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#ifdef CONFIG_HAVE_ARCH_HASH 45/* This header may use the GOLDEN_RATIO_xx constants */ 46#include <asm/hash.h> 47#endif 48 49/* 50 * The _generic versions exist only so lib/test_hash.c can compare 51 * the arch-optimized versions with the generic. 52 * 53 * Note that if you change these, any <asm/hash.h> that aren't updated 54 * to match need to have their HAVE_ARCH_* define values updated so the 55 * self-test will not false-positive. 56 */ 57#ifndef HAVE_ARCH__HASH_32 58#define __hash_32 __hash_32_generic 59#endif 60static inline u32 __hash_32_generic(u32 val) 61{ 62 return val * GOLDEN_RATIO_32; 63} 64 65#ifndef HAVE_ARCH_HASH_32 66#define hash_32 hash_32_generic 67#endif 68static inline u32 hash_32_generic(u32 val, unsigned int bits) 69{ 70 /* High bits are more random, so use them. */ 71 return __hash_32(val) >> (32 - bits); 72} 73 74#ifndef HAVE_ARCH_HASH_64 75#define hash_64 hash_64_generic 76#endif 77static __always_inline u32 hash_64_generic(u64 val, unsigned int bits) 78{ 79#if BITS_PER_LONG == 64 80 /* 64x64-bit multiply is efficient on all 64-bit processors */ 81 return val * GOLDEN_RATIO_64 >> (64 - bits); 82#else 83 /* Hash 64 bits using only 32x32-bit multiply. */ 84 return hash_32((u32)val ^ __hash_32(val >> 32), bits); 85#endif 86} 87 88static inline u32 hash_ptr(const void *ptr, unsigned int bits) 89{ 90 return hash_long((unsigned long)ptr, bits); 91} 92 93/* This really should be called fold32_ptr; it does no hashing to speak of. */ 94static inline u32 hash32_ptr(const void *ptr) 95{ 96 unsigned long val = (unsigned long)ptr; 97 98#if BITS_PER_LONG == 64 99 val ^= (val >> 32); 100#endif 101 return (u32)val; 102} 103 104#endif /* _LINUX_HASH_H */ 105