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-rw-r--r--gc/backgraph.c447
1 files changed, 447 insertions, 0 deletions
diff --git a/gc/backgraph.c b/gc/backgraph.c
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+/*
+ * Copyright (c) 2001 by Hewlett-Packard Company. All rights reserved.
+ *
+ * THIS MATERIAL IS PROVIDED AS IS, WITH ABSOLUTELY NO WARRANTY EXPRESSED
+ * OR IMPLIED. ANY USE IS AT YOUR OWN RISK.
+ *
+ * Permission is hereby granted to use or copy this program
+ * for any purpose, provided the above notices are retained on all copies.
+ * Permission to modify the code and to distribute modified code is granted,
+ * provided the above notices are retained, and a notice that the code was
+ * modified is included with the above copyright notice.
+ *
+ */
+
+/*
+ * This implements a full, though not well-tuned, representation of the
+ * backwards points-to graph. This is used to test for non-GC-robust
+ * data structures; the code is not used during normal garbage collection.
+ *
+ * One restriction is that we drop all back-edges from nodes with very
+ * high in-degree, and simply add them add them to a list of such
+ * nodes. They are then treated as permanent roots. Id this by itself
+ * doesn't introduce a space leak, then such nodes can't contribute to
+ * a growing space leak.
+ */
+
+#ifdef MAKE_BACK_GRAPH
+
+#define MAX_IN 10 /* Maximum in-degree we handle directly */
+
+#include "private/dbg_mlc.h"
+#include <unistd.h>
+
+#if !defined(DBG_HDRS_ALL) || (ALIGNMENT != CPP_WORDSZ/8) || !defined(UNIX_LIKE)
+# error Configuration doesnt support MAKE_BACK_GRAPH
+#endif
+
+/* We store single back pointers directly in the object's oh_bg_ptr field. */
+/* If there is more than one ptr to an object, we store q | FLAG_MANY, */
+/* where q is a pointer to a back_edges object. */
+/* Every once in a while we use a back_edges object even for a single */
+/* pointer, since we need the other fields in the back_edges structure to */
+/* be present in some fraction of the objects. Otherwise we get serious */
+/* performance issues. */
+#define FLAG_MANY 2
+
+typedef struct back_edges_struct {
+ word n_edges; /* Number of edges, including those in continuation */
+ /* structures. */
+ unsigned short flags;
+# define RETAIN 1 /* Directly points to a reachable object; */
+ /* retain for next GC. */
+ unsigned short height_gc_no;
+ /* If height > 0, then the GC_gc_no value when it */
+ /* was computed. If it was computed this cycle, then */
+ /* it is current. If it was computed during the */
+ /* last cycle, then it represents the old height, */
+ /* which is only saved for live objects referenced by */
+ /* dead ones. This may grow due to refs from newly */
+ /* dead objects. */
+ signed_word height;
+ /* Longest path through unreachable nodes to this node */
+ /* that we found using depth first search. */
+
+# define HEIGHT_UNKNOWN ((signed_word)(-2))
+# define HEIGHT_IN_PROGRESS ((signed_word)(-1))
+ ptr_t edges[MAX_IN];
+ struct back_edges_struct *cont;
+ /* Pointer to continuation structure; we use only the */
+ /* edges field in the continuation. */
+ /* also used as free list link. */
+} back_edges;
+
+/* Allocate a new back edge structure. Should be more sophisticated */
+/* if this were production code. */
+#define MAX_BACK_EDGE_STRUCTS 100000
+static back_edges *back_edge_space = 0;
+int GC_n_back_edge_structs = 0; /* Serves as pointer to never used */
+ /* back_edges space. */
+static back_edges *avail_back_edges = 0;
+ /* Pointer to free list of deallocated */
+ /* back_edges structures. */
+
+static back_edges * new_back_edges(void)
+{
+ if (0 == back_edge_space) {
+ back_edge_space = (back_edges *)
+ sbrk(MAX_BACK_EDGE_STRUCTS*sizeof(back_edges));
+ }
+ if (0 != avail_back_edges) {
+ back_edges * result = avail_back_edges;
+ avail_back_edges = result -> cont;
+ result -> cont = 0;
+ return result;
+ }
+ if (GC_n_back_edge_structs >= MAX_BACK_EDGE_STRUCTS - 1) {
+ ABORT("needed too much space for back edges: adjust "
+ "MAX_BACK_EDGE_STRUCTS");
+ }
+ return back_edge_space + (GC_n_back_edge_structs++);
+}
+
+/* Deallocate p and its associated continuation structures. */
+static void deallocate_back_edges(back_edges *p)
+{
+ back_edges *last = p;
+
+ while (0 != last -> cont) last = last -> cont;
+ last -> cont = avail_back_edges;
+ avail_back_edges = p;
+}
+
+/* Table of objects that are currently on the depth-first search */
+/* stack. Only objects with in-degree one are in this table. */
+/* Other objects are identified using HEIGHT_IN_PROGRESS. */
+/* This data structure NEEDS IMPROVEMENT. */
+#define MAX_IN_PROGRESS 10000
+static ptr_t * in_progress_space = 0;
+static int n_in_progress = 0;
+
+static void push_in_progress(ptr_t p)
+{
+ if (in_progress_space == 0)
+ in_progress_space = sbrk(MAX_IN_PROGRESS * sizeof(ptr_t));
+ if (n_in_progress == MAX_IN_PROGRESS)
+ ABORT("Exceeded MAX_IN_PROGRESS");
+ in_progress_space[n_in_progress++] = p;
+}
+
+static GC_bool is_in_progress(ptr_t p)
+{
+ int i;
+ for (i = 0; i < n_in_progress; ++i) {
+ if (in_progress_space[i] == p) return TRUE;
+ }
+ return FALSE;
+}
+
+static void pop_in_progress(ptr_t p)
+{
+ --n_in_progress;
+ GC_ASSERT(in_progress_space[n_in_progress] == p);
+}
+
+#define GET_OH_BG_PTR(p) \
+ (ptr_t)REVEAL_POINTER(((oh *)(p)) -> oh_bg_ptr)
+#define SET_OH_BG_PTR(p,q) (((oh *)(p)) -> oh_bg_ptr) = HIDE_POINTER(q)
+
+/* Execute s once for each predecessor q of p in the points-to graph. */
+/* s should be a bracketed statement. We declare q. */
+#define FOR_EACH_PRED(q, p, s) \
+ { \
+ ptr_t q = GET_OH_BG_PTR(p); \
+ if (!((word)q & FLAG_MANY)) { \
+ if (q && !((word)q & 1)) s \
+ /* !((word)q & 1) checks for a misnterpreted freelist link */ \
+ } else { \
+ back_edges *orig_be_ = (back_edges *)((word)q & ~FLAG_MANY); \
+ back_edges *be_ = orig_be_; \
+ int total_, local_; \
+ int n_edges_ = be_ -> n_edges; \
+ for (total_ = 0, local_ = 0; total_ < n_edges_; ++local_, ++total_) { \
+ if (local_ == MAX_IN) { \
+ be_ = be_ -> cont; \
+ local_ = 0; \
+ } \
+ q = be_ -> edges[local_]; s \
+ } \
+ } \
+ }
+
+/* Ensure that p has a back_edges structure associated with it. */
+static void ensure_struct(ptr_t p)
+{
+ ptr_t old_back_ptr = GET_OH_BG_PTR(p);
+
+ if (!((word)old_back_ptr & FLAG_MANY)) {
+ back_edges *be = new_back_edges();
+ be -> flags = 0;
+ if (0 == old_back_ptr) {
+ be -> n_edges = 0;
+ } else {
+ be -> n_edges = 1;
+ be -> edges[0] = old_back_ptr;
+ }
+ be -> height = HEIGHT_UNKNOWN;
+ be -> height_gc_no = GC_gc_no - 1;
+ GC_ASSERT(be >= back_edge_space);
+ SET_OH_BG_PTR(p, (word)be | FLAG_MANY);
+ }
+}
+
+/* Add the (forward) edge from p to q to the backward graph. Both p */
+/* q are pointers to the object base, i.e. pointers to an oh. */
+static void add_edge(ptr_t p, ptr_t q)
+{
+ ptr_t old_back_ptr = GET_OH_BG_PTR(q);
+ back_edges * be, *be_cont;
+ word i;
+ static unsigned random_number = 13;
+# define GOT_LUCKY_NUMBER (((++random_number) & 0x7f) == 0)
+ /* A not very random number we use to occasionally allocate a */
+ /* back_edges structure even for a single backward edge. This */
+ /* prevents us from repeatedly tracing back through very long */
+ /* chains, since we will have some place to store height and */
+ /* in_progress flags along the way. */
+
+ GC_ASSERT(p == GC_base(p) && q == GC_base(q));
+ if (!GC_HAS_DEBUG_INFO(q) || !GC_HAS_DEBUG_INFO(p)) {
+ /* This is really a misinterpreted free list link, since we saw */
+ /* a pointer to a free list. Dont overwrite it! */
+ return;
+ }
+ if (0 == old_back_ptr) {
+ SET_OH_BG_PTR(q, p);
+ if (GOT_LUCKY_NUMBER) ensure_struct(q);
+ return;
+ }
+ /* Check whether it was already in the list of predecessors. */
+ FOR_EACH_PRED(pred, q, { if (p == pred) return; });
+ ensure_struct(q);
+ old_back_ptr = GET_OH_BG_PTR(q);
+ be = (back_edges *)((word)old_back_ptr & ~FLAG_MANY);
+ for (i = be -> n_edges, be_cont = be; i > MAX_IN;
+ be_cont = be_cont -> cont, i -= MAX_IN) {}
+ if (i == MAX_IN) {
+ be_cont -> cont = new_back_edges();
+ be_cont = be_cont -> cont;
+ i = 0;
+ }
+ be_cont -> edges[i] = p;
+ be -> n_edges++;
+ if (be -> n_edges == 100) {
+# if 0
+ if (GC_print_stats) {
+ GC_err_printf0("The following object has in-degree >= 100:\n");
+ GC_print_heap_obj(q);
+ }
+# endif
+ }
+}
+
+typedef void (*per_object_func)(ptr_t p, word n_words, word gc_descr);
+
+static void per_object_helper(struct hblk *h, word fn)
+{
+ hdr * hhdr = HDR(h);
+ word sz = hhdr -> hb_sz;
+ word descr = hhdr -> hb_descr;
+ per_object_func f = (per_object_func)fn;
+ int i = 0;
+
+ do {
+ f((ptr_t)(h -> hb_body + i), sz, descr);
+ i += sz;
+ } while (i + sz <= BYTES_TO_WORDS(HBLKSIZE));
+}
+
+void GC_apply_to_each_object(per_object_func f)
+{
+ GC_apply_to_all_blocks(per_object_helper, (word)f);
+}
+
+static void reset_back_edge(ptr_t p, word n_words, word gc_descr)
+{
+ /* Skip any free list links, or dropped blocks */
+ if (GC_HAS_DEBUG_INFO(p)) {
+ ptr_t old_back_ptr = GET_OH_BG_PTR(p);
+ if ((word)old_back_ptr & FLAG_MANY) {
+ back_edges *be = (back_edges *)((word)old_back_ptr & ~FLAG_MANY);
+ if (!(be -> flags & RETAIN)) {
+ deallocate_back_edges(be);
+ SET_OH_BG_PTR(p, 0);
+ } else {
+ word *currentp;
+
+ GC_ASSERT(GC_is_marked(p));
+
+ /* Back edges may point to objects that will not be retained. */
+ /* Delete them for now, but remember the height. */
+ /* Some will be added back at next GC. */
+ be -> n_edges = 0;
+ if (0 != be -> cont) {
+ deallocate_back_edges(be -> cont);
+ be -> cont = 0;
+ }
+
+ GC_ASSERT(GC_is_marked(p));
+
+ /* We only retain things for one GC cycle at a time. */
+ be -> flags &= ~RETAIN;
+ }
+ } else /* Simple back pointer */ {
+ /* Clear to avoid dangling pointer. */
+ SET_OH_BG_PTR(p, 0);
+ }
+ }
+}
+
+static void add_back_edges(ptr_t p, word n_words, word gc_descr)
+{
+ word *currentp = (word *)(p + sizeof(oh));
+
+ /* For now, fix up non-length descriptors conservatively. */
+ if((gc_descr & GC_DS_TAGS) != GC_DS_LENGTH) {
+ gc_descr = WORDS_TO_BYTES(n_words);
+ }
+ while (currentp < (word *)(p + gc_descr)) {
+ word current = *currentp++;
+ if (current >= (word)GC_least_plausible_heap_addr &&
+ current <= (word)GC_greatest_plausible_heap_addr) {
+ ptr_t target = GC_base((GC_PTR)current);
+ if (0 != target) {
+ add_edge(p, target);
+ }
+ }
+ }
+}
+
+/* Rebuild the reprentation of the backward reachability graph. */
+/* Does not examine mark bits. Can be called before GC. */
+void GC_build_back_graph(void)
+{
+ GC_apply_to_each_object(add_back_edges);
+}
+
+/* Return an approximation to the length of the longest simple path */
+/* through unreachable objects to p. We refer to this as the height */
+/* of p. */
+static word backwards_height(ptr_t p)
+{
+ word result;
+ ptr_t back_ptr = GET_OH_BG_PTR(p);
+ back_edges *be;
+
+ if (0 == back_ptr) return 1;
+ if (!((word)back_ptr & FLAG_MANY)) {
+ if (is_in_progress(p)) return 0; /* DFS back edge, i.e. we followed */
+ /* an edge to an object already */
+ /* on our stack: ignore */
+ push_in_progress(p);
+ result = backwards_height(back_ptr)+1;
+ pop_in_progress(p);
+ return result;
+ }
+ be = (back_edges *)((word)back_ptr & ~FLAG_MANY);
+ if (be -> height >= 0 && be -> height_gc_no == GC_gc_no)
+ return be -> height;
+ /* Ignore back edges in DFS */
+ if (be -> height == HEIGHT_IN_PROGRESS) return 0;
+ result = (be -> height > 0? be -> height : 1);
+ be -> height = HEIGHT_IN_PROGRESS;
+ FOR_EACH_PRED(q, p, {
+ word this_height;
+ if (GC_is_marked(q) && !(FLAG_MANY & (word)GET_OH_BG_PTR(p))) {
+ if (GC_print_stats)
+ GC_printf2("Found bogus pointer from 0x%lx to 0x%lx\n", q, p);
+ /* Reachable object "points to" unreachable one. */
+ /* Could be caused by our lax treatment of GC descriptors. */
+ this_height = 1;
+ } else {
+ this_height = backwards_height(q);
+ }
+ if (this_height >= result) result = this_height + 1;
+ });
+ be -> height = result;
+ be -> height_gc_no = GC_gc_no;
+ return result;
+}
+
+word GC_max_height;
+ptr_t GC_deepest_obj;
+
+/* Compute the maximum height of every unreachable predecessor p of a */
+/* reachable object. Arrange to save the heights of all such objects p */
+/* so that they can be used in calculating the height of objects in the */
+/* next GC. */
+/* Set GC_max_height to be the maximum height we encounter, and */
+/* GC_deepest_obj to be the corresponding object. */
+static void update_max_height(ptr_t p, word n_words, word gc_descr)
+{
+ if (GC_is_marked(p) && GC_HAS_DEBUG_INFO(p)) {
+ int i;
+ word p_height = 0;
+ ptr_t p_deepest_obj = 0;
+ ptr_t back_ptr;
+ back_edges *be = 0;
+
+ /* If we remembered a height last time, use it as a minimum. */
+ /* It may have increased due to newly unreachable chains pointing */
+ /* to p, but it can't have decreased. */
+ back_ptr = GET_OH_BG_PTR(p);
+ if (0 != back_ptr && ((word)back_ptr & FLAG_MANY)) {
+ be = (back_edges *)((word)back_ptr & ~FLAG_MANY);
+ if (be -> height != HEIGHT_UNKNOWN) p_height = be -> height;
+ }
+ FOR_EACH_PRED(q, p, {
+ if (!GC_is_marked(q) && GC_HAS_DEBUG_INFO(q)) {
+ word q_height;
+
+ q_height = backwards_height(q);
+ if (q_height > p_height) {
+ p_height = q_height;
+ p_deepest_obj = q;
+ }
+ }
+ });
+ if (p_height > 0) {
+ /* Remember the height for next time. */
+ if (be == 0) {
+ ensure_struct(p);
+ back_ptr = GET_OH_BG_PTR(p);
+ be = (back_edges *)((word)back_ptr & ~FLAG_MANY);
+ }
+ be -> flags |= RETAIN;
+ be -> height = p_height;
+ be -> height_gc_no = GC_gc_no;
+ }
+ if (p_height > GC_max_height) {
+ GC_max_height = p_height;
+ GC_deepest_obj = p_deepest_obj;
+ }
+ }
+}
+
+void GC_traverse_back_graph(void)
+{
+ static word max_max_height = 0;
+ GC_max_height = 0;
+ GC_apply_to_each_object(update_max_height);
+ GC_printf2("Maximum backwards height of reachable objects at GC %lu is %ld\n",
+ (unsigned long) GC_gc_no, GC_max_height);
+ if (GC_max_height > max_max_height) {
+ max_max_height = GC_max_height;
+ GC_printf0("The following unreachable object is last in a longest chain "
+ "of unreachable objects:\n");
+ GC_print_heap_obj(GC_deepest_obj);
+ }
+ if (GC_print_stats) {
+ GC_printf1("Needed max total of %ld back-edge structs\n",
+ GC_n_back_edge_structs);
+ }
+ GC_apply_to_each_object(reset_back_edge);
+ GC_deepest_obj = 0;
+}
+
+#endif /* MAKE_BACK_GRAPH */
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