Re: [6/n] Use build_vector_type_for_mode in get_vectype_for_scalar_type_and_size

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

On Fri, Oct 25, 2019 at 2:32 PM Richard Sandiford
<richard.sandiford@arm.com> wrote:
>
> Except for one case, get_vectype_for_scalar_type_and_size calculates
> what the vector mode should be and then calls build_vector_type,
> which recomputes the mode from scratch.  This patch makes it use
> build_vector_type_for_mode instead.
>
> The exception mentioned above is when preferred_simd_mode returns
> an integer mode, which it does if no appropriate vector mode exists.
> The integer mode in question is usually word_mode, although epiphany
> can return a doubleword mode in some cases.
>
> There's no guarantee that this integer mode is appropriate, since for
> example the scalar type could be a float.  The traditional behaviour is
> therefore to use the integer mode to determine a size only, and leave
> mode_for_vector to pick the TYPE_MODE.  (Note that it can actually end
> up picking a vector mode if the target defines a disabled vector mode.
> We therefore still need to check TYPE_MODE after building the type.)

OK.

Thanks,
Richard.

>
> 2019-10-24  Richard Sandiford  <richard.sandiford@arm.com>
>
> gcc/
>         * tree-vect-stmts.c (get_vectype_for_scalar_type_and_size): If
>         targetm.vectorize.preferred_simd_mode returns an integer mode,
>         use mode_for_vector to decide what the vector type's mode
>         should actually be.  Use build_vector_type_for_mode instead
>         of build_vector_type.
>
> Index: gcc/tree-vect-stmts.c
> ===================================================================
> --- gcc/tree-vect-stmts.c       2019-10-25 13:26:59.309877555 +0100
> +++ gcc/tree-vect-stmts.c       2019-10-25 13:27:08.653811531 +0100
> @@ -11162,16 +11162,31 @@ get_vectype_for_scalar_type_and_size (tr
>    /* If no size was supplied use the mode the target prefers.   Otherwise
>       lookup a vector mode of the specified size.  */
>    if (known_eq (size, 0U))
> -    simd_mode = targetm.vectorize.preferred_simd_mode (inner_mode);
> +    {
> +      simd_mode = targetm.vectorize.preferred_simd_mode (inner_mode);
> +      if (SCALAR_INT_MODE_P (simd_mode))
> +       {
> +         /* Traditional behavior is not to take the integer mode
> +            literally, but simply to use it as a way of determining
> +            the vector size.  It is up to mode_for_vector to decide
> +            what the TYPE_MODE should be.
> +
> +            Note that nunits == 1 is allowed in order to support single
> +            element vector types.  */
> +         if (!multiple_p (GET_MODE_SIZE (simd_mode), nbytes, &nunits)
> +             || !mode_for_vector (inner_mode, nunits).exists (&simd_mode))
> +           return NULL_TREE;
> +       }
> +    }
>    else if (!multiple_p (size, nbytes, &nunits)
>            || !mode_for_vector (inner_mode, nunits).exists (&simd_mode))
>      return NULL_TREE;
> -  /* NOTE: nunits == 1 is allowed to support single element vector types.  */
> -  if (!multiple_p (GET_MODE_SIZE (simd_mode), nbytes, &nunits))
> -    return NULL_TREE;
>
> -  vectype = build_vector_type (scalar_type, nunits);
> +  vectype = build_vector_type_for_mode (scalar_type, simd_mode);
>
> +  /* In cases where the mode was chosen by mode_for_vector, check that
> +     the target actually supports the chosen mode, or that it at least
> +     allows the vector mode to be replaced by a like-sized integer.  */
>    if (!VECTOR_MODE_P (TYPE_MODE (vectype))
>        && !INTEGRAL_MODE_P (TYPE_MODE (vectype)))
>      return NULL_TREE;