Re: [PATCH v2 2/3] x86: Do not prefer ERMS for memset on Zen3+

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

On 06/02/24 16:01, Noah Goldstein wrote:
> On Tue, Feb 6, 2024 at 5:43 PM Adhemerval Zanella
> <adhemerval.zanella@linaro.org> wrote:
>>
>> For AMD Zen3+ architecture, the performance of the vectorized loop is
>> slightly better than ERMS.
>>
>> Checked on x86_64-linux-gnu on Zen3.
>> ---
>>  sysdeps/x86/dl-cacheinfo.h | 16 +++++++++++-----
>>  1 file changed, 11 insertions(+), 5 deletions(-)
>>
>> diff --git a/sysdeps/x86/dl-cacheinfo.h b/sysdeps/x86/dl-cacheinfo.h
>> index 74b804c5e6..f2cd6f179d 100644
>> --- a/sysdeps/x86/dl-cacheinfo.h
>> +++ b/sysdeps/x86/dl-cacheinfo.h
>> @@ -1010,11 +1010,17 @@ dl_init_cacheinfo (struct cpu_features *cpu_features)
>>    if (tunable_size > minimum_rep_movsb_threshold)
>>      rep_movsb_threshold = tunable_size;
>>
>> -  /* NB: The default value of the x86_rep_stosb_threshold tunable is the
>> -     same as the default value of __x86_rep_stosb_threshold and the
>> -     minimum value is fixed.  */
>> -  rep_stosb_threshold = TUNABLE_GET (x86_rep_stosb_threshold,
>> -                                    long int, NULL);
>> +  /* For AMD Zen3+ architecture, the performance of the vectorized loop is
>> +     slightly better than ERMS.  */
>> +  if (cpu_features->basic.kind == arch_kind_amd)
>> +    rep_stosb_threshold = SIZE_MAX;
>> +
>> +  if (TUNABLE_IS_INITIALIZED (x86_rep_stosb_threshold))
>> +    /* NB: The default value of the x86_rep_stosb_threshold tunable is the
>> +       same as the default value of __x86_rep_stosb_threshold and the
>> +       minimum value is fixed.  */
>> +    rep_stosb_threshold = TUNABLE_GET (x86_rep_stosb_threshold,
>> +                                      long int, NULL);
> Can we keep the unconditional
>   rep_stosb_threshold = TUNABLE_GET (x86_rep_stosb_threshold,
>                                     long int, NULL);
> for targets other than AMD?
> Its easier to control the defaults that way.

Alright, I will change to

  /* NB: The default value of the x86_rep_stosb_threshold tunable is the
     same as the default value of __x86_rep_stosb_threshold and the
     minimum value is fixed.  */
  rep_stosb_threshold = TUNABLE_GET (x86_rep_stosb_threshold,
                                     long int, NULL);
  if (cpu_features->basic.kind == arch_kind_amd
      && !TUNABLE_IS_INITIALIZED (x86_rep_stosb_threshold))
    /* For AMD Zen3+ architecture, the performance of the vectorized loop is
       slightly better than ERMS.  */
    rep_stosb_threshold = SIZE_MAX;