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

[Noah Goldstein <goldstein.w.n@gmail.com>]

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.

>
>    TUNABLE_SET_WITH_BOUNDS (x86_data_cache_size, data, 0, SIZE_MAX);
>    TUNABLE_SET_WITH_BOUNDS (x86_shared_cache_size, shared, 0, SIZE_MAX);
> --
> 2.34.1
>