```   1#ifndef _LINUX_HASH_H
2#define _LINUX_HASH_H
3/* Fast hashing routine for ints,  longs and pointers.
4   (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 */
16
17#include <asm/types.h>
18
19/* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */
20#define GOLDEN_RATIO_PRIME_32 0x9e370001UL
21/*  2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */
22#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
30#else
31#error Wordsize not 32 or 64
32#endif
33
34static inline u64 hash_64(u64 val, unsigned int bits)
35{
36        u64 hash = val;
37
38        /*  Sigh, gcc can't optimise this alone like it does for 32 bits. */
39        u64 n = hash;
40        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;
52
53        /* High bits are more random, so use them. */
54        return hash >> (64 - bits);
55}
56
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
66static inline unsigned long hash_ptr(const void *ptr, unsigned int bits)
67{
68        return hash_long((unsigned long)ptr, bits);
69}
70#endif /* _LINUX_HASH_H */
71```
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