Re: [PATCH] Add __builtin_convertvector support (PR c++/85052)

[Richard Sandiford]

Jakub Jelinek <ja...@redhat.com> writes:
> +      /* Can't use get_compute_type here, as supportable_convert_operation
> +      doesn't necessarily use an optab and needs two arguments.  */
> +      tree vector_compute_type
> +     = type_for_widest_vector_mode (TREE_TYPE (arg_type), mov_optab);
> +      unsigned HOST_WIDE_INT nelts;
> +      if (vector_compute_type
> +       && VECTOR_MODE_P (TYPE_MODE (vector_compute_type))
> +       && subparts_gt (arg_type, vector_compute_type)
> +       && TYPE_VECTOR_SUBPARTS (vector_compute_type).is_constant (&nelts))
> +     {
> +       while (nelts > 1)
> +         {
> +           tree ret1_type = build_vector_type (TREE_TYPE (ret_type), nelts);
> +           tree arg1_type = build_vector_type (TREE_TYPE (arg_type), nelts);
> +           if (supportable_convert_operation (code, ret1_type, arg1_type,
> +                                              &decl, &code1))
> +             {
> +               new_rhs = expand_vector_piecewise (gsi, do_vec_conversion,
> +                                                  ret_type, arg1_type, arg,
> +                                                  decl, code1);
> +               g = gimple_build_assign (lhs, new_rhs);
> +               gsi_replace (gsi, g, false);
> +               return;
> +             }
> +           nelts = nelts / 2;
> +         }
> +     }

I think for this it would be better to use:

      if (vector_compute_type
          && VECTOR_MODE_P (TYPE_MODE (vector_compute_type))
          && subparts_gt (arg_type, vector_compute_type))
        {
          unsigned HOST_WIDE_INT nelts = constant_lower_bound
            (TYPE_VECTOR_SUBPARTS (vector_compute_type));

since the loop is self-checking.

E.g. this will make the Advanced SIMD handling on AArch64 the same
regardless of whether SVE is also enabled.

Thanks,
Richard