pre_read fixes
[fio.git] / hash.h
... / ...
CommitLineData
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 */