## File: hash.h

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fio 1.38-1
 `1234567891011121314151617181920212223242526272829303132333435363738394041424344454647484950515253545556575859606162` ``````#ifndef _LINUX_HASH_H #define _LINUX_HASH_H #include "arch/arch.h" /* Fast hashing routine for a long. (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. */ #if BITS_PER_LONG == 32 /* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */ #define GOLDEN_RATIO_PRIME 0x9e370001UL #elif BITS_PER_LONG == 64 /* 2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */ #define GOLDEN_RATIO_PRIME 0x9e37fffffffc0001UL #else #error Define GOLDEN_RATIO_PRIME for your wordsize. #endif static inline unsigned long hash_long(unsigned long val, unsigned int bits) { unsigned long hash = val; #if BITS_PER_LONG == 64 /* Sigh, gcc can't optimise this alone like it does for 32 bits. */ unsigned long 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; #else /* On some cpus multiply is faster, on others gcc will do shifts */ hash *= GOLDEN_RATIO_PRIME; #endif /* High bits are more random, so use them. */ return hash >> (BITS_PER_LONG - bits); } static inline unsigned long hash_ptr(void *ptr, unsigned int bits) { return hash_long((unsigned long)ptr, bits); } #endif /* _LINUX_HASH_H */ ``````