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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#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
26static 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
54static inline unsigned long hash_ptr(void *ptr, unsigned int bits)
55{
56 return hash_long((unsigned long)ptr, bits);
57}
58#endif /* _LINUX_HASH_H */