libguile/guardians.c

   1 /*      Copyright (C) 1998, 1999, 2000 Free Software Foundation, Inc.
   2  *
   3  * This program is free software; you can redistribute it and/or modify
   4  * it under the terms of the GNU General Public License as published by
   5  * the Free Software Foundation; either version 2, or (at your option)
   6  * any later version.
   7  *
   8  * This program is distributed in the hope that it will be useful,
   9  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  10  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  11  * GNU General Public License for more details.
  12  *
  13  * You should have received a copy of the GNU General Public License
  14  * along with this software; see the file COPYING.  If not, write to
  15  * the Free Software Foundation, Inc., 59 Temple Place, Suite 330,
  16  * Boston, MA 02111-1307 USA
  17  *
  18  * As a special exception, the Free Software Foundation gives permission
  19  * for additional uses of the text contained in its release of GUILE.
  20  *
  21  * The exception is that, if you link the GUILE library with other files
  22  * to produce an executable, this does not by itself cause the
  23  * resulting executable to be covered by the GNU General Public License.
  24  * Your use of that executable is in no way restricted on account of
  25  * linking the GUILE library code into it.
  26  *
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  28  * the executable file might be covered by the GNU General Public License.
  29  *
  30  * This exception applies only to the code released by the
  31  * Free Software Foundation under the name GUILE.  If you copy
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  40  * If you do not wish that, delete this exception notice.  */
  41
  42 /* Software engineering face-lift by Greg J. Badros, 11-Dec-1999,
  43    gjb@cs.washington.edu, http://www.cs.washington.edu/homes/gjb */
  44
  45 \f
  46
  47 /* This is an implementation of guardians as described in
  48  * R. Kent Dybvig, Carl Bruggeman, and David Eby (1993) "Guardians in
  49  * a Generation-Based Garbage Collector" ACM SIGPLAN Conference on
  50  * Programming Language Design and Implementation, June 1993
  51  * ftp://ftp.cs.indiana.edu/pub/scheme-repository/doc/pubs/guardians.ps.gz
  52  *
  53  * Author:      Michael N. Livshin
  54  * Modified by: Mikael Djurfeldt
  55  */
  56
  57 #include <stdio.h>
  58 #include <assert.h>
  59
  60 #include "_scm.h"
  61 #include "ports.h"
  62 #include "print.h"
  63 #include "smob.h"
  64 #include "vectors.h"
  65
  66 #include "validate.h"
  67 #include "guardians.h"
  68
  69 static long scm_tc16_guardian;
  70
  71 /* The live and zombies FIFOs are implemented as tconcs as described
  72    in Dybvig's paper.  This decouples addition and removal of elements
  73    so that no synchronization between these needs to take place.
  74 */
  75 #define TCONC_IN(tc, obj, pair) \
  76 do { \
  77   SCM_SETCAR ((tc).tail, obj); \
  78   SCM_SETCAR (pair, SCM_BOOL_F); \
  79   SCM_SETCDR (pair, SCM_EOL); \
  80   SCM_SETCDR ((tc).tail, pair); \
  81   (tc).tail = pair; \
  82 } while (0)
  83
  84 #define TCONC_OUT(tc, res) \
  85 do { \
  86   (res) = SCM_CAR ((tc).head); \
  87   (tc).head = SCM_CDR ((tc).head); \
  88 } while (0)
  89
  90 #define TCONC_EMPTYP(tc) (SCM_EQ_P ((tc).head, (tc).tail))
  91
  92 typedef struct tconc_t
  93 {
  94   SCM head;
  95   SCM tail;
  96 } tconc_t;
  97
  98 typedef struct guardian_t
  99 {
 100   tconc_t live;
 101   tconc_t zombies;
 102   struct guardian_t *next;
 103 } guardian_t;
 104
 105 #define GUARDIAN(x) ((guardian_t *) SCM_CELL_WORD_1 (x))
 106 #define GUARDIAN_LIVE(x) (GUARDIAN (x)->live)
 107 #define GUARDIAN_ZOMBIES(x) (GUARDIAN (x)->zombies)
 108 #define GUARDIAN_NEXT(x) (GUARDIAN (x)->next)
 109
 110 #define CCLO_G(cclo) (SCM_VELTS (cclo)[1])
 111
 112 /* subr constructed from guard below.  */
 113 static SCM guard1;
 114
 115 /* this is wrapped in a compiled closure and is the Scheme entry point
 116    for each guardian: if arg is an object, it's added to the
 117    guardian's live list.  if arg is unbound, the next available
 118    zombified object (or #f if none) is returned.  */
 119 static SCM
 120 guard (SCM cclo, SCM arg)
 121 {
 122   if (!SCM_UNBNDP (arg))
 123     {
 124       scm_guard (cclo, arg);
 125       return SCM_UNSPECIFIED;
 126     }
 127   else
 128     return scm_get_one_zombie (cclo);
 129 }
 130
 131 void
 132 scm_guard (SCM guardian, SCM obj)
 133 {
 134   SCM g = CCLO_G (guardian);
 135
 136   if (SCM_NIMP (obj))
 137     {
 138       SCM z;
 139
 140       SCM_NEWCELL (z);
 141
 142       /* This critical section barrier will be replaced by a mutex. */
 143       SCM_DEFER_INTS;
 144       TCONC_IN (GUARDIAN_LIVE (g), obj, z);
 145       SCM_ALLOW_INTS;
 146     }
 147 }
 148
 149 SCM
 150 scm_get_one_zombie (SCM guardian)
 151 {
 152   SCM g = CCLO_G (guardian);
 153   SCM res = SCM_BOOL_F;
 154
 155   /* This critical section barrier will be replaced by a mutex. */
 156   SCM_DEFER_INTS;
 157   if (!TCONC_EMPTYP (GUARDIAN_ZOMBIES (g)))
 158     TCONC_OUT (GUARDIAN_ZOMBIES (g), res);
 159   SCM_ALLOW_INTS;
 160   return res;
 161 }
 162
 163 SCM_DEFINE (scm_make_guardian, "make-guardian", 0, 0, 0,
 164             (),
 165             "Create a new guardian.\n"
 166             "A guardian protects a set of objects from garbage collection,\n"
 167             "allowing a program to apply cleanup or other actions.\n\n"
 168
 169             "make-guardian returns a procedure representing the guardian.\n"
 170             "Calling the guardian procedure with an argument adds the\n"
 171             "argument to the guardian's set of protected objects.\n"
 172             "Calling the guardian procedure without an argument returns\n"
 173             "one of the protected objects which are ready for garbage\n"
 174             "collection or @code{#f} if no such object is available.\n"
 175             "Objects which are returned in this way are removed from\n"
 176             "the guardian.\n\n".
 177
 178             "See R. Kent Dybvig, Carl Bruggeman, and David Eby (1993)\n"
 179             "\"Guardians in a Generation-Based Garbage Collector\".\n"
 180             "ACM SIGPLAN Conference on Programming Language Design\n"
 181             "and Implementation, June 1993.")
 182 #define FUNC_NAME s_scm_make_guardian
 183 {
 184   SCM cclo = scm_makcclo (guard1, 2L);
 185   guardian_t *g = SCM_MUST_MALLOC_TYPE(guardian_t);
 186   SCM z1 = scm_cons (SCM_BOOL_F, SCM_EOL);
 187   SCM z2 = scm_cons (SCM_BOOL_F, SCM_EOL);
 188   SCM z;
 189
 190   /* A tconc starts out with one tail pair. */
 191   g->live.head = g->live.tail = z1;
 192   g->zombies.head = g->zombies.tail = z2;
 193
 194   SCM_NEWSMOB (z, scm_tc16_guardian, g);
 195
 196   CCLO_G (cclo) = z;
 197
 198   return cclo;
 199 }
 200 #undef FUNC_NAME
 201
 202 /* during the gc mark phase, live guardians are linked into a list
 203    here.  */
 204 static guardian_t *first_live_guardian = NULL;
 205 static guardian_t **current_link_field = NULL;
 206
 207 /* called before gc mark phase begins to initialise the live guardian
 208    list.  */
 209 void
 210 scm_guardian_gc_init()
 211 {
 212   current_link_field = &first_live_guardian;
 213   first_live_guardian = NULL;
 214 }
 215
 216 /* mark a guardian by adding it to the live guardian list.  */
 217 static SCM
 218 g_mark (SCM ptr)
 219 {
 220   *current_link_field = GUARDIAN (ptr);
 221   current_link_field = &GUARDIAN_NEXT (ptr);
 222   GUARDIAN_NEXT (ptr) = NULL;
 223
 224   /* the objects protected by the guardian are not marked here: that
 225      would prevent them from ever getting collected.  instead marking
 226      is done at the end of the mark phase by scm_guardian_zombify.  */
 227   return SCM_BOOL_F;
 228 }
 229
 230 /* this is called by the garbage collector between the mark and sweep
 231    phases.  for each marked guardian, it moves any unmarked object in
 232    its live list (tconc) to its zombie list (tconc).  */
 233 void scm_guardian_zombify (void)
 234 {
 235   guardian_t *first_guardian;
 236   guardian_t **link_field = &first_live_guardian;
 237
 238   /* Note that new guardians may be stuck on the end of the live
 239      guardian list as we run this loop.  As we move unmarked objects
 240      to the zombie list and mark them, we may find some guarded
 241      guardians.  The guardian mark function will stick them on the end
 242      of this list, so they'll be processed properly.  */
 243
 244   do {
 245     guardian_t *g;
 246
 247     first_guardian = *link_field;
 248     link_field = current_link_field;
 249
 250     /* first, scan all the guardians that are currently known to be live
 251        and move their unmarked objects to zombie lists. */
 252
 253     for (g = first_guardian; g; g = g->next)
 254       {
 255         SCM tconc_tail = g->live.tail;
 256         SCM *prev_ptr = &g->live.head;
 257         SCM pair = g->live.head;
 258
 259         while (! SCM_EQ_P (pair, tconc_tail))
 260           {
 261             SCM next_pair = SCM_CDR (pair);
 262
 263             if (SCM_NMARKEDP (SCM_CAR (pair)))
 264               {
 265                 /* got you, zombie! */
 266
 267                 /* out of the live list! */
 268                 *prev_ptr = next_pair;
 269
 270                 /* into the zombie list! */
 271                 TCONC_IN (g->zombies, SCM_CAR (pair), pair);
 272               }
 273             else
 274               prev_ptr = SCM_CDRLOC (pair);
 275
 276             pair = next_pair;
 277           }
 278
 279         /* Mark the cells of the live list (yes, the cells in the list,
 280            even though we don't care about objects pointed to by the list
 281            cars, since we know they are already marked).  */
 282         for (pair = g->live.head; SCM_NIMP (pair); pair = SCM_GCCDR (pair))
 283           SCM_SETGCMARK (pair);
 284       }
 285
 286     /* ghouston: Doesn't it seem a bit disturbing that if a zombie
 287        is returned to full life after getting returned from the
 288        guardian procedure, it may reference objects which are in a
 289        guardian's zombie list?  Is it not necessary to move such
 290        zombies back to the live list, to avoid allowing the
 291        guardian procedure to return an object which is referenced,
 292        so not collectable?  The paper doesn't give this
 293        impression.
 294
 295        cmm: the paper does explicitly say that an object that is
 296        guarded more than once should be returned more than once.
 297        I believe this covers the above scenario. */
 298
 299     /* Preserve the zombies in their undead state, by marking to
 300        prevent collection.  Note that this may uncover zombified
 301        guardians -- if so, they'll be processed in the next loop. */
 302
 303     for (g = first_guardian; g && (!*link_field || g != *link_field); g = g->next)
 304       scm_gc_mark (g->zombies.head);
 305
 306   } while (current_link_field != link_field);
 307 }
 308
 309 /* not generally used, since guardian smob is wrapped in a closure.
 310    maybe useful for debugging.  */
 311 static int
 312 g_print (SCM exp, SCM port, scm_print_state *pstate)
 313 {
 314   char buf[256];
 315   sprintf (buf, "#<guardian live objs: %lu zombies: %lu>",
 316            scm_ilength (SCM_CDR (GUARDIAN_LIVE (exp).head)),
 317            scm_ilength (SCM_CDR (GUARDIAN_ZOMBIES (exp).head)));
 318   scm_puts (buf, port);
 319
 320   return 1;
 321 }
 322
 323 void
 324 scm_init_guardian()
 325 {
 326   scm_tc16_guardian = scm_make_smob_type_mfpe ("guardian", sizeof (guardian_t),
 327                                                g_mark, NULL, g_print, NULL);
 328   guard1 = scm_make_subr_opt ("guardian", scm_tc7_subr_2o, guard, 0);
 329
 330 #include "guardians.x"
 331 }
 332
 333 /*
 334   Local Variables:
 335   c-file-style: "gnu"
 336   End:
 337 */