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