hash.h revision 5921e80c5dfc9f96d2f21da6ae58f2b5d3a0b373 
1#ifndef _LINUX_HASH_H
2#define _LINUX_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
17#ifdef __WORDSIZE
18#define BITS_PER_LONG	__WORDSIZE
19#else
20#define BITS_PER_LONG	32
21#endif
22
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
29#else
30#error Define GOLDEN_RATIO_PRIME for your wordsize.
31#endif
32
33static inline unsigned long hash_long(unsigned long val, unsigned int bits)
34{
35	unsigned long hash = val;
36
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;
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#else
53	/* On some cpus multiply is faster, on others gcc will do shifts */
54	hash *= GOLDEN_RATIO_PRIME;
55#endif
56
57	/* High bits are more random, so use them. */
58	return hash >> (BITS_PER_LONG - bits);
59}
60
61static inline unsigned long hash_ptr(void *ptr, unsigned int bits)
62{
63	return hash_long((unsigned long)ptr, bits);
64}
65#endif /* _LINUX_HASH_H */
66