Improve generic rawmemchr
Commit Message
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.
--
Comments
On Wed, Nov 16, 2016 at 06:53:03PM +0000, Wilco Dijkstra wrote:
> 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.
> --
>
looks ok.
LGTM.
On 16/11/2016 16:53, Wilco Dijkstra wrote:
> 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)
>
@@ -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)