Re: [PATCH v2] aarch64: disable LDP via tuning structure for -mcpu=ampere1/1a

[Philipp Tomsich <philipp.tomsich@vrull.eu>]

On Mon, 17 Apr 2023 at 17:07, Kyrylo Tkachov <Kyrylo.Tkachov@arm.com> wrote:
>
>
>
> > -----Original Message-----
> > From: Philipp Tomsich <philipp.tomsich@vrull.eu>
> > Sent: Monday, April 17, 2023 11:22 AM
> > To: Kyrylo Tkachov <Kyrylo.Tkachov@arm.com>
> > Cc: gcc-patches@gcc.gnu.org; Di Zhao <di.zhao@amperecomputing.com>
> > Subject: Re: [PATCH v2] aarch64: disable LDP via tuning structure for -
> > mcpu=ampere1/1a
> >
> > OK for backport?
> > This will be all the way down to GCC10, as I just realized that we
> > need to backport the entire ampere1/1a support to GCC10 (we stopped at
> > GCC11 for some unexplainable reason)...
>
> Ok, under the principle that we'd already backported the ampere1 support and this is a small and unintrusive change.
> But I suppose the change for the branches shouldn't include the TODO note as we would not be extending the LDP restriction support there.

Thanks for the reminder to drop the TODO.
I would have done a straight cherry-pick without this... and regretted it.

Philipp.

>
> Thanks,
> Kyrill
>
> >
> > Philipp.
> >
> >
> > On Mon, 17 Apr 2023 at 12:20, Philipp Tomsich <philipp.tomsich@vrull.eu>
> > wrote:
> > >
> > > Applied to master, thanks!
> > > Philipp.
> > >
> > > On Mon, 17 Apr 2023 at 11:56, Kyrylo Tkachov <Kyrylo.Tkachov@arm.com>
> > wrote:
> > >>
> > >>
> > >>
> > >> > -----Original Message-----
> > >> > From: Philipp Tomsich <philipp.tomsich@vrull.eu>
> > >> > Sent: Friday, April 14, 2023 7:06 PM
> > >> > To: gcc-patches@gcc.gnu.org
> > >> > Cc: Kyrylo Tkachov <Kyrylo.Tkachov@arm.com>; Philipp Tomsich
> > >> > <philipp.tomsich@vrull.eu>; Di Zhao <di.zhao@amperecomputing.com>
> > >> > Subject: [PATCH v2] aarch64: disable LDP via tuning structure for -
> > >> > mcpu=ampere1/1a
> > >> >
> > >> > AmpereOne (-mcpu=ampere1) breaks LDP instructions into two uops.
> > >> > Given the chance that this causes instructions to slip into the next
> > >> > decoding cycle and the additional overheads when handling
> > >> > cacheline-crossing LDP instructions, we disable the generation of LDP
> > >> > isntructions through the tuning structure from instruction combining
> > >> > (such as in peephole2).
> > >> >
> > >> > Given the code-density benefits in builtins and prologue/epilogue
> > >> > expansion, we allow LDPs there.
> > >> >
> > >> > This commit:
> > >> >  * adds a new tuning option AARCH64_EXTRA_TUNE_NO_LDP_COMBINE
> > >> >  * allows -moverride=tune=... to override this
> > >> >
> > >> > These changes are benchmark-driven, yielding the following changes
> > >> > (with a net-overall improvement):
> > >> >    503.bwaves_r.      -0.88%
> > >> >    507.cactuBSSN_r     0.35%
> > >> >    508.namd_r          3.09%
> > >> >    510.parest_r       -2.99%
> > >> >    511.povray_r        5.54%
> > >> >    519.lbm_r          15.83%
> > >> >    521.wrf_r           0.56%
> > >> >    526.blender_r       2.47%
> > >> >    527.cam4_r          0.70%
> > >> >    538.imagick_r       0.00%
> > >> >    544.nab_r          -0.33%
> > >> >    549.fotonik3d_r.   -0.42%
> > >> >    554.roms_r          0.00%
> > >> >    -------------------------
> > >> >    = total             1.79%
> > >> >
> > >> > Signed-off-by: Philipp Tomsich <philipp.tomsich@vrull.eu>
> > >> > Co-Authored-By: Di Zhao <di.zhao@amperecomputing.com>
> > >>
> > >> Ok.
> > >> Thanks,
> > >> Kyrill
> > >>
> > >> >
> > >> > gcc/ChangeLog:
> > >> >
> > >> >       * config/aarch64/aarch64-tuning-flags.def
> > >> > (AARCH64_EXTRA_TUNING_OPTION):
> > >> >       Add AARCH64_EXTRA_TUNE_NO_LDP_COMBINE.
> > >> >       * config/aarch64/aarch64.cc (aarch64_operands_ok_for_ldpstp):
> > >> >       Check for the above tuning option when processing loads.
> > >> >
> > >> > gcc/testsuite/ChangeLog:
> > >> >
> > >> >       * gcc.target/aarch64/ampere1-no_ldp_combine.c: New test.
> > >> >
> > >> > ---
> > >> >
> > >> > Changes in v2:
> > >> > - apply both to -mcpu=ampere1 and -mcpu=ampere1a
> > >> > - add TODO: tag, per discussions on the mailing list
> > >> > - add testcase
> > >> >
> > >> >  gcc/config/aarch64/aarch64-tuning-flags.def    |  3 +++
> > >> >  gcc/config/aarch64/aarch64.cc                  | 18 ++++++++++++++++--
> > >> >  .../aarch64/ampere1-no_ldp_combine.c           | 11 +++++++++++
> > >> >  3 files changed, 30 insertions(+), 2 deletions(-)
> > >> >  create mode 100644 gcc/testsuite/gcc.target/aarch64/ampere1-
> > >> > no_ldp_combine.c
> > >> >
> > >> > diff --git a/gcc/config/aarch64/aarch64-tuning-flags.def
> > >> > b/gcc/config/aarch64/aarch64-tuning-flags.def
> > >> > index 712895a5263..52112ba7c48 100644
> > >> > --- a/gcc/config/aarch64/aarch64-tuning-flags.def
> > >> > +++ b/gcc/config/aarch64/aarch64-tuning-flags.def
> > >> > @@ -44,6 +44,9 @@ AARCH64_EXTRA_TUNING_OPTION
> > >> > ("cheap_shift_extend", CHEAP_SHIFT_EXTEND)
> > >> >  /* Disallow load/store pair instructions on Q-registers.  */
> > >> >  AARCH64_EXTRA_TUNING_OPTION ("no_ldp_stp_qregs",
> > >> > NO_LDP_STP_QREGS)
> > >> >
> > >> > +/* Disallow load-pair instructions to be formed in combine/peephole.
> > */
> > >> > +AARCH64_EXTRA_TUNING_OPTION ("no_ldp_combine",
> > >> > NO_LDP_COMBINE)
> > >> > +
> > >> >  AARCH64_EXTRA_TUNING_OPTION ("rename_load_regs",
> > >> > RENAME_LOAD_REGS)
> > >> >
> > >> >  AARCH64_EXTRA_TUNING_OPTION ("cse_sve_vl_constants",
> > >> > CSE_SVE_VL_CONSTANTS)
> > >> > diff --git a/gcc/config/aarch64/aarch64.cc
> > b/gcc/config/aarch64/aarch64.cc
> > >> > index f4ef22ce02f..0f04ab9fba0 100644
> > >> > --- a/gcc/config/aarch64/aarch64.cc
> > >> > +++ b/gcc/config/aarch64/aarch64.cc
> > >> > @@ -1933,7 +1933,7 @@ static const struct tune_params
> > ampere1_tunings =
> > >> >    2, /* min_div_recip_mul_df.  */
> > >> >    0, /* max_case_values.  */
> > >> >    tune_params::AUTOPREFETCHER_WEAK,  /* autoprefetcher_model.  */
> > >> > -  (AARCH64_EXTRA_TUNE_NONE),         /* tune_flags.  */
> > >> > +  (AARCH64_EXTRA_TUNE_NO_LDP_COMBINE),       /* tune_flags.  */
> > >> >    &ampere1_prefetch_tune
> > >> >  };
> > >> >
> > >> > @@ -1971,7 +1971,7 @@ static const struct tune_params
> > ampere1a_tunings
> > >> > =
> > >> >    2, /* min_div_recip_mul_df.  */
> > >> >    0, /* max_case_values.  */
> > >> >    tune_params::AUTOPREFETCHER_WEAK,  /* autoprefetcher_model.  */
> > >> > -  (AARCH64_EXTRA_TUNE_NONE),         /* tune_flags.  */
> > >> > +  (AARCH64_EXTRA_TUNE_NO_LDP_COMBINE),       /* tune_flags.  */
> > >> >    &ampere1_prefetch_tune
> > >> >  };
> > >> >
> > >> > @@ -26053,6 +26053,20 @@ aarch64_operands_ok_for_ldpstp (rtx
> > >> > *operands, bool load,
> > >> >    enum reg_class rclass_1, rclass_2;
> > >> >    rtx mem_1, mem_2, reg_1, reg_2;
> > >> >
> > >> > +  /* Allow the tuning structure to disable LDP instruction formation
> > >> > +     from combining instructions (e.g., in peephole2).
> > >> > +     TODO: Implement fine-grained tuning control for LDP and STP:
> > >> > +        1. control policies for load and store separately;
> > >> > +        2. support the following policies:
> > >> > +           - default (use what is in the tuning structure)
> > >> > +           - always
> > >> > +           - never
> > >> > +           - aligned (only if the compiler can prove that the
> > >> > +             load will be aligned to 2 * element_size)  */
> > >> > +  if (load && (aarch64_tune_params.extra_tuning_flags
> > >> > +            & AARCH64_EXTRA_TUNE_NO_LDP_COMBINE))
> > >> > +    return false;
> > >> > +
> > >> >    if (load)
> > >> >      {
> > >> >        mem_1 = operands[1];
> > >> > diff --git a/gcc/testsuite/gcc.target/aarch64/ampere1-no_ldp_combine.c
> > >> > b/gcc/testsuite/gcc.target/aarch64/ampere1-no_ldp_combine.c
> > >> > new file mode 100644
> > >> > index 00000000000..bc871f4481d
> > >> > --- /dev/null
> > >> > +++ b/gcc/testsuite/gcc.target/aarch64/ampere1-no_ldp_combine.c
> > >> > @@ -0,0 +1,11 @@
> > >> > +/* { dg-options "-O3 -mtune=ampere1" } */
> > >> > +
> > >> > +long
> > >> > +foo (long a[])
> > >> > +{
> > >> > +  return a[0] + a[1];
> > >> > +}
> > >> > +
> > >> > +/* We should see two ldrs instead of one ldp. */
> > >> > +/* { dg-final { scan-assembler {\tldr\t} } } */
> > >> > +/* { dg-final { scan-assembler-not {\tldp\t} } } */
> > >> > --
> > >> > 2.34.1
> > >>