[PATCH 07/25] [pr82089] Don't sign-extend SFV 1 in BImode

[Richard Sandiford]

<ams@codesourcery.com> writes:
> This is an update of the patch posted to PR82089 long ago.  We ran into the
> same bug on GCN, so we need this fixed as part of this series.
>
> 2018-09-05  Andrew Stubbs  <ams@codesourcery.com>
>             Tom de Vries  <tom@codesourcery.com>
>
> 	PR82089
>
> 	gcc/
> 	* expmed.c (emit_cstore): Fix handling of result_mode == BImode and
> 	STORE_FLAG_VALUE == 1.
> ---
>  gcc/expmed.c | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
>
> diff --git a/gcc/expmed.c b/gcc/expmed.c
> index 29ce10b..0b87fdc 100644
> --- a/gcc/expmed.c
> +++ b/gcc/expmed.c
> @@ -5464,11 +5464,18 @@ emit_cstore (rtx target, enum insn_code icode, enum rtx_code code,
>       If STORE_FLAG_VALUE does not have the sign bit set when
>       interpreted in MODE, we can do this conversion as unsigned, which
>       is usually more efficient.  */
> -  if (GET_MODE_SIZE (int_target_mode) > GET_MODE_SIZE (result_mode))
> +  if (GET_MODE_SIZE (int_target_mode) > GET_MODE_SIZE (result_mode)
> +      || (result_mode == BImode && int_target_mode != BImode))

Would be better to test GET_MODE_PRECISION instead of GET_MODE_SIZE,
if that works, instead of treating BImode as a special case.

>      {
> -      convert_move (target, subtarget,
> -		    val_signbit_known_clear_p (result_mode,
> -					       STORE_FLAG_VALUE));
> +      gcc_assert (GET_MODE_SIZE (result_mode) != 1
> +		  || STORE_FLAG_VALUE == 1 || STORE_FLAG_VALUE == -1);
> +      bool unsignedp
> +	= (GET_MODE_SIZE (result_mode) == 1
> +	   ? STORE_FLAG_VALUE == 1
> +	   : val_signbit_known_clear_p (result_mode, STORE_FLAG_VALUE));
> +
> +      convert_move (target, subtarget, unsignedp);
> +

GET_MODE_SIZE == 1 would also trigger for QImode, which shouldn't be treated
differently from HImode etc.

The original val_signbit_known_clear_p test seems like it might be an
abstraction too far.  In practice STORE_FLAG_VALUE has to fit within
the mode of a natural (unextended) condition result, so I think we can
simply test STORE_FLAG_VALUE >= 0 for all modes to see whether the target
wants the result to be treated as signed or unsigned.

Thanks,
Richard