X-Git-Url: http://erislabs.net/gitweb/?a=blobdiff_plain;f=lib%2Fmemchr.c;h=6c2b2d6c7dc15cdc1bf15664653cdaecae7e18a7;hb=ec919e79c7278b5b69aced77c453e5b165accfcf;hp=cb8d4a22acd6e9cc3d810957c0f0c71fa8ce68ac;hpb=87e2683e04a8ef7d18ea8f515e65d8885e5a2355;p=gnulib.git diff --git a/lib/memchr.c b/lib/memchr.c index cb8d4a22a..6c2b2d6c7 100644 --- a/lib/memchr.c +++ b/lib/memchr.c @@ -1,145 +1,172 @@ -/* Copyright (C) 1991 Free Software Foundation, Inc. - Based on strlen implemention by Torbjorn Granlund (tege@sics.se), +/* Copyright (C) 1991, 1993, 1996-1997, 1999-2000, 2003-2004, 2006, 2008-2010 + Free Software Foundation, Inc. + + 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 Library General Public License as -published by the Free Software Foundation; either version 2 of the -License, or (at your option) any later version. +NOTE: The canonical source of this file is maintained with the GNU C Library. +Bugs can be reported to bug-glibc@prep.ai.mit.edu. + +This program is free software: you can redistribute it and/or modify it +under the terms of the GNU General Public License as published by the +Free Software Foundation; either version 3 of the License, or any +later version. -The GNU C Library is distributed in the hope that it will be useful, +This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of -MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU -Library General Public License for more details. +MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the +GNU General Public License for more details. -You should have received a copy of the GNU Library General Public -License along with the GNU C Library; see the file COPYING.LIB. If -not, write to the Free Software Foundation, Inc., 675 Mass Ave, -Cambridge, MA 02139, USA. */ +You should have received a copy of the GNU General Public License +along with this program. If not, see . */ +#ifndef _LIBC +# include +#endif +#include +#include -/* Search no more than N bytes of S for C. */ +#if defined _LIBC +# include +#else +# define reg_char char +#endif -char * -memchr(s, c, n) - unsigned char * s ; - int c ; - unsigned n; -{ - unsigned char *char_ptr; - unsigned long int *longword_ptr; - unsigned long int longword, magic_bits, charmask; +#include - c = (unsigned char) c; +#if HAVE_BP_SYM_H || defined _LIBC +# include +#else +# define BP_SYM(sym) sym +#endif - /* Handle the first few characters by reading one character at a time. - Do this until CHAR_PTR is aligned on a 4-byte border. */ - for (char_ptr = s; n > 0 && ((unsigned long int) char_ptr & 3) != 0; +#undef __memchr +#ifdef _LIBC +# undef memchr +#endif + +#ifndef weak_alias +# define __memchr memchr +#endif + +/* Search no more than N bytes of S for C. */ +void * +__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; + + const unsigned char *char_ptr; + const longword *longword_ptr; + longword repeated_one; + longword repeated_c; + unsigned reg_char c; + + c = (unsigned char) c_in; + + /* 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 (char *) char_ptr; - - longword_ptr = (unsigned long int *) char_ptr; + return (void *) 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 + longword_ptr = (const longword *) char_ptr; - 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 = 0x7efefeff; + /* All these elucidatory comments refer to 4-byte longwords, + but the theory applies equally well to any size longwords. */ - /* Set up a longword, each of whose bytes is C. */ - charmask = c | (c << 8); - charmask |= charmask << 16; + /* 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) + { + 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; + } + } + } - /* 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 (n >= 4) + /* 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)) { - /* 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. */ - - unsigned char *cp = ( unsigned char *) (longword_ptr - 1); - - if (cp[0] == c) - return (char *) cp; - if (cp[1] == c) - return (char *) &cp[1]; - if (cp[2] == c) - return (char *) &cp[2]; - if (cp[3] == c) - return (char *) &cp[3]; - } - - n -= 4; + longword longword1 = *longword_ptr ^ repeated_c; + + if ((((longword1 - repeated_one) & ~longword1) + & (repeated_one << 7)) != 0) + break; + longword_ptr++; + n -= sizeof (longword); } - char_ptr = ( unsigned char *) longword_ptr; + 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. */ - while (n-- > 0) + for (; n > 0; --n, ++char_ptr) { if (*char_ptr == c) - return (char *) char_ptr; - else - ++char_ptr; + return (void *) char_ptr; } - return 0; + return NULL; } +#ifdef weak_alias +weak_alias (__memchr, BP_SYM (memchr)) +#endif