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 static __always_inline u64 hash_64(u64 val, unsigned int bits) 36 { 37 u64 hash = val; 38 39 /* Sigh, gcc can't optimise this alone like it does for 32 bits. */ 40 u64 n = hash; 41 n <<= 18; 42 hash -= n; 43 n <<= 33; 44 hash -= n; 45 n <<= 3; 46 hash += n; 47 n <<= 3; 48 hash -= n; 49 n <<= 4; 50 hash += n; 51 n <<= 2; 52 hash += n; 53 54 /* High bits are more random, so use them. */ 55 return hash >> (64 - bits); 56 } 57 58 static inline u32 hash_32(u32 val, unsigned int bits) 59 { 60 /* On some cpus multiply is faster, on others gcc will do shifts */ 61 u32 hash = val * GOLDEN_RATIO_PRIME_32; 62 63 /* High bits are more random, so use them. */ 64 return hash >> (32 - bits); 65 } 66 67 static inline unsigned long hash_ptr(const void *ptr, unsigned int bits) 68 { 69 return hash_long((unsigned long)ptr, bits); 70 } 71 72 static inline u32 hash32_ptr(const void *ptr) 73 { 74 unsigned long val = (unsigned long)ptr; 75 76 #if BITS_PER_LONG == 64 77 val ^= (val >> 32); 78 #endif 79 return (u32)val; 80 } 81 #endif /* _LINUX_HASH_H */ 82
