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) 1999-2013 Free Software Foundation, Inc.
7 This file is part of GDB.
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.
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.
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/>. */
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.
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.
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!
46 You shouldn't modify the strings you get from a bcache, because:
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
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.
65 Size of bcache VS hashtab:
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
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)
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
80 4 (slot) / 4 (chain length) + 1 (length) + 4 (chain) = 6 bytes
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:
86 4 (slot) x 4/3 (occupancy) + 1 (length) = 6 1/3 bytes
88 So a perfect hashtab has just slightly larger than an average
91 It turns out that an average hashtab is far worse. Two things
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.
97 - the string structure needs to be aligned to 8 bytes which for
98 hashtab wastes 7 bytes, while for bcache wastes only 3.
102 hashtab: 4 x 2 + 1 + 7 = 16 bytes
104 bcache 4 / 4 + 1 + 4 + 3 = 9 bytes
106 The numbers of GDB debugging GDB support this. ~40% vs ~70% overhead.
109 Speed of bcache VS hashtab (the half hash hack):
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:
116 a.length == b.length && memcmp (a.data, b.data, a.length) == 0
118 That is lengths are checked before doing the memcmp.
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:
126 a.half_hash == b.half_hash && a.length == b.length && memcmp
127 (a.data, b.data, a.length)
129 The numbers from GDB debugging GDB show this to be a remarkable
130 100% effective (only necessary length and memcmp tests being
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 :-(
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
147 extern const void *bcache (const void *addr
, int length
,
148 struct bcache
*bcache
);
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
);
156 /* Free all the storage used by BCACHE. */
157 extern void bcache_xfree (struct bcache
*bcache
);
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
));
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
);
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
,
176 #endif /* BCACHE_H */
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