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 /* ANDROID_CHANGE_BEGIN */
18 #ifndef __APPLE__
19 #include <asm/types.h>
20 #endif
21 /* ANDROID_CHANGE_END */
22
23 /* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */
24 #define GOLDEN_RATIO_PRIME_32 0x9e370001UL
25 /* 2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */
26 #define GOLDEN_RATIO_PRIME_64 0x9e37fffffffc0001UL
27
28 #if BITS_PER_LONG == 32
29 #define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_32
30 #define hash_long(val, bits) hash_32(val, bits)
31 #elif BITS_PER_LONG == 64
32 #define hash_long(val, bits) hash_64(val, bits)
33 #define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_64
34 #else
35 #error Wordsize not 32 or 64
36 #endif
37
hash_64(u64 val,unsigned int bits)38 static inline u64 hash_64(u64 val, unsigned int bits)
39 {
40 u64 hash = val;
41
42 /* Sigh, gcc can't optimise this alone like it does for 32 bits. */
43 u64 n = hash;
44 n <<= 18;
45 hash -= n;
46 n <<= 33;
47 hash -= n;
48 n <<= 3;
49 hash += n;
50 n <<= 3;
51 hash -= n;
52 n <<= 4;
53 hash += n;
54 n <<= 2;
55 hash += n;
56
57 /* High bits are more random, so use them. */
58 return hash >> (64 - bits);
59 }
60
hash_32(u32 val,unsigned int bits)61 static inline u32 hash_32(u32 val, unsigned int bits)
62 {
63 /* On some cpus multiply is faster, on others gcc will do shifts */
64 u32 hash = val * GOLDEN_RATIO_PRIME_32;
65
66 /* High bits are more random, so use them. */
67 return hash >> (32 - bits);
68 }
69
hash_ptr(void * ptr,unsigned int bits)70 static inline unsigned long hash_ptr(void *ptr, unsigned int bits)
71 {
72 return hash_long((unsigned long)ptr, bits);
73 }
74 #endif /* _LINUX_HASH_H */
75