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
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. */
47 typedef struct back_edges_struct {
48 word n_edges; /* Number of edges, including those in continuation */
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 */
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))
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. */
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 STATIC int GC_n_back_edge_structs = 0;
79 /* Serves as pointer to never used */
80 /* back_edges space. */
81 static back_edges *avail_back_edges = 0;
82 /* Pointer to free list of deallocated */
83 /* back_edges structures. */
85 static back_edges * new_back_edges(void)
87 if (0 == back_edge_space) {
88 back_edge_space = (back_edges *)
89 GET_MEM(MAX_BACK_EDGE_STRUCTS*sizeof(back_edges));
90 GC_add_to_our_memory((ptr_t)back_edge_space,
91 MAX_BACK_EDGE_STRUCTS*sizeof(back_edges));
93 if (0 != avail_back_edges) {
94 back_edges * result = avail_back_edges;
95 avail_back_edges = result -> cont;
99 if (GC_n_back_edge_structs >= MAX_BACK_EDGE_STRUCTS - 1) {
100 ABORT("needed too much space for back edges: adjust "
101 "MAX_BACK_EDGE_STRUCTS");
103 return back_edge_space + (GC_n_back_edge_structs++);
106 /* Deallocate p and its associated continuation structures. */
107 static void deallocate_back_edges(back_edges *p)
109 back_edges *last = p;
111 while (0 != last -> cont) last = last -> cont;
112 last -> cont = avail_back_edges;
113 avail_back_edges = p;
116 /* Table of objects that are currently on the depth-first search */
117 /* stack. Only objects with in-degree one are in this table. */
118 /* Other objects are identified using HEIGHT_IN_PROGRESS. */
119 /* FIXME: This data structure NEEDS IMPROVEMENT. */
120 #define INITIAL_IN_PROGRESS 10000
121 static ptr_t * in_progress_space = 0;
122 static size_t in_progress_size = 0;
123 static size_t n_in_progress = 0;
125 static void push_in_progress(ptr_t p)
127 if (n_in_progress >= in_progress_size) {
128 if (in_progress_size == 0) {
129 in_progress_size = INITIAL_IN_PROGRESS;
130 in_progress_space = (ptr_t *)GET_MEM(in_progress_size * sizeof(ptr_t));
131 GC_add_to_our_memory((ptr_t)in_progress_space,
132 in_progress_size * sizeof(ptr_t));
134 ptr_t * new_in_progress_space;
135 in_progress_size *= 2;
136 new_in_progress_space = (ptr_t *)
137 GET_MEM(in_progress_size * sizeof(ptr_t));
138 GC_add_to_our_memory((ptr_t)new_in_progress_space,
139 in_progress_size * sizeof(ptr_t));
140 BCOPY(in_progress_space, new_in_progress_space,
141 n_in_progress * sizeof(ptr_t));
142 in_progress_space = new_in_progress_space;
143 /* FIXME: This just drops the old space. */
146 if (in_progress_space == 0)
147 ABORT("MAKE_BACK_GRAPH: Out of in-progress space: "
148 "Huge linear data structure?");
149 in_progress_space[n_in_progress++] = p;
152 static GC_bool is_in_progress(ptr_t p)
155 for (i = 0; i < n_in_progress; ++i) {
156 if (in_progress_space[i] == p) return TRUE;
161 static void pop_in_progress(ptr_t p)
164 GC_ASSERT(in_progress_space[n_in_progress] == p);
167 #define GET_OH_BG_PTR(p) \
168 (ptr_t)REVEAL_POINTER(((oh *)(p)) -> oh_bg_ptr)
169 #define SET_OH_BG_PTR(p,q) (((oh *)(p)) -> oh_bg_ptr) = HIDE_POINTER(q)
171 /* Execute s once for each predecessor q of p in the points-to graph. */
172 /* s should be a bracketed statement. We declare q. */
173 #define FOR_EACH_PRED(q, p, s) \
175 ptr_t q = GET_OH_BG_PTR(p); \
176 if (!((word)q & FLAG_MANY)) { \
177 if (q && !((word)q & 1)) s \
178 /* !((word)q & 1) checks for a misnterpreted freelist link */ \
180 back_edges *orig_be_ = (back_edges *)((word)q & ~FLAG_MANY); \
181 back_edges *be_ = orig_be_; \
184 word n_edges_ = be_ -> n_edges; \
185 for (total_ = 0, local_ = 0; total_ < n_edges_; ++local_, ++total_) { \
186 if (local_ == MAX_IN) { \
190 q = be_ -> edges[local_]; s \
195 /* Ensure that p has a back_edges structure associated with it. */
196 static void ensure_struct(ptr_t p)
198 ptr_t old_back_ptr = GET_OH_BG_PTR(p);
200 if (!((word)old_back_ptr & FLAG_MANY)) {
201 back_edges *be = new_back_edges();
203 if (0 == old_back_ptr) {
207 be -> edges[0] = old_back_ptr;
209 be -> height = HEIGHT_UNKNOWN;
210 be -> height_gc_no = (unsigned short)(GC_gc_no - 1);
211 GC_ASSERT(be >= back_edge_space);
212 SET_OH_BG_PTR(p, (word)be | FLAG_MANY);
216 /* Add the (forward) edge from p to q to the backward graph. Both p */
217 /* q are pointers to the object base, i.e. pointers to an oh. */
218 static void add_edge(ptr_t p, ptr_t q)
220 ptr_t old_back_ptr = GET_OH_BG_PTR(q);
221 back_edges * be, *be_cont;
223 static unsigned random_number = 13;
224 # define GOT_LUCKY_NUMBER (((++random_number) & 0x7f) == 0)
225 /* A not very random number we use to occasionally allocate a */
226 /* back_edges structure even for a single backward edge. This */
227 /* prevents us from repeatedly tracing back through very long */
228 /* chains, since we will have some place to store height and */
229 /* in_progress flags along the way. */
231 GC_ASSERT(p == GC_base(p) && q == GC_base(q));
232 if (!GC_HAS_DEBUG_INFO(q) || !GC_HAS_DEBUG_INFO(p)) {
233 /* This is really a misinterpreted free list link, since we saw */
234 /* a pointer to a free list. Dont overwrite it! */
237 if (0 == old_back_ptr) {
239 if (GOT_LUCKY_NUMBER) ensure_struct(q);
242 /* Check whether it was already in the list of predecessors. */
243 FOR_EACH_PRED(pred, q, { if (p == pred) return; });
245 old_back_ptr = GET_OH_BG_PTR(q);
246 be = (back_edges *)((word)old_back_ptr & ~FLAG_MANY);
247 for (i = be -> n_edges, be_cont = be; i > MAX_IN;
248 be_cont = be_cont -> cont, i -= MAX_IN) {}
250 be_cont -> cont = new_back_edges();
251 be_cont = be_cont -> cont;
254 be_cont -> edges[i] = p;
256 if (be -> n_edges == 100) {
258 if (GC_print_stats) {
259 GC_err_printf("The following object has in-degree >= 100:\n");
260 GC_print_heap_obj(q);
266 typedef void (*per_object_func)(ptr_t p, size_t n_bytes, word gc_descr);
268 static void per_object_helper(struct hblk *h, word fn)
271 size_t sz = hhdr -> hb_sz;
272 word descr = hhdr -> hb_descr;
273 per_object_func f = (per_object_func)fn;
277 f((ptr_t)(h -> hb_body + i), sz, descr);
279 } while (i + (int)sz <= BYTES_TO_WORDS(HBLKSIZE));
282 void GC_apply_to_each_object(per_object_func f)
284 GC_apply_to_all_blocks(per_object_helper, (word)f);
288 static void reset_back_edge(ptr_t p, size_t n_bytes, word gc_descr)
290 /* Skip any free list links, or dropped blocks */
291 if (GC_HAS_DEBUG_INFO(p)) {
292 ptr_t old_back_ptr = GET_OH_BG_PTR(p);
293 if ((word)old_back_ptr & FLAG_MANY) {
294 back_edges *be = (back_edges *)((word)old_back_ptr & ~FLAG_MANY);
295 if (!(be -> flags & RETAIN)) {
296 deallocate_back_edges(be);
300 GC_ASSERT(GC_is_marked(p));
302 /* Back edges may point to objects that will not be retained. */
303 /* Delete them for now, but remember the height. */
304 /* Some will be added back at next GC. */
306 if (0 != be -> cont) {
307 deallocate_back_edges(be -> cont);
311 GC_ASSERT(GC_is_marked(p));
313 /* We only retain things for one GC cycle at a time. */
314 be -> flags &= ~RETAIN;
316 } else /* Simple back pointer */ {
317 /* Clear to avoid dangling pointer. */
323 static void add_back_edges(ptr_t p, size_t n_bytes, word gc_descr)
325 word *currentp = (word *)(p + sizeof(oh));
327 /* For now, fix up non-length descriptors conservatively. */
328 if((gc_descr & GC_DS_TAGS) != GC_DS_LENGTH) {
331 while (currentp < (word *)(p + gc_descr)) {
332 word current = *currentp++;
333 FIXUP_POINTER(current);
334 if (current >= (word)GC_least_plausible_heap_addr &&
335 current <= (word)GC_greatest_plausible_heap_addr) {
336 ptr_t target = GC_base((void *)current);
344 /* Rebuild the representation of the backward reachability graph. */
345 /* Does not examine mark bits. Can be called before GC. */
346 void GC_build_back_graph(void)
348 GC_apply_to_each_object(add_back_edges);
351 /* Return an approximation to the length of the longest simple path */
352 /* through unreachable objects to p. We refer to this as the height */
354 static word backwards_height(ptr_t p)
357 ptr_t back_ptr = GET_OH_BG_PTR(p);
360 if (0 == back_ptr) return 1;
361 if (!((word)back_ptr & FLAG_MANY)) {
362 if (is_in_progress(p)) return 0; /* DFS back edge, i.e. we followed */
363 /* an edge to an object already */
364 /* on our stack: ignore */
366 result = backwards_height(back_ptr)+1;
370 be = (back_edges *)((word)back_ptr & ~FLAG_MANY);
371 if (be -> height >= 0 && be -> height_gc_no == (unsigned short)GC_gc_no)
373 /* Ignore back edges in DFS */
374 if (be -> height == HEIGHT_IN_PROGRESS) return 0;
375 result = (be -> height > 0? be -> height : 1);
376 be -> height = HEIGHT_IN_PROGRESS;
377 FOR_EACH_PRED(q, p, {
379 if (GC_is_marked(q) && !(FLAG_MANY & (word)GET_OH_BG_PTR(p))) {
381 GC_log_printf("Found bogus pointer from %p to %p\n", q, p);
382 /* Reachable object "points to" unreachable one. */
383 /* Could be caused by our lax treatment of GC descriptors. */
386 this_height = backwards_height(q);
388 if (this_height >= result) result = this_height + 1;
390 be -> height = result;
391 be -> height_gc_no = (unsigned short)GC_gc_no;
395 STATIC word GC_max_height;
396 STATIC ptr_t GC_deepest_obj;
398 /* Compute the maximum height of every unreachable predecessor p of a */
399 /* reachable object. Arrange to save the heights of all such objects p */
400 /* so that they can be used in calculating the height of objects in the */
402 /* Set GC_max_height to be the maximum height we encounter, and */
403 /* GC_deepest_obj to be the corresponding object. */
405 static void update_max_height(ptr_t p, size_t n_bytes, word gc_descr)
407 if (GC_is_marked(p) && GC_HAS_DEBUG_INFO(p)) {
409 ptr_t p_deepest_obj = 0;
413 /* If we remembered a height last time, use it as a minimum. */
414 /* It may have increased due to newly unreachable chains pointing */
415 /* to p, but it can't have decreased. */
416 back_ptr = GET_OH_BG_PTR(p);
417 if (0 != back_ptr && ((word)back_ptr & FLAG_MANY)) {
418 be = (back_edges *)((word)back_ptr & ~FLAG_MANY);
419 if (be -> height != HEIGHT_UNKNOWN) p_height = be -> height;
421 FOR_EACH_PRED(q, p, {
422 if (!GC_is_marked(q) && GC_HAS_DEBUG_INFO(q)) {
425 q_height = backwards_height(q);
426 if (q_height > p_height) {
433 /* Remember the height for next time. */
436 back_ptr = GET_OH_BG_PTR(p);
437 be = (back_edges *)((word)back_ptr & ~FLAG_MANY);
439 be -> flags |= RETAIN;
440 be -> height = p_height;
441 be -> height_gc_no = (unsigned short)GC_gc_no;
443 if (p_height > GC_max_height) {
444 GC_max_height = p_height;
445 GC_deepest_obj = p_deepest_obj;
450 STATIC word GC_max_max_height = 0;
452 void GC_traverse_back_graph(void)
455 GC_apply_to_each_object(update_max_height);
456 if (0 != GC_deepest_obj)
457 GC_set_mark_bit(GC_deepest_obj); /* Keep it until we can print it. */
460 void GC_print_back_graph_stats(void)
462 GC_printf("Maximum backwards height of reachable objects at GC %lu is %ld\n",
463 (unsigned long) GC_gc_no, (unsigned long)GC_max_height);
464 if (GC_max_height > GC_max_max_height) {
465 GC_max_max_height = GC_max_height;
466 GC_printf("The following unreachable object is last in a longest chain "
467 "of unreachable objects:\n");
468 GC_print_heap_obj(GC_deepest_obj);
470 if (GC_print_stats) {
471 GC_log_printf("Needed max total of %d back-edge structs\n",
472 GC_n_back_edge_structs);
474 GC_apply_to_each_object(reset_back_edge);
478 #else /* !MAKE_BACK_GRAPH */
481 /* ANSI C doesn't allow translation units to be empty. */
483 #endif /* !MAKE_BACK_GRAPH */