Commit | Line | Data |
---|---|---|
1da177e4 LT |
1 | #ifndef _LINUX_HASH_H |
2 | #define _LINUX_HASH_H | |
4e701482 | 3 | /* Fast hashing routine for ints, longs and pointers. |
6d49e352 | 4 | (C) 2002 Nadia Yvette Chambers, IBM */ |
1da177e4 LT |
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 | */ | |
4e701482 MW |
16 | |
17 | #include <asm/types.h> | |
65c10553 | 18 | #include <linux/compiler.h> |
4e701482 | 19 | |
1da177e4 | 20 | /* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */ |
4e701482 | 21 | #define GOLDEN_RATIO_PRIME_32 0x9e370001UL |
1da177e4 | 22 | /* 2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */ |
4e701482 MW |
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 | |
1da177e4 | 31 | #else |
4e701482 | 32 | #error Wordsize not 32 or 64 |
1da177e4 LT |
33 | #endif |
34 | ||
65c10553 | 35 | static __always_inline u64 hash_64(u64 val, unsigned int bits) |
1da177e4 | 36 | { |
4e701482 | 37 | u64 hash = val; |
1da177e4 | 38 | |
1da177e4 | 39 | /* Sigh, gcc can't optimise this alone like it does for 32 bits. */ |
4e701482 | 40 | u64 n = hash; |
1da177e4 LT |
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; | |
4e701482 MW |
53 | |
54 | /* High bits are more random, so use them. */ | |
55 | return hash >> (64 - bits); | |
56 | } | |
57 | ||
58 | static inline u32 hash_32(u32 val, unsigned int bits) | |
59 | { | |
1da177e4 | 60 | /* On some cpus multiply is faster, on others gcc will do shifts */ |
4e701482 | 61 | u32 hash = val * GOLDEN_RATIO_PRIME_32; |
1da177e4 LT |
62 | |
63 | /* High bits are more random, so use them. */ | |
4e701482 | 64 | return hash >> (32 - bits); |
1da177e4 | 65 | } |
4e701482 | 66 | |
f9918794 | 67 | static inline unsigned long hash_ptr(const void *ptr, unsigned int bits) |
1da177e4 LT |
68 | { |
69 | return hash_long((unsigned long)ptr, bits); | |
70 | } | |
b14f243a PE |
71 | |
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 | } | |
1da177e4 | 81 | #endif /* _LINUX_HASH_H */ |