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 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
  34 static 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
  57 static 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
  66 static inline unsigned long hash_ptr(void *ptr, unsigned int bits)
  67 {
  68         return hash_long((unsigned long)ptr, bits);
  69 }
  70 #endif /* _LINUX_HASH_H */