x86_64: Prefer EVEX512 code-path on AMD Zen5 CPUs

  Introduced a synthetic architecture preference flag (Prefer_EVEX512)
and enabled it for AMD Zen5 (CPUID Family 0x1A) when AVX-512F, AVX-512VL,
and AVX-512BW are supported.

This flag ensures that IFUNC dispatch selects the 512-bit EVEX variants
on Zen5, leveraging its native 512-bit AVX-512 execution for higher
throughput. EVEX (256-bit) implementations are chosen only when this
flag is not set, while EVEX512 entries remain intact.

Affected functions:
  - strchr
  - strlen
  - strnlen
  - strrchr
  - strchrnul
  - memchr

Benchmarks conducted on AMD Zen5 hardware demonstrate significant
performance improvements across all affected functions:

Function    Baseline   Patched    Avg         Avg        Avg      Max
            Variant    Variant    Baseline    Patched    Change   Improve
                                  (ns)        (ns)       %        %
------------+----------+----------+-----------+----------+--------+--------
STRCHR      evex       evex512    16.408      12.293     25.08%   37.69%
STRLEN      evex       evex512    16.862      11.436     32.18%   56.74%
STRNLEN     evex       evex512    18.493      11.762     36.40%   64.40%
STRRCHR     evex       evex512    15.154      10.874     28.24%   44.38%
STRCHRNUL   evex       evex512    16.464      12.605     23.44%   45.56%
MEMCHR      evex       evex512    9.984       8.268      17.19%   39.99%
  

On Thu, Feb 19, 2026 at 2:09 AM Sajan Karumanchi
<sajan.karumanchi@gmail.com> wrote:
>
> Introduced a synthetic architecture preference flag (Prefer_EVEX512)
> and enabled it for AMD Zen5 (CPUID Family 0x1A) when AVX-512F, AVX-512VL,
> and AVX-512BW are supported.
>
> This flag ensures that IFUNC dispatch selects the 512-bit EVEX variants
> on Zen5, leveraging its native 512-bit AVX-512 execution for higher
> throughput. EVEX (256-bit) implementations are chosen only when this
> flag is not set, while EVEX512 entries remain intact.
>
> Affected functions:
>   - strchr
>   - strlen
>   - strnlen
>   - strrchr
>   - strchrnul
>   - memchr
>
> Benchmarks conducted on AMD Zen5 hardware demonstrate significant
> performance improvements across all affected functions:
>
> Function    Baseline   Patched    Avg         Avg        Avg      Max
>             Variant    Variant    Baseline    Patched    Change   Improve
>                                   (ns)        (ns)       %        %
> ------------+----------+----------+-----------+----------+--------+--------
> STRCHR      evex       evex512    16.408      12.293     25.08%   37.69%
> STRLEN      evex       evex512    16.862      11.436     32.18%   56.74%
> STRNLEN     evex       evex512    18.493      11.762     36.40%   64.40%
> STRRCHR     evex       evex512    15.154      10.874     28.24%   44.38%
> STRCHRNUL   evex       evex512    16.464      12.605     23.44%   45.56%
> MEMCHR      evex       evex512    9.984       8.268      17.19%   39.99%
>
>
>
> diff --git a/sysdeps/x86/cpu-features.c b/sysdeps/x86/cpu-features.c
> index ccfef55652..882e28743a 100644
> --- a/sysdeps/x86/cpu-features.c
> +++ b/sysdeps/x86/cpu-features.c
> @@ -1009,6 +1009,17 @@ disable_tsx:
>        cpu_features->preferred[index_arch_Avoid_Non_Temporal_Memset]
>           &= ~bit_arch_Avoid_Non_Temporal_Memset;
>
> +      /* Prefer EVEX512 string/memory variants on AMD Zen5 (Family 0x1A)
> +         when AVX-512F/VL/BW are usable. */
> +      if (family == 0x1A
> +          && CPU_FEATURE_USABLE_P (cpu_features, AVX512F)
> +          && CPU_FEATURE_USABLE_P (cpu_features, AVX512VL)
> +          && CPU_FEATURE_USABLE_P (cpu_features, AVX512BW))
> +        {
> +          cpu_features->preferred[index_arch_Prefer_EVEX512]
> +            |= bit_arch_Prefer_EVEX512;
> +        }
> +
>        if (CPU_FEATURE_USABLE_P (cpu_features, AVX))
>         {
>           /* Since the FMA4 bit is in CPUID_INDEX_80000001 and
> diff --git a/sysdeps/x86/include/cpu-features-preferred_feature_index_1.def b/sysdeps/x86/include/cpu-features-preferred_feature_index_1.def
> index 8cab2ae248..74acb3fde1 100644
> --- a/sysdeps/x86/include/cpu-features-preferred_feature_index_1.def
> +++ b/sysdeps/x86/include/cpu-features-preferred_feature_index_1.def
> @@ -35,3 +35,4 @@ BIT (Prefer_FSRM)
>  BIT (Avoid_Short_Distance_REP_MOVSB)
>  BIT (Avoid_Non_Temporal_Memset)
>  BIT (Avoid_STOSB)
> +BIT (Prefer_EVEX512)
> diff --git a/sysdeps/x86_64/multiarch/ifunc-evex512.h b/sysdeps/x86_64/multiarch/ifunc-evex512.h
> new file mode 100644
> index 0000000000..54ea6f9ae3
> --- /dev/null
> +++ b/sysdeps/x86_64/multiarch/ifunc-evex512.h
> @@ -0,0 +1,60 @@
> +/* Common definition for ifunc selection optimized with EVEX512.
> +   All versions must be listed in ifunc-impl-list.c.
> +   Copyright (C) 2026 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +#include <init-arch.h>
> +
> +extern __typeof (REDIRECT_NAME) OPTIMIZE (evex) attribute_hidden;
> +extern __typeof (REDIRECT_NAME) OPTIMIZE (evex512) attribute_hidden;
> +
> +extern __typeof (REDIRECT_NAME) OPTIMIZE (avx2) attribute_hidden;
> +extern __typeof (REDIRECT_NAME) OPTIMIZE (avx2_rtm) attribute_hidden;
> +
> +extern __typeof (REDIRECT_NAME) OPTIMIZE (sse2) attribute_hidden;
> +
> +static inline void *
> +IFUNC_SELECTOR (void)
> +{
> +  const struct cpu_features *cpu_features = __get_cpu_features ();
> +
> +  if (X86_ISA_CPU_FEATURE_USABLE_P (cpu_features, AVX2)
> +      && X86_ISA_CPU_FEATURE_USABLE_P (cpu_features, BMI1)
> +      && X86_ISA_CPU_FEATURE_USABLE_P (cpu_features, BMI2)
> +      && X86_ISA_CPU_FEATURE_USABLE_P (cpu_features, LZCNT)
> +      && X86_ISA_CPU_FEATURES_ARCH_P (cpu_features,
> +                                     AVX_Fast_Unaligned_Load, ))
> +    {
> +      if (X86_ISA_CPU_FEATURE_USABLE_P (cpu_features, AVX512VL)
> +         && X86_ISA_CPU_FEATURE_USABLE_P (cpu_features, AVX512BW))
> +       {
> +         if (CPU_FEATURES_ARCH_P (cpu_features, Prefer_EVEX512))
> +           return OPTIMIZE (evex512);

Do we really need this?  Can you change ifunc-evex.h to check Prefer_EVEX512
instead?

> +
> +         return OPTIMIZE (evex);
> +       }
> +
> +      if (CPU_FEATURE_USABLE_P (cpu_features, RTM))
> +       return OPTIMIZE (avx2_rtm);
> +
> +      if (X86_ISA_CPU_FEATURES_ARCH_P (cpu_features,
> +                                      Prefer_No_VZEROUPPER, !))
> +       return OPTIMIZE (avx2);
> +    }
> +
> +  return OPTIMIZE (sse2);
> +}
> diff --git a/sysdeps/x86_64/multiarch/ifunc-impl-list.c b/sysdeps/x86_64/multiarch/ifunc-impl-list.c
> index 40d5a8819a..a721ecce60 100644
> --- a/sysdeps/x86_64/multiarch/ifunc-impl-list.c
> +++ b/sysdeps/x86_64/multiarch/ifunc-impl-list.c
> @@ -58,8 +58,11 @@ __libc_ifunc_impl_list (const char *name, struct libc_ifunc_impl *array,
>
>    /* Support sysdeps/x86_64/multiarch/memchr.c.  */
>    IFUNC_IMPL (i, name, memchr,
> +             /* EVEX only when Prefer_EVEX512 is NOT set. */
>               X86_IFUNC_IMPL_ADD_V4 (array, i, memchr,
> -                                    (CPU_FEATURE_USABLE (AVX512VL)
> +                                    (!CPU_FEATURES_ARCH_P (__get_cpu_features (),
> +                                                           Prefer_EVEX512)
> +                                     && CPU_FEATURE_USABLE (AVX512VL)

Don't change this file since all available implementations should be
included.

>                                       && CPU_FEATURE_USABLE (AVX512BW)
>                                       && CPU_FEATURE_USABLE (BMI2)),
>                                      __memchr_evex)
> @@ -369,8 +372,11 @@ __libc_ifunc_impl_list (const char *name, struct libc_ifunc_impl *array,
>
>    /* Support sysdeps/x86_64/multiarch/strlen.c.  */
>    IFUNC_IMPL (i, name, strlen,
> +             /* EVEX only when Prefer_EVEX512 is NOT set. */
>               X86_IFUNC_IMPL_ADD_V4 (array, i, strlen,
> -                                    (CPU_FEATURE_USABLE (AVX512VL)
> +                                    (!CPU_FEATURES_ARCH_P (__get_cpu_features (),
> +                                                           Prefer_EVEX512)
> +                                     && CPU_FEATURE_USABLE (AVX512VL)
>                                       && CPU_FEATURE_USABLE (AVX512BW)
>                                       && CPU_FEATURE_USABLE (BMI2)),
>                                      __strlen_evex)
> @@ -396,8 +402,11 @@ __libc_ifunc_impl_list (const char *name, struct libc_ifunc_impl *array,
>
>    /* Support sysdeps/x86_64/multiarch/strnlen.c.  */
>    IFUNC_IMPL (i, name, strnlen,
> +             /* EVEX only when Prefer_EVEX512 is NOT set. */
>               X86_IFUNC_IMPL_ADD_V4 (array, i, strnlen,
> -                                    (CPU_FEATURE_USABLE (AVX512VL)
> +                                    (!CPU_FEATURES_ARCH_P (__get_cpu_features (),
> +                                                           Prefer_EVEX512)
> +                                     && CPU_FEATURE_USABLE (AVX512VL)
>                                       && CPU_FEATURE_USABLE (AVX512BW)
>                                       && CPU_FEATURE_USABLE (BMI2)),
>                                      __strnlen_evex)
> @@ -539,8 +548,11 @@ __libc_ifunc_impl_list (const char *name, struct libc_ifunc_impl *array,
>
>    /* Support sysdeps/x86_64/multiarch/strchr.c.  */
>    IFUNC_IMPL (i, name, strchr,
> +             /* EVEX only when Prefer_EVEX512 is NOT set. */
>               X86_IFUNC_IMPL_ADD_V4 (array, i, strchr,
> -                                    (CPU_FEATURE_USABLE (AVX512VL)
> +                                    (!CPU_FEATURES_ARCH_P (__get_cpu_features (),
> +                                                           Prefer_EVEX512)
> +                                     && CPU_FEATURE_USABLE (AVX512VL)
>                                       && CPU_FEATURE_USABLE (AVX512BW)
>                                       && CPU_FEATURE_USABLE (BMI2)),
>                                      __strchr_evex)
> @@ -568,8 +580,11 @@ __libc_ifunc_impl_list (const char *name, struct libc_ifunc_impl *array,
>
>    /* Support sysdeps/x86_64/multiarch/strchrnul.c.  */
>    IFUNC_IMPL (i, name, strchrnul,
> +             /* EVEX only when Prefer_EVEX512 is NOT set. */
>               X86_IFUNC_IMPL_ADD_V4 (array, i, strchrnul,
> -                                    (CPU_FEATURE_USABLE (AVX512VL)
> +                                    (!CPU_FEATURES_ARCH_P (__get_cpu_features (),
> +                                                           Prefer_EVEX512)
> +                                     && CPU_FEATURE_USABLE (AVX512VL)
>                                       && CPU_FEATURE_USABLE (AVX512BW)
>                                       && CPU_FEATURE_USABLE (BMI2)),
>                                      __strchrnul_evex)
> @@ -594,8 +609,11 @@ __libc_ifunc_impl_list (const char *name, struct libc_ifunc_impl *array,
>
>    /* Support sysdeps/x86_64/multiarch/strrchr.c.  */
>    IFUNC_IMPL (i, name, strrchr,
> +             /* EVEX only when Prefer_EVEX512 is NOT set. */
>               X86_IFUNC_IMPL_ADD_V4 (array, i, strrchr,
> -                                    (CPU_FEATURE_USABLE (AVX512VL)
> +                                    (!CPU_FEATURES_ARCH_P (__get_cpu_features (),
> +                                                           Prefer_EVEX512)
> +                                     && CPU_FEATURE_USABLE (AVX512VL)
>                                       && CPU_FEATURE_USABLE (AVX512BW)
>                                       && CPU_FEATURE_USABLE (BMI1)
>                                       && CPU_FEATURE_USABLE (BMI2)),
> diff --git a/sysdeps/x86_64/multiarch/memchr.c b/sysdeps/x86_64/multiarch/memchr.c
> index cea0518787..2b4f0f0689 100644
> --- a/sysdeps/x86_64/multiarch/memchr.c
> +++ b/sysdeps/x86_64/multiarch/memchr.c
> @@ -24,7 +24,53 @@
>  # undef memchr
>
>  # define SYMBOL_NAME memchr
> -# include "ifunc-evex.h"
> +# include <init-arch.h>
> +
> +extern __typeof (REDIRECT_NAME) OPTIMIZE (evex) attribute_hidden;
> +extern __typeof (REDIRECT_NAME) OPTIMIZE (evex512) attribute_hidden;
> +extern __typeof (REDIRECT_NAME) OPTIMIZE (evex_rtm) attribute_hidden;
> +
> +extern __typeof (REDIRECT_NAME) OPTIMIZE (avx2) attribute_hidden;
> +extern __typeof (REDIRECT_NAME) OPTIMIZE (avx2_rtm) attribute_hidden;
> +
> +extern __typeof (REDIRECT_NAME) OPTIMIZE (sse2) attribute_hidden;
> +
> +static inline void *
> +IFUNC_SELECTOR (void)
> +{
> +  const struct cpu_features *cpu_features = __get_cpu_features ();
> +
> +  /* NB: The X86_ISA_* feature check macros are evaluated at
> +     compile time.  */
> +  if (X86_ISA_CPU_FEATURE_USABLE_P (cpu_features, AVX2)
> +      && X86_ISA_CPU_FEATURE_USABLE_P (cpu_features, BMI2)
> +      && X86_ISA_CPU_FEATURES_ARCH_P (cpu_features,
> +                                     AVX_Fast_Unaligned_Load, ))
> +    {
> +      if (X86_ISA_CPU_FEATURE_USABLE_P (cpu_features, AVX512VL)
> +         && X86_ISA_CPU_FEATURE_USABLE_P (cpu_features, AVX512BW))
> +       {
> +         if (CPU_FEATURES_ARCH_P (cpu_features, Prefer_EVEX512))
> +           return OPTIMIZE (evex512);
> +
> +         if (CPU_FEATURE_USABLE_P (cpu_features, RTM))
> +           return OPTIMIZE (evex_rtm);
> +
> +         return OPTIMIZE (evex);
> +       }
> +
> +      if (CPU_FEATURE_USABLE_P (cpu_features, RTM))
> +       return OPTIMIZE (avx2_rtm);
> +
> +      if (X86_ISA_CPU_FEATURES_ARCH_P (cpu_features,
> +                                      Prefer_No_VZEROUPPER, !))
> +       return OPTIMIZE (avx2);
> +    }
> +
> +  /* This is unreachable (compile time checked) if ISA level >= 3
> +     so no need for a robust fallback here.  */
> +  return OPTIMIZE (sse2);
> +}
>
>  libc_ifunc_redirected (__redirect_memchr, memchr, IFUNC_SELECTOR ());
>  strong_alias (memchr, __memchr)
> diff --git a/sysdeps/x86_64/multiarch/strchr.c b/sysdeps/x86_64/multiarch/strchr.c
> index 1064bc8e5b..f6bd36ba73 100644
> --- a/sysdeps/x86_64/multiarch/strchr.c
> +++ b/sysdeps/x86_64/multiarch/strchr.c
> @@ -27,6 +27,7 @@
>  # include <init-arch.h>
>
>  extern __typeof (REDIRECT_NAME) OPTIMIZE (evex) attribute_hidden;
> +extern __typeof (REDIRECT_NAME) OPTIMIZE (evex512) attribute_hidden;
>
>  extern __typeof (REDIRECT_NAME) OPTIMIZE (avx2) attribute_hidden;
>  extern __typeof (REDIRECT_NAME) OPTIMIZE (avx2_rtm) attribute_hidden;
> @@ -46,7 +47,12 @@ IFUNC_SELECTOR (void)
>      {
>        if (X86_ISA_CPU_FEATURE_USABLE_P (cpu_features, AVX512VL)
>           && X86_ISA_CPU_FEATURE_USABLE_P (cpu_features, AVX512BW))
> -       return OPTIMIZE (evex);
> +       {
> +         if (CPU_FEATURES_ARCH_P (cpu_features, Prefer_EVEX512))
> +           return OPTIMIZE (evex512);
> +
> +         return OPTIMIZE (evex);
> +       }
>
>        if (CPU_FEATURE_USABLE_P (cpu_features, RTM))
>         return OPTIMIZE (avx2_rtm);
> diff --git a/sysdeps/x86_64/multiarch/strchrnul.c b/sysdeps/x86_64/multiarch/strchrnul.c
> index f4bfc28c7b..81ea03b91b 100644
> --- a/sysdeps/x86_64/multiarch/strchrnul.c
> +++ b/sysdeps/x86_64/multiarch/strchrnul.c
> @@ -26,10 +26,10 @@
>  # undef strchrnul
>
>  # define SYMBOL_NAME strchrnul
> -# include "ifunc-avx2.h"
> +# include "ifunc-evex512.h"

Please update ifunc-avx2.h instead.

>
>  libc_ifunc_redirected (__redirect_strchrnul, __strchrnul,
> -                      IFUNC_SELECTOR ());
> +                      IFUNC_SELECTOR ());
>  weak_alias (__strchrnul, strchrnul)
>  # ifdef SHARED
>  __hidden_ver1 (__strchrnul, __GI___strchrnul, __redirect_strchrnul)
> diff --git a/sysdeps/x86_64/multiarch/strlen.c b/sysdeps/x86_64/multiarch/strlen.c
> index 95aa48e5d3..76f51b077a 100644
> --- a/sysdeps/x86_64/multiarch/strlen.c
> +++ b/sysdeps/x86_64/multiarch/strlen.c
> @@ -24,7 +24,7 @@
>  # undef strlen
>
>  # define SYMBOL_NAME strlen
> -# include "ifunc-avx2.h"
> +# include "ifunc-evex512.h"
>
>  libc_ifunc_redirected (__redirect_strlen, strlen, IFUNC_SELECTOR ());
>  # ifdef SHARED
> diff --git a/sysdeps/x86_64/multiarch/strnlen.c b/sysdeps/x86_64/multiarch/strnlen.c
> index bfe4d7abf0..47ab8a5932 100644
> --- a/sysdeps/x86_64/multiarch/strnlen.c
> +++ b/sysdeps/x86_64/multiarch/strnlen.c
> @@ -26,7 +26,7 @@
>  # undef strnlen
>
>  # define SYMBOL_NAME strnlen
> -# include "ifunc-avx2.h"
> +# include "ifunc-evex512.h"
>
>  libc_ifunc_redirected (__redirect_strnlen, __strnlen, IFUNC_SELECTOR ());
>  weak_alias (__strnlen, strnlen);
> diff --git a/sysdeps/x86_64/multiarch/strrchr.c b/sysdeps/x86_64/multiarch/strrchr.c
> index 66d3f5b1d1..15c6443902 100644
> --- a/sysdeps/x86_64/multiarch/strrchr.c
> +++ b/sysdeps/x86_64/multiarch/strrchr.c
> @@ -23,7 +23,7 @@
>  # undef strrchr
>
>  # define SYMBOL_NAME strrchr
> -# include "ifunc-avx2.h"
> +# include "ifunc-evex512.h"
>
>  libc_ifunc_redirected (__redirect_strrchr, strrchr, IFUNC_SELECTOR ());
>  weak_alias (strrchr, rindex);
> --
> 2.34.1
>