On Fri, 14 Apr 2023 at 11:31, Philipp Tomsich wrote: > Kyrylo, > > On Fri, 14 Apr 2023 at 11:21, Kyrylo Tkachov > wrote: > > > > Hi Philipp, > > > > > -----Original Message----- > > > From: Philipp Tomsich > > > Sent: Friday, April 14, 2023 12:22 AM > > > To: gcc-patches@gcc.gnu.org > > > Cc: Kyrylo Tkachov ; Philipp Tomsich > > > ; Di Zhao > > > Subject: [PATCH] aarch64: disable LDP via tuning structure for - > > > mcpu=ampere1 > > > > > > 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. > > > > LDPs are indeed quite an important part of the ISA for code density and > there are, in principle, second-order benefits from using them, like > keeping the instruction cache footprint low (which can be important for > large workloads). > > Did you gather some benchmarks showing a benefit of disabling them in > this manner? > > This has been benchmark-driven, but I need to follow up separately (as > I the final numbers are with the folks that have access to the > benchmark machines).. > Here are the numbers for the submitted change for AmpereOne: 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% > > > > > This commit: > > > * adds a new tuning option AARCH64_EXTRA_TUNE_NO_LDP_COMBINE > > > * allows -moverride=tune=... to override this > > > > > > Signed-off-by: Philipp Tomsich > > > Co-Authored-By: Di Zhao > > > > > > 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/config/aarch64/aarch64-tuning-flags.def | 3 +++ > > > gcc/config/aarch64/aarch64.cc | 8 +++++++- > > > 2 files changed, 10 insertions(+), 1 deletion(-) > > > > > > 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..8dc1a9ceb17 100644 > > > --- a/gcc/config/aarch64/aarch64.cc > > > +++ b/gcc/config/aarch64/aarch64.cc > > > @@ -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. */ > > > &ere1_prefetch_tune > > > }; > > > > > > @@ -26053,6 +26053,12 @@ 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). */ > > > + if (load && (aarch64_tune_params.extra_tuning_flags > > > + & AARCH64_EXTRA_TUNE_NO_LDP_COMBINE)) > > > + return false; > > > > If we do decide to do this, I think this is not a complete approach. See > the similar tuning flag AARCH64_EXTRA_TUNE_NO_LDP_STP_QREGS. > > There's various other places in the backend that would need to be > adjusted to avoid bringing loads together for the peephole2s to merge (the > sched_fusion stuff). > > Plus there's the cpymem expansions that would generate load pairs too... > > I have add-on patches for these, but given that I don't have direct > access to the benchmarking machine and the benchmarks have been run > with this functionality only, I didn't submit them for the time being. > Do you see a path to get this in during the current cycle and defer > the add-on patches (happy to resubmit as a series) only? > > > We'd want some testcases added to check that LDPs are blocked too... > > > > Thanks, > > Kyrill > > > > > + > > > if (load) > > > { > > > mem_1 = operands[1]; > > > -- > > > 2.34.1 > > >