| 1 | #ifndef _LINUX_HASH_H
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| 2 | #define _LINUX_HASH_H
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| 3 | |
| 4 | (C) 2002 Nadia Yvette Chambers, IBM
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| 5 |
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| 6 | #include <asm/types.h>
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| 7 | #include <linux/compiler.h>
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| 8 |
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| 9 | |
| 10 | * The "GOLDEN_RATIO_PRIME" is used in ifs/btrfs/brtfs_inode.h and
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| 11 | * fs/inode.c. It's not actually prime any more (the previous primes
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| 12 | * were actively bad for hashing), but the name remains.
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| 13 |
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| 14 | #if BITS_PER_LONG == 32
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| 15 | #define GOLDEN_RATIO_PRIME GOLDEN_RATIO_32
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| 16 | #define hash_long(val, bits) hash_32(val, bits)
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| 17 | #elif BITS_PER_LONG == 64
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| 18 | #define hash_long(val, bits) hash_64(val, bits)
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| 19 | #define GOLDEN_RATIO_PRIME GOLDEN_RATIO_64
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| 20 | #else
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| 21 | #error Wordsize not 32 or 64
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| 22 | #endif
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| 23 |
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| 24 | |
| 25 | * This hash multiplies the input by a large odd number and takes the
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| 26 | * high bits. Since multiplication propagates changes to the most
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| 27 | * significant end only, it is essential that the high bits of the
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| 28 | * product be used for the hash value.
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| 29 | *
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| 30 | * Chuck Lever verified the effectiveness of this technique:
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| 31 | * http://www.citi.umich.edu/techreports/reports/citi-tr-00-1.pdf
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| 32 | *
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| 33 | * Although a random odd number will do, it turns out that the golden
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| 34 | * ratio phi = (sqrt(5)-1)/2, or its negative, has particularly nice
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| 35 | * properties. (See Knuth vol 3, section 6.4, exercise 9.)
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| 36 | *
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| 37 | * These are the negative, (1 - phi) = phi**2 = (3 - sqrt(5))/2,
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| 38 | * which is very slightly easier to multiply by and makes no
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| 39 | * difference to the hash distribution.
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| 40 |
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| 41 | #define GOLDEN_RATIO_32 0x61C88647
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| 42 | #define GOLDEN_RATIO_64 0x61C8864680B583EBull
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| 43 |
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| 44 | #ifdef CONFIG_HAVE_ARCH_HASH
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| 45 |
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| 46 | #include <asm/hash.h>
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| 47 | #endif
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| 48 |
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| 49 | |
| 50 | * The _generic versions exist only so lib/test_hash.c can compare
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| 51 | * the arch-optimized versions with the generic.
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| 52 | *
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| 53 | * Note that if you change these, any <asm/hash.h> that aren't updated
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| 54 | * to match need to have their HAVE_ARCH_* define values updated so the
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| 55 | * self-test will not false-positive.
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| 56 |
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| 57 | #ifndef HAVE_ARCH__HASH_32
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| 58 | #define __hash_32 __hash_32_generic
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| 59 | #endif
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| 60 | static inline u32 __hash_32_generic(u32 val)
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| 61 | {
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| 62 | return val * GOLDEN_RATIO_32;
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| 63 | }
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| 64 |
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| 65 | #ifndef HAVE_ARCH_HASH_32
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| 66 | #define hash_32 hash_32_generic
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| 67 | #endif
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| 68 | static inline u32 hash_32_generic(u32 val, unsigned int bits)
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| 69 | {
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| 70 |
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| 71 | return __hash_32(val) >> (32 - bits);
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| 72 | }
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| 73 |
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| 74 | #ifndef HAVE_ARCH_HASH_64
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| 75 | #define hash_64 hash_64_generic
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| 76 | #endif
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| 77 | static __always_inline u32 hash_64_generic(u64 val, unsigned int bits)
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| 78 | {
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| 79 | #if BITS_PER_LONG == 64
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| 80 |
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| 81 | return val * GOLDEN_RATIO_64 >> (64 - bits);
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| 82 | #else
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| 83 |
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| 84 | return hash_32((u32)val ^ __hash_32(val >> 32), bits);
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| 85 | #endif
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| 86 | }
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| 87 |
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| 88 | static inline u32 hash_ptr(const void *ptr, unsigned int bits)
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| 89 | {
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| 90 | return hash_long((unsigned long)ptr, bits);
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| 91 | }
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| 92 |
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| 93 |
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| 94 | static inline u32 hash32_ptr(const void *ptr)
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| 95 | {
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| 96 | unsigned long val = (unsigned long)ptr;
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| 97 |
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| 98 | #if BITS_PER_LONG == 64
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| 99 | val ^= (val >> 32);
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| 100 | #endif
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| 101 | return (u32)val;
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| 102 | }
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| 103 |
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| 104 | #endif
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