diff --git a/include/bitops-emulated/generic-ilog2.h b/include/bitops-emulated/generic-ilog2.h new file mode 100644 index 0000000..c3a9aad --- /dev/null +++ b/include/bitops-emulated/generic-ilog2.h @@ -0,0 +1,60 @@ +/* generic-ilog2.h - generic ilog2 implementations. + * + * Copyright (C) 2024 Bruno Raoult ("br") + * Licensed under the GNU General Public License v3.0 or later. + * Some rights reserved. See COPYING. + * + * You should have received a copy of the GNU General Public License along with this + * program. If not, see . + * + * SPDX-License-Identifier: GPL-3.0-or-later + * + */ +#ifndef _GENERIC_ILOG2_H_ +#define _GENERIC_ILOG2_H_ + +#include "br.h" + +/* + * See https://stackoverflow.com/a/11398748/3079831 + */ +static const int __ilog2_32_table[32] = { + 0, 9, 1, 10, 13, 21, 2, 29, + 11, 14, 16, 18, 22, 25, 3, 30, + 8, 12, 20, 28, 15, 17, 24, 7, + 19, 27, 23, 6, 26, 5, 4, 31 +}; + +static const int __ilog2_64_table[64] = { + 63, 0, 58, 1, 59, 47, 53, 2, + 60, 39, 48, 27, 54, 33, 42, 3, + 61, 51, 37, 40, 49, 18, 28, 20, + 55, 30, 34, 11, 43, 14, 22, 4, + 62, 57, 46, 52, 38, 26, 32, 41, + 50, 36, 17, 19, 29, 10, 13, 21, + 56, 45, 25, 31, 35, 16, 9, 12, + 44, 24, 15, 8, 23, 7, 6, 5 +}; + +static __always_inline __const int __ilog2_32_emulated (u32 n) +{ + n |= n >> 1; + n |= n >> 2; + n |= n >> 4; + n |= n >> 8; + n |= n >> 16; + return __ilog2_32_table[(n*0x07C4ACDD) >> 27]; +} + +static __always_inline __const int __ilog2_64_emulated (u64 n) +{ + n |= n >> 1; + n |= n >> 2; + n |= n >> 4; + n |= n >> 8; + n |= n >> 16; + n |= n >> 32; + return __ilog2_64_table[((n - (n >> 1))*0x07EDD5E59A4E28C2LL) >> 58]; +} + +#endif /* _GENERIC_ILOG2_H_ */