```   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
35static __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
58static 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
67static inline unsigned long hash_ptr(const void *ptr, unsigned int bits)
68{
69        return hash_long((unsigned long)ptr, bits);
70}
71
72static 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```
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