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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.

 */

#include "gc.h" /* For configuration information. */

#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 *)

                        GET_MEM(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.               */

/* 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;

static void push_in_progress(ptr_t p)

{

  if (n_in_progress >= in_progress_size)

    if (in_progress_size == 0) {

      in_progress_size = INITIAL_IN_PROGRESS;

      in_progress_space = (ptr_t *)GET_MEM(in_progress_size * sizeof(ptr_t));

    } else {

      ptr_t * new_in_progress_space;

      in_progress_size *= 2;

      new_in_progress_space = (ptr_t *)

                                GET_MEM(in_progress_size * sizeof(ptr_t));

      BCOPY(in_progress_space, new_in_progress_space,

            n_in_progress * sizeof(ptr_t));

      in_progress_space = new_in_progress_space;

      /* FIXME: This just drops the old space.  */

    }

  if (in_progress_space == 0)

      ABORT("MAKE_BACK_GRAPH: Out of in-progress space: "

            "Huge linear data structure?");

  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 representation 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;

    }

  }

}

word GC_max_max_height = 0;

void GC_traverse_back_graph(void)

{

  GC_max_height = 0;

  GC_apply_to_each_object(update_max_height);

}

void GC_print_back_graph_stats(void)

{

  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 > GC_max_max_height) {

    GC_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 */