mono/libgc/backgraph.c

/*
 * 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 */