Harald Welte | 7101ca2 | 2020-12-04 21:44:44 +0100 | [diff] [blame] | 1 | #pragma once |
| 2 | #include <osmocom/core/log2.h> |
| 3 | /* Fast hashing routine for ints, longs and pointers. |
| 4 | (C) 2002 Nadia Yvette Chambers, IBM */ |
| 5 | |
| 6 | #include <limits.h> |
| 7 | #if ULONG_MAX == 4294967295 |
| 8 | #define BITS_PER_LONG 32 |
| 9 | #else |
| 10 | #define BITS_PER_LONG 64 |
| 11 | #endif |
| 12 | |
| 13 | /* |
| 14 | * The "GOLDEN_RATIO_PRIME" is used in ifs/btrfs/brtfs_inode.h and |
| 15 | * fs/inode.c. It's not actually prime any more (the previous primes |
| 16 | * were actively bad for hashing), but the name remains. |
| 17 | */ |
| 18 | #if BITS_PER_LONG == 32 |
| 19 | #define GOLDEN_RATIO_PRIME GOLDEN_RATIO_32 |
| 20 | #define hash_long(val, bits) hash_32(val, bits) |
| 21 | #elif BITS_PER_LONG == 64 |
| 22 | #define hash_long(val, bits) hash_64(val, bits) |
| 23 | #define GOLDEN_RATIO_PRIME GOLDEN_RATIO_64 |
| 24 | #else |
| 25 | #error Wordsize not 32 or 64 |
| 26 | #endif |
| 27 | |
| 28 | /* |
| 29 | * This hash multiplies the input by a large odd number and takes the |
| 30 | * high bits. Since multiplication propagates changes to the most |
| 31 | * significant end only, it is essential that the high bits of the |
| 32 | * product be used for the hash value. |
| 33 | * |
| 34 | * Chuck Lever verified the effectiveness of this technique: |
| 35 | * http://www.citi.umich.edu/techreports/reports/citi-tr-00-1.pdf |
| 36 | * |
| 37 | * Although a random odd number will do, it turns out that the golden |
| 38 | * ratio phi = (sqrt(5)-1)/2, or its negative, has particularly nice |
| 39 | * properties. (See Knuth vol 3, section 6.4, exercise 9.) |
| 40 | * |
| 41 | * These are the negative, (1 - phi) = phi**2 = (3 - sqrt(5))/2, |
| 42 | * which is very slightly easier to multiply by and makes no |
| 43 | * difference to the hash distribution. |
| 44 | */ |
| 45 | #define GOLDEN_RATIO_32 0x61C88647 |
| 46 | #define GOLDEN_RATIO_64 0x61C8864680B583EBull |
| 47 | |
| 48 | /* |
| 49 | * The _generic versions exist only so lib/test_hash.c can compare |
| 50 | * the arch-optimized versions with the generic. |
| 51 | * |
| 52 | * Note that if you change these, any <asm/hash.h> that aren't updated |
| 53 | * to match need to have their HAVE_ARCH_* define values updated so the |
| 54 | * self-test will not false-positive. |
| 55 | */ |
| 56 | #ifndef HAVE_ARCH__HASH_32 |
| 57 | #define __hash_32 __hash_32_generic |
| 58 | #endif |
| 59 | static inline uint32_t __hash_32_generic(uint32_t val) |
| 60 | { |
| 61 | return val * GOLDEN_RATIO_32; |
| 62 | } |
| 63 | |
| 64 | #ifndef HAVE_ARCH_HASH_32 |
| 65 | #define hash_32 hash_32_generic |
| 66 | #endif |
| 67 | static inline uint32_t hash_32_generic(uint32_t val, unsigned int bits) |
| 68 | { |
| 69 | /* High bits are more random, so use them. */ |
| 70 | return __hash_32(val) >> (32 - bits); |
| 71 | } |
| 72 | |
| 73 | #ifndef HAVE_ARCH_HASH_64 |
| 74 | #define hash_64 hash_64_generic |
| 75 | #endif |
| 76 | static __always_inline uint32_t hash_64_generic(uint64_t val, unsigned int bits) |
| 77 | { |
| 78 | #if BITS_PER_LONG == 64 |
| 79 | /* 64x64-bit multiply is efficient on all 64-bit processors */ |
| 80 | return val * GOLDEN_RATIO_64 >> (64 - bits); |
| 81 | #else |
| 82 | /* Hash 64 bits using only 32x32-bit multiply. */ |
| 83 | return hash_32((uint32_t)val ^ __hash_32(val >> 32), bits); |
| 84 | #endif |
| 85 | } |
| 86 | |
| 87 | static inline uint32_t hash_ptr(const void *ptr, unsigned int bits) |
| 88 | { |
| 89 | return hash_long((unsigned long)ptr, bits); |
| 90 | } |
| 91 | |
| 92 | /* This really should be called fold32_ptr; it does no hashing to speak of. */ |
| 93 | static inline uint32_t hash32_ptr(const void *ptr) |
| 94 | { |
| 95 | unsigned long val = (unsigned long)ptr; |
| 96 | |
| 97 | #if BITS_PER_LONG == 64 |
| 98 | val ^= (val >> 32); |
| 99 | #endif |
| 100 | return (uint32_t)val; |
| 101 | } |