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 35static __always_inline u64 hash_64(u64 val, unsigned int bits) 36{ 37 u64 hash = val; 38 39#if defined(CONFIG_ARCH_HAS_FAST_MULTIPLIER) && BITS_PER_LONG == 64 40 hash = hash * GOLDEN_RATIO_PRIME_64; 41#else 42 /* Sigh, gcc can't optimise this alone like it does for 32 bits. */ 43 u64 n = hash; 44 n <<= 18; 45 hash -= n; 46 n <<= 33; 47 hash -= n; 48 n <<= 3; 49 hash += n; 50 n <<= 3; 51 hash -= n; 52 n <<= 4; 53 hash += n; 54 n <<= 2; 55 hash += n; 56#endif 57 58 /* High bits are more random, so use them. */ 59 return hash >> (64 - bits); 60} 61 62static inline u32 hash_32(u32 val, unsigned int bits) 63{ 64 /* On some cpus multiply is faster, on others gcc will do shifts */ 65 u32 hash = val * GOLDEN_RATIO_PRIME_32; 66 67 /* High bits are more random, so use them. */ 68 return hash >> (32 - bits); 69} 70 71static inline unsigned long hash_ptr(const void *ptr, unsigned int bits) 72{ 73 return hash_long((unsigned long)ptr, bits); 74} 75 76static inline u32 hash32_ptr(const void *ptr) 77{ 78 unsigned long val = (unsigned long)ptr; 79 80#if BITS_PER_LONG == 64 81 val ^= (val >> 32); 82#endif 83 return (u32)val; 84} 85 86#endif /* _LINUX_HASH_H */ 87