hash.h

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#ifndef _LINUX_HASH_H
#define _LINUX_HASH_H
/* Fast hashing routine for ints,  longs and pointers.
   (C) 2002 William Lee Irwin III, IBM */

/*
 * Knuth recommends primes in approximately golden ratio to the maximum
 * integer representable by a machine word for multiplicative hashing.
 * Chuck Lever verified the effectiveness of this technique:
 * http://www.citi.umich.edu/techreports/reports/citi-tr-00-1.pdf
 *
 * These primes are chosen to be bit-sparse, that is operations on
 * them can use shifts and additions instead of multiplications for
 * machines where multiplications are slow.
 */

#include <asm/types.h>

/* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */
#define GOLDEN_RATIO_PRIME_32 0x9e370001UL
/*  2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */
#define GOLDEN_RATIO_PRIME_64 0x9e37fffffffc0001UL

#if BITS_PER_LONG == 32
#define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_32
#define hash_long(val, bits) hash_32(val, bits)
#elif BITS_PER_LONG == 64
#define hash_long(val, bits) hash_64(val, bits)
#define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_64
#else
#error Wordsize not 32 or 64
#endif

static inline u64 hash_64(u64 val, unsigned int bits)
{
    u64 hash = val;

    /*  Sigh, gcc can't optimise this alone like it does for 32 bits. */
    u64 n = hash;
    n <<= 18;
    hash -= n;
    n <<= 33;
    hash -= n;
    n <<= 3;
    hash += n;
    n <<= 3;
    hash -= n;
    n <<= 4;
    hash += n;
    n <<= 2;
    hash += n;

    /* High bits are more random, so use them. */
    return hash >> (64 - bits);
}

static inline u32 hash_32(u32 val, unsigned int bits)
{
    /* On some cpus multiply is faster, on others gcc will do shifts */
    u32 hash = val * GOLDEN_RATIO_PRIME_32;

    /* High bits are more random, so use them. */
    return hash >> (32 - bits);
}

static inline unsigned long hash_ptr(const void *ptr, unsigned int bits)
{
    return hash_long((unsigned long)ptr, bits);
}

static inline u32 hash32_ptr(const void *ptr)
{
    unsigned long val = (unsigned long)ptr;

#if BITS_PER_LONG == 64
    val ^= (val >> 32);
#endif
    return (u32)val;
}
#endif /* _LINUX_HASH_H */