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| Rev | Author | Line No. | Line |
|---|---|---|---|
| 422 | giacomo | 1 | #ifndef _LINUX_HASH_H |
| 2 | #define _LINUX_HASH_H |
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| 3 | /* Fast hashing routine for a long. |
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| 4 | (C) 2002 William Lee Irwin III, IBM */ |
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| 5 | |||
| 6 | /* |
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| 7 | * Knuth recommends primes in approximately golden ratio to the maximum |
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| 8 | * integer representable by a machine word for multiplicative hashing. |
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| 9 | * Chuck Lever verified the effectiveness of this technique: |
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| 10 | * http://www.citi.umich.edu/techreports/reports/citi-tr-00-1.pdf |
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| 11 | * |
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| 12 | * These primes are chosen to be bit-sparse, that is operations on |
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| 13 | * them can use shifts and additions instead of multiplications for |
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| 14 | * machines where multiplications are slow. |
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| 15 | */ |
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| 16 | #if BITS_PER_LONG == 32 |
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| 17 | /* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */ |
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| 18 | #define GOLDEN_RATIO_PRIME 0x9e370001UL |
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| 19 | #elif BITS_PER_LONG == 64 |
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| 20 | /* 2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */ |
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| 21 | #define GOLDEN_RATIO_PRIME 0x9e37fffffffc0001UL |
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| 22 | #else |
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| 23 | #error Define GOLDEN_RATIO_PRIME for your wordsize. |
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| 24 | #endif |
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| 25 | |||
| 26 | static inline unsigned long hash_long(unsigned long val, unsigned int bits) |
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| 27 | { |
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| 28 | unsigned long hash = val; |
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| 29 | |||
| 30 | #if BITS_PER_LONG == 64 |
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| 31 | /* Sigh, gcc can't optimise this alone like it does for 32 bits. */ |
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| 32 | unsigned long n = hash; |
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| 33 | n <<= 18; |
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| 34 | hash -= n; |
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| 35 | n <<= 33; |
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| 36 | hash -= n; |
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| 37 | n <<= 3; |
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| 38 | hash += n; |
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| 39 | n <<= 3; |
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| 40 | hash -= n; |
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| 41 | n <<= 4; |
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| 42 | hash += n; |
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| 43 | n <<= 2; |
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| 44 | hash += n; |
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| 45 | #else |
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| 46 | /* On some cpus multiply is faster, on others gcc will do shifts */ |
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| 47 | hash *= GOLDEN_RATIO_PRIME; |
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| 48 | #endif |
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| 49 | |||
| 50 | /* High bits are more random, so use them. */ |
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| 51 | return hash >> (BITS_PER_LONG - bits); |
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| 52 | } |
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| 53 | |||
| 54 | static inline unsigned long hash_ptr(void *ptr, unsigned int bits) |
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| 55 | { |
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| 56 | return hash_long((unsigned long)ptr, bits); |
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| 57 | } |
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| 58 | #endif /* _LINUX_HASH_H */ |