boehm-gc/backgraph.c

   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                         sbrk(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 /* This data structure NEEDS IMPROVEMENT.                               */
 117 #define MAX_IN_PROGRESS 10000
 118 static ptr_t * in_progress_space = 0;
 119 static int n_in_progress = 0;
 120
 121 static void push_in_progress(ptr_t p)
 122 {
 123   if (in_progress_space == 0)
 124       in_progress_space = sbrk(MAX_IN_PROGRESS * sizeof(ptr_t));
 125   if (n_in_progress == MAX_IN_PROGRESS)
 126       ABORT("Exceeded MAX_IN_PROGRESS");
 127   in_progress_space[n_in_progress++] = p;
 128 }
 129
 130 static GC_bool is_in_progress(ptr_t p)
 131 {
 132   int i;
 133   for (i = 0; i < n_in_progress; ++i) {
 134     if (in_progress_space[i] == p) return TRUE;
 135   }
 136   return FALSE;
 137 }
 138
 139 static void pop_in_progress(ptr_t p)
 140 {
 141   --n_in_progress;
 142   GC_ASSERT(in_progress_space[n_in_progress] == p);
 143 }
 144
 145 #define GET_OH_BG_PTR(p) \
 146                 (ptr_t)REVEAL_POINTER(((oh *)(p)) -> oh_bg_ptr)
 147 #define SET_OH_BG_PTR(p,q) (((oh *)(p)) -> oh_bg_ptr) = HIDE_POINTER(q)
 148
 149 /* Execute s once for each predecessor q of p in the points-to graph.   */
 150 /* s should be a bracketed statement.  We declare q.                    */
 151 #define FOR_EACH_PRED(q, p, s) \
 152   { \
 153     ptr_t q = GET_OH_BG_PTR(p); \
 154     if (!((word)q & FLAG_MANY)) { \
 155       if (q && !((word)q & 1)) s \
 156               /* !((word)q & 1) checks for a misnterpreted freelist link */ \
 157     } else { \
 158       back_edges *orig_be_ = (back_edges *)((word)q & ~FLAG_MANY); \
 159       back_edges *be_ = orig_be_; \
 160       int total_, local_; \
 161       int n_edges_ = be_ -> n_edges; \
 162       for (total_ = 0, local_ = 0; total_ < n_edges_; ++local_, ++total_) { \
 163           if (local_ == MAX_IN) { \
 164               be_ = be_ -> cont; \
 165               local_ = 0; \
 166           } \
 167           q = be_ -> edges[local_]; s \
 168       } \
 169     } \
 170   }
 171
 172 /* Ensure that p has a back_edges structure associated with it. */
 173 static void ensure_struct(ptr_t p)
 174 {
 175   ptr_t old_back_ptr = GET_OH_BG_PTR(p);
 176
 177   if (!((word)old_back_ptr & FLAG_MANY)) {
 178     back_edges *be = new_back_edges();
 179     be -> flags = 0;
 180     if (0 == old_back_ptr) {
 181       be -> n_edges = 0;
 182     } else {
 183       be -> n_edges = 1;
 184       be -> edges[0] = old_back_ptr;
 185     }
 186     be -> height = HEIGHT_UNKNOWN;
 187     be -> height_gc_no = GC_gc_no - 1;
 188     GC_ASSERT(be >= back_edge_space);
 189     SET_OH_BG_PTR(p, (word)be | FLAG_MANY);
 190   }
 191 }
 192
 193 /* Add the (forward) edge from p to q to the backward graph.  Both p    */
 194 /* q are pointers to the object base, i.e. pointers to an oh.           */
 195 static void add_edge(ptr_t p,  ptr_t q)
 196 {
 197     ptr_t old_back_ptr = GET_OH_BG_PTR(q);
 198     back_edges * be, *be_cont;
 199     word i;
 200     static unsigned random_number = 13;
 201 #   define GOT_LUCKY_NUMBER (((++random_number) & 0x7f) == 0)
 202       /* A not very random number we use to occasionally allocate a     */
 203       /* back_edges structure even for a single backward edge.  This    */
 204       /* prevents us from repeatedly tracing back through very long     */
 205       /* chains, since we will have some place to store height and      */
 206       /* in_progress flags along the way.                               */
 207
 208     GC_ASSERT(p == GC_base(p) && q == GC_base(q));
 209     if (!GC_HAS_DEBUG_INFO(q) || !GC_HAS_DEBUG_INFO(p)) {
 210       /* This is really a misinterpreted free list link, since we saw */
 211       /* a pointer to a free list.  Dont overwrite it!                */
 212       return;
 213     }
 214     if (0 == old_back_ptr) {
 215         SET_OH_BG_PTR(q, p);
 216         if (GOT_LUCKY_NUMBER) ensure_struct(q);
 217         return;
 218     }
 219     /* Check whether it was already in the list of predecessors. */
 220       FOR_EACH_PRED(pred, q, { if (p == pred) return; });
 221     ensure_struct(q);
 222     old_back_ptr = GET_OH_BG_PTR(q);
 223     be = (back_edges *)((word)old_back_ptr & ~FLAG_MANY);
 224     for (i = be -> n_edges, be_cont = be; i > MAX_IN;
 225         be_cont = be_cont -> cont, i -= MAX_IN) {}
 226     if (i == MAX_IN) {
 227         be_cont -> cont = new_back_edges();
 228         be_cont = be_cont -> cont;
 229         i = 0;
 230     }
 231     be_cont -> edges[i] = p;
 232     be -> n_edges++;
 233     if (be -> n_edges == 100) {
 234 #       if 0
 235           if (GC_print_stats) {
 236             GC_err_printf0("The following object has in-degree >= 100:\n");
 237             GC_print_heap_obj(q);
 238           }
 239 #       endif
 240     }
 241 }
 242
 243 typedef void (*per_object_func)(ptr_t p, word n_words, word gc_descr);
 244
 245 static void per_object_helper(struct hblk *h, word fn)
 246 {
 247   hdr * hhdr = HDR(h);
 248   word sz = hhdr -> hb_sz;
 249   word descr = hhdr -> hb_descr;
 250   per_object_func f = (per_object_func)fn;
 251   int i = 0;
 252
 253   do {
 254     f((ptr_t)(h -> hb_body + i), sz, descr);
 255     i += sz;
 256   } while (i + sz <= BYTES_TO_WORDS(HBLKSIZE));
 257 }
 258
 259 void GC_apply_to_each_object(per_object_func f)
 260 {
 261   GC_apply_to_all_blocks(per_object_helper, (word)f);
 262 }
 263
 264 static void reset_back_edge(ptr_t p, word n_words, word gc_descr)
 265 {
 266   /* Skip any free list links, or dropped blocks */
 267   if (GC_HAS_DEBUG_INFO(p)) {
 268     ptr_t old_back_ptr = GET_OH_BG_PTR(p);
 269     if ((word)old_back_ptr & FLAG_MANY) {
 270       back_edges *be = (back_edges *)((word)old_back_ptr & ~FLAG_MANY);
 271       if (!(be -> flags & RETAIN)) {
 272         deallocate_back_edges(be);
 273         SET_OH_BG_PTR(p, 0);
 274       } else {
 275         word *currentp;
 276
 277         GC_ASSERT(GC_is_marked(p));
 278
 279         /* Back edges may point to objects that will not be retained.   */
 280         /* Delete them for now, but remember the height.                */
 281         /* Some will be added back at next GC.                          */
 282           be -> n_edges = 0;
 283           if (0 != be -> cont) {
 284             deallocate_back_edges(be -> cont);
 285             be -> cont = 0;
 286           }
 287
 288         GC_ASSERT(GC_is_marked(p));
 289
 290         /* We only retain things for one GC cycle at a time.            */
 291           be -> flags &= ~RETAIN;
 292       }
 293     } else /* Simple back pointer */ {
 294       /* Clear to avoid dangling pointer. */
 295       SET_OH_BG_PTR(p, 0);
 296     }
 297   }
 298 }
 299
 300 static void add_back_edges(ptr_t p, word n_words, word gc_descr)
 301 {
 302   word *currentp = (word *)(p + sizeof(oh));
 303
 304   /* For now, fix up non-length descriptors conservatively.     */
 305     if((gc_descr & GC_DS_TAGS) != GC_DS_LENGTH) {
 306       gc_descr = WORDS_TO_BYTES(n_words);
 307     }
 308   while (currentp < (word *)(p + gc_descr)) {
 309     word current = *currentp++;
 310     if (current >= (word)GC_least_plausible_heap_addr &&
 311         current <= (word)GC_greatest_plausible_heap_addr) {
 312        ptr_t target = GC_base((GC_PTR)current);
 313        if (0 != target) {
 314          add_edge(p, target);
 315        }
 316     }
 317   }
 318 }
 319
 320 /* Rebuild the reprentation of the backward reachability graph. */
 321 /* Does not examine mark bits.  Can be called before GC.        */
 322 void GC_build_back_graph(void)
 323 {
 324   GC_apply_to_each_object(add_back_edges);
 325 }
 326
 327 /* Return an approximation to the length of the longest simple path     */
 328 /* through unreachable objects to p.  We refer to this as the height    */
 329 /* of p.                                                                */
 330 static word backwards_height(ptr_t p)
 331 {
 332   word result;
 333   ptr_t back_ptr = GET_OH_BG_PTR(p);
 334   back_edges *be;
 335
 336   if (0 == back_ptr) return 1;
 337   if (!((word)back_ptr & FLAG_MANY)) {
 338     if (is_in_progress(p)) return 0;  /* DFS back edge, i.e. we followed  */
 339                                       /* an edge to an object already     */
 340                                       /* on our stack: ignore             */
 341     push_in_progress(p);
 342     result = backwards_height(back_ptr)+1;
 343     pop_in_progress(p);
 344     return result;
 345   }
 346   be = (back_edges *)((word)back_ptr & ~FLAG_MANY);
 347   if (be -> height >= 0 && be -> height_gc_no == GC_gc_no)
 348       return be -> height;
 349   /* Ignore back edges in DFS */
 350     if (be -> height == HEIGHT_IN_PROGRESS) return 0;
 351   result = (be -> height > 0? be -> height : 1);
 352   be -> height = HEIGHT_IN_PROGRESS;
 353   FOR_EACH_PRED(q, p, {
 354     word this_height;
 355     if (GC_is_marked(q) && !(FLAG_MANY & (word)GET_OH_BG_PTR(p))) {
 356       if (GC_print_stats)
 357           GC_printf2("Found bogus pointer from 0x%lx to 0x%lx\n", q, p);
 358         /* Reachable object "points to" unreachable one.                */
 359         /* Could be caused by our lax treatment of GC descriptors.      */
 360       this_height = 1;
 361     } else {
 362         this_height = backwards_height(q);
 363     }
 364     if (this_height >= result) result = this_height + 1;
 365   });
 366   be -> height = result;
 367   be -> height_gc_no = GC_gc_no;
 368   return result;
 369 }
 370
 371 word GC_max_height;
 372 ptr_t GC_deepest_obj;
 373
 374 /* Compute the maximum height of every unreachable predecessor p of  a  */
 375 /* reachable object.  Arrange to save the heights of all such objects p */
 376 /* so that they can be used in calculating the height of objects in the */
 377 /* next GC.                                                             */
 378 /* Set GC_max_height to be the maximum height we encounter, and         */
 379 /* GC_deepest_obj to be the corresponding object.                       */
 380 static void update_max_height(ptr_t p, word n_words, word gc_descr)
 381 {
 382   if (GC_is_marked(p) && GC_HAS_DEBUG_INFO(p)) {
 383     int i;
 384     word p_height = 0;
 385     ptr_t p_deepest_obj = 0;
 386     ptr_t back_ptr;
 387     back_edges *be = 0;
 388
 389     /* If we remembered a height last time, use it as a minimum.        */
 390     /* It may have increased due to newly unreachable chains pointing   */
 391     /* to p, but it can't have decreased.                               */
 392     back_ptr = GET_OH_BG_PTR(p);
 393     if (0 != back_ptr && ((word)back_ptr & FLAG_MANY)) {
 394       be = (back_edges *)((word)back_ptr & ~FLAG_MANY);
 395       if (be -> height != HEIGHT_UNKNOWN) p_height = be -> height;
 396     }
 397     FOR_EACH_PRED(q, p, {
 398       if (!GC_is_marked(q) && GC_HAS_DEBUG_INFO(q)) {
 399         word q_height;
 400
 401         q_height = backwards_height(q);
 402         if (q_height > p_height) {
 403           p_height = q_height;
 404           p_deepest_obj = q;
 405         }
 406       }
 407     });
 408     if (p_height > 0) {
 409       /* Remember the height for next time. */
 410         if (be == 0) {
 411           ensure_struct(p);
 412           back_ptr = GET_OH_BG_PTR(p);
 413           be = (back_edges *)((word)back_ptr & ~FLAG_MANY);
 414         }
 415         be -> flags |= RETAIN;
 416         be -> height = p_height;
 417         be -> height_gc_no = GC_gc_no;
 418     }
 419     if (p_height > GC_max_height) {
 420         GC_max_height = p_height;
 421         GC_deepest_obj = p_deepest_obj;
 422     }
 423   }
 424 }
 425
 426 void GC_traverse_back_graph(void)
 427 {
 428   static word max_max_height = 0;
 429   GC_max_height = 0;
 430   GC_apply_to_each_object(update_max_height);
 431   GC_printf2("Maximum backwards height of reachable objects at GC %lu is %ld\n",
 432              (unsigned long) GC_gc_no, GC_max_height);
 433   if (GC_max_height > max_max_height) {
 434     max_max_height = GC_max_height;
 435     GC_printf0("The following unreachable object is last in a longest chain "
 436                "of unreachable objects:\n");
 437     GC_print_heap_obj(GC_deepest_obj);
 438   }
 439   if (GC_print_stats) {
 440     GC_printf1("Needed max total of %ld back-edge structs\n",
 441                GC_n_back_edge_structs);
 442   }
 443   GC_apply_to_each_object(reset_back_edge);
 444   GC_deepest_obj = 0;
 445 }
 446
 447 #endif /* MAKE_BACK_GRAPH */