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Diffstat (limited to 'gl/rawmemchr.c')
-rw-r--r-- | gl/rawmemchr.c | 136 |
1 files changed, 0 insertions, 136 deletions
diff --git a/gl/rawmemchr.c b/gl/rawmemchr.c deleted file mode 100644 index a0298ce64e..0000000000 --- a/gl/rawmemchr.c +++ /dev/null @@ -1,136 +0,0 @@ -/* Searching in a string. - Copyright (C) 2008-2013 Free Software Foundation, Inc. - - 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 - (at your option) 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> - -/* Specification. */ -#include <string.h> - -/* Find the first occurrence of C in S. */ -void * -rawmemchr (const void *s, int c_in) -{ - /* 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 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; - (size_t) char_ptr % sizeof (longword) != 0; - ++char_ptr) - if (*char_ptr == c) - return (void *) char_ptr; - - longword_ptr = (const longword *) char_ptr; - - /* All these elucidatory comments refer to 4-byte longwords, - but the theory applies equally well to any size longwords. */ - - /* 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 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 NUL or - 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. - - The test whether any byte in longword1 is zero is equivalent - to testing whether tmp is nonzero. - - This test can read beyond the end of a string, depending on where - C_IN is encountered. However, this is considered safe since the - initialization phase ensured that the read will be aligned, - therefore, the read will not cross page boundaries and will not - cause a fault. */ - - while (1) - { - longword longword1 = *longword_ptr ^ repeated_c; - - if ((((longword1 - repeated_one) & ~longword1) - & (repeated_one << 7)) != 0) - break; - longword_ptr++; - } - - char_ptr = (const unsigned char *) longword_ptr; - - /* At this point, we know that 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. */ - - char_ptr = (unsigned char *) longword_ptr; - while (*char_ptr != c) - char_ptr++; - return (void *) char_ptr; -} |