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 #ifndef _LINUX_HASH_H #define _LINUX_HASH_H /* Fast hashing routine for ints, longs and pointers. (C) 2002 Nadia Yvette Chambers, 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 #include /* 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 __always_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(const void *ptr, unsigned int bits) { return hash_long((unsigned long)ptr, bits); } static inline u32 hash32_ptr(const void *ptr) { unsigned long val = (unsigned long)ptr; #if BITS_PER_LONG == 64 val ^= (val >> 32); #endif return (u32)val; } #endif /* _LINUX_HASH_H */