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 /* Sigh, gcc can't optimise this alone like it does for 32 bits. */ 41 u64 n = hash; 42 n <<= 18; 43 hash -= n; 44 n <<= 33; 45 hash -= n; 46 n <<= 3; 47 hash += n; 48 n <<= 3; 49 hash -= n; 50 n <<= 4; 51 hash += n; 52 n <<= 2; 53 hash += n; 54 55 /* High bits are more random, so use them. */ 56 return hash >> (64 - bits); 57 } 58 59 static inline u32 hash_32(u32 val, unsigned int bits) 60 { 61 /* On some cpus multiply is faster, on others gcc will do shifts */ 62 u32 hash = val * GOLDEN_RATIO_PRIME_32; 63 64 /* High bits are more random, so use them. */ 65 return hash >> (32 - bits); 66 } 67 68 static inline unsigned long hash_ptr(const void *ptr, unsigned int bits) 69 { 70 return hash_long((unsigned long)ptr, bits); 71 } 72 73 static inline u32 hash32_ptr(const void *ptr) 74 { 75 unsigned long val = (unsigned long)ptr; 76 77 #if BITS_PER_LONG == 64 78 val ^= (val >> 32); 79 #endif 80 return (u32)val; 81 } 82 83 struct fast_hash_ops { 84 u32 (*hash)(const void *data, u32 len, u32 seed); 85 u32 (*hash2)(const u32 *data, u32 len, u32 seed); 86 }; 87 88 /** 89 * arch_fast_hash - Caclulates a hash over a given buffer that can have 90 * arbitrary size. This function will eventually use an 91 * architecture-optimized hashing implementation if 92 * available, and trades off distribution for speed. 93 * 94 * @data: buffer to hash 95 * @len: length of buffer in bytes 96 * @seed: start seed 97 * 98 * Returns 32bit hash. 99 */ 100 extern u32 arch_fast_hash(const void *data, u32 len, u32 seed); 101 102 /** 103 * arch_fast_hash2 - Caclulates a hash over a given buffer that has a 104 * size that is of a multiple of 32bit words. This 105 * function will eventually use an architecture- 106 * optimized hashing implementation if available, 107 * and trades off distribution for speed. 108 * 109 * @data: buffer to hash (must be 32bit padded) 110 * @len: number of 32bit words 111 * @seed: start seed 112 * 113 * Returns 32bit hash. 114 */ 115 extern u32 arch_fast_hash2(const u32 *data, u32 len, u32 seed); 116 117 #endif /* _LINUX_HASH_H */ 118