Re: [PATCH] vect: Handle demoting FLOAT and promoting FIX_TRUNC.

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

On Thu, Jul 13, 2023 at 12:31 PM Robin Dapp via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> Hi,
>
> the recent changes that allowed multi-step conversions for
> "non-packing/unpacking", i.e. modifier == NONE targets included
> promoting to-float and demoting to-int variants.  This patch
> adds demoting to-float and promoting to-int handling.

Can you add testcases?  Also the current restriction is because
the variants you add are not always correct and I don't see any
checks that the intermediate type doesn't lose significant bits?

Richard.

> Bootstrapped and regtested on x86 and aarch64.
>
> A question that seems related: Why do we require !flag_trapping_math
> for the "NONE" multistep conversion but not for the "NARROW_DST"
> case when both seem to handle float -> int and there are float
> values that do not have an int representation?  If a backend
> can guarantee that the conversion traps, should it just implement
> a multistep conversion in a matching expander?
>
> Regards
>  Robin
>
>
> gcc/ChangeLog:
>
>         * tree-vect-stmts.cc (vectorizable_conversion): Handle
>         more demotion/promotion for modifier == NONE.
> ---
>  gcc/tree-vect-stmts.cc | 40 +++++++++++++++++++++++++++++-----------
>  1 file changed, 29 insertions(+), 11 deletions(-)
>
> diff --git a/gcc/tree-vect-stmts.cc b/gcc/tree-vect-stmts.cc
> index 10e71178ce7..78e0510be7e 100644
> --- a/gcc/tree-vect-stmts.cc
> +++ b/gcc/tree-vect-stmts.cc
> @@ -5324,28 +5324,46 @@ vectorizable_conversion (vec_info *vinfo,
>         break;
>        }
>
> -      /* For conversions between float and smaller integer types try whether we
> -        can use intermediate signed integer types to support the
> +      /* For conversions between float and larger integer types try whether
> +        we can use intermediate signed integer types to support the
>          conversion.  */
>        if ((code == FLOAT_EXPR
> -          && GET_MODE_SIZE (lhs_mode) > GET_MODE_SIZE (rhs_mode))
> +          && GET_MODE_SIZE (lhs_mode) != GET_MODE_SIZE (rhs_mode))
>           || (code == FIX_TRUNC_EXPR
> -             && GET_MODE_SIZE (rhs_mode) > GET_MODE_SIZE (lhs_mode)
> -             && !flag_trapping_math))
> +             && ((GET_MODE_SIZE (rhs_mode) > GET_MODE_SIZE (lhs_mode)
> +                 && !flag_trapping_math)
> +                 || GET_MODE_SIZE (rhs_mode) < GET_MODE_SIZE (lhs_mode))))
>         {
> +         bool demotion = GET_MODE_SIZE (rhs_mode) > GET_MODE_SIZE (lhs_mode);
>           bool float_expr_p = code == FLOAT_EXPR;
> -         scalar_mode imode = float_expr_p ? rhs_mode : lhs_mode;
> -         fltsz = GET_MODE_SIZE (float_expr_p ? lhs_mode : rhs_mode);
> +         unsigned short target_size;
> +         scalar_mode intermediate_mode;
> +         if (demotion)
> +           {
> +             intermediate_mode = lhs_mode;
> +             target_size = GET_MODE_SIZE (rhs_mode);
> +           }
> +         else
> +           {
> +             target_size = GET_MODE_SIZE (lhs_mode);
> +             tree itype
> +               = build_nonstandard_integer_type (GET_MODE_BITSIZE
> +                                                 (rhs_mode), 0);
> +             intermediate_mode = SCALAR_TYPE_MODE (itype);
> +           }
>           code1 = float_expr_p ? code : NOP_EXPR;
>           codecvt1 = float_expr_p ? NOP_EXPR : code;
> -         FOR_EACH_2XWIDER_MODE (rhs_mode_iter, imode)
> +         opt_scalar_mode mode_iter;
> +         FOR_EACH_2XWIDER_MODE (mode_iter, intermediate_mode)
>             {
> -             imode = rhs_mode_iter.require ();
> -             if (GET_MODE_SIZE (imode) > fltsz)
> +             intermediate_mode = mode_iter.require ();
> +
> +             if (GET_MODE_SIZE (intermediate_mode) > target_size)
>                 break;
>
>               cvt_type
> -               = build_nonstandard_integer_type (GET_MODE_BITSIZE (imode),
> +               = build_nonstandard_integer_type (GET_MODE_BITSIZE
> +                                                 (intermediate_mode),
>                                                   0);
>               cvt_type = get_vectype_for_scalar_type (vinfo, cvt_type,
>                                                       slp_node);
> --
> 2.41.0
>