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 <asm/hash.h> 19 #include <linux/compiler.h> 20 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 25 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 32 #else 33 #error Wordsize not 32 or 64 34 #endif 35 36 static __always_inline u64 hash_64(u64 val, unsigned int bits) 37 { 38 u64 hash = val; 39 40 #if defined(CONFIG_ARCH_HAS_FAST_MULTIPLIER) && BITS_PER_LONG == 64 41 hash = hash * GOLDEN_RATIO_PRIME_64; 42 #else 43 /* Sigh, gcc can't optimise this alone like it does for 32 bits. */ 44 u64 n = hash; 45 n <<= 18; 46 hash -= n; 47 n <<= 33; 48 hash -= n; 49 n <<= 3; 50 hash += n; 51 n <<= 3; 52 hash -= n; 53 n <<= 4; 54 hash += n; 55 n <<= 2; 56 hash += n; 57 #endif 58 59 /* High bits are more random, so use them. */ 60 return hash >> (64 - bits); 61 } 62 63 static inline u32 hash_32(u32 val, unsigned int bits) 64 { 65 /* On some cpus multiply is faster, on others gcc will do shifts */ 66 u32 hash = val * GOLDEN_RATIO_PRIME_32; 67 68 /* High bits are more random, so use them. */ 69 return hash >> (32 - bits); 70 } 71 72 static inline unsigned long hash_ptr(const void *ptr, unsigned int bits) 73 { 74 return hash_long((unsigned long)ptr, bits); 75 } 76 77 static inline u32 hash32_ptr(const void *ptr) 78 { 79 unsigned long val = (unsigned long)ptr; 80 81 #if BITS_PER_LONG == 64 82 val ^= (val >> 32); 83 #endif 84 return (u32)val; 85 } 86 87 #endif /* _LINUX_HASH_H */ 88