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 <linux/compiler.h>
19
20 /* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */
21 #define GOLDEN_RATIO_PRIME_32 0x9e370001UL
22 /* 2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */
23 #define GOLDEN_RATIO_PRIME_64 0x9e37fffffffc0001UL
24
25 #if BITS_PER_LONG == 32
26 #define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_32
27 #define hash_long(val, bits) hash_32(val, bits)
28 #elif BITS_PER_LONG == 64
29 #define hash_long(val, bits) hash_64(val, bits)
30 #define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_64
31 #else
32 #error Wordsize not 32 or 64
33 #endif
34
hash_64(u64 val,unsigned int bits)35 static __always_inline u64 hash_64(u64 val, unsigned int bits)
36 {
37 u64 hash = val;
38
39 /* Sigh, gcc can't optimise this alone like it does for 32 bits. */
40 u64 n = hash;
41 n <<= 18;
42 hash -= n;
43 n <<= 33;
44 hash -= n;
45 n <<= 3;
46 hash += n;
47 n <<= 3;
48 hash -= n;
49 n <<= 4;
50 hash += n;
51 n <<= 2;
52 hash += n;
53
54 /* High bits are more random, so use them. */
55 return hash >> (64 - bits);
56 }
57
hash_32(u32 val,unsigned int bits)58 static inline u32 hash_32(u32 val, unsigned int bits)
59 {
60 /* On some cpus multiply is faster, on others gcc will do shifts */
61 u32 hash = val * GOLDEN_RATIO_PRIME_32;
62
63 /* High bits are more random, so use them. */
64 return hash >> (32 - bits);
65 }
66
hash_ptr(const void * ptr,unsigned int bits)67 static inline unsigned long hash_ptr(const void *ptr, unsigned int bits)
68 {
69 return hash_long((unsigned long)ptr, bits);
70 }
71
hash32_ptr(const void * ptr)72 static inline u32 hash32_ptr(const void *ptr)
73 {
74 unsigned long val = (unsigned long)ptr;
75
76 #if BITS_PER_LONG == 64
77 val ^= (val >> 32);
78 #endif
79 return (u32)val;
80 }
81 #endif /* _LINUX_HASH_H */
82