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