3 /* Fast hashing routine for ints, longs and pointers.
4 (C) 2002 William Lee Irwin III, IBM */
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
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.
17 #include <asm/types.h>
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
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
31 #error Wordsize not 32 or 64
34 static inline u64
hash_64(u64 val
, unsigned int bits
)
38 /* Sigh, gcc can't optimise this alone like it does for 32 bits. */
53 /* High bits are more random, so use them. */
54 return hash
>> (64 - bits
);
57 static inline u32
hash_32(u32 val
, unsigned int bits
)
59 /* On some cpus multiply is faster, on others gcc will do shifts */
60 u32 hash
= val
* GOLDEN_RATIO_PRIME_32
;
62 /* High bits are more random, so use them. */
63 return hash
>> (32 - bits
);
66 static inline unsigned long hash_ptr(void *ptr
, unsigned int bits
)
68 return hash_long((unsigned long)ptr
, bits
);
70 #endif /* _LINUX_HASH_H */
This page took 0.030899 seconds and 5 git commands to generate.