Line data Source code
1 : #ifndef _LINUX_HASH_H
2 : #define _LINUX_HASH_H
3 : /* Fast hashing routine for ints, longs and pointers.
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 : #include <asm/types.h>
18 :
19 : /* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */
20 : #define GOLDEN_RATIO_PRIME_32 0x9e370001UL
21 : /* 2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */
22 : #define GOLDEN_RATIO_PRIME_64 0x9e37fffffffc0001UL
23 :
24 : #if BITS_PER_LONG == 32
25 : #define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_32
26 : #define hash_long(val, bits) hash_32(val, bits)
27 : #elif BITS_PER_LONG == 64
28 : #define hash_long(val, bits) hash_64(val, bits)
29 : #define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_64
30 : #else
31 : #error Wordsize not 32 or 64
32 : #endif
33 :
34 : static inline u64 hash_64(u64 val, unsigned int bits)
35 : {
36 0 : u64 hash = val;
37 0 :
38 : /* Sigh, gcc can't optimise this alone like it does for 32 bits. */
39 0 : u64 n = hash;
40 0 : n <<= 18;
41 0 : hash -= n;
42 0 : n <<= 33;
43 0 : hash -= n;
44 0 : n <<= 3;
45 0 : hash += n;
46 0 : n <<= 3;
47 0 : hash -= n;
48 0 : n <<= 4;
49 0 : hash += n;
50 0 : n <<= 2;
51 0 : hash += n;
52 :
53 : /* High bits are more random, so use them. */
54 0 : return hash >> (64 - bits);
55 : }
56 :
57 : static inline u32 hash_32(u32 val, unsigned int bits)
58 : {
59 : /* On some cpus multiply is faster, on others gcc will do shifts */
60 : u32 hash = val * GOLDEN_RATIO_PRIME_32;
61 :
62 : /* High bits are more random, so use them. */
63 : return hash >> (32 - bits);
64 : }
65 :
66 : static inline unsigned long hash_ptr(void *ptr, unsigned int bits)
67 : {
68 : return hash_long((unsigned long)ptr, bits);
69 : }
70 : #endif /* _LINUX_HASH_H */
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