Hi Philipp, > -----Original Message----- > From: Philipp Tomsich <philipp.toms...@vrull.eu> > Sent: Friday, April 14, 2023 12:22 AM > To: gcc-patches@gcc.gnu.org > Cc: Kyrylo Tkachov <kyrylo.tkac...@arm.com>; Philipp Tomsich > <philipp.toms...@vrull.eu>; Di Zhao <di.z...@amperecomputing.com> > 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 commit: > * adds a new tuning option AARCH64_EXTRA_TUNE_NO_LDP_COMBINE > * allows -moverride=tune=... to override this > > Signed-off-by: Philipp Tomsich <philipp.toms...@vrull.eu> > Co-Authored-By: Di Zhao <di.z...@amperecomputing.com> > > 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... We'd want some testcases added to check that LDPs are blocked too... Thanks, Kyrill > + > if (load) > { > mem_1 = operands[1]; > -- > 2.34.1