3 /* Fast hashing routine for a long.
4 (C) 2002 William Lee Irwin III, IBM */
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
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.
18 #define BITS_PER_LONG __WORDSIZE
20 #define BITS_PER_LONG 32
23 #if BITS_PER_LONG == 32
24 /* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */
25 #define GOLDEN_RATIO_PRIME 0x9e370001UL
26 #elif BITS_PER_LONG == 64
27 /* 2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */
28 #define GOLDEN_RATIO_PRIME 0x9e37fffffffc0001UL
30 #error Define GOLDEN_RATIO_PRIME for your wordsize.
33 static inline unsigned long hash_long(unsigned long val, unsigned int bits)
35 unsigned long hash = val;
37 #if BITS_PER_LONG == 64
38 /* Sigh, gcc can't optimise this alone like it does for 32 bits. */
39 unsigned long n = hash;
53 /* On some cpus multiply is faster, on others gcc will do shifts */
54 hash *= GOLDEN_RATIO_PRIME;
57 /* High bits are more random, so use them. */
58 return hash >> (BITS_PER_LONG - bits);
61 static inline unsigned long hash_ptr(void *ptr, unsigned int bits)
63 return hash_long((unsigned long)ptr, bits);
65 #endif /* _LINUX_HASH_H */