bits_ctz.h

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#ifndef __WSUTIL_BITS_CTZ_H__
#define __WSUTIL_BITS_CTZ_H__
 
#include <inttypes.h>
 
/* ws_ctz == trailing zeros == position of lowest set bit [0..63] */
/* ws_ilog2 == position of highest set bit == 63 - leading zeros [0..63] */
 
/* The return value of both ws_ctz and ws_ilog2 is undefined for x == 0 */
 
#if defined(__GNUC__) && ((__GNUC__ > 3) || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4))
 
static inline int
ws_ctz(uint64_t x)
{
    return __builtin_ctzll(x);
}
 
static inline int
ws_ilog2(uint64_t x)
{
    return 63 - __builtin_clzll(x);
}
 
#else
 
static inline int
__ws_ctz32(uint32_t x)
{
    /* From http://graphics.stanford.edu/~seander/bithacks.html#ZerosOnRightMultLookup */
    static const uint8_t table[32] = {
        0,   1, 28,  2, 29, 14, 24, 3, 30, 22, 20, 15, 25, 17,  4, 8,
        31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18,  6, 11,  5, 10, 9
    };
 
    return table[((uint32_t)((x & -(int32_t)x) * 0x077CB531U)) >> 27];
}
 
static inline int
ws_ctz(uint64_t x)
{
    uint32_t hi = x >> 32;
    uint32_t lo = (uint32_t) x;
 
    if (lo == 0)
        return 32 + __ws_ctz32(hi);
    else
        return __ws_ctz32(lo);
}
 
static inline int
__ws_ilog2_32(uint32_t x)
{
    /* From http://graphics.stanford.edu/~seander/bithacks.html#IntegerLogDeBruijn */
    static const uint8_t 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
    };
 
    x |= x >> 1;
    x |= x >> 2;
    x |= x >> 4;
    x |= x >> 8;
    x |= x >> 16;
 
    return table[((uint32_t)(x * 0x07C4ACDDU)) >> 27];
}
 
static inline int
ws_ilog2(uint64_t x)
{
    uint32_t hi = x >> 32;
    uint32_t lo = (uint32_t) x;
 
    if (hi == 0)
        return __ws_ilog2_32(lo);
    else
        return 32 + __ws_ilog2_32(hi);
}
 
#endif
 
#endif /* __WSUTIL_BITS_CTZ_H__ */