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 #include <asm/types.h> 7 #include <linux/compiler.h> 8 9 /* 10 * The "GOLDEN_RATIO_PRIME" is used in ifs/btrfs/brtfs_inode.h and 11 * fs/inode.c. It's not actually prime any more (the previous primes 12 * were actively bad for hashing), but the name remains. 13 */ 14 #if BITS_PER_LONG == 32 15 #define GOLDEN_RATIO_PRIME GOLDEN_RATIO_32 16 #define hash_long(val, bits) hash_32(val, bits) 17 #elif BITS_PER_LONG == 64 18 #define hash_long(val, bits) hash_64(val, bits) 19 #define GOLDEN_RATIO_PRIME GOLDEN_RATIO_64 20 #else 21 #error Wordsize not 32 or 64 22 #endif 23 24 /* 25 * This hash multiplies the input by a large odd number and takes the 26 * high bits. Since multiplication propagates changes to the most 27 * significant end only, it is essential that the high bits of the 28 * product be used for the hash value. 29 * 30 * Chuck Lever verified the effectiveness of this technique: 31 * http://www.citi.umich.edu/techreports/reports/citi-tr-00-1.pdf 32 * 33 * Although a random odd number will do, it turns out that the golden 34 * ratio phi = (sqrt(5)-1)/2, or its negative, has particularly nice 35 * properties. (See Knuth vol 3, section 6.4, exercise 9.) 36 * 37 * These are the negative, (1 - phi) = phi**2 = (3 - sqrt(5))/2, 38 * which is very slightly easier to multiply by and makes no 39 * difference to the hash distribution. 40 */ 41 #define GOLDEN_RATIO_32 0x61C88647 42 #define GOLDEN_RATIO_64 0x61C8864680B583EBull 43 44 45 static inline u32 __hash_32(u32 val) 46 { 47 return val * GOLDEN_RATIO_32; 48 } 49 50 static inline u32 hash_32(u32 val, unsigned int bits) 51 { 52 /* High bits are more random, so use them. */ 53 return __hash_32(val) >> (32 - bits); 54 } 55 56 static __always_inline u32 hash_64(u64 val, unsigned int bits) 57 { 58 #if BITS_PER_LONG == 64 59 /* 64x64-bit multiply is efficient on all 64-bit processors */ 60 return val * GOLDEN_RATIO_64 >> (64 - bits); 61 #else 62 /* Hash 64 bits using only 32x32-bit multiply. */ 63 return hash_32((u32)val ^ __hash_32(val >> 32), bits); 64 #endif 65 } 66 67 static inline u32 hash_ptr(const void *ptr, unsigned int bits) 68 { 69 return hash_long((unsigned long)ptr, bits); 70 } 71 72 /* This really should be called fold32_ptr; it does no hashing to speak of. */ 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 #endif /* _LINUX_HASH_H */ 84