/* Include file cached obstack implementation.
- Written by Fred Fish (fnf@cygnus.com)
- Copyright 1995 Free Software Foundation, Inc.
+ Written by Fred Fish <fnf@cygnus.com>
+ Rewritten by Jim Blandy <jimb@cygnus.com>
-This file is part of GDB.
+ Copyright (C) 1999-2020 Free Software Foundation, Inc.
-This program is free software; you can redistribute it and/or modify
-it under the terms of the GNU General Public License as published by
-the Free Software Foundation; either version 2 of the License, or
-(at your option) any later version.
+ This file is part of GDB.
-This program is distributed in the hope that it will be useful,
-but WITHOUT ANY WARRANTY; without even the implied warranty of
-MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
-GNU General Public License for more details.
+ This program is free software; you can redistribute it and/or modify
+ it under the terms of the GNU General Public License as published by
+ the Free Software Foundation; either version 3 of the License, or
+ (at your option) any later version.
-You should have received a copy of the GNU General Public License
-along with this program; if not, write to the Free Software
-Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. */
+ This program is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ GNU General Public License for more details.
+
+ You should have received a copy of the GNU General Public License
+ along with this program. If not, see <http://www.gnu.org/licenses/>. */
#ifndef BCACHE_H
#define BCACHE_H 1
-#define BCACHE_HASHLENGTH 12 /* Number of bits in hash value */
-#define BCACHE_HASHSIZE (1 << BCACHE_HASHLENGTH)
-#define BCACHE_MAXLENGTH 128
+/* A bcache is a data structure for factoring out duplication in
+ read-only structures. You give the bcache some string of bytes S.
+ If the bcache already contains a copy of S, it hands you back a
+ pointer to its copy. Otherwise, it makes a fresh copy of S, and
+ hands you back a pointer to that. In either case, you can throw
+ away your copy of S, and use the bcache's.
-struct hashlink {
- struct hashlink *next;
- char data[1];
-};
+ The "strings" in question are arbitrary strings of bytes --- they
+ can contain zero bytes. You pass in the length explicitly when you
+ call the bcache function.
+
+ This means that you can put ordinary C objects in a bcache.
+ However, if you do this, remember that structs can contain `holes'
+ between members, added for alignment. These bytes usually contain
+ garbage. If you try to bcache two objects which are identical from
+ your code's point of view, but have different garbage values in the
+ structure's holes, then the bcache will treat them as separate
+ strings, and you won't get the nice elimination of duplicates you
+ were hoping for. So, remember to memset your structures full of
+ zeros before bcaching them!
+
+ You shouldn't modify the strings you get from a bcache, because:
+
+ - You don't necessarily know who you're sharing space with. If I
+ stick eight bytes of text in a bcache, and then stick an eight-byte
+ structure in the same bcache, there's no guarantee those two
+ objects don't actually comprise the same sequence of bytes. If
+ they happen to, the bcache will use a single byte string for both
+ of them. Then, modifying the structure will change the string. In
+ bizarre ways.
+
+ - Even if you know for some other reason that all that's okay,
+ there's another problem. A bcache stores all its strings in a hash
+ table. If you modify a string's contents, you will probably change
+ its hash value. This means that the modified string is now in the
+ wrong place in the hash table, and future bcache probes will never
+ find it. So by mutating a string, you give up any chance of
+ sharing its space with future duplicates.
+
+
+ Size of bcache VS hashtab:
+
+ For bcache, the most critical cost is size (or more exactly the
+ overhead added by the bcache). It turns out that the bcache is
+ remarkably efficient.
+
+ Assuming a 32-bit system (the hash table slots are 4 bytes),
+ ignoring alignment, and limit strings to 255 bytes (1 byte length)
+ we get ...
+
+ bcache: This uses a separate linked list to track the hash chain.
+ The numbers show roughly 100% occupancy of the hash table and an
+ average chain length of 4. Spreading the slot cost over the 4
+ chain elements:
+
+ 4 (slot) / 4 (chain length) + 1 (length) + 4 (chain) = 6 bytes
+
+ hashtab: This uses a more traditional re-hash algorithm where the
+ chain is maintained within the hash table. The table occupancy is
+ kept below 75% but we'll assume its perfect:
+
+ 4 (slot) x 4/3 (occupancy) + 1 (length) = 6 1/3 bytes
+
+ So a perfect hashtab has just slightly larger than an average
+ bcache.
+
+ It turns out that an average hashtab is far worse. Two things
+ hurt:
+
+ - Hashtab's occupancy is more like 50% (it ranges between 38% and
+ 75%) giving a per slot cost of 4x2 vs 4x4/3.
+
+ - the string structure needs to be aligned to 8 bytes which for
+ hashtab wastes 7 bytes, while for bcache wastes only 3.
+
+ This gives:
+
+ hashtab: 4 x 2 + 1 + 7 = 16 bytes
+
+ bcache 4 / 4 + 1 + 4 + 3 = 9 bytes
+
+ The numbers of GDB debugging GDB support this. ~40% vs ~70% overhead.
+
+
+ Speed of bcache VS hashtab (the half hash hack):
+
+ While hashtab has a typical chain length of 1, bcache has a chain
+ length of round 4. This means that the bcache will require
+ something like double the number of compares after that initial
+ hash. In both cases the comparison takes the form:
+
+ a.length == b.length && memcmp (a.data, b.data, a.length) == 0
+
+ That is lengths are checked before doing the memcmp.
+
+ For GDB debugging GDB, it turned out that all lengths were 24 bytes
+ (no C++ so only psymbols were cached) and hence, all compares
+ required a call to memcmp. As a hack, two bytes of padding
+ (mentioned above) are used to store the upper 16 bits of the
+ string's hash value and then that is used in the comparison vis:
+
+ a.half_hash == b.half_hash && a.length == b.length && memcmp
+ (a.data, b.data, a.length)
+
+ The numbers from GDB debugging GDB show this to be a remarkable
+ 100% effective (only necessary length and memcmp tests being
+ performed).
+
+ Mind you, looking at the wall clock, the same GDB debugging GDB
+ showed only marginal speed up (0.780 vs 0.773s). Seems GDB is too
+ busy doing something else :-(
+
+*/
+
+namespace gdb {
+
+struct bstring;
+
+struct bcache
+{
+ /* Allocate a bcache. HASH_FN and COMPARE_FN can be used to pass in
+ custom hash, and compare functions to be used by this bcache. If
+ HASH_FUNCTION is NULL fast_hash() is used and if COMPARE_FUNCTION is
+ NULL memcmp() is used. */
+
+ explicit bcache (unsigned long (*hash_fn)(const void *,
+ int length) = nullptr,
+ int (*compare_fn)(const void *, const void *,
+ int length) = nullptr)
+ : m_hash_function (hash_fn == nullptr ? default_hash : hash_fn),
+ m_compare_function (compare_fn == nullptr ? compare : compare_fn)
+ {
+ }
+
+ ~bcache ();
+
+ /* Find a copy of the LENGTH bytes at ADDR in BCACHE. If BCACHE has
+ never seen those bytes before, add a copy of them to BCACHE. In
+ either case, return a pointer to BCACHE's copy of that string.
+ Since the cached value is ment to be read-only, return a const
+ buffer. If ADDED is not NULL, set *ADDED to true if the bytes
+ were newly added to the cache, or to false if the bytes were
+ found in the cache. */
+
+ const void *insert (const void *addr, int length, int *added = nullptr);
+
+ /* Print statistics on this bcache's memory usage and efficacity at
+ eliminating duplication. TYPE should be a string describing the
+ kind of data this bcache holds. Statistics are printed using
+ `printf_filtered' and its ilk. */
+ void print_statistics (const char *type);
+ int memory_used ();
+
+private:
+
+ /* All the bstrings are allocated here. */
+ struct obstack m_cache {};
+
+ /* How many hash buckets we're using. */
+ unsigned int m_num_buckets = 0;
+
+ /* Hash buckets. This table is allocated using malloc, so when we
+ grow the table we can return the old table to the system. */
+ struct bstring **m_bucket = nullptr;
+
+ /* Statistics. */
+ unsigned long m_unique_count = 0; /* number of unique strings */
+ long m_total_count = 0; /* total number of strings cached, including dups */
+ long m_unique_size = 0; /* size of unique strings, in bytes */
+ long m_total_size = 0; /* total number of bytes cached, including dups */
+ long m_structure_size = 0; /* total size of bcache, including infrastructure */
+ /* Number of times that the hash table is expanded and hence
+ re-built, and the corresponding number of times that a string is
+ [re]hashed as part of entering it into the expanded table. The
+ total number of hashes can be computed by adding TOTAL_COUNT to
+ expand_hash_count. */
+ unsigned long m_expand_count = 0;
+ unsigned long m_expand_hash_count = 0;
+ /* Number of times that the half-hash compare hit (compare the upper
+ 16 bits of hash values) hit, but the corresponding combined
+ length/data compare missed. */
+ unsigned long m_half_hash_miss_count = 0;
+
+ /* Hash function to be used for this bcache object. */
+ unsigned long (*m_hash_function)(const void *addr, int length);
+
+ /* Compare function to be used for this bcache object. */
+ int (*m_compare_function)(const void *, const void *, int length);
+
+ /* Default compare function. */
+ static int compare (const void *addr1, const void *addr2, int length);
+
+ /* Default hash function. */
+ static unsigned long default_hash (const void *ptr, int length)
+ {
+ return fast_hash (ptr, length, 0);
+ }
-struct bcache {
- struct obstack cache;
- struct hashlink **indextable[BCACHE_MAXLENGTH];
- int cache_hits;
- int cache_misses;
- int cache_bytes;
- int cache_savings;
- int bcache_overflows;
+ /* Expand the hash table. */
+ void expand_hash_table ();
};
-extern void *
-bcache PARAMS ((void *bytes, int count, struct bcache *bcachep));
+} /* namespace gdb */
#endif /* BCACHE_H */
projects / deliverable / binutils-gdb.git / blobdiff