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104 lines
3.0 KiB
104 lines
3.0 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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#include <asm/types.h> |
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#include <linux/compiler.h> |
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/* |
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* The "GOLDEN_RATIO_PRIME" is used in ifs/btrfs/brtfs_inode.h and |
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* fs/inode.c. It's not actually prime any more (the previous primes |
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* were actively bad for hashing), but the name remains. |
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*/ |
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#if BITS_PER_LONG == 32 |
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#define GOLDEN_RATIO_PRIME GOLDEN_RATIO_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_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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* This hash multiplies the input by a large odd number and takes the |
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* high bits. Since multiplication propagates changes to the most |
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* significant end only, it is essential that the high bits of the |
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* product be used for the hash value. |
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* |
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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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* 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. (See Knuth vol 3, section 6.4, exercise 9.) |
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* |
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* These are the negative, (1 - phi) = phi**2 = (3 - sqrt(5))/2, |
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* which is very slightly easier to multiply by and makes no |
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* difference to the hash distribution. |
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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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#ifdef CONFIG_HAVE_ARCH_HASH |
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/* This header may use the GOLDEN_RATIO_xx constants */ |
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#include <asm/hash.h> |
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#endif |
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/* |
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* The _generic versions exist only so lib/test_hash.c can compare |
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* the arch-optimized versions with the generic. |
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* |
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* Note that if you change these, any <asm/hash.h> that aren't updated |
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* to match need to have their HAVE_ARCH_* define values updated so the |
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* self-test will not false-positive. |
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*/ |
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#ifndef HAVE_ARCH__HASH_32 |
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#define __hash_32 __hash_32_generic |
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#endif |
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static inline u32 __hash_32_generic(u32 val) |
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{ |
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return val * GOLDEN_RATIO_32; |
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} |
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#ifndef HAVE_ARCH_HASH_32 |
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#define hash_32 hash_32_generic |
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#endif |
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static inline u32 hash_32_generic(u32 val, unsigned int bits) |
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{ |
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/* High bits are more random, so use them. */ |
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return __hash_32(val) >> (32 - bits); |
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} |
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#ifndef HAVE_ARCH_HASH_64 |
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#define hash_64 hash_64_generic |
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#endif |
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static __always_inline u32 hash_64_generic(u64 val, unsigned int bits) |
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{ |
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#if BITS_PER_LONG == 64 |
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/* 64x64-bit multiply is efficient on all 64-bit processors */ |
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return val * GOLDEN_RATIO_64 >> (64 - bits); |
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#else |
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/* Hash 64 bits using only 32x32-bit multiply. */ |
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return hash_32((u32)val ^ __hash_32(val >> 32), bits); |
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#endif |
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} |
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static inline u32 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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/* This really should be called fold32_ptr; it does no hashing to speak of. */ |
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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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