x86: Add comment explaining no Slow_SSE4_2 check in ifunc-sse4_2

Just for clarities sake and so that if a future implementation is
added we remember to add the check.

diff --git a/sysdeps/x86_64/multiarch/ifunc-sse4_2.h b/sysdeps/x86_64/multiarch/ifunc-sse4_2.h
index ee36525..f8b5693 100644 (file)
--- a/sysdeps/x86_64/multiarch/ifunc-sse4_2.h
+++ b/sysdeps/x86_64/multiarch/ifunc-sse4_2.h
@@ -27,6 +27,12 @@ IFUNC_SELECTOR (void)
 {
   const struct cpu_features* cpu_features = __get_cpu_features ();
 
+  /* This function uses the `pcmpstri` sse4.2 instruction which can be
+     slow on some CPUs.  This normally would be guarded by a
+     Slow_SSE4_2 check, but since there is no other optimized
+     implementation its best to keep it regardless.  If an optimized
+     fallback is added add a X86_ISA_CPU_FEATURE_ARCH_P (cpu_features,
+     Slow_SSE4_2) check.  */
   if (CPU_FEATURE_USABLE_P (cpu_features, SSE4_2))
     return OPTIMIZE (sse42);