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

[Richard Henderson <rth@twiddle.net>]

On 1/9/23 12:35, Adhemerval Zanella Netto 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).

You'd need to update the extract arithmetic too.  I'm sure it's still using ctz and the 
aligned pointer.  Which might be why I used the slightly more complex arithmetic here, so 
that the tail of the function didn't need changing...


r~