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Linus Torvalds1da177e2005-04-16 15:20:36 -07001#ifndef _LINUX_HASH_H
2#define _LINUX_HASH_H
Matthew Wilcox4e701482008-02-06 01:36:14 -08003/* Fast hashing routine for ints, longs and pointers.
Linus Torvalds1da177e2005-04-16 15:20:36 -07004 (C) 2002 William Lee Irwin III, 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 */
Matthew Wilcox4e701482008-02-06 01:36:14 -080016
17#include <asm/types.h>
18
Linus Torvalds1da177e2005-04-16 15:20:36 -070019/* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */
Matthew Wilcox4e701482008-02-06 01:36:14 -080020#define GOLDEN_RATIO_PRIME_32 0x9e370001UL
Linus Torvalds1da177e2005-04-16 15:20:36 -070021/* 2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */
Matthew Wilcox4e701482008-02-06 01:36:14 -080022#define GOLDEN_RATIO_PRIME_64 0x9e37fffffffc0001UL
23
24#if BITS_PER_LONG == 32
25#define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_32
26#define hash_long(val, bits) hash_32(val, bits)
27#elif BITS_PER_LONG == 64
28#define hash_long(val, bits) hash_64(val, bits)
29#define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_64
Linus Torvalds1da177e2005-04-16 15:20:36 -070030#else
Matthew Wilcox4e701482008-02-06 01:36:14 -080031#error Wordsize not 32 or 64
Linus Torvalds1da177e2005-04-16 15:20:36 -070032#endif
33
Matthew Wilcox4e701482008-02-06 01:36:14 -080034static inline u64 hash_64(u64 val, unsigned int bits)
Linus Torvalds1da177e2005-04-16 15:20:36 -070035{
Matthew Wilcox4e701482008-02-06 01:36:14 -080036 u64 hash = val;
Linus Torvalds1da177e2005-04-16 15:20:36 -070037
Linus Torvalds1da177e2005-04-16 15:20:36 -070038 /* Sigh, gcc can't optimise this alone like it does for 32 bits. */
Matthew Wilcox4e701482008-02-06 01:36:14 -080039 u64 n = hash;
Linus Torvalds1da177e2005-04-16 15:20:36 -070040 n <<= 18;
41 hash -= n;
42 n <<= 33;
43 hash -= n;
44 n <<= 3;
45 hash += n;
46 n <<= 3;
47 hash -= n;
48 n <<= 4;
49 hash += n;
50 n <<= 2;
51 hash += n;
Linus Torvalds1da177e2005-04-16 15:20:36 -070052
53 /* High bits are more random, so use them. */
Matthew Wilcox4e701482008-02-06 01:36:14 -080054 return hash >> (64 - bits);
Linus Torvalds1da177e2005-04-16 15:20:36 -070055}
Matthew Wilcox4e701482008-02-06 01:36:14 -080056
57static inline u32 hash_32(u32 val, unsigned int bits)
58{
59 /* On some cpus multiply is faster, on others gcc will do shifts */
60 u32 hash = val * GOLDEN_RATIO_PRIME_32;
61
62 /* High bits are more random, so use them. */
63 return hash >> (32 - bits);
64}
65
Ian Campbellf9918792011-08-17 13:45:09 +010066static inline unsigned long hash_ptr(const void *ptr, unsigned int bits)
Linus Torvalds1da177e2005-04-16 15:20:36 -070067{
68 return hash_long((unsigned long)ptr, bits);
69}
70#endif /* _LINUX_HASH_H */