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 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
  35static __always_inline u64 hash_64(u64 val, unsigned int bits)
  36{
  37        u64 hash = val;
  38
  39        /*  Sigh, gcc can't optimise this alone like it does for 32 bits. */
  40        u64 n = hash;
  41        n <<= 18;
  42        hash -= n;
  43        n <<= 33;
  44        hash -= n;
  45        n <<= 3;
  46        hash += n;
  47        n <<= 3;
  48        hash -= n;
  49        n <<= 4;
  50        hash += n;
  51        n <<= 2;
  52        hash += n;
  53
  54        /* High bits are more random, so use them. */
  55        return hash >> (64 - bits);
  56}
  57
  58static inline u32 hash_32(u32 val, unsigned int bits)
  59{
  60        /* On some cpus multiply is faster, on others gcc will do shifts */
  61        u32 hash = val * GOLDEN_RATIO_PRIME_32;
  62
  63        /* High bits are more random, so use them. */
  64        return hash >> (32 - bits);
  65}
  66
  67static inline unsigned long hash_ptr(const void *ptr, unsigned int bits)
  68{
  69        return hash_long((unsigned long)ptr, bits);
  70}
  71
  72static inline u32 hash32_ptr(const void *ptr)
  73{
  74        unsigned long val = (unsigned long)ptr;
  75
  76#if BITS_PER_LONG == 64
  77        val ^= (val >> 32);
  78#endif
  79        return (u32)val;
  80}
  81#endif /* _LINUX_HASH_H */
  82
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