1 /* Include file cached obstack implementation.
2 Written by Fred Fish <fnf@cygnus.com>
3 Rewritten by Jim Blandy <jimb@cygnus.com>
5 Copyright (C) 1999, 2000, 2002, 2003, 2007, 2008
6 Free Software Foundation, Inc.
8 This file is part of GDB.
10 This program is free software; you can redistribute it and/or modify
11 it under the terms of the GNU General Public License as published by
12 the Free Software Foundation; either version 3 of the License, or
13 (at your option) any later version.
15 This program is distributed in the hope that it will be useful,
16 but WITHOUT ANY WARRANTY; without even the implied warranty of
17 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
18 GNU General Public License for more details.
20 You should have received a copy of the GNU General Public License
21 along with this program. If not, see <http://www.gnu.org/licenses/>. */
26 /* A bcache is a data structure for factoring out duplication in
27 read-only structures. You give the bcache some string of bytes S.
28 If the bcache already contains a copy of S, it hands you back a
29 pointer to its copy. Otherwise, it makes a fresh copy of S, and
30 hands you back a pointer to that. In either case, you can throw
31 away your copy of S, and use the bcache's.
33 The "strings" in question are arbitrary strings of bytes --- they
34 can contain zero bytes. You pass in the length explicitly when you
35 call the bcache function.
37 This means that you can put ordinary C objects in a bcache.
38 However, if you do this, remember that structs can contain `holes'
39 between members, added for alignment. These bytes usually contain
40 garbage. If you try to bcache two objects which are identical from
41 your code's point of view, but have different garbage values in the
42 structure's holes, then the bcache will treat them as separate
43 strings, and you won't get the nice elimination of duplicates you
44 were hoping for. So, remember to memset your structures full of
45 zeros before bcaching them!
47 You shouldn't modify the strings you get from a bcache, because:
49 - You don't necessarily know who you're sharing space with. If I
50 stick eight bytes of text in a bcache, and then stick an eight-byte
51 structure in the same bcache, there's no guarantee those two
52 objects don't actually comprise the same sequence of bytes. If
53 they happen to, the bcache will use a single byte string for both
54 of them. Then, modifying the structure will change the string. In
57 - Even if you know for some other reason that all that's okay,
58 there's another problem. A bcache stores all its strings in a hash
59 table. If you modify a string's contents, you will probably change
60 its hash value. This means that the modified string is now in the
61 wrong place in the hash table, and future bcache probes will never
62 find it. So by mutating a string, you give up any chance of
63 sharing its space with future duplicates.
66 Size of bcache VS hashtab:
68 For bcache, the most critical cost is size (or more exactly the
69 overhead added by the bcache). It turns out that the bcache is
72 Assuming a 32-bit system (the hash table slots are 4 bytes),
73 ignoring alignment, and limit strings to 255 bytes (1 byte length)
76 bcache: This uses a separate linked list to track the hash chain.
77 The numbers show roughly 100% occupancy of the hash table and an
78 average chain length of 4. Spreading the slot cost over the 4
81 4 (slot) / 4 (chain length) + 1 (length) + 4 (chain) = 6 bytes
83 hashtab: This uses a more traditional re-hash algorithm where the
84 chain is maintained within the hash table. The table occupancy is
85 kept below 75% but we'll assume its perfect:
87 4 (slot) x 4/3 (occupancy) + 1 (length) = 6 1/3 bytes
89 So a perfect hashtab has just slightly larger than an average
92 It turns out that an average hashtab is far worse. Two things
95 - Hashtab's occupancy is more like 50% (it ranges between 38% and
96 75%) giving a per slot cost of 4x2 vs 4x4/3.
98 - the string structure needs to be aligned to 8 bytes which for
99 hashtab wastes 7 bytes, while for bcache wastes only 3.
103 hashtab: 4 x 2 + 1 + 7 = 16 bytes
105 bcache 4 / 4 + 1 + 4 + 3 = 9 bytes
107 The numbers of GDB debugging GDB support this. ~40% vs ~70% overhead.
110 Speed of bcache VS hashtab (the half hash hack):
112 While hashtab has a typical chain length of 1, bcache has a chain
113 length of round 4. This means that the bcache will require
114 something like double the number of compares after that initial
115 hash. In both cases the comparison takes the form:
117 a.length == b.length && memcmp (a.data, b.data, a.length) == 0
119 That is lengths are checked before doing the memcmp.
121 For GDB debugging GDB, it turned out that all lengths were 24 bytes
122 (no C++ so only psymbols were cached) and hence, all compares
123 required a call to memcmp. As a hack, two bytes of padding
124 (mentioned above) are used to store the upper 16 bits of the
125 string's hash value and then that is used in the comparison vis:
127 a.half_hash == b.half_hash && a.length == b.length && memcmp
128 (a.data, b.data, a.length)
130 The numbers from GDB debugging GDB show this to be a remarkable
131 100% effective (only necessary length and memcmp tests being
134 Mind you, looking at the wall clock, the same GDB debugging GDB
135 showed only marginal speed up (0.780 vs 0.773s). Seems GDB is too
136 busy doing something else :-(
143 /* Find a copy of the LENGTH bytes at ADDR in BCACHE. If BCACHE has
144 never seen those bytes before, add a copy of them to BCACHE. In
145 either case, return a pointer to BCACHE's copy of that string.
146 Since the cached value is ment to be read-only, return a const
148 extern const void *bcache (const void *addr
, int length
,
149 struct bcache
*bcache
);
151 /* Like bcache, but if ADDED is not NULL, set *ADDED to true if the
152 bytes were newly added to the cache, or to false if the bytes were
153 found in the cache. */
154 extern const void *bcache_full (const void *addr
, int length
,
155 struct bcache
*bcache
, int *added
);
157 /* Free all the storage used by BCACHE. */
158 extern void bcache_xfree (struct bcache
*bcache
);
160 /* Create a new bcache object. */
161 extern struct bcache
*bcache_xmalloc (void);
163 /* Print statistics on BCACHE's memory usage and efficacity at
164 eliminating duplication. TYPE should be a string describing the
165 kind of data BCACHE holds. Statistics are printed using
166 `printf_filtered' and its ilk. */
167 extern void print_bcache_statistics (struct bcache
*bcache
, char *type
);
168 extern int bcache_memory_used (struct bcache
*bcache
);
170 /* The hash function */
171 extern unsigned long hash(const void *addr
, int length
);
173 #endif /* BCACHE_H */
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