| 1 | /* Include file cached obstack implementation. |
| 2 | Written by Fred Fish <fnf@cygnus.com> |
| 3 | Rewritten by Jim Blandy <jimb@cygnus.com> |
| 4 | |
| 5 | Copyright (C) 1999-2017 Free Software Foundation, Inc. |
| 6 | |
| 7 | This file is part of GDB. |
| 8 | |
| 9 | This program is free software; you can redistribute it and/or modify |
| 10 | it under the terms of the GNU General Public License as published by |
| 11 | the Free Software Foundation; either version 3 of the License, or |
| 12 | (at your option) any later version. |
| 13 | |
| 14 | This program is distributed in the hope that it will be useful, |
| 15 | but WITHOUT ANY WARRANTY; without even the implied warranty of |
| 16 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the |
| 17 | GNU General Public License for more details. |
| 18 | |
| 19 | You should have received a copy of the GNU General Public License |
| 20 | along with this program. If not, see <http://www.gnu.org/licenses/>. */ |
| 21 | |
| 22 | #ifndef BCACHE_H |
| 23 | #define BCACHE_H 1 |
| 24 | |
| 25 | /* A bcache is a data structure for factoring out duplication in |
| 26 | read-only structures. You give the bcache some string of bytes S. |
| 27 | If the bcache already contains a copy of S, it hands you back a |
| 28 | pointer to its copy. Otherwise, it makes a fresh copy of S, and |
| 29 | hands you back a pointer to that. In either case, you can throw |
| 30 | away your copy of S, and use the bcache's. |
| 31 | |
| 32 | The "strings" in question are arbitrary strings of bytes --- they |
| 33 | can contain zero bytes. You pass in the length explicitly when you |
| 34 | call the bcache function. |
| 35 | |
| 36 | This means that you can put ordinary C objects in a bcache. |
| 37 | However, if you do this, remember that structs can contain `holes' |
| 38 | between members, added for alignment. These bytes usually contain |
| 39 | garbage. If you try to bcache two objects which are identical from |
| 40 | your code's point of view, but have different garbage values in the |
| 41 | structure's holes, then the bcache will treat them as separate |
| 42 | strings, and you won't get the nice elimination of duplicates you |
| 43 | were hoping for. So, remember to memset your structures full of |
| 44 | zeros before bcaching them! |
| 45 | |
| 46 | You shouldn't modify the strings you get from a bcache, because: |
| 47 | |
| 48 | - You don't necessarily know who you're sharing space with. If I |
| 49 | stick eight bytes of text in a bcache, and then stick an eight-byte |
| 50 | structure in the same bcache, there's no guarantee those two |
| 51 | objects don't actually comprise the same sequence of bytes. If |
| 52 | they happen to, the bcache will use a single byte string for both |
| 53 | of them. Then, modifying the structure will change the string. In |
| 54 | bizarre ways. |
| 55 | |
| 56 | - Even if you know for some other reason that all that's okay, |
| 57 | there's another problem. A bcache stores all its strings in a hash |
| 58 | table. If you modify a string's contents, you will probably change |
| 59 | its hash value. This means that the modified string is now in the |
| 60 | wrong place in the hash table, and future bcache probes will never |
| 61 | find it. So by mutating a string, you give up any chance of |
| 62 | sharing its space with future duplicates. |
| 63 | |
| 64 | |
| 65 | Size of bcache VS hashtab: |
| 66 | |
| 67 | For bcache, the most critical cost is size (or more exactly the |
| 68 | overhead added by the bcache). It turns out that the bcache is |
| 69 | remarkably efficient. |
| 70 | |
| 71 | Assuming a 32-bit system (the hash table slots are 4 bytes), |
| 72 | ignoring alignment, and limit strings to 255 bytes (1 byte length) |
| 73 | we get ... |
| 74 | |
| 75 | bcache: This uses a separate linked list to track the hash chain. |
| 76 | The numbers show roughly 100% occupancy of the hash table and an |
| 77 | average chain length of 4. Spreading the slot cost over the 4 |
| 78 | chain elements: |
| 79 | |
| 80 | 4 (slot) / 4 (chain length) + 1 (length) + 4 (chain) = 6 bytes |
| 81 | |
| 82 | hashtab: This uses a more traditional re-hash algorithm where the |
| 83 | chain is maintained within the hash table. The table occupancy is |
| 84 | kept below 75% but we'll assume its perfect: |
| 85 | |
| 86 | 4 (slot) x 4/3 (occupancy) + 1 (length) = 6 1/3 bytes |
| 87 | |
| 88 | So a perfect hashtab has just slightly larger than an average |
| 89 | bcache. |
| 90 | |
| 91 | It turns out that an average hashtab is far worse. Two things |
| 92 | hurt: |
| 93 | |
| 94 | - Hashtab's occupancy is more like 50% (it ranges between 38% and |
| 95 | 75%) giving a per slot cost of 4x2 vs 4x4/3. |
| 96 | |
| 97 | - the string structure needs to be aligned to 8 bytes which for |
| 98 | hashtab wastes 7 bytes, while for bcache wastes only 3. |
| 99 | |
| 100 | This gives: |
| 101 | |
| 102 | hashtab: 4 x 2 + 1 + 7 = 16 bytes |
| 103 | |
| 104 | bcache 4 / 4 + 1 + 4 + 3 = 9 bytes |
| 105 | |
| 106 | The numbers of GDB debugging GDB support this. ~40% vs ~70% overhead. |
| 107 | |
| 108 | |
| 109 | Speed of bcache VS hashtab (the half hash hack): |
| 110 | |
| 111 | While hashtab has a typical chain length of 1, bcache has a chain |
| 112 | length of round 4. This means that the bcache will require |
| 113 | something like double the number of compares after that initial |
| 114 | hash. In both cases the comparison takes the form: |
| 115 | |
| 116 | a.length == b.length && memcmp (a.data, b.data, a.length) == 0 |
| 117 | |
| 118 | That is lengths are checked before doing the memcmp. |
| 119 | |
| 120 | For GDB debugging GDB, it turned out that all lengths were 24 bytes |
| 121 | (no C++ so only psymbols were cached) and hence, all compares |
| 122 | required a call to memcmp. As a hack, two bytes of padding |
| 123 | (mentioned above) are used to store the upper 16 bits of the |
| 124 | string's hash value and then that is used in the comparison vis: |
| 125 | |
| 126 | a.half_hash == b.half_hash && a.length == b.length && memcmp |
| 127 | (a.data, b.data, a.length) |
| 128 | |
| 129 | The numbers from GDB debugging GDB show this to be a remarkable |
| 130 | 100% effective (only necessary length and memcmp tests being |
| 131 | performed). |
| 132 | |
| 133 | Mind you, looking at the wall clock, the same GDB debugging GDB |
| 134 | showed only marginal speed up (0.780 vs 0.773s). Seems GDB is too |
| 135 | busy doing something else :-( |
| 136 | |
| 137 | */ |
| 138 | |
| 139 | |
| 140 | struct bcache; |
| 141 | |
| 142 | /* Find a copy of the LENGTH bytes at ADDR in BCACHE. If BCACHE has |
| 143 | never seen those bytes before, add a copy of them to BCACHE. In |
| 144 | either case, return a pointer to BCACHE's copy of that string. |
| 145 | Since the cached value is ment to be read-only, return a const |
| 146 | buffer. */ |
| 147 | extern const void *bcache (const void *addr, int length, |
| 148 | struct bcache *bcache); |
| 149 | |
| 150 | /* Like bcache, but if ADDED is not NULL, set *ADDED to true if the |
| 151 | bytes were newly added to the cache, or to false if the bytes were |
| 152 | found in the cache. */ |
| 153 | extern const void *bcache_full (const void *addr, int length, |
| 154 | struct bcache *bcache, int *added); |
| 155 | |
| 156 | /* Free all the storage used by BCACHE. */ |
| 157 | extern void bcache_xfree (struct bcache *bcache); |
| 158 | |
| 159 | /* Create a new bcache object. */ |
| 160 | extern struct bcache *bcache_xmalloc ( |
| 161 | unsigned long (*hash_function)(const void *, int length), |
| 162 | int (*compare_function)(const void *, const void *, int length)); |
| 163 | |
| 164 | /* Print statistics on BCACHE's memory usage and efficacity at |
| 165 | eliminating duplication. TYPE should be a string describing the |
| 166 | kind of data BCACHE holds. Statistics are printed using |
| 167 | `printf_filtered' and its ilk. */ |
| 168 | extern void print_bcache_statistics (struct bcache *bcache, char *type); |
| 169 | extern int bcache_memory_used (struct bcache *bcache); |
| 170 | |
| 171 | /* The hash functions */ |
| 172 | extern unsigned long hash(const void *addr, int length); |
| 173 | extern unsigned long hash_continue (const void *addr, int length, |
| 174 | unsigned long h); |
| 175 | |
| 176 | #endif /* BCACHE_H */ |