LXR linux/include/linux/hash.h

   1#ifndef _LINUX_HASH_H
   2#define _LINUX_HASH_H
   3/* Fast hashing routine for ints,  longs and pointers.
   4   (C) 2002 Nadia Yvette Chambers, IBM */
   5
   6/*
   7 * Knuth recommends primes in approximately golden ratio to the maximum
   8 * integer representable by a machine word for multiplicative hashing.
   9 * Chuck Lever verified the effectiveness of this technique:
  10 * http://www.citi.umich.edu/techreports/reports/citi-tr-00-1.pdf
  11 *
  12 * These primes are chosen to be bit-sparse, that is operations on
  13 * them can use shifts and additions instead of multiplications for
  14 * machines where multiplications are slow.
  15 */
  16
  17#include <asm/types.h>
  18#include <asm/hash.h>
  19#include <linux/compiler.h>
  20
  21/* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */
  22#define GOLDEN_RATIO_PRIME_32 0x9e370001UL
  23/*  2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */
  24#define GOLDEN_RATIO_PRIME_64 0x9e37fffffffc0001UL
  25
  26#if BITS_PER_LONG == 32
  27#define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_32
  28#define hash_long(val, bits) hash_32(val, bits)
  29#elif BITS_PER_LONG == 64
  30#define hash_long(val, bits) hash_64(val, bits)
  31#define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_64
  32#else
  33#error Wordsize not 32 or 64
  34#endif
  35
  36static __always_inline u64 hash_64(u64 val, unsigned int bits)
  37{
  38        u64 hash = val;
  39
  40#if defined(CONFIG_ARCH_HAS_FAST_MULTIPLIER) && BITS_PER_LONG == 64
  41        hash = hash * GOLDEN_RATIO_PRIME_64;
  42#else
  43        /*  Sigh, gcc can't optimise this alone like it does for 32 bits. */
  44        u64 n = hash;
  45        n <<= 18;
  46        hash -= n;
  47        n <<= 33;
  48        hash -= n;
  49        n <<= 3;
  50        hash += n;
  51        n <<= 3;
  52        hash -= n;
  53        n <<= 4;
  54        hash += n;
  55        n <<= 2;
  56        hash += n;
  57#endif
  58
  59        /* High bits are more random, so use them. */
  60        return hash >> (64 - bits);
  61}
  62
  63static inline u32 hash_32(u32 val, unsigned int bits)
  64{
  65        /* On some cpus multiply is faster, on others gcc will do shifts */
  66        u32 hash = val * GOLDEN_RATIO_PRIME_32;
  67
  68        /* High bits are more random, so use them. */
  69        return hash >> (32 - bits);
  70}
  71
  72static inline unsigned long hash_ptr(const void *ptr, unsigned int bits)
  73{
  74        return hash_long((unsigned long)ptr, bits);
  75}
  76
  77static inline u32 hash32_ptr(const void *ptr)
  78{
  79        unsigned long val = (unsigned long)ptr;
  80
  81#if BITS_PER_LONG == 64
  82        val ^= (val >> 32);
  83#endif
  84        return (u32)val;
  85}
  86
  87struct fast_hash_ops {
  88        u32 (*hash)(const void *data, u32 len, u32 seed);
  89        u32 (*hash2)(const u32 *data, u32 len, u32 seed);
  90};
  91
  92/**
  93 *      arch_fast_hash - Caclulates a hash over a given buffer that can have
  94 *                       arbitrary size. This function will eventually use an
  95 *                       architecture-optimized hashing implementation if
  96 *                       available, and trades off distribution for speed.
  97 *
  98 *      @data: buffer to hash
  99 *      @len: length of buffer in bytes
 100 *      @seed: start seed
 101 *
 102 *      Returns 32bit hash.
 103 */
 104extern u32 arch_fast_hash(const void *data, u32 len, u32 seed);
 105
 106/**
 107 *      arch_fast_hash2 - Caclulates a hash over a given buffer that has a
 108 *                        size that is of a multiple of 32bit words. This
 109 *                        function will eventually use an architecture-
 110 *                        optimized hashing implementation if available,
 111 *                        and trades off distribution for speed.
 112 *
 113 *      @data: buffer to hash (must be 32bit padded)
 114 *      @len: number of 32bit words
 115 *      @seed: start seed
 116 *
 117 *      Returns 32bit hash.
 118 */
 119extern u32 arch_fast_hash2(const u32 *data, u32 len, u32 seed);
 120
 121#endif /* _LINUX_HASH_H */
 122