/*
* 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.
*
*/
#include "private/dbg_mlc.h"
/*
* 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. If 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 <unistd.h> */
#if (!defined(DBG_HDRS_ALL) || (ALIGNMENT != CPP_WORDSZ/8) \
/* || !defined(UNIX_LIKE) */) && !defined(CPPCHECK)
# error The configuration does not 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 (-2)
# define HEIGHT_IN_PROGRESS (-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;
STATIC 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. */
GC_ASSERT(GC_page_size != 0);
back_edge_space = (back_edges *)GET_MEM(bytes_to_get);
if (NULL == back_edge_space)
ABORT("Insufficient memory for back edges");
GC_add_to_our_memory((ptr_t)back_edge_space, bytes_to_get);
}
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. */
/* FIXME: This data structure NEEDS IMPROVEMENT. */
#define INITIAL_IN_PROGRESS 10000
static ptr_t * in_progress_space = 0;
static size_t in_progress_size = 0;
static size_t n_in_progress = 0;
/* 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 = (unsigned short)(GC_gc_no - 1);
GC_ASSERT((word)be >= (word)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 pred = GET_OH_BG_PTR(q);
back_edges * be, *be_cont;
word i;
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. Don't overwrite it! */
return;
}
if (NULL == pred) {
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. */
SET_OH_BG_PTR(q, p);
if (GOT_LUCKY_NUMBER) ensure_struct(q);
return;
}
/* Check whether it was already in the list of predecessors. */
{
back_edges *e = (back_edges *)((word)pred & ~(word)FLAG_MANY);
word n_edges;
word total;
int local = 0;
if (((word)pred & FLAG_MANY) != 0) {
n_edges = e -> n_edges;
} else if (((word)COVERT_DATAFLOW(pred) & 1) == 0) {
/* A misinterpreted freelist link. */
n_edges = 1;
local = -1;
} else {
n_edges = 0;
}
for (total = 0; total < n_edges; ++total) {
if (local == MAX_IN) {
e = e -> cont;
local = 0;
}
if (local >= 0)
pred = e -> edges[local++];
if (pred == p)
return;
}
}
ensure_struct(q);
be = (back_edges *)((word)GET_OH_BG_PTR(q) & ~(word)FLAG_MANY);
for (i = be -> n_edges, be_cont = be; i > MAX_IN; i -= MAX_IN)
be_cont = be_cont -> cont;
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++;
# ifdef DEBUG_PRINT_BIG_N_EDGES
if (GC_print_stats == VERBOSE && be -> n_edges == 100) {
GC_err_printf("The following object has big in-degree:\n");
GC_print_heap_obj(q);
}
# endif
}
typedef void (*per_object_func)(ptr_t p, size_t n_bytes, word gc_descr);
static void per_object_helper(struct hblk *h, word fn)
{
hdr * hhdr = HDR(h);
size_t sz = (size_t)hhdr->hb_sz;
word descr = hhdr -> hb_descr;
per_object_func f = (per_object_func)fn;
size_t i = 0;
do {
f((ptr_t)(h -> hb_body + i), sz, descr);
i += sz;
} while (i + sz <= HBLKSIZE);
}
static void reset_back_edge(ptr_t p, size_t n_bytes GC_ATTR_UNUSED,
word gc_descr GC_ATTR_UNUSED)
{
/* 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 & ~(word)FLAG_MANY);
if (!(be -> flags & RETAIN)) {
deallocate_back_edges(be);
SET_OH_BG_PTR(p, 0);
} else {
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, size_t n_bytes, 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 = n_bytes;
}
while ((word)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 = (ptr_t)GC_base((void *)current);
if (0 != target) {
add_edge(p, target);
}
}
}
}
/* Rebuild the representation of the backward reachability graph. */
/* Does not examine mark bits. Can be called before GC. */
GC_INNER void GC_build_back_graph(void)
{
GC_ASSERT(I_HOLD_LOCK());
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 pred = GET_OH_BG_PTR(p);
back_edges *be;
if (NULL == pred)
return 1;
if (((word)pred & FLAG_MANY) == 0) {
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(pred) + 1;
pop_in_progress(p);
return result;
}
be = (back_edges *)((word)pred & ~(word)FLAG_MANY);
if (be -> height >= 0 && be -> height_gc_no == (unsigned short)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;
{
back_edges *e = be;
word n_edges;
word total;
int local = 0;
if (((word)pred & FLAG_MANY) != 0) {
n_edges = e -> n_edges;
} else if (((word)pred & 1) == 0) {
/* A misinterpreted freelist link. */
n_edges = 1;
local = -1;
} else {
n_edges = 0;
}
for (total = 0; total < n_edges; ++total) {
word this_height;
if (local == MAX_IN) {
e = e -> cont;
local = 0;
}
if (local >= 0)
pred = e -> edges[local++];
/* Execute the following once for each predecessor pred of p */
/* in the points-to graph. */
if (GC_is_marked(pred) && ((word)GET_OH_BG_PTR(p) & FLAG_MANY) == 0) {
GC_COND_LOG_PRINTF("Found bogus pointer from %p to %p\n",
(void *)pred, (void *)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(pred);
}
if (this_height >= result)
result = this_height + 1;
}
}
be -> height = result;
be -> height_gc_no = (unsigned short)GC_gc_no;
return result;
}
STATIC word GC_max_height = 0;
STATIC ptr_t GC_deepest_obj = NULL;
/* 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, size_t n_bytes GC_ATTR_UNUSED,
word gc_descr GC_ATTR_UNUSED)
{
if (GC_is_marked(p) && GC_HAS_DEBUG_INFO(p)) {
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 & ~(word)FLAG_MANY);
if (be -> height != HEIGHT_UNKNOWN) p_height = be -> height;
}
{
ptr_t pred = GET_OH_BG_PTR(p);
back_edges *e = (back_edges *)((word)pred & ~(word)FLAG_MANY);
word n_edges;
word total;
int local = 0;
if (((word)pred & FLAG_MANY) != 0) {
n_edges = e -> n_edges;
} else if (pred != NULL && ((word)pred & 1) == 0) {
/* A misinterpreted freelist link. */
n_edges = 1;
local = -1;
} else {
n_edges = 0;
}
for (total = 0; total < n_edges; ++total) {
if (local == MAX_IN) {
e = e -> cont;
local = 0;
}
if (local >= 0)
pred = e -> edges[local++];
/* Execute the following once for each predecessor pred of p */
/* in the points-to graph. */
if (!GC_is_marked(pred) && GC_HAS_DEBUG_INFO(pred)) {
word this_height = backwards_height(pred);
if (this_height > p_height) {
p_height = this_height;
p_deepest_obj = pred;
}
}
}
}
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 & ~(word)FLAG_MANY);
}
be -> flags |= RETAIN;
be -> height = p_height;
be -> height_gc_no = (unsigned short)GC_gc_no;
}
if (p_height > GC_max_height) {
GC_max_height = p_height;
GC_deepest_obj = p_deepest_obj;
}
}
}
STATIC word GC_max_max_height = 0;
GC_INNER void GC_traverse_back_graph(void)
{
GC_ASSERT(I_HOLD_LOCK());
GC_max_height = 0;
GC_apply_to_each_object(update_max_height);
if (0 != GC_deepest_obj)
GC_set_mark_bit(GC_deepest_obj); /* Keep it until we can print it. */
}
void GC_print_back_graph_stats(void)
{
GC_ASSERT(I_HOLD_LOCK());
GC_printf("Maximum backwards height of reachable objects"
" at GC #%lu is %lu\n",
(unsigned long)GC_gc_no, (unsigned long)GC_max_height);
if (GC_max_height > GC_max_max_height) {
ptr_t obj = GC_deepest_obj;
GC_max_max_height = GC_max_height;
UNLOCK();
GC_err_printf(
"The following unreachable object is last in a longest chain "
"of unreachable objects:\n");
GC_print_heap_obj(obj);
LOCK();
}
GC_COND_LOG_PRINTF("Needed max total of %d back-edge structs\n",
GC_n_back_edge_structs);
GC_apply_to_each_object(reset_back_edge);
GC_deepest_obj = 0;
}
