Re: [PATCH v11 06/29] string: Improve generic strchr

[Noah Goldstein <goldstein.w.n@gmail.com>]

On Wed, Feb 1, 2023 at 11:04 AM Adhemerval Zanella
<adhemerval.zanella@linaro.org> wrote:
>
> New algorithm now calls strchrnul.
>
> Checked on x86_64-linux-gnu, i686-linux-gnu, powerpc-linux-gnu,
> and powerpc64-linux-gnu by removing the arch-specific assembly
> implementation and disabling multi-arch (it covers both LE and BE
> for 64 and 32 bits).
>
> Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>  string/strchr.c         | 159 ++--------------------------------------
>  sysdeps/s390/strchr-c.c |  11 +--
>  2 files changed, 14 insertions(+), 156 deletions(-)
>
> diff --git a/string/strchr.c b/string/strchr.c
> index 1572b8b42e..30c3eb10f2 100644
> --- a/string/strchr.c
> +++ b/string/strchr.c
> @@ -21,165 +21,22 @@
>     <https://www.gnu.org/licenses/>.  */
>
>  #include <string.h>
> -#include <stdlib.h>
>
>  #undef strchr
> +#undef index
>
> -#ifndef STRCHR
> -# define STRCHR strchr
> +#ifdef STRCHR
> +# define strchr STRCHR
>  #endif
>
>  /* Find the first occurrence of C in S.  */
>  char *
> -STRCHR (const char *s, int c_in)
> +strchr (const char *s, int c_in)
>  {
> -  const unsigned char *char_ptr;
> -  const unsigned long int *longword_ptr;
> -  unsigned long int longword, magic_bits, charmask;
> -  unsigned char c;
> -
> -  c = (unsigned char) c_in;
> -
> -  /* 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 = (const unsigned char *) s;
> -       ((unsigned long int) char_ptr & (sizeof (longword) - 1)) != 0;
> -       ++char_ptr)
> -    if (*char_ptr == c)
> -      return (void *) char_ptr;
> -    else if (*char_ptr == '\0')
> -      return NULL;
> -
> -  /* 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.  */
> -  magic_bits = -1;
> -  magic_bits = magic_bits / 0xff * 0xfe << 1 >> 1 | 1;
> -
> -  /* Set up a longword, each of whose bytes is C.  */
> -  charmask = c | (c << 8);
> -  charmask |= charmask << 16;
> -  if (sizeof (longword) > 4)
> -    /* Do the shift in two steps to avoid a warning if long has 32 bits.  */
> -    charmask |= (charmask << 16) << 16;
> -  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.  */
> -  for (;;)
> -    {
> -      /* 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.
> -
> -        3) But wait!  Aren't we looking for C as well as zero?
> -        Good point.  So what we do is XOR LONGWORD with a longword,
> -        each of whose bytes is C.  This turns each byte that is C
> -        into a zero.  */
> -
> -      longword = *longword_ptr++;
> -
> -      /* Add MAGIC_BITS to LONGWORD.  */
> -      if ((((longword + magic_bits)
> -
> -           /* Set those bits that were unchanged by the addition.  */
> -           ^ ~longword)
> -
> -          /* Look at only the hole bits.  If any of the hole bits
> -             are unchanged, most likely one of the bytes was a
> -             zero.  */
> -          & ~magic_bits) != 0
> -
> -         /* That caught zeroes.  Now test for C.  */
> -         || ((((longword ^ charmask) + magic_bits) ^ ~(longword ^ charmask))
> -             & ~magic_bits) != 0)
> -       {
> -         /* Which of the bytes was C or zero?
> -            If none of them were, it was a misfire; continue the search.  */
> -
> -         const unsigned char *cp = (const unsigned char *) (longword_ptr - 1);
> -
> -         if (*cp == c)
> -           return (char *) cp;
> -         else if (*cp == '\0')
> -           return NULL;
> -         if (*++cp == c)
> -           return (char *) cp;
> -         else if (*cp == '\0')
> -           return NULL;
> -         if (*++cp == c)
> -           return (char *) cp;
> -         else if (*cp == '\0')
> -           return NULL;
> -         if (*++cp == c)
> -           return (char *) cp;
> -         else if (*cp == '\0')
> -           return NULL;
> -         if (sizeof (longword) > 4)
> -           {
> -             if (*++cp == c)
> -               return (char *) cp;
> -             else if (*cp == '\0')
> -               return NULL;
> -             if (*++cp == c)
> -               return (char *) cp;
> -             else if (*cp == '\0')
> -               return NULL;
> -             if (*++cp == c)
> -               return (char *) cp;
> -             else if (*cp == '\0')
> -               return NULL;
> -             if (*++cp == c)
> -               return (char *) cp;
> -             else if (*cp == '\0')
> -               return NULL;
> -           }
> -       }
> -    }
> -
> -  return NULL;
> +  char *r = __strchrnul (s, c_in);
> +  return (*(unsigned char *)r == (unsigned char)c_in) ? r : NULL;
>  }
> -
> -#ifdef weak_alias
> -# undef index
> +#ifndef STRCHR
>  weak_alias (strchr, index)
> -#endif
>  libc_hidden_builtin_def (strchr)
> +#endif
> diff --git a/sysdeps/s390/strchr-c.c b/sysdeps/s390/strchr-c.c
> index c00f2cceea..90822ae0f4 100644
> --- a/sysdeps/s390/strchr-c.c
> +++ b/sysdeps/s390/strchr-c.c
> @@ -21,13 +21,14 @@
>  #if HAVE_STRCHR_C
>  # if HAVE_STRCHR_IFUNC
>  #  define STRCHR STRCHR_C
> -#  undef weak_alias
> +# endif
> +
> +# include <string/strchr.c>
> +
> +# if HAVE_STRCHR_IFUNC
>  #  if defined SHARED && IS_IN (libc)
> -#   undef libc_hidden_builtin_def
> -#   define libc_hidden_builtin_def(name)                       \
> -     __hidden_ver1 (__strchr_c, __GI_strchr, __strchr_c);
> +__hidden_ver1 (__strchr_c, __GI_strchr, __strchr_c);
>  #  endif
>  # endif
>
> -# include <string/strchr.c>
>  #endif
> --
> 2.34.1
>
NIT: Imo this patch should come after the strchrnul one.