1 #ifndef _LINUX_HASH_H 2 #define _LINUX_HASH_H 3 /* Fast hashing routine for a long. 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 #if BITS_PER_LONG == 32 17 /* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */ 18 #define GOLDEN_RATIO_PRIME 0x9e370001UL 19 #elif BITS_PER_LONG == 64 20 /* 2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */ 21 #define GOLDEN_RATIO_PRIME 0x9e37fffffffc0001UL 22 #else 23 #error Define GOLDEN_RATIO_PRIME for your wordsize. 24 #endif 25 26 static inline unsigned long hash_long(unsigned long val, unsigned int bits) 27 { 28 unsigned long hash = val; 29 30 #if BITS_PER_LONG == 64 31 /* Sigh, gcc can't optimise this alone like it does for 32 bits. */ 32 unsigned long n = hash; 33 n <<= 18; 34 hash -= n; 35 n <<= 33; 36 hash -= n; 37 n <<= 3; 38 hash += n; 39 n <<= 3; 40 hash -= n; 41 n <<= 4; 42 hash += n; 43 n <<= 2; 44 hash += n; 45 #else 46 /* On some cpus multiply is faster, on others gcc will do shifts */ 47 hash *= GOLDEN_RATIO_PRIME; 48 #endif 49 50 /* High bits are more random, so use them. */ 51 return hash >> (BITS_PER_LONG - bits); 52 } 53 54 static inline unsigned long hash_ptr(void *ptr, unsigned int bits) 55 { 56 return hash_long((unsigned long)ptr, bits); 57 } 58 #endif /* _LINUX_HASH_H */ 59