RE: [PATCH] aarch64: disable LDP via tuning structure for -mcpu=ampere1

[Kyrylo Tkachov <Kyrylo.Tkachov@arm.com>]

Hi Philipp,

> -----Original Message-----
> From: Philipp Tomsich <philipp.tomsich@vrull.eu>
> Sent: Friday, April 14, 2023 12:22 AM
> 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] 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.tomsich@vrull.eu>
> Co-Authored-By: Di Zhao <di.zhao@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.  */
>    &ampere1_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