1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
|
#ifndef _LINUX_HASH_H
#define _LINUX_HASH_H
/* Fast hashing routine for a long.
(C) 2002 William Lee Irwin III, IBM */
/*
* Knuth recommends primes in approximately golden ratio to the maximum
* integer representable by a machine word for multiplicative hashing.
* Chuck Lever verified the effectiveness of this technique:
* http://www.citi.umich.edu/techreports/reports/citi-tr-00-1.pdf
*
* These primes are chosen to be bit-sparse, that is operations on
* them can use shifts and additions instead of multiplications for
* machines where multiplications are slow.
*/
#if BITS_PER_LONG == 32
/* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */
#define GOLDEN_RATIO_PRIME 0x9e370001UL
#elif BITS_PER_LONG == 64
/* 2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */
#define GOLDEN_RATIO_PRIME 0x9e37fffffffc0001UL
#else
#error Define GOLDEN_RATIO_PRIME for your wordsize.
#endif
static inline unsigned long hash_long(unsigned long val, unsigned int bits)
{
unsigned long hash = val;
#if BITS_PER_LONG == 64
/* Sigh, gcc can't optimise this alone like it does for 32 bits. */
unsigned long n = hash;
n <<= 18;
hash -= n;
n <<= 33;
hash -= n;
n <<= 3;
hash += n;
n <<= 3;
hash -= n;
n <<= 4;
hash += n;
n <<= 2;
hash += n;
#else
/* On some cpus multiply is faster, on others gcc will do shifts */
hash *= GOLDEN_RATIO_PRIME;
#endif
/* High bits are more random, so use them. */
return hash >> (BITS_PER_LONG - bits);
}
static inline unsigned long hash_ptr(void *ptr, unsigned int bits)
{
return hash_long((unsigned long)ptr, bits);
}
#endif /* _LINUX_HASH_H */
|
