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1da177e4 LT |
1 | #ifndef _LINUX_HASH_H |
2 | #define _LINUX_HASH_H | |
4e701482 | 3 | /* Fast hashing routine for ints, longs and pointers. |
6d49e352 | 4 | (C) 2002 Nadia Yvette Chambers, IBM */ |
1da177e4 LT |
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 | */ | |
4e701482 MW |
16 | |
17 | #include <asm/types.h> | |
65c10553 | 18 | #include <linux/compiler.h> |
4e701482 | 19 | |
1da177e4 | 20 | /* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */ |
4e701482 | 21 | #define GOLDEN_RATIO_PRIME_32 0x9e370001UL |
1da177e4 | 22 | /* 2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */ |
4e701482 MW |
23 | #define GOLDEN_RATIO_PRIME_64 0x9e37fffffffc0001UL |
24 | ||
25 | #if BITS_PER_LONG == 32 | |
26 | #define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_32 | |
27 | #define hash_long(val, bits) hash_32(val, bits) | |
28 | #elif BITS_PER_LONG == 64 | |
29 | #define hash_long(val, bits) hash_64(val, bits) | |
30 | #define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_64 | |
1da177e4 | 31 | #else |
4e701482 | 32 | #error Wordsize not 32 or 64 |
1da177e4 LT |
33 | #endif |
34 | ||
689de1d6 LT |
35 | /* |
36 | * The above primes are actively bad for hashing, since they are | |
37 | * too sparse. The 32-bit one is mostly ok, the 64-bit one causes | |
38 | * real problems. Besides, the "prime" part is pointless for the | |
39 | * multiplicative hash. | |
40 | * | |
41 | * Although a random odd number will do, it turns out that the golden | |
42 | * ratio phi = (sqrt(5)-1)/2, or its negative, has particularly nice | |
43 | * properties. | |
44 | * | |
45 | * These are the negative, (1 - phi) = (phi^2) = (3 - sqrt(5))/2. | |
46 | * (See Knuth vol 3, section 6.4, exercise 9.) | |
47 | */ | |
48 | #define GOLDEN_RATIO_32 0x61C88647 | |
49 | #define GOLDEN_RATIO_64 0x61C8864680B583EBull | |
50 | ||
65c10553 | 51 | static __always_inline u64 hash_64(u64 val, unsigned int bits) |
1da177e4 | 52 | { |
4e701482 | 53 | u64 hash = val; |
1da177e4 | 54 | |
689de1d6 LT |
55 | #if BITS_PER_LONG == 64 |
56 | hash = hash * GOLDEN_RATIO_64; | |
23d0db76 | 57 | #else |
1da177e4 | 58 | /* Sigh, gcc can't optimise this alone like it does for 32 bits. */ |
4e701482 | 59 | u64 n = hash; |
1da177e4 LT |
60 | n <<= 18; |
61 | hash -= n; | |
62 | n <<= 33; | |
63 | hash -= n; | |
64 | n <<= 3; | |
65 | hash += n; | |
66 | n <<= 3; | |
67 | hash -= n; | |
68 | n <<= 4; | |
69 | hash += n; | |
70 | n <<= 2; | |
71 | hash += n; | |
23d0db76 | 72 | #endif |
4e701482 MW |
73 | |
74 | /* High bits are more random, so use them. */ | |
75 | return hash >> (64 - bits); | |
76 | } | |
77 | ||
78 | static inline u32 hash_32(u32 val, unsigned int bits) | |
79 | { | |
1da177e4 | 80 | /* On some cpus multiply is faster, on others gcc will do shifts */ |
4e701482 | 81 | u32 hash = val * GOLDEN_RATIO_PRIME_32; |
1da177e4 LT |
82 | |
83 | /* High bits are more random, so use them. */ | |
4e701482 | 84 | return hash >> (32 - bits); |
1da177e4 | 85 | } |
4e701482 | 86 | |
f9918794 | 87 | static inline unsigned long hash_ptr(const void *ptr, unsigned int bits) |
1da177e4 LT |
88 | { |
89 | return hash_long((unsigned long)ptr, bits); | |
90 | } | |
b14f243a PE |
91 | |
92 | static inline u32 hash32_ptr(const void *ptr) | |
93 | { | |
94 | unsigned long val = (unsigned long)ptr; | |
95 | ||
96 | #if BITS_PER_LONG == 64 | |
97 | val ^= (val >> 32); | |
98 | #endif | |
99 | return (u32)val; | |
100 | } | |
71ae8aac | 101 | |
1da177e4 | 102 | #endif /* _LINUX_HASH_H */ |