Re: [PATCH v5 08/17] string: Improve generic strchrnul

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

On Mon, Jan 9, 2023 at 12:35 PM Adhemerval Zanella Netto
<adhemerval.zanella@linaro.org> wrote:
>
>
>
> On 05/01/23 20:17, Noah Goldstein wrote:
> > On Mon, Sep 19, 2022 at 1:04 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} functions.
> >>
> >> Checked on x86_64-linux-gnu, i686-linux-gnu, powerpc64-linux-gnu,
> >> and powerpc-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/strchrnul.c                            | 156 +++---------------
> >>  .../power4/multiarch/strchrnul-ppc32.c        |   4 -
> >>  sysdeps/s390/strchrnul-c.c                    |   2 -
> >>  3 files changed, 24 insertions(+), 138 deletions(-)
> >>
> >> diff --git a/string/strchrnul.c b/string/strchrnul.c
> >> index 0cc1fc6bb0..67defa3dab 100644
> >> --- a/string/strchrnul.c
> >> +++ b/string/strchrnul.c
> >> @@ -1,10 +1,5 @@
> >>  /* Copyright (C) 1991-2022 Free Software Foundation, Inc.
> >>     This file is part of the GNU C Library.
> >> -   Based on strlen implementation by Torbjorn Granlund (tege@sics.se),
> >> -   with help from Dan Sahlin (dan@sics.se) and
> >> -   bug fix and commentary by Jim Blandy (jimb@ai.mit.edu);
> >> -   adaptation to strchr suggested by Dick Karpinski (dick@cca.ucsf.edu),
> >> -   and implemented by Roland McGrath (roland@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
> >> @@ -21,146 +16,43 @@
> >>     <https://www.gnu.org/licenses/>.  */
> >>
> >>  #include <string.h>
> >> -#include <memcopy.h>
> >>  #include <stdlib.h>
> >> +#include <stdint.h>
> >> +#include <string-fza.h>
> >> +#include <string-fzb.h>
> >> +#include <string-fzi.h>
> >> +#include <string-maskoff.h>
> >>
> >>  #undef __strchrnul
> >>  #undef strchrnul
> >>
> >> -#ifndef STRCHRNUL
> >> -# define STRCHRNUL __strchrnul
> >> +#ifdef STRCHRNUL
> >> +# define __strchrnul STRCHRNUL
> >>  #endif
> >>
> >>  /* Find the first occurrence of C in S or the final NUL byte.  */
> >>  char *
> >> -STRCHRNUL (const char *s, int c_in)
> >> +__strchrnul (const char *str, 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 || *char_ptr == '\0')
> >> -      return (void *) char_ptr;
> >> -
> >> -  /* 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.  */
> >> +  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) str;
> >> +  const op_t *word_ptr = word_containing (str);
> >>
> >> -        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));
> >
> > Think much clearer (and probably better codegen) is:
> > find_zero_eq_low/all(word, repeated) >> (s_int * CHAR_BIT)
>
> It does not seem to work, at least not replacing the two lines with:
>
>   op_t word = find_zero_eq_all/low (*word_ptr, repeated_c) >> (s_int * CHAR_BIT);
>
> The loader itself can not loader anything (which means strchr is failing
> somewhere).

The following works for me (only checking test-strchrnul, so maybe its missing
a case), also as I have it here its only little endian. (Would need an
ifdef here
or API for shifting out out-of-bounds bits for cross platform).
```
  op_t word = *word_ptr;
  op_t mask = find_zero_eq_low(word, repeated_c) >> (CHAR_BIT * (s_int
% sizeof(uintptr_t)));
  if(mask) {
      return (char *) str + index_first_(mask);
  }

  while (! has_zero_eq (word, repeated_c))
    word = *++word_ptr;

  op_t found = index_first_zero_eq (word, repeated_c);
  return (char *) (word_ptr) + found;
```