131 lines
5.4 KiB
C
131 lines
5.4 KiB
C
/* Include file cached obstack implementation.
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Written by Fred Fish <fnf@cygnus.com>
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Rewritten by Jim Blandy <jimb@cygnus.com>
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Copyright 1999, 2000 Free Software Foundation, Inc.
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This file is part of GDB.
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 2 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program; if not, write to the Free Software
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Foundation, Inc., 59 Temple Place - Suite 330,
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Boston, MA 02111-1307, USA. */
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#ifndef BCACHE_H
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#define BCACHE_H 1
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/* A bcache is a data structure for factoring out duplication in
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read-only structures. You give the bcache some string of bytes S.
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If the bcache already contains a copy of S, it hands you back a
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pointer to its copy. Otherwise, it makes a fresh copy of S, and
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hands you back a pointer to that. In either case, you can throw
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away your copy of S, and use the bcache's.
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The "strings" in question are arbitrary strings of bytes --- they
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can contain zero bytes. You pass in the length explicitly when you
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call the bcache function.
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This means that you can put ordinary C objects in a bcache.
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However, if you do this, remember that structs can contain `holes'
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between members, added for alignment. These bytes usually contain
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garbage. If you try to bcache two objects which are identical from
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your code's point of view, but have different garbage values in the
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structure's holes, then the bcache will treat them as separate
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strings, and you won't get the nice elimination of duplicates you
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were hoping for. So, remember to memset your structures full of
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zeros before bcaching them!
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You shouldn't modify the strings you get from a bcache, because:
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- You don't necessarily know who you're sharing space with. If I
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stick eight bytes of text in a bcache, and then stick an
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eight-byte structure in the same bcache, there's no guarantee
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those two objects don't actually comprise the same sequence of
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bytes. If they happen to, the bcache will use a single byte
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string for both of them. Then, modifying the structure will
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change the string. In bizarre ways.
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- Even if you know for some other reason that all that's okay,
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there's another problem. A bcache stores all its strings in a
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hash table. If you modify a string's contents, you will probably
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change its hash value. This means that the modified string is
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now in the wrong place in the hash table, and future bcache
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probes will never find it. So by mutating a string, you give up
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any chance of sharing its space with future duplicates. */
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/* The type used to hold a single bcache string. The user data is
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stored in d.data. Since it can be any type, it needs to have the
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same alignment as the most strict alignment of any type on the host
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machine. I don't know of any really correct way to do this in
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stock ANSI C, so just do it the same way obstack.h does.
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It would be nicer to have this stuff hidden away in bcache.c, but
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struct objstack contains a struct bcache directly --- not a pointer
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to one --- and then the memory-mapped stuff makes this a real pain.
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We don't strictly need to expose struct bstring, but it's better to
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have it all in one place. */
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struct bstring {
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struct bstring *next;
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size_t length;
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union
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{
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char data[1];
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double dummy;
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}
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d;
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};
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/* The structure for a bcache itself.
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To initialize a bcache, just fill it with zeros. */
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struct bcache {
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/* All the bstrings are allocated here. */
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struct obstack cache;
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/* How many hash buckets we're using. */
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unsigned int num_buckets;
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/* Hash buckets. This table is allocated using malloc, so when we
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grow the table we can return the old table to the system. */
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struct bstring **bucket;
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/* Statistics. */
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unsigned long unique_count; /* number of unique strings */
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long total_count; /* total number of strings cached, including dups */
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long unique_size; /* size of unique strings, in bytes */
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long total_size; /* total number of bytes cached, including dups */
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long structure_size; /* total size of bcache, including infrastructure */
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};
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/* Find a copy of the LENGTH bytes at ADDR in BCACHE. If BCACHE has
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never seen those bytes before, add a copy of them to BCACHE. In
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either case, return a pointer to BCACHE's copy of that string. */
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extern void *bcache (const void *addr, int length, struct bcache *bcache);
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/* Free all the storage that BCACHE refers to. The result is a valid,
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but empty, bcache. This does not free BCACHE itself, since that
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might be part of some larger object. */
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extern void free_bcache (struct bcache *bcache);
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/* Print statistics on BCACHE's memory usage and efficacity at
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eliminating duplication. TYPE should be a string describing the
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kind of data BCACHE holds. Statistics are printed using
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`printf_filtered' and its ilk. */
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extern void print_bcache_statistics (struct bcache *bcache, char *type);
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/* The hash function */
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extern unsigned long hash(const void *addr, int length);
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#endif /* BCACHE_H */
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