[glibc] x86: Use 64MB as nt-store threshold if no cacheinfo [BZ #30429]

[glibc] x86: Use 64MB as nt-store threshold if no cacheinfo [BZ #30429]

[Noah Goldstein]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=ed2f9dc9420c4c61436328778a70459d0a35556a

commit ed2f9dc9420c4c61436328778a70459d0a35556a
Author: Noah Goldstein <goldstein.w.n@gmail.com>
Date:   Mon May 8 22:10:20 2023 -0500

    x86: Use 64MB as nt-store threshold if no cacheinfo [BZ #30429]
    
    If `non_temporal_threshold` is below `minimum_non_temporal_threshold`,
    it almost certainly means we failed to read the systems cache info.
    
    In this case, rather than defaulting the minimum correct value, we
    should default to a value that gets at least reasonable
    performance. 64MB is chosen conservatively to be at the very high
    end. This should never cause non-temporal stores when, if we had read
    cache info, we wouldn't have otherwise.
    Reviewed-by: Florian Weimer <fweimer@redhat.com>

Diff:
---
 sysdeps/x86/dl-cacheinfo.h | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/sysdeps/x86/dl-cacheinfo.h b/sysdeps/x86/dl-cacheinfo.h
index ec88945b39..877e73d700 100644
--- a/sysdeps/x86/dl-cacheinfo.h
+++ b/sysdeps/x86/dl-cacheinfo.h
@@ -749,8 +749,16 @@ dl_init_cacheinfo (struct cpu_features *cpu_features)
      reflected in the manual.  */
   unsigned long int maximum_non_temporal_threshold = SIZE_MAX >> 4;
   unsigned long int minimum_non_temporal_threshold = 0x4040;
+
+  /* If `non_temporal_threshold` less than `minimum_non_temporal_threshold`
+     it most likely means we failed to detect the cache info. We don't want
+     to default to `minimum_non_temporal_threshold` as such a small value,
+     while correct, has bad performance. We default to 64MB as reasonable
+     default bound. 64MB is likely conservative in that most/all systems would
+     choose a lower value so it should never forcing non-temporal stores when
+     they otherwise wouldn't be used.  */
   if (non_temporal_threshold < minimum_non_temporal_threshold)
-    non_temporal_threshold = minimum_non_temporal_threshold;
+    non_temporal_threshold = 64 * 1024 * 1024;
   else if (non_temporal_threshold > maximum_non_temporal_threshold)
     non_temporal_threshold = maximum_non_temporal_threshold;