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1#ifndef _LINUX_HASH_H
2#define _LINUX_HASH_H
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3
4#include "arch/arch.h"
5
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6/* Fast hashing routine for a long.
7 (C) 2002 William Lee Irwin III, IBM */
8
9/*
10 * Knuth recommends primes in approximately golden ratio to the maximum
11 * integer representable by a machine word for multiplicative hashing.
12 * Chuck Lever verified the effectiveness of this technique:
13 * http://www.citi.umich.edu/techreports/reports/citi-tr-00-1.pdf
14 *
15 * These primes are chosen to be bit-sparse, that is operations on
16 * them can use shifts and additions instead of multiplications for
17 * machines where multiplications are slow.
18 */
5921e80c 19
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20#if BITS_PER_LONG == 32
21/* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */
22#define GOLDEN_RATIO_PRIME 0x9e370001UL
23#elif BITS_PER_LONG == 64
24/* 2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */
25#define GOLDEN_RATIO_PRIME 0x9e37fffffffc0001UL
26#else
27#error Define GOLDEN_RATIO_PRIME for your wordsize.
28#endif
29
30static inline unsigned long hash_long(unsigned long val, unsigned int bits)
31{
32 unsigned long hash = val;
33
34#if BITS_PER_LONG == 64
35 /* Sigh, gcc can't optimise this alone like it does for 32 bits. */
36 unsigned long n = hash;
37 n <<= 18;
38 hash -= n;
39 n <<= 33;
40 hash -= n;
41 n <<= 3;
42 hash += n;
43 n <<= 3;
44 hash -= n;
45 n <<= 4;
46 hash += n;
47 n <<= 2;
48 hash += n;
49#else
50 /* On some cpus multiply is faster, on others gcc will do shifts */
51 hash *= GOLDEN_RATIO_PRIME;
52#endif
53
54 /* High bits are more random, so use them. */
55 return hash >> (BITS_PER_LONG - bits);
56}
57
58static inline unsigned long hash_ptr(void *ptr, unsigned int bits)
59{
60 return hash_long((unsigned long)ptr, bits);
61}
62#endif /* _LINUX_HASH_H */