174 lines
6.8 KiB
C
174 lines
6.8 KiB
C
/* Include file cached obstack implementation.
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Written by Fred Fish <fnf@cygnus.com>
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Rewritten by Jim Blandy <jimb@cygnus.com>
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Copyright (C) 1999, 2000, 2002, 2003, 2007, 2008, 2009, 2010
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Free Software Foundation, Inc.
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This file is part of GDB.
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 3 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see <http://www.gnu.org/licenses/>. */
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#ifndef BCACHE_H
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#define BCACHE_H 1
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/* A bcache is a data structure for factoring out duplication in
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read-only structures. You give the bcache some string of bytes S.
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If the bcache already contains a copy of S, it hands you back a
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pointer to its copy. Otherwise, it makes a fresh copy of S, and
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hands you back a pointer to that. In either case, you can throw
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away your copy of S, and use the bcache's.
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The "strings" in question are arbitrary strings of bytes --- they
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can contain zero bytes. You pass in the length explicitly when you
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call the bcache function.
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This means that you can put ordinary C objects in a bcache.
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However, if you do this, remember that structs can contain `holes'
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between members, added for alignment. These bytes usually contain
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garbage. If you try to bcache two objects which are identical from
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your code's point of view, but have different garbage values in the
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structure's holes, then the bcache will treat them as separate
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strings, and you won't get the nice elimination of duplicates you
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were hoping for. So, remember to memset your structures full of
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zeros before bcaching them!
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You shouldn't modify the strings you get from a bcache, because:
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- You don't necessarily know who you're sharing space with. If I
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stick eight bytes of text in a bcache, and then stick an eight-byte
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structure in the same bcache, there's no guarantee those two
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objects don't actually comprise the same sequence of bytes. If
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they happen to, the bcache will use a single byte string for both
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of them. Then, modifying the structure will change the string. In
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bizarre ways.
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- Even if you know for some other reason that all that's okay,
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there's another problem. A bcache stores all its strings in a hash
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table. If you modify a string's contents, you will probably change
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its hash value. This means that the modified string is now in the
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wrong place in the hash table, and future bcache probes will never
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find it. So by mutating a string, you give up any chance of
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sharing its space with future duplicates.
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Size of bcache VS hashtab:
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For bcache, the most critical cost is size (or more exactly the
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overhead added by the bcache). It turns out that the bcache is
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remarkably efficient.
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Assuming a 32-bit system (the hash table slots are 4 bytes),
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ignoring alignment, and limit strings to 255 bytes (1 byte length)
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we get ...
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bcache: This uses a separate linked list to track the hash chain.
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The numbers show roughly 100% occupancy of the hash table and an
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average chain length of 4. Spreading the slot cost over the 4
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chain elements:
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4 (slot) / 4 (chain length) + 1 (length) + 4 (chain) = 6 bytes
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hashtab: This uses a more traditional re-hash algorithm where the
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chain is maintained within the hash table. The table occupancy is
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kept below 75% but we'll assume its perfect:
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4 (slot) x 4/3 (occupancy) + 1 (length) = 6 1/3 bytes
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So a perfect hashtab has just slightly larger than an average
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bcache.
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It turns out that an average hashtab is far worse. Two things
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hurt:
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- Hashtab's occupancy is more like 50% (it ranges between 38% and
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75%) giving a per slot cost of 4x2 vs 4x4/3.
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- the string structure needs to be aligned to 8 bytes which for
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hashtab wastes 7 bytes, while for bcache wastes only 3.
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This gives:
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hashtab: 4 x 2 + 1 + 7 = 16 bytes
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bcache 4 / 4 + 1 + 4 + 3 = 9 bytes
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The numbers of GDB debugging GDB support this. ~40% vs ~70% overhead.
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Speed of bcache VS hashtab (the half hash hack):
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While hashtab has a typical chain length of 1, bcache has a chain
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length of round 4. This means that the bcache will require
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something like double the number of compares after that initial
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hash. In both cases the comparison takes the form:
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a.length == b.length && memcmp (a.data, b.data, a.length) == 0
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That is lengths are checked before doing the memcmp.
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For GDB debugging GDB, it turned out that all lengths were 24 bytes
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(no C++ so only psymbols were cached) and hence, all compares
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required a call to memcmp. As a hack, two bytes of padding
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(mentioned above) are used to store the upper 16 bits of the
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string's hash value and then that is used in the comparison vis:
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a.half_hash == b.half_hash && a.length == b.length && memcmp
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(a.data, b.data, a.length)
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The numbers from GDB debugging GDB show this to be a remarkable
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100% effective (only necessary length and memcmp tests being
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performed).
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Mind you, looking at the wall clock, the same GDB debugging GDB
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showed only marginal speed up (0.780 vs 0.773s). Seems GDB is too
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busy doing something else :-(
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*/
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struct bcache;
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/* Find a copy of the LENGTH bytes at ADDR in BCACHE. If BCACHE has
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never seen those bytes before, add a copy of them to BCACHE. In
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either case, return a pointer to BCACHE's copy of that string.
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Since the cached value is ment to be read-only, return a const
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buffer. */
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extern const void *bcache (const void *addr, int length,
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struct bcache *bcache);
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/* Like bcache, but if ADDED is not NULL, set *ADDED to true if the
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bytes were newly added to the cache, or to false if the bytes were
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found in the cache. */
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extern const void *bcache_full (const void *addr, int length,
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struct bcache *bcache, int *added);
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/* Free all the storage used by BCACHE. */
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extern void bcache_xfree (struct bcache *bcache);
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/* Create a new bcache object. */
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extern struct bcache *bcache_xmalloc (void);
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/* Print statistics on BCACHE's memory usage and efficacity at
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eliminating duplication. TYPE should be a string describing the
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kind of data BCACHE holds. Statistics are printed using
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`printf_filtered' and its ilk. */
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extern void print_bcache_statistics (struct bcache *bcache, char *type);
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extern int bcache_memory_used (struct bcache *bcache);
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/* The hash function */
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extern unsigned long hash(const void *addr, int length);
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#endif /* BCACHE_H */
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