Line data Source code
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 */
5 :
6 : /*
7 : * Knuth recommends primes in approximately golden ratio to the maximum
8 : * integer representable by a machine word for multiplicative hashing.
9 : * Chuck Lever verified the effectiveness of this technique:
10 : * http://www.citi.umich.edu/techreports/reports/citi-tr-00-1.pdf
11 : *
12 : * These primes are chosen to be bit-sparse, that is operations on
13 : * them can use shifts and additions instead of multiplications for
14 : * machines where multiplications are slow.
15 : */
16 :
17 : #include <asm/types.h>
18 : #include <asm/hash.h>
19 : #include <linux/compiler.h>
20 :
21 : /* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */
22 : #define GOLDEN_RATIO_PRIME_32 0x9e370001UL
23 : /* 2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */
24 : #define GOLDEN_RATIO_PRIME_64 0x9e37fffffffc0001UL
25 :
26 : #if BITS_PER_LONG == 32
27 : #define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_32
28 : #define hash_long(val, bits) hash_32(val, bits)
29 : #elif BITS_PER_LONG == 64
30 : #define hash_long(val, bits) hash_64(val, bits)
31 : #define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_64
32 : #else
33 : #error Wordsize not 32 or 64
34 : #endif
35 :
36 : static __always_inline u64 hash_64(u64 val, unsigned int bits)
37 : {
38 : u64 hash = val;
39 :
40 : #if defined(CONFIG_ARCH_HAS_FAST_MULTIPLIER) && BITS_PER_LONG == 64
41 298 : hash = hash * GOLDEN_RATIO_PRIME_64;
42 : #else
43 : /* Sigh, gcc can't optimise this alone like it does for 32 bits. */
44 : u64 n = hash;
45 : n <<= 18;
46 : hash -= n;
47 : n <<= 33;
48 : hash -= n;
49 : n <<= 3;
50 : hash += n;
51 : n <<= 3;
52 : hash -= n;
53 : n <<= 4;
54 : hash += n;
55 : n <<= 2;
56 : hash += n;
57 : #endif
58 :
59 : /* High bits are more random, so use them. */
60 298 : return hash >> (64 - bits);
61 : }
62 :
63 : static inline u32 hash_32(u32 val, unsigned int bits)
64 : {
65 : /* On some cpus multiply is faster, on others gcc will do shifts */
66 : u32 hash = val * GOLDEN_RATIO_PRIME_32;
67 :
68 : /* High bits are more random, so use them. */
69 : return hash >> (32 - bits);
70 : }
71 :
72 : static inline unsigned long hash_ptr(const void *ptr, unsigned int bits)
73 : {
74 : return hash_long((unsigned long)ptr, bits);
75 : }
76 :
77 : static inline u32 hash32_ptr(const void *ptr)
78 : {
79 : unsigned long val = (unsigned long)ptr;
80 :
81 : #if BITS_PER_LONG == 64
82 : val ^= (val >> 32);
83 : #endif
84 : return (u32)val;
85 : }
86 :
87 : struct fast_hash_ops {
88 : u32 (*hash)(const void *data, u32 len, u32 seed);
89 : u32 (*hash2)(const u32 *data, u32 len, u32 seed);
90 : };
91 :
92 : /**
93 : * arch_fast_hash - Caclulates a hash over a given buffer that can have
94 : * arbitrary size. This function will eventually use an
95 : * architecture-optimized hashing implementation if
96 : * available, and trades off distribution for speed.
97 : *
98 : * @data: buffer to hash
99 : * @len: length of buffer in bytes
100 : * @seed: start seed
101 : *
102 : * Returns 32bit hash.
103 : */
104 : extern u32 arch_fast_hash(const void *data, u32 len, u32 seed);
105 :
106 : /**
107 : * arch_fast_hash2 - Caclulates a hash over a given buffer that has a
108 : * size that is of a multiple of 32bit words. This
109 : * function will eventually use an architecture-
110 : * optimized hashing implementation if available,
111 : * and trades off distribution for speed.
112 : *
113 : * @data: buffer to hash (must be 32bit padded)
114 : * @len: number of 32bit words
115 : * @seed: start seed
116 : *
117 : * Returns 32bit hash.
118 : */
119 : extern u32 arch_fast_hash2(const u32 *data, u32 len, u32 seed);
120 :
121 : #endif /* _LINUX_HASH_H */
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