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[bpt/guile.git] / libguile / gc.c

/*      Copyright (C) 1995, 1996, 1997, 1998 Free Software Foundation, Inc.
 * 
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2, or (at your option)
 * any later version.
 * 
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 * 
 * You should have received a copy of the GNU General Public License
 * along with this software; see the file COPYING.  If not, write to
 * the Free Software Foundation, Inc., 59 Temple Place, Suite 330,
 * Boston, MA 02111-1307 USA
 *
 * As a special exception, the Free Software Foundation gives permission
 * for additional uses of the text contained in its release of GUILE.
 *
 * The exception is that, if you link the GUILE library with other files
 * to produce an executable, this does not by itself cause the
 * resulting executable to be covered by the GNU General Public License.
 * Your use of that executable is in no way restricted on account of
 * linking the GUILE library code into it.
 *
 * This exception does not however invalidate any other reasons why
 * the executable file might be covered by the GNU General Public License.
 *
 * This exception applies only to the code released by the
 * Free Software Foundation under the name GUILE.  If you copy
 * code from other Free Software Foundation releases into a copy of
 * GUILE, as the General Public License permits, the exception does
 * not apply to the code that you add in this way.  To avoid misleading
 * anyone as to the status of such modified files, you must delete
 * this exception notice from them.
 *
 * If you write modifications of your own for GUILE, it is your choice
 * whether to permit this exception to apply to your modifications.
 * If you do not wish that, delete this exception notice.  */
\f
#include <stdio.h>
#include "_scm.h"
#include "stime.h"
#include "stackchk.h"
#include "struct.h"
#include "genio.h"
#include "weaks.h"
#include "guardians.h"
#include "smob.h"
#include "unif.h"
#include "async.h"

#include "gc.h"

#ifdef HAVE_MALLOC_H
#include <malloc.h>
#endif

#ifdef HAVE_UNISTD_H
#include <unistd.h>
#endif

#ifdef __STDC__
#include <stdarg.h>
#define var_start(x, y) va_start(x, y)
#else
#include <varargs.h>
#define var_start(x, y) va_start(x)
#endif

\f
/* {heap tuning parameters}
 * 
 * These are parameters for controlling memory allocation.  The heap
 * is the area out of which scm_cons, and object headers are allocated.
 *
 * Each heap cell is 8 bytes on a 32 bit machine and 16 bytes on a
 * 64 bit machine.  The units of the _SIZE parameters are bytes.
 * Cons pairs and object headers occupy one heap cell.
 *
 * SCM_INIT_HEAP_SIZE is the initial size of heap.  If this much heap is
 * allocated initially the heap will grow by half its current size
 * each subsequent time more heap is needed.
 *
 * If SCM_INIT_HEAP_SIZE heap cannot be allocated initially, SCM_HEAP_SEG_SIZE
 * will be used, and the heap will grow by SCM_HEAP_SEG_SIZE when more
 * heap is needed.  SCM_HEAP_SEG_SIZE must fit into type scm_sizet.  This code
 * is in scm_init_storage() and alloc_some_heap() in sys.c
 * 
 * If SCM_INIT_HEAP_SIZE can be allocated initially, the heap will grow by
 * SCM_EXPHEAP(scm_heap_size) when more heap is needed.
 *
 * SCM_MIN_HEAP_SEG_SIZE is minimum size of heap to accept when more heap
 * is needed.
 *
 * INIT_MALLOC_LIMIT is the initial amount of malloc usage which will
 * trigger a GC. 
 *
 * SCM_MTRIGGER_HYSTERESIS is the amount of malloc storage that must be
 * reclaimed by a GC triggered by must_malloc. If less than this is
 * reclaimed, the trigger threshold is raised. [I don't know what a
 * good value is. I arbitrarily chose 1/10 of the INIT_MALLOC_LIMIT to
 * work around a oscillation that caused almost constant GC.]  
 */

#define SCM_INIT_HEAP_SIZE (32768L*sizeof(scm_cell))
#define SCM_MIN_HEAP_SEG_SIZE (2048L*sizeof(scm_cell))
#ifdef _QC
# define SCM_HEAP_SEG_SIZE 32768L
#else
# ifdef sequent
#  define SCM_HEAP_SEG_SIZE (7000L*sizeof(scm_cell))
# else
#  define SCM_HEAP_SEG_SIZE (16384L*sizeof(scm_cell))
# endif
#endif
#define SCM_EXPHEAP(scm_heap_size) (scm_heap_size*2)
#define SCM_INIT_MALLOC_LIMIT 100000
#define SCM_MTRIGGER_HYSTERESIS (SCM_INIT_MALLOC_LIMIT/10)

/* CELL_UP and CELL_DN are used by scm_init_heap_seg to find scm_cell aligned inner
   bounds for allocated storage */

#ifdef PROT386
/*in 386 protected mode we must only adjust the offset */
# define CELL_UP(p) MK_FP(FP_SEG(p), ~7&(FP_OFF(p)+7))
# define CELL_DN(p) MK_FP(FP_SEG(p), ~7&FP_OFF(p))
#else
# ifdef _UNICOS
#  define CELL_UP(p) (SCM_CELLPTR)(~1L & ((long)(p)+1L))
#  define CELL_DN(p) (SCM_CELLPTR)(~1L & (long)(p))
# else
#  define CELL_UP(p) (SCM_CELLPTR)(~(sizeof(scm_cell)-1L) & ((long)(p)+sizeof(scm_cell)-1L))
#  define CELL_DN(p) (SCM_CELLPTR)(~(sizeof(scm_cell)-1L) & (long)(p))
# endif                         /* UNICOS */
#endif                          /* PROT386 */


\f
/* scm_freelist
 * is the head of freelist of cons pairs.
 */
SCM scm_freelist = SCM_EOL;

/* scm_mtrigger
 * is the number of bytes of must_malloc allocation needed to trigger gc.
 */
unsigned long scm_mtrigger;


/* scm_gc_heap_lock
 * If set, don't expand the heap.  Set only during gc, during which no allocation
 * is supposed to take place anyway.
 */
int scm_gc_heap_lock = 0;

/* GC Blocking
 * Don't pause for collection if this is set -- just
 * expand the heap.
 */

int scm_block_gc = 1;

/* If fewer than MIN_GC_YIELD cells are recovered during a garbage
 * collection (GC) more space is allocated for the heap.
 */
#define MIN_GC_YIELD (scm_heap_size/4)

/* During collection, this accumulates objects holding
 * weak references.
 */
SCM *scm_weak_vectors;
int scm_weak_size;
int scm_n_weak;

/* GC Statistics Keeping
 */
unsigned long scm_cells_allocated = 0;
unsigned long scm_mallocated = 0;
unsigned long scm_gc_cells_collected;
unsigned long scm_gc_malloc_collected;
unsigned long scm_gc_ports_collected;
unsigned long scm_gc_rt;
unsigned long scm_gc_time_taken = 0;

SCM_SYMBOL (sym_cells_allocated, "cells-allocated");
SCM_SYMBOL (sym_heap_size, "cell-heap-size");
SCM_SYMBOL (sym_mallocated, "bytes-malloced");
SCM_SYMBOL (sym_mtrigger, "gc-malloc-threshold");
SCM_SYMBOL (sym_heap_segments, "cell-heap-segments");
SCM_SYMBOL (sym_gc_time_taken, "gc-time-taken");


struct scm_heap_seg_data
{
  /* lower and upper bounds of the segment */
  SCM_CELLPTR bounds[2];

  /* address of the head-of-freelist pointer for this segment's cells.
     All segments usually point to the same one, scm_freelist.  */
  SCM *freelistp;

  /* number of SCM words per object in this segment */
  int ncells;

  /* If SEG_DATA->valid is non-zero, the conservative marking
     functions will apply SEG_DATA->valid to the purported pointer and
     SEG_DATA, and mark the object iff the function returns non-zero.
     At the moment, I don't think anyone uses this.  */
  int (*valid) ();
};




static void scm_mark_weak_vector_spines SCM_P ((void));
static scm_sizet init_heap_seg SCM_P ((SCM_CELLPTR, scm_sizet, int, SCM *));
static void alloc_some_heap SCM_P ((int, SCM *));


\f
/* Debugging functions.  */

#ifdef DEBUG_FREELIST

/* Return the number of the heap segment containing CELL.  */
static int
which_seg (SCM cell)
{
  int i;

  for (i = 0; i < scm_n_heap_segs; i++)
    if (SCM_PTR_LE (scm_heap_table[i].bounds[0], (SCM_CELLPTR) cell)
        && SCM_PTR_GT (scm_heap_table[i].bounds[1], (SCM_CELLPTR) cell))
      return i;
  fprintf (stderr, "which_seg: can't find segment containing cell %lx\n",
           cell);
  abort ();
}


SCM_PROC (s_map_free_list, "map-free-list", 0, 0, 0, scm_map_free_list);
SCM
scm_map_free_list ()
{
  int last_seg = -1, count = 0;
  SCM f;
  
  fprintf (stderr, "%d segments total\n", scm_n_heap_segs);
  for (f = scm_freelist; SCM_NIMP (f); f = SCM_CDR (f))
    {
      int this_seg = which_seg (f);

      if (this_seg != last_seg)
        {
          if (last_seg != -1)
            fprintf (stderr, "  %5d cells in segment %d\n", count, last_seg);
          last_seg = this_seg;
          count = 0;
        }
      count++;
    }
  if (last_seg != -1)
    fprintf (stderr, "  %5d cells in segment %d\n", count, last_seg);

  fflush (stderr);

  return SCM_UNSPECIFIED;
}


/* Number of calls to SCM_NEWCELL since startup.  */
static unsigned long scm_newcell_count;

/* Search freelist for anything that isn't marked as a free cell.
   Abort if we find something.  */
static void
scm_check_freelist ()
{
  SCM f;
  int i = 0;

  for (f = scm_freelist; SCM_NIMP (f); f = SCM_CDR (f), i++)
    if (SCM_CAR (f) != (SCM) scm_tc_free_cell)
      {
        fprintf (stderr, "Bad cell in freelist on newcell %lu: %d'th elt\n",
                 scm_newcell_count, i);
        fflush (stderr);
        abort ();
      }
}

static int scm_debug_check_freelist = 0;
SCM
scm_debug_newcell (void)
{
  SCM new;

  scm_newcell_count++;
  if (scm_debug_check_freelist)
    scm_check_freelist ();

  /* The rest of this is supposed to be identical to the SCM_NEWCELL
     macro.  */
  if (SCM_IMP (scm_freelist))
    new = scm_gc_for_newcell ();
  else
    {
      new = scm_freelist;
      scm_freelist = SCM_CDR (scm_freelist);
      ++scm_cells_allocated;
    }

  return new;
}

#endif /* DEBUG_FREELIST */

\f

/* {Scheme Interface to GC}
 */

SCM_PROC (s_gc_stats, "gc-stats", 0, 0, 0, scm_gc_stats);
SCM
scm_gc_stats ()
{
  int i;
  int n;
  SCM heap_segs;
  SCM local_scm_mtrigger;
  SCM local_scm_mallocated;
  SCM local_scm_heap_size;
  SCM local_scm_cells_allocated;
  SCM local_scm_gc_time_taken;
  SCM answer;

  SCM_DEFER_INTS;
  scm_block_gc = 1;
 retry:
  heap_segs = SCM_EOL;
  n = scm_n_heap_segs;
  for (i = scm_n_heap_segs; i--; )
    heap_segs = scm_cons (scm_cons (scm_ulong2num ((unsigned long)scm_heap_table[i].bounds[1]),
                                    scm_ulong2num ((unsigned long)scm_heap_table[i].bounds[0])),
                          heap_segs);
  if (scm_n_heap_segs != n)
    goto retry;
  scm_block_gc = 0;

  local_scm_mtrigger = scm_mtrigger;
  local_scm_mallocated = scm_mallocated;
  local_scm_heap_size = scm_heap_size;
  local_scm_cells_allocated = scm_cells_allocated;
  local_scm_gc_time_taken = scm_gc_time_taken;

  answer = scm_listify (scm_cons (sym_gc_time_taken, scm_ulong2num (local_scm_gc_time_taken)),
                        scm_cons (sym_cells_allocated, scm_ulong2num (local_scm_cells_allocated)),
                        scm_cons (sym_heap_size, scm_ulong2num (local_scm_heap_size)),
                        scm_cons (sym_mallocated, scm_ulong2num (local_scm_mallocated)),
                        scm_cons (sym_mtrigger, scm_ulong2num (local_scm_mtrigger)),
                        scm_cons (sym_heap_segments, heap_segs),
                        SCM_UNDEFINED);
  SCM_ALLOW_INTS;
  return answer;
}


void 
scm_gc_start (what)
     char *what;
{
  scm_gc_rt = SCM_INUM (scm_get_internal_run_time ());
  scm_gc_cells_collected = 0;
  scm_gc_malloc_collected = 0;
  scm_gc_ports_collected = 0;
}

void 
scm_gc_end ()
{
  scm_gc_rt = SCM_INUM (scm_get_internal_run_time ()) - scm_gc_rt;
  scm_gc_time_taken = scm_gc_time_taken + scm_gc_rt;
  scm_system_async_mark (scm_gc_async);
}


SCM_PROC(s_object_address, "object-address", 1, 0, 0, scm_object_addr);
SCM
scm_object_addr (obj)
     SCM obj;
{
  return scm_ulong2num ((unsigned long)obj);
}


SCM_PROC(s_gc, "gc", 0, 0, 0, scm_gc);
SCM 
scm_gc ()
{
  SCM_DEFER_INTS;
  scm_igc ("call");
  SCM_ALLOW_INTS;
  return SCM_UNSPECIFIED;
}


\f
/* {C Interface For When GC is Triggered}
 */

void
scm_gc_for_alloc (ncells, freelistp)
     int ncells;
     SCM * freelistp;
{
  SCM_REDEFER_INTS;
  scm_igc ("cells");
  if ((scm_gc_cells_collected < MIN_GC_YIELD) || SCM_IMP (*freelistp))
    {
      alloc_some_heap (ncells, freelistp);
    }
  SCM_REALLOW_INTS;
}


SCM 
scm_gc_for_newcell ()
{
  SCM fl;
  scm_gc_for_alloc (1, &scm_freelist);
  fl = scm_freelist;
  scm_freelist = SCM_CDR (fl);
  return fl;
}

void
scm_igc (what)
     char *what;
{
  int j;

#ifdef USE_THREADS
  /* During the critical section, only the current thread may run. */
  SCM_THREAD_CRITICAL_SECTION_START;
#endif

  /* fprintf (stderr, "gc: %s\n", what); */

  scm_gc_start (what);
  if (!scm_stack_base || scm_block_gc)
    {
      scm_gc_end ();
      return;
    }

  ++scm_gc_heap_lock;
  scm_n_weak = 0;

  scm_guardian_gc_init ();

  /* unprotect any struct types with no instances */
#if 0
  {
    SCM type_list;
    SCM * pos;

    pos = &scm_type_obj_list;
    type_list = scm_type_obj_list;
    while (type_list != SCM_EOL)
      if (SCM_VELTS (SCM_CAR (type_list))[scm_struct_i_refcnt])
        {
          pos = SCM_CDRLOC (type_list);
          type_list = SCM_CDR (type_list);
        }
      else
        {
          *pos = SCM_CDR (type_list);
          type_list = SCM_CDR (type_list);
        }
  }
#endif

  /* flush dead entries from the continuation stack */
  {
    int x;
    int bound;
    SCM * elts;
    elts = SCM_VELTS (scm_continuation_stack);
    bound = SCM_LENGTH (scm_continuation_stack);
    x = SCM_INUM (scm_continuation_stack_ptr);
    while (x < bound)
      {
        elts[x] = SCM_BOOL_F;
        ++x;
      }
  }

#ifndef USE_THREADS
  
  /* Protect from the C stack.  This must be the first marking
   * done because it provides information about what objects
   * are "in-use" by the C code.   "in-use" objects are  those
   * for which the values from SCM_LENGTH and SCM_CHARS must remain
   * usable.   This requirement is stricter than a liveness
   * requirement -- in particular, it constrains the implementation
   * of scm_vector_set_length_x.
   */
  SCM_FLUSH_REGISTER_WINDOWS;
  /* This assumes that all registers are saved into the jmp_buf */
  setjmp (scm_save_regs_gc_mark);
  scm_mark_locations ((SCM_STACKITEM *) scm_save_regs_gc_mark,
                      (   (scm_sizet) (sizeof (SCM_STACKITEM) - 1 +
                                       sizeof scm_save_regs_gc_mark)
                          / sizeof (SCM_STACKITEM)));

  {
    /* stack_len is long rather than scm_sizet in order to guarantee that
       &stack_len is long aligned */
#ifdef SCM_STACK_GROWS_UP
#ifdef nosve
    long stack_len = (SCM_STACKITEM *) (&stack_len) - scm_stack_base;
#else
    long stack_len = scm_stack_size (scm_stack_base);
#endif
    scm_mark_locations (scm_stack_base, (scm_sizet) stack_len);
#else
#ifdef nosve
    long stack_len = scm_stack_base - (SCM_STACKITEM *) (&stack_len);
#else
    long stack_len = scm_stack_size (scm_stack_base);
#endif
    scm_mark_locations ((scm_stack_base - stack_len), (scm_sizet) stack_len);
#endif
  }

#else /* USE_THREADS */

  /* Mark every thread's stack and registers */
  scm_threads_mark_stacks();

#endif /* USE_THREADS */

  /* FIXME: insert a phase to un-protect string-data preserved
   * in scm_vector_set_length_x.
   */

  j = SCM_NUM_PROTECTS;
  while (j--)
    scm_gc_mark (scm_sys_protects[j]);

#ifndef USE_THREADS
  scm_gc_mark (scm_root->handle);
#endif
  
  scm_mark_weak_vector_spines ();

  scm_guardian_zombify ();

  scm_gc_sweep ();

  --scm_gc_heap_lock;
  scm_gc_end ();

#ifdef USE_THREADS
  SCM_THREAD_CRITICAL_SECTION_END;
#endif
}

\f
/* {Mark/Sweep} 
 */



/* Mark an object precisely.
 */
void 
scm_gc_mark (p)
     SCM p;
{
  register long i;
  register SCM ptr;

  ptr = p;

gc_mark_loop:
  if (SCM_IMP (ptr))
    return;

gc_mark_nimp:
  if (SCM_NCELLP (ptr))
    scm_wta (ptr, "rogue pointer in heap", NULL);

  switch (SCM_TYP7 (ptr))
    {
    case scm_tcs_cons_nimcar:
      if (SCM_GCMARKP (ptr))
        break;
      SCM_SETGCMARK (ptr);
      if (SCM_IMP (SCM_CDR (ptr))) /* SCM_IMP works even with a GC mark */
        {
          ptr = SCM_CAR (ptr);
          goto gc_mark_nimp;
        }
      scm_gc_mark (SCM_CAR (ptr));
      ptr = SCM_GCCDR (ptr);
      goto gc_mark_nimp;
    case scm_tcs_cons_imcar:
      if (SCM_GCMARKP (ptr))
        break;
      SCM_SETGCMARK (ptr);
      ptr = SCM_GCCDR (ptr);
      goto gc_mark_loop;
    case scm_tcs_cons_gloc:
      if (SCM_GCMARKP (ptr))
        break;
      SCM_SETGCMARK (ptr);
      {
        SCM vcell;
        vcell = SCM_CAR (ptr) - 1L;
        switch (SCM_CDR (vcell))
          {
          default:
            scm_gc_mark (vcell);
            ptr = SCM_GCCDR (ptr);
            goto gc_mark_loop;
          case 1:               /* ! */
          case 0:               /* ! */
            {
              SCM layout;
              SCM * vtable_data;
              int len;
              char * fields_desc;
              register SCM * mem;
              register int x;

              vtable_data = (SCM *)vcell;
              layout = vtable_data[scm_vtable_index_layout];
              len = SCM_LENGTH (layout);
              fields_desc = SCM_CHARS (layout);
              /* We're using SCM_GCCDR here like STRUCT_DATA, except
                 that it removes the mark */
              mem = (SCM *)SCM_GCCDR (ptr);
              
              if (vtable_data[scm_struct_i_flags] & SCM_STRUCTF_ENTITY)
                {
                  scm_gc_mark (mem[scm_struct_i_proc + 0]);
                  scm_gc_mark (mem[scm_struct_i_proc + 1]);
                  scm_gc_mark (mem[scm_struct_i_proc + 2]);
                  scm_gc_mark (mem[scm_struct_i_proc + 3]);
                }
              if (len)
                {
                  for (x = 0; x < len - 2; x += 2, ++mem)
                    if (fields_desc[x] == 'p')
                      scm_gc_mark (*mem);
                  if (fields_desc[x] == 'p')
                    {
                      if (SCM_LAYOUT_TAILP (fields_desc[x + 1]))
                        for (x = *mem; x; --x)
                          scm_gc_mark (*++mem);
                      else
                        scm_gc_mark (*mem);
                    }
                }
              if (!SCM_CDR (vcell))
                {
                  SCM_SETGCMARK (vcell);
                  ptr = vtable_data[scm_vtable_index_vtable];
                  goto gc_mark_loop;
                }
            }
          }
      }
      break;
    case scm_tcs_closures:
      if (SCM_GCMARKP (ptr))
        break;
      SCM_SETGCMARK (ptr);
      if (SCM_IMP (SCM_CDR (ptr)))
        {
          ptr = SCM_CLOSCAR (ptr);
          goto gc_mark_nimp;
        }
      scm_gc_mark (SCM_CLOSCAR (ptr));
      ptr = SCM_GCCDR (ptr);
      goto gc_mark_nimp;
    case scm_tc7_vector:
    case scm_tc7_lvector:
#ifdef CCLO
    case scm_tc7_cclo:
#endif
      if (SCM_GC8MARKP (ptr))
        break;
      SCM_SETGC8MARK (ptr);
      i = SCM_LENGTH (ptr);
      if (i == 0)
        break;
      while (--i > 0)
        if (SCM_NIMP (SCM_VELTS (ptr)[i]))
          scm_gc_mark (SCM_VELTS (ptr)[i]);
      ptr = SCM_VELTS (ptr)[0];
      goto gc_mark_loop;
    case scm_tc7_contin:
      if SCM_GC8MARKP
        (ptr) break;
      SCM_SETGC8MARK (ptr);
      if (SCM_VELTS (ptr))
        scm_mark_locations (SCM_VELTS (ptr),
                            (scm_sizet)
                            (SCM_LENGTH (ptr) +
                             (sizeof (SCM_STACKITEM) + -1 +
                              sizeof (scm_contregs)) /
                             sizeof (SCM_STACKITEM)));
      break;
    case scm_tc7_bvect:
    case scm_tc7_byvect:
    case scm_tc7_ivect:
    case scm_tc7_uvect:
    case scm_tc7_fvect:
    case scm_tc7_dvect:
    case scm_tc7_cvect:
    case scm_tc7_svect:
#ifdef LONGLONGS
    case scm_tc7_llvect:
#endif

    case scm_tc7_string:
      SCM_SETGC8MARK (ptr);
      break;

    case scm_tc7_substring:
      if (SCM_GC8MARKP(ptr))
        break;
      SCM_SETGC8MARK (ptr);
      ptr = SCM_CDR (ptr);
      goto gc_mark_loop;

    case scm_tc7_wvect:
      if (SCM_GC8MARKP(ptr))
        break;
      scm_weak_vectors[scm_n_weak++] = ptr;
      if (scm_n_weak >= scm_weak_size)
        {
          SCM_SYSCALL (scm_weak_vectors =
                       (SCM *) realloc ((char *) scm_weak_vectors,
                                        sizeof (SCM) * (scm_weak_size *= 2)));
          if (scm_weak_vectors == NULL)
            {
              scm_puts ("weak vector table", scm_cur_errp);
              scm_puts ("\nFATAL ERROR DURING CRITICAL SCM_CODE SECTION\n",
                        scm_cur_errp);
              exit(SCM_EXIT_FAILURE);
            }
        }
      SCM_SETGC8MARK (ptr);
      if (SCM_IS_WHVEC_ANY (ptr))
        {
          int x;
          int len;
          int weak_keys;
          int weak_values;

          len = SCM_LENGTH (ptr);
          weak_keys = SCM_IS_WHVEC (ptr) || SCM_IS_WHVEC_B (ptr);
          weak_values = SCM_IS_WHVEC_V (ptr) || SCM_IS_WHVEC_B (ptr);
          
          for (x = 0; x < len; ++x)
            {
              SCM alist;
              alist = SCM_VELTS (ptr)[x];
              /* mark everything on the alist
               * except the keys or values, according to weak_values and weak_keys.
               */
              while (   SCM_NIMP (alist)
                     && SCM_CONSP (alist)
                     && !SCM_GCMARKP (alist)
                     && SCM_NIMP (SCM_CAR (alist))
                     && SCM_CONSP (SCM_CAR (alist)))
                {
                  SCM kvpair;
                  SCM next_alist;

                  kvpair = SCM_CAR (alist);
                  next_alist = SCM_CDR (alist);
                  /* 
                   * Do not do this:
                   *    SCM_SETGCMARK (alist);
                   *    SCM_SETGCMARK (kvpair);
                   *
                   * It may be that either the key or value is protected by
                   * an escaped reference to part of the spine of this alist.
                   * If we mark the spine here, and only mark one or neither of the
                   * key and value, they may never be properly marked.
                   * This leads to a horrible situation in which an alist containing
                   * freelist cells is exported.
                   *
                   * So only mark the spines of these arrays last of all marking.
                   * If somebody confuses us by constructing a weak vector
                   * with a circular alist then we are hosed, but at least we
                   * won't prematurely drop table entries.
                   */
                  if (!weak_keys)
                    scm_gc_mark (SCM_CAR (kvpair));
                  if (!weak_values)
                    scm_gc_mark (SCM_GCCDR (kvpair));
                  alist = next_alist;
                }
              if (SCM_NIMP (alist))
                scm_gc_mark (alist);
            }
        }
      break;

    case scm_tc7_msymbol:
      if (SCM_GC8MARKP(ptr))
        break;
      SCM_SETGC8MARK (ptr);
      scm_gc_mark (SCM_SYMBOL_FUNC (ptr));
      ptr = SCM_SYMBOL_PROPS (ptr);
      goto gc_mark_loop;
    case scm_tc7_ssymbol:
      if (SCM_GC8MARKP(ptr))
        break;
      SCM_SETGC8MARK (ptr);
      break;
    case scm_tcs_subrs:
      ptr = (SCM)(scm_heap_org + (((unsigned long)SCM_CAR (ptr)) >> 8));
      goto gc_mark_loop;
    case scm_tc7_port:
      i = SCM_PTOBNUM (ptr);
      if (!(i < scm_numptob))
        goto def;
      if (SCM_GC8MARKP (ptr))
        break;
      SCM_SETGC8MARK (ptr);
      if (SCM_PTAB_ENTRY(ptr))
        scm_gc_mark (SCM_PTAB_ENTRY(ptr)->file_name);
      if (scm_ptobs[i].mark)
        {
          ptr = (scm_ptobs[i].mark) (ptr);
          goto gc_mark_loop;
        }
      else
        return;
      break;
    case scm_tc7_smob:
      if (SCM_GC8MARKP (ptr))
        break;
      SCM_SETGC8MARK (ptr);
      switch SCM_GCTYP16 (ptr)
        { /* should be faster than going through scm_smobs */
        case scm_tc_free_cell:
          /* printf("found free_cell %X ", ptr); fflush(stdout); */
          SCM_SETCDR (ptr, SCM_EOL);
          break;
        case scm_tcs_bignums:
        case scm_tc16_flo:
          break;
        default:
          i = SCM_SMOBNUM (ptr);
          if (!(i < scm_numsmob))
            goto def;
          if (scm_smobs[i].mark)
            {
              ptr = (scm_smobs[i].mark) (ptr);
              goto gc_mark_loop;
            }
          else
            return;
        }
      break;
    default:
    def:scm_wta (ptr, "unknown type in ", "gc_mark");
    }
}


/* Mark a Region Conservatively
 */

void 
scm_mark_locations (x, n)
     SCM_STACKITEM x[];
     scm_sizet n;
{
  register long m = n;
  register int i, j;
  register SCM_CELLPTR ptr;

  while (0 <= --m)
    if SCM_CELLP (*(SCM **) & x[m])
      {
        ptr = (SCM_CELLPTR) SCM2PTR ((*(SCM **) & x[m]));
        i = 0;
        j = scm_n_heap_segs - 1;
        if (   SCM_PTR_LE (scm_heap_table[i].bounds[0], ptr)
            && SCM_PTR_GT (scm_heap_table[j].bounds[1], ptr))
          {
            while (i <= j)
              {
                int seg_id;
                seg_id = -1;
                if (   (i == j)
                    || SCM_PTR_GT (scm_heap_table[i].bounds[1], ptr))
                  seg_id = i;
                else if (SCM_PTR_LE (scm_heap_table[j].bounds[0], ptr))
                  seg_id = j;
                else
                  {
                    int k;
                    k = (i + j) / 2;
                    if (k == i)
                      break;
                    if (SCM_PTR_GT (scm_heap_table[k].bounds[1], ptr))
                      {
                        j = k;
                        ++i;
                        if (SCM_PTR_LE (scm_heap_table[i].bounds[0], ptr))
                          continue;
                        else
                          break;
                      }
                    else if (SCM_PTR_LE (scm_heap_table[k].bounds[0], ptr))
                      {
                        i = k;
                        --j;
                        if (SCM_PTR_GT (scm_heap_table[j].bounds[1], ptr))
                          continue;
                        else
                          break;
                      }
                  }
                if (   !scm_heap_table[seg_id].valid
                    || scm_heap_table[seg_id].valid (ptr,
                                                     &scm_heap_table[seg_id]))
                  scm_gc_mark (*(SCM *) & x[m]);
                break;
              }

          }
      }
}


/* The following is a C predicate which determines if an SCM value can be
   regarded as a pointer to a cell on the heap.  The code is duplicated
   from scm_mark_locations.  */


int
scm_cellp (value)
     SCM value;
{
  register int i, j;
  register SCM_CELLPTR ptr;
  
  if SCM_CELLP (*(SCM **) & value)
    {
      ptr = (SCM_CELLPTR) SCM2PTR ((*(SCM **) & value));
      i = 0;
      j = scm_n_heap_segs - 1;
      if (   SCM_PTR_LE (scm_heap_table[i].bounds[0], ptr)
             && SCM_PTR_GT (scm_heap_table[j].bounds[1], ptr))
        {
          while (i <= j)
            {
              int seg_id;
              seg_id = -1;
              if (   (i == j)
                     || SCM_PTR_GT (scm_heap_table[i].bounds[1], ptr))
                seg_id = i;
              else if (SCM_PTR_LE (scm_heap_table[j].bounds[0], ptr))
                seg_id = j;
              else
                {
                  int k;
                  k = (i + j) / 2;
                  if (k == i)
                    break;
                  if (SCM_PTR_GT (scm_heap_table[k].bounds[1], ptr))
                    {
                      j = k;
                      ++i;
                      if (SCM_PTR_LE (scm_heap_table[i].bounds[0], ptr))
                        continue;
                      else
                        break;
                    }
                  else if (SCM_PTR_LE (scm_heap_table[k].bounds[0], ptr))
                    {
                      i = k;
                      --j;
                      if (SCM_PTR_GT (scm_heap_table[j].bounds[1], ptr))
                        continue;
                      else
                        break;
                    }
                }
              if (   !scm_heap_table[seg_id].valid
                     || scm_heap_table[seg_id].valid (ptr,
                                                      &scm_heap_table[seg_id]))
                return 1;
              break;
            }

        }
    }
  return 0;
}


static void
scm_mark_weak_vector_spines ()
{
  int i;

  for (i = 0; i < scm_n_weak; ++i)
    {
      if (SCM_IS_WHVEC_ANY (scm_weak_vectors[i]))
        {
          SCM *ptr;
          SCM obj;
          int j;
          int n;

          obj = scm_weak_vectors[i];
          ptr = SCM_VELTS (scm_weak_vectors[i]);
          n = SCM_LENGTH (scm_weak_vectors[i]);
          for (j = 0; j < n; ++j)
            {
              SCM alist;

              alist = ptr[j];
              while (   SCM_NIMP (alist)
                     && SCM_CONSP (alist)
                     && !SCM_GCMARKP (alist) 
                     && SCM_NIMP (SCM_CAR (alist))
                     && SCM_CONSP  (SCM_CAR (alist)))
                {
                  SCM_SETGCMARK (alist);
                  SCM_SETGCMARK (SCM_CAR (alist));
                  alist = SCM_GCCDR (alist);
                }
            }
        }
    }
}



void 
scm_gc_sweep ()
{
  register SCM_CELLPTR ptr;
#ifdef SCM_POINTERS_MUNGED
  register SCM scmptr;
#else
#undef scmptr
#define scmptr (SCM)ptr
#endif
  register SCM nfreelist;
  register SCM *hp_freelist;
  register long m;
  register int span;
  long i;
  scm_sizet seg_size;

  m = 0;

  /* Reset all free list pointers.  We'll reconstruct them completely
     while scanning.  */
  for (i = 0; i < scm_n_heap_segs; i++)
    *scm_heap_table[i].freelistp = SCM_EOL;

  for (i = 0; i < scm_n_heap_segs; i++)
    {
      register scm_sizet n = 0;
      register scm_sizet j;

      /* Unmarked cells go onto the front of the freelist this heap
         segment points to.  Rather than updating the real freelist
         pointer as we go along, we accumulate the new head in
         nfreelist.  Then, if it turns out that the entire segment is
         free, we free (i.e., malloc's free) the whole segment, and
         simply don't assign nfreelist back into the real freelist.  */
      hp_freelist = scm_heap_table[i].freelistp;
      nfreelist = *hp_freelist;

      span = scm_heap_table[i].ncells;
      ptr = CELL_UP (scm_heap_table[i].bounds[0]);
      seg_size = CELL_DN (scm_heap_table[i].bounds[1]) - ptr;
      for (j = seg_size + span; j -= span; ptr += span)
        {
#ifdef SCM_POINTERS_MUNGED
          scmptr = PTR2SCM (ptr);
#endif
          switch SCM_TYP7 (scmptr)
            {
            case scm_tcs_cons_gloc:
              if (SCM_GCMARKP (scmptr))
                {
                  if (SCM_CDR (SCM_CAR (scmptr) - 1) == (SCM)1)
                    SCM_SETCDR (SCM_CAR (scmptr) - 1, (SCM) 0);
                  goto cmrkcontinue;
                }
              {
                SCM vcell;
                vcell = SCM_CAR (scmptr) - 1L;

                if ((SCM_CDR (vcell) == 0) || (SCM_CDR (vcell) == 1))
                  {
                    SCM *p = (SCM *) SCM_GCCDR (scmptr);
                    if (((SCM*) vcell)[scm_vtable_index_layout]
                        & SCM_STRUCTF_LIGHT)
                      {
                        SCM layout = ((SCM*)vcell)[scm_vtable_index_layout];
                        m += SCM_LENGTH (layout) / 2;
                        free ((char *) p);
                      }
                    else
                      {
                        m += p[scm_struct_i_n_words] * sizeof (SCM);
                        /* I feel like I'm programming in BCPL here... */
                        free ((char *) p[scm_struct_i_ptr]);
                      }
                  }
              }
              break;
            case scm_tcs_cons_imcar:
            case scm_tcs_cons_nimcar:
            case scm_tcs_closures:
              if (SCM_GCMARKP (scmptr))
                goto cmrkcontinue;
              break;
            case scm_tc7_wvect:
              if (SCM_GC8MARKP (scmptr))
                {
                  goto c8mrkcontinue;
                }
              else
                {
                  m += (1 + SCM_LENGTH (scmptr)) * sizeof (SCM);
                  scm_must_free ((char *)(SCM_VELTS (scmptr) - 1));
                  break;
                }

            case scm_tc7_vector:
            case scm_tc7_lvector:
#ifdef CCLO
            case scm_tc7_cclo:
#endif
              if (SCM_GC8MARKP (scmptr))
                goto c8mrkcontinue;

              m += (SCM_LENGTH (scmptr) * sizeof (SCM));
            freechars:
              scm_must_free (SCM_CHARS (scmptr));
              /*        SCM_SETCHARS(scmptr, 0);*/
              break;
            case scm_tc7_bvect:
              if SCM_GC8MARKP (scmptr)
                goto c8mrkcontinue;
              m += sizeof (long) * ((SCM_HUGE_LENGTH (scmptr) + SCM_LONG_BIT - 1) / SCM_LONG_BIT);
              goto freechars;
            case scm_tc7_byvect:
              if SCM_GC8MARKP (scmptr)
                goto c8mrkcontinue;
              m += SCM_HUGE_LENGTH (scmptr) * sizeof (char);
              goto freechars;
            case scm_tc7_ivect:
            case scm_tc7_uvect:
              if SCM_GC8MARKP (scmptr)
                goto c8mrkcontinue;
              m += SCM_HUGE_LENGTH (scmptr) * sizeof (long);
              goto freechars;
            case scm_tc7_svect:
              if SCM_GC8MARKP (scmptr)
                goto c8mrkcontinue;
              m += SCM_HUGE_LENGTH (scmptr) * sizeof (short);
              goto freechars;
#ifdef LONGLONGS
            case scm_tc7_llvect:
              if SCM_GC8MARKP (scmptr)
                goto c8mrkcontinue;
              m += SCM_HUGE_LENGTH (scmptr) * sizeof (long_long);
              goto freechars;
#endif
            case scm_tc7_fvect:
              if SCM_GC8MARKP (scmptr)
                goto c8mrkcontinue;
              m += SCM_HUGE_LENGTH (scmptr) * sizeof (float);
              goto freechars;
            case scm_tc7_dvect:
              if SCM_GC8MARKP (scmptr)
                goto c8mrkcontinue;
              m += SCM_HUGE_LENGTH (scmptr) * sizeof (double);
              goto freechars;
            case scm_tc7_cvect:
              if SCM_GC8MARKP (scmptr)
                goto c8mrkcontinue;
              m += SCM_HUGE_LENGTH (scmptr) * 2 * sizeof (double);
              goto freechars;
            case scm_tc7_substring:
              if (SCM_GC8MARKP (scmptr))
                goto c8mrkcontinue;
              break;
            case scm_tc7_string:
              if (SCM_GC8MARKP (scmptr))
                goto c8mrkcontinue;
              m += SCM_HUGE_LENGTH (scmptr) + 1;
              goto freechars;
            case scm_tc7_msymbol:
              if (SCM_GC8MARKP (scmptr))
                goto c8mrkcontinue;
              m += (  SCM_LENGTH (scmptr)
                    + 1
                    + sizeof (SCM) * ((SCM *)SCM_CHARS (scmptr) - SCM_SLOTS(scmptr)));
              scm_must_free ((char *)SCM_SLOTS (scmptr));
              break;
            case scm_tc7_contin:
              if SCM_GC8MARKP (scmptr)
                goto c8mrkcontinue;
              m += SCM_LENGTH (scmptr) * sizeof (SCM_STACKITEM) + sizeof (scm_contregs);
              if (SCM_VELTS (scmptr))
                goto freechars;
            case scm_tc7_ssymbol:
              if SCM_GC8MARKP(scmptr)
                goto c8mrkcontinue;
              break;
            case scm_tcs_subrs:
              continue;
            case scm_tc7_port:
              if SCM_GC8MARKP (scmptr)
                goto c8mrkcontinue;
              if SCM_OPENP (scmptr)
                {
                  int k = SCM_PTOBNUM (scmptr);
                  if (!(k < scm_numptob))
                    goto sweeperr;
                  /* Keep "revealed" ports alive.  */
                  if (scm_revealed_count(scmptr) > 0)
                    continue;
                  /* Yes, I really do mean scm_ptobs[k].free */
                  /* rather than ftobs[k].close.  .close */
                  /* is for explicit CLOSE-PORT by user */
                  (scm_ptobs[k].free) (scmptr);
                  SCM_SETSTREAM (scmptr, 0);
                  scm_remove_from_port_table (scmptr);
                  scm_gc_ports_collected++;
                  SCM_SETAND_CAR (scmptr, ~SCM_OPN);
                }
              break;
            case scm_tc7_smob:
              switch SCM_GCTYP16 (scmptr)
                {
                case scm_tc_free_cell:
                  if SCM_GC8MARKP (scmptr)
                    goto c8mrkcontinue;
                  break;
#ifdef SCM_BIGDIG
                case scm_tcs_bignums:
                  if SCM_GC8MARKP (scmptr)
                    goto c8mrkcontinue;
                  m += (SCM_NUMDIGS (scmptr) * SCM_BITSPERDIG / SCM_CHAR_BIT);
                  goto freechars;
#endif /* def SCM_BIGDIG */
                case scm_tc16_flo:
                  if SCM_GC8MARKP (scmptr)
                    goto c8mrkcontinue;
                  switch ((int) (SCM_CAR (scmptr) >> 16))
                    {
                    case (SCM_IMAG_PART | SCM_REAL_PART) >> 16:
                      m += sizeof (double);
                    case SCM_REAL_PART >> 16:
                    case SCM_IMAG_PART >> 16:
                      m += sizeof (double);
                      goto freechars;
                    case 0:
                      break;
                    default:
                      goto sweeperr;
                    }
                  break;
                default:
                  if SCM_GC8MARKP (scmptr)
                    goto c8mrkcontinue;

                  {
                    int k;
                    k = SCM_SMOBNUM (scmptr);
                    if (!(k < scm_numsmob))
                      goto sweeperr;
                    m += (scm_smobs[k].free) ((SCM) scmptr);
                    break;
                  }
                }
              break;
            default:
            sweeperr:scm_wta (scmptr, "unknown type in ", "gc_sweep");
            }
          n += span;
#if 0
          if (SCM_CAR (scmptr) == (SCM) scm_tc_free_cell)
            exit (2);
#endif
          /* Stick the new cell on the front of nfreelist.  It's
             critical that we mark this cell as freed; otherwise, the
             conservative collector might trace it as some other type
             of object.  */
          SCM_SETCAR (scmptr, (SCM) scm_tc_free_cell);
          SCM_SETCDR (scmptr, nfreelist);
          nfreelist = scmptr;

          continue;
        c8mrkcontinue:
          SCM_CLRGC8MARK (scmptr);
          continue;
        cmrkcontinue:
          SCM_CLRGCMARK (scmptr);
        }
#ifdef GC_FREE_SEGMENTS
      if (n == seg_size)
        {
          register long j;

          scm_heap_size -= seg_size;
          free ((char *) scm_heap_table[i].bounds[0]);
          scm_heap_table[i].bounds[0] = 0;
          for (j = i + 1; j < scm_n_heap_segs; j++)
            scm_heap_table[j - 1] = scm_heap_table[j];
          scm_n_heap_segs -= 1;
          i--;          /* We need to scan the segment just moved.  */
        }
      else
#endif /* ifdef GC_FREE_SEGMENTS */
        /* Update the real freelist pointer to point to the head of
           the list of free cells we've built for this segment.  */
        *hp_freelist = nfreelist;

#ifdef DEBUG_FREELIST
      scm_check_freelist ();
      scm_map_free_list ();
#endif

      scm_gc_cells_collected += n;
    }
  /* Scan weak vectors. */
  {
    SCM *ptr;
    for (i = 0; i < scm_n_weak; ++i)
      {
        if (!SCM_IS_WHVEC_ANY (scm_weak_vectors[i]))
          {
            register long j, n;

            ptr = SCM_VELTS (scm_weak_vectors[i]);
            n = SCM_LENGTH (scm_weak_vectors[i]);
            for (j = 0; j < n; ++j)
              if (SCM_NIMP (ptr[j]) && SCM_FREEP (ptr[j]))
                ptr[j] = SCM_BOOL_F;
          }
        else /* if (SCM_IS_WHVEC_ANY (scm_weak_vectors[i])) */
          {
            SCM obj = scm_weak_vectors[i];
            register long n = SCM_LENGTH (scm_weak_vectors[i]);
            register long j;

            ptr = SCM_VELTS (scm_weak_vectors[i]);

            for (j = 0; j < n; ++j)
              {
                SCM * fixup;
                SCM alist;
                int weak_keys;
                int weak_values;
                
                weak_keys = SCM_IS_WHVEC (obj) || SCM_IS_WHVEC_B (obj);
                weak_values = SCM_IS_WHVEC_V (obj) || SCM_IS_WHVEC_B (obj);

                fixup = ptr + j;
                alist = *fixup;

                while (SCM_NIMP (alist)
                       && SCM_CONSP (alist)
                       && SCM_NIMP (SCM_CAR (alist))
                       && SCM_CONSP (SCM_CAR (alist)))
                  {
                    SCM key;
                    SCM value;

                    key = SCM_CAAR (alist);
                    value = SCM_CDAR (alist);
                    if (   (weak_keys && SCM_NIMP (key) && SCM_FREEP (key))
                        || (weak_values && SCM_NIMP (value) && SCM_FREEP (value)))
                      {
                        *fixup = SCM_CDR (alist);
                      }
                    else
                      fixup = SCM_CDRLOC (alist);
                    alist = SCM_CDR (alist);
                  }
              }
          }
      }
  }
  scm_cells_allocated = (scm_heap_size - scm_gc_cells_collected);
  scm_mallocated -= m;
  scm_gc_malloc_collected = m;
}


\f

/* {Front end to malloc}
 *
 * scm_must_malloc, scm_must_realloc, scm_must_free, scm_done_malloc
 *
 * These functions provide services comperable to malloc, realloc, and
 * free.  They are for allocating malloced parts of scheme objects.
 * The primary purpose of the front end is to impose calls to gc.
 */

/* scm_must_malloc
 * Return newly malloced storage or throw an error.
 *
 * The parameter WHAT is a string for error reporting.
 * If the threshold scm_mtrigger will be passed by this 
 * allocation, or if the first call to malloc fails,
 * garbage collect -- on the presumption that some objects
 * using malloced storage may be collected.
 *
 * The limit scm_mtrigger may be raised by this allocation.
 */
char *
scm_must_malloc (len, what)
     scm_sizet len;
     char *what;
{
  char *ptr;
  scm_sizet size = len;
  unsigned long nm = scm_mallocated + size;
  if (len != size)
  malerr:
    scm_wta (SCM_MAKINUM (len), (char *) SCM_NALLOC, what);
  if ((nm <= scm_mtrigger))
    {
      SCM_SYSCALL (ptr = (char *) malloc (size));
      if (NULL != ptr)
        {
          scm_mallocated = nm;
          return ptr;
        }
    }

  scm_igc (what);
  nm = scm_mallocated + size;
  SCM_SYSCALL (ptr = (char *) malloc (size));
  if (NULL != ptr)
    {
      scm_mallocated = nm;
      if (nm > scm_mtrigger - SCM_MTRIGGER_HYSTERESIS) {
        if (nm > scm_mtrigger)
          scm_mtrigger = nm + nm / 2;
        else
          scm_mtrigger += scm_mtrigger / 2;
      }
      return ptr;
    }
  goto malerr;
}


/* scm_must_realloc
 * is similar to scm_must_malloc.
 */
char *
scm_must_realloc (char *where,
                  scm_sizet olen,
                  scm_sizet len,
                  char *what)
{
  char *ptr;
  scm_sizet size = len;
  scm_sizet nm = scm_mallocated + size - olen;
  if (len != size)
  ralerr:
    scm_wta (SCM_MAKINUM (len), (char *) SCM_NALLOC, what);
  if ((nm <= scm_mtrigger))
    {
      SCM_SYSCALL (ptr = (char *) realloc (where, size));
      if (NULL != ptr)
        {
          scm_mallocated = nm;
          return ptr;
        }
    }
  scm_igc (what);
  nm = scm_mallocated + size - olen;
  SCM_SYSCALL (ptr = (char *) realloc (where, size));
  if (NULL != ptr)
    {
      scm_mallocated = nm;
      if (nm > scm_mtrigger - SCM_MTRIGGER_HYSTERESIS) {
        if (nm > scm_mtrigger)
          scm_mtrigger = nm + nm / 2;
        else
          scm_mtrigger += scm_mtrigger / 2;
      }
      return ptr;
    }
  goto ralerr;
}

void 
scm_must_free (obj)
     char *obj;
{
  if (obj)
    free (obj);
  else
    scm_wta (SCM_INUM0, "already free", "");
}

/* Announce that there has been some malloc done that will be freed
 * during gc.  A typical use is for a smob that uses some malloced
 * memory but can not get it from scm_must_malloc (for whatever
 * reason).  When a new object of this smob is created you call
 * scm_done_malloc with the size of the object.  When your smob free
 * function is called, be sure to include this size in the return
 * value. */

void
scm_done_malloc (size)
     long size;
{
  scm_mallocated += size;

  if (scm_mallocated > scm_mtrigger)
    {
      scm_igc ("foreign mallocs");
      if (scm_mallocated > scm_mtrigger - SCM_MTRIGGER_HYSTERESIS)
        {
          if (scm_mallocated > scm_mtrigger)
            scm_mtrigger = scm_mallocated + scm_mallocated / 2;
          else
            scm_mtrigger += scm_mtrigger / 2;
        }
    }
}


\f

/* {Heap Segments}
 *
 * Each heap segment is an array of objects of a particular size.
 * Every segment has an associated (possibly shared) freelist.
 * A table of segment records is kept that records the upper and
 * lower extents of the segment;  this is used during the conservative
 * phase of gc to identify probably gc roots (because they point
 * into valid segments at reasonable offsets).  */

/* scm_expmem
 * is true if the first segment was smaller than INIT_HEAP_SEG.
 * If scm_expmem is set to one, subsequent segment allocations will
 * allocate segments of size SCM_EXPHEAP(scm_heap_size).
 */
int scm_expmem = 0;

/* scm_heap_org
 * is the lowest base address of any heap segment.
 */
SCM_CELLPTR scm_heap_org;

struct scm_heap_seg_data * scm_heap_table = 0;
int scm_n_heap_segs = 0;

/* scm_heap_size
 * is the total number of cells in heap segments.
 */
unsigned long scm_heap_size = 0;

/* init_heap_seg
 * initializes a new heap segment and return the number of objects it contains.
 *
 * The segment origin, segment size in bytes, and the span of objects
 * in cells are input parameters.  The freelist is both input and output.
 *
 * This function presume that the scm_heap_table has already been expanded
 * to accomodate a new segment record.
 */


static scm_sizet 
init_heap_seg (seg_org, size, ncells, freelistp)
     SCM_CELLPTR seg_org;
     scm_sizet size;
     int ncells;
     SCM *freelistp;
{
  register SCM_CELLPTR ptr;
#ifdef SCM_POINTERS_MUNGED
  register SCM scmptr;
#else
#undef scmptr
#define scmptr ptr
#endif
  SCM_CELLPTR seg_end;
  int new_seg_index;
  int n_new_objects;
  
  if (seg_org == NULL)
    return 0;

  ptr = seg_org;

  /* Compute the ceiling on valid object pointers w/in this segment. 
   */
  seg_end = CELL_DN ((char *) ptr + size);

  /* Find the right place and insert the segment record. 
   *
   */
  for (new_seg_index = 0;
       (   (new_seg_index < scm_n_heap_segs)
        && SCM_PTR_LE (scm_heap_table[new_seg_index].bounds[0], seg_org));
       new_seg_index++)
    ;

  {
    int i;
    for (i = scm_n_heap_segs; i > new_seg_index; --i)
      scm_heap_table[i] = scm_heap_table[i - 1];
  }
  
  ++scm_n_heap_segs;

  scm_heap_table[new_seg_index].valid = 0;
  scm_heap_table[new_seg_index].ncells = ncells;
  scm_heap_table[new_seg_index].freelistp = freelistp;
  scm_heap_table[new_seg_index].bounds[0] = (SCM_CELLPTR)ptr;
  scm_heap_table[new_seg_index].bounds[1] = (SCM_CELLPTR)seg_end;


  /* Compute the least valid object pointer w/in this segment 
   */
  ptr = CELL_UP (ptr);


  n_new_objects = seg_end - ptr;

  /* Prepend objects in this segment to the freelist. 
   */
  while (ptr < seg_end)
    {
#ifdef SCM_POINTERS_MUNGED
      scmptr = PTR2SCM (ptr);
#endif
      SCM_SETCAR (scmptr, (SCM) scm_tc_free_cell);
      SCM_SETCDR (scmptr, PTR2SCM (ptr + ncells));
      ptr += ncells;
    }

  ptr -= ncells;

  /* Patch up the last freelist pointer in the segment
   * to join it to the input freelist.
   */
  SCM_SETCDR (PTR2SCM (ptr), *freelistp);
  *freelistp = PTR2SCM (CELL_UP (seg_org));

  scm_heap_size += (ncells * n_new_objects);
  return size;
#ifdef scmptr
#undef scmptr
#endif
}


static void 
alloc_some_heap (ncells, freelistp)
     int ncells;
     SCM * freelistp;
{
  struct scm_heap_seg_data * tmptable;
  SCM_CELLPTR ptr;
  scm_sizet len;

  /* Critical code sections (such as the garbage collector)
   * aren't supposed to add heap segments.
   */
  if (scm_gc_heap_lock)
    scm_wta (SCM_UNDEFINED, "need larger initial", "heap");

  /* Expand the heap tables to have room for the new segment.
   * Do not yet increment scm_n_heap_segs -- that is done by init_heap_seg
   * only if the allocation of the segment itself succeeds.
   */
  len = (1 + scm_n_heap_segs) * sizeof (struct scm_heap_seg_data);

  SCM_SYSCALL (tmptable = ((struct scm_heap_seg_data *)
                       realloc ((char *)scm_heap_table, len)));
  if (!tmptable)
    scm_wta (SCM_UNDEFINED, "could not grow", "hplims");
  else
    scm_heap_table = tmptable;


  /* Pick a size for the new heap segment.
   * The rule for picking the size of a segment is explained in 
   * gc.h
   */
  if (scm_expmem)
    {
      len = (scm_sizet) (SCM_EXPHEAP (scm_heap_size) * sizeof (scm_cell));
      if ((scm_sizet) (SCM_EXPHEAP (scm_heap_size) * sizeof (scm_cell)) != len)
        len = 0;
    }
  else
    len = SCM_HEAP_SEG_SIZE;

  {
    scm_sizet smallest;

    smallest = (ncells * sizeof (scm_cell));
    if (len < smallest)
      len = (ncells * sizeof (scm_cell));

    /* Allocate with decaying ambition. */
    while ((len >= SCM_MIN_HEAP_SEG_SIZE)
           && (len >= smallest))
      {
        SCM_SYSCALL (ptr = (SCM_CELLPTR) malloc (len));
        if (ptr)
          {
            init_heap_seg (ptr, len, ncells, freelistp);
            return;
          }
        len /= 2;
      }
  }

  scm_wta (SCM_UNDEFINED, "could not grow", "heap");
}



SCM_PROC (s_unhash_name, "unhash-name", 1, 0, 0, scm_unhash_name);
SCM
scm_unhash_name (name)
     SCM name;
{
  int x;
  int bound;
  SCM_ASSERT (SCM_NIMP (name) && SCM_SYMBOLP (name), name, SCM_ARG1, s_unhash_name);
  SCM_DEFER_INTS;
  bound = scm_n_heap_segs;
  for (x = 0; x < bound; ++x)
    {
      SCM_CELLPTR p;
      SCM_CELLPTR pbound;
      p  = (SCM_CELLPTR)scm_heap_table[x].bounds[0];
      pbound = (SCM_CELLPTR)scm_heap_table[x].bounds[1];
      while (p < pbound)
        {
          SCM incar;
          incar = p->car;
          if (1 == (7 & (int)incar))
            {
              --incar;
              if (   ((name == SCM_BOOL_T) || (SCM_CAR (incar) == name))
                  && (SCM_CDR (incar) != 0)
                  && (SCM_CDR (incar) != 1))
                {
                  p->car = name;
                }
            }
          ++p;
        }
    }
  SCM_ALLOW_INTS;
  return name;
}


\f
/* {GC Protection Helper Functions}
 */


void
scm_remember (ptr)
     SCM * ptr;
{}


SCM
scm_return_first (SCM elt, ...)
{
  return elt;
}


SCM
scm_permanent_object (obj)
     SCM obj;
{
  SCM_REDEFER_INTS;
  scm_permobjs = scm_cons (obj, scm_permobjs);
  SCM_REALLOW_INTS;
  return obj;
}


/* Protect OBJ from the garbage collector.  OBJ will not be freed,
   even if all other references are dropped, until someone applies
   scm_unprotect_object to it.  This function returns OBJ.

   Calls to scm_protect_object nest.  For every object O, there is a
   counter which scm_protect_object(O) increments and
   scm_unprotect_object(O) decrements, if it is greater than zero.  If
   an object's counter is greater than zero, the garbage collector
   will not free it.

   Of course, that's not how it's implemented.  scm_protect_object and
   scm_unprotect_object just maintain a list of references to things.
   Since the GC knows about this list, all objects it mentions stay
   alive.  scm_protect_object adds its argument to the list;
   scm_unprotect_object removes the first occurrence of its argument
   to the list.  */
SCM
scm_protect_object (obj)
     SCM obj;
{
  scm_protects = scm_cons (obj, scm_protects);

  return obj;
}


/* Remove any protection for OBJ established by a prior call to
   scm_protect_object.  This function returns OBJ.

   See scm_protect_object for more information.  */
SCM
scm_unprotect_object (obj)
     SCM obj;
{
  SCM *tail_ptr = &scm_protects;

  while (SCM_NIMP (*tail_ptr) && SCM_CONSP (*tail_ptr))
    if (SCM_CAR (*tail_ptr) == obj)
      {
        *tail_ptr = SCM_CDR (*tail_ptr);
        break;
      }
    else
      tail_ptr = SCM_CDRLOC (*tail_ptr);

  return obj;
}


\f
int
scm_init_storage (scm_sizet init_heap_size)
{
  scm_sizet j;

  j = SCM_NUM_PROTECTS;
  while (j)
    scm_sys_protects[--j] = SCM_BOOL_F;
  scm_block_gc = 1;
  scm_freelist = SCM_EOL;
  scm_expmem = 0;

  j = SCM_HEAP_SEG_SIZE;
  scm_mtrigger = SCM_INIT_MALLOC_LIMIT;
  scm_heap_table = ((struct scm_heap_seg_data *)
                    scm_must_malloc (sizeof (struct scm_heap_seg_data), "hplims"));
  if (0L == init_heap_size)
    init_heap_size = SCM_INIT_HEAP_SIZE;
  j = init_heap_size;
  if ((init_heap_size != j)
      || !init_heap_seg ((SCM_CELLPTR) malloc (j), j, 1, &scm_freelist))
    {
      j = SCM_HEAP_SEG_SIZE;
      if (!init_heap_seg ((SCM_CELLPTR) malloc (j), j, 1, &scm_freelist))
        return 1;
    }
  else
    scm_expmem = 1;
  scm_heap_org = CELL_UP (scm_heap_table[0].bounds[0]);
  /* scm_hplims[0] can change. do not remove scm_heap_org */
  if (!(scm_weak_vectors = (SCM *) malloc ((scm_weak_size = 32) * sizeof(SCM))))
    return 1;

  /* Initialise the list of ports.  */
  scm_port_table = (struct scm_port_table **) malloc ((long) (sizeof (struct scm_port_table)
                                                              * scm_port_table_room));
  if (!scm_port_table)
    return 1;


  scm_undefineds = scm_cons (SCM_UNDEFINED, SCM_EOL);
  SCM_SETCDR (scm_undefineds, scm_undefineds);

  scm_listofnull = scm_cons (SCM_EOL, SCM_EOL);
  scm_nullstr = scm_makstr (0L, 0);
  scm_nullvect = scm_make_vector (SCM_INUM0, SCM_UNDEFINED);
  scm_symhash = scm_make_vector ((SCM) SCM_MAKINUM (scm_symhash_dim), SCM_EOL);
  scm_weak_symhash = scm_make_weak_key_hash_table ((SCM) SCM_MAKINUM (scm_symhash_dim));
  scm_symhash_vars = scm_make_vector ((SCM) SCM_MAKINUM (scm_symhash_dim), SCM_EOL);
  scm_stand_in_procs = SCM_EOL;
  scm_permobjs = SCM_EOL;
  scm_protects = SCM_EOL;
  scm_asyncs = SCM_EOL;
  scm_sysintern ("most-positive-fixnum", (SCM) SCM_MAKINUM (SCM_MOST_POSITIVE_FIXNUM));
  scm_sysintern ("most-negative-fixnum", (SCM) SCM_MAKINUM (SCM_MOST_NEGATIVE_FIXNUM));
#ifdef SCM_BIGDIG
  scm_sysintern ("bignum-radix", SCM_MAKINUM (SCM_BIGRAD));
#endif
  return 0;
}
\f

void
scm_init_gc ()
{
#include "gc.x"
}