105 lines
3 KiB
C
105 lines
3 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 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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