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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 */
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 /* Sigh, gcc can't optimise this alone like it does for 32 bits. */
41 u64 n = hash;
42 n <<= 18;
43 hash -= n;
44 n <<= 33;
45 hash -= n;
46 n <<= 3;
47 hash += n;
48 n <<= 3;
49 hash -= n;
50 n <<= 4;
51 hash += n;
52 n <<= 2;
53 hash += n;
54
55 /* High bits are more random, so use them. */
56 return hash >> (64 - bits);
57 }
58
59 static inline u32 hash_32(u32 val, unsigned int bits)
60 {
61 /* On some cpus multiply is faster, on others gcc will do shifts */
62 u32 hash = val * GOLDEN_RATIO_PRIME_32;
63
64 /* High bits are more random, so use them. */
65 return hash >> (32 - bits);
66 }
67
68 static inline unsigned long hash_ptr(const void *ptr, unsigned int bits)
69 {
70 return hash_long((unsigned long)ptr, bits);
71 }
72
73 static inline u32 hash32_ptr(const void *ptr)
74 {
75 unsigned long val = (unsigned long)ptr;
76
77 #if BITS_PER_LONG == 64
78 val ^= (val >> 32);
79 #endif
80 return (u32)val;
81 }
82
83 struct fast_hash_ops {
84 u32 (*hash)(const void *data, u32 len, u32 seed);
85 u32 (*hash2)(const u32 *data, u32 len, u32 seed);
86 };
87
88 /**
89 * arch_fast_hash - Caclulates a hash over a given buffer that can have
90 * arbitrary size. This function will eventually use an
91 * architecture-optimized hashing implementation if
92 * available, and trades off distribution for speed.
93 *
94 * @data: buffer to hash
95 * @len: length of buffer in bytes
96 * @seed: start seed
97 *
98 * Returns 32bit hash.
99 */
100 extern u32 arch_fast_hash(const void *data, u32 len, u32 seed);
101
102 /**
103 * arch_fast_hash2 - Caclulates a hash over a given buffer that has a
104 * size that is of a multiple of 32bit words. This
105 * function will eventually use an architecture-
106 * optimized hashing implementation if available,
107 * and trades off distribution for speed.
108 *
109 * @data: buffer to hash (must be 32bit padded)
110 * @len: number of 32bit words
111 * @seed: start seed
112 *
113 * Returns 32bit hash.
114 */
115 extern u32 arch_fast_hash2(const u32 *data, u32 len, u32 seed);
116
117 #endif /* _LINUX_HASH_H */