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gcc/boehm-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++;
FIXUP_POINTER(current);
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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