gdb/bcache.h

   1 /* Include file cached obstack implementation.
   2    Written by Fred Fish <fnf@cygnus.com>
   3    Rewritten by Jim Blandy <jimb@cygnus.com>
   4    Copyright 1999 Free Software Foundation, Inc.
   5
   6    This file is part of GDB.
   7
   8    This program is free software; you can redistribute it and/or modify
   9    it under the terms of the GNU General Public License as published by
  10    the Free Software Foundation; either version 2 of the License, or
  11    (at your option) any later version.
  12
  13    This program is distributed in the hope that it will be useful,
  14    but WITHOUT ANY WARRANTY; without even the implied warranty of
  15    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  16    GNU General Public License for more details.
  17
  18    You should have received a copy of the GNU General Public License
  19    along with this program; if not, write to the Free Software
  20    Foundation, Inc., 59 Temple Place - Suite 330,
  21    Boston, MA 02111-1307, USA.  */
  22
  23 #ifndef BCACHE_H
  24 #define BCACHE_H 1
  25
  26 /* A bcache is a data structure for factoring out duplication in
  27    read-only structures.  You give the bcache some string of bytes S.
  28    If the bcache already contains a copy of S, it hands you back a
  29    pointer to its copy.  Otherwise, it makes a fresh copy of S, and
  30    hands you back a pointer to that.  In either case, you can throw
  31    away your copy of S, and use the bcache's.
  32
  33    The "strings" in question are arbitrary strings of bytes --- they
  34    can contain zero bytes.  You pass in the length explicitly when you
  35    call the bcache function.
  36
  37    This means that you can put ordinary C objects in a bcache.
  38    However, if you do this, remember that structs can contain `holes'
  39    between members, added for alignment.  These bytes usually contain
  40    garbage.  If you try to bcache two objects which are identical from
  41    your code's point of view, but have different garbage values in the
  42    structure's holes, then the bcache will treat them as separate
  43    strings, and you won't get the nice elimination of duplicates you
  44    were hoping for.  So, remember to memset your structures full of
  45    zeros before bcaching them!
  46
  47    You shouldn't modify the strings you get from a bcache, because:
  48
  49    - You don't necessarily know who you're sharing space with.  If I
  50      stick eight bytes of text in a bcache, and then stick an
  51      eight-byte structure in the same bcache, there's no guarantee
  52      those two objects don't actually comprise the same sequence of
  53      bytes.  If they happen to, the bcache will use a single byte
  54      string for both of them.  Then, modifying the structure will
  55      change the string.  In bizarre ways.
  56
  57    - Even if you know for some other reason that all that's okay,
  58      there's another problem.  A bcache stores all its strings in a
  59      hash table.  If you modify a string's contents, you will probably
  60      change its hash value.  This means that the modified string is
  61      now in the wrong place in the hash table, and future bcache
  62      probes will never find it.  So by mutating a string, you give up
  63      any chance of sharing its space with future duplicates.  */
  64
  65
  66 /* The type used to hold a single bcache string.  The user data is
  67    stored in d.data.  Since it can be any type, it needs to have the
  68    same alignment as the most strict alignment of any type on the host
  69    machine.  I don't know of any really correct way to do this in
  70    stock ANSI C, so just do it the same way obstack.h does.
  71
  72    It would be nicer to have this stuff hidden away in bcache.c, but
  73    struct objstack contains a struct bcache directly --- not a pointer
  74    to one --- and then the memory-mapped stuff makes this a real pain.
  75    We don't strictly need to expose struct bstring, but it's better to
  76    have it all in one place.  */
  77
  78 struct bstring {
  79   struct bstring *next;
  80   size_t length;
  81
  82   union
  83   {
  84     char data[1];
  85     double dummy;
  86   }
  87   d;
  88 };
  89
  90
  91 /* The structure for a bcache itself.
  92    To initialize a bcache, just fill it with zeros.  */
  93 struct bcache {
  94   /* All the bstrings are allocated here.  */
  95   struct obstack cache;
  96
  97   /* How many hash buckets we're using.  */
  98   unsigned int num_buckets;
  99
 100   /* Hash buckets.  This table is allocated using malloc, so when we
 101      grow the table we can return the old table to the system.  */
 102   struct bstring **bucket;
 103
 104   /* Statistics.  */
 105   unsigned long unique_count;   /* number of unique strings */
 106   long total_count;     /* total number of strings cached, including dups */
 107   long unique_size;     /* size of unique strings, in bytes */
 108   long total_size;      /* total number of bytes cached, including dups */
 109   long structure_size;  /* total size of bcache, including infrastructure */
 110 };
 111
 112
 113 /* Find a copy of the LENGTH bytes at ADDR in BCACHE.  If BCACHE has
 114    never seen those bytes before, add a copy of them to BCACHE.  In
 115    either case, return a pointer to BCACHE's copy of that string.  */
 116 extern void *bcache (void *addr, int length, struct bcache *bcache);
 117
 118 /* Free all the storage that BCACHE refers to.  The result is a valid,
 119    but empty, bcache.  This does not free BCACHE itself, since that
 120    might be part of some larger object.  */
 121 extern void free_bcache (struct bcache *bcache);
 122
 123 /* Print statistics on BCACHE's memory usage and efficacity at
 124    eliminating duplication.  TYPE should be a string describing the
 125    kind of data BCACHE holds.  Statistics are printed using
 126    `printf_filtered' and its ilk.  */
 127 extern void print_bcache_statistics (struct bcache *bcache, char *type);
 128 /* The hash function */
 129 extern unsigned long hash(void *addr, int length);
 130 #endif /* BCACHE_H */