2 * Copyright (c) 2001 by Hewlett-Packard Company. All rights reserved.
4 * THIS MATERIAL IS PROVIDED AS IS, WITH ABSOLUTELY NO WARRANTY EXPRESSED
5 * OR IMPLIED. ANY USE IS AT YOUR OWN RISK.
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.
15 #include "private/dbg_mlc.h"
18 * This implements a full, though not well-tuned, representation of the
19 * backwards points-to graph. This is used to test for non-GC-robust
20 * data structures; the code is not used during normal garbage collection.
22 * One restriction is that we drop all back-edges from nodes with very
23 * high in-degree, and simply add them add them to a list of such
24 * nodes. They are then treated as permanent roots. Id this by itself
25 * doesn't introduce a space leak, then such nodes can't contribute to
26 * a growing space leak.
29 #ifdef MAKE_BACK_GRAPH
31 #define MAX_IN 10 /* Maximum in-degree we handle directly */
33 /* #include <unistd.h> */
35 #if !defined(DBG_HDRS_ALL) || (ALIGNMENT != CPP_WORDSZ/8) /* || !defined(UNIX_LIKE) */
36 # error The configuration does not support MAKE_BACK_GRAPH
39 /* We store single back pointers directly in the object's oh_bg_ptr field. */
40 /* If there is more than one ptr to an object, we store q | FLAG_MANY, */
41 /* where q is a pointer to a back_edges object. */
42 /* Every once in a while we use a back_edges object even for a single */
43 /* pointer, since we need the other fields in the back_edges structure to */
44 /* be present in some fraction of the objects. Otherwise we get serious */
45 /* performance issues. */
48 typedef struct back_edges_struct {
49 word n_edges; /* Number of edges, including those in continuation */
52 # define RETAIN 1 /* Directly points to a reachable object; */
53 /* retain for next GC. */
54 unsigned short height_gc_no;
55 /* If height > 0, then the GC_gc_no value when it */
56 /* was computed. If it was computed this cycle, then */
57 /* it is current. If it was computed during the */
58 /* last cycle, then it represents the old height, */
59 /* which is only saved for live objects referenced by */
60 /* dead ones. This may grow due to refs from newly */
63 /* Longest path through unreachable nodes to this node */
64 /* that we found using depth first search. */
66 # define HEIGHT_UNKNOWN ((signed_word)(-2))
67 # define HEIGHT_IN_PROGRESS ((signed_word)(-1))
69 struct back_edges_struct *cont;
70 /* Pointer to continuation structure; we use only the */
71 /* edges field in the continuation. */
72 /* also used as free list link. */
75 /* Allocate a new back edge structure. Should be more sophisticated */
76 /* if this were production code. */
77 #define MAX_BACK_EDGE_STRUCTS 100000
78 static back_edges *back_edge_space = 0;
79 STATIC int GC_n_back_edge_structs = 0;
80 /* Serves as pointer to never used */
81 /* back_edges space. */
82 static back_edges *avail_back_edges = 0;
83 /* Pointer to free list of deallocated */
84 /* back_edges structures. */
86 static back_edges * new_back_edges(void)
88 if (0 == back_edge_space) {
89 back_edge_space = (back_edges *)
90 GET_MEM(MAX_BACK_EDGE_STRUCTS*sizeof(back_edges));
91 GC_add_to_our_memory((ptr_t)back_edge_space,
92 MAX_BACK_EDGE_STRUCTS*sizeof(back_edges));
94 if (0 != avail_back_edges) {
95 back_edges * result = avail_back_edges;
96 avail_back_edges = result -> cont;
100 if (GC_n_back_edge_structs >= MAX_BACK_EDGE_STRUCTS - 1) {
101 ABORT("needed too much space for back edges: adjust "
102 "MAX_BACK_EDGE_STRUCTS");
104 return back_edge_space + (GC_n_back_edge_structs++);
107 /* Deallocate p and its associated continuation structures. */
108 static void deallocate_back_edges(back_edges *p)
110 back_edges *last = p;
112 while (0 != last -> cont) last = last -> cont;
113 last -> cont = avail_back_edges;
114 avail_back_edges = p;
117 /* Table of objects that are currently on the depth-first search */
118 /* stack. Only objects with in-degree one are in this table. */
119 /* Other objects are identified using HEIGHT_IN_PROGRESS. */
120 /* FIXME: This data structure NEEDS IMPROVEMENT. */
121 #define INITIAL_IN_PROGRESS 10000
122 static ptr_t * in_progress_space = 0;
123 static size_t in_progress_size = 0;
124 static size_t n_in_progress = 0;
126 static void push_in_progress(ptr_t p)
128 if (n_in_progress >= in_progress_size) {
129 if (in_progress_size == 0) {
130 in_progress_size = INITIAL_IN_PROGRESS;
131 in_progress_space = (ptr_t *)GET_MEM(in_progress_size * sizeof(ptr_t));
132 GC_add_to_our_memory((ptr_t)in_progress_space,
133 in_progress_size * sizeof(ptr_t));
135 ptr_t * new_in_progress_space;
136 in_progress_size *= 2;
137 new_in_progress_space = (ptr_t *)
138 GET_MEM(in_progress_size * sizeof(ptr_t));
139 GC_add_to_our_memory((ptr_t)new_in_progress_space,
140 in_progress_size * sizeof(ptr_t));
141 BCOPY(in_progress_space, new_in_progress_space,
142 n_in_progress * sizeof(ptr_t));
143 in_progress_space = new_in_progress_space;
144 /* FIXME: This just drops the old space. */
147 if (in_progress_space == 0)
148 ABORT("MAKE_BACK_GRAPH: Out of in-progress space: "
149 "Huge linear data structure?");
150 in_progress_space[n_in_progress++] = p;
153 static GC_bool is_in_progress(ptr_t p)
156 for (i = 0; i < n_in_progress; ++i) {
157 if (in_progress_space[i] == p) return TRUE;
162 GC_INLINE void pop_in_progress(ptr_t p)
165 GC_ASSERT(in_progress_space[n_in_progress] == p);
168 #define GET_OH_BG_PTR(p) \
169 (ptr_t)GC_REVEAL_POINTER(((oh *)(p)) -> oh_bg_ptr)
170 #define SET_OH_BG_PTR(p,q) (((oh *)(p)) -> oh_bg_ptr = GC_HIDE_POINTER(q))
172 /* Execute s once for each predecessor q of p in the points-to graph. */
173 /* s should be a bracketed statement. We declare q. */
174 #define FOR_EACH_PRED(q, p, s) \
176 ptr_t q = GET_OH_BG_PTR(p); \
177 if (!((word)q & FLAG_MANY)) { \
178 if (q && !((word)q & 1)) s \
179 /* !((word)q & 1) checks for a misnterpreted freelist link */ \
181 back_edges *orig_be_ = (back_edges *)((word)q & ~FLAG_MANY); \
182 back_edges *be_ = orig_be_; \
185 word n_edges_ = be_ -> n_edges; \
186 for (total_ = 0, local_ = 0; total_ < n_edges_; ++local_, ++total_) { \
187 if (local_ == MAX_IN) { \
191 q = be_ -> edges[local_]; s \
196 /* Ensure that p has a back_edges structure associated with it. */
197 static void ensure_struct(ptr_t p)
199 ptr_t old_back_ptr = GET_OH_BG_PTR(p);
201 if (!((word)old_back_ptr & FLAG_MANY)) {
202 back_edges *be = new_back_edges();
204 if (0 == old_back_ptr) {
208 be -> edges[0] = old_back_ptr;
210 be -> height = HEIGHT_UNKNOWN;
211 be -> height_gc_no = (unsigned short)(GC_gc_no - 1);
212 GC_ASSERT(be >= back_edge_space);
213 SET_OH_BG_PTR(p, (word)be | FLAG_MANY);
217 /* Add the (forward) edge from p to q to the backward graph. Both p */
218 /* q are pointers to the object base, i.e. pointers to an oh. */
219 static void add_edge(ptr_t p, ptr_t q)
221 ptr_t old_back_ptr = GET_OH_BG_PTR(q);
222 back_edges * be, *be_cont;
224 static unsigned random_number = 13;
225 # define GOT_LUCKY_NUMBER (((++random_number) & 0x7f) == 0)
226 /* A not very random number we use to occasionally allocate a */
227 /* back_edges structure even for a single backward edge. This */
228 /* prevents us from repeatedly tracing back through very long */
229 /* chains, since we will have some place to store height and */
230 /* in_progress flags along the way. */
232 GC_ASSERT(p == GC_base(p) && q == GC_base(q));
233 if (!GC_HAS_DEBUG_INFO(q) || !GC_HAS_DEBUG_INFO(p)) {
234 /* This is really a misinterpreted free list link, since we saw */
235 /* a pointer to a free list. Don't overwrite it! */
238 if (0 == old_back_ptr) {
240 if (GOT_LUCKY_NUMBER) ensure_struct(q);
243 /* Check whether it was already in the list of predecessors. */
244 FOR_EACH_PRED(pred, q, { if (p == pred) return; });
246 old_back_ptr = GET_OH_BG_PTR(q);
247 be = (back_edges *)((word)old_back_ptr & ~FLAG_MANY);
248 for (i = be -> n_edges, be_cont = be; i > MAX_IN; i -= MAX_IN)
249 be_cont = be_cont -> cont;
251 be_cont -> cont = new_back_edges();
252 be_cont = be_cont -> cont;
255 be_cont -> edges[i] = p;
257 if (be -> n_edges == 100) {
259 if (GC_print_stats) {
260 GC_err_printf("The following object has in-degree >= 100:\n");
261 GC_print_heap_obj(q);
267 typedef void (*per_object_func)(ptr_t p, size_t n_bytes, word gc_descr);
269 static void per_object_helper(struct hblk *h, word fn)
272 size_t sz = hhdr -> hb_sz;
273 word descr = hhdr -> hb_descr;
274 per_object_func f = (per_object_func)fn;
278 f((ptr_t)(h -> hb_body + i), sz, descr);
280 } while (i + (int)sz <= BYTES_TO_WORDS(HBLKSIZE));
283 GC_INLINE void GC_apply_to_each_object(per_object_func f)
285 GC_apply_to_all_blocks(per_object_helper, (word)f);
289 static void reset_back_edge(ptr_t p, size_t n_bytes, word gc_descr)
291 /* Skip any free list links, or dropped blocks */
292 if (GC_HAS_DEBUG_INFO(p)) {
293 ptr_t old_back_ptr = GET_OH_BG_PTR(p);
294 if ((word)old_back_ptr & FLAG_MANY) {
295 back_edges *be = (back_edges *)((word)old_back_ptr & ~FLAG_MANY);
296 if (!(be -> flags & RETAIN)) {
297 deallocate_back_edges(be);
301 GC_ASSERT(GC_is_marked(p));
303 /* Back edges may point to objects that will not be retained. */
304 /* Delete them for now, but remember the height. */
305 /* Some will be added back at next GC. */
307 if (0 != be -> cont) {
308 deallocate_back_edges(be -> cont);
312 GC_ASSERT(GC_is_marked(p));
314 /* We only retain things for one GC cycle at a time. */
315 be -> flags &= ~RETAIN;
317 } else /* Simple back pointer */ {
318 /* Clear to avoid dangling pointer. */
324 static void add_back_edges(ptr_t p, size_t n_bytes, word gc_descr)
326 word *currentp = (word *)(p + sizeof(oh));
328 /* For now, fix up non-length descriptors conservatively. */
329 if((gc_descr & GC_DS_TAGS) != GC_DS_LENGTH) {
332 while (currentp < (word *)(p + gc_descr)) {
333 word current = *currentp++;
334 FIXUP_POINTER(current);
335 if (current >= (word)GC_least_plausible_heap_addr &&
336 current <= (word)GC_greatest_plausible_heap_addr) {
337 ptr_t target = GC_base((void *)current);
345 /* Rebuild the representation of the backward reachability graph. */
346 /* Does not examine mark bits. Can be called before GC. */
347 GC_INNER void GC_build_back_graph(void)
349 GC_apply_to_each_object(add_back_edges);
352 /* Return an approximation to the length of the longest simple path */
353 /* through unreachable objects to p. We refer to this as the height */
355 static word backwards_height(ptr_t p)
358 ptr_t back_ptr = GET_OH_BG_PTR(p);
361 if (0 == back_ptr) return 1;
362 if (!((word)back_ptr & FLAG_MANY)) {
363 if (is_in_progress(p)) return 0; /* DFS back edge, i.e. we followed */
364 /* an edge to an object already */
365 /* on our stack: ignore */
367 result = backwards_height(back_ptr)+1;
371 be = (back_edges *)((word)back_ptr & ~FLAG_MANY);
372 if (be -> height >= 0 && be -> height_gc_no == (unsigned short)GC_gc_no)
374 /* Ignore back edges in DFS */
375 if (be -> height == HEIGHT_IN_PROGRESS) return 0;
376 result = (be -> height > 0? be -> height : 1);
377 be -> height = HEIGHT_IN_PROGRESS;
378 FOR_EACH_PRED(q, p, {
380 if (GC_is_marked(q) && !(FLAG_MANY & (word)GET_OH_BG_PTR(p))) {
382 GC_log_printf("Found bogus pointer from %p to %p\n", q, p);
383 /* Reachable object "points to" unreachable one. */
384 /* Could be caused by our lax treatment of GC descriptors. */
387 this_height = backwards_height(q);
389 if (this_height >= result) result = this_height + 1;
391 be -> height = result;
392 be -> height_gc_no = (unsigned short)GC_gc_no;
396 STATIC word GC_max_height = 0;
397 STATIC ptr_t GC_deepest_obj = NULL;
399 /* Compute the maximum height of every unreachable predecessor p of a */
400 /* reachable object. Arrange to save the heights of all such objects p */
401 /* so that they can be used in calculating the height of objects in the */
403 /* Set GC_max_height to be the maximum height we encounter, and */
404 /* GC_deepest_obj to be the corresponding object. */
406 static void update_max_height(ptr_t p, size_t n_bytes, word gc_descr)
408 if (GC_is_marked(p) && GC_HAS_DEBUG_INFO(p)) {
410 ptr_t p_deepest_obj = 0;
414 /* If we remembered a height last time, use it as a minimum. */
415 /* It may have increased due to newly unreachable chains pointing */
416 /* to p, but it can't have decreased. */
417 back_ptr = GET_OH_BG_PTR(p);
418 if (0 != back_ptr && ((word)back_ptr & FLAG_MANY)) {
419 be = (back_edges *)((word)back_ptr & ~FLAG_MANY);
420 if (be -> height != HEIGHT_UNKNOWN) p_height = be -> height;
422 FOR_EACH_PRED(q, p, {
423 if (!GC_is_marked(q) && GC_HAS_DEBUG_INFO(q)) {
426 q_height = backwards_height(q);
427 if (q_height > p_height) {
434 /* Remember the height for next time. */
437 back_ptr = GET_OH_BG_PTR(p);
438 be = (back_edges *)((word)back_ptr & ~FLAG_MANY);
440 be -> flags |= RETAIN;
441 be -> height = p_height;
442 be -> height_gc_no = (unsigned short)GC_gc_no;
444 if (p_height > GC_max_height) {
445 GC_max_height = p_height;
446 GC_deepest_obj = p_deepest_obj;
451 STATIC word GC_max_max_height = 0;
453 GC_INNER void GC_traverse_back_graph(void)
456 GC_apply_to_each_object(update_max_height);
457 if (0 != GC_deepest_obj)
458 GC_set_mark_bit(GC_deepest_obj); /* Keep it until we can print it. */
461 void GC_print_back_graph_stats(void)
463 GC_printf("Maximum backwards height of reachable objects at GC %lu is %ld\n",
464 (unsigned long) GC_gc_no, (unsigned long)GC_max_height);
465 if (GC_max_height > GC_max_max_height) {
466 GC_max_max_height = GC_max_height;
467 GC_printf("The following unreachable object is last in a longest chain "
468 "of unreachable objects:\n");
469 GC_print_heap_obj(GC_deepest_obj);
471 if (GC_print_stats) {
472 GC_log_printf("Needed max total of %d back-edge structs\n",
473 GC_n_back_edge_structs);
475 GC_apply_to_each_object(reset_back_edge);
479 #endif /* MAKE_BACK_GRAPH */