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
35 * The above primes are actively bad for hashing, since they are
36 * too sparse. The 32-bit one is mostly ok, the 64-bit one causes
37 * real problems. Besides, the "prime" part is pointless for the
38 * multiplicative hash.
40 * Although a random odd number will do, it turns out that the golden
41 * ratio phi = (sqrt(5)-1)/2, or its negative, has particularly nice
44 * These are the negative, (1 - phi) = (phi^2) = (3 - sqrt(5))/2.
45 * (See Knuth vol 3, section 6.4, exercise 9.)
47 #define GOLDEN_RATIO_32 0x61C88647
48 #define GOLDEN_RATIO_64 0x61C8864680B583EBull
50 static inline u64 hash_64(u64 val, unsigned int bits)
54 #if BITS_PER_LONG == 64
55 hash = hash * GOLDEN_RATIO_64;
57 /* Sigh, gcc can't optimise this alone like it does for 32 bits. */
73 /* High bits are more random, so use them. */
74 return hash >> (64 - bits);
77 static inline u32 hash_32(u32 val, unsigned int bits)
79 /* On some cpus multiply is faster, on others gcc will do shifts */
80 u32 hash = val * GOLDEN_RATIO_PRIME_32;
82 /* High bits are more random, so use them. */
83 return hash >> (32 - bits);
86 static inline unsigned long hash_ptr(const void *ptr, unsigned int bits)
88 return hash_long((unsigned long)ptr, bits);
90 #endif /* _LINUX_HASH_H */