Re: [PATCH, rs6000] Tweak modulo define_insns to eliminate register copy

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

Hi!

On Mon, Feb 27, 2023 at 09:11:37AM -0600, Pat Haugen wrote:
> The define_insns for the modulo operation currently force the target 
> register
> to a distinct reg in preparation for a possible future peephole combining
> div/mod. But this can lead to cases of a needless copy being inserted. Fixed
> with the following patch.

Have you verified those peepholes still match?

Do those peepholes actually improve performance?  On new CPUs?  The code
here says
;; On machines with modulo support, do a combined div/mod the old fashioned
;; method, since the multiply/subtract is faster than doing the mod instruction
;; after a divide.
but that really should not be true: we can do the div and mod in
parallel (except in SMT4 perhaps, which we never schedule for anyway),
so that should always be strictly faster.

> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/powerpc/mod-no_copy.c
> @@ -0,0 +1,17 @@
> +/* { dg-do compile { target { powerpc*-*-*  } } } */

All files in gcc.target/powerpc/ test for this already.  Just leave off
the target clause here?

> +/* { dg-require-effective-target powerpc_p9modulo_ok } */

Leave out this line, because ...

> +/* { dg-options "-mdejagnu-cpu=power9 -O2" } */

... the -mcpu= forces it to true always.

> +/* Verify r3 is used as source and target, no copy inserted. */

> +/* { dg-final { scan-assembler-not {\mmr\M} } } */

That is probably good enough, yeah, since the test results in only a
handful of insns.


Segher