Richard.

> Thanks,
> Richard


> 2018-05-24  Richard Sandiford  <richard.sandiford@linaro.org>

> gcc/
>          * tree-vect-patterns.c: Include predict.h.
>          (vect_recog_divmod_pattern): Restrict check for division support
>          to when optimizing for size.

> gcc/testsuite/
>          * gcc.dg/vect/bb-slp-div-1.c: New XFAILed test.

> Index: gcc/tree-vect-patterns.c
> ===================================================================
> *** gcc/tree-vect-patterns.c    2018-05-24 13:50:21.656138942 +0100
> --- gcc/tree-vect-patterns.c    2018-05-24 13:50:21.819134113 +0100
> *************** vect_recog_divmod_pattern (vec<gimple *>
> *** 2674,2688 ****
>      if (vectype == NULL_TREE)
>        return NULL;

> !   /* If the target can handle vectorized division or modulo natively,
> !      don't attempt to optimize this.  */
> !   optab = optab_for_tree_code (rhs_code, vectype, optab_default);
> !   if (optab != unknown_optab)
>        {
> !       machine_mode vec_mode = TYPE_MODE (vectype);
> !       int icode = (int) optab_handler (optab, vec_mode);
> !       if (icode != CODE_FOR_nothing)
> !       return NULL;
>        }

>      prec = TYPE_PRECISION (itype);
> --- 2674,2692 ----
>      if (vectype == NULL_TREE)
>        return NULL;

> !   if (optimize_bb_for_size_p (gimple_bb (last_stmt)))
>        {
> !       /* If the target can handle vectorized division or modulo
natively,
> !        don't attempt to optimize this, since native division is likely
> !        to give smaller code.  */
> !       optab = optab_for_tree_code (rhs_code, vectype, optab_default);
> !       if (optab != unknown_optab)
> !       {
> !         machine_mode vec_mode = TYPE_MODE (vectype);
> !         int icode = (int) optab_handler (optab, vec_mode);
> !         if (icode != CODE_FOR_nothing)
> !           return NULL;
> !       }
>        }

>      prec = TYPE_PRECISION (itype);
> Index: gcc/testsuite/gcc.dg/vect/bb-slp-div-1.c
> ===================================================================
> *** /dev/null   2018-04-20 16:19:46.369131350 +0100
> --- gcc/testsuite/gcc.dg/vect/bb-slp-div-1.c    2018-05-24
13:50:21.818134143 +0100
> ***************
> *** 0 ****
> --- 1,19 ----
> + /* { dg-do compile } */
> + /* { dg-additional-options "-msve-vector-bits=256" { target aarch64_sve
} } */
> +
> + int x[8];
> +
> + void
> + f (void)
> + {
> +   x[0] /= 2;
> +   x[1] /= 3;
> +   x[2] /= 4;
> +   x[3] /= 5;
> +   x[4] /= 6;
> +   x[5] /= 7;
> +   x[6] /= 8;
> +   x[7] /= 9;
> + }
> +
> + /* { dg-final { scan-tree-dump "basic block vectorized" "slp2" { xfail
*-*-* } } } */