/* 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>
+
+ Copyright (C) 1999-2000, 2002-2003, 2007-2012 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 2 of the License, or
+ 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,
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, write to the Free Software
- Foundation, Inc., 59 Temple Place - Suite 330,
- Boston, MA 02111-1307, USA. */
+ 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
-
-/* Note that the user data is stored in data[]. Since it can be any type,
- it needs to have the same alignment as the most strict alignment of
- any type on the host machine. So do it the same way obstack does. */
-
-struct hashlink
- {
- struct hashlink *next;
- union
- {
- char data[1];
- double dummy;
- }
- d;
- };
-
-/* BCACHE_DATA is used to get the address of the cached data. */
-
-#define BCACHE_DATA(p) ((p)->d.data)
-
-/* BCACHE_DATA_ALIGNMENT is used to get the offset of the start of
- cached data within the hashlink struct. This value, plus the
- size of the cached data, is the amount of space to allocate for
- a hashlink struct to hold the next pointer and the data. */
-
-#define BCACHE_DATA_ALIGNMENT \
- (((char *) BCACHE_DATA((struct hashlink*) 0) - (char *) 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;
- };
-
-extern void *
- bcache PARAMS ((void *bytes, int count, struct bcache * bcachep));
-
-extern void
-print_bcache_statistics PARAMS ((struct bcache *, char *));
+/* 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 const void *bcache (const void *addr, int length,
+ struct bcache *bcache);
+
+/* Like bcache, but 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. */
+extern const void *bcache_full (const void *addr, int length,
+ struct bcache *bcache, int *added);
+
+/* Free all the storage used by BCACHE. */
+extern void bcache_xfree (struct bcache *bcache);
+
+/* Create a new bcache object. */
+extern struct bcache *bcache_xmalloc (
+ unsigned long (*hash_function)(const void *, int length),
+ int (*compare_function)(const void *, const void *, int length));
+
+/* 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 functions */
+extern unsigned long hash(const void *addr, int length);
+extern unsigned long hash_continue (const void *addr, int length,
+ unsigned long h);
#endif /* BCACHE_H */
projects / external / binutils.git / blobdiff