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 34static inline u64 hash_64(u64 val, unsigned int bits) 35{ 36 u64 hash = val; 37 38 /* Sigh, gcc can't optimise this alone like it does for 32 bits. */ 39 u64 n = hash; 40 n <<= 18; 41 hash -= n; 42 n <<= 33; 43 hash -= n; 44 n <<= 3; 45 hash += n; 46 n <<= 3; 47 hash -= n; 48 n <<= 4; 49 hash += n; 50 n <<= 2; 51 hash += n; 52 53 /* High bits are more random, so use them. */ 54 return hash >> (64 - bits); 55} 56 57static 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 66static 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 */ 71