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1 | #ifndef _LINUX_HASH_H |
2 | #define _LINUX_HASH_H | |
3 | /* Fast hashing routine for ints, longs and pointers. | |
4 | (C) 2002 Nadia Yvette Chambers, IBM */ | |
0e55fa11 | 5 | |
ae3c14a0 ACM |
6 | #include <asm/types.h> |
7 | #include <linux/compiler.h> | |
8 | ||
9 | /* | |
10 | * The "GOLDEN_RATIO_PRIME" is used in ifs/btrfs/brtfs_inode.h and | |
11 | * fs/inode.c. It's not actually prime any more (the previous primes | |
12 | * were actively bad for hashing), but the name remains. | |
13 | */ | |
14 | #if BITS_PER_LONG == 32 | |
15 | #define GOLDEN_RATIO_PRIME GOLDEN_RATIO_32 | |
16 | #define hash_long(val, bits) hash_32(val, bits) | |
17 | #elif BITS_PER_LONG == 64 | |
18 | #define hash_long(val, bits) hash_64(val, bits) | |
19 | #define GOLDEN_RATIO_PRIME GOLDEN_RATIO_64 | |
20 | #else | |
21 | #error Wordsize not 32 or 64 | |
22 | #endif | |
23 | ||
24 | /* | |
25 | * This hash multiplies the input by a large odd number and takes the | |
26 | * high bits. Since multiplication propagates changes to the most | |
27 | * significant end only, it is essential that the high bits of the | |
28 | * product be used for the hash value. | |
29 | * | |
30 | * Chuck Lever verified the effectiveness of this technique: | |
31 | * http://www.citi.umich.edu/techreports/reports/citi-tr-00-1.pdf | |
32 | * | |
33 | * Although a random odd number will do, it turns out that the golden | |
34 | * ratio phi = (sqrt(5)-1)/2, or its negative, has particularly nice | |
35 | * properties. (See Knuth vol 3, section 6.4, exercise 9.) | |
36 | * | |
37 | * These are the negative, (1 - phi) = phi**2 = (3 - sqrt(5))/2, | |
38 | * which is very slightly easier to multiply by and makes no | |
39 | * difference to the hash distribution. | |
40 | */ | |
41 | #define GOLDEN_RATIO_32 0x61C88647 | |
42 | #define GOLDEN_RATIO_64 0x61C8864680B583EBull | |
43 | ||
44 | #ifdef CONFIG_HAVE_ARCH_HASH | |
45 | /* This header may use the GOLDEN_RATIO_xx constants */ | |
46 | #include <asm/hash.h> | |
47 | #endif | |
48 | ||
49 | /* | |
50 | * The _generic versions exist only so lib/test_hash.c can compare | |
51 | * the arch-optimized versions with the generic. | |
52 | * | |
53 | * Note that if you change these, any <asm/hash.h> that aren't updated | |
54 | * to match need to have their HAVE_ARCH_* define values updated so the | |
55 | * self-test will not false-positive. | |
56 | */ | |
57 | #ifndef HAVE_ARCH__HASH_32 | |
58 | #define __hash_32 __hash_32_generic | |
59 | #endif | |
60 | static inline u32 __hash_32_generic(u32 val) | |
61 | { | |
62 | return val * GOLDEN_RATIO_32; | |
63 | } | |
64 | ||
65 | #ifndef HAVE_ARCH_HASH_32 | |
66 | #define hash_32 hash_32_generic | |
0e55fa11 | 67 | #endif |
ae3c14a0 ACM |
68 | static inline u32 hash_32_generic(u32 val, unsigned int bits) |
69 | { | |
70 | /* High bits are more random, so use them. */ | |
71 | return __hash_32(val) >> (32 - bits); | |
72 | } | |
73 | ||
74 | #ifndef HAVE_ARCH_HASH_64 | |
75 | #define hash_64 hash_64_generic | |
76 | #endif | |
77 | static __always_inline u32 hash_64_generic(u64 val, unsigned int bits) | |
78 | { | |
79 | #if BITS_PER_LONG == 64 | |
80 | /* 64x64-bit multiply is efficient on all 64-bit processors */ | |
81 | return val * GOLDEN_RATIO_64 >> (64 - bits); | |
82 | #else | |
83 | /* Hash 64 bits using only 32x32-bit multiply. */ | |
84 | return hash_32((u32)val ^ __hash_32(val >> 32), bits); | |
85 | #endif | |
86 | } | |
87 | ||
88 | static inline u32 hash_ptr(const void *ptr, unsigned int bits) | |
89 | { | |
90 | return hash_long((unsigned long)ptr, bits); | |
91 | } | |
92 | ||
93 | /* This really should be called fold32_ptr; it does no hashing to speak of. */ | |
94 | static inline u32 hash32_ptr(const void *ptr) | |
95 | { | |
96 | unsigned long val = (unsigned long)ptr; | |
97 | ||
98 | #if BITS_PER_LONG == 64 | |
99 | val ^= (val >> 32); | |
100 | #endif | |
101 | return (u32)val; | |
102 | } | |
103 | ||
104 | #endif /* _LINUX_HASH_H */ |