x86: Add thresholds for "rep movsb/stosb" to tunables

Add x86_rep_movsb_threshold and x86_rep_stosb_threshold to tunables
to update thresholds for "rep movsb" and "rep stosb" at run-time.

Note that the user specified threshold for "rep movsb" smaller than
the minimum threshold will be ignored.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

1 files changed, 24 insertions, 0 deletions

diff --git a/sysdeps/x86/dl-tunables.list b/sysdeps/x86/dl-tunables.list
index 251b926..1a4a93a 100644
--- a/sysdeps/x86/dl-tunables.list
+++ b/sysdeps/x86/dl-tunables.list
@@ -30,6 +30,30 @@ glibc {
     x86_non_temporal_threshold {
       type: SIZE_T
     }
+    x86_rep_movsb_threshold {
+      type: SIZE_T
+      # Since there is overhead to set up REP MOVSB operation, REP MOVSB
+      # isn't faster on short data.  The memcpy micro benchmark in glibc
+      # shows that 2KB is the approximate value above which REP MOVSB
+      # becomes faster than SSE2 optimization on processors with Enhanced
+      # REP MOVSB.  Since larger register size can move more data with a
+      # single load and store, the threshold is higher with larger register
+      # size.  Note: Since the REP MOVSB threshold must be greater than 8
+      # times of vector size, the minium value must be updated at run-time.
+      minval: 1
+      default: 2048
+    }
+    x86_rep_stosb_threshold {
+      type: SIZE_T
+      # Since there is overhead to set up REP STOSB operation, REP STOSB
+      # isn't faster on short data.  The memset micro benchmark in glibc
+      # shows that 2KB is the approximate value above which REP STOSB
+      # becomes faster on processors with Enhanced REP STOSB.  Since the
+      # stored value is fixed, larger register size has minimal impact
+      # on threshold.
+      minval: 1
+      default: 2048
+    }
     x86_data_cache_size {
       type: SIZE_T
     }