LXR linux/include/linux/hash.h

   1#ifndef _LINUX_HASH_H
   2#define _LINUX_HASH_H
   3/* Fast hashing routine for a long.
   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#if BITS_PER_LONG == 32
  17/* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */
  18#define GOLDEN_RATIO_PRIME 0x9e370001UL
  19#elif BITS_PER_LONG == 64
  20/*  2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */
  21#define GOLDEN_RATIO_PRIME 0x9e37fffffffc0001UL
  22#else
  23#error Define GOLDEN_RATIO_PRIME for your wordsize.
  24#endif
  25
  26static inline unsigned long hash_long(unsigned long val, unsigned int bits)
  27{
  28        unsigned long hash = val;
  29
  30#if BITS_PER_LONG == 64
  31        /*  Sigh, gcc can't optimise this alone like it does for 32 bits. */
  32        unsigned long n = hash;
  33        n <<= 18;
  34        hash -= n;
  35        n <<= 33;
  36        hash -= n;
  37        n <<= 3;
  38        hash += n;
  39        n <<= 3;
  40        hash -= n;
  41        n <<= 4;
  42        hash += n;
  43        n <<= 2;
  44        hash += n;
  45#else
  46        /* On some cpus multiply is faster, on others gcc will do shifts */
  47        hash *= GOLDEN_RATIO_PRIME;
  48#endif
  49
  50        /* High bits are more random, so use them. */
  51        return hash >> (BITS_PER_LONG - bits);
  52}
  53        
  54static inline unsigned long hash_ptr(void *ptr, unsigned int bits)
  55{
  56        return hash_long((unsigned long)ptr, bits);
  57}
  58#endif /* _LINUX_HASH_H */
  59