59 lines
1.6 KiB
C
59 lines
1.6 KiB
C
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#ifndef _LINUX_HASH_H
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#define _LINUX_HASH_H
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/* Fast hashing routine for a long.
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(C) 2002 William Lee Irwin III, IBM */
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/*
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* Knuth recommends primes in approximately golden ratio to the maximum
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* integer representable by a machine word for multiplicative hashing.
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* Chuck Lever verified the effectiveness of this technique:
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* http://www.citi.umich.edu/techreports/reports/citi-tr-00-1.pdf
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*
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* These primes are chosen to be bit-sparse, that is operations on
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* them can use shifts and additions instead of multiplications for
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* machines where multiplications are slow.
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*/
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#if BITS_PER_LONG == 32
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/* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */
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#define GOLDEN_RATIO_PRIME 0x9e370001UL
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#elif BITS_PER_LONG == 64
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/* 2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */
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#define GOLDEN_RATIO_PRIME 0x9e37fffffffc0001UL
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#else
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#error Define GOLDEN_RATIO_PRIME for your wordsize.
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#endif
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static inline unsigned long hash_long(unsigned long val, unsigned int bits)
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{
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unsigned long hash = val;
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#if BITS_PER_LONG == 64
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/* Sigh, gcc can't optimise this alone like it does for 32 bits. */
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unsigned long n = hash;
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n <<= 18;
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hash -= n;
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n <<= 33;
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hash -= n;
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n <<= 3;
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hash += n;
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n <<= 3;
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hash -= n;
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n <<= 4;
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hash += n;
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n <<= 2;
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hash += n;
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#else
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/* On some cpus multiply is faster, on others gcc will do shifts */
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hash *= GOLDEN_RATIO_PRIME;
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#endif
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/* High bits are more random, so use them. */
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return hash >> (BITS_PER_LONG - bits);
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}
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static inline unsigned long hash_ptr(void *ptr, unsigned int bits)
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{
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return hash_long((unsigned long)ptr, bits);
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}
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#endif /* _LINUX_HASH_H */
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