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 /*
37 * The above primes are actively bad for hashing, since they are
38 * too sparse. The 32-bit one is mostly ok, the 64-bit one causes
39 * real problems. Besides, the "prime" part is pointless for the
40 * multiplicative hash.
41 *
42 * Although a random odd number will do, it turns out that the golden
43 * ratio phi = (sqrt(5)-1)/2, or its negative, has particularly nice
44 * properties.
45 *
46 * These are the negative, (1 - phi) = (phi^2) = (3 - sqrt(5))/2.
47 * (See Knuth vol 3, section 6.4, exercise 9.)
48 */
49 #define GOLDEN_RATIO_32 0x61C88647
50 #define GOLDEN_RATIO_64 0x61C8864680B583EBull
51
hash_64(u64 val,unsigned int bits)52 static __always_inline u64 hash_64(u64 val, unsigned int bits)
53 {
54 u64 hash = val;
55
56 #if BITS_PER_LONG == 64
57 hash = hash * GOLDEN_RATIO_64;
58 #else
59 /* Sigh, gcc can't optimise this alone like it does for 32 bits. */
60 u64 n = hash;
61 n <<= 18;
62 hash -= n;
63 n <<= 33;
64 hash -= n;
65 n <<= 3;
66 hash += n;
67 n <<= 3;
68 hash -= n;
69 n <<= 4;
70 hash += n;
71 n <<= 2;
72 hash += n;
73 #endif
74
75 /* High bits are more random, so use them. */
76 return hash >> (64 - bits);
77 }
78
hash_32(u32 val,unsigned int bits)79 static inline u32 hash_32(u32 val, unsigned int bits)
80 {
81 /* On some cpus multiply is faster, on others gcc will do shifts */
82 u32 hash = val * GOLDEN_RATIO_PRIME_32;
83
84 /* High bits are more random, so use them. */
85 return hash >> (32 - bits);
86 }
87
hash_ptr(const void * ptr,unsigned int bits)88 static inline unsigned long hash_ptr(const void *ptr, unsigned int bits)
89 {
90 return hash_long((unsigned long)ptr, bits);
91 }
92
hash32_ptr(const void * ptr)93 static inline u32 hash32_ptr(const void *ptr)
94 {
95 unsigned long val = (unsigned long)ptr;
96
97 #if BITS_PER_LONG == 64
98 val ^= (val >> 32);
99 #endif
100 return (u32)val;
101 }
102
103 struct fast_hash_ops {
104 u32 (*hash)(const void *data, u32 len, u32 seed);
105 u32 (*hash2)(const u32 *data, u32 len, u32 seed);
106 };
107
108 /**
109 * arch_fast_hash - Caclulates a hash over a given buffer that can have
110 * arbitrary size. This function will eventually use an
111 * architecture-optimized hashing implementation if
112 * available, and trades off distribution for speed.
113 *
114 * @data: buffer to hash
115 * @len: length of buffer in bytes
116 * @seed: start seed
117 *
118 * Returns 32bit hash.
119 */
120 extern u32 arch_fast_hash(const void *data, u32 len, u32 seed);
121
122 /**
123 * arch_fast_hash2 - Caclulates a hash over a given buffer that has a
124 * size that is of a multiple of 32bit words. This
125 * function will eventually use an architecture-
126 * optimized hashing implementation if available,
127 * and trades off distribution for speed.
128 *
129 * @data: buffer to hash (must be 32bit padded)
130 * @len: number of 32bit words
131 * @seed: start seed
132 *
133 * Returns 32bit hash.
134 */
135 extern u32 arch_fast_hash2(const u32 *data, u32 len, u32 seed);
136
137 #endif /* _LINUX_HASH_H */
138