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102 lines
2.7 KiB
102 lines
2.7 KiB
#ifndef _LINUX_HASH_H |
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#define _LINUX_HASH_H |
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/* Fast hashing routine for ints, longs and pointers. |
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(C) 2002 Nadia Yvette Chambers, 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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#include <asm/types.h> |
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#include <linux/compiler.h> |
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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_32 0x9e370001UL |
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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_64 0x9e37fffffffc0001UL |
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#if BITS_PER_LONG == 32 |
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#define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_32 |
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#define hash_long(val, bits) hash_32(val, bits) |
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#elif BITS_PER_LONG == 64 |
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#define hash_long(val, bits) hash_64(val, bits) |
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#define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_64 |
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#else |
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#error Wordsize not 32 or 64 |
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#endif |
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/* |
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* The above primes are actively bad for hashing, since they are |
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* too sparse. The 32-bit one is mostly ok, the 64-bit one causes |
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* real problems. Besides, the "prime" part is pointless for the |
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* multiplicative hash. |
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* |
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* Although a random odd number will do, it turns out that the golden |
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* ratio phi = (sqrt(5)-1)/2, or its negative, has particularly nice |
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* properties. |
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* |
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* These are the negative, (1 - phi) = (phi^2) = (3 - sqrt(5))/2. |
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* (See Knuth vol 3, section 6.4, exercise 9.) |
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*/ |
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#define GOLDEN_RATIO_32 0x61C88647 |
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#define GOLDEN_RATIO_64 0x61C8864680B583EBull |
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static __always_inline u64 hash_64(u64 val, unsigned int bits) |
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{ |
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u64 hash = val; |
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#if BITS_PER_LONG == 64 |
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hash = hash * GOLDEN_RATIO_64; |
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#else |
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/* Sigh, gcc can't optimise this alone like it does for 32 bits. */ |
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u64 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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#endif |
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/* High bits are more random, so use them. */ |
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return hash >> (64 - bits); |
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} |
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static inline u32 hash_32(u32 val, unsigned int bits) |
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{ |
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/* On some cpus multiply is faster, on others gcc will do shifts */ |
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u32 hash = val * GOLDEN_RATIO_PRIME_32; |
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/* High bits are more random, so use them. */ |
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return hash >> (32 - bits); |
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} |
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static inline unsigned long hash_ptr(const 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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static inline u32 hash32_ptr(const void *ptr) |
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{ |
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unsigned long val = (unsigned long)ptr; |
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#if BITS_PER_LONG == 64 |
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val ^= (val >> 32); |
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#endif |
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return (u32)val; |
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} |
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#endif /* _LINUX_HASH_H */
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