Improve generic rawmemchr

  ping


From: Wilco Dijkstra
Sent: 16 November 2016 18:53
To: libc-alpha@sourceware.org
Cc: nd
Subject: [PATCH] Improve generic rawmemchr
    
Improve generic rawmemchr for targets that don't have an
assembler version by tailcalling memchr with the maximum size.
If a target has an optimized memchr this is significantly faster
(~3x on AArch64), if not, then this makes little difference.
Also optimize the special case of zero to use strlen as this is
typically faster than memchr.

ChangeLog:
2015-11-16  Wilco Dijkstra  <wdijkstr@arm.com>

        * string/rawmemchr.c (RAWMEMCHR): Use faster memchr/strlen.
--
  

LGMT [1].

[1] https://sourceware.org/ml/libc-alpha/2016-11/msg00989.html

On 13/12/2016 10:49, Wilco Dijkstra wrote:
> ping
> 
> 
> From: Wilco Dijkstra
> Sent: 16 November 2016 18:53
> To: libc-alpha@sourceware.org
> Cc: nd
> Subject: [PATCH] Improve generic rawmemchr
>     
> Improve generic rawmemchr for targets that don't have an
> assembler version by tailcalling memchr with the maximum size.
> If a target has an optimized memchr this is significantly faster
> (~3x on AArch64), if not, then this makes little difference.
> Also optimize the special case of zero to use strlen as this is
> typically faster than memchr.
> 
> ChangeLog:
> 2015-11-16  Wilco Dijkstra  <wdijkstr@arm.com>
> 
>         * string/rawmemchr.c (RAWMEMCHR): Use faster memchr/strlen.
> --
> 
> diff --git a/string/rawmemchr.c b/string/rawmemchr.c
> index fa3176d6ac7e25490be415af0459807509d1e02b..1a146af980619ac9a37a3c9d8df3917e7ce5db12 100644
> --- a/string/rawmemchr.c
> +++ b/string/rawmemchr.c
> @@ -1,10 +1,5 @@
>  /* Copyright (C) 1991-2016 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
> @@ -20,157 +15,19 @@
>     License along with the GNU C Library; if not, see
>     <http://www.gnu.org/licenses/>.  */
>  
> -#ifdef HAVE_CONFIG_H
> -#include <config.h>
> -#endif
> -
> -#undef __ptr_t
> -#define __ptr_t void *
> -
> -#if defined (_LIBC)
> -# include <string.h>
> -# include <memcopy.h>
> -# include <stdlib.h>
> -#endif
> -
> -#if defined (HAVE_LIMITS_H) || defined (_LIBC)
> -# include <limits.h>
> -#endif
> -
> -#define LONG_MAX_32_BITS 2147483647
> -
> -#ifndef LONG_MAX
> -#define LONG_MAX LONG_MAX_32_BITS
> -#endif
> -
> -#include <sys/types.h>
> -
> -#undef memchr
> +#include <string.h>
>  
>  #ifndef RAWMEMCHR
>  # define RAWMEMCHR __rawmemchr
>  #endif
>  
>  /* Find the first occurrence of C in S.  */
> -__ptr_t
> -RAWMEMCHR (const __ptr_t s, int c_in)
> +void *
> +RAWMEMCHR (const void *s, int c)
>  {
> -  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 (__ptr_t) 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 LONG_MAX > LONG_MAX_32_BITS
> -  charmask |= charmask << 32;
> -#endif
> -
> -  /* 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.  */
> -  while (1)
> -    {
> -      /* 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.
> -
> -        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.
> -
> -        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!
> -
> -        So it ignores everything except 128's, when they're aligned
> -        properly.
> -
> -        3) But wait!  Aren't we looking for C, not 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++ ^ charmask;
> -
> -      /* 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)
> -       {
> -         /* Which of the bytes was C?  If none of them were, it was
> -            a misfire; continue the search.  */
> -
> -         const unsigned char *cp = (const unsigned char *) (longword_ptr - 1);
> -
> -         if (cp[0] == c)
> -           return (__ptr_t) cp;
> -         if (cp[1] == c)
> -           return (__ptr_t) &cp[1];
> -         if (cp[2] == c)
> -           return (__ptr_t) &cp[2];
> -         if (cp[3] == c)
> -           return (__ptr_t) &cp[3];
> -#if LONG_MAX > 2147483647
> -         if (cp[4] == c)
> -           return (__ptr_t) &cp[4];
> -         if (cp[5] == c)
> -           return (__ptr_t) &cp[5];
> -         if (cp[6] == c)
> -           return (__ptr_t) &cp[6];
> -         if (cp[7] == c)
> -           return (__ptr_t) &cp[7];
> -#endif
> -       }
> -    }
> +  if (c != '\0')
> +    return memchr (s, c, (size_t)-1);
> +  return (char *)s + strlen (s);
>  }
>  libc_hidden_def (__rawmemchr)
>  weak_alias (__rawmemchr, rawmemchr)
> 
>     
>