2004-12-24 Ben Maurer <bmaurer@ximian.com>

[mono-project.git] / libgc / mark.c

/*
 * Copyright 1988, 1989 Hans-J. Boehm, Alan J. Demers
 * Copyright (c) 1991-1995 by Xerox Corporation.  All rights reserved.
 * Copyright (c) 2000 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 <stdio.h>
# include "private/gc_pmark.h"

#if defined(MSWIN32) && defined(__GNUC__)
# include <excpt.h>
#endif

/* We put this here to minimize the risk of inlining. */
/*VARARGS*/
#ifdef __WATCOMC__
  void GC_noop(void *p, ...) {}
#else
  void GC_noop() {}
#endif

/* Single argument version, robust against whole program analysis. */
void GC_noop1(x)
word x;
{
    static VOLATILE word sink;

    sink = x;
}

/* mark_proc GC_mark_procs[MAX_MARK_PROCS] = {0} -- declared in gc_priv.h */

word GC_n_mark_procs = GC_RESERVED_MARK_PROCS;

/* Initialize GC_obj_kinds properly and standard free lists properly.   */
/* This must be done statically since they may be accessed before       */
/* GC_init is called.                                                   */
/* It's done here, since we need to deal with mark descriptors.         */
struct obj_kind GC_obj_kinds[MAXOBJKINDS] = {
/* PTRFREE */ { &GC_aobjfreelist[0], 0 /* filled in dynamically */,
                0 | GC_DS_LENGTH, FALSE, FALSE },
/* NORMAL  */ { &GC_objfreelist[0], 0,
                0 | GC_DS_LENGTH,  /* Adjusted in GC_init_inner for EXTRA_BYTES */
                TRUE /* add length to descr */, TRUE },
/* UNCOLLECTABLE */
              { &GC_uobjfreelist[0], 0,
                0 | GC_DS_LENGTH, TRUE /* add length to descr */, TRUE },
# ifdef ATOMIC_UNCOLLECTABLE
   /* AUNCOLLECTABLE */
              { &GC_auobjfreelist[0], 0,
                0 | GC_DS_LENGTH, FALSE /* add length to descr */, FALSE },
# endif
# ifdef STUBBORN_ALLOC
/*STUBBORN*/ { &GC_sobjfreelist[0], 0,
                0 | GC_DS_LENGTH, TRUE /* add length to descr */, TRUE },
# endif
};

# ifdef ATOMIC_UNCOLLECTABLE
#   ifdef STUBBORN_ALLOC
      int GC_n_kinds = 5;
#   else
      int GC_n_kinds = 4;
#   endif
# else
#   ifdef STUBBORN_ALLOC
      int GC_n_kinds = 4;
#   else
      int GC_n_kinds = 3;
#   endif
# endif


# ifndef INITIAL_MARK_STACK_SIZE
#   define INITIAL_MARK_STACK_SIZE (1*HBLKSIZE)
                /* INITIAL_MARK_STACK_SIZE * sizeof(mse) should be a    */
                /* multiple of HBLKSIZE.                                */
                /* The incremental collector actually likes a larger    */
                /* size, since it want to push all marked dirty objs    */
                /* before marking anything new.  Currently we let it    */
                /* grow dynamically.                                    */
# endif

/*
 * Limits of stack for GC_mark routine.
 * All ranges between GC_mark_stack(incl.) and GC_mark_stack_top(incl.) still
 * need to be marked from.
 */

word GC_n_rescuing_pages;       /* Number of dirty pages we marked from */
                                /* excludes ptrfree pages, etc.         */

mse * GC_mark_stack;

mse * GC_mark_stack_limit;

word GC_mark_stack_size = 0;
 
#ifdef PARALLEL_MARK
  mse * VOLATILE GC_mark_stack_top;
#else
  mse * GC_mark_stack_top;
#endif

static struct hblk * scan_ptr;

mark_state_t GC_mark_state = MS_NONE;

GC_bool GC_mark_stack_too_small = FALSE;

GC_bool GC_objects_are_marked = FALSE;  /* Are there collectable marked */
                                        /* objects in the heap?         */

/* Is a collection in progress?  Note that this can return true in the  */
/* nonincremental case, if a collection has been abandoned and the      */
/* mark state is now MS_INVALID.                                        */
GC_bool GC_collection_in_progress()
{
    return(GC_mark_state != MS_NONE);
}

/* clear all mark bits in the header */
void GC_clear_hdr_marks(hhdr)
register hdr * hhdr;
{
#   ifdef USE_MARK_BYTES
      BZERO(hhdr -> hb_marks, MARK_BITS_SZ);
#   else
      BZERO(hhdr -> hb_marks, MARK_BITS_SZ*sizeof(word));
#   endif
}

/* Set all mark bits in the header.  Used for uncollectable blocks. */
void GC_set_hdr_marks(hhdr)
register hdr * hhdr;
{
    register int i;

    for (i = 0; i < MARK_BITS_SZ; ++i) {
#     ifdef USE_MARK_BYTES
        hhdr -> hb_marks[i] = 1;
#     else
        hhdr -> hb_marks[i] = ONES;
#     endif
    }
}

/*
 * Clear all mark bits associated with block h.
 */
/*ARGSUSED*/
# if defined(__STDC__) || defined(__cplusplus)
    static void clear_marks_for_block(struct hblk *h, word dummy)
# else
    static void clear_marks_for_block(h, dummy)
    struct hblk *h;
    word dummy;
# endif
{
    register hdr * hhdr = HDR(h);
    
    if (IS_UNCOLLECTABLE(hhdr -> hb_obj_kind)) return;
        /* Mark bit for these is cleared only once the object is        */
        /* explicitly deallocated.  This either frees the block, or     */
        /* the bit is cleared once the object is on the free list.      */
    GC_clear_hdr_marks(hhdr);
}

/* Slow but general routines for setting/clearing/asking about mark bits */
void GC_set_mark_bit(p)
ptr_t p;
{
    register struct hblk *h = HBLKPTR(p);
    register hdr * hhdr = HDR(h);
    register int word_no = (word *)p - (word *)h;
    
    set_mark_bit_from_hdr(hhdr, word_no);
}

void GC_clear_mark_bit(p)
ptr_t p;
{
    register struct hblk *h = HBLKPTR(p);
    register hdr * hhdr = HDR(h);
    register int word_no = (word *)p - (word *)h;
    
    clear_mark_bit_from_hdr(hhdr, word_no);
}

GC_bool GC_is_marked(p)
ptr_t p;
{
    register struct hblk *h = HBLKPTR(p);
    register hdr * hhdr = HDR(h);
    register int word_no = (word *)p - (word *)h;
    
    return(mark_bit_from_hdr(hhdr, word_no));
}


/*
 * Clear mark bits in all allocated heap blocks.  This invalidates
 * the marker invariant, and sets GC_mark_state to reflect this.
 * (This implicitly starts marking to reestablish the invariant.)
 */
void GC_clear_marks()
{
    GC_apply_to_all_blocks(clear_marks_for_block, (word)0);
    GC_objects_are_marked = FALSE;
    GC_mark_state = MS_INVALID;
    scan_ptr = 0;
#   ifdef GATHERSTATS
        /* Counters reflect currently marked objects: reset here */
        GC_composite_in_use = 0;
        GC_atomic_in_use = 0;
#   endif

}

/* Initiate a garbage collection.  Initiates a full collection if the   */
/* mark state is invalid.                                               */
/*ARGSUSED*/
void GC_initiate_gc()
{
    if (GC_dirty_maintained) GC_read_dirty();
#   ifdef STUBBORN_ALLOC
        GC_read_changed();
#   endif
#   ifdef CHECKSUMS
        {
            extern void GC_check_dirty();
            
            if (GC_dirty_maintained) GC_check_dirty();
        }
#   endif
    GC_n_rescuing_pages = 0;
    if (GC_mark_state == MS_NONE) {
        GC_mark_state = MS_PUSH_RESCUERS;
    } else if (GC_mark_state != MS_INVALID) {
        ABORT("unexpected state");
    } /* else this is really a full collection, and mark        */
      /* bits are invalid.                                      */
    scan_ptr = 0;
}


static void alloc_mark_stack();

/* Perform a small amount of marking.                   */
/* We try to touch roughly a page of memory.            */
/* Return TRUE if we just finished a mark phase.        */
/* Cold_gc_frame is an address inside a GC frame that   */
/* remains valid until all marking is complete.         */
/* A zero value indicates that it's OK to miss some     */
/* register values.                                     */
/* We hold the allocation lock.  In the case of         */
/* incremental collection, the world may not be stopped.*/
#ifdef MSWIN32
  /* For win32, this is called after we establish a structured  */
  /* exception handler, in case Windows unmaps one of our root  */
  /* segments.  See below.  In either case, we acquire the      */
  /* allocator lock long before we get here.                    */
  GC_bool GC_mark_some_inner(cold_gc_frame)
  ptr_t cold_gc_frame;
#else
  GC_bool GC_mark_some(cold_gc_frame)
  ptr_t cold_gc_frame;
#endif
{
    switch(GC_mark_state) {
        case MS_NONE:
            return(FALSE);
            
        case MS_PUSH_RESCUERS:
            if (GC_mark_stack_top
                >= GC_mark_stack_limit - INITIAL_MARK_STACK_SIZE/2) {
                /* Go ahead and mark, even though that might cause us to */
                /* see more marked dirty objects later on.  Avoid this   */
                /* in the future.                                        */
                GC_mark_stack_too_small = TRUE;
                MARK_FROM_MARK_STACK();
                return(FALSE);
            } else {
                scan_ptr = GC_push_next_marked_dirty(scan_ptr);
                if (scan_ptr == 0) {
#                   ifdef CONDPRINT
                      if (GC_print_stats) {
                        GC_printf1("Marked from %lu dirty pages\n",
                                   (unsigned long)GC_n_rescuing_pages);
                      }
#                   endif
                    GC_push_roots(FALSE, cold_gc_frame);
                    GC_objects_are_marked = TRUE;
                    if (GC_mark_state != MS_INVALID) {
                        GC_mark_state = MS_ROOTS_PUSHED;
                    }
                }
            }
            return(FALSE);
        
        case MS_PUSH_UNCOLLECTABLE:
            if (GC_mark_stack_top
                >= GC_mark_stack + GC_mark_stack_size/4) {
#               ifdef PARALLEL_MARK
                  /* Avoid this, since we don't parallelize the marker  */
                  /* here.                                              */
                  if (GC_parallel) GC_mark_stack_too_small = TRUE;
#               endif
                MARK_FROM_MARK_STACK();
                return(FALSE);
            } else {
                scan_ptr = GC_push_next_marked_uncollectable(scan_ptr);
                if (scan_ptr == 0) {
                    GC_push_roots(TRUE, cold_gc_frame);
                    GC_objects_are_marked = TRUE;
                    if (GC_mark_state != MS_INVALID) {
                        GC_mark_state = MS_ROOTS_PUSHED;
                    }
                }
            }
            return(FALSE);
        
        case MS_ROOTS_PUSHED:
#           ifdef PARALLEL_MARK
              /* In the incremental GC case, this currently doesn't     */
              /* quite do the right thing, since it runs to             */
              /* completion.  On the other hand, starting a             */
              /* parallel marker is expensive, so perhaps it is         */
              /* the right thing?                                       */
              /* Eventually, incremental marking should run             */
              /* asynchronously in multiple threads, without grabbing   */
              /* the allocation lock.                                   */
                if (GC_parallel) {
                  GC_do_parallel_mark();
                  GC_ASSERT(GC_mark_stack_top < GC_first_nonempty);
                  GC_mark_stack_top = GC_mark_stack - 1;
                  if (GC_mark_stack_too_small) {
                    alloc_mark_stack(2*GC_mark_stack_size);
                  }
                  if (GC_mark_state == MS_ROOTS_PUSHED) {
                    GC_mark_state = MS_NONE;
                    return(TRUE);
                  } else {
                    return(FALSE);
                  }
                }
#           endif
            if (GC_mark_stack_top >= GC_mark_stack) {
                MARK_FROM_MARK_STACK();
                return(FALSE);
            } else {
                GC_mark_state = MS_NONE;
                if (GC_mark_stack_too_small) {
                    alloc_mark_stack(2*GC_mark_stack_size);
                }
                return(TRUE);
            }
            
        case MS_INVALID:
        case MS_PARTIALLY_INVALID:
            if (!GC_objects_are_marked) {
                GC_mark_state = MS_PUSH_UNCOLLECTABLE;
                return(FALSE);
            }
            if (GC_mark_stack_top >= GC_mark_stack) {
                MARK_FROM_MARK_STACK();
                return(FALSE);
            }
            if (scan_ptr == 0 && GC_mark_state == MS_INVALID) {
                /* About to start a heap scan for marked objects. */
                /* Mark stack is empty.  OK to reallocate.        */
                if (GC_mark_stack_too_small) {
                    alloc_mark_stack(2*GC_mark_stack_size);
                }
                GC_mark_state = MS_PARTIALLY_INVALID;
            }
            scan_ptr = GC_push_next_marked(scan_ptr);
            if (scan_ptr == 0 && GC_mark_state == MS_PARTIALLY_INVALID) {
                GC_push_roots(TRUE, cold_gc_frame);
                GC_objects_are_marked = TRUE;
                if (GC_mark_state != MS_INVALID) {
                    GC_mark_state = MS_ROOTS_PUSHED;
                }
            }
            return(FALSE);
        default:
            ABORT("GC_mark_some: bad state");
            return(FALSE);
    }
}


#ifdef MSWIN32

# ifdef __GNUC__

    typedef struct {
      EXCEPTION_REGISTRATION ex_reg;
      void *alt_path;
    } ext_ex_regn;


    static EXCEPTION_DISPOSITION mark_ex_handler(
        struct _EXCEPTION_RECORD *ex_rec, 
        void *est_frame,
        struct _CONTEXT *context,
        void *disp_ctxt)
    {
        if (ex_rec->ExceptionCode == STATUS_ACCESS_VIOLATION) {
          ext_ex_regn *xer = (ext_ex_regn *)est_frame;

          /* Unwind from the inner function assuming the standard */
          /* function prologue.                                   */
          /* Assumes code has not been compiled with              */
          /* -fomit-frame-pointer.                                */
          context->Esp = context->Ebp;
          context->Ebp = *((DWORD *)context->Esp);
          context->Esp = context->Esp - 8;

          /* Resume execution at the "real" handler within the    */
          /* wrapper function.                                    */
          context->Eip = (DWORD )(xer->alt_path);

          return ExceptionContinueExecution;

        } else {
            return ExceptionContinueSearch;
        }
    }
# endif /* __GNUC__ */


  GC_bool GC_mark_some(cold_gc_frame)
  ptr_t cold_gc_frame;
  {
      GC_bool ret_val;

#   ifndef __GNUC__
      /* Windows 98 appears to asynchronously create and remove  */
      /* writable memory mappings, for reasons we haven't yet    */
      /* understood.  Since we look for writable regions to      */
      /* determine the root set, we may try to mark from an      */
      /* address range that disappeared since we started the     */
      /* collection.  Thus we have to recover from faults here.  */
      /* This code does not appear to be necessary for Windows   */
      /* 95/NT/2000. Note that this code should never generate   */
      /* an incremental GC write fault.                          */

      __try {

#   else /* __GNUC__ */

      /* Manually install an exception handler since GCC does    */
      /* not yet support Structured Exception Handling (SEH) on  */
      /* Win32.                                                  */

      ext_ex_regn er;

      er.alt_path = &&handle_ex;
      er.ex_reg.handler = mark_ex_handler;
      asm volatile ("movl %%fs:0, %0" : "=r" (er.ex_reg.prev));
      asm volatile ("movl %0, %%fs:0" : : "r" (&er));

#   endif /* __GNUC__ */

          ret_val = GC_mark_some_inner(cold_gc_frame);

#   ifndef __GNUC__

      } __except (GetExceptionCode() == EXCEPTION_ACCESS_VIOLATION ?
                EXCEPTION_EXECUTE_HANDLER : EXCEPTION_CONTINUE_SEARCH) {

#   else /* __GNUC__ */

          /* Prevent GCC from considering the following code unreachable */
          /* and thus eliminating it.                                    */
          if (er.alt_path != 0)
              goto rm_handler;

handle_ex:
          /* Execution resumes from here on an access violation. */

#   endif /* __GNUC__ */

#         ifdef CONDPRINT
            if (GC_print_stats) {
              GC_printf0("Caught ACCESS_VIOLATION in marker. "
                         "Memory mapping disappeared.\n");
            }
#         endif /* CONDPRINT */

          /* We have bad roots on the stack.  Discard mark stack.  */
          /* Rescan from marked objects.  Redetermine roots.     */
          GC_invalidate_mark_state();   
          scan_ptr = 0;

          ret_val = FALSE;

#   ifndef __GNUC__

      }

#   else /* __GNUC__ */

rm_handler:
      /* Uninstall the exception handler */
      asm volatile ("mov %0, %%fs:0" : : "r" (er.ex_reg.prev));

#   endif /* __GNUC__ */

      return ret_val;
  }
#endif /* MSWIN32 */


GC_bool GC_mark_stack_empty()
{
    return(GC_mark_stack_top < GC_mark_stack);
}       

#ifdef PROF_MARKER
    word GC_prof_array[10];
#   define PROF(n) GC_prof_array[n]++
#else
#   define PROF(n)
#endif

/* Given a pointer to someplace other than a small object page or the   */
/* first page of a large object, either:                                */
/*      - return a pointer to somewhere in the first page of the large  */
/*        object, if current points to a large object.                  */
/*        In this case *hhdr is replaced with a pointer to the header   */
/*        for the large object.                                         */
/*      - just return current if it does not point to a large object.   */
/*ARGSUSED*/
ptr_t GC_find_start(current, hhdr, new_hdr_p)
register ptr_t current;
register hdr *hhdr, **new_hdr_p;
{
    if (GC_all_interior_pointers) {
        if (hhdr != 0) {
            register ptr_t orig = current;
            
            current = (ptr_t)HBLKPTR(current);
            do {
              current = current - HBLKSIZE*(word)hhdr;
              hhdr = HDR(current);
            } while(IS_FORWARDING_ADDR_OR_NIL(hhdr));
            /* current points to near the start of the large object */
            if (hhdr -> hb_flags & IGNORE_OFF_PAGE) return(orig);
            if ((word *)orig - (word *)current
                 >= (ptrdiff_t)(hhdr->hb_sz)) {
                /* Pointer past the end of the block */
                return(orig);
            }
            *new_hdr_p = hhdr;
            return(current);
        } else {
            return(current);
        }
    } else {
        return(current);
    }
}

void GC_invalidate_mark_state()
{
    GC_mark_state = MS_INVALID;
    GC_mark_stack_top = GC_mark_stack-1;
}

mse * GC_signal_mark_stack_overflow(msp)
mse * msp;
{
    GC_mark_state = MS_INVALID;
    GC_mark_stack_too_small = TRUE;
#   ifdef CONDPRINT
      if (GC_print_stats) {
        GC_printf1("Mark stack overflow; current size = %lu entries\n",
                    GC_mark_stack_size);
      }
#   endif
    return(msp - GC_MARK_STACK_DISCARDS);
}

/*
 * Mark objects pointed to by the regions described by
 * mark stack entries between GC_mark_stack and GC_mark_stack_top,
 * inclusive.  Assumes the upper limit of a mark stack entry
 * is never 0.  A mark stack entry never has size 0.
 * We try to traverse on the order of a hblk of memory before we return.
 * Caller is responsible for calling this until the mark stack is empty.
 * Note that this is the most performance critical routine in the
 * collector.  Hence it contains all sorts of ugly hacks to speed
 * things up.  In particular, we avoid procedure calls on the common
 * path, we take advantage of peculiarities of the mark descriptor
 * encoding, we optionally maintain a cache for the block address to
 * header mapping, we prefetch when an object is "grayed", etc. 
 */
mse * GC_mark_from(mark_stack_top, mark_stack, mark_stack_limit)
mse * mark_stack_top;
mse * mark_stack;
mse * mark_stack_limit;
{
  int credit = HBLKSIZE;        /* Remaining credit for marking work    */
  register word * current_p;    /* Pointer to current candidate ptr.    */
  register word current;        /* Candidate pointer.                   */
  register word * limit;        /* (Incl) limit of current candidate    */
                                /* range                                */
  register word descr;
  register ptr_t greatest_ha = GC_greatest_plausible_heap_addr;
  register ptr_t least_ha = GC_least_plausible_heap_addr;
  DECLARE_HDR_CACHE;

# define SPLIT_RANGE_WORDS 128  /* Must be power of 2.          */

  GC_objects_are_marked = TRUE;
  INIT_HDR_CACHE;
# ifdef OS2 /* Use untweaked version to circumvent compiler problem */
  while (mark_stack_top >= mark_stack && credit >= 0) {
# else
  while ((((ptr_t)mark_stack_top - (ptr_t)mark_stack) | credit)
        >= 0) {
# endif
    current_p = mark_stack_top -> mse_start;
    descr = mark_stack_top -> mse_descr;
  retry:
    /* current_p and descr describe the current object.         */
    /* *mark_stack_top is vacant.                               */
    /* The following is 0 only for small objects described by a simple  */
    /* length descriptor.  For many applications this is the common     */
    /* case, so we try to detect it quickly.                            */
    if (descr & ((~(WORDS_TO_BYTES(SPLIT_RANGE_WORDS) - 1)) | GC_DS_TAGS)) {
      word tag = descr & GC_DS_TAGS;
      
      switch(tag) {
        case GC_DS_LENGTH:
          /* Large length.                                              */
          /* Process part of the range to avoid pushing too much on the */
          /* stack.                                                     */
          GC_ASSERT(descr < (word)GC_greatest_plausible_heap_addr
                            - (word)GC_least_plausible_heap_addr);
#         ifdef PARALLEL_MARK
#           define SHARE_BYTES 2048
            if (descr > SHARE_BYTES && GC_parallel
                && mark_stack_top < mark_stack_limit - 1) {
              int new_size = (descr/2) & ~(sizeof(word)-1);
              mark_stack_top -> mse_start = current_p;
              mark_stack_top -> mse_descr = new_size + sizeof(word);
                                        /* makes sure we handle         */
                                        /* misaligned pointers.         */
              mark_stack_top++;
              current_p = (word *) ((char *)current_p + new_size);
              descr -= new_size;
              goto retry;
            }
#         endif /* PARALLEL_MARK */
          mark_stack_top -> mse_start =
                limit = current_p + SPLIT_RANGE_WORDS-1;
          mark_stack_top -> mse_descr =
                        descr - WORDS_TO_BYTES(SPLIT_RANGE_WORDS-1);
          /* Make sure that pointers overlapping the two ranges are     */
          /* considered.                                                */
          limit = (word *)((char *)limit + sizeof(word) - ALIGNMENT);
          break;
        case GC_DS_BITMAP:
          mark_stack_top--;
          descr &= ~GC_DS_TAGS;
          credit -= WORDS_TO_BYTES(WORDSZ/2); /* guess */
          while (descr != 0) {
            if ((signed_word)descr < 0) {
              current = *current_p;
              FIXUP_POINTER(current);
              if ((ptr_t)current >= least_ha && (ptr_t)current < greatest_ha) {
                PREFETCH((ptr_t)current);
                HC_PUSH_CONTENTS((ptr_t)current, mark_stack_top,
                              mark_stack_limit, current_p, exit1);
              }
            }
            descr <<= 1;
            ++ current_p;
          }
          continue;
        case GC_DS_PROC:
          mark_stack_top--;
          credit -= GC_PROC_BYTES;
          mark_stack_top =
              (*PROC(descr))
                    (current_p, mark_stack_top,
                    mark_stack_limit, ENV(descr));
          continue;
        case GC_DS_PER_OBJECT:
          if ((signed_word)descr >= 0) {
            /* Descriptor is in the object.     */
            descr = *(word *)((ptr_t)current_p + descr - GC_DS_PER_OBJECT);
          } else {
            /* Descriptor is in type descriptor pointed to by first     */
            /* word in object.                                          */
            ptr_t type_descr = *(ptr_t *)current_p;
            /* type_descr is either a valid pointer to the descriptor   */
            /* structure, or this object was on a free list.  If it     */
            /* it was anything but the last object on the free list,    */
            /* we will misinterpret the next object on the free list as */
            /* the type descriptor, and get a 0 GC descriptor, which    */
            /* is ideal.  Unfortunately, we need to check for the last  */
            /* object case explicitly.                                  */
            if (0 == type_descr) {
                /* Rarely executed.     */
                mark_stack_top--;
                continue;
            }
            descr = *(word *)(type_descr
                              - (descr - (GC_DS_PER_OBJECT
                                          - GC_INDIR_PER_OBJ_BIAS)));
          }
          if (0 == descr) {
              /* Can happen either because we generated a 0 descriptor  */
              /* or we saw a pointer to a free object.                  */
              mark_stack_top--;
              continue;
          }
          goto retry;
      }
    } else /* Small object with length descriptor */ {
      mark_stack_top--;
      limit = (word *)(((ptr_t)current_p) + (word)descr);
    }
    /* The simple case in which we're scanning a range. */
    GC_ASSERT(!((word)current_p & (ALIGNMENT-1)));
    credit -= (ptr_t)limit - (ptr_t)current_p;
    limit -= 1;
    {
#     define PREF_DIST 4

#     ifndef SMALL_CONFIG
        word deferred;

        /* Try to prefetch the next pointer to be examined asap.        */
        /* Empirically, this also seems to help slightly without        */
        /* prefetches, at least on linux/X86.  Presumably this loop     */
        /* ends up with less register pressure, and gcc thus ends up    */
        /* generating slightly better code.  Overall gcc code quality   */
        /* for this loop is still not great.                            */
        for(;;) {
          PREFETCH((ptr_t)limit - PREF_DIST*CACHE_LINE_SIZE);
          GC_ASSERT(limit >= current_p);
          deferred = *limit;
          FIXUP_POINTER(deferred);
          limit = (word *)((char *)limit - ALIGNMENT);
          if ((ptr_t)deferred >= least_ha && (ptr_t)deferred <  greatest_ha) {
            PREFETCH((ptr_t)deferred);
            break;
          }
          if (current_p > limit) goto next_object;
          /* Unroll once, so we don't do too many of the prefetches     */
          /* based on limit.                                            */
          deferred = *limit;
          FIXUP_POINTER(deferred);
          limit = (word *)((char *)limit - ALIGNMENT);
          if ((ptr_t)deferred >= least_ha && (ptr_t)deferred <  greatest_ha) {
            PREFETCH((ptr_t)deferred);
            break;
          }
          if (current_p > limit) goto next_object;
        }
#     endif

      while (current_p <= limit) {
        /* Empirically, unrolling this loop doesn't help a lot. */
        /* Since HC_PUSH_CONTENTS expands to a lot of code,     */
        /* we don't.                                            */
        current = *current_p;
        FIXUP_POINTER(current);
        PREFETCH((ptr_t)current_p + PREF_DIST*CACHE_LINE_SIZE);
        if ((ptr_t)current >= least_ha && (ptr_t)current <  greatest_ha) {
          /* Prefetch the contents of the object we just pushed.  It's  */
          /* likely we will need them soon.                             */
          PREFETCH((ptr_t)current);
          HC_PUSH_CONTENTS((ptr_t)current, mark_stack_top,
                           mark_stack_limit, current_p, exit2);
        }
        current_p = (word *)((char *)current_p + ALIGNMENT);
      }

#     ifndef SMALL_CONFIG
        /* We still need to mark the entry we previously prefetched.    */
        /* We alrady know that it passes the preliminary pointer        */
        /* validity test.                                               */
        HC_PUSH_CONTENTS((ptr_t)deferred, mark_stack_top,
                         mark_stack_limit, current_p, exit4);
        next_object:;
#     endif
    }
  }
  return mark_stack_top;
}

#ifdef PARALLEL_MARK

/* We assume we have an ANSI C Compiler.        */
GC_bool GC_help_wanted = FALSE;
unsigned GC_helper_count = 0;
unsigned GC_active_count = 0;
mse * VOLATILE GC_first_nonempty;
word GC_mark_no = 0;

#define LOCAL_MARK_STACK_SIZE HBLKSIZE
        /* Under normal circumstances, this is big enough to guarantee  */
        /* We don't overflow half of it in a single call to             */
        /* GC_mark_from.                                                */


/* Steal mark stack entries starting at mse low into mark stack local   */
/* until we either steal mse high, or we have max entries.              */
/* Return a pointer to the top of the local mark stack.                 */
/* *next is replaced by a pointer to the next unscanned mark stack      */
/* entry.                                                               */
mse * GC_steal_mark_stack(mse * low, mse * high, mse * local,
                          unsigned max, mse **next)
{
    mse *p;
    mse *top = local - 1;
    unsigned i = 0;

    /* Make sure that prior writes to the mark stack are visible. */
    /* On some architectures, the fact that the reads are         */
    /* volatile should suffice.                                   */
#   if !defined(IA64) && !defined(HP_PA) && !defined(I386)
      GC_memory_barrier();
#   endif
    GC_ASSERT(high >= low-1 && high - low + 1 <= GC_mark_stack_size);
    for (p = low; p <= high && i <= max; ++p) {
        word descr = *(volatile word *) &(p -> mse_descr);
        /* In the IA64 memory model, the following volatile store is    */
        /* ordered after this read of descr.  Thus a thread must read   */
        /* the original nonzero value.  HP_PA appears to be similar,    */
        /* and if I'm reading the P4 spec correctly, X86 is probably    */
        /* also OK.  In some other cases we need a barrier.             */
#       if !defined(IA64) && !defined(HP_PA) && !defined(I386)
          GC_memory_barrier();
#       endif
        if (descr != 0) {
            *(volatile word *) &(p -> mse_descr) = 0;
            /* More than one thread may get this entry, but that's only */
            /* a minor performance problem.                             */
            ++top;
            top -> mse_descr = descr;
            top -> mse_start = p -> mse_start;
            GC_ASSERT(  top -> mse_descr & GC_DS_TAGS != GC_DS_LENGTH || 
                        top -> mse_descr < GC_greatest_plausible_heap_addr
                                           - GC_least_plausible_heap_addr);
            /* If this is a big object, count it as                     */
            /* size/256 + 1 objects.                                    */
            ++i;
            if ((descr & GC_DS_TAGS) == GC_DS_LENGTH) i += (descr >> 8);
        }
    }
    *next = p;
    return top;
}

/* Copy back a local mark stack.        */
/* low and high are inclusive bounds.   */
void GC_return_mark_stack(mse * low, mse * high)
{
    mse * my_top;
    mse * my_start;
    size_t stack_size;

    if (high < low) return;
    stack_size = high - low + 1;
    GC_acquire_mark_lock();
    my_top = GC_mark_stack_top;
    my_start = my_top + 1;
    if (my_start - GC_mark_stack + stack_size > GC_mark_stack_size) {
#     ifdef CONDPRINT
        if (GC_print_stats) {
          GC_printf0("No room to copy back mark stack.");
        }
#     endif
      GC_mark_state = MS_INVALID;
      GC_mark_stack_too_small = TRUE;
      /* We drop the local mark stack.  We'll fix things later. */
    } else {
      BCOPY(low, my_start, stack_size * sizeof(mse));
      GC_ASSERT(GC_mark_stack_top = my_top);
#     if !defined(IA64) && !defined(HP_PA)
        GC_memory_barrier();
#     endif
        /* On IA64, the volatile write acts as a release barrier. */
      GC_mark_stack_top = my_top + stack_size;
    }
    GC_release_mark_lock();
    GC_notify_all_marker();
}

/* Mark from the local mark stack.              */
/* On return, the local mark stack is empty.    */
/* But this may be achieved by copying the      */
/* local mark stack back into the global one.   */
void GC_do_local_mark(mse *local_mark_stack, mse *local_top)
{
    unsigned n;
#   define N_LOCAL_ITERS 1

#   ifdef GC_ASSERTIONS
      /* Make sure we don't hold mark lock. */
        GC_acquire_mark_lock();
        GC_release_mark_lock();
#   endif
    for (;;) {
        for (n = 0; n < N_LOCAL_ITERS; ++n) {
            local_top = GC_mark_from(local_top, local_mark_stack,
                                     local_mark_stack + LOCAL_MARK_STACK_SIZE);
            if (local_top < local_mark_stack) return;
            if (local_top - local_mark_stack >= LOCAL_MARK_STACK_SIZE/2) {
                GC_return_mark_stack(local_mark_stack, local_top);
                return;
            }
        }
        if (GC_mark_stack_top < GC_first_nonempty &&
            GC_active_count < GC_helper_count
            && local_top > local_mark_stack + 1) {
            /* Try to share the load, since the main stack is empty,    */
            /* and helper threads are waiting for a refill.             */
            /* The entries near the bottom of the stack are likely      */
            /* to require more work.  Thus we return those, eventhough  */
            /* it's harder.                                             */
            mse * p;
            mse * new_bottom = local_mark_stack
                                + (local_top - local_mark_stack)/2;
            GC_ASSERT(new_bottom > local_mark_stack
                      && new_bottom < local_top);
            GC_return_mark_stack(local_mark_stack, new_bottom - 1);
            memmove(local_mark_stack, new_bottom,
                    (local_top - new_bottom + 1) * sizeof(mse));
            local_top -= (new_bottom - local_mark_stack);
        }
    }
}

#define ENTRIES_TO_GET 5

long GC_markers = 2;            /* Normally changed by thread-library-  */
                                /* -specific code.                      */

/* Mark using the local mark stack until the global mark stack is empty */
/* and there are no active workers. Update GC_first_nonempty to reflect */
/* progress.                                                            */
/* Caller does not hold mark lock.                                      */
/* Caller has already incremented GC_helper_count.  We decrement it,    */
/* and maintain GC_active_count.                                        */
void GC_mark_local(mse *local_mark_stack, int id)
{
    mse * my_first_nonempty;

    GC_acquire_mark_lock();
    GC_active_count++;
    my_first_nonempty = GC_first_nonempty;
    GC_ASSERT(GC_first_nonempty >= GC_mark_stack && 
              GC_first_nonempty <= GC_mark_stack_top + 1);
#   ifdef PRINTSTATS
        GC_printf1("Starting mark helper %lu\n", (unsigned long)id);
#   endif
    GC_release_mark_lock();
    for (;;) {
        size_t n_on_stack;
        size_t n_to_get;
        mse *next;
        mse * my_top;
        mse * local_top;
        mse * global_first_nonempty = GC_first_nonempty;

        GC_ASSERT(my_first_nonempty >= GC_mark_stack && 
                  my_first_nonempty <= GC_mark_stack_top + 1);
        GC_ASSERT(global_first_nonempty >= GC_mark_stack && 
                  global_first_nonempty <= GC_mark_stack_top + 1);
        if (my_first_nonempty < global_first_nonempty) {
            my_first_nonempty = global_first_nonempty;
        } else if (global_first_nonempty < my_first_nonempty) {
            GC_compare_and_exchange((word *)(&GC_first_nonempty), 
                                   (word) global_first_nonempty,
                                   (word) my_first_nonempty);
            /* If this fails, we just go ahead, without updating        */
            /* GC_first_nonempty.                                       */
        }
        /* Perhaps we should also update GC_first_nonempty, if it */
        /* is less.  But that would require using atomic updates. */
        my_top = GC_mark_stack_top;
        n_on_stack = my_top - my_first_nonempty + 1;
        if (0 == n_on_stack) {
            GC_acquire_mark_lock();
            my_top = GC_mark_stack_top;
            n_on_stack = my_top - my_first_nonempty + 1;
            if (0 == n_on_stack) {
                GC_active_count--;
                GC_ASSERT(GC_active_count <= GC_helper_count);
                /* Other markers may redeposit objects  */
                /* on the stack.                                */
                if (0 == GC_active_count) GC_notify_all_marker();
                while (GC_active_count > 0
                       && GC_first_nonempty > GC_mark_stack_top) {
                    /* We will be notified if either GC_active_count    */
                    /* reaches zero, or if more objects are pushed on   */
                    /* the global mark stack.                           */
                    GC_wait_marker();
                }
                if (GC_active_count == 0 &&
                    GC_first_nonempty > GC_mark_stack_top) { 
                    GC_bool need_to_notify = FALSE;
                    /* The above conditions can't be falsified while we */
                    /* hold the mark lock, since neither                */
                    /* GC_active_count nor GC_mark_stack_top can        */
                    /* change.  GC_first_nonempty can only be           */
                    /* incremented asynchronously.  Thus we know that   */
                    /* both conditions actually held simultaneously.    */
                    GC_helper_count--;
                    if (0 == GC_helper_count) need_to_notify = TRUE;
#                   ifdef PRINTSTATS
                      GC_printf1(
                        "Finished mark helper %lu\n", (unsigned long)id);
#                   endif
                    GC_release_mark_lock();
                    if (need_to_notify) GC_notify_all_marker();
                    return;
                }
                /* else there's something on the stack again, or        */
                /* another helper may push something.                   */
                GC_active_count++;
                GC_ASSERT(GC_active_count > 0);
                GC_release_mark_lock();
                continue;
            } else {
                GC_release_mark_lock();
            }
        }
        n_to_get = ENTRIES_TO_GET;
        if (n_on_stack < 2 * ENTRIES_TO_GET) n_to_get = 1;
        local_top = GC_steal_mark_stack(my_first_nonempty, my_top,
                                        local_mark_stack, n_to_get,
                                        &my_first_nonempty);
        GC_ASSERT(my_first_nonempty >= GC_mark_stack && 
                  my_first_nonempty <= GC_mark_stack_top + 1);
        GC_do_local_mark(local_mark_stack, local_top);
    }
}

/* Perform Parallel mark.                       */
/* We hold the GC lock, not the mark lock.      */
/* Currently runs until the mark stack is       */
/* empty.                                       */
void GC_do_parallel_mark()
{
    mse local_mark_stack[LOCAL_MARK_STACK_SIZE];
    mse * local_top;
    mse * my_top;

    GC_acquire_mark_lock();
    GC_ASSERT(I_HOLD_LOCK());
    /* This could be a GC_ASSERT, but it seems safer to keep it on      */
    /* all the time, especially since it's cheap.                       */
    if (GC_help_wanted || GC_active_count != 0 || GC_helper_count != 0)
        ABORT("Tried to start parallel mark in bad state");
#   ifdef PRINTSTATS
        GC_printf1("Starting marking for mark phase number %lu\n",
                   (unsigned long)GC_mark_no);
#   endif
    GC_first_nonempty = GC_mark_stack;
    GC_active_count = 0;
    GC_helper_count = 1;
    GC_help_wanted = TRUE;
    GC_release_mark_lock();
    GC_notify_all_marker();
        /* Wake up potential helpers.   */
    GC_mark_local(local_mark_stack, 0);
    GC_acquire_mark_lock();
    GC_help_wanted = FALSE;
    /* Done; clean up.  */
    while (GC_helper_count > 0) GC_wait_marker();
    /* GC_helper_count cannot be incremented while GC_help_wanted == FALSE */
#   ifdef PRINTSTATS
        GC_printf1(
            "Finished marking for mark phase number %lu\n",
            (unsigned long)GC_mark_no);
#   endif
    GC_mark_no++;
    GC_release_mark_lock();
    GC_notify_all_marker();
}


/* Try to help out the marker, if it's running.         */
/* We do not hold the GC lock, but the requestor does.  */
void GC_help_marker(word my_mark_no)
{
    mse local_mark_stack[LOCAL_MARK_STACK_SIZE];
    unsigned my_id;
    mse * my_first_nonempty;

    if (!GC_parallel) return;
    GC_acquire_mark_lock();
    while (GC_mark_no < my_mark_no
           || !GC_help_wanted && GC_mark_no == my_mark_no) {
      GC_wait_marker();
    }
    my_id = GC_helper_count;
    if (GC_mark_no != my_mark_no || my_id >= GC_markers) {
      /* Second test is useful only if original threads can also        */
      /* act as helpers.  Under Linux they can't.                       */
      GC_release_mark_lock();
      return;
    }
    GC_helper_count = my_id + 1;
    GC_release_mark_lock();
    GC_mark_local(local_mark_stack, my_id);
    /* GC_mark_local decrements GC_helper_count. */
}

#endif /* PARALLEL_MARK */

/* Allocate or reallocate space for mark stack of size s words  */
/* May silently fail.                                           */
static void alloc_mark_stack(n)
word n;
{
    mse * new_stack = (mse *)GC_scratch_alloc(n * sizeof(struct GC_ms_entry));
    
    GC_mark_stack_too_small = FALSE;
    if (GC_mark_stack_size != 0) {
        if (new_stack != 0) {
          word displ = (word)GC_mark_stack & (GC_page_size - 1);
          signed_word size = GC_mark_stack_size * sizeof(struct GC_ms_entry);
          
          /* Recycle old space */
              if (0 != displ) displ = GC_page_size - displ;
              size = (size - displ) & ~(GC_page_size - 1);
              if (size > 0) {
                GC_add_to_heap((struct hblk *)
                                ((word)GC_mark_stack + displ), (word)size);
              }
          GC_mark_stack = new_stack;
          GC_mark_stack_size = n;
          GC_mark_stack_limit = new_stack + n;
#         ifdef CONDPRINT
            if (GC_print_stats) {
              GC_printf1("Grew mark stack to %lu frames\n",
                         (unsigned long) GC_mark_stack_size);
            }
#         endif
        } else {
#         ifdef CONDPRINT
            if (GC_print_stats) {
              GC_printf1("Failed to grow mark stack to %lu frames\n",
                         (unsigned long) n);
            }
#         endif
        }
    } else {
        if (new_stack == 0) {
            GC_err_printf0("No space for mark stack\n");
            EXIT();
        }
        GC_mark_stack = new_stack;
        GC_mark_stack_size = n;
        GC_mark_stack_limit = new_stack + n;
    }
    GC_mark_stack_top = GC_mark_stack-1;
}

void GC_mark_init()
{
    alloc_mark_stack(INITIAL_MARK_STACK_SIZE);
}

/*
 * Push all locations between b and t onto the mark stack.
 * b is the first location to be checked. t is one past the last
 * location to be checked.
 * Should only be used if there is no possibility of mark stack
 * overflow.
 */
void GC_push_all(bottom, top)
ptr_t bottom;
ptr_t top;
{
    register word length;
    
    bottom = (ptr_t)(((word) bottom + ALIGNMENT-1) & ~(ALIGNMENT-1));
    top = (ptr_t)(((word) top) & ~(ALIGNMENT-1));
    if (top == 0 || bottom == top) return;
    GC_mark_stack_top++;
    if (GC_mark_stack_top >= GC_mark_stack_limit) {
        ABORT("unexpected mark stack overflow");
    }
    length = top - bottom;
#   if GC_DS_TAGS > ALIGNMENT - 1
        length += GC_DS_TAGS;
        length &= ~GC_DS_TAGS;
#   endif
    GC_mark_stack_top -> mse_start = (word *)bottom;
    GC_mark_stack_top -> mse_descr = length;
}

/*
 * Analogous to the above, but push only those pages h with dirty_fn(h) != 0.
 * We use push_fn to actually push the block.
 * Used both to selectively push dirty pages, or to push a block
 * in piecemeal fashion, to allow for more marking concurrency.
 * Will not overflow mark stack if push_fn pushes a small fixed number
 * of entries.  (This is invoked only if push_fn pushes a single entry,
 * or if it marks each object before pushing it, thus ensuring progress
 * in the event of a stack overflow.)
 */
void GC_push_selected(bottom, top, dirty_fn, push_fn)
ptr_t bottom;
ptr_t top;
int (*dirty_fn) GC_PROTO((struct hblk * h));
void (*push_fn) GC_PROTO((ptr_t bottom, ptr_t top));
{
    register struct hblk * h;

    bottom = (ptr_t)(((long) bottom + ALIGNMENT-1) & ~(ALIGNMENT-1));
    top = (ptr_t)(((long) top) & ~(ALIGNMENT-1));

    if (top == 0 || bottom == top) return;
    h = HBLKPTR(bottom + HBLKSIZE);
    if (top <= (ptr_t) h) {
        if ((*dirty_fn)(h-1)) {
            (*push_fn)(bottom, top);
        }
        return;
    }
    if ((*dirty_fn)(h-1)) {
        (*push_fn)(bottom, (ptr_t)h);
    }
    while ((ptr_t)(h+1) <= top) {
        if ((*dirty_fn)(h)) {
            if ((word)(GC_mark_stack_top - GC_mark_stack)
                > 3 * GC_mark_stack_size / 4) {
                /* Danger of mark stack overflow */
                (*push_fn)((ptr_t)h, top);
                return;
            } else {
                (*push_fn)((ptr_t)h, (ptr_t)(h+1));
            }
        }
        h++;
    }
    if ((ptr_t)h != top) {
        if ((*dirty_fn)(h)) {
            (*push_fn)((ptr_t)h, top);
        }
    }
    if (GC_mark_stack_top >= GC_mark_stack_limit) {
        ABORT("unexpected mark stack overflow");
    }
}

# ifndef SMALL_CONFIG

#ifdef PARALLEL_MARK
    /* Break up root sections into page size chunks to better spread    */
    /* out work.                                                        */
    GC_bool GC_true_func(struct hblk *h) { return TRUE; }
#   define GC_PUSH_ALL(b,t) GC_push_selected(b,t,GC_true_func,GC_push_all);
#else
#   define GC_PUSH_ALL(b,t) GC_push_all(b,t);
#endif


void GC_push_conditional(bottom, top, all)
ptr_t bottom;
ptr_t top;
int all;
{
    if (all) {
      if (GC_dirty_maintained) {
#       ifdef PROC_VDB
            /* Pages that were never dirtied cannot contain pointers    */
            GC_push_selected(bottom, top, GC_page_was_ever_dirty, GC_push_all);
#       else
            GC_push_all(bottom, top);
#       endif
      } else {
        GC_push_all(bottom, top);
      }
    } else {
        GC_push_selected(bottom, top, GC_page_was_dirty, GC_push_all);
    }
}
#endif

# if defined(MSWIN32) || defined(MSWINCE)
  void __cdecl GC_push_one(p)
# else
  void GC_push_one(p)
# endif
word p;
{
    GC_PUSH_ONE_STACK(p, MARKED_FROM_REGISTER);
}

struct GC_ms_entry *GC_mark_and_push(obj, mark_stack_ptr, mark_stack_limit, src)
GC_PTR obj;
struct GC_ms_entry * mark_stack_ptr;
struct GC_ms_entry * mark_stack_limit;
GC_PTR *src;
{
   PREFETCH(obj);
   PUSH_CONTENTS(obj, mark_stack_ptr /* modified */, mark_stack_limit, src,
                 was_marked /* internally generated exit label */);
   return mark_stack_ptr;
}

# ifdef __STDC__
#   define BASE(p) (word)GC_base((void *)(p))
# else
#   define BASE(p) (word)GC_base((char *)(p))
# endif

/* Mark and push (i.e. gray) a single object p onto the main    */
/* mark stack.  Consider p to be valid if it is an interior     */
/* pointer.                                                     */
/* The object p has passed a preliminary pointer validity       */
/* test, but we do not definitely know whether it is valid.     */
/* Mark bits are NOT atomically updated.  Thus this must be the */
/* only thread setting them.                                    */
# if defined(PRINT_BLACK_LIST) || defined(KEEP_BACK_PTRS)
    void GC_mark_and_push_stack(p, source)
    ptr_t source;
# else
    void GC_mark_and_push_stack(p)
#   define source 0
# endif
register word p;
{
    register word r;
    register hdr * hhdr; 
    register int displ;
  
    GET_HDR(p, hhdr);
    if (IS_FORWARDING_ADDR_OR_NIL(hhdr)) {
        if (hhdr != 0) {
          r = BASE(p);
          hhdr = HDR(r);
          displ = BYTES_TO_WORDS(HBLKDISPL(r));
        }
    } else {
        register map_entry_type map_entry;
        
        displ = HBLKDISPL(p);
        map_entry = MAP_ENTRY((hhdr -> hb_map), displ);
        if (map_entry >= MAX_OFFSET) {
          if (map_entry == OFFSET_TOO_BIG || !GC_all_interior_pointers) {
              r = BASE(p);
              displ = BYTES_TO_WORDS(HBLKDISPL(r));
              if (r == 0) hhdr = 0;
          } else {
              /* Offset invalid, but map reflects interior pointers     */
              hhdr = 0;
          }
        } else {
          displ = BYTES_TO_WORDS(displ);
          displ -= map_entry;
          r = (word)((word *)(HBLKPTR(p)) + displ);
        }
    }
    /* If hhdr != 0 then r == GC_base(p), only we did it faster. */
    /* displ is the word index within the block.                 */
    if (hhdr == 0) {
#       ifdef PRINT_BLACK_LIST
          GC_add_to_black_list_stack(p, source);
#       else
          GC_add_to_black_list_stack(p);
#       endif
#       undef source  /* In case we had to define it. */
    } else {
        if (!mark_bit_from_hdr(hhdr, displ)) {
            set_mark_bit_from_hdr(hhdr, displ);
            GC_STORE_BACK_PTR(source, (ptr_t)r);
            PUSH_OBJ((word *)r, hhdr, GC_mark_stack_top,
                     GC_mark_stack_limit);
        }
    }
}

# ifdef TRACE_BUF

# define TRACE_ENTRIES 1000

struct trace_entry {
    char * kind;
    word gc_no;
    word words_allocd;
    word arg1;
    word arg2;
} GC_trace_buf[TRACE_ENTRIES];

int GC_trace_buf_ptr = 0;

void GC_add_trace_entry(char *kind, word arg1, word arg2)
{
    GC_trace_buf[GC_trace_buf_ptr].kind = kind;
    GC_trace_buf[GC_trace_buf_ptr].gc_no = GC_gc_no;
    GC_trace_buf[GC_trace_buf_ptr].words_allocd = GC_words_allocd;
    GC_trace_buf[GC_trace_buf_ptr].arg1 = arg1 ^ 0x80000000;
    GC_trace_buf[GC_trace_buf_ptr].arg2 = arg2 ^ 0x80000000;
    GC_trace_buf_ptr++;
    if (GC_trace_buf_ptr >= TRACE_ENTRIES) GC_trace_buf_ptr = 0;
}

void GC_print_trace(word gc_no, GC_bool lock)
{
    int i;
    struct trace_entry *p;
    
    if (lock) LOCK();
    for (i = GC_trace_buf_ptr-1; i != GC_trace_buf_ptr; i--) {
        if (i < 0) i = TRACE_ENTRIES-1;
        p = GC_trace_buf + i;
        if (p -> gc_no < gc_no || p -> kind == 0) return;
        printf("Trace:%s (gc:%d,words:%d) 0x%X, 0x%X\n",
                p -> kind, p -> gc_no, p -> words_allocd,
                (p -> arg1) ^ 0x80000000, (p -> arg2) ^ 0x80000000);
    }
    printf("Trace incomplete\n");
    if (lock) UNLOCK();
}

# endif /* TRACE_BUF */

/*
 * A version of GC_push_all that treats all interior pointers as valid
 * and scans the entire region immediately, in case the contents
 * change.
 */
void GC_push_all_eager(bottom, top)
ptr_t bottom;
ptr_t top;
{
    word * b = (word *)(((long) bottom + ALIGNMENT-1) & ~(ALIGNMENT-1));
    word * t = (word *)(((long) top) & ~(ALIGNMENT-1));
    register word *p;
    register word q;
    register word *lim;
    register ptr_t greatest_ha = GC_greatest_plausible_heap_addr;
    register ptr_t least_ha = GC_least_plausible_heap_addr;
#   define GC_greatest_plausible_heap_addr greatest_ha
#   define GC_least_plausible_heap_addr least_ha

    if (top == 0) return;
    /* check all pointers in range and push if they appear      */
    /* to be valid.                                             */
      lim = t - 1 /* longword */;
      for (p = b; p <= lim; p = (word *)(((char *)p) + ALIGNMENT)) {
        q = *p;
        GC_PUSH_ONE_STACK(q, p);
      }
#   undef GC_greatest_plausible_heap_addr
#   undef GC_least_plausible_heap_addr
}

#ifndef THREADS
/*
 * A version of GC_push_all that treats all interior pointers as valid
 * and scans part of the area immediately, to make sure that saved
 * register values are not lost.
 * Cold_gc_frame delimits the stack section that must be scanned
 * eagerly.  A zero value indicates that no eager scanning is needed.
 */
void GC_push_all_stack_partially_eager(bottom, top, cold_gc_frame)
ptr_t bottom;
ptr_t top;
ptr_t cold_gc_frame;
{
  if (!NEED_FIXUP_POINTER && GC_all_interior_pointers) {
#   define EAGER_BYTES 1024
    /* Push the hot end of the stack eagerly, so that register values   */
    /* saved inside GC frames are marked before they disappear.         */
    /* The rest of the marking can be deferred until later.             */
    if (0 == cold_gc_frame) {
        GC_push_all_stack(bottom, top);
        return;
    }
    GC_ASSERT(bottom <= cold_gc_frame && cold_gc_frame <= top);
#   ifdef STACK_GROWS_DOWN
        GC_push_all(cold_gc_frame - sizeof(ptr_t), top);
        GC_push_all_eager(bottom, cold_gc_frame);
#   else /* STACK_GROWS_UP */
        GC_push_all(bottom, cold_gc_frame + sizeof(ptr_t));
        GC_push_all_eager(cold_gc_frame, top);
#   endif /* STACK_GROWS_UP */
  } else {
    GC_push_all_eager(bottom, top);
  }
# ifdef TRACE_BUF
      GC_add_trace_entry("GC_push_all_stack", bottom, top);
# endif
}
#endif /* !THREADS */

void GC_push_all_stack(bottom, top)
ptr_t bottom;
ptr_t top;
{
  if (!NEED_FIXUP_POINTER && GC_all_interior_pointers) {
    GC_push_all(bottom, top);
  } else {
    GC_push_all_eager(bottom, top);
  }
}

#if !defined(SMALL_CONFIG) && !defined(USE_MARK_BYTES)
/* Push all objects reachable from marked objects in the given block */
/* of size 1 objects.                                                */
void GC_push_marked1(h, hhdr)
struct hblk *h;
register hdr * hhdr;
{
    word * mark_word_addr = &(hhdr->hb_marks[0]);
    register word *p;
    word *plim;
    register int i;
    register word q;
    register word mark_word;
    register ptr_t greatest_ha = GC_greatest_plausible_heap_addr;
    register ptr_t least_ha = GC_least_plausible_heap_addr;
    register mse * mark_stack_top = GC_mark_stack_top;
    register mse * mark_stack_limit = GC_mark_stack_limit;
#   define GC_mark_stack_top mark_stack_top
#   define GC_mark_stack_limit mark_stack_limit
#   define GC_greatest_plausible_heap_addr greatest_ha
#   define GC_least_plausible_heap_addr least_ha
    
    p = (word *)(h->hb_body);
    plim = (word *)(((word)h) + HBLKSIZE);

    /* go through all words in block */
        while( p < plim )  {
            mark_word = *mark_word_addr++;
            i = 0;
            while(mark_word != 0) {
              if (mark_word & 1) {
                  q = p[i];
                  GC_PUSH_ONE_HEAP(q, p + i);
              }
              i++;
              mark_word >>= 1;
            }
            p += WORDSZ;
        }
#   undef GC_greatest_plausible_heap_addr
#   undef GC_least_plausible_heap_addr        
#   undef GC_mark_stack_top
#   undef GC_mark_stack_limit
    GC_mark_stack_top = mark_stack_top;
}


#ifndef UNALIGNED

/* Push all objects reachable from marked objects in the given block */
/* of size 2 objects.                                                */
void GC_push_marked2(h, hhdr)
struct hblk *h;
register hdr * hhdr;
{
    word * mark_word_addr = &(hhdr->hb_marks[0]);
    register word *p;
    word *plim;
    register int i;
    register word q;
    register word mark_word;
    register ptr_t greatest_ha = GC_greatest_plausible_heap_addr;
    register ptr_t least_ha = GC_least_plausible_heap_addr;
    register mse * mark_stack_top = GC_mark_stack_top;
    register mse * mark_stack_limit = GC_mark_stack_limit;
#   define GC_mark_stack_top mark_stack_top
#   define GC_mark_stack_limit mark_stack_limit
#   define GC_greatest_plausible_heap_addr greatest_ha
#   define GC_least_plausible_heap_addr least_ha
    
    p = (word *)(h->hb_body);
    plim = (word *)(((word)h) + HBLKSIZE);

    /* go through all words in block */
        while( p < plim )  {
            mark_word = *mark_word_addr++;
            i = 0;
            while(mark_word != 0) {
              if (mark_word & 1) {
                  q = p[i];
                  GC_PUSH_ONE_HEAP(q, p + i);
                  q = p[i+1];
                  GC_PUSH_ONE_HEAP(q, p + i);
              }
              i += 2;
              mark_word >>= 2;
            }
            p += WORDSZ;
        }
#   undef GC_greatest_plausible_heap_addr
#   undef GC_least_plausible_heap_addr        
#   undef GC_mark_stack_top
#   undef GC_mark_stack_limit
    GC_mark_stack_top = mark_stack_top;
}

/* Push all objects reachable from marked objects in the given block */
/* of size 4 objects.                                                */
/* There is a risk of mark stack overflow here.  But we handle that. */
/* And only unmarked objects get pushed, so it's not very likely.    */
void GC_push_marked4(h, hhdr)
struct hblk *h;
register hdr * hhdr;
{
    word * mark_word_addr = &(hhdr->hb_marks[0]);
    register word *p;
    word *plim;
    register int i;
    register word q;
    register word mark_word;
    register ptr_t greatest_ha = GC_greatest_plausible_heap_addr;
    register ptr_t least_ha = GC_least_plausible_heap_addr;
    register mse * mark_stack_top = GC_mark_stack_top;
    register mse * mark_stack_limit = GC_mark_stack_limit;
#   define GC_mark_stack_top mark_stack_top
#   define GC_mark_stack_limit mark_stack_limit
#   define GC_greatest_plausible_heap_addr greatest_ha
#   define GC_least_plausible_heap_addr least_ha
    
    p = (word *)(h->hb_body);
    plim = (word *)(((word)h) + HBLKSIZE);

    /* go through all words in block */
        while( p < plim )  {
            mark_word = *mark_word_addr++;
            i = 0;
            while(mark_word != 0) {
              if (mark_word & 1) {
                  q = p[i];
                  GC_PUSH_ONE_HEAP(q, p + i);
                  q = p[i+1];
                  GC_PUSH_ONE_HEAP(q, p + i + 1);
                  q = p[i+2];
                  GC_PUSH_ONE_HEAP(q, p + i + 2);
                  q = p[i+3];
                  GC_PUSH_ONE_HEAP(q, p + i + 3);
              }
              i += 4;
              mark_word >>= 4;
            }
            p += WORDSZ;
        }
#   undef GC_greatest_plausible_heap_addr
#   undef GC_least_plausible_heap_addr        
#   undef GC_mark_stack_top
#   undef GC_mark_stack_limit
    GC_mark_stack_top = mark_stack_top;
}

#endif /* UNALIGNED */

#endif /* SMALL_CONFIG */

/* Push all objects reachable from marked objects in the given block */
void GC_push_marked(h, hhdr)
struct hblk *h;
register hdr * hhdr;
{
    register int sz = hhdr -> hb_sz;
    register int descr = hhdr -> hb_descr;
    register word * p;
    register int word_no;
    register word * lim;
    register mse * GC_mark_stack_top_reg;
    register mse * mark_stack_limit = GC_mark_stack_limit;
    
    /* Some quick shortcuts: */
        if ((0 | GC_DS_LENGTH) == descr) return;
        if (GC_block_empty(hhdr)/* nothing marked */) return;
    GC_n_rescuing_pages++;
    GC_objects_are_marked = TRUE;
    if (sz > MAXOBJSZ) {
        lim = (word *)h;
    } else {
        lim = (word *)(h + 1) - sz;
    }
    
    switch(sz) {
#   if !defined(SMALL_CONFIG) && !defined(USE_MARK_BYTES)   
     case 1:
       GC_push_marked1(h, hhdr);
       break;
#   endif
#   if !defined(SMALL_CONFIG) && !defined(UNALIGNED) && \
       !defined(USE_MARK_BYTES)
     case 2:
       GC_push_marked2(h, hhdr);
       break;
     case 4:
       GC_push_marked4(h, hhdr);
       break;
#   endif       
     default:
      GC_mark_stack_top_reg = GC_mark_stack_top;
      for (p = (word *)h, word_no = 0; p <= lim; p += sz, word_no += sz) {
         if (mark_bit_from_hdr(hhdr, word_no)) {
           /* Mark from fields inside the object */
             PUSH_OBJ((word *)p, hhdr, GC_mark_stack_top_reg, mark_stack_limit);
#            ifdef GATHERSTATS
                /* Subtract this object from total, since it was        */
                /* added in twice.                                      */
                GC_composite_in_use -= sz;
#            endif
         }
      }
      GC_mark_stack_top = GC_mark_stack_top_reg;
    }
}

#ifndef SMALL_CONFIG
/* Test whether any page in the given block is dirty    */
GC_bool GC_block_was_dirty(h, hhdr)
struct hblk *h;
register hdr * hhdr;
{
    register int sz = hhdr -> hb_sz;
    
    if (sz < MAXOBJSZ) {
         return(GC_page_was_dirty(h));
    } else {
         register ptr_t p = (ptr_t)h;
         sz = WORDS_TO_BYTES(sz);
         while (p < (ptr_t)h + sz) {
             if (GC_page_was_dirty((struct hblk *)p)) return(TRUE);
             p += HBLKSIZE;
         }
         return(FALSE);
    }
}
#endif /* SMALL_CONFIG */

/* Similar to GC_push_next_marked, but return address of next block     */
struct hblk * GC_push_next_marked(h)
struct hblk *h;
{
    register hdr * hhdr;
    
    h = GC_next_used_block(h);
    if (h == 0) return(0);
    hhdr = HDR(h);
    GC_push_marked(h, hhdr);
    return(h + OBJ_SZ_TO_BLOCKS(hhdr -> hb_sz));
}

#ifndef SMALL_CONFIG
/* Identical to above, but mark only from dirty pages   */
struct hblk * GC_push_next_marked_dirty(h)
struct hblk *h;
{
    register hdr * hhdr;
    
    if (!GC_dirty_maintained) { ABORT("dirty bits not set up"); }
    for (;;) {
        h = GC_next_used_block(h);
        if (h == 0) return(0);
        hhdr = HDR(h);
#       ifdef STUBBORN_ALLOC
          if (hhdr -> hb_obj_kind == STUBBORN) {
            if (GC_page_was_changed(h) && GC_block_was_dirty(h, hhdr)) {
                break;
            }
          } else {
            if (GC_block_was_dirty(h, hhdr)) break;
          }
#       else
          if (GC_block_was_dirty(h, hhdr)) break;
#       endif
        h += OBJ_SZ_TO_BLOCKS(hhdr -> hb_sz);
    }
    GC_push_marked(h, hhdr);
    return(h + OBJ_SZ_TO_BLOCKS(hhdr -> hb_sz));
}
#endif

/* Similar to above, but for uncollectable pages.  Needed since we      */
/* do not clear marks for such pages, even for full collections.        */
struct hblk * GC_push_next_marked_uncollectable(h)
struct hblk *h;
{
    register hdr * hhdr = HDR(h);
    
    for (;;) {
        h = GC_next_used_block(h);
        if (h == 0) return(0);
        hhdr = HDR(h);
        if (hhdr -> hb_obj_kind == UNCOLLECTABLE) break;
        h += OBJ_SZ_TO_BLOCKS(hhdr -> hb_sz);
    }
    GC_push_marked(h, hhdr);
    return(h + OBJ_SZ_TO_BLOCKS(hhdr -> hb_sz));
}