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 /* 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 #include <asm/types.h> 18 #include <linux/compiler.h> 19 20 /* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */ 21 #define GOLDEN_RATIO_PRIME_32 0x9e370001UL 22 /* 2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */ 23 #define GOLDEN_RATIO_PRIME_64 0x9e37fffffffc0001UL 24 25 #if BITS_PER_LONG == 32 26 #define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_32 27 #define hash_long(val, bits) hash_32(val, bits) 28 #elif BITS_PER_LONG == 64 29 #define hash_long(val, bits) hash_64(val, bits) 30 #define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_64 31 #else 32 #error Wordsize not 32 or 64 33 #endif 34 35 /* 36 * The above primes are actively bad for hashing, since they are 37 * too sparse. The 32-bit one is mostly ok, the 64-bit one causes 38 * real problems. Besides, the "prime" part is pointless for the 39 * multiplicative hash. 40 * 41 * Although a random odd number will do, it turns out that the golden 42 * ratio phi = (sqrt(5)-1)/2, or its negative, has particularly nice 43 * properties. 44 * 45 * These are the negative, (1 - phi) = (phi^2) = (3 - sqrt(5))/2. 46 * (See Knuth vol 3, section 6.4, exercise 9.) 47 */ 48 #define GOLDEN_RATIO_32 0x61C88647 49 #define GOLDEN_RATIO_64 0x61C8864680B583EBull 50 51 static __always_inline u64 hash_64(u64 val, unsigned int bits) 52 { 53 u64 hash = val; 54 55 #if BITS_PER_LONG == 64 56 hash = hash * GOLDEN_RATIO_64; 57 #else 58 /* Sigh, gcc can't optimise this alone like it does for 32 bits. */ 59 u64 n = hash; 60 n <<= 18; 61 hash -= n; 62 n <<= 33; 63 hash -= n; 64 n <<= 3; 65 hash += n; 66 n <<= 3; 67 hash -= n; 68 n <<= 4; 69 hash += n; 70 n <<= 2; 71 hash += n; 72 #endif 73 74 /* High bits are more random, so use them. */ 75 return hash >> (64 - bits); 76 } 77 78 static inline u32 hash_32(u32 val, unsigned int bits) 79 { 80 /* On some cpus multiply is faster, on others gcc will do shifts */ 81 u32 hash = val * GOLDEN_RATIO_PRIME_32; 82 83 /* High bits are more random, so use them. */ 84 return hash >> (32 - bits); 85 } 86 87 static inline unsigned long hash_ptr(const void *ptr, unsigned int bits) 88 { 89 return hash_long((unsigned long)ptr, bits); 90 } 91 92 static inline u32 hash32_ptr(const void *ptr) 93 { 94 unsigned long val = (unsigned long)ptr; 95 96 #if BITS_PER_LONG == 64 97 val ^= (val >> 32); 98 #endif 99 return (u32)val; 100 } 101 102 #endif /* _LINUX_HASH_H */ 103