linux/include/linux/hash.h
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   1#ifndef _LINUX_HASH_H
   2#define _LINUX_HASH_H
   3/* Fast hashing routine for ints,  longs and pointers.
   4   (C) 2002 William Lee Irwin III, 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
  19/* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */
  20#define GOLDEN_RATIO_PRIME_32 0x9e370001UL
  21/*  2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */
  22#define GOLDEN_RATIO_PRIME_64 0x9e37fffffffc0001UL
  23
  24#if BITS_PER_LONG == 32
  25#define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_32
  26#define hash_long(val, bits) hash_32(val, bits)
  27#elif BITS_PER_LONG == 64
  28#define hash_long(val, bits) hash_64(val, bits)
  29#define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_64
  30#else
  31#error Wordsize not 32 or 64
  32#endif
  33
  34static inline u64 hash_64(u64 val, unsigned int bits)
  35{
  36        u64 hash = val;
  37
  38        /*  Sigh, gcc can't optimise this alone like it does for 32 bits. */
  39        u64 n = hash;
  40        n <<= 18;
  41        hash -= n;
  42        n <<= 33;
  43        hash -= n;
  44        n <<= 3;
  45        hash += n;
  46        n <<= 3;
  47        hash -= n;
  48        n <<= 4;
  49        hash += n;
  50        n <<= 2;
  51        hash += n;
  52
  53        /* High bits are more random, so use them. */
  54        return hash >> (64 - bits);
  55}
  56
  57static inline u32 hash_32(u32 val, unsigned int bits)
  58{
  59        /* On some cpus multiply is faster, on others gcc will do shifts */
  60        u32 hash = val * GOLDEN_RATIO_PRIME_32;
  61
  62        /* High bits are more random, so use them. */
  63        return hash >> (32 - bits);
  64}
  65
  66static inline unsigned long hash_ptr(const void *ptr, unsigned int bits)
  67{
  68        return hash_long((unsigned long)ptr, bits);
  69}
  70
  71static inline u32 hash32_ptr(const void *ptr)
  72{
  73        unsigned long val = (unsigned long)ptr;
  74
  75#if BITS_PER_LONG == 64
  76        val ^= (val >> 32);
  77#endif
  78        return (u32)val;
  79}
  80#endif /* _LINUX_HASH_H */
  81
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