Re: [PATCH 1/3] Add IEEE 128-bit min/max support on PowerPC.

[Segher Boessenkool <segher@kernel.crashing.org>]

On Tue, Jun 08, 2021 at 08:21:25PM -0400, Michael Meissner wrote:
> GCC will not convert ternary operations into using min/max instructions
> provided in this patch unless the user uses -Ofast or similar switches due to
> issues with NaNs.

It will not do it because it is *incorrect* to do :-)

(The RTL operators smin/smax are undefined for any NaN inputs (unless
both are the same bit pattern), and for different sign zeroes; it isn't
only NaNs even).

> gcc/
> 2021-06-08  Michael Meissner  <meissner@linux.ibm.com>
> 
> 	* config/rs6000/rs6000.c (rs6000_emit_minmax): Add support for ISA
> 	3.1 IEEE 128-bit floating point xsmaxcqp and xsmincqp
> 	instructions.

Please don't randomly break lines.

> --- a/gcc/config/rs6000/rs6000.c
> +++ b/gcc/config/rs6000/rs6000.c
> @@ -16103,7 +16103,8 @@ rs6000_emit_minmax (rtx dest, enum rtx_code code, rtx op0, rtx op1)
>    /* VSX/altivec have direct min/max insns.  */
>    if ((code == SMAX || code == SMIN)
>        && (VECTOR_UNIT_ALTIVEC_OR_VSX_P (mode)
> -	  || (mode == SFmode && VECTOR_UNIT_VSX_P (DFmode))))
> +	  || (mode == SFmode && VECTOR_UNIT_VSX_P (DFmode))
> +	  || (TARGET_POWER10 && TARGET_FLOAT128_HW && FLOAT128_IEEE_P (mode))))

The actual insns only check TARGET_POWER10 (so no TARGET_FLOAT128_HW).
Which is right, this or that?

> +/* { dg-require-effective-target ppc_float128_hw } */
> +/* { dg-require-effective-target power10_ok } */
> +/* { dg-options "-mdejagnu-cpu=power10 -O2 -ffast-math" } */

In testcases we can assume that float128_hw is set whenever we have a
p10; we don't manually disable it to make live hard for ourselves ;-)

We could disallow disabling QP float separately from all other float.
We will still need to test if float is enabled at all so it won't help
all that much immediately, alas.

With that TARGET_POWER10 condition fixed: okay for trunk, and for 11
once it is tested for trunk on all systems.  Thanks!


Segher