[glibc] x86: Do not prefer ERMS for memset on Zen3+

[glibc] x86: Do not prefer ERMS for memset on Zen3+

[H.J. Lu]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=272708884cb750f12f5c74a00e6620c19dc6d567

commit 272708884cb750f12f5c74a00e6620c19dc6d567
Author: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Date:   Thu Feb 8 10:08:39 2024 -0300

    x86: Do not prefer ERMS for memset on Zen3+
    
    For AMD Zen3+ architecture, the performance of the vectorized loop is
    slightly better than ERMS.
    
    Checked on x86_64-linux-gnu on Zen3.
    Reviewed-by: H.J. Lu <hjl.tools@gmail.com>

Diff:
---
 sysdeps/x86/dl-cacheinfo.h | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/sysdeps/x86/dl-cacheinfo.h b/sysdeps/x86/dl-cacheinfo.h
index f34d12846c..5a98f70364 100644
--- a/sysdeps/x86/dl-cacheinfo.h
+++ b/sysdeps/x86/dl-cacheinfo.h
@@ -1021,6 +1021,11 @@ dl_init_cacheinfo (struct cpu_features *cpu_features)
      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;
 
   TUNABLE_SET_WITH_BOUNDS (x86_data_cache_size, data, 0, SIZE_MAX);
   TUNABLE_SET_WITH_BOUNDS (x86_shared_cache_size, shared, 0, SIZE_MAX);