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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, 2008, 2009, 2010

   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 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 (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 */