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

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

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).