1 #ifndef _LINUX_HASH_H 2 #define _LINUX_HASH_H 3 /* Fast hashing routine for ints, longs and pointers. 4 (C) 2002 William Lee Irwin III, 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 /* ANDROID_CHANGE_BEGIN */ 18 #ifndef __APPLE__ 19 #include <asm/types.h> 20 #endif 21 /* ANDROID_CHANGE_END */ 22 23 /* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */ 24 #define GOLDEN_RATIO_PRIME_32 0x9e370001UL 25 /* 2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */ 26 #define GOLDEN_RATIO_PRIME_64 0x9e37fffffffc0001UL 27 28 #if BITS_PER_LONG == 32 29 #define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_32 30 #define hash_long(val, bits) hash_32(val, bits) 31 #elif BITS_PER_LONG == 64 32 #define hash_long(val, bits) hash_64(val, bits) 33 #define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_64 34 #else 35 #error Wordsize not 32 or 64 36 #endif 37 38 static inline u64 hash_64(u64 val, unsigned int bits) 39 { 40 u64 hash = val; 41 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 57 /* High bits are more random, so use them. */ 58 return hash >> (64 - bits); 59 } 60 61 static inline u32 hash_32(u32 val, unsigned int bits) 62 { 63 /* On some cpus multiply is faster, on others gcc will do shifts */ 64 u32 hash = val * GOLDEN_RATIO_PRIME_32; 65 66 /* High bits are more random, so use them. */ 67 return hash >> (32 - bits); 68 } 69 70 static inline unsigned long hash_ptr(void *ptr, unsigned int bits) 71 { 72 return hash_long((unsigned long)ptr, bits); 73 } 74 #endif /* _LINUX_HASH_H */ 75
