x86-64: Require BMI2 for AVX2 strcmp implementation

The AVX2 strcmp implementation uses the 'bzhi' instruction, which
belongs to the BMI2 CPU feature.

NB: It also uses the 'tzcnt' BMI1 instruction, but it is executed as BSF
as BSF if the CPU doesn't support TZCNT, and produces the same result
for non-zero input.

Partially fixes: b77b06e0e296 ("x86: Optimize strcmp-avx2.S")
Partially resolves: BZ #29611

Reviewed-by: Noah Goldstein  <goldstein.w.n@gmail.com>
(cherry picked from commit 4d64c6445735e9b34e2ac8e369312cbfc2f88e17)

diff --git a/sysdeps/x86_64/multiarch/ifunc-impl-list.c b/sysdeps/x86_64/multiarch/ifunc-impl-list.c
index ca64b34c146a76f919cda01000167d8a0230a2c2..70931f15985334afbb728d2b6e665855ff59a254 100644 (file)
--- a/sysdeps/x86_64/multiarch/ifunc-impl-list.c
+++ b/sysdeps/x86_64/multiarch/ifunc-impl-list.c
@@ -503,10 +503,12 @@ __libc_ifunc_impl_list (const char *name, struct libc_ifunc_impl *array,
   /* Support sysdeps/x86_64/multiarch/strcmp.c.  */
   IFUNC_IMPL (i, name, strcmp,
              IFUNC_IMPL_ADD (array, i, strcmp,
-                             CPU_FEATURE_USABLE (AVX2),
+                             (CPU_FEATURE_USABLE (AVX2)
+                              && CPU_FEATURE_USABLE (BMI2)),
                              __strcmp_avx2)
              IFUNC_IMPL_ADD (array, i, strcmp,
                              (CPU_FEATURE_USABLE (AVX2)
+                              && CPU_FEATURE_USABLE (BMI2)
                               && CPU_FEATURE_USABLE (RTM)),
                              __strcmp_avx2_rtm)
              IFUNC_IMPL_ADD (array, i, strcmp,

diff --git a/sysdeps/x86_64/multiarch/strcmp.c b/sysdeps/x86_64/multiarch/strcmp.c
index b457fb4c150e44072ddecf5b181ef33f58e660f5..0c0cd20a03278a2b107bdeb89b59659f687e8845 100644 (file)
--- a/sysdeps/x86_64/multiarch/strcmp.c
+++ b/sysdeps/x86_64/multiarch/strcmp.c
@@ -40,11 +40,11 @@ IFUNC_SELECTOR (void)
   const struct cpu_features* cpu_features = __get_cpu_features ();
 
   if (CPU_FEATURE_USABLE_P (cpu_features, AVX2)
+      && CPU_FEATURE_USABLE_P (cpu_features, BMI2)
       && CPU_FEATURES_ARCH_P (cpu_features, AVX_Fast_Unaligned_Load))
     {
       if (CPU_FEATURE_USABLE_P (cpu_features, AVX512VL)
-         && CPU_FEATURE_USABLE_P (cpu_features, AVX512BW)
-         && CPU_FEATURE_USABLE_P (cpu_features, BMI2))
+         && CPU_FEATURE_USABLE_P (cpu_features, AVX512BW))
        return OPTIMIZE (evex);
 
       if (CPU_FEATURE_USABLE_P (cpu_features, RTM))