Mercurial > gnulib
view lib/ffs.c @ 40086:d6de7c427079
stdopen: modernize and simplify
* lib/stdopen.c: Update copyright date
Do not include sys/types.h; no longer needed these days.
(stdopen): Use C99-style decl in loop. Return int errno
value, rather than just a bool. Do not worry about fd mismatches,
since the caller cares only if 0, 1, 2 are occupied.
* lib/stdopen.h: No need to include <stdbool.h>.
* m4/stdopen.m4: Remove.
* modules/stdopen: New file.
author | Paul Eggert <eggert@cs.ucla.edu> |
---|---|
date | Sat, 05 Jan 2019 18:02:32 -0800 |
parents | b06060465f09 |
children |
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/* ffs.c -- find the first set bit in a word. Copyright (C) 2011-2019 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 <https://www.gnu.org/licenses/>. */ /* Written by Eric Blake. */ #include <config.h> /* Specification. */ #include <strings.h> #include <limits.h> int ffs (int i) { #if __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4) return __builtin_ffs (i); #else /* <https://github.com/gibsjose/BitHacks> gives this deBruijn constant for a branch-less computation, although that table counted trailing zeros rather than bit position. This requires 32-bit int, we fall back to a naive algorithm on the rare platforms where that assumption is not true. */ if (CHAR_BIT * sizeof i == 32) { static unsigned int table[] = { 1, 2, 29, 3, 30, 15, 25, 4, 31, 23, 21, 16, 26, 18, 5, 9, 32, 28, 14, 24, 22, 20, 17, 8, 27, 13, 19, 7, 12, 6, 11, 10 }; unsigned int u = i; unsigned int bit = u & -u; return table[(bit * 0x077cb531U) >> 27] - !i; } else { unsigned int j; for (j = 0; j < CHAR_BIT * sizeof i; j++) if (i & (1U << j)) return j + 1; return 0; } #endif }