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From: Aurelien Jarno <>
Subject: [glibc/release/2.33/master] x86-64: Require BMI2 for AVX2 strncmp implementation
Date: Mon,  3 Oct 2022 22:05:01 +0000 (GMT)	[thread overview]
Message-ID: <> (raw);h=050d652900d5f88447af335b20b621344f16e192

commit 050d652900d5f88447af335b20b621344f16e192
Author: Aurelien Jarno <>
Date:   Mon Oct 3 23:16:46 2022 +0200

    x86-64: Require BMI2 for AVX2 strncmp implementation
    The AVX2 strncmp implementations 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  <>
    (cherry picked from commit fc7de1d9b99ae1676bc626ddca422d7abee0eb48)

 sysdeps/x86_64/multiarch/ifunc-impl-list.c | 7 +++++--
 sysdeps/x86_64/multiarch/strncmp.c         | 4 ++--
 2 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/sysdeps/x86_64/multiarch/ifunc-impl-list.c b/sysdeps/x86_64/multiarch/ifunc-impl-list.c
index 70931f1598..34d5f6efe5 100644
--- a/sysdeps/x86_64/multiarch/ifunc-impl-list.c
+++ b/sysdeps/x86_64/multiarch/ifunc-impl-list.c
@@ -1022,15 +1022,18 @@ __libc_ifunc_impl_list (const char *name, struct libc_ifunc_impl *array,
   /* Support sysdeps/x86_64/multiarch/strncmp.c.  */
   IFUNC_IMPL (i, name, strncmp,
 	      IFUNC_IMPL_ADD (array, i, strncmp,
+			       && CPU_FEATURE_USABLE (BMI2)),
 	      IFUNC_IMPL_ADD (array, i, strncmp,
+			       && CPU_FEATURE_USABLE (BMI2)
 			       && CPU_FEATURE_USABLE (RTM)),
 	      IFUNC_IMPL_ADD (array, i, strncmp,
-			       && CPU_FEATURE_USABLE (AVX512BW)),
+			       && CPU_FEATURE_USABLE (AVX512BW)
+			       && CPU_FEATURE_USABLE (BMI2)),
 	      IFUNC_IMPL_ADD (array, i, strncmp, CPU_FEATURE_USABLE (SSE4_2),
diff --git a/sysdeps/x86_64/multiarch/strncmp.c b/sysdeps/x86_64/multiarch/strncmp.c
index f94a421784..7632d7b2dd 100644
--- a/sysdeps/x86_64/multiarch/strncmp.c
+++ b/sysdeps/x86_64/multiarch/strncmp.c
@@ -39,11 +39,11 @@ 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))

                 reply	other threads:[~2022-10-03 22:05 UTC|newest]

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