Re: [PATCH V4, rs6000] Disable generation of scalar modulo instructions

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

Hi!

On Fri, Jun 30, 2023 at 02:26:35PM -0500, Pat Haugen wrote:
> gcc/
>     * config/rs6000/rs6000.cc (rs6000_rtx_costs): Check if disabling
>     scalar modulo.

"Check whether the modulo instruction is disabled?"

>     * config/rs6000/rs6000.md (mod<mode>3, *mod<mode>3): Disable.
>     (define_expand umod<mode>3): New.
>     (define_insn umod<mode>3): Rename to *umod<mode>3 and disable.
>     (umodti3, modti3): Disable.

None of these patterns are disabled!  Instead, the new TARGET_* thing
is used.

> +/* Disable generation of scalar modulo instructions due to performance issues
> +   with certain input values.  This can be removed in the future when the
> +   issues have been resolved.  */
> +#define RS6000_DISABLE_SCALAR_MODULO 1

I think that is a bit optimistic -- in the future we will still want to
support older cores ;-)

> -  "TARGET_POWER10 && TARGET_POWERPC64"
> +  "TARGET_POWER10 && TARGET_POWERPC64 && !RS6000_DISABLE_SCALAR_MODULO"
>    "vmoduq %0,%1,%2"

Did we ever test if this insn in fact is slower as well?  I don't mean
either way, orthogonality is good, but just for my enlightenment.

> +++ b/gcc/testsuite/gcc.target/powerpc/clone1.c

> +/* { Fail due to RS6000_DISABLE_SCALAR_MODULO. */

Xfail, but heh.  No need to change that.

> +/* { dg-final { scan-assembler-times {\mdivd\M}  1 { xfail *-*-* } } } */

> +/* { Fail due to RS6000_DISABLE_SCALAR_MODULO. */
> +/* { dg-final { scan-assembler-times {\mmodsw\M} 1 { xfail *-*-* } } } */

Thanks for the \m \M, it is much harder to determine if the tests
actually work, without that :-)

With improved changelog: okay for trunk.  Okay for all backports as
well (after some soak time).

Thank you!


Segher