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/* Include file cached obstack implementation.
Written by Fred Fish <fnf@cygnus.com>
Rewritten by Jim Blandy <jimb@cygnus.com>
Copyright (C) 1999, 2000, 2002, 2003, 2007 Free Software Foundation, Inc.
This file is part of GDB.
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.
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
/* 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.
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 :-(
*/
struct 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. */
extern void *deprecated_bcache (const void *addr, int length,
struct bcache *bcache);
extern const void *bcache (const void *addr, int length,
struct bcache *bcache);
/* Free all the storage used by BCACHE. */
extern void bcache_xfree (struct bcache *bcache);
/* Create a new bcache object. */
extern struct bcache *bcache_xmalloc (void);
/* Print statistics on BCACHE's memory usage and efficacity at
eliminating duplication. TYPE should be a string describing the
kind of data BCACHE holds. Statistics are printed using
`printf_filtered' and its ilk. */
extern void print_bcache_statistics (struct bcache *bcache, char *type);
extern int bcache_memory_used (struct bcache *bcache);
/* The hash function */
extern unsigned long hash(const void *addr, int length);
#endif /* BCACHE_H */
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