log2.h

Go to the documentation of this file.

/* SPDX-License-Identifier: GPL-2.0-or-later */
/* Integer base 2 logarithm calculation
 *
 * Copyright (C) 2006 Red Hat, Inc. All Rights Reserved.
 * Written by David Howells (dhowells@redhat.com)
 */
 
#pragma once
#include <stdint.h>
 
/* from linux/asm-generic/bitops/{fls,fls64}.h - could later be enhanced
 * with architecture specific optimized versions */
 
static inline __attribute__((always_inline)) int fls(unsigned int x)
{
        int r = 32;
 
        if (!x)
                return 0;
        if (!(x & 0xffff0000u)) {
                x <<= 16;
                r -= 16;
        }
        if (!(x & 0xff000000u)) {
                x <<= 8;
                r -= 8;
        }
        if (!(x & 0xf0000000u)) {
                x <<= 4;
                r -= 4;
        }
        if (!(x & 0xc0000000u)) {
                x <<= 2;
                r -= 2;
        }
        if (!(x & 0x80000000u)) {
                x <<= 1;
                r -= 1;
        }
        return r;
}
 
static inline __attribute__((always_inline)) int fls64(uint64_t x)
{
        uint32_t h = x >> 32;
        if (h)
                return fls(h) + 32;
        return fls(x);
}
 
/*
 * non-constant log of base 2 calculators
 * - the arch may override these in asm/bitops.h if they can be implemented
 *   more efficiently than using fls() and fls64()
 * - the arch is not required to handle n==0 if implementing the fallback
 */
#ifndef CONFIG_ARCH_HAS_ILOG2_U32
static inline __attribute__((const))
int __ilog2_u32(uint32_t n)
{
        return fls(n) - 1;
}
#endif
 
#ifndef CONFIG_ARCH_HAS_ILOG2_U64
static inline __attribute__((const))
int __ilog2_u64(uint64_t n)
{
        return fls64(n) - 1;
}
#endif
 
#define const_ilog2(n)                          \
(                                               \
        __builtin_constant_p(n) ? (             \
                (n) < 2 ? 0 :                   \
                (n) & (1ULL << 63) ? 63 :       \
                (n) & (1ULL << 62) ? 62 :       \
                (n) & (1ULL << 61) ? 61 :       \
                (n) & (1ULL << 60) ? 60 :       \
                (n) & (1ULL << 59) ? 59 :       \
                (n) & (1ULL << 58) ? 58 :       \
                (n) & (1ULL << 57) ? 57 :       \
                (n) & (1ULL << 56) ? 56 :       \
                (n) & (1ULL << 55) ? 55 :       \
                (n) & (1ULL << 54) ? 54 :       \
                (n) & (1ULL << 53) ? 53 :       \
                (n) & (1ULL << 52) ? 52 :       \
                (n) & (1ULL << 51) ? 51 :       \
                (n) & (1ULL << 50) ? 50 :       \
                (n) & (1ULL << 49) ? 49 :       \
                (n) & (1ULL << 48) ? 48 :       \
                (n) & (1ULL << 47) ? 47 :       \
                (n) & (1ULL << 46) ? 46 :       \
                (n) & (1ULL << 45) ? 45 :       \
                (n) & (1ULL << 44) ? 44 :       \
                (n) & (1ULL << 43) ? 43 :       \
                (n) & (1ULL << 42) ? 42 :       \
                (n) & (1ULL << 41) ? 41 :       \
                (n) & (1ULL << 40) ? 40 :       \
                (n) & (1ULL << 39) ? 39 :       \
                (n) & (1ULL << 38) ? 38 :       \
                (n) & (1ULL << 37) ? 37 :       \
                (n) & (1ULL << 36) ? 36 :       \
                (n) & (1ULL << 35) ? 35 :       \
                (n) & (1ULL << 34) ? 34 :       \
                (n) & (1ULL << 33) ? 33 :       \
                (n) & (1ULL << 32) ? 32 :       \
                (n) & (1ULL << 31) ? 31 :       \
                (n) & (1ULL << 30) ? 30 :       \
                (n) & (1ULL << 29) ? 29 :       \
                (n) & (1ULL << 28) ? 28 :       \
                (n) & (1ULL << 27) ? 27 :       \
                (n) & (1ULL << 26) ? 26 :       \
                (n) & (1ULL << 25) ? 25 :       \
                (n) & (1ULL << 24) ? 24 :       \
                (n) & (1ULL << 23) ? 23 :       \
                (n) & (1ULL << 22) ? 22 :       \
                (n) & (1ULL << 21) ? 21 :       \
                (n) & (1ULL << 20) ? 20 :       \
                (n) & (1ULL << 19) ? 19 :       \
                (n) & (1ULL << 18) ? 18 :       \
                (n) & (1ULL << 17) ? 17 :       \
                (n) & (1ULL << 16) ? 16 :       \
                (n) & (1ULL << 15) ? 15 :       \
                (n) & (1ULL << 14) ? 14 :       \
                (n) & (1ULL << 13) ? 13 :       \
                (n) & (1ULL << 12) ? 12 :       \
                (n) & (1ULL << 11) ? 11 :       \
                (n) & (1ULL << 10) ? 10 :       \
                (n) & (1ULL <<  9) ?  9 :       \
                (n) & (1ULL <<  8) ?  8 :       \
                (n) & (1ULL <<  7) ?  7 :       \
                (n) & (1ULL <<  6) ?  6 :       \
                (n) & (1ULL <<  5) ?  5 :       \
                (n) & (1ULL <<  4) ?  4 :       \
                (n) & (1ULL <<  3) ?  3 :       \
                (n) & (1ULL <<  2) ?  2 :       \
                1) :                            \
        -1)
 
#define ilog2(n) \
( \
        __builtin_constant_p(n) ?       \
        const_ilog2(n) :                \
        (sizeof(n) <= 4) ?              \
        __ilog2_u32(n) :                \
        __ilog2_u64(n)                  \
 )