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. If 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) \
36 /* || !defined(UNIX_LIKE) */) && !defined(CPPCHECK)
37 # error The configuration does not support MAKE_BACK_GRAPH
40 /* We store single back pointers directly in the object's oh_bg_ptr field. */
41 /* If there is more than one ptr to an object, we store q | FLAG_MANY, */
42 /* where q is a pointer to a back_edges object. */
43 /* Every once in a while we use a back_edges object even for a single */
44 /* pointer, since we need the other fields in the back_edges structure to */
45 /* be present in some fraction of the objects. Otherwise we get serious */
46 /* performance issues. */
49 typedef struct back_edges_struct {
50 word n_edges; /* Number of edges, including those in continuation */
53 # define RETAIN 1 /* Directly points to a reachable object; */
54 /* retain for next GC. */
55 unsigned short height_gc_no;
56 /* If height > 0, then the GC_gc_no value when it */
57 /* was computed. If it was computed this cycle, then */
58 /* it is current. If it was computed during the */
59 /* last cycle, then it represents the old height, */
60 /* which is only saved for live objects referenced by */
61 /* dead ones. This may grow due to refs from newly */
64 /* Longest path through unreachable nodes to this node */
65 /* that we found using depth first search. */
66 # define HEIGHT_UNKNOWN (-2)
67 # define HEIGHT_IN_PROGRESS (-1)
70 struct back_edges_struct *cont;
71 /* Pointer to continuation structure; we use only the */
72 /* edges field in the continuation. */
73 /* also used as free list link. */
76 /* Allocate a new back edge structure. Should be more sophisticated */
77 /* if this were production code. */
78 #define MAX_BACK_EDGE_STRUCTS 100000
79 static back_edges *back_edge_space = 0;
80 STATIC int GC_n_back_edge_structs = 0;
81 /* Serves as pointer to never used */
82 /* back_edges space. */
83 static back_edges *avail_back_edges = 0;
84 /* Pointer to free list of deallocated */
85 /* back_edges structures. */
87 static back_edges * new_back_edges(void)
89 if (0 == back_edge_space) {
90 size_t bytes_to_get = ROUNDUP_PAGESIZE_IF_MMAP(MAX_BACK_EDGE_STRUCTS
91 * sizeof(back_edges));
93 back_edge_space = (back_edges *)GET_MEM(bytes_to_get);
94 if (NULL == back_edge_space)
95 ABORT("Insufficient memory for back edges");
96 GC_add_to_our_memory((ptr_t)back_edge_space, bytes_to_get);
98 if (0 != avail_back_edges) {
99 back_edges * result = avail_back_edges;
100 avail_back_edges = result -> cont;
104 if (GC_n_back_edge_structs >= MAX_BACK_EDGE_STRUCTS - 1) {
105 ABORT("Needed too much space for back edges: adjust "
106 "MAX_BACK_EDGE_STRUCTS");
108 return back_edge_space + (GC_n_back_edge_structs++);
111 /* Deallocate p and its associated continuation structures. */
112 static void deallocate_back_edges(back_edges *p)
114 back_edges *last = p;
116 while (0 != last -> cont) last = last -> cont;
117 last -> cont = avail_back_edges;
118 avail_back_edges = p;
121 /* Table of objects that are currently on the depth-first search */
122 /* stack. Only objects with in-degree one are in this table. */
123 /* Other objects are identified using HEIGHT_IN_PROGRESS. */
124 /* FIXME: This data structure NEEDS IMPROVEMENT. */
125 #define INITIAL_IN_PROGRESS 10000
126 static ptr_t * in_progress_space = 0;
127 static size_t in_progress_size = 0;
128 static size_t n_in_progress = 0;
130 static void push_in_progress(ptr_t p)
132 if (n_in_progress >= in_progress_size) {
133 ptr_t * new_in_progress_space;
135 if (NULL == in_progress_space) {
136 in_progress_size = ROUNDUP_PAGESIZE_IF_MMAP(INITIAL_IN_PROGRESS
139 new_in_progress_space =
140 (ptr_t *)GET_MEM(in_progress_size * sizeof(ptr_t));
142 in_progress_size *= 2;
143 new_in_progress_space = (ptr_t *)
144 GET_MEM(in_progress_size * sizeof(ptr_t));
145 if (new_in_progress_space != NULL)
146 BCOPY(in_progress_space, new_in_progress_space,
147 n_in_progress * sizeof(ptr_t));
149 GC_add_to_our_memory((ptr_t)new_in_progress_space,
150 in_progress_size * sizeof(ptr_t));
152 GC_scratch_recycle_no_gww(in_progress_space,
153 n_in_progress * sizeof(ptr_t));
154 # elif defined(LINT2)
155 /* TODO: implement GWW-aware recycling as in alloc_mark_stack */
156 GC_noop1((word)in_progress_space);
158 in_progress_space = new_in_progress_space;
160 if (in_progress_space == 0)
161 ABORT("MAKE_BACK_GRAPH: Out of in-progress space: "
162 "Huge linear data structure?");
163 in_progress_space[n_in_progress++] = p;
166 static GC_bool is_in_progress(ptr_t p)
169 for (i = 0; i < n_in_progress; ++i) {
170 if (in_progress_space[i] == p) return TRUE;
175 GC_INLINE void pop_in_progress(ptr_t p GC_ATTR_UNUSED)
178 GC_ASSERT(in_progress_space[n_in_progress] == p);
181 #define GET_OH_BG_PTR(p) \
182 (ptr_t)GC_REVEAL_POINTER(((oh *)(p)) -> oh_bg_ptr)
183 #define SET_OH_BG_PTR(p,q) (((oh *)(p)) -> oh_bg_ptr = GC_HIDE_POINTER(q))
185 /* Ensure that p has a back_edges structure associated with it. */
186 static void ensure_struct(ptr_t p)
188 ptr_t old_back_ptr = GET_OH_BG_PTR(p);
190 if (!((word)old_back_ptr & FLAG_MANY)) {
191 back_edges *be = new_back_edges();
193 if (0 == old_back_ptr) {
197 be -> edges[0] = old_back_ptr;
199 be -> height = HEIGHT_UNKNOWN;
200 be -> height_gc_no = (unsigned short)(GC_gc_no - 1);
201 GC_ASSERT((word)be >= (word)back_edge_space);
202 SET_OH_BG_PTR(p, (word)be | FLAG_MANY);
206 /* Add the (forward) edge from p to q to the backward graph. Both p */
207 /* q are pointers to the object base, i.e. pointers to an oh. */
208 static void add_edge(ptr_t p, ptr_t q)
210 ptr_t pred = GET_OH_BG_PTR(q);
211 back_edges * be, *be_cont;
214 GC_ASSERT(p == GC_base(p) && q == GC_base(q));
215 if (!GC_HAS_DEBUG_INFO(q) || !GC_HAS_DEBUG_INFO(p)) {
216 /* This is really a misinterpreted free list link, since we saw */
217 /* a pointer to a free list. Don't overwrite it! */
221 static unsigned random_number = 13;
222 # define GOT_LUCKY_NUMBER (((++random_number) & 0x7f) == 0)
223 /* A not very random number we use to occasionally allocate a */
224 /* back_edges structure even for a single backward edge. This */
225 /* prevents us from repeatedly tracing back through very long */
226 /* chains, since we will have some place to store height and */
227 /* in_progress flags along the way. */
230 if (GOT_LUCKY_NUMBER) ensure_struct(q);
234 /* Check whether it was already in the list of predecessors. */
236 back_edges *e = (back_edges *)((word)pred & ~FLAG_MANY);
241 if (((word)pred & FLAG_MANY) != 0) {
242 n_edges = e -> n_edges;
243 } else if (((word)COVERT_DATAFLOW(pred) & 1) == 0) {
244 /* A misinterpreted freelist link. */
250 for (total = 0; total < n_edges; ++total) {
251 if (local == MAX_IN) {
256 pred = e -> edges[local++];
263 be = (back_edges *)((word)GET_OH_BG_PTR(q) & ~FLAG_MANY);
264 for (i = be -> n_edges, be_cont = be; i > MAX_IN; i -= MAX_IN)
265 be_cont = be_cont -> cont;
267 be_cont -> cont = new_back_edges();
268 be_cont = be_cont -> cont;
271 be_cont -> edges[i] = p;
273 # ifdef DEBUG_PRINT_BIG_N_EDGES
274 if (GC_print_stats == VERBOSE && be -> n_edges == 100) {
275 GC_err_printf("The following object has big in-degree:\n");
276 GC_print_heap_obj(q);
281 typedef void (*per_object_func)(ptr_t p, size_t n_bytes, word gc_descr);
283 static void per_object_helper(struct hblk *h, word fn)
286 size_t sz = (size_t)hhdr->hb_sz;
287 word descr = hhdr -> hb_descr;
288 per_object_func f = (per_object_func)fn;
292 f((ptr_t)(h -> hb_body + i), sz, descr);
294 } while (i + sz <= BYTES_TO_WORDS(HBLKSIZE));
297 GC_INLINE void GC_apply_to_each_object(per_object_func f)
299 GC_apply_to_all_blocks(per_object_helper, (word)f);
302 static void reset_back_edge(ptr_t p, size_t n_bytes GC_ATTR_UNUSED,
303 word gc_descr GC_ATTR_UNUSED)
305 /* Skip any free list links, or dropped blocks */
306 if (GC_HAS_DEBUG_INFO(p)) {
307 ptr_t old_back_ptr = GET_OH_BG_PTR(p);
308 if ((word)old_back_ptr & FLAG_MANY) {
309 back_edges *be = (back_edges *)((word)old_back_ptr & ~FLAG_MANY);
310 if (!(be -> flags & RETAIN)) {
311 deallocate_back_edges(be);
315 GC_ASSERT(GC_is_marked(p));
317 /* Back edges may point to objects that will not be retained. */
318 /* Delete them for now, but remember the height. */
319 /* Some will be added back at next GC. */
321 if (0 != be -> cont) {
322 deallocate_back_edges(be -> cont);
326 GC_ASSERT(GC_is_marked(p));
328 /* We only retain things for one GC cycle at a time. */
329 be -> flags &= ~RETAIN;
331 } else /* Simple back pointer */ {
332 /* Clear to avoid dangling pointer. */
338 static void add_back_edges(ptr_t p, size_t n_bytes, word gc_descr)
340 word *currentp = (word *)(p + sizeof(oh));
342 /* For now, fix up non-length descriptors conservatively. */
343 if((gc_descr & GC_DS_TAGS) != GC_DS_LENGTH) {
346 while ((word)currentp < (word)(p + gc_descr)) {
347 word current = *currentp++;
348 FIXUP_POINTER(current);
349 if (current >= (word)GC_least_plausible_heap_addr &&
350 current <= (word)GC_greatest_plausible_heap_addr) {
351 ptr_t target = (ptr_t)GC_base((void *)current);
359 /* Rebuild the representation of the backward reachability graph. */
360 /* Does not examine mark bits. Can be called before GC. */
361 GC_INNER void GC_build_back_graph(void)
363 GC_ASSERT(I_HOLD_LOCK());
364 GC_apply_to_each_object(add_back_edges);
367 /* Return an approximation to the length of the longest simple path */
368 /* through unreachable objects to p. We refer to this as the height */
370 static word backwards_height(ptr_t p)
373 ptr_t pred = GET_OH_BG_PTR(p);
378 if (((word)pred & FLAG_MANY) == 0) {
379 if (is_in_progress(p)) return 0; /* DFS back edge, i.e. we followed */
380 /* an edge to an object already */
381 /* on our stack: ignore */
383 result = backwards_height(pred) + 1;
387 be = (back_edges *)((word)pred & ~FLAG_MANY);
388 if (be -> height >= 0 && be -> height_gc_no == (unsigned short)GC_gc_no)
390 /* Ignore back edges in DFS */
391 if (be -> height == HEIGHT_IN_PROGRESS) return 0;
392 result = (be -> height > 0? be -> height : 1);
393 be -> height = HEIGHT_IN_PROGRESS;
401 if (((word)pred & FLAG_MANY) != 0) {
402 n_edges = e -> n_edges;
403 } else if (((word)pred & 1) == 0) {
404 /* A misinterpreted freelist link. */
410 for (total = 0; total < n_edges; ++total) {
412 if (local == MAX_IN) {
417 pred = e -> edges[local++];
419 /* Execute the following once for each predecessor pred of p */
420 /* in the points-to graph. */
421 if (GC_is_marked(pred) && ((word)GET_OH_BG_PTR(p) & FLAG_MANY) == 0) {
422 GC_COND_LOG_PRINTF("Found bogus pointer from %p to %p\n",
423 (void *)pred, (void *)p);
424 /* Reachable object "points to" unreachable one. */
425 /* Could be caused by our lax treatment of GC descriptors. */
428 this_height = backwards_height(pred);
430 if (this_height >= result)
431 result = this_height + 1;
435 be -> height = result;
436 be -> height_gc_no = (unsigned short)GC_gc_no;
440 STATIC word GC_max_height = 0;
441 STATIC ptr_t GC_deepest_obj = NULL;
443 /* Compute the maximum height of every unreachable predecessor p of a */
444 /* reachable object. Arrange to save the heights of all such objects p */
445 /* so that they can be used in calculating the height of objects in the */
447 /* Set GC_max_height to be the maximum height we encounter, and */
448 /* GC_deepest_obj to be the corresponding object. */
449 static void update_max_height(ptr_t p, size_t n_bytes GC_ATTR_UNUSED,
450 word gc_descr GC_ATTR_UNUSED)
452 if (GC_is_marked(p) && GC_HAS_DEBUG_INFO(p)) {
454 ptr_t p_deepest_obj = 0;
458 /* If we remembered a height last time, use it as a minimum. */
459 /* It may have increased due to newly unreachable chains pointing */
460 /* to p, but it can't have decreased. */
461 back_ptr = GET_OH_BG_PTR(p);
462 if (0 != back_ptr && ((word)back_ptr & FLAG_MANY)) {
463 be = (back_edges *)((word)back_ptr & ~FLAG_MANY);
464 if (be -> height != HEIGHT_UNKNOWN) p_height = be -> height;
468 ptr_t pred = GET_OH_BG_PTR(p);
469 back_edges *e = (back_edges *)((word)pred & ~FLAG_MANY);
474 if (((word)pred & FLAG_MANY) != 0) {
475 n_edges = e -> n_edges;
476 } else if (pred != NULL && ((word)pred & 1) == 0) {
477 /* A misinterpreted freelist link. */
483 for (total = 0; total < n_edges; ++total) {
484 if (local == MAX_IN) {
489 pred = e -> edges[local++];
491 /* Execute the following once for each predecessor pred of p */
492 /* in the points-to graph. */
493 if (!GC_is_marked(pred) && GC_HAS_DEBUG_INFO(pred)) {
494 word this_height = backwards_height(pred);
495 if (this_height > p_height) {
496 p_height = this_height;
497 p_deepest_obj = pred;
504 /* Remember the height for next time. */
507 back_ptr = GET_OH_BG_PTR(p);
508 be = (back_edges *)((word)back_ptr & ~FLAG_MANY);
510 be -> flags |= RETAIN;
511 be -> height = p_height;
512 be -> height_gc_no = (unsigned short)GC_gc_no;
514 if (p_height > GC_max_height) {
515 GC_max_height = p_height;
516 GC_deepest_obj = p_deepest_obj;
521 STATIC word GC_max_max_height = 0;
523 GC_INNER void GC_traverse_back_graph(void)
525 GC_ASSERT(I_HOLD_LOCK());
527 GC_apply_to_each_object(update_max_height);
528 if (0 != GC_deepest_obj)
529 GC_set_mark_bit(GC_deepest_obj); /* Keep it until we can print it. */
532 void GC_print_back_graph_stats(void)
534 GC_ASSERT(I_HOLD_LOCK());
535 GC_printf("Maximum backwards height of reachable objects at GC %lu is %lu\n",
536 (unsigned long) GC_gc_no, (unsigned long)GC_max_height);
537 if (GC_max_height > GC_max_max_height) {
538 ptr_t obj = GC_deepest_obj;
540 GC_max_max_height = GC_max_height;
543 "The following unreachable object is last in a longest chain "
544 "of unreachable objects:\n");
545 GC_print_heap_obj(obj);
548 GC_COND_LOG_PRINTF("Needed max total of %d back-edge structs\n",
549 GC_n_back_edge_structs);
550 GC_apply_to_each_object(reset_back_edge);
554 #endif /* MAKE_BACK_GRAPH */