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
35/*
36 * The above primes are actively bad for hashing, since they are
37 * too sparse. The 32-bit one is mostly ok, the 64-bit one causes
38 * real problems. Besides, the "prime" part is pointless for the
39 * multiplicative hash.
40 *
41 * Although a random odd number will do, it turns out that the golden
42 * ratio phi = (sqrt(5)-1)/2, or its negative, has particularly nice
43 * properties.
44 *
45 * These are the negative, (1 - phi) = (phi^2) = (3 - sqrt(5))/2.
46 * (See Knuth vol 3, section 6.4, exercise 9.)
47 */
48#define GOLDEN_RATIO_32 0x61C88647
49#define GOLDEN_RATIO_64 0x61C8864680B583EBull
50
51static __always_inline u64 hash_64(u64 val, unsigned int bits)
52{
53	u64 hash = val;
54
55#if BITS_PER_LONG == 64
56	hash = hash * GOLDEN_RATIO_64;
57#else
58	/*  Sigh, gcc can't optimise this alone like it does for 32 bits. */
59	u64 n = hash;
60	n <<= 18;
61	hash -= n;
62	n <<= 33;
63	hash -= n;
64	n <<= 3;
65	hash += n;
66	n <<= 3;
67	hash -= n;
68	n <<= 4;
69	hash += n;
70	n <<= 2;
71	hash += n;
72#endif
73
74	/* High bits are more random, so use them. */
75	return hash >> (64 - bits);
76}
77
78static inline u32 hash_32(u32 val, unsigned int bits)
79{
80	/* On some cpus multiply is faster, on others gcc will do shifts */
81	u32 hash = val * GOLDEN_RATIO_PRIME_32;
82
83	/* High bits are more random, so use them. */
84	return hash >> (32 - bits);
85}
86
87static inline unsigned long hash_ptr(const void *ptr, unsigned int bits)
88{
89	return hash_long((unsigned long)ptr, bits);
90}
91
92static inline u32 hash32_ptr(const void *ptr)
93{
94	unsigned long val = (unsigned long)ptr;
95
96#if BITS_PER_LONG == 64
97	val ^= (val >> 32);
98#endif
99	return (u32)val;
100}
101
102#endif /* _LINUX_HASH_H */
103