kernel / pub / scm / linux / kernel / git / mraynal / linux / refs/tags/v2.6.30-rc5 / . / include / linux / hash.h

#ifndef _LINUX_HASH_H | |

#define _LINUX_HASH_H | |

/* Fast hashing routine for ints, longs and pointers. | |

(C) 2002 William Lee Irwin III, IBM */ | |

/* | |

* Knuth recommends primes in approximately golden ratio to the maximum | |

* integer representable by a machine word for multiplicative hashing. | |

* Chuck Lever verified the effectiveness of this technique: | |

* http://www.citi.umich.edu/techreports/reports/citi-tr-00-1.pdf | |

* | |

* These primes are chosen to be bit-sparse, that is operations on | |

* them can use shifts and additions instead of multiplications for | |

* machines where multiplications are slow. | |

*/ | |

#include <asm/types.h> | |

/* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */ | |

#define GOLDEN_RATIO_PRIME_32 0x9e370001UL | |

/* 2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */ | |

#define GOLDEN_RATIO_PRIME_64 0x9e37fffffffc0001UL | |

#if BITS_PER_LONG == 32 | |

#define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_32 | |

#define hash_long(val, bits) hash_32(val, bits) | |

#elif BITS_PER_LONG == 64 | |

#define hash_long(val, bits) hash_64(val, bits) | |

#define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_64 | |

#else | |

#error Wordsize not 32 or 64 | |

#endif | |

static inline u64 hash_64(u64 val, unsigned int bits) | |

{ | |

u64 hash = val; | |

/* Sigh, gcc can't optimise this alone like it does for 32 bits. */ | |

u64 n = hash; | |

n <<= 18; | |

hash -= n; | |

n <<= 33; | |

hash -= n; | |

n <<= 3; | |

hash += n; | |

n <<= 3; | |

hash -= n; | |

n <<= 4; | |

hash += n; | |

n <<= 2; | |

hash += n; | |

/* High bits are more random, so use them. */ | |

return hash >> (64 - bits); | |

} | |

static inline u32 hash_32(u32 val, unsigned int bits) | |

{ | |

/* On some cpus multiply is faster, on others gcc will do shifts */ | |

u32 hash = val * GOLDEN_RATIO_PRIME_32; | |

/* High bits are more random, so use them. */ | |

return hash >> (32 - bits); | |

} | |

static inline unsigned long hash_ptr(void *ptr, unsigned int bits) | |

{ | |

return hash_long((unsigned long)ptr, bits); | |

} | |

#endif /* _LINUX_HASH_H */ |