Written by Fred Fish <fnf@cygnus.com>
Rewritten by Jim Blandy <jimb@cygnus.com>
- Copyright 1999, 2000, 2002 Free Software Foundation, Inc.
+ Copyright (C) 1999, 2000, 2002, 2003, 2007 Free Software Foundation, Inc.
This file is part of GDB.
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. */
+ Foundation, Inc., 51 Franklin Street, Fifth Floor,
+ Boston, MA 02110-1301, USA. */
#ifndef BCACHE_H
#define BCACHE_H 1
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.
+ 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. */
+ 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. */
-extern void *bcache (const void *addr, int length, struct bcache *bcache);
+ 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);
projects / thirdparty / binutils-gdb.git / blobdiff