projects / fio.git / blame
| Commit | Line | Data |
|---|---|---|
| bdc7211e JA |
1 | #ifndef _LINUX_HASH_H |
| 2 | #define _LINUX_HASH_H | |
| daaa166f JA |
3 | |
| 4 | #include "arch/arch.h" | |
| 5 | ||
| bdc7211e JA |
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 | |
| bdc7211e JA |
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 | ||
| 30 | static 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 | ||
| 58 | static 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 */ |