x86: Adding an upper bound for Enhanced REP MOVSB.

In the process of optimizing memcpy for AMD machines, we have found the
vector move operations are outperforming enhanced REP MOVSB for data
transfers above the L2 cache size on Zen3 architectures.
To handle this use case, we are adding an upper bound parameter on
enhanced REP MOVSB:'__x86_rep_movsb_stop_threshold'.
As per large-bench results, we are configuring this parameter to the
L2 cache size for AMD machines and applicable from Zen3 architecture
supporting the ERMS feature.
For architectures other than AMD, it is the computed value of
non-temporal threshold parameter.

Reviewed-by: Premachandra Mallappa <premachandra.mallappa@amd.com>
(cherry picked from commit 6e02b3e9327b7dbb063958d2b124b64fcb4bbe3f)

diff --git a/sysdeps/x86/cacheinfo.h b/sysdeps/x86/cacheinfo.h
index 0f850bdf12087e151c4232dbc0790fe062c262c7..ee057de50d3890a4e03b4bd1f1e9c210b3dc5e21 100644 (file)
--- a/sysdeps/x86/cacheinfo.h
+++ b/sysdeps/x86/cacheinfo.h
@@ -54,6 +54,9 @@ long int __x86_rep_movsb_threshold attribute_hidden = 2048;
 /* Threshold to use Enhanced REP STOSB.  */
 long int __x86_rep_stosb_threshold attribute_hidden = 2048;
 
+/* Threshold to stop using Enhanced REP MOVSB.  */
+long int __x86_rep_movsb_stop_threshold attribute_hidden;
+
 static void
 init_cacheinfo (void)
 {
@@ -85,5 +88,6 @@ init_cacheinfo (void)
 
   __x86_rep_movsb_threshold = cpu_features->rep_movsb_threshold;
   __x86_rep_stosb_threshold = cpu_features->rep_stosb_threshold;
+  __x86_rep_movsb_stop_threshold =  cpu_features->rep_movsb_stop_threshold;
 }
 #endif

diff --git a/sysdeps/x86/dl-cacheinfo.h b/sysdeps/x86/dl-cacheinfo.h
index 2ab3acd83e1cfd97251bb13415303c693299f9f1..d9944250fc87732333545eb8d333c1352f0e37b7 100644 (file)
--- a/sysdeps/x86/dl-cacheinfo.h
+++ b/sysdeps/x86/dl-cacheinfo.h
@@ -704,7 +704,7 @@ dl_init_cacheinfo (struct cpu_features *cpu_features)
   int max_cpuid_ex;
   long int data = -1;
   long int shared = -1;
-  long int core;
+  long int core = -1;
   unsigned int threads = 0;
   unsigned long int level1_icache_size = -1;
   unsigned long int level1_icache_linesize = -1;
@@ -892,6 +892,18 @@ dl_init_cacheinfo (struct cpu_features *cpu_features)
 #endif
     }
 
+  unsigned long int rep_movsb_stop_threshold;
+  /* ERMS feature is implemented from AMD Zen3 architecture and it is
+     performing poorly for data above L2 cache size. Henceforth, adding
+     an upper bound threshold parameter to limit the usage of Enhanced
+     REP MOVSB operations and setting its value to L2 cache size.  */
+  if (cpu_features->basic.kind == arch_kind_amd)
+    rep_movsb_stop_threshold = core;
+  /* Setting the upper bound of ERMS to the computed value of
+     non-temporal threshold for architectures other than AMD.  */
+  else
+    rep_movsb_stop_threshold = non_temporal_threshold;
+
   /* The default threshold to use Enhanced REP STOSB.  */
   unsigned long int rep_stosb_threshold = 2048;
 
@@ -938,4 +950,5 @@ dl_init_cacheinfo (struct cpu_features *cpu_features)
   cpu_features->non_temporal_threshold = non_temporal_threshold;
   cpu_features->rep_movsb_threshold = rep_movsb_threshold;
   cpu_features->rep_stosb_threshold = rep_stosb_threshold;
+  cpu_features->rep_movsb_stop_threshold = rep_movsb_stop_threshold;
 }

diff --git a/sysdeps/x86/include/cpu-features.h b/sysdeps/x86/include/cpu-features.h
index bfe4fe231eabca3f164e3562869ae8b67bc09d12..74408fde5c7ab56eaaa919ab7b10cfb8f2128c72 100644 (file)
--- a/sysdeps/x86/include/cpu-features.h
+++ b/sysdeps/x86/include/cpu-features.h
@@ -855,6 +855,8 @@ struct cpu_features
   unsigned long int non_temporal_threshold;
   /* Threshold to use "rep movsb".  */
   unsigned long int rep_movsb_threshold;
+  /* Threshold to stop using "rep movsb".  */
+  unsigned long int rep_movsb_stop_threshold;
   /* Threshold to use "rep stosb".  */
   unsigned long int rep_stosb_threshold;
   /* _SC_LEVEL1_ICACHE_SIZE.  */

diff --git a/sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S b/sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S
index 03a2e4dfb0910e497d1f857685f1024350a8cd9b..897a3d976275b7bf3cd7b76d3ffaef2bd43ece59 100644 (file)
--- a/sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S
+++ b/sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S
@@ -30,7 +30,10 @@
       load and aligned store.  Load the last 4 * VEC and first VEC
       before the loop and store them after the loop to support
       overlapping addresses.
-   6. If size >= __x86_shared_non_temporal_threshold and there is no
+   6. On machines with ERMS feature, if size greater than equal or to
+      __x86_rep_movsb_threshold and less than
+      __x86_rep_movsb_stop_threshold, then REP MOVSB will be used.
+   7. If size >= __x86_shared_non_temporal_threshold and there is no
       overlap between destination and source, use non-temporal store
       instead of aligned store.  */
 
@@ -252,7 +255,7 @@ L(return):
 #endif
 
 L(movsb):
-       cmp     __x86_shared_non_temporal_threshold(%rip), %RDX_LP
+       cmp     __x86_rep_movsb_stop_threshold(%rip), %RDX_LP
        jae     L(more_8x_vec)
        cmpq    %rsi, %rdi
        jb      1f