6 /* Fast hashing routine for a long.
7 (C) 2002 William Lee Irwin III, IBM */
10 * Knuth recommends primes in approximately golden ratio to the maximum
11 * integer representable by a machine word for multiplicative hashing.
12 * Chuck Lever verified the effectiveness of this technique:
13 * http://www.citi.umich.edu/techreports/reports/citi-tr-00-1.pdf
15 * These primes are chosen to be bit-sparse, that is operations on
16 * them can use shifts and additions instead of multiplications for
17 * machines where multiplications are slow.
20 #if BITS_PER_LONG == 32
21 /* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */
22 #define GOLDEN_RATIO_PRIME 0x9e370001UL
23 #elif BITS_PER_LONG == 64
24 /* 2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */
25 #define GOLDEN_RATIO_PRIME 0x9e37fffffffc0001UL
27 #error Define GOLDEN_RATIO_PRIME for your wordsize.
30 static inline unsigned long hash_long(unsigned long val, unsigned int bits)
32 unsigned long hash = val;
34 #if BITS_PER_LONG == 64
35 /* Sigh, gcc can't optimise this alone like it does for 32 bits. */
36 unsigned long n = hash;
50 /* On some cpus multiply is faster, on others gcc will do shifts */
51 hash *= GOLDEN_RATIO_PRIME;
54 /* High bits are more random, so use them. */
55 return hash >> (BITS_PER_LONG - bits);
58 static inline unsigned long hash_ptr(void *ptr, unsigned int bits)
60 return hash_long((unsigned long)ptr, bits);
62 #endif /* _LINUX_HASH_H */