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

[Richard Sandiford <richard.sandiford@arm.com>]

Jakub Jelinek <jakub@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