1 /* SPDX-License-Identifier: GPL-2.0 */ 2 #ifndef _LINUX_HASH_H 3 #define _LINUX_HASH_H 4 /* Fast hashing routine for ints, longs and pointers. 5 (C) 2002 Nadia Yvette Chambers, IBM */ 6 7 #include <asm/types.h> 8 #include <linux/compiler.h> 9 10 /* 11 * The "GOLDEN_RATIO_PRIME" is used in ifs/btrfs/brtfs_inode.h and 12 * fs/inode.c. It's not actually prime any more (the previous primes 13 * were actively bad for hashing), but the name remains. 14 */ 15 #if BITS_PER_LONG == 32 16 #define GOLDEN_RATIO_PRIME GOLDEN_RATIO_32 17 #define hash_long(val, bits) hash_32(val, bits) 18 #elif BITS_PER_LONG == 64 19 #define hash_long(val, bits) hash_64(val, bits) 20 #define GOLDEN_RATIO_PRIME GOLDEN_RATIO_64 21 #else 22 #error Wordsize not 32 or 64 23 #endif 24 25 /* 26 * This hash multiplies the input by a large odd number and takes the 27 * high bits. Since multiplication propagates changes to the most 28 * significant end only, it is essential that the high bits of the 29 * product be used for the hash value. 30 * 31 * Chuck Lever verified the effectiveness of this technique: 32 * http://www.citi.umich.edu/techreports/reports/citi-tr-00-1.pdf 33 * 34 * Although a random odd number will do, it turns out that the golden 35 * ratio phi = (sqrt(5)-1)/2, or its negative, has particularly nice 36 * properties. (See Knuth vol 3, section 6.4, exercise 9.) 37 * 38 * These are the negative, (1 - phi) = phi**2 = (3 - sqrt(5))/2, 39 * which is very slightly easier to multiply by and makes no 40 * difference to the hash distribution. 41 */ 42 #define GOLDEN_RATIO_32 0x61C88647 43 #define GOLDEN_RATIO_64 0x61C8864680B583EBull 44 45 #ifdef CONFIG_HAVE_ARCH_HASH 46 /* This header may use the GOLDEN_RATIO_xx constants */ 47 #include <asm/hash.h> 48 #endif 49 50 /* 51 * The _generic versions exist only so lib/test_hash.c can compare 52 * the arch-optimized versions with the generic. 53 * 54 * Note that if you change these, any <asm/hash.h> that aren't updated 55 * to match need to have their HAVE_ARCH_* define values updated so the 56 * self-test will not false-positive. 57 */ 58 #ifndef HAVE_ARCH__HASH_32 59 #define __hash_32 __hash_32_generic 60 #endif 61 static inline u32 __hash_32_generic(u32 val) 62 { 63 return val * GOLDEN_RATIO_32; 64 } 65 66 #ifndef HAVE_ARCH_HASH_32 67 #define hash_32 hash_32_generic 68 #endif 69 static inline u32 hash_32_generic(u32 val, unsigned int bits) 70 { 71 /* High bits are more random, so use them. */ 72 return __hash_32(val) >> (32 - bits); 73 } 74 75 #ifndef HAVE_ARCH_HASH_64 76 #define hash_64 hash_64_generic 77 #endif 78 static __always_inline u32 hash_64_generic(u64 val, unsigned int bits) 79 { 80 #if BITS_PER_LONG == 64 81 /* 64x64-bit multiply is efficient on all 64-bit processors */ 82 return val * GOLDEN_RATIO_64 >> (64 - bits); 83 #else 84 /* Hash 64 bits using only 32x32-bit multiply. */ 85 return hash_32((u32)val ^ __hash_32(val >> 32), bits); 86 #endif 87 } 88 89 static inline u32 hash_ptr(const void *ptr, unsigned int bits) 90 { 91 return hash_long((unsigned long)ptr, bits); 92 } 93 94 /* This really should be called fold32_ptr; it does no hashing to speak of. */ 95 static inline u32 hash32_ptr(const void *ptr) 96 { 97 unsigned long val = (unsigned long)ptr; 98 99 #if BITS_PER_LONG == 64 100 val ^= (val >> 32); 101 #endif 102 return (u32)val; 103 } 104 105 #endif /* _LINUX_HASH_H */ 106