* ChangeLog: Follow spelling conventions.
[official-gcc.git] / boehm-gc / backgraph.c
blob01ab738f2f2316d57d01f86e94a956932aa3b9d8
1 /*
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.
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.
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.
27 #ifdef MAKE_BACK_GRAPH
29 #define MAX_IN 10 /* Maximum in-degree we handle directly */
31 #include "private/dbg_mlc.h"
32 #include <unistd.h>
34 #if !defined(DBG_HDRS_ALL) || (ALIGNMENT != CPP_WORDSZ/8) || !defined(UNIX_LIKE)
35 # error Configuration doesnt support MAKE_BACK_GRAPH
36 #endif
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
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. */
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;
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. */
84 static back_edges * new_back_edges(void)
86 if (0 == back_edge_space) {
87 back_edge_space = (back_edges *)
88 sbrk(MAX_BACK_EDGE_STRUCTS*sizeof(back_edges));
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;
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");
100 return back_edge_space + (GC_n_back_edge_structs++);
103 /* Deallocate p and its associated continuation structures. */
104 static void deallocate_back_edges(back_edges *p)
106 back_edges *last = p;
108 while (0 != last -> cont) last = last -> cont;
109 last -> cont = avail_back_edges;
110 avail_back_edges = p;
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;
121 static void push_in_progress(ptr_t p)
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;
130 static GC_bool is_in_progress(ptr_t p)
132 int i;
133 for (i = 0; i < n_in_progress; ++i) {
134 if (in_progress_space[i] == p) return TRUE;
136 return FALSE;
139 static void pop_in_progress(ptr_t p)
141 --n_in_progress;
142 GC_ASSERT(in_progress_space[n_in_progress] == p);
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)
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) \
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; \
167 q = be_ -> edges[local_]; s \
172 /* Ensure that p has a back_edges structure associated with it. */
173 static void ensure_struct(ptr_t p)
175 ptr_t old_back_ptr = GET_OH_BG_PTR(p);
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;
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);
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)
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. */
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;
214 if (0 == old_back_ptr) {
215 SET_OH_BG_PTR(q, p);
216 if (GOT_LUCKY_NUMBER) ensure_struct(q);
217 return;
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;
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);
239 # endif
243 typedef void (*per_object_func)(ptr_t p, word n_words, word gc_descr);
245 static void per_object_helper(struct hblk *h, word fn)
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;
253 do {
254 f((ptr_t)(h -> hb_body + i), sz, descr);
255 i += sz;
256 } while (i + sz <= BYTES_TO_WORDS(HBLKSIZE));
259 void GC_apply_to_each_object(per_object_func f)
261 GC_apply_to_all_blocks(per_object_helper, (word)f);
264 static void reset_back_edge(ptr_t p, word n_words, word gc_descr)
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;
277 GC_ASSERT(GC_is_marked(p));
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;
288 GC_ASSERT(GC_is_marked(p));
290 /* We only retain things for one GC cycle at a time. */
291 be -> flags &= ~RETAIN;
293 } else /* Simple back pointer */ {
294 /* Clear to avoid dangling pointer. */
295 SET_OH_BG_PTR(p, 0);
300 static void add_back_edges(ptr_t p, word n_words, word gc_descr)
302 word *currentp = (word *)(p + sizeof(oh));
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);
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);
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)
324 GC_apply_to_each_object(add_back_edges);
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)
332 word result;
333 ptr_t back_ptr = GET_OH_BG_PTR(p);
334 back_edges *be;
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;
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);
364 if (this_height >= result) result = this_height + 1;
366 be -> height = result;
367 be -> height_gc_no = GC_gc_no;
368 return result;
371 word GC_max_height;
372 ptr_t GC_deepest_obj;
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)
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;
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;
397 FOR_EACH_PRED(q, p, {
398 if (!GC_is_marked(q) && GC_HAS_DEBUG_INFO(q)) {
399 word q_height;
401 q_height = backwards_height(q);
402 if (q_height > p_height) {
403 p_height = q_height;
404 p_deepest_obj = q;
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);
415 be -> flags |= RETAIN;
416 be -> height = p_height;
417 be -> height_gc_no = GC_gc_no;
419 if (p_height > GC_max_height) {
420 GC_max_height = p_height;
421 GC_deepest_obj = p_deepest_obj;
426 void GC_traverse_back_graph(void)
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);
439 if (GC_print_stats) {
440 GC_printf1("Needed max total of %ld back-edge structs\n",
441 GC_n_back_edge_structs);
443 GC_apply_to_each_object(reset_back_edge);
444 GC_deepest_obj = 0;
447 #endif /* MAKE_BACK_GRAPH */