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        /*  Sigh, gcc can't optimise this alone like it does for 32 bits. */
  41        u64 n = hash;
  42        n <<= 18;
  43        hash -= n;
  44        n <<= 33;
  45        hash -= n;
  46        n <<= 3;
  47        hash += n;
  48        n <<= 3;
  49        hash -= n;
  50        n <<= 4;
  51        hash += n;
  52        n <<= 2;
  53        hash += n;
  54
  55        /* High bits are more random, so use them. */
  56        return hash >> (64 - bits);
  57}
  58
  59static inline u32 hash_32(u32 val, unsigned int bits)
  60{
  61        /* On some cpus multiply is faster, on others gcc will do shifts */
  62        u32 hash = val * GOLDEN_RATIO_PRIME_32;
  63
  64        /* High bits are more random, so use them. */
  65        return hash >> (32 - bits);
  66}
  67
  68static inline unsigned long hash_ptr(const void *ptr, unsigned int bits)
  69{
  70        return hash_long((unsigned long)ptr, bits);
  71}
  72
  73static inline u32 hash32_ptr(const void *ptr)
  74{
  75        unsigned long val = (unsigned long)ptr;
  76
  77#if BITS_PER_LONG == 64
  78        val ^= (val >> 32);
  79#endif
  80        return (u32)val;
  81}
  82
  83struct fast_hash_ops {
  84        u32 (*hash)(const void *data, u32 len, u32 seed);
  85        u32 (*hash2)(const u32 *data, u32 len, u32 seed);
  86};
  87
  88/**
  89 *      arch_fast_hash - Caclulates a hash over a given buffer that can have
  90 *                       arbitrary size. This function will eventually use an
  91 *                       architecture-optimized hashing implementation if
  92 *                       available, and trades off distribution for speed.
  93 *
  94 *      @data: buffer to hash
  95 *      @len: length of buffer in bytes
  96 *      @seed: start seed
  97 *
  98 *      Returns 32bit hash.
  99 */
 100extern u32 arch_fast_hash(const void *data, u32 len, u32 seed);
 101
 102/**
 103 *      arch_fast_hash2 - Caclulates a hash over a given buffer that has a
 104 *                        size that is of a multiple of 32bit words. This
 105 *                        function will eventually use an architecture-
 106 *                        optimized hashing implementation if available,
 107 *                        and trades off distribution for speed.
 108 *
 109 *      @data: buffer to hash (must be 32bit padded)
 110 *      @len: number of 32bit words
 111 *      @seed: start seed
 112 *
 113 *      Returns 32bit hash.
 114 */
 115extern u32 arch_fast_hash2(const u32 *data, u32 len, u32 seed);
 116
 117#endif /* _LINUX_HASH_H */
 118