1 #ifndef _XEN_HASH_H
2 #define _XEN_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
hash_long(unsigned long val,unsigned int bits)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
hash_ptr(void * ptr,unsigned int bits)54 static inline unsigned long hash_ptr(void *ptr, unsigned int bits)
55 {
56 return hash_long((unsigned long)ptr, bits);
57 }
58 #endif /* _XEN_HASH_H */
59