blob: 68d5d48daf21eb4dc5d19670b1fcb861b41700d8 [file] [log] [blame]
 /* ffs.c -- find the first set bit in a word. Copyright (C) 2011-2021 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 . */ /* Written by Eric Blake. */ #include /* Specification. */ #include #include #if defined _MSC_VER && !(__clang_major__ >= 4) # include #endif int ffs (int i) { #if __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4) || (__clang_major__ >= 4) return __builtin_ffs (i); #elif defined _MSC_VER /* _BitScanForward */ unsigned long bit; if (_BitScanForward (&bit, i)) return bit + 1; else return 0; #else /* 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 }