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author | Eric Blake <ebb9@byu.net> | 2008-03-01 06:54:29 -0700 |
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committer | Eric Blake <ebb9@byu.net> | 2008-03-01 06:54:29 -0700 |
commit | f91b9f973226f2b434ba24e32845f252a3d6e64b (patch) | |
tree | 50642406adf757906b8fb656f493e18c9551c02f /lib/memchr2.c | |
parent | 9a51440f6b90dd7eeba4758587af5d2d8d9ddb4b (diff) | |
download | gnulib-f91b9f973226f2b434ba24e32845f252a3d6e64b.tar.gz |
New module 'memchr2'.
* modules/memchr2: New file.
* modules/memchr2-tests: Likewise.
* lib/memchr2.h: Likewise.
* lib/memchr2.c: Likewise, based on memchr.c.
* tests/test-memchr2.c: New test.
* MODULES.html.sh (String handling): Add memchr2.
Signed-off-by: Eric Blake <ebb9@byu.net>
Diffstat (limited to 'lib/memchr2.c')
-rw-r--r-- | lib/memchr2.c | 194 |
1 files changed, 194 insertions, 0 deletions
diff --git a/lib/memchr2.c b/lib/memchr2.c new file mode 100644 index 0000000000..540ed9fc3b --- /dev/null +++ b/lib/memchr2.c @@ -0,0 +1,194 @@ +/* Copyright (C) 1991, 1993, 1996, 1997, 1999, 2000, 2003, 2004, 2006, + 2008 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 in glibc by Roland McGrath (roland@ai.mit.edu). + Extension to memchr2 implemented by Eric Blake (ebb9@byu.net). + +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. + +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 General Public License for more details. + +You should have received a copy of the GNU General Public License +along with this program. If not, see <http://www.gnu.org/licenses/>. */ + +#include <config.h> + +#include "memchr2.h" + +#include <limits.h> +#include <stdint.h> +#include <string.h> + +/* Return the first address of either C1 or C2 (treated as unsigned + char) that occurs within N bytes of the memory region S. If + neither byte appears, return NULL. */ +void * +memchr2 (void const *s, int c1_in, int c2_in, size_t n) +{ + const unsigned char *char_ptr; + const uintmax_t *longword_ptr; + uintmax_t longword1; + uintmax_t longword2; + uintmax_t magic_bits; + uintmax_t charmask1; + uintmax_t charmask2; + unsigned char c1; + unsigned char c2; + int i; + + c1 = (unsigned char) c1_in; + c2 = (unsigned char) c2_in; + + if (c1 == c2) + return memchr (s, c1, n); + + /* 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; + n > 0 && (size_t) char_ptr % sizeof longword1 != 0; + --n, ++char_ptr) + if (*char_ptr == c1 || *char_ptr == c2) + return (void *) char_ptr; + + /* All these elucidatory comments refer to 4-byte longwords, + but the theory applies equally well to any size longwords. */ + + longword_ptr = (const uintmax_t *) 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. */ + + /* Set MAGIC_BITS to be this pattern of 1 and 0 bits. + Set CHARMASK to be a longword, each of whose bytes is C. */ + + magic_bits = 0xfefefefe; + charmask1 = c1 | (c1 << 8); + charmask2 = c2 | (c2 << 8); + charmask1 |= charmask2 << 16; + charmask1 |= charmask2 << 16; +#if 0xffffffffU < UINTMAX_MAX + magic_bits |= magic_bits << 32; + charmask1 |= charmask1 << 32; + charmask2 |= charmask2 << 32; + if (8 < sizeof longword1) + for (i = 64; i < sizeof longword1 * 8; i *= 2) + { + magic_bits |= magic_bits << i; + charmask1 |= charmask1 << i; + charmask2 |= charmask2 << i; + } +#endif + magic_bits = (UINTMAX_MAX >> 1) & (magic_bits | 1); + + /* 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 >= sizeof longword1) + { + /* 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. */ + + longword1 = *longword_ptr ^ charmask1; + longword2 = *longword_ptr++ ^ charmask2; + + /* Add MAGIC_BITS to LONGWORD. */ + if ((((longword1 + magic_bits) + + /* Set those bits that were unchanged by the addition. */ + ^ ~longword1) + + /* 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 + || (((longword2 + magic_bits) ^ ~longword2) & ~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] == c1 || cp[0] == c2) + return (void *) cp; + if (cp[1] == c1 || cp[1] == c2) + return (void *) &cp[1]; + if (cp[2] == c1 || cp[2] == c2) + return (void *) &cp[2]; + if (cp[3] == c1 || cp[3] == c2) + return (void *) &cp[3]; + if (4 < sizeof longword1 && (cp[4] == c1 || cp[4] == c2)) + return (void *) &cp[4]; + if (5 < sizeof longword1 && (cp[5] == c1 || cp[5] == c2)) + return (void *) &cp[5]; + if (6 < sizeof longword1 && (cp[6] == c1 || cp[6] == c2)) + return (void *) &cp[6]; + if (7 < sizeof longword1 && (cp[7] == c1 || cp[7] == c2)) + return (void *) &cp[7]; + if (8 < sizeof longword1) + for (i = 8; i < sizeof longword1; i++) + if (cp[i] == c1 || cp[i] == c2) + return (void *) &cp[i]; + } + + n -= sizeof longword1; + } + + char_ptr = (const unsigned char *) longword_ptr; + + while (n-- > 0) + { + if (*char_ptr == c1 || *char_ptr == c2) + return (void *) char_ptr; + ++char_ptr; + } + + return 0; +} |