1 /* Include file cached obstack implementation.
2 Written by Fred Fish <fnf@cygnus.com>
3 Rewritten by Jim Blandy <jimb@cygnus.com>
5 Copyright 1999, 2000, 2002, 2003 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 2 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, write to the Free Software
21 Foundation, Inc., 59 Temple Place - Suite 330,
22 Boston, MA 02111-1307, USA. */
27 /* A bcache is a data structure for factoring out duplication in
28 read-only structures. You give the bcache some string of bytes S.
29 If the bcache already contains a copy of S, it hands you back a
30 pointer to its copy. Otherwise, it makes a fresh copy of S, and
31 hands you back a pointer to that. In either case, you can throw
32 away your copy of S, and use the bcache's.
34 The "strings" in question are arbitrary strings of bytes --- they
35 can contain zero bytes. You pass in the length explicitly when you
36 call the bcache function.
38 This means that you can put ordinary C objects in a bcache.
39 However, if you do this, remember that structs can contain `holes'
40 between members, added for alignment. These bytes usually contain
41 garbage. If you try to bcache two objects which are identical from
42 your code's point of view, but have different garbage values in the
43 structure's holes, then the bcache will treat them as separate
44 strings, and you won't get the nice elimination of duplicates you
45 were hoping for. So, remember to memset your structures full of
46 zeros before bcaching them!
48 You shouldn't modify the strings you get from a bcache, because:
50 - You don't necessarily know who you're sharing space with. If I
51 stick eight bytes of text in a bcache, and then stick an eight-byte
52 structure in the same bcache, there's no guarantee those two
53 objects don't actually comprise the same sequence of bytes. If
54 they happen to, the bcache will use a single byte string for both
55 of them. Then, modifying the structure will change the string. In
58 - Even if you know for some other reason that all that's okay,
59 there's another problem. A bcache stores all its strings in a hash
60 table. If you modify a string's contents, you will probably change
61 its hash value. This means that the modified string is now in the
62 wrong place in the hash table, and future bcache probes will never
63 find it. So by mutating a string, you give up any chance of
64 sharing its space with future duplicates.
67 Size of bcache VS hashtab:
69 For bcache, the most critical cost is size (or more exactly the
70 overhead added by the bcache). It turns out that the bcache is
73 Assuming a 32-bit system (the hash table slots are 4 bytes),
74 ignoring alignment, and limit strings to 255 bytes (1 byte length)
77 bcache: This uses a separate linked list to track the hash chain.
78 The numbers show roughly 100% occupancy of the hash table and an
79 average chain length of 4. Spreading the slot cost over the 4
82 4 (slot) / 4 (chain length) + 1 (length) + 4 (chain) = 6 bytes
84 hashtab: This uses a more traditional re-hash algorithm where the
85 chain is maintained within the hash table. The table occupancy is
86 kept below 75% but we'll assume its perfect:
88 4 (slot) x 4/3 (occupancy) + 1 (length) = 6 1/3 bytes
90 So a perfect hashtab has just slightly larger than an average
93 It turns out that an average hashtab is far worse. Two things
96 - Hashtab's occupancy is more like 50% (it ranges between 38% and
97 75%) giving a per slot cost of 4x2 vs 4x4/3.
99 - the string structure needs to be aligned to 8 bytes which for
100 hashtab wastes 7 bytes, while for bcache wastes only 3.
104 hashtab: 4 x 2 + 1 + 7 = 16 bytes
106 bcache 4 / 4 + 1 + 4 + 3 = 9 bytes
108 The numbers of GDB debugging GDB support this. ~40% vs ~70% overhead.
111 Speed of bcache VS hashtab (the half hash hack):
113 While hashtab has a typical chain length of 1, bcache has a chain
114 length of round 4. This means that the bcache will require
115 something like double the number of compares after that initial
116 hash. In both cases the comparison takes the form:
118 a.length == b.length && memcmp (a.data, b.data, a.length) == 0
120 That is lengths are checked before doing the memcmp.
122 For GDB debugging GDB, it turned out that all lengths were 24 bytes
123 (no C++ so only psymbols were cached) and hence, all compares
124 required a call to memcmp. As a hack, two bytes of padding
125 (mentioned above) are used to store the upper 16 bits of the
126 string's hash value and then that is used in the comparison vis:
128 a.half_hash == b.half_hash && a.length == b.length && memcmp
129 (a.data, b.data, a.length)
131 The numbers from GDB debugging GDB show this to be a remarkable
132 100% effective (only necessary length and memcmp tests being
135 Mind you, looking at the wall clock, the same GDB debugging GDB
136 showed only marginal speed up (0.780 vs 0.773s). Seems GDB is too
137 busy doing something else :-(
144 /* Find a copy of the LENGTH bytes at ADDR in BCACHE. If BCACHE has
145 never seen those bytes before, add a copy of them to BCACHE. In
146 either case, return a pointer to BCACHE's copy of that string.
147 Since the cached value is ment to be read-only, return a const
149 extern void *deprecated_bcache (const void *addr
, int length
,
150 struct bcache
*bcache
);
151 extern const void *bcache (const void *addr
, int length
,
152 struct bcache
*bcache
);
154 /* Free all the storage used by BCACHE. */
155 extern void bcache_xfree (struct bcache
*bcache
);
157 /* Create a new bcache object. */
158 extern struct bcache
*bcache_xmalloc (void);
160 /* Print statistics on BCACHE's memory usage and efficacity at
161 eliminating duplication. TYPE should be a string describing the
162 kind of data BCACHE holds. Statistics are printed using
163 `printf_filtered' and its ilk. */
164 extern void print_bcache_statistics (struct bcache
*bcache
, char *type
);
165 extern int bcache_memory_used (struct bcache
*bcache
);
167 /* The hash function */
168 extern unsigned long hash(const void *addr
, int length
);
170 #endif /* BCACHE_H */
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