| /* Include file cached obstack implementation. |
| Written by Fred Fish <fnf@cygnus.com> |
| Rewritten by Jim Blandy <jimb@cygnus.com> |
| |
| Copyright 1999, 2000, 2002 Free Software Foundation, Inc. |
| |
| This file is part of GDB. |
| |
| This program is free software; you can redistribute it and/or modify |
| it under the terms of the GNU General Public License as published by |
| the Free Software Foundation; either version 2 of the License, or |
| (at your option) any later version. |
| |
| This program is distributed in the hope that it will be useful, |
| but WITHOUT ANY WARRANTY; without even the implied warranty of |
| MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the |
| GNU General Public License for more details. |
| |
| You should have received a copy of the GNU General Public License |
| along with this program; if not, write to the Free Software |
| Foundation, Inc., 59 Temple Place - Suite 330, |
| Boston, MA 02111-1307, USA. */ |
| |
| #ifndef BCACHE_H |
| #define BCACHE_H 1 |
| |
| /* A bcache is a data structure for factoring out duplication in |
| read-only structures. You give the bcache some string of bytes S. |
| If the bcache already contains a copy of S, it hands you back a |
| pointer to its copy. Otherwise, it makes a fresh copy of S, and |
| hands you back a pointer to that. In either case, you can throw |
| away your copy of S, and use the bcache's. |
| |
| The "strings" in question are arbitrary strings of bytes --- they |
| can contain zero bytes. You pass in the length explicitly when you |
| call the bcache function. |
| |
| This means that you can put ordinary C objects in a bcache. |
| However, if you do this, remember that structs can contain `holes' |
| between members, added for alignment. These bytes usually contain |
| garbage. If you try to bcache two objects which are identical from |
| your code's point of view, but have different garbage values in the |
| structure's holes, then the bcache will treat them as separate |
| strings, and you won't get the nice elimination of duplicates you |
| were hoping for. So, remember to memset your structures full of |
| zeros before bcaching them! |
| |
| You shouldn't modify the strings you get from a bcache, because: |
| |
| - You don't necessarily know who you're sharing space with. If I |
| stick eight bytes of text in a bcache, and then stick an |
| eight-byte structure in the same bcache, there's no guarantee |
| those two objects don't actually comprise the same sequence of |
| bytes. If they happen to, the bcache will use a single byte |
| string for both of them. Then, modifying the structure will |
| change the string. In bizarre ways. |
| |
| - Even if you know for some other reason that all that's okay, |
| there's another problem. A bcache stores all its strings in a |
| hash table. If you modify a string's contents, you will probably |
| change its hash value. This means that the modified string is |
| now in the wrong place in the hash table, and future bcache |
| probes will never find it. So by mutating a string, you give up |
| any chance of sharing its space with future duplicates. */ |
| |
| |
| struct bcache; |
| |
| /* Find a copy of the LENGTH bytes at ADDR in BCACHE. If BCACHE has |
| never seen those bytes before, add a copy of them to BCACHE. In |
| either case, return a pointer to BCACHE's copy of that string. */ |
| extern void *bcache (const void *addr, int length, struct bcache *bcache); |
| |
| /* Free all the storage used by BCACHE. */ |
| extern void bcache_xfree (struct bcache *bcache); |
| |
| /* Create a new bcache object. */ |
| extern struct bcache *bcache_xmalloc (void); |
| |
| /* Print statistics on BCACHE's memory usage and efficacity at |
| eliminating duplication. TYPE should be a string describing the |
| kind of data BCACHE holds. Statistics are printed using |
| `printf_filtered' and its ilk. */ |
| extern void print_bcache_statistics (struct bcache *bcache, char *type); |
| extern int bcache_memory_used (struct bcache *bcache); |
| |
| /* The hash function */ |
| extern unsigned long hash(const void *addr, int length); |
| |
| #endif /* BCACHE_H */ |