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[binutils-gdb.git] / gdb / bcache.h
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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) 1999-2022 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/>. */
22 #ifndef BCACHE_H
23 #define BCACHE_H 1
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
54 bizarre ways.
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
69 remarkably efficient.
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)
73 we get ...
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
78 chain elements:
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
89 bcache.
91 It turns out that an average hashtab is far worse. Two things
92 hurt:
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.
100 This gives:
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
131 performed).
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 :-(
139 namespace gdb {
141 struct bstring;
143 struct bcache
145 virtual ~bcache ();
147 /* Find a copy of the LENGTH bytes at ADDR in BCACHE. If BCACHE has
148 never seen those bytes before, add a copy of them to BCACHE. In
149 either case, return a pointer to BCACHE's copy of that string.
150 Since the cached value is meant to be read-only, return a const
151 buffer. If ADDED is not NULL, set *ADDED to true if the bytes
152 were newly added to the cache, or to false if the bytes were
153 found in the cache. */
155 const void *insert (const void *addr, int length, bool *added = nullptr);
157 /* Print statistics on this bcache's memory usage and efficacity at
158 eliminating duplication. TYPE should be a string describing the
159 kind of data this bcache holds. Statistics are printed using
160 `gdb_printf' and its ilk. */
161 void print_statistics (const char *type);
162 int memory_used ();
164 protected:
166 /* Hash function to be used for this bcache object. Defaults to
167 fast_hash. */
168 virtual unsigned long hash (const void *addr, int length);
170 /* Compare function to be used for this bcache object. Defaults to
171 memcmp. */
172 virtual int compare (const void *left, const void *right, int length);
174 private:
176 /* All the bstrings are allocated here. */
177 struct obstack m_cache {};
179 /* How many hash buckets we're using. */
180 unsigned int m_num_buckets = 0;
182 /* Hash buckets. This table is allocated using malloc, so when we
183 grow the table we can return the old table to the system. */
184 struct bstring **m_bucket = nullptr;
186 /* Statistics. */
187 unsigned long m_unique_count = 0; /* number of unique strings */
188 long m_total_count = 0; /* total number of strings cached, including dups */
189 long m_unique_size = 0; /* size of unique strings, in bytes */
190 long m_total_size = 0; /* total number of bytes cached, including dups */
191 long m_structure_size = 0; /* total size of bcache, including infrastructure */
192 /* Number of times that the hash table is expanded and hence
193 re-built, and the corresponding number of times that a string is
194 [re]hashed as part of entering it into the expanded table. The
195 total number of hashes can be computed by adding TOTAL_COUNT to
196 expand_hash_count. */
197 unsigned long m_expand_count = 0;
198 unsigned long m_expand_hash_count = 0;
199 /* Number of times that the half-hash compare hit (compare the upper
200 16 bits of hash values) hit, but the corresponding combined
201 length/data compare missed. */
202 unsigned long m_half_hash_miss_count = 0;
204 /* Expand the hash table. */
205 void expand_hash_table ();
208 } /* namespace gdb */
210 #endif /* BCACHE_H */