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
  26 static 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
  54 static 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 */