root/gc/backgraph.c

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DEFINITIONS

This source file includes following definitions.
  1. back_edges
  2. new_back_edges
  3. deallocate_back_edges
  4. push_in_progress
  5. is_in_progress
  6. pop_in_progress
  7. ensure_struct
  8. add_edge
  9. per_object_helper
  10. GC_apply_to_each_object
  11. reset_back_edge
  12. add_back_edges
  13. GC_build_back_graph
  14. backwards_height
  15. update_max_height
  16. GC_traverse_back_graph
  17. GC_print_back_graph_stats

   1 /*
   2  * Copyright (c) 2001 by Hewlett-Packard Company. All rights reserved.
   3  *
   4  * THIS MATERIAL IS PROVIDED AS IS, WITH ABSOLUTELY NO WARRANTY EXPRESSED
   5  * OR IMPLIED.  ANY USE IS AT YOUR OWN RISK.
   6  *
   7  * Permission is hereby granted to use or copy this program
   8  * for any purpose,  provided the above notices are retained on all copies.
   9  * Permission to modify the code and to distribute modified code is granted,
  10  * provided the above notices are retained, and a notice that the code was
  11  * modified is included with the above copyright notice.
  12  *
  13  */
  14 
  15 /*
  16  * This implements a full, though not well-tuned, representation of the
  17  * backwards points-to graph.  This is used to test for non-GC-robust
  18  * data structures; the code is not used during normal garbage collection.
  19  *
  20  * One restriction is that we drop all back-edges from nodes with very
  21  * high in-degree, and simply add them add them to a list of such
  22  * nodes.  They are then treated as permanent roots.  Id this by itself
  23  * doesn't introduce a space leak, then such nodes can't contribute to
  24  * a growing space leak.
  25  */
  26 
  27 #ifdef MAKE_BACK_GRAPH
  28 
  29 #define MAX_IN  10      /* Maximum in-degree we handle directly */
  30 
  31 #include "private/dbg_mlc.h"
  32 #include <unistd.h>
  33 
  34 #if !defined(DBG_HDRS_ALL) || (ALIGNMENT != CPP_WORDSZ/8) || !defined(UNIX_LIKE)
  35 # error Configuration doesnt support MAKE_BACK_GRAPH
  36 #endif
  37 
  38 /* We store single back pointers directly in the object's oh_bg_ptr field.   */
  39 /* If there is more than one ptr to an object, we store q | FLAG_MANY,       */
  40 /* where q is a pointer to a back_edges object.                              */
  41 /* Every once in a while we use a back_edges object even for a single        */
  42 /* pointer, since we need the other fields in the back_edges structure to    */
  43 /* be present in some fraction of the objects.  Otherwise we get serious     */
  44 /* performance issues.                                                       */
  45 #define FLAG_MANY 2
  46 
  47 typedef struct back_edges_struct {
  48   word n_edges; /* Number of edges, including those in continuation     */
  49                 /* structures.                                          */
  50   unsigned short flags;
  51 #       define RETAIN 1 /* Directly points to a reachable object;       */
  52                         /* retain for next GC.                          */
  53   unsigned short height_gc_no;
  54                 /* If height > 0, then the GC_gc_no value when it       */
  55                 /* was computed.  If it was computed this cycle, then   */
  56                 /* it is current.  If it was computed during the        */
  57                 /* last cycle, then it represents the old height,       */
  58                 /* which is only saved for live objects referenced by   */
  59                 /* dead ones.  This may grow due to refs from newly     */
  60                 /* dead objects.                                        */
  61   signed_word height;
  62                 /* Longest path through unreachable nodes to this node  */
  63                 /* that we found using depth first search.              */
  64   
  65 #   define HEIGHT_UNKNOWN ((signed_word)(-2))
  66 #   define HEIGHT_IN_PROGRESS ((signed_word)(-1))
  67   ptr_t edges[MAX_IN];
  68   struct back_edges_struct *cont;
  69                 /* Pointer to continuation structure; we use only the   */
  70                 /* edges field in the continuation.                     */
  71                 /* also used as free list link.                         */
  72 } back_edges;
  73 
  74 /* Allocate a new back edge structure.  Should be more sophisticated    */
  75 /* if this were production code.                                        */
  76 #define MAX_BACK_EDGE_STRUCTS 100000
  77 static back_edges *back_edge_space = 0;
  78 int GC_n_back_edge_structs = 0; /* Serves as pointer to never used      */
  79                                 /* back_edges space.                    */
  80 static back_edges *avail_back_edges = 0;
  81                                 /* Pointer to free list of deallocated  */
  82                                 /* back_edges structures.               */
  83 
  84 static back_edges * new_back_edges(void)
  85 {
  86   if (0 == back_edge_space) {
  87     back_edge_space = (back_edges *)
  88                         GET_MEM(MAX_BACK_EDGE_STRUCTS*sizeof(back_edges));
  89   }
  90   if (0 != avail_back_edges) {
  91     back_edges * result = avail_back_edges;
  92     avail_back_edges = result -> cont;
  93     result -> cont = 0;
  94     return result;
  95   }
  96   if (GC_n_back_edge_structs >= MAX_BACK_EDGE_STRUCTS - 1) {
  97     ABORT("needed too much space for back edges: adjust "
  98           "MAX_BACK_EDGE_STRUCTS");
  99   }
 100   return back_edge_space + (GC_n_back_edge_structs++);
 101 }
 102 
 103 /* Deallocate p and its associated continuation structures.     */
 104 static void deallocate_back_edges(back_edges *p)
 105 {
 106    back_edges *last = p;
 107 
 108    while (0 != last -> cont) last = last -> cont;
 109    last -> cont = avail_back_edges;
 110    avail_back_edges = p;
 111 }
 112 
 113 /* Table of objects that are currently on the depth-first search        */
 114 /* stack.  Only objects with in-degree one are in this table.           */
 115 /* Other objects are identified using HEIGHT_IN_PROGRESS.               */
 116 /* FIXME: This data structure NEEDS IMPROVEMENT.                        */
 117 #define INITIAL_IN_PROGRESS 10000
 118 static ptr_t * in_progress_space = 0;
 119 static size_t in_progress_size = 0;
 120 static size_t n_in_progress = 0;
 121 
 122 static void push_in_progress(ptr_t p)
 123 {
 124   if (n_in_progress >= in_progress_size) 
 125     if (in_progress_size == 0) {
 126       in_progress_size = INITIAL_IN_PROGRESS;
 127       in_progress_space = (ptr_t *)GET_MEM(in_progress_size * sizeof(ptr_t));
 128     } else {
 129       ptr_t * new_in_progress_space;
 130       in_progress_size *= 2;
 131       new_in_progress_space = (ptr_t *)
 132                                 GET_MEM(in_progress_size * sizeof(ptr_t));
 133       BCOPY(in_progress_space, new_in_progress_space,
 134             n_in_progress * sizeof(ptr_t));
 135       in_progress_space = new_in_progress_space;
 136       /* FIXME: This just drops the old space.  */
 137     }
 138   if (in_progress_space == 0)
 139       ABORT("MAKE_BACK_GRAPH: Out of in-progress space: "
 140             "Huge linear data structure?");
 141   in_progress_space[n_in_progress++] = p;
 142 }
 143 
 144 static GC_bool is_in_progress(ptr_t p)
 145 {
 146   int i;
 147   for (i = 0; i < n_in_progress; ++i) {
 148     if (in_progress_space[i] == p) return TRUE;
 149   }
 150   return FALSE;
 151 }
 152 
 153 static void pop_in_progress(ptr_t p)
 154 {
 155   --n_in_progress;
 156   GC_ASSERT(in_progress_space[n_in_progress] == p);
 157 }
 158 
 159 #define GET_OH_BG_PTR(p) \
 160                 (ptr_t)REVEAL_POINTER(((oh *)(p)) -> oh_bg_ptr)
 161 #define SET_OH_BG_PTR(p,q) (((oh *)(p)) -> oh_bg_ptr) = HIDE_POINTER(q)
 162 
 163 /* Execute s once for each predecessor q of p in the points-to graph.   */
 164 /* s should be a bracketed statement.  We declare q.                    */
 165 #define FOR_EACH_PRED(q, p, s) \
 166   { \
 167     ptr_t q = GET_OH_BG_PTR(p); \
 168     if (!((word)q & FLAG_MANY)) { \
 169       if (q && !((word)q & 1)) s \
 170               /* !((word)q & 1) checks for a misnterpreted freelist link */ \
 171     } else { \
 172       back_edges *orig_be_ = (back_edges *)((word)q & ~FLAG_MANY); \
 173       back_edges *be_ = orig_be_; \
 174       int total_, local_; \
 175       int n_edges_ = be_ -> n_edges; \
 176       for (total_ = 0, local_ = 0; total_ < n_edges_; ++local_, ++total_) { \
 177           if (local_ == MAX_IN) { \
 178               be_ = be_ -> cont; \
 179               local_ = 0; \
 180           } \
 181           q = be_ -> edges[local_]; s \
 182       } \
 183     } \
 184   }
 185 
 186 /* Ensure that p has a back_edges structure associated with it. */
 187 static void ensure_struct(ptr_t p)
 188 {
 189   ptr_t old_back_ptr = GET_OH_BG_PTR(p);
 190 
 191   if (!((word)old_back_ptr & FLAG_MANY)) {
 192     back_edges *be = new_back_edges();
 193     be -> flags = 0;
 194     if (0 == old_back_ptr) {
 195       be -> n_edges = 0;
 196     } else {
 197       be -> n_edges = 1;
 198       be -> edges[0] = old_back_ptr;
 199     }
 200     be -> height = HEIGHT_UNKNOWN;
 201     be -> height_gc_no = GC_gc_no - 1;
 202     GC_ASSERT(be >= back_edge_space);
 203     SET_OH_BG_PTR(p, (word)be | FLAG_MANY);
 204   }
 205 }
 206 
 207 /* Add the (forward) edge from p to q to the backward graph.  Both p    */
 208 /* q are pointers to the object base, i.e. pointers to an oh.           */
 209 static void add_edge(ptr_t p,  ptr_t q)
 210 {
 211     ptr_t old_back_ptr = GET_OH_BG_PTR(q);
 212     back_edges * be, *be_cont;
 213     word i;
 214     static unsigned random_number = 13;
 215 #   define GOT_LUCKY_NUMBER (((++random_number) & 0x7f) == 0)
 216       /* A not very random number we use to occasionally allocate a     */
 217       /* back_edges structure even for a single backward edge.  This    */
 218       /* prevents us from repeatedly tracing back through very long     */
 219       /* chains, since we will have some place to store height and      */
 220       /* in_progress flags along the way.                               */
 221 
 222     GC_ASSERT(p == GC_base(p) && q == GC_base(q));
 223     if (!GC_HAS_DEBUG_INFO(q) || !GC_HAS_DEBUG_INFO(p)) {
 224       /* This is really a misinterpreted free list link, since we saw */
 225       /* a pointer to a free list.  Dont overwrite it!                */
 226       return;
 227     }
 228     if (0 == old_back_ptr) {
 229         SET_OH_BG_PTR(q, p);
 230         if (GOT_LUCKY_NUMBER) ensure_struct(q);
 231         return;
 232     }
 233     /* Check whether it was already in the list of predecessors. */
 234       FOR_EACH_PRED(pred, q, { if (p == pred) return; });
 235     ensure_struct(q);
 236     old_back_ptr = GET_OH_BG_PTR(q);
 237     be = (back_edges *)((word)old_back_ptr & ~FLAG_MANY);
 238     for (i = be -> n_edges, be_cont = be; i > MAX_IN;
 239         be_cont = be_cont -> cont, i -= MAX_IN) {}
 240     if (i == MAX_IN) {
 241         be_cont -> cont = new_back_edges();
 242         be_cont = be_cont -> cont;
 243         i = 0;
 244     }
 245     be_cont -> edges[i] = p;
 246     be -> n_edges++;
 247     if (be -> n_edges == 100) {
 248 #       if 0
 249           if (GC_print_stats) {
 250             GC_err_printf0("The following object has in-degree >= 100:\n");
 251             GC_print_heap_obj(q);
 252           }
 253 #       endif
 254     }
 255 }
 256 
 257 typedef void (*per_object_func)(ptr_t p, word n_words, word gc_descr);
 258 
 259 static void per_object_helper(struct hblk *h, word fn)
 260 {
 261   hdr * hhdr = HDR(h);
 262   word sz = hhdr -> hb_sz;
 263   word descr = hhdr -> hb_descr;
 264   per_object_func f = (per_object_func)fn;
 265   int i = 0;
 266 
 267   do {
 268     f((ptr_t)(h -> hb_body + i), sz, descr);
 269     i += sz;
 270   } while (i + sz <= BYTES_TO_WORDS(HBLKSIZE));
 271 }
 272 
 273 void GC_apply_to_each_object(per_object_func f)
 274 {
 275   GC_apply_to_all_blocks(per_object_helper, (word)f);
 276 }
 277 
 278 static void reset_back_edge(ptr_t p, word n_words, word gc_descr)
 279 {
 280   /* Skip any free list links, or dropped blocks */
 281   if (GC_HAS_DEBUG_INFO(p)) {
 282     ptr_t old_back_ptr = GET_OH_BG_PTR(p);
 283     if ((word)old_back_ptr & FLAG_MANY) {
 284       back_edges *be = (back_edges *)((word)old_back_ptr & ~FLAG_MANY);
 285       if (!(be -> flags & RETAIN)) {
 286         deallocate_back_edges(be);
 287         SET_OH_BG_PTR(p, 0); 
 288       } else {
 289         word *currentp;
 290 
 291         GC_ASSERT(GC_is_marked(p));
 292 
 293         /* Back edges may point to objects that will not be retained.   */
 294         /* Delete them for now, but remember the height.                */
 295         /* Some will be added back at next GC.                          */
 296           be -> n_edges = 0;
 297           if (0 != be -> cont) {
 298             deallocate_back_edges(be -> cont);
 299             be -> cont = 0;
 300           }
 301 
 302         GC_ASSERT(GC_is_marked(p));
 303 
 304         /* We only retain things for one GC cycle at a time.            */
 305           be -> flags &= ~RETAIN;
 306       }
 307     } else /* Simple back pointer */ {
 308       /* Clear to avoid dangling pointer. */
 309       SET_OH_BG_PTR(p, 0);
 310     }
 311   }
 312 }
 313 
 314 static void add_back_edges(ptr_t p, word n_words, word gc_descr)
 315 {
 316   word *currentp = (word *)(p + sizeof(oh));
 317 
 318   /* For now, fix up non-length descriptors conservatively.     */
 319     if((gc_descr & GC_DS_TAGS) != GC_DS_LENGTH) {
 320       gc_descr = WORDS_TO_BYTES(n_words);
 321     }
 322   while (currentp < (word *)(p + gc_descr)) {
 323     word current = *currentp++;
 324     FIXUP_POINTER(current);
 325     if (current >= (word)GC_least_plausible_heap_addr && 
 326         current <= (word)GC_greatest_plausible_heap_addr) {
 327        ptr_t target = GC_base((GC_PTR)current);
 328        if (0 != target) {
 329          add_edge(p, target);
 330        }
 331     }
 332   }
 333 }
 334 
 335 /* Rebuild the representation of the backward reachability graph.       */
 336 /* Does not examine mark bits.  Can be called before GC.                */
 337 void GC_build_back_graph(void)
 338 {
 339   GC_apply_to_each_object(add_back_edges);
 340 }
 341 
 342 /* Return an approximation to the length of the longest simple path     */
 343 /* through unreachable objects to p.  We refer to this as the height    */
 344 /* of p.                                                                */
 345 static word backwards_height(ptr_t p)
 346 {
 347   word result;
 348   ptr_t back_ptr = GET_OH_BG_PTR(p);
 349   back_edges *be;
 350 
 351   if (0 == back_ptr) return 1;
 352   if (!((word)back_ptr & FLAG_MANY)) {
 353     if (is_in_progress(p)) return 0;  /* DFS back edge, i.e. we followed  */
 354                                       /* an edge to an object already     */
 355                                       /* on our stack: ignore             */
 356     push_in_progress(p);
 357     result = backwards_height(back_ptr)+1;
 358     pop_in_progress(p);
 359     return result;
 360   }
 361   be = (back_edges *)((word)back_ptr & ~FLAG_MANY);
 362   if (be -> height >= 0 && be -> height_gc_no == GC_gc_no)
 363       return be -> height;
 364   /* Ignore back edges in DFS */
 365     if (be -> height == HEIGHT_IN_PROGRESS) return 0;
 366   result = (be -> height > 0? be -> height : 1);
 367   be -> height = HEIGHT_IN_PROGRESS;
 368   FOR_EACH_PRED(q, p, {
 369     word this_height;
 370     if (GC_is_marked(q) && !(FLAG_MANY & (word)GET_OH_BG_PTR(p))) {
 371       if (GC_print_stats)
 372           GC_printf2("Found bogus pointer from 0x%lx to 0x%lx\n", q, p);
 373         /* Reachable object "points to" unreachable one.                */
 374         /* Could be caused by our lax treatment of GC descriptors.      */
 375       this_height = 1;
 376     } else {
 377         this_height = backwards_height(q);
 378     }
 379     if (this_height >= result) result = this_height + 1;
 380   });
 381   be -> height = result;
 382   be -> height_gc_no = GC_gc_no;
 383   return result;
 384 }
 385 
 386 word GC_max_height;
 387 ptr_t GC_deepest_obj;
 388 
 389 /* Compute the maximum height of every unreachable predecessor p of  a  */
 390 /* reachable object.  Arrange to save the heights of all such objects p */
 391 /* so that they can be used in calculating the height of objects in the */
 392 /* next GC.                                                             */
 393 /* Set GC_max_height to be the maximum height we encounter, and         */
 394 /* GC_deepest_obj to be the corresponding object.                       */
 395 static void update_max_height(ptr_t p, word n_words, word gc_descr)
 396 {
 397   if (GC_is_marked(p) && GC_HAS_DEBUG_INFO(p)) {
 398     int i;
 399     word p_height = 0;
 400     ptr_t p_deepest_obj = 0;
 401     ptr_t back_ptr;
 402     back_edges *be = 0;
 403 
 404     /* If we remembered a height last time, use it as a minimum.        */
 405     /* It may have increased due to newly unreachable chains pointing   */
 406     /* to p, but it can't have decreased.                               */
 407     back_ptr = GET_OH_BG_PTR(p);
 408     if (0 != back_ptr && ((word)back_ptr & FLAG_MANY)) {
 409       be = (back_edges *)((word)back_ptr & ~FLAG_MANY);
 410       if (be -> height != HEIGHT_UNKNOWN) p_height = be -> height;
 411     }
 412     FOR_EACH_PRED(q, p, {
 413       if (!GC_is_marked(q) && GC_HAS_DEBUG_INFO(q)) {
 414         word q_height;
 415 
 416         q_height = backwards_height(q);
 417         if (q_height > p_height) {
 418           p_height = q_height;
 419           p_deepest_obj = q;
 420         }
 421       }
 422     });
 423     if (p_height > 0) {
 424       /* Remember the height for next time. */
 425         if (be == 0) {
 426           ensure_struct(p);
 427           back_ptr = GET_OH_BG_PTR(p);
 428           be = (back_edges *)((word)back_ptr & ~FLAG_MANY);
 429         }
 430         be -> flags |= RETAIN;
 431         be -> height = p_height;
 432         be -> height_gc_no = GC_gc_no;
 433     }
 434     if (p_height > GC_max_height) {
 435         GC_max_height = p_height;
 436         GC_deepest_obj = p_deepest_obj;
 437     }
 438   }
 439 }
 440 
 441 word GC_max_max_height = 0;
 442 
 443 void GC_traverse_back_graph(void)
 444 {
 445   GC_max_height = 0;
 446   GC_apply_to_each_object(update_max_height);
 447 }
 448 
 449 void GC_print_back_graph_stats(void)
 450 {
 451   GC_printf2("Maximum backwards height of reachable objects at GC %lu is %ld\n",
 452              (unsigned long) GC_gc_no, GC_max_height);
 453   if (GC_max_height > GC_max_max_height) {
 454     GC_max_max_height = GC_max_height;
 455     GC_printf0("The following unreachable object is last in a longest chain "
 456                "of unreachable objects:\n");
 457     GC_print_heap_obj(GC_deepest_obj);
 458   }
 459   if (GC_print_stats) {
 460     GC_printf1("Needed max total of %ld back-edge structs\n",
 461                GC_n_back_edge_structs);
 462   }
 463   GC_apply_to_each_object(reset_back_edge);
 464   GC_deepest_obj = 0;
 465 }
 466 
 467 #endif /* MAKE_BACK_GRAPH */

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