Annotation of OpenXM_contrib2/asir2000/gc/backgraph.c, Revision 1.2
1.1 noro 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++;
1.2 ! noro 310: FIXUP_POINTER(current);
1.1 noro 311: if (current >= (word)GC_least_plausible_heap_addr &&
312: current <= (word)GC_greatest_plausible_heap_addr) {
313: ptr_t target = GC_base((GC_PTR)current);
314: if (0 != target) {
315: add_edge(p, target);
316: }
317: }
318: }
319: }
320:
321: /* Rebuild the reprentation of the backward reachability graph. */
322: /* Does not examine mark bits. Can be called before GC. */
323: void GC_build_back_graph(void)
324: {
325: GC_apply_to_each_object(add_back_edges);
326: }
327:
328: /* Return an approximation to the length of the longest simple path */
329: /* through unreachable objects to p. We refer to this as the height */
330: /* of p. */
331: static word backwards_height(ptr_t p)
332: {
333: word result;
334: ptr_t back_ptr = GET_OH_BG_PTR(p);
335: back_edges *be;
336:
337: if (0 == back_ptr) return 1;
338: if (!((word)back_ptr & FLAG_MANY)) {
339: if (is_in_progress(p)) return 0; /* DFS back edge, i.e. we followed */
340: /* an edge to an object already */
341: /* on our stack: ignore */
342: push_in_progress(p);
343: result = backwards_height(back_ptr)+1;
344: pop_in_progress(p);
345: return result;
346: }
347: be = (back_edges *)((word)back_ptr & ~FLAG_MANY);
348: if (be -> height >= 0 && be -> height_gc_no == GC_gc_no)
349: return be -> height;
350: /* Ignore back edges in DFS */
351: if (be -> height == HEIGHT_IN_PROGRESS) return 0;
352: result = (be -> height > 0? be -> height : 1);
353: be -> height = HEIGHT_IN_PROGRESS;
354: FOR_EACH_PRED(q, p, {
355: word this_height;
356: if (GC_is_marked(q) && !(FLAG_MANY & (word)GET_OH_BG_PTR(p))) {
357: if (GC_print_stats)
358: GC_printf2("Found bogus pointer from 0x%lx to 0x%lx\n", q, p);
359: /* Reachable object "points to" unreachable one. */
360: /* Could be caused by our lax treatment of GC descriptors. */
361: this_height = 1;
362: } else {
363: this_height = backwards_height(q);
364: }
365: if (this_height >= result) result = this_height + 1;
366: });
367: be -> height = result;
368: be -> height_gc_no = GC_gc_no;
369: return result;
370: }
371:
372: word GC_max_height;
373: ptr_t GC_deepest_obj;
374:
375: /* Compute the maximum height of every unreachable predecessor p of a */
376: /* reachable object. Arrange to save the heights of all such objects p */
377: /* so that they can be used in calculating the height of objects in the */
378: /* next GC. */
379: /* Set GC_max_height to be the maximum height we encounter, and */
380: /* GC_deepest_obj to be the corresponding object. */
381: static void update_max_height(ptr_t p, word n_words, word gc_descr)
382: {
383: if (GC_is_marked(p) && GC_HAS_DEBUG_INFO(p)) {
384: int i;
385: word p_height = 0;
386: ptr_t p_deepest_obj = 0;
387: ptr_t back_ptr;
388: back_edges *be = 0;
389:
390: /* If we remembered a height last time, use it as a minimum. */
391: /* It may have increased due to newly unreachable chains pointing */
392: /* to p, but it can't have decreased. */
393: back_ptr = GET_OH_BG_PTR(p);
394: if (0 != back_ptr && ((word)back_ptr & FLAG_MANY)) {
395: be = (back_edges *)((word)back_ptr & ~FLAG_MANY);
396: if (be -> height != HEIGHT_UNKNOWN) p_height = be -> height;
397: }
398: FOR_EACH_PRED(q, p, {
399: if (!GC_is_marked(q) && GC_HAS_DEBUG_INFO(q)) {
400: word q_height;
401:
402: q_height = backwards_height(q);
403: if (q_height > p_height) {
404: p_height = q_height;
405: p_deepest_obj = q;
406: }
407: }
408: });
409: if (p_height > 0) {
410: /* Remember the height for next time. */
411: if (be == 0) {
412: ensure_struct(p);
413: back_ptr = GET_OH_BG_PTR(p);
414: be = (back_edges *)((word)back_ptr & ~FLAG_MANY);
415: }
416: be -> flags |= RETAIN;
417: be -> height = p_height;
418: be -> height_gc_no = GC_gc_no;
419: }
420: if (p_height > GC_max_height) {
421: GC_max_height = p_height;
422: GC_deepest_obj = p_deepest_obj;
423: }
424: }
425: }
426:
427: void GC_traverse_back_graph(void)
428: {
429: static word max_max_height = 0;
430: GC_max_height = 0;
431: GC_apply_to_each_object(update_max_height);
432: GC_printf2("Maximum backwards height of reachable objects at GC %lu is %ld\n",
433: (unsigned long) GC_gc_no, GC_max_height);
434: if (GC_max_height > max_max_height) {
435: max_max_height = GC_max_height;
436: GC_printf0("The following unreachable object is last in a longest chain "
437: "of unreachable objects:\n");
438: GC_print_heap_obj(GC_deepest_obj);
439: }
440: if (GC_print_stats) {
441: GC_printf1("Needed max total of %ld back-edge structs\n",
442: GC_n_back_edge_structs);
443: }
444: GC_apply_to_each_object(reset_back_edge);
445: GC_deepest_obj = 0;
446: }
447:
448: #endif /* MAKE_BACK_GRAPH */
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