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 <linux/compiler.h>
  19
  20/* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */
  21#define GOLDEN_RATIO_PRIME_32 0x9e370001UL
  22/*  2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */
  23#define GOLDEN_RATIO_PRIME_64 0x9e37fffffffc0001UL
  24
  25#if BITS_PER_LONG == 32
  26#define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_32
  27#define hash_long(val, bits) hash_32(val, bits)
  28#elif BITS_PER_LONG == 64
  29#define hash_long(val, bits) hash_64(val, bits)
  30#define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_64
  31#else
  32#error Wordsize not 32 or 64
  33#endif
  34
  35/*
  36 * The above primes are actively bad for hashing, since they are
  37 * too sparse. The 32-bit one is mostly ok, the 64-bit one causes
  38 * real problems. Besides, the "prime" part is pointless for the
  39 * multiplicative hash.
  40 *
  41 * Although a random odd number will do, it turns out that the golden
  42 * ratio phi = (sqrt(5)-1)/2, or its negative, has particularly nice
  43 * properties.
  44 *
  45 * These are the negative, (1 - phi) = (phi^2) = (3 - sqrt(5))/2.
  46 * (See Knuth vol 3, section 6.4, exercise 9.)
  47 */
  48#define GOLDEN_RATIO_32 0x61C88647
  49#define GOLDEN_RATIO_64 0x61C8864680B583EBull
  50
  51static __always_inline u64 hash_64(u64 val, unsigned int bits)
  52{
  53        u64 hash = val;
  54
  55#if BITS_PER_LONG == 64
  56        hash = hash * GOLDEN_RATIO_64;
  57#else
  58        /*  Sigh, gcc can't optimise this alone like it does for 32 bits. */
  59        u64 n = hash;
  60        n <<= 18;
  61        hash -= n;
  62        n <<= 33;
  63        hash -= n;
  64        n <<= 3;
  65        hash += n;
  66        n <<= 3;
  67        hash -= n;
  68        n <<= 4;
  69        hash += n;
  70        n <<= 2;
  71        hash += n;
  72#endif
  73
  74        /* High bits are more random, so use them. */
  75        return hash >> (64 - bits);
  76}
  77
  78static inline u32 hash_32(u32 val, unsigned int bits)
  79{
  80        /* On some cpus multiply is faster, on others gcc will do shifts */
  81        u32 hash = val * GOLDEN_RATIO_PRIME_32;
  82
  83        /* High bits are more random, so use them. */
  84        return hash >> (32 - bits);
  85}
  86
  87static inline unsigned long hash_ptr(const void *ptr, unsigned int bits)
  88{
  89        return hash_long((unsigned long)ptr, bits);
  90}
  91
  92static inline u32 hash32_ptr(const void *ptr)
  93{
  94        unsigned long val = (unsigned long)ptr;
  95
  96#if BITS_PER_LONG == 64
  97        val ^= (val >> 32);
  98#endif
  99        return (u32)val;
 100}
 101
 102#endif /* _LINUX_HASH_H */
 103