/* Include file cached obstack implementation.
Written by Fred Fish <fnf@cygnus.com>
Rewritten by Jim Blandy <jimb@cygnus.com>
- Copyright 1999 Free Software Foundation, Inc.
+
+ 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
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. */
-
-
-/* The type used to hold a single bcache string. The user data is
- stored in d.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. I don't know of any really correct way to do this in
- stock ANSI C, so just do it the same way obstack.h does.
-
- It would be nicer to have this stuff hidden away in bcache.c, but
- struct objstack contains a struct bcache directly --- not a pointer
- to one --- and then the memory-mapped stuff makes this a real pain.
- We don't strictly need to expose struct bstring, but it's better to
- have it all in one place. */
-
-struct bstring {
- struct bstring *next;
- size_t length;
-
- union
- {
- char data[1];
- double dummy;
- }
- d;
-};
-
-
-/* The structure for a bcache itself.
- To initialize a bcache, just fill it with zeros. */
-struct bcache {
- /* All the bstrings are allocated here. */
- struct obstack cache;
-
- /* How many hash buckets we're using. */
- unsigned int num_buckets;
+ 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 :-(
- /* 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 **bucket;
+*/
- /* Statistics. */
- unsigned long unique_count; /* number of unique strings */
- long total_count; /* total number of strings cached, including dups */
- long unique_size; /* size of unique strings, in bytes */
- long total_size; /* total number of bytes cached, including dups */
- long structure_size; /* total size of bcache, including infrastructure */
-};
+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 (void *addr, int length, struct bcache *bcache);
-
-/* Free all the storage that BCACHE refers to. The result is a valid,
- but empty, bcache. This does not free BCACHE itself, since that
- might be part of some larger object. */
-extern void free_bcache (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 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);
-/* The hash function */
-extern unsigned long hash(void *addr, int length);
+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