```   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#include <asm/types.h>
7#include <linux/compiler.h>
8
9/*
10 * The "GOLDEN_RATIO_PRIME" is used in ifs/btrfs/brtfs_inode.h and
11 * fs/inode.c.  It's not actually prime any more (the previous primes
12 * were actively bad for hashing), but the name remains.
13 */
14#if BITS_PER_LONG == 32
15#define GOLDEN_RATIO_PRIME GOLDEN_RATIO_32
16#define hash_long(val, bits) hash_32(val, bits)
17#elif BITS_PER_LONG == 64
18#define hash_long(val, bits) hash_64(val, bits)
19#define GOLDEN_RATIO_PRIME GOLDEN_RATIO_64
20#else
21#error Wordsize not 32 or 64
22#endif
23
24/*
25 * This hash multiplies the input by a large odd number and takes the
26 * high bits.  Since multiplication propagates changes to the most
27 * significant end only, it is essential that the high bits of the
28 * product be used for the hash value.
29 *
30 * Chuck Lever verified the effectiveness of this technique:
31 * http://www.citi.umich.edu/techreports/reports/citi-tr-00-1.pdf
32 *
33 * Although a random odd number will do, it turns out that the golden
34 * ratio phi = (sqrt(5)-1)/2, or its negative, has particularly nice
35 * properties.  (See Knuth vol 3, section 6.4, exercise 9.)
36 *
37 * These are the negative, (1 - phi) = phi**2 = (3 - sqrt(5))/2,
38 * which is very slightly easier to multiply by and makes no
39 * difference to the hash distribution.
40 */
41#define GOLDEN_RATIO_32 0x61C88647
42#define GOLDEN_RATIO_64 0x61C8864680B583EBull
43
44#ifdef CONFIG_HAVE_ARCH_HASH
45/* This header may use the GOLDEN_RATIO_xx constants */
46#include <asm/hash.h>
47#endif
48
49/*
50 * The _generic versions exist only so lib/test_hash.c can compare
51 * the arch-optimized versions with the generic.
52 *
53 * Note that if you change these, any <asm/hash.h> that aren't updated
54 * to match need to have their HAVE_ARCH_* define values updated so the
55 * self-test will not false-positive.
56 */
57#ifndef HAVE_ARCH__HASH_32
58#define __hash_32 __hash_32_generic
59#endif
60static inline u32 __hash_32_generic(u32 val)
61{
62        return val * GOLDEN_RATIO_32;
63}
64
65static inline u32 hash_32(u32 val, unsigned int bits)
66{
67        /* High bits are more random, so use them. */
68        return __hash_32(val) >> (32 - bits);
69}
70
71#ifndef HAVE_ARCH_HASH_64
72#define hash_64 hash_64_generic
73#endif
74static __always_inline u32 hash_64_generic(u64 val, unsigned int bits)
75{
76#if BITS_PER_LONG == 64
77        /* 64x64-bit multiply is efficient on all 64-bit processors */
78        return val * GOLDEN_RATIO_64 >> (64 - bits);
79#else
80        /* Hash 64 bits using only 32x32-bit multiply. */
81        return hash_32((u32)val ^ __hash_32(val >> 32), bits);
82#endif
83}
84
85static inline u32 hash_ptr(const void *ptr, unsigned int bits)
86{
87        return hash_long((unsigned long)ptr, bits);
88}
89
90/* This really should be called fold32_ptr; it does no hashing to speak of. */
91static inline u32 hash32_ptr(const void *ptr)
92{
93        unsigned long val = (unsigned long)ptr;
94
95#if BITS_PER_LONG == 64
96        val ^= (val >> 32);
97#endif
98        return (u32)val;
99}
100
101#endif /* _LINUX_HASH_H */
102```