Re: [PATCH v5 10/17] string: Improve generic memchr

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

On 05/01/23 20:47, Noah Goldstein wrote:
> On Mon, Sep 19, 2022 at 1:05 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-opthr functions.
>>
>> 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/memchr.c                               | 168 +++++-------------
>>  .../powerpc32/power4/multiarch/memchr-ppc32.c |  14 +-
>>  .../powerpc64/multiarch/memchr-ppc64.c        |   9 +-
>>  3 files changed, 48 insertions(+), 143 deletions(-)
>>
>> diff --git a/string/memchr.c b/string/memchr.c
>> index 422bcd0cd6..08d518b02d 100644
>> --- a/string/memchr.c
>> +++ b/string/memchr.c
>> @@ -1,10 +1,6 @@
>> -/* Copyright (C) 1991-2022 Free Software Foundation, Inc.
>> +/* Scan memory for a character.  Generic version
>> +   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
>> -   commentary by Jim Blandy (jimb@ai.mit.edu);
>> -   adaptation to memchr 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
>> @@ -20,143 +16,65 @@
>>     License along with the GNU C Library; if not, see
>>     <https://www.gnu.org/licenses/>.  */
>>
>> -#ifndef _LIBC
>> -# include <config.h>
>> -#endif
>> -
>> +#include <intprops.h>
>> +#include <string-fza.h>
>> +#include <string-fzb.h>
>> +#include <string-fzi.h>
>> +#include <string-maskoff.h>
>> +#include <string-opthr.h>
>>  #include <string.h>
>>
>> -#include <stddef.h>
>> +#undef memchr
>>
>> -#include <limits.h>
>> -
>> -#undef __memchr
>> -#ifdef _LIBC
>> -# undef memchr
>> +#ifdef MEMCHR
>> +# define __memchr MEMCHR
>>  #endif
>>
>> -#ifndef weak_alias
>> -# define __memchr memchr
>> -#endif
>> -
>> -#ifndef MEMCHR
>> -# define MEMCHR __memchr
>> -#endif
>> +static inline const char *
>> +sadd (uintptr_t x, uintptr_t y)
>> +{
>> +  uintptr_t ret = INT_ADD_OVERFLOW (x, y) ? (uintptr_t)-1 : x + y;
>> +  return (const char *)ret;
>> +}
>>
>>  /* Search no more than N bytes of S for C.  */
>>  void *
>> -MEMCHR (void const *s, int c_in, size_t n)
>> +__memchr (void const *s, int c_in, size_t n)
>>  {
>> -  /* On 32-bit hardware, choosing longword to be a 32-bit unsigned
>> -     long instead of a 64-bit uintmax_t tends to give better
>> -     performance.  On 64-bit hardware, unsigned long is generally 64
>> -     bits already.  Change this typedef to experiment with
>> -     performance.  */
>> -  typedef unsigned long int longword;
>> +  if (__glibc_unlikely (n == 0))
>> +    return NULL;
>>
>> -  const unsigned char *char_ptr;
>> -  const longword *longword_ptr;
>> -  longword repeated_one;
>> -  longword repeated_c;
>> -  unsigned char c;
>> +  uintptr_t s_int = (uintptr_t) s;
>>
>> -  c = (unsigned char) c_in;
>> +  /* Set up a word, each of whose bytes is C.  */
>> +  op_t repeated_c = repeat_bytes (c_in);
>> +  op_t before_mask = create_mask (s_int);
>>
>> -  /* Handle the first few bytes by reading one byte at a time.
>> -     Do this until CHAR_PTR is aligned on a longword boundary.  */
>> -  for (char_ptr = (const unsigned char *) s;
>> -       n > 0 && (size_t) char_ptr % sizeof (longword) != 0;
>> -       --n, ++char_ptr)
>> -    if (*char_ptr == c)
>> -      return (void *) char_ptr;
>> +  /* Compute the address of the last byte taking in consideration possible
>> +     overflow.  */
>> +  const char *lbyte = sadd (s_int, n - 1);
> 
> Do you need this? The comparison in the loop is == so letting it
> overflow should be fine no?

Do you mean the saturation add or the last lbyte check? For saturation add
I recall that it requires for memchr (..., SIZE_MAX), otherwise the last
byte/word would be incorrect (I fixed some assembly routines that triggered
this issue in the past).

>>
>> -  longword_ptr = (const longword *) char_ptr;
>> +  /* Compute the address of the word containing the last byte. */
>> +  const op_t *lword = word_containing (lbyte);
>>
>> -  /* All these elucidatory comments refer to 4-byte longwords,
>> -     but the theory applies equally well to any size longwords.  */
>> +  /* Read the first word, but munge it so that bytes before the array
>> +     will not match goal.  */
>> +  const op_t *word_ptr = word_containing (s);
>> +  op_t word = (*word_ptr | before_mask) ^ (repeated_c & before_mask);
> 
> Likewise, prefer just shifting out the invalid comparisons on the first word.

I will need to check why this is not really working, I think I suggest it
on previous iteration and I could not make it work for some reason.

>>
>> -  /* Compute auxiliary longword values:
>> -     repeated_one is a value which has a 1 in every byte.
>> -     repeated_c has c in every byte.  */
>> -  repeated_one = 0x01010101;
>> -  repeated_c = c | (c << 8);
>> -  repeated_c |= repeated_c << 16;
>> -  if (0xffffffffU < (longword) -1)
>> +  while (has_eq (word, repeated_c) == 0)
>>      {
>> -      repeated_one |= repeated_one << 31 << 1;
>> -      repeated_c |= repeated_c << 31 << 1;
>> -      if (8 < sizeof (longword))
>> -       {
>> -         size_t i;
>> -
>> -         for (i = 64; i < sizeof (longword) * 8; i *= 2)
>> -           {
>> -             repeated_one |= repeated_one << i;
>> -             repeated_c |= repeated_c << i;
>> -           }
>> -       }
>> +      if (word_ptr == lword)
>> +       return NULL;
>> +      word = *++word_ptr;
>>      }
>>
>> -  /* Instead of the traditional loop which tests each byte, 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 equal to c.  We first use an xor
>> -     with repeated_c.  This reduces the task to testing whether *any of the
>> -     four* bytes in longword1 is zero.
>> -
>> -     We compute tmp =
>> -       ((longword1 - repeated_one) & ~longword1) & (repeated_one << 7).
>> -     That is, we perform the following operations:
>> -       1. Subtract repeated_one.
>> -       2. & ~longword1.
>> -       3. & a mask consisting of 0x80 in every byte.
>> -     Consider what happens in each byte:
>> -       - If a byte of longword1 is zero, step 1 and 2 transform it into 0xff,
>> -        and step 3 transforms it into 0x80.  A carry can also be propagated
>> -        to more significant bytes.
>> -       - If a byte of longword1 is nonzero, let its lowest 1 bit be at
>> -        position k (0 <= k <= 7); so the lowest k bits are 0.  After step 1,
>> -        the byte ends in a single bit of value 0 and k bits of value 1.
>> -        After step 2, the result is just k bits of value 1: 2^k - 1.  After
>> -        step 3, the result is 0.  And no carry is produced.
>> -     So, if longword1 has only non-zero bytes, tmp is zero.
>> -     Whereas if longword1 has a zero byte, call j the position of the least
>> -     significant zero byte.  Then the result has a zero at positions 0, ...,
>> -     j-1 and a 0x80 at position j.  We cannot predict the result at the more
>> -     significant bytes (positions j+1..3), but it does not matter since we
>> -     already have a non-zero bit at position 8*j+7.
>> -
>> -     So, the test whether any byte in longword1 is zero is equivalent to
>> -     testing whether tmp is nonzero.  */
>> -
>> -  while (n >= sizeof (longword))
>> -    {
>> -      longword longword1 = *longword_ptr ^ repeated_c;
>> -
>> -      if ((((longword1 - repeated_one) & ~longword1)
>> -          & (repeated_one << 7)) != 0)
>> -       break;
>> -      longword_ptr++;
>> -      n -= sizeof (longword);
>> -    }
>> -
>> -  char_ptr = (const unsigned char *) longword_ptr;
>> -
>> -  /* At this point, we know that either n < sizeof (longword), or one of the
>> -     sizeof (longword) bytes starting at char_ptr is == c.  On little-endian
>> -     machines, we could determine the first such byte without any further
>> -     memory accesses, just by looking at the tmp result from the last loop
>> -     iteration.  But this does not work on big-endian machines.  Choose code
>> -     that works in both cases.  */
>> -
>> -  for (; n > 0; --n, ++char_ptr)
>> -    {
>> -      if (*char_ptr == c)
>> -       return (void *) char_ptr;
>> -    }
>> -
>> -  return NULL;
>> +  /* We found a match, but it might be in a byte past the end
>> +     of the array.  */
>> +  char *ret = (char *) word_ptr + index_first_eq (word, repeated_c);
>> +  return (ret <= lbyte) ? ret : NULL;
>>  }
>> -#ifdef weak_alias
>> +#ifndef MEMCHR
>>  weak_alias (__memchr, memchr)
>> -#endif
>>  libc_hidden_builtin_def (memchr)
>> +#endif
>> diff --git a/sysdeps/powerpc/powerpc32/power4/multiarch/memchr-ppc32.c b/sysdeps/powerpc/powerpc32/power4/multiarch/memchr-ppc32.c
>> index fc69df54b3..02877d3c98 100644
>> --- a/sysdeps/powerpc/powerpc32/power4/multiarch/memchr-ppc32.c
>> +++ b/sysdeps/powerpc/powerpc32/power4/multiarch/memchr-ppc32.c
>> @@ -18,17 +18,11 @@
>>
>>  #include <string.h>
>>
>> -#define MEMCHR  __memchr_ppc
>> +extern __typeof (memchr) __memchr_ppc attribute_hidden;
>>
>> -#undef weak_alias
>> -#define weak_alias(a, b)
>> +#define MEMCHR  __memchr_ppc
>> +#include <string/memchr.c>
>>
>>  #ifdef SHARED
>> -# undef libc_hidden_builtin_def
>> -# define libc_hidden_builtin_def(name) \
>> -  __hidden_ver1(__memchr_ppc, __GI_memchr, __memchr_ppc);
>> +__hidden_ver1(__memchr_ppc, __GI_memchr, __memchr_ppc);
>>  #endif
>> -
>> -extern __typeof (memchr) __memchr_ppc attribute_hidden;
>> -
>> -#include <string/memchr.c>
>> diff --git a/sysdeps/powerpc/powerpc64/multiarch/memchr-ppc64.c b/sysdeps/powerpc/powerpc64/multiarch/memchr-ppc64.c
>> index 3c966f4403..15beca787b 100644
>> --- a/sysdeps/powerpc/powerpc64/multiarch/memchr-ppc64.c
>> +++ b/sysdeps/powerpc/powerpc64/multiarch/memchr-ppc64.c
>> @@ -18,14 +18,7 @@
>>
>>  #include <string.h>
>>
>> -#define MEMCHR  __memchr_ppc
>> -
>> -#undef weak_alias
>> -#define weak_alias(a, b)
>> -
>> -# undef libc_hidden_builtin_def
>> -# define libc_hidden_builtin_def(name)
>> -
>>  extern __typeof (memchr) __memchr_ppc attribute_hidden;
>>
>> +#define MEMCHR  __memchr_ppc
>>  #include <string/memchr.c>
>> --
>> 2.34.1
>>