[v11,05/29] string: Improve generic strnlen with memchr

  It also cleanups the multiple inclusion by leaving the ifunc
implementation to undef the weak_alias and libc_hidden_def.

Co-authored-by: Richard Henderson  <richard.henderson@linaro.org>
---
 string/strnlen.c                              | 137 +-----------------
 sysdeps/i386/i686/multiarch/strnlen-c.c       |  14 +-
 .../power4/multiarch/strnlen-ppc32.c          |  14 +-
 sysdeps/s390/strnlen-c.c                      |  14 +-
 4 files changed, 27 insertions(+), 152 deletions(-)
  

On Wed, Feb 1, 2023 at 11:04 AM Adhemerval Zanella
<adhemerval.zanella@linaro.org> wrote:
>
> It also cleanups the multiple inclusion by leaving the ifunc
> implementation to undef the weak_alias and libc_hidden_def.
>
> Co-authored-by: Richard Henderson  <richard.henderson@linaro.org>
> ---
>  string/strnlen.c                              | 137 +-----------------
>  sysdeps/i386/i686/multiarch/strnlen-c.c       |  14 +-
>  .../power4/multiarch/strnlen-ppc32.c          |  14 +-
>  sysdeps/s390/strnlen-c.c                      |  14 +-
>  4 files changed, 27 insertions(+), 152 deletions(-)
>
> diff --git a/string/strnlen.c b/string/strnlen.c
> index 6ff294eab1..dc23354ec8 100644
> --- a/string/strnlen.c
> +++ b/string/strnlen.c
> @@ -1,10 +1,6 @@
>  /* Find the length of STRING, but scan at most MAXLEN characters.
>     Copyright (C) 1991-2023 Free Software Foundation, Inc.
>
> -   Based on strlen written by Torbjorn Granlund (tege@sics.se),
> -   with help from Dan Sahlin (dan@sics.se);
> -   commentary by Jim Blandy (jimb@ai.mit.edu).
> -
>     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
> @@ -20,7 +16,6 @@
>     not, see <https://www.gnu.org/licenses/>.  */
>
>  #include <string.h>
> -#include <stdlib.h>
>
>  /* Find the length of S, but scan at most MAXLEN characters.  If no
>     '\0' terminator is found in that many characters, return MAXLEN.  */
> @@ -32,134 +27,12 @@
>  size_t
>  __strnlen (const char *str, size_t maxlen)
>  {
> -  const char *char_ptr, *end_ptr = str + maxlen;
> -  const unsigned long int *longword_ptr;
> -  unsigned long int longword, himagic, lomagic;
> -
> -  if (maxlen == 0)
> -    return 0;
> -
> -  if (__glibc_unlikely (end_ptr < str))
> -    end_ptr = (const char *) ~0UL;
> -
> -  /* Handle the first few characters by reading one character at a time.
> -     Do this until CHAR_PTR is aligned on a longword boundary.  */
> -  for (char_ptr = str; ((unsigned long int) char_ptr
> -                       & (sizeof (longword) - 1)) != 0;
> -       ++char_ptr)
> -    if (*char_ptr == '\0')
> -      {
> -       if (char_ptr > end_ptr)
> -         char_ptr = end_ptr;
> -       return char_ptr - str;
> -      }
> -
> -  /* All these elucidatory comments refer to 4-byte longwords,
> -     but the theory applies equally well to 8-byte longwords.  */
> -
> -  longword_ptr = (unsigned long int *) char_ptr;
> -
> -  /* Bits 31, 24, 16, and 8 of this number are zero.  Call these bits
> -     the "holes."  Note that there is a hole just to the left of
> -     each byte, with an extra at the end:
> -
> -     bits:  01111110 11111110 11111110 11111111
> -     bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
> -
> -     The 1-bits make sure that carries propagate to the next 0-bit.
> -     The 0-bits provide holes for carries to fall into.  */
> -  himagic = 0x80808080L;
> -  lomagic = 0x01010101L;
> -  if (sizeof (longword) > 4)
> -    {
> -      /* 64-bit version of the magic.  */
> -      /* Do the shift in two steps to avoid a warning if long has 32 bits.  */
> -      himagic = ((himagic << 16) << 16) | himagic;
> -      lomagic = ((lomagic << 16) << 16) | lomagic;
> -    }
> -  if (sizeof (longword) > 8)
> -    abort ();
> -
> -  /* Instead of the traditional loop which tests each character,
> -     we will test a longword at a time.  The tricky part is testing
> -     if *any of the four* bytes in the longword in question are zero.  */
> -  while (longword_ptr < (unsigned long int *) end_ptr)
> -    {
> -      /* We tentatively exit the loop if adding MAGIC_BITS to
> -        LONGWORD fails to change any of the hole bits of LONGWORD.
> -
> -        1) Is this safe?  Will it catch all the zero bytes?
> -        Suppose there is a byte with all zeros.  Any carry bits
> -        propagating from its left will fall into the hole at its
> -        least significant bit and stop.  Since there will be no
> -        carry from its most significant bit, the LSB of the
> -        byte to the left will be unchanged, and the zero will be
> -        detected.
> -
> -        2) Is this worthwhile?  Will it ignore everything except
> -        zero bytes?  Suppose every byte of LONGWORD has a bit set
> -        somewhere.  There will be a carry into bit 8.  If bit 8
> -        is set, this will carry into bit 16.  If bit 8 is clear,
> -        one of bits 9-15 must be set, so there will be a carry
> -        into bit 16.  Similarly, there will be a carry into bit
> -        24.  If one of bits 24-30 is set, there will be a carry
> -        into bit 31, so all of the hole bits will be changed.
> -
> -        The one misfire occurs when bits 24-30 are clear and bit
> -        31 is set; in this case, the hole at bit 31 is not
> -        changed.  If we had access to the processor carry flag,
> -        we could close this loophole by putting the fourth hole
> -        at bit 32!
> -
> -        So it ignores everything except 128's, when they're aligned
> -        properly.  */
> -
> -      longword = *longword_ptr++;
> -
> -      if ((longword - lomagic) & himagic)
> -       {
> -         /* Which of the bytes was the zero?  If none of them were, it was
> -            a misfire; continue the search.  */
> -
> -         const char *cp = (const char *) (longword_ptr - 1);
> -
> -         char_ptr = cp;
> -         if (cp[0] == 0)
> -           break;
> -         char_ptr = cp + 1;
> -         if (cp[1] == 0)
> -           break;
> -         char_ptr = cp + 2;
> -         if (cp[2] == 0)
> -           break;
> -         char_ptr = cp + 3;
> -         if (cp[3] == 0)
> -           break;
> -         if (sizeof (longword) > 4)
> -           {
> -             char_ptr = cp + 4;
> -             if (cp[4] == 0)
> -               break;
> -             char_ptr = cp + 5;
> -             if (cp[5] == 0)
> -               break;
> -             char_ptr = cp + 6;
> -             if (cp[6] == 0)
> -               break;
> -             char_ptr = cp + 7;
> -             if (cp[7] == 0)
> -               break;
> -           }
> -       }
> -      char_ptr = end_ptr;
> -    }
> -
> -  if (char_ptr > end_ptr)
> -    char_ptr = end_ptr;
> -  return char_ptr - str;
> +  const char *found = memchr (str, '\0', maxlen);
> +  return found ? found - str : maxlen;
>  }
> +
>  #ifndef STRNLEN
> -libc_hidden_def (__strnlen)
>  weak_alias (__strnlen, strnlen)
> -#endif
> +libc_hidden_def (__strnlen)
>  libc_hidden_def (strnlen)
> +#endif
> diff --git a/sysdeps/i386/i686/multiarch/strnlen-c.c b/sysdeps/i386/i686/multiarch/strnlen-c.c
> index 351e939a93..beb0350d53 100644
> --- a/sysdeps/i386/i686/multiarch/strnlen-c.c
> +++ b/sysdeps/i386/i686/multiarch/strnlen-c.c
> @@ -1,10 +1,10 @@
>  #define STRNLEN  __strnlen_ia32
> +#include <string/strnlen.c>
> +
>  #ifdef SHARED
> -# undef libc_hidden_def
> -# define libc_hidden_def(name)  \
> -    __hidden_ver1 (__strnlen_ia32, __GI_strnlen, __strnlen_ia32); \
> -    strong_alias (__strnlen_ia32, __strnlen_ia32_1); \
> -    __hidden_ver1 (__strnlen_ia32_1, __GI___strnlen, __strnlen_ia32_1);
> +/* Alias for internal symbol to avoid PLT generation, it redirects the
> +   libc_hidden_def (__strnlen/strlen) to default implementation.  */
> +__hidden_ver1 (__strnlen_ia32, __GI_strnlen, __strnlen_ia32);
> +strong_alias (__strnlen_ia32, __strnlen_ia32_1);
> +__hidden_ver1 (__strnlen_ia32_1, __GI___strnlen, __strnlen_ia32_1);
>  #endif
> -
> -#include "string/strnlen.c"
> diff --git a/sysdeps/powerpc/powerpc32/power4/multiarch/strnlen-ppc32.c b/sysdeps/powerpc/powerpc32/power4/multiarch/strnlen-ppc32.c
> index 957b9b99e8..2ca1cd7181 100644
> --- a/sysdeps/powerpc/powerpc32/power4/multiarch/strnlen-ppc32.c
> +++ b/sysdeps/powerpc/powerpc32/power4/multiarch/strnlen-ppc32.c
> @@ -17,12 +17,12 @@
>     <https://www.gnu.org/licenses/>.  */
>
>  #define STRNLEN  __strnlen_ppc
> +#include <string/strnlen.c>
> +
>  #ifdef SHARED
> -# undef libc_hidden_def
> -# define libc_hidden_def(name)  \
> -    __hidden_ver1 (__strnlen_ppc, __GI_strnlen, __strnlen_ppc); \
> -    strong_alias (__strnlen_ppc, __strnlen_ppc_1); \
> -    __hidden_ver1 (__strnlen_ppc_1, __GI___strnlen, __strnlen_ppc_1);
> +/* Alias for internal symbol to avoid PLT generation, it redirects the
> +   libc_hidden_def (__strnlen/strlen) to default implementation.  */
> +__hidden_ver1 (__strnlen_ppc, __GI_strnlen, __strnlen_ppc); \
> +strong_alias (__strnlen_ppc, __strnlen_ppc_1); \
> +__hidden_ver1 (__strnlen_ppc_1, __GI___strnlen, __strnlen_ppc_1);
>  #endif
> -
> -#include <string/strnlen.c>
> diff --git a/sysdeps/s390/strnlen-c.c b/sysdeps/s390/strnlen-c.c
> index 172fcc7caa..95156a0ff5 100644
> --- a/sysdeps/s390/strnlen-c.c
> +++ b/sysdeps/s390/strnlen-c.c
> @@ -21,14 +21,16 @@
>  #if HAVE_STRNLEN_C
>  # if HAVE_STRNLEN_IFUNC
>  #  define STRNLEN STRNLEN_C
> +# endif
> +
> +# include <string/strnlen.c>
> +
> +# if HAVE_STRNLEN_IFUNC
>  #  if defined SHARED && IS_IN (libc)
> -#   undef libc_hidden_def
> -#   define libc_hidden_def(name)                                       \
> -  __hidden_ver1 (__strnlen_c, __GI_strnlen, __strnlen_c);      \
> -  strong_alias (__strnlen_c, __strnlen_c_1);                   \
> -  __hidden_ver1 (__strnlen_c_1, __GI___strnlen, __strnlen_c_1);
> +__hidden_ver1 (__strnlen_c, __GI_strnlen, __strnlen_c);
> +strong_alias (__strnlen_c, __strnlen_c_1);
> +__hidden_ver1 (__strnlen_c_1, __GI___strnlen, __strnlen_c_1);
>  #  endif
>  # endif
>
> -# include <string/strnlen.c>
>  #endif
> --
> 2.34.1
NIT: I think this patch should come after the memchr improvement.
  

On Wed, Feb 1, 2023 at 11:04 AM Adhemerval Zanella
<adhemerval.zanella@linaro.org> wrote:
>
> It also cleanups the multiple inclusion by leaving the ifunc
> implementation to undef the weak_alias and libc_hidden_def.
>
> Co-authored-by: Richard Henderson  <richard.henderson@linaro.org>
> ---
>  string/strnlen.c                              | 137 +-----------------
>  sysdeps/i386/i686/multiarch/strnlen-c.c       |  14 +-
>  .../power4/multiarch/strnlen-ppc32.c          |  14 +-
>  sysdeps/s390/strnlen-c.c                      |  14 +-
>  4 files changed, 27 insertions(+), 152 deletions(-)
>
> diff --git a/string/strnlen.c b/string/strnlen.c
> index 6ff294eab1..dc23354ec8 100644
> --- a/string/strnlen.c
> +++ b/string/strnlen.c
> @@ -1,10 +1,6 @@
>  /* Find the length of STRING, but scan at most MAXLEN characters.
>     Copyright (C) 1991-2023 Free Software Foundation, Inc.
>
> -   Based on strlen written by Torbjorn Granlund (tege@sics.se),
> -   with help from Dan Sahlin (dan@sics.se);
> -   commentary by Jim Blandy (jimb@ai.mit.edu).
> -
>     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
> @@ -20,7 +16,6 @@
>     not, see <https://www.gnu.org/licenses/>.  */
>
>  #include <string.h>
> -#include <stdlib.h>
>
>  /* Find the length of S, but scan at most MAXLEN characters.  If no
>     '\0' terminator is found in that many characters, return MAXLEN.  */
> @@ -32,134 +27,12 @@
>  size_t
>  __strnlen (const char *str, size_t maxlen)
>  {
> -  const char *char_ptr, *end_ptr = str + maxlen;
> -  const unsigned long int *longword_ptr;
> -  unsigned long int longword, himagic, lomagic;
> -
> -  if (maxlen == 0)
> -    return 0;
> -
> -  if (__glibc_unlikely (end_ptr < str))
> -    end_ptr = (const char *) ~0UL;
> -
> -  /* Handle the first few characters by reading one character at a time.
> -     Do this until CHAR_PTR is aligned on a longword boundary.  */
> -  for (char_ptr = str; ((unsigned long int) char_ptr
> -                       & (sizeof (longword) - 1)) != 0;
> -       ++char_ptr)
> -    if (*char_ptr == '\0')
> -      {
> -       if (char_ptr > end_ptr)
> -         char_ptr = end_ptr;
> -       return char_ptr - str;
> -      }
> -
> -  /* All these elucidatory comments refer to 4-byte longwords,
> -     but the theory applies equally well to 8-byte longwords.  */
> -
> -  longword_ptr = (unsigned long int *) char_ptr;
> -
> -  /* Bits 31, 24, 16, and 8 of this number are zero.  Call these bits
> -     the "holes."  Note that there is a hole just to the left of
> -     each byte, with an extra at the end:
> -
> -     bits:  01111110 11111110 11111110 11111111
> -     bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
> -
> -     The 1-bits make sure that carries propagate to the next 0-bit.
> -     The 0-bits provide holes for carries to fall into.  */
> -  himagic = 0x80808080L;
> -  lomagic = 0x01010101L;
> -  if (sizeof (longword) > 4)
> -    {
> -      /* 64-bit version of the magic.  */
> -      /* Do the shift in two steps to avoid a warning if long has 32 bits.  */
> -      himagic = ((himagic << 16) << 16) | himagic;
> -      lomagic = ((lomagic << 16) << 16) | lomagic;
> -    }
> -  if (sizeof (longword) > 8)
> -    abort ();
> -
> -  /* Instead of the traditional loop which tests each character,
> -     we will test a longword at a time.  The tricky part is testing
> -     if *any of the four* bytes in the longword in question are zero.  */
> -  while (longword_ptr < (unsigned long int *) end_ptr)
> -    {
> -      /* We tentatively exit the loop if adding MAGIC_BITS to
> -        LONGWORD fails to change any of the hole bits of LONGWORD.
> -
> -        1) Is this safe?  Will it catch all the zero bytes?
> -        Suppose there is a byte with all zeros.  Any carry bits
> -        propagating from its left will fall into the hole at its
> -        least significant bit and stop.  Since there will be no
> -        carry from its most significant bit, the LSB of the
> -        byte to the left will be unchanged, and the zero will be
> -        detected.
> -
> -        2) Is this worthwhile?  Will it ignore everything except
> -        zero bytes?  Suppose every byte of LONGWORD has a bit set
> -        somewhere.  There will be a carry into bit 8.  If bit 8
> -        is set, this will carry into bit 16.  If bit 8 is clear,
> -        one of bits 9-15 must be set, so there will be a carry
> -        into bit 16.  Similarly, there will be a carry into bit
> -        24.  If one of bits 24-30 is set, there will be a carry
> -        into bit 31, so all of the hole bits will be changed.
> -
> -        The one misfire occurs when bits 24-30 are clear and bit
> -        31 is set; in this case, the hole at bit 31 is not
> -        changed.  If we had access to the processor carry flag,
> -        we could close this loophole by putting the fourth hole
> -        at bit 32!
> -
> -        So it ignores everything except 128's, when they're aligned
> -        properly.  */
> -
> -      longword = *longword_ptr++;
> -
> -      if ((longword - lomagic) & himagic)
> -       {
> -         /* Which of the bytes was the zero?  If none of them were, it was
> -            a misfire; continue the search.  */
> -
> -         const char *cp = (const char *) (longword_ptr - 1);
> -
> -         char_ptr = cp;
> -         if (cp[0] == 0)
> -           break;
> -         char_ptr = cp + 1;
> -         if (cp[1] == 0)
> -           break;
> -         char_ptr = cp + 2;
> -         if (cp[2] == 0)
> -           break;
> -         char_ptr = cp + 3;
> -         if (cp[3] == 0)
> -           break;
> -         if (sizeof (longword) > 4)
> -           {
> -             char_ptr = cp + 4;
> -             if (cp[4] == 0)
> -               break;
> -             char_ptr = cp + 5;
> -             if (cp[5] == 0)
> -               break;
> -             char_ptr = cp + 6;
> -             if (cp[6] == 0)
> -               break;
> -             char_ptr = cp + 7;
> -             if (cp[7] == 0)
> -               break;
> -           }
> -       }
> -      char_ptr = end_ptr;
> -    }
> -
> -  if (char_ptr > end_ptr)
> -    char_ptr = end_ptr;
> -  return char_ptr - str;
> +  const char *found = memchr (str, '\0', maxlen);
Can be __memchr to skip PLT no?
> +  return found ? found - str : maxlen;
>  }
> +
>  #ifndef STRNLEN
> -libc_hidden_def (__strnlen)
>  weak_alias (__strnlen, strnlen)
> -#endif
> +libc_hidden_def (__strnlen)
>  libc_hidden_def (strnlen)
> +#endif
> diff --git a/sysdeps/i386/i686/multiarch/strnlen-c.c b/sysdeps/i386/i686/multiarch/strnlen-c.c
> index 351e939a93..beb0350d53 100644
> --- a/sysdeps/i386/i686/multiarch/strnlen-c.c
> +++ b/sysdeps/i386/i686/multiarch/strnlen-c.c
> @@ -1,10 +1,10 @@
>  #define STRNLEN  __strnlen_ia32
> +#include <string/strnlen.c>
> +
>  #ifdef SHARED
> -# undef libc_hidden_def
> -# define libc_hidden_def(name)  \
> -    __hidden_ver1 (__strnlen_ia32, __GI_strnlen, __strnlen_ia32); \
> -    strong_alias (__strnlen_ia32, __strnlen_ia32_1); \
> -    __hidden_ver1 (__strnlen_ia32_1, __GI___strnlen, __strnlen_ia32_1);
> +/* Alias for internal symbol to avoid PLT generation, it redirects the
> +   libc_hidden_def (__strnlen/strlen) to default implementation.  */
> +__hidden_ver1 (__strnlen_ia32, __GI_strnlen, __strnlen_ia32);
> +strong_alias (__strnlen_ia32, __strnlen_ia32_1);
> +__hidden_ver1 (__strnlen_ia32_1, __GI___strnlen, __strnlen_ia32_1);
>  #endif
> -
> -#include "string/strnlen.c"
> diff --git a/sysdeps/powerpc/powerpc32/power4/multiarch/strnlen-ppc32.c b/sysdeps/powerpc/powerpc32/power4/multiarch/strnlen-ppc32.c
> index 957b9b99e8..2ca1cd7181 100644
> --- a/sysdeps/powerpc/powerpc32/power4/multiarch/strnlen-ppc32.c
> +++ b/sysdeps/powerpc/powerpc32/power4/multiarch/strnlen-ppc32.c
> @@ -17,12 +17,12 @@
>     <https://www.gnu.org/licenses/>.  */
>
>  #define STRNLEN  __strnlen_ppc
> +#include <string/strnlen.c>
> +
>  #ifdef SHARED
> -# undef libc_hidden_def
> -# define libc_hidden_def(name)  \
> -    __hidden_ver1 (__strnlen_ppc, __GI_strnlen, __strnlen_ppc); \
> -    strong_alias (__strnlen_ppc, __strnlen_ppc_1); \
> -    __hidden_ver1 (__strnlen_ppc_1, __GI___strnlen, __strnlen_ppc_1);
> +/* Alias for internal symbol to avoid PLT generation, it redirects the
> +   libc_hidden_def (__strnlen/strlen) to default implementation.  */
> +__hidden_ver1 (__strnlen_ppc, __GI_strnlen, __strnlen_ppc); \
> +strong_alias (__strnlen_ppc, __strnlen_ppc_1); \
> +__hidden_ver1 (__strnlen_ppc_1, __GI___strnlen, __strnlen_ppc_1);
>  #endif
> -
> -#include <string/strnlen.c>
> diff --git a/sysdeps/s390/strnlen-c.c b/sysdeps/s390/strnlen-c.c
> index 172fcc7caa..95156a0ff5 100644
> --- a/sysdeps/s390/strnlen-c.c
> +++ b/sysdeps/s390/strnlen-c.c
> @@ -21,14 +21,16 @@
>  #if HAVE_STRNLEN_C
>  # if HAVE_STRNLEN_IFUNC
>  #  define STRNLEN STRNLEN_C
> +# endif
> +
> +# include <string/strnlen.c>
> +
> +# if HAVE_STRNLEN_IFUNC
>  #  if defined SHARED && IS_IN (libc)
> -#   undef libc_hidden_def
> -#   define libc_hidden_def(name)                                       \
> -  __hidden_ver1 (__strnlen_c, __GI_strnlen, __strnlen_c);      \
> -  strong_alias (__strnlen_c, __strnlen_c_1);                   \
> -  __hidden_ver1 (__strnlen_c_1, __GI___strnlen, __strnlen_c_1);
> +__hidden_ver1 (__strnlen_c, __GI_strnlen, __strnlen_c);
> +strong_alias (__strnlen_c, __strnlen_c_1);
> +__hidden_ver1 (__strnlen_c_1, __GI___strnlen, __strnlen_c_1);
>  #  endif
>  # endif
>
> -# include <string/strnlen.c>
>  #endif
> --
> 2.34.1
>
  

On 01/02/23 16:36, Noah Goldstein wrote:
>> --
>> 2.34.1
> NIT: I think this patch should come after the memchr improvement.

Hum sounds, reasonable. I will change it.
  

On 01/02/23 16:39, Noah Goldstein wrote:
> On Wed, Feb 1, 2023 at 11:04 AM Adhemerval Zanella
> <adhemerval.zanella@linaro.org> wrote:
>>
>> It also cleanups the multiple inclusion by leaving the ifunc
>> implementation to undef the weak_alias and libc_hidden_def.
>>
>> Co-authored-by: Richard Henderson  <richard.henderson@linaro.org>
>> ---
>>  string/strnlen.c                              | 137 +-----------------
>>  sysdeps/i386/i686/multiarch/strnlen-c.c       |  14 +-
>>  .../power4/multiarch/strnlen-ppc32.c          |  14 +-
>>  sysdeps/s390/strnlen-c.c                      |  14 +-
>>  4 files changed, 27 insertions(+), 152 deletions(-)
>>
>> diff --git a/string/strnlen.c b/string/strnlen.c
>> index 6ff294eab1..dc23354ec8 100644
>> --- a/string/strnlen.c
>> +++ b/string/strnlen.c
>> @@ -1,10 +1,6 @@
>>  /* Find the length of STRING, but scan at most MAXLEN characters.
>>     Copyright (C) 1991-2023 Free Software Foundation, Inc.
>>
>> -   Based on strlen written by Torbjorn Granlund (tege@sics.se),
>> -   with help from Dan Sahlin (dan@sics.se);
>> -   commentary by Jim Blandy (jimb@ai.mit.edu).
>> -
>>     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
>> @@ -20,7 +16,6 @@
>>     not, see <https://www.gnu.org/licenses/>.  */
>>
>>  #include <string.h>
>> -#include <stdlib.h>
>>
>>  /* Find the length of S, but scan at most MAXLEN characters.  If no
>>     '\0' terminator is found in that many characters, return MAXLEN.  */
>> @@ -32,134 +27,12 @@
>>  size_t
>>  __strnlen (const char *str, size_t maxlen)
>>  {
>> -  const char *char_ptr, *end_ptr = str + maxlen;
>> -  const unsigned long int *longword_ptr;
>> -  unsigned long int longword, himagic, lomagic;
>> -
>> -  if (maxlen == 0)
>> -    return 0;
>> -
>> -  if (__glibc_unlikely (end_ptr < str))
>> -    end_ptr = (const char *) ~0UL;
>> -
>> -  /* Handle the first few characters by reading one character at a time.
>> -     Do this until CHAR_PTR is aligned on a longword boundary.  */
>> -  for (char_ptr = str; ((unsigned long int) char_ptr
>> -                       & (sizeof (longword) - 1)) != 0;
>> -       ++char_ptr)
>> -    if (*char_ptr == '\0')
>> -      {
>> -       if (char_ptr > end_ptr)
>> -         char_ptr = end_ptr;
>> -       return char_ptr - str;
>> -      }
>> -
>> -  /* All these elucidatory comments refer to 4-byte longwords,
>> -     but the theory applies equally well to 8-byte longwords.  */
>> -
>> -  longword_ptr = (unsigned long int *) char_ptr;
>> -
>> -  /* Bits 31, 24, 16, and 8 of this number are zero.  Call these bits
>> -     the "holes."  Note that there is a hole just to the left of
>> -     each byte, with an extra at the end:
>> -
>> -     bits:  01111110 11111110 11111110 11111111
>> -     bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
>> -
>> -     The 1-bits make sure that carries propagate to the next 0-bit.
>> -     The 0-bits provide holes for carries to fall into.  */
>> -  himagic = 0x80808080L;
>> -  lomagic = 0x01010101L;
>> -  if (sizeof (longword) > 4)
>> -    {
>> -      /* 64-bit version of the magic.  */
>> -      /* Do the shift in two steps to avoid a warning if long has 32 bits.  */
>> -      himagic = ((himagic << 16) << 16) | himagic;
>> -      lomagic = ((lomagic << 16) << 16) | lomagic;
>> -    }
>> -  if (sizeof (longword) > 8)
>> -    abort ();
>> -
>> -  /* Instead of the traditional loop which tests each character,
>> -     we will test a longword at a time.  The tricky part is testing
>> -     if *any of the four* bytes in the longword in question are zero.  */
>> -  while (longword_ptr < (unsigned long int *) end_ptr)
>> -    {
>> -      /* We tentatively exit the loop if adding MAGIC_BITS to
>> -        LONGWORD fails to change any of the hole bits of LONGWORD.
>> -
>> -        1) Is this safe?  Will it catch all the zero bytes?
>> -        Suppose there is a byte with all zeros.  Any carry bits
>> -        propagating from its left will fall into the hole at its
>> -        least significant bit and stop.  Since there will be no
>> -        carry from its most significant bit, the LSB of the
>> -        byte to the left will be unchanged, and the zero will be
>> -        detected.
>> -
>> -        2) Is this worthwhile?  Will it ignore everything except
>> -        zero bytes?  Suppose every byte of LONGWORD has a bit set
>> -        somewhere.  There will be a carry into bit 8.  If bit 8
>> -        is set, this will carry into bit 16.  If bit 8 is clear,
>> -        one of bits 9-15 must be set, so there will be a carry
>> -        into bit 16.  Similarly, there will be a carry into bit
>> -        24.  If one of bits 24-30 is set, there will be a carry
>> -        into bit 31, so all of the hole bits will be changed.
>> -
>> -        The one misfire occurs when bits 24-30 are clear and bit
>> -        31 is set; in this case, the hole at bit 31 is not
>> -        changed.  If we had access to the processor carry flag,
>> -        we could close this loophole by putting the fourth hole
>> -        at bit 32!
>> -
>> -        So it ignores everything except 128's, when they're aligned
>> -        properly.  */
>> -
>> -      longword = *longword_ptr++;
>> -
>> -      if ((longword - lomagic) & himagic)
>> -       {
>> -         /* Which of the bytes was the zero?  If none of them were, it was
>> -            a misfire; continue the search.  */
>> -
>> -         const char *cp = (const char *) (longword_ptr - 1);
>> -
>> -         char_ptr = cp;
>> -         if (cp[0] == 0)
>> -           break;
>> -         char_ptr = cp + 1;
>> -         if (cp[1] == 0)
>> -           break;
>> -         char_ptr = cp + 2;
>> -         if (cp[2] == 0)
>> -           break;
>> -         char_ptr = cp + 3;
>> -         if (cp[3] == 0)
>> -           break;
>> -         if (sizeof (longword) > 4)
>> -           {
>> -             char_ptr = cp + 4;
>> -             if (cp[4] == 0)
>> -               break;
>> -             char_ptr = cp + 5;
>> -             if (cp[5] == 0)
>> -               break;
>> -             char_ptr = cp + 6;
>> -             if (cp[6] == 0)
>> -               break;
>> -             char_ptr = cp + 7;
>> -             if (cp[7] == 0)
>> -               break;
>> -           }
>> -       }
>> -      char_ptr = end_ptr;
>> -    }
>> -
>> -  if (char_ptr > end_ptr)
>> -    char_ptr = end_ptr;
>> -  return char_ptr - str;
>> +  const char *found = memchr (str, '\0', maxlen);
> Can be __memchr to skip PLT no?

The internal memchr alias is defined with libc_hidden_builtin_def (memchr), so
calling 'memchr' will issue the internal alias.  For instance, on loongarch 
(which uses generic strnlen):

$ objdump -t string/strnlen.os | grep memchr
0000000000000000         *UND*  0000000000000000 __GI_memchr