3 /* Fast hashing routine for ints, longs and pointers.
4 (C) 2002 Nadia Yvette Chambers, 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 #include <linux/compiler.h>
21 /* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */
22 #define GOLDEN_RATIO_PRIME_32 0x9e370001UL
23 /* 2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */
24 #define GOLDEN_RATIO_PRIME_64 0x9e37fffffffc0001UL
26 #if BITS_PER_LONG == 32
27 #define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_32
28 #define hash_long(val, bits) hash_32(val, bits)
29 #elif BITS_PER_LONG == 64
30 #define hash_long(val, bits) hash_64(val, bits)
31 #define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_64
33 #error Wordsize not 32 or 64
37 * The above primes are actively bad for hashing, since they are
38 * too sparse. The 32-bit one is mostly ok, the 64-bit one causes
39 * real problems. Besides, the "prime" part is pointless for the
40 * multiplicative hash.
42 * Although a random odd number will do, it turns out that the golden
43 * ratio phi = (sqrt(5)-1)/2, or its negative, has particularly nice
46 * These are the negative, (1 - phi) = (phi^2) = (3 - sqrt(5))/2.
47 * (See Knuth vol 3, section 6.4, exercise 9.)
49 #define GOLDEN_RATIO_32 0x61C88647
50 #define GOLDEN_RATIO_64 0x61C8864680B583EBull
52 static __always_inline u64 hash_64(u64 val, unsigned int bits)
56 #if BITS_PER_LONG == 64
57 hash = hash * GOLDEN_RATIO_64;
59 /* Sigh, gcc can't optimise this alone like it does for 32 bits. */
75 /* High bits are more random, so use them. */
76 return hash >> (64 - bits);
79 static inline u32 hash_32(u32 val, unsigned int bits)
81 /* On some cpus multiply is faster, on others gcc will do shifts */
82 u32 hash = val * GOLDEN_RATIO_PRIME_32;
84 /* High bits are more random, so use them. */
85 return hash >> (32 - bits);
88 static inline unsigned long hash_ptr(const void *ptr, unsigned int bits)
90 return hash_long((unsigned long)ptr, bits);
93 static inline u32 hash32_ptr(const void *ptr)
95 unsigned long val = (unsigned long)ptr;
97 #if BITS_PER_LONG == 64
103 struct fast_hash_ops {
104 u32 (*hash)(const void *data, u32 len, u32 seed);
105 u32 (*hash2)(const u32 *data, u32 len, u32 seed);
109 * arch_fast_hash - Caclulates a hash over a given buffer that can have
110 * arbitrary size. This function will eventually use an
111 * architecture-optimized hashing implementation if
112 * available, and trades off distribution for speed.
114 * @data: buffer to hash
115 * @len: length of buffer in bytes
118 * Returns 32bit hash.
120 extern u32 arch_fast_hash(const void *data, u32 len, u32 seed);
123 * arch_fast_hash2 - Caclulates a hash over a given buffer that has a
124 * size that is of a multiple of 32bit words. This
125 * function will eventually use an architecture-
126 * optimized hashing implementation if available,
127 * and trades off distribution for speed.
129 * @data: buffer to hash (must be 32bit padded)
130 * @len: number of 32bit words
133 * Returns 32bit hash.
135 extern u32 arch_fast_hash2(const u32 *data, u32 len, u32 seed);
137 #endif /* _LINUX_HASH_H */