x86: Add `Avoid_STOSB` tunable to allow NT memset without ERMS

The goal of this flag is to allow targets which don't prefer/have ERMS
to still access the non-temporal memset implementation.

There are 4 cases for tuning memset:
    1) `Avoid_STOSB && Avoid_Non_Temporal_Memset`
        - Memset with temporal stores
    2) `Avoid_STOSB && !Avoid_Non_Temporal_Memset`
        - Memset with temporal/non-temporal stores. Non-temporal path
          goes through `rep stosb` path. We accomplish this by setting
          `x86_rep_stosb_threshold` to
          `x86_memset_non_temporal_threshold`.
    3) `!Avoid_STOSB && Avoid_Non_Temporal_Memset`
        - Memset with temporal stores/`rep stosb`
    3) `!Avoid_STOSB && !Avoid_Non_Temporal_Memset`
        - Memset with temporal stores/`rep stosb`/non-temporal stores.
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/sysdeps/x86/cpu-features.c b/sysdeps/x86/cpu-features.c
index a4786d23c7..0fb50f9432 100644
--- a/sysdeps/x86/cpu-features.c
+++ b/sysdeps/x86/cpu-features.c
@@ -1119,6 +1119,10 @@ https://www.intel.com/content/www/us/en/support/articles/000059422/processors.ht
   if (CPU_FEATURES_CPU_P (cpu_features, CMOV))
     cpu_features->preferred[index_arch_I686] |= bit_arch_I686;
 
+  /* No ERMS, we want to avoid stosb for memset.  */
+  if (!CPU_FEATURE_USABLE_P (cpu_features, ERMS))
+    cpu_features->preferred[index_arch_Avoid_STOSB] |= bit_arch_Avoid_STOSB;
+
 #if !HAS_CPUID
 no_cpuid:
 #endif