[deliverable/binutils-gdb.git] / gdb / bcache.h

/* Include file cached obstack implementation.
   Written by Fred Fish <fnf@cygnus.com>
   Rewritten by Jim Blandy <jimb@cygnus.com>

   Copyright 1999, 2000, 2002 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
   (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, write to the Free Software
   Foundation, Inc., 59 Temple Place - Suite 330,
   Boston, MA 02111-1307, USA.  */

#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.  */


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);

/* 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 */
Commit	Line	Data
c906108c	1	/* Include file cached obstack implementation.
c2d11a7d JM	2	Written by Fred Fish <fnf@cygnus.com>
c2d11a7d JM	3	Rewritten by Jim Blandy <jimb@cygnus.com>
af5f3db6 AC	4
af5f3db6 AC	5	Copyright 1999, 2000, 2002 Free Software Foundation, Inc.
c906108c	6
c5aa993b	7	This file is part of GDB.
c906108c	8
c5aa993b JM	9	This program is free software; you can redistribute it and/or modify
	10	it under the terms of the GNU General Public License as published by
	11	the Free Software Foundation; either version 2 of the License, or
	12	(at your option) any later version.
c906108c	13
c5aa993b JM	14	This program is distributed in the hope that it will be useful,
	15	but WITHOUT ANY WARRANTY; without even the implied warranty of
	16	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
	17	GNU General Public License for more details.
c906108c	18
c5aa993b JM	19	You should have received a copy of the GNU General Public License
	20	along with this program; if not, write to the Free Software
	21	Foundation, Inc., 59 Temple Place - Suite 330,
	22	Boston, MA 02111-1307, USA. */
c906108c SS	23
	24	#ifndef BCACHE_H
	25	#define BCACHE_H 1
	26
c2d11a7d JM	27	/* A bcache is a data structure for factoring out duplication in
	28	read-only structures. You give the bcache some string of bytes S.
	29	If the bcache already contains a copy of S, it hands you back a
	30	pointer to its copy. Otherwise, it makes a fresh copy of S, and
	31	hands you back a pointer to that. In either case, you can throw
	32	away your copy of S, and use the bcache's.
	33
	34	The "strings" in question are arbitrary strings of bytes --- they
	35	can contain zero bytes. You pass in the length explicitly when you
	36	call the bcache function.
	37
	38	This means that you can put ordinary C objects in a bcache.
	39	However, if you do this, remember that structs can contain `holes'
	40	between members, added for alignment. These bytes usually contain
	41	garbage. If you try to bcache two objects which are identical from
	42	your code's point of view, but have different garbage values in the
	43	structure's holes, then the bcache will treat them as separate
	44	strings, and you won't get the nice elimination of duplicates you
	45	were hoping for. So, remember to memset your structures full of
	46	zeros before bcaching them!
	47
	48	You shouldn't modify the strings you get from a bcache, because:
	49
	50	- You don't necessarily know who you're sharing space with. If I
	51	stick eight bytes of text in a bcache, and then stick an
	52	eight-byte structure in the same bcache, there's no guarantee
	53	those two objects don't actually comprise the same sequence of
	54	bytes. If they happen to, the bcache will use a single byte
	55	string for both of them. Then, modifying the structure will
	56	change the string. In bizarre ways.
	57
	58	- Even if you know for some other reason that all that's okay,
	59	there's another problem. A bcache stores all its strings in a
	60	hash table. If you modify a string's contents, you will probably
	61	change its hash value. This means that the modified string is
	62	now in the wrong place in the hash table, and future bcache
	63	probes will never find it. So by mutating a string, you give up
	64	any chance of sharing its space with future duplicates. */
	65
	66
af5f3db6	67	struct bcache;
c2d11a7d JM	68
	69	/* Find a copy of the LENGTH bytes at ADDR in BCACHE. If BCACHE has
	70	never seen those bytes before, add a copy of them to BCACHE. In
	71	either case, return a pointer to BCACHE's copy of that string. */
d85a5daf	72	extern void bcache (const void addr, int length, struct bcache *bcache);
c2d11a7d	73
af5f3db6 AC	74	/* Free all the storage used by BCACHE. */
	75	extern void bcache_xfree (struct bcache *bcache);
	76
	77	/* Create a new bcache object. */
	78	extern struct bcache *bcache_xmalloc (void);
c2d11a7d JM	79
	80	/* Print statistics on BCACHE's memory usage and efficacity at
	81	eliminating duplication. TYPE should be a string describing the
	82	kind of data BCACHE holds. Statistics are printed using
	83	`printf_filtered' and its ilk. */
	84	extern void print_bcache_statistics (struct bcache bcache, char type);
af5f3db6 AC	85	extern int bcache_memory_used (struct bcache *bcache);
af5f3db6 AC	86
357e46e7	87	/* The hash function */
d85a5daf	88	extern unsigned long hash(const void *addr, int length);
af5f3db6	89
c906108c	90	#endif /* BCACHE_H */