Re: [PATCH v5 07/17] string: Improve generic strchr

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

On 05/01/23 20:19, Noah Goldstein wrote:
> On Thu, Jan 5, 2023 at 3:09 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>>
>> On Mon, Sep 19, 2022 at 1:01 PM Adhemerval Zanella via Libc-alpha
>> <libc-alpha@sourceware.org> wrote:
>>>
>>> New algorithm have the following key differences:
>>>
>>>   - Reads first word unaligned and use string-maskoff function to
>>>     remove unwanted data.  This strategy follow arch-specific
>>>     optimization used on aarch64 and powerpc.
>>>
>>>   - Use string-fz{b,i} and string-extbyte function.
>>>
>>> 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).
>>>
>>> Co-authored-by: Richard Henderson  <rth@twiddle.net>
>>> ---
>>>  string/strchr.c         | 172 +++++++---------------------------------
>>>  sysdeps/s390/strchr-c.c |  11 +--
>>>  2 files changed, 34 insertions(+), 149 deletions(-)
>>>
>>> diff --git a/string/strchr.c b/string/strchr.c
>>> index bfd0c4e4bc..6bbee7f79d 100644
>>> --- a/string/strchr.c
>>> +++ b/string/strchr.c
>>> @@ -22,164 +22,48 @@
>>>
>>>  #include <string.h>
>>>  #include <stdlib.h>
>>> +#include <stdint.h>
>>> +#include <string-fza.h>
>>> +#include <string-fzb.h>
>>> +#include <string-fzi.h>
>>> +#include <string-extbyte.h>
>>> +#include <string-maskoff.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.
>>> +  /* Set up a word, each of whose bytes is C.  */
>>> +  unsigned char c = (unsigned char) c_in;
>>> +  op_t repeated_c = repeat_bytes (c_in);
>>>
>>> -        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.
>>> +  /* Align the input address to op_t.  */
>>> +  uintptr_t s_int = (uintptr_t) s;
>>> +  const op_t *word_ptr = word_containing (s);
>>>
>>> -        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!
>>> +  /* Read the first aligned word, but force bytes before the string to
>>> +     match neither zero nor goal (we make sure the high bit of each byte
>>> +     is 1, and the low 7 bits are all the opposite of the goal byte).  */
>>> +  op_t bmask = create_mask (s_int);
>>> +  op_t word = (*word_ptr | bmask) ^ (repeated_c & highbit_mask (bmask));
>>>
>>> -        So it ignores everything except 128's, when they're aligned
>>> -        properly.
>>> +  while (! has_zero_eq (word, repeated_c))
>>> +    word = *++word_ptr;
>>>
>>> -        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;
>>> -           }
>>> -       }
>>> -    }
>>> +  op_t found = index_first_zero_eq (word, repeated_c);
>>>
>>> +  if (extractbyte (word, found) == c)
>>> +    return (char *) (word_ptr) + found;
>>>    return 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 4ac3a62fba..a5a1781b1c 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
>>>
>>
>> Can this just be implemented as:
>>
>> char * r = strchrnul(p, c);
>> return *r ? r : NULL;
> Thats wrong, should be: `return (*r == c) ? r : NULL;`
>>
>> then only have strchrnul impl to worry about?

Yes, although I think strchr is a more used symbol than strchrnul. However, 
we can optimize it later by adding a __strchrnul_inline and expand it 
on both strchr and strchrnul.  I will change to use strchrnul as you suggested.