text: 
  /*
   * 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(word x)
  {
      static volatile word sink;
  
      sink = x;
  }
  
  /* mark_proc GC_mark_procs[MAX_MARK_PROCS] = {0} -- declared in gc_priv.h */
  
  unsigned 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
        unsigned GC_n_kinds = 5;
  #   else
        unsigned GC_n_kinds = 4;
  #   endif
  # else
  #   ifdef STUBBORN_ALLOC
        unsigned GC_n_kinds = 4;
  #   else
        unsigned 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;
  
  size_t GC_mark_stack_size = 0;
  
  #ifdef PARALLEL_MARK
  # include "atomic_ops.h"
  
    mse * volatile GC_mark_stack_top;
    /* Updated only with mark lock held, but read asynchronously.	*/
    volatile AO_t GC_first_nonempty;
  	/* Lowest entry on mark stack	*/
  	/* that may be nonempty.	*/
  	/* Updated only by initiating 	*/
  	/* thread.			*/
  #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(void)
  {
      return(GC_mark_state != MS_NONE);
  }
  
  /* clear all mark bits in the header */
  void GC_clear_hdr_marks(hdr *hhdr)
  {
      size_t last_bit = FINAL_MARK_BIT(hhdr -> hb_sz);
  
  #   ifdef USE_MARK_BYTES
        BZERO(hhdr -> hb_marks, MARK_BITS_SZ);
        hhdr -> hb_marks[last_bit] = 1;
  #   else
        BZERO(hhdr -> hb_marks, MARK_BITS_SZ*sizeof(word));
        set_mark_bit_from_hdr(hhdr, last_bit);
  #   endif
      hhdr -> hb_n_marks = 0;
  }
  
  /* Set all mark bits in the header.  Used for uncollectable blocks. */
  void GC_set_hdr_marks(hdr *hhdr)
  {
      unsigned i;
      size_t sz = hhdr -> hb_sz;
      size_t n_marks = FINAL_MARK_BIT(sz);
  
  #   ifdef USE_MARK_BYTES
        for (i = 0; i <= n_marks; i += MARK_BIT_OFFSET(sz)) {
  	hhdr -> hb_marks[i] = 1;
        }
  #   else
        for (i = 0; i < divWORDSZ(n_marks + WORDSZ); ++i) {
  	hhdr -> hb_marks[i] = ONES;
        }
  #   endif
  #   ifdef MARK_BIT_PER_OBJ
        hhdr -> hb_n_marks = n_marks - 1;
  #   else
        hhdr -> hb_n_marks = HBLK_OBJS(sz);
  #   endif
  }
  
  /*
   * Clear all mark bits associated with block h.
   */
  /*ARGSUSED*/
  static void clear_marks_for_block(struct hblk *h, word dummy)
  {
      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(ptr_t p)
  {
      struct hblk *h = HBLKPTR(p);
      hdr * hhdr = HDR(h);
      word bit_no = MARK_BIT_NO(p - (ptr_t)h, hhdr -> hb_sz);
  
      if (!mark_bit_from_hdr(hhdr, bit_no)) {
        set_mark_bit_from_hdr(hhdr, bit_no);
        ++hhdr -> hb_n_marks;
      }
  }
  
  void GC_clear_mark_bit(ptr_t p)
  {
      struct hblk *h = HBLKPTR(p);
      hdr * hhdr = HDR(h);
      word bit_no = MARK_BIT_NO(p - (ptr_t)h, hhdr -> hb_sz);
  
      if (mark_bit_from_hdr(hhdr, bit_no)) {
        size_t n_marks;
        clear_mark_bit_from_hdr(hhdr, bit_no);
        n_marks = hhdr -> hb_n_marks - 1;
  #     ifdef PARALLEL_MARK
          if (n_marks != 0)
            hhdr -> hb_n_marks = n_marks;
          /* Don't decrement to zero.  The counts are approximate due to	*/
          /* concurrency issues, but we need to ensure that a count of 	*/
          /* zero implies an empty block.					*/
  #     else
            hhdr -> hb_n_marks = n_marks;
  #     endif
      }
  }
  
  GC_bool GC_is_marked(ptr_t p)
  {
      struct hblk *h = HBLKPTR(p);
      hdr * hhdr = HDR(h);
      word bit_no = MARK_BIT_NO(p - (ptr_t)h, hhdr -> hb_sz);
  
      return((GC_bool)mark_bit_from_hdr(hhdr, bit_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(void)
  {
      GC_apply_to_all_blocks(clear_marks_for_block, (word)0);
      GC_objects_are_marked = FALSE;
      GC_mark_state = MS_INVALID;
      scan_ptr = 0;
  }
  
  /* Initiate a garbage collection.  Initiates a full collection if the	*/
  /* mark	state is invalid.						*/
  /*ARGSUSED*/
  void GC_initiate_gc(void)
  {
      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(size_t);
  
  # if defined(MSWIN32) || defined(USE_PROC_FOR_LIBRARIES) && defined(THREADS)
      /* Under rare conditions, we may end up marking from nonexistent memory. */
      /* Hence we need to be prepared to recover by running GC_mark_some	     */
      /* with a suitable handler in place.				     */
  #   define WRAP_MARK_SOME
  # endif
  
  /* 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 WRAP_MARK_SOME
    /* 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(ptr_t cold_gc_frame)
  #else
    GC_bool GC_mark_some(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) {
  		    if (GC_print_stats) {
  			GC_log_printf("Marked from %u dirty pages\n",
  				      GC_n_rescuing_pages);
  		    }
  		    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 < (mse *)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);
      }
  }
  
  
  #if defined(MSWIN32) && defined(__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__  && MSWIN32 */
  
  #ifdef GC_WIN32_THREADS
    extern GC_bool GC_started_thread_while_stopped(void);
    /* In win32_threads.c.  Did we invalidate mark phase with an	*/
    /* unexpected thread start?					*/
  #endif
  
  # ifdef WRAP_MARK_SOME
    GC_bool GC_mark_some(ptr_t cold_gc_frame)
    {
        GC_bool ret_val;
  
  #   ifdef MSWIN32
  #    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.                          */
        /* It's conceivable that this is the same issue with	 */
        /* terminating threads that we see with Linux and		 */
        /* USE_PROC_FOR_LIBRARIES.				 */
  
        __try {
            ret_val = GC_mark_some_inner(cold_gc_frame);
        } __except (GetExceptionCode() == EXCEPTION_ACCESS_VIOLATION ?
                  EXCEPTION_EXECUTE_HANDLER : EXCEPTION_CONTINUE_SEARCH) {
  	  goto handle_ex;
        }
  #     ifdef GC_WIN32_THREADS
  	/* With DllMain-based thread tracking, a thread may have	*/
  	/* started while we were marking.  This is logically equivalent	*/
  	/* to the exception case; our results are invalid and we have	*/
  	/* to start over.  This cannot be prevented since we can't	*/
  	/* block in DllMain.						*/
  	if (GC_started_thread_while_stopped()) goto handle_ex;
  #     endif
       rm_handler:
        return ret_val;
  
  #    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));
        ret_val = GC_mark_some_inner(cold_gc_frame);
        /* Prevent GCC from considering the following code unreachable */
        /* and thus eliminating it.                                    */
          if (er.alt_path == 0)
            goto handle_ex;
      rm_handler:
        /* Uninstall the exception handler */
        asm volatile ("mov %0, %%fs:0" : : "r" (er.ex_reg.prev));
        return ret_val;
  
  #    endif /* __GNUC__ */
  #   else /* !MSWIN32 */
        /* Here we are handling the case in which /proc is used for root	*/
        /* finding, and we have threads.  We may find a stack for a	*/
        /* thread that is in the process of exiting, and disappears	*/
        /* while we are marking it.  This seems extremely difficult to	*/
        /* avoid otherwise.						*/
        if (GC_incremental)
  	      WARN("Incremental GC incompatible with /proc roots\n", 0);
  	/* I'm not sure if this could still work ...	*/
        GC_setup_temporary_fault_handler();
        if(SETJMP(GC_jmp_buf) != 0) goto handle_ex;
        ret_val = GC_mark_some_inner(cold_gc_frame);
      rm_handler:
        GC_reset_fault_handler();
        return ret_val;
  
  #   endif /* !MSWIN32 */
  
  handle_ex:
      /* Exception handler starts here for all cases. */
        if (GC_print_stats) {
          GC_log_printf("Caught ACCESS_VIOLATION in marker. "
  		      "Memory mapping disappeared.\n");
        }
  
        /* 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;
        goto rm_handler;  // Back to platform-specific code.
    }
  #endif /* WRAP_MARK_SOME */
  
  
  GC_bool GC_mark_stack_empty(void)
  {
      return(GC_mark_stack_top < GC_mark_stack);
  }
  
  void GC_invalidate_mark_state(void)
  {
      GC_mark_state = MS_INVALID;
      GC_mark_stack_top = GC_mark_stack-1;
  }
  
  mse * GC_signal_mark_stack_overflow(mse *msp)
  {
      GC_mark_state = MS_INVALID;
      GC_mark_stack_too_small = TRUE;
      if (GC_print_stats) {
  	GC_log_printf("Mark stack overflow; current size = %lu entries\n",
  		      GC_mark_stack_size);
      }
      return(msp - GC_MARK_STACK_DISCARDS);
  }
  
  /*
   * Mark objects pointed to by the regions described by
   * mark stack entries between mark_stack and 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(mse *mark_stack_top, mse *mark_stack, mse *mark_stack_limit)
  {
    signed_word credit = HBLKSIZE;  /* Remaining credit for marking work	*/
    ptr_t current_p;	/* Pointer to current candidate ptr.	*/
    word current;	/* Candidate pointer.			*/
    ptr_t limit;	/* (Incl) limit of current candidate 	*/
  				/* range				*/
    word descr;
    ptr_t greatest_ha = GC_greatest_plausible_heap_addr;
    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 ENABLE_TRACE
  	    if (GC_trace_addr >= current_p
  		&& GC_trace_addr < current_p + descr) {
  		GC_log_printf("GC:%d Large section; start %p len %lu\n",
  			      GC_gc_no, current_p, (unsigned long) descr);
  	    }
  #	  endif /* ENABLE_TRACE */
  #	  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++;
  #	      ifdef ENABLE_TRACE
  	        if (GC_trace_addr >= current_p
  		    && GC_trace_addr < current_p + descr) {
  		    GC_log_printf("GC:%d splitting (parallel) %p at %p\n",
  				  GC_gc_no, current_p, current_p + new_size);
  	        }
  #	      endif /* ENABLE_TRACE */
  	      current_p += new_size;
  	      descr -= new_size;
  	      goto retry;
  	    }
  #	  endif /* PARALLEL_MARK */
            mark_stack_top -> mse_start =
  		limit = current_p + WORDS_TO_BYTES(SPLIT_RANGE_WORDS-1);
            mark_stack_top -> mse_descr =
  			descr - WORDS_TO_BYTES(SPLIT_RANGE_WORDS-1);
  #	  ifdef ENABLE_TRACE
  	    if (GC_trace_addr >= current_p
  		&& GC_trace_addr < current_p + descr) {
  		GC_log_printf("GC:%d splitting %p at %p\n",
  			      GC_gc_no, current_p, limit);
  	    }
  #	  endif /* ENABLE_TRACE */
            /* Make sure that pointers overlapping the two ranges are	*/
            /* considered. 						*/
            limit += sizeof(word) - ALIGNMENT;
            break;
          case GC_DS_BITMAP:
            mark_stack_top--;
  #	  ifdef ENABLE_TRACE
  	    if (GC_trace_addr >= current_p
  		&& GC_trace_addr < current_p + WORDS_TO_BYTES(WORDSZ-2)) {
  		GC_log_printf("GC:%d Tracing from %p bitmap descr %lu\n",
  			      GC_gc_no, current_p, (unsigned long) descr);
  	    }
  #	  endif /* ENABLE_TRACE */
            descr &= ~GC_DS_TAGS;
            credit -= WORDS_TO_BYTES(WORDSZ/2); /* guess */
            while (descr != 0) {
              if ((signed_word)descr < 0) {
                current = *(word *)current_p;
  	      FIXUP_POINTER(current);
  	      if ((ptr_t)current >= least_ha && (ptr_t)current < greatest_ha) {
  		PREFETCH((ptr_t)current);
  #               ifdef ENABLE_TRACE
  	          if (GC_trace_addr == current_p) {
  	            GC_log_printf("GC:%d Considering(3) %p -> %p\n",
  			          GC_gc_no, current_p, (ptr_t) current);
  	          }
  #               endif /* ENABLE_TRACE */
                  PUSH_CONTENTS((ptr_t)current, mark_stack_top,
  			      mark_stack_limit, current_p, exit1);
  	      }
              }
  	    descr <<= 1;
  	    current_p += sizeof(word);
            }
            continue;
          case GC_DS_PROC:
            mark_stack_top--;
  #	  ifdef ENABLE_TRACE
  	    if (GC_trace_addr >= current_p
  		&& GC_base(current_p) != 0
  		&& GC_base(current_p) == GC_base(GC_trace_addr)) {
  		GC_log_printf("GC:%d Tracing from %p proc descr %lu\n",
  			      GC_gc_no, current_p, (unsigned long) descr);
  	    }
  #	  endif /* ENABLE_TRACE */
            credit -= GC_PROC_BYTES;
            mark_stack_top =
                (*PROC(descr))
  		    ((word *)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 *)(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 = current_p + (word)descr;
      }
  #   ifdef ENABLE_TRACE
  	if (GC_trace_addr >= current_p
  	    && GC_trace_addr < limit) {
  	    GC_log_printf("GC:%d Tracing from %p len %lu\n",
  			  GC_gc_no, current_p, (unsigned long) descr);
  	}
  #   endif /* ENABLE_TRACE */
      /* The simple case in which we're scanning a range.	*/
      GC_ASSERT(!((word)current_p & (ALIGNMENT-1)));
      credit -= limit - current_p;
      limit -= sizeof(word);
      {
  #     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(limit - PREF_DIST*CACHE_LINE_SIZE);
  	  GC_ASSERT(limit >= current_p);
  	  deferred = *(word *)limit;
  	  FIXUP_POINTER(deferred);
  	  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 = *(word *)limit;
  	  FIXUP_POINTER(deferred);
  	  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 PUSH_CONTENTS expands to a lot of code,	*/
  	/* we don't.						*/
          current = *(word *)current_p;
  	FIXUP_POINTER(current);
          PREFETCH(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);
  #         ifdef ENABLE_TRACE
  	    if (GC_trace_addr == current_p) {
  	        GC_log_printf("GC:%d Considering(1) %p -> %p\n",
  			      GC_gc_no, current_p, (ptr_t) current);
  	    }
  #         endif /* ENABLE_TRACE */
            PUSH_CONTENTS((ptr_t)current, mark_stack_top,
  		           mark_stack_limit, current_p, exit2);
          }
          current_p += ALIGNMENT;
        }
  
  #     ifndef SMALL_CONFIG
  	/* We still need to mark the entry we previously prefetched.	*/
  	/* We already know that it passes the preliminary pointer	*/
  	/* validity test.						*/
  #       ifdef ENABLE_TRACE
  	    if (GC_trace_addr == current_p) {
  	        GC_log_printf("GC:%d Considering(2) %p -> %p\n",
  			      GC_gc_no, current_p, (ptr_t) deferred);
  	    }
  #       endif /* ENABLE_TRACE */
          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;
  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;
  
      GC_ASSERT(high >= low-1 && high - low + 1 <= GC_mark_stack_size);
      for (p = low; p <= high && i <= max; ++p) {
  	word descr = AO_load((volatile AO_t *) &(p -> mse_descr));
  	if (descr != 0) {
  	    /* Must be ordered after read of descr: */
  	    AO_store_release_write((volatile AO_t *) &(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 < (ptr_t)GC_greatest_plausible_heap_addr
  			                 - (ptr_t)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;	/* Concurrent modification impossible. */
      my_start = my_top + 1;
      if (my_start - GC_mark_stack + stack_size > GC_mark_stack_size) {
        if (GC_print_stats) {
  	  GC_log_printf("No room to copy back mark stack.");
        }
        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((mse *)AO_load((volatile AO_t *)(&GC_mark_stack_top))
  		== my_top);
        AO_store_release_write((volatile AO_t *)(&GC_mark_stack_top),
  		             (AO_t)(my_top + stack_size));
  		/* Ensures visibility of previously written stack contents. */
      }
      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 ((mse *)AO_load((volatile AO_t *)(&GC_mark_stack_top))
  	    < (mse *)AO_load(&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 * 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 = (mse *)AO_load(&GC_first_nonempty);
      GC_ASSERT((mse *)AO_load(&GC_first_nonempty) >= GC_mark_stack &&
  	      (mse *)AO_load(&GC_first_nonempty) <=
  	      (mse *)AO_load((volatile AO_t *)(&GC_mark_stack_top)) + 1);
      if (GC_print_stats == VERBOSE)
  	GC_log_printf("Starting mark helper %lu\n", (unsigned long)id);
      GC_release_mark_lock();
      for (;;) {
  	size_t n_on_stack;
          size_t n_to_get;
  	mse * my_top;
  	mse * local_top;
          mse * global_first_nonempty = (mse *)AO_load(&GC_first_nonempty);
  
  	GC_ASSERT(my_first_nonempty >= GC_mark_stack &&
  		  my_first_nonempty <=
  		  (mse *)AO_load((volatile AO_t *)(&GC_mark_stack_top))  + 1);
  	GC_ASSERT(global_first_nonempty >= GC_mark_stack &&
  		  global_first_nonempty <=
  		  (mse *)AO_load((volatile AO_t *)(&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) {
  	    AO_compare_and_swap(&GC_first_nonempty,
  				(AO_t) global_first_nonempty,
  				(AO_t) 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 = (mse *)AO_load_acquire((volatile AO_t *)(&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;
  		/* Asynchronous modification impossible here, 	*/
  	        /* since we hold mark lock. 			*/
              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
  		       && (mse *)AO_load(&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 &&
  		    (mse *)AO_load(&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;
  		    if (GC_print_stats == VERBOSE)
  		      GC_log_printf(
  		        "Finished mark helper %lu\n", (unsigned long)id);
  		    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 <=
  		    (mse *)AO_load((volatile AO_t *)(&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];
  
      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");
      if (GC_print_stats == VERBOSE)
  	GC_log_printf("Starting marking for mark phase number %lu\n",
  		   (unsigned long)GC_mark_no);
      GC_first_nonempty = (AO_t)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 */
      if (GC_print_stats == VERBOSE)
  	GC_log_printf(
  	    "Finished marking for mark phase number %lu\n",
  	    (unsigned long)GC_mark_no);
      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;
  
      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 n entries.  */
  /* May silently fail.						   */
  static void alloc_mark_stack(size_t n)
  {
      mse * new_stack = (mse *)GC_scratch_alloc(n * sizeof(struct GC_ms_entry));
  #   ifdef GWW_VDB
        /* Don't recycle a stack segment obtained with the wrong flags. 	*/
        /* Win32 GetWriteWatch requires the right kind of memory.		*/
        static GC_bool GC_incremental_at_stack_alloc = 0;
        GC_bool recycle_old = (!GC_incremental || GC_incremental_at_stack_alloc);
  
        GC_incremental_at_stack_alloc = GC_incremental;
  #   else
  #     define recycle_old 1
  #   endif
  
      GC_mark_stack_too_small = FALSE;
      if (GC_mark_stack_size != 0) {
          if (new_stack != 0) {
  	  if (recycle_old) {
              /* Recycle old space */
                size_t page_offset = (word)GC_mark_stack & (GC_page_size - 1);
                size_t size = GC_mark_stack_size * sizeof(struct GC_ms_entry);
  	      size_t displ = 0;
  
  	      if (0 != page_offset) displ = GC_page_size - page_offset;
  	      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;
  	  if (GC_print_stats) {
  	      GC_log_printf("Grew mark stack to %lu frames\n",
  			    (unsigned long) GC_mark_stack_size);
  	  }
          } else {
  	  if (GC_print_stats) {
  	      GC_log_printf("Failed to grow mark stack to %lu frames\n",
  			    (unsigned long) n);
  	  }
          }
      } else {
          if (new_stack == 0) {
              GC_err_printf("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(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 = 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(ptr_t bottom, ptr_t top,
  		      int (*dirty_fn) (struct hblk *),
  		      void (*push_fn) (ptr_t, ptr_t))
  {
      struct hblk * h;
  
      bottom = (ptr_t)(((word) bottom + ALIGNMENT-1) & ~(ALIGNMENT-1));
      top = (ptr_t)(((word) 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(ptr_t bottom, ptr_t top, GC_bool 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(word p)
  # else
    void GC_push_one(word p)
  # endif
  {
      GC_PUSH_ONE_STACK((ptr_t)p, MARKED_FROM_REGISTER);
  }
  
  struct GC_ms_entry *GC_mark_and_push(void *obj,
  				     mse *mark_stack_ptr,
  				     mse *mark_stack_limit,
  				     void **src)
  {
      hdr * hhdr;
  
      PREFETCH(obj);
      GET_HDR(obj, hhdr);
      if (EXPECT(IS_FORWARDING_ADDR_OR_NIL(hhdr),FALSE)) {
        if (GC_all_interior_pointers) {
  	hhdr = GC_find_header(GC_base(obj));
  	if (hhdr == 0) {
            GC_ADD_TO_BLACK_LIST_NORMAL(obj, src);
  	  return mark_stack_ptr;
  	}
        } else {
          GC_ADD_TO_BLACK_LIST_NORMAL(obj, src);
  	return mark_stack_ptr;
        }
      }
      if (EXPECT(HBLK_IS_FREE(hhdr),0)) {
  	GC_ADD_TO_BLACK_LIST_NORMAL(obj, src);
  	return mark_stack_ptr;
      }
  
      PUSH_CONTENTS_HDR(obj, mark_stack_ptr /* modified */, mark_stack_limit,
  		      src, was_marked, hhdr, TRUE);
   was_marked:
      return mark_stack_ptr;
  }
  
  /* 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(ptr_t p, ptr_t source)
  # else
      void GC_mark_and_push_stack(ptr_t p)
  #   define source 0
  # endif
  {
      hdr * hhdr;
      ptr_t r = p;
  
      PREFETCH(p);
      GET_HDR(p, hhdr);
      if (EXPECT(IS_FORWARDING_ADDR_OR_NIL(hhdr),FALSE)) {
          if (hhdr != 0) {
            r = GC_base(p);
  	  hhdr = HDR(r);
  	}
          if (hhdr == 0) {
              GC_ADD_TO_BLACK_LIST_STACK(p, source);
  	    return;
  	}
      }
      if (EXPECT(HBLK_IS_FREE(hhdr),0)) {
  	GC_ADD_TO_BLACK_LIST_NORMAL(p, src);
  	return;
      }
  #   if defined(MANUAL_VDB) && defined(THREADS)
        /* Pointer is on the stack.  We may have dirtied the object	*/
        /* it points to, but not yet have called GC_dirty();	*/
        GC_dirty(p);	/* Implicitly affects entire object.	*/
  #   endif
      PUSH_CONTENTS_HDR(r, GC_mark_stack_top, GC_mark_stack_limit,
  		      source, mark_and_push_exit, hhdr, FALSE);
    mark_and_push_exit: ;
      /* We silently ignore pointers to near the end of a block,	*/
      /* which is very mildly suboptimal.				*/
      /* FIXME: We should probably add a header word to address	*/
      /* this.							*/
  }
  
  # ifdef TRACE_BUF
  
  # define TRACE_ENTRIES 1000
  
  struct trace_entry {
      char * kind;
      word gc_no;
      word bytes_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].bytes_allocd = GC_bytes_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,bytes:%d) 0x%X, 0x%X\n",
  		p -> kind, p -> gc_no, p -> bytes_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(ptr_t bottom, ptr_t top)
  {
      word * b = (word *)(((word) bottom + ALIGNMENT-1) & ~(ALIGNMENT-1));
      word * t = (word *)(((word) top) & ~(ALIGNMENT-1));
      register word *p;
      register ptr_t 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 *)(((ptr_t)p) + ALIGNMENT)) {
  	q = (ptr_t)(*p);
  	GC_PUSH_ONE_STACK((ptr_t)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.
   * We don't need to worry about the MANUAL_VDB case here, since this
   * is only called in the single-threaded case.  We assume that we
   * cannot collect between an assignment and the corresponding
   * GC_dirty() call.
   */
  void GC_push_all_stack_partially_eager(ptr_t bottom, ptr_t top,
  				       ptr_t cold_gc_frame)
  {
    if (!NEED_FIXUP_POINTER && GC_all_interior_pointers) {
      /* 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(ptr_t bottom, ptr_t top)
  {
  # if defined(THREADS) && defined(MPROTECT_VDB)
      GC_push_all_eager(bottom, top);
  # else
      if (!NEED_FIXUP_POINTER && GC_all_interior_pointers) {
        GC_push_all(bottom, top);
      } else {
        GC_push_all_eager(bottom, top);
      }
  # endif
  }
  
  #if !defined(SMALL_CONFIG) && !defined(USE_MARK_BYTES) && \
      defined(MARK_BIT_PER_GRANULE)
  # if GC_GRANULE_WORDS == 1
  #   define USE_PUSH_MARKED_ACCELERATORS
  #   define PUSH_GRANULE(q) \
  		{ ptr_t qcontents = (ptr_t)((q)[0]); \
  	          GC_PUSH_ONE_HEAP(qcontents, (q)); }
  # elif GC_GRANULE_WORDS == 2
  #   define USE_PUSH_MARKED_ACCELERATORS
  #   define PUSH_GRANULE(q) \
  		{ ptr_t qcontents = (ptr_t)((q)[0]); \
  	          GC_PUSH_ONE_HEAP(qcontents, (q)); \
  		  qcontents = (ptr_t)((q)[1]); \
  	          GC_PUSH_ONE_HEAP(qcontents, (q)+1); }
  # elif GC_GRANULE_WORDS == 4
  #   define USE_PUSH_MARKED_ACCELERATORS
  #   define PUSH_GRANULE(q) \
  		{ ptr_t qcontents = (ptr_t)((q)[0]); \
  	          GC_PUSH_ONE_HEAP(qcontents, (q)); \
  		  qcontents = (ptr_t)((q)[1]); \
  	          GC_PUSH_ONE_HEAP(qcontents, (q)+1); \
  		  qcontents = (ptr_t)((q)[2]); \
  	          GC_PUSH_ONE_HEAP(qcontents, (q)+2); \
  		  qcontents = (ptr_t)((q)[3]); \
  	          GC_PUSH_ONE_HEAP(qcontents, (q)+3); }
  # endif
  #endif
  
  #ifdef USE_PUSH_MARKED_ACCELERATORS
  /* Push all objects reachable from marked objects in the given block */
  /* containing objects of size 1 granule.			     */
  void GC_push_marked1(struct hblk *h, hdr *hhdr)
  {
      word * mark_word_addr = &(hhdr->hb_marks[0]);
      word *p;
      word *plim;
      word *q;
      word mark_word;
  
      /* Allow registers to be used for some frequently acccessed	*/
      /* global variables.  Otherwise aliasing issues are likely	*/
      /* to prevent that.						*/
      ptr_t greatest_ha = GC_greatest_plausible_heap_addr;
      ptr_t least_ha = GC_least_plausible_heap_addr;
      mse * mark_stack_top = GC_mark_stack_top;
      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++;
  	    q = p;
  	    while(mark_word != 0) {
  	      if (mark_word & 1) {
  		  PUSH_GRANULE(q);
  	      }
  	      q += GC_GRANULE_WORDS;
  	      mark_word >>= 1;
  	    }
  	    p += WORDSZ*GC_GRANULE_WORDS;
  	}
  
  #   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 (granules) objects.				     */
  void GC_push_marked2(struct hblk *h, hdr *hhdr)
  {
      word * mark_word_addr = &(hhdr->hb_marks[0]);
      word *p;
      word *plim;
      word *q;
      word mark_word;
  
      ptr_t greatest_ha = GC_greatest_plausible_heap_addr;
      ptr_t least_ha = GC_least_plausible_heap_addr;
      mse * mark_stack_top = GC_mark_stack_top;
      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++;
  	    q = p;
  	    while(mark_word != 0) {
  	      if (mark_word & 1) {
  		  PUSH_GRANULE(q);
  		  PUSH_GRANULE(q + GC_GRANULE_WORDS);
  	      }
  	      q += 2 * GC_GRANULE_WORDS;
  	      mark_word >>= 2;
  	    }
  	    p += WORDSZ*GC_GRANULE_WORDS;
  	}
  
  #   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;
  }
  
  # if GC_GRANULE_WORDS < 4
  /* Push all objects reachable from marked objects in the given block */
  /* of size 4 (granules) 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(struct hblk *h, hdr *hhdr)
  {
      word * mark_word_addr = &(hhdr->hb_marks[0]);
      word *p;
      word *plim;
      word *q;
      word mark_word;
  
      ptr_t greatest_ha = GC_greatest_plausible_heap_addr;
      ptr_t least_ha = GC_least_plausible_heap_addr;
      mse * mark_stack_top = GC_mark_stack_top;
      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++;
  	    q = p;
  	    while(mark_word != 0) {
  	      if (mark_word & 1) {
  		  PUSH_GRANULE(q);
  		  PUSH_GRANULE(q + GC_GRANULE_WORDS);
  		  PUSH_GRANULE(q + 2*GC_GRANULE_WORDS);
  		  PUSH_GRANULE(q + 3*GC_GRANULE_WORDS);
  	      }
  	      q += 4 * GC_GRANULE_WORDS;
  	      mark_word >>= 4;
  	    }
  	    p += WORDSZ*GC_GRANULE_WORDS;
  	}
  #   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 /* GC_GRANULE_WORDS < 4 */
  
  #endif /* UNALIGNED */
  
  #endif /* USE_PUSH_MARKED_ACCELERATORS */
  
  /* Push all objects reachable from marked objects in the given block */
  void GC_push_marked(struct hblk *h, hdr *hhdr)
  {
      size_t sz = hhdr -> hb_sz;
      word descr = hhdr -> hb_descr;
      ptr_t p;
      word bit_no;
      ptr_t lim;
      mse * GC_mark_stack_top_reg;
      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 > MAXOBJBYTES) {
          lim = h -> hb_body;
      } else {
          lim = (h + 1)->hb_body - sz;
      }
  
      switch(BYTES_TO_GRANULES(sz)) {
  #   if defined(USE_PUSH_MARKED_ACCELERATORS)
       case 1:
         GC_push_marked1(h, hhdr);
         break;
  #    if !defined(UNALIGNED)
         case 2:
           GC_push_marked2(h, hhdr);
           break;
  #     if GC_GRANULE_WORDS < 4
         case 4:
           GC_push_marked4(h, hhdr);
           break;
  #     endif
  #    endif
  #   endif
       default:
        GC_mark_stack_top_reg = GC_mark_stack_top;
        for (p = h -> hb_body, bit_no = 0; p <= lim;
  	   p += sz, bit_no += MARK_BIT_OFFSET(sz)) {
           if (mark_bit_from_hdr(hhdr, bit_no)) {
             /* Mark from fields inside the object */
               PUSH_OBJ(p, hhdr, GC_mark_stack_top_reg, mark_stack_limit);
           }
        }
        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(struct hblk *h, hdr *hhdr)
  {
      size_t sz = hhdr -> hb_sz;
  
      if (sz <= MAXOBJBYTES) {
           return(GC_page_was_dirty(h));
      } else {
  	 ptr_t p = (ptr_t)h;
           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(struct hblk *h)
  {
      hdr * hhdr = HDR(h);
  
      if (EXPECT(IS_FORWARDING_ADDR_OR_NIL(hhdr), FALSE)) {
        h = GC_next_used_block(h);
        if (h == 0) return(0);
        hhdr = GC_find_header((ptr_t)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(struct hblk *h)
  {
      hdr * hhdr = HDR(h);
  
      if (!GC_dirty_maintained) { ABORT("dirty bits not set up"); }
      for (;;) {
  	if (EXPECT(IS_FORWARDING_ADDR_OR_NIL(hhdr), FALSE)) {
            h = GC_next_used_block(h);
            if (h == 0) return(0);
            hhdr = GC_find_header((ptr_t)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);
  	hhdr = HDR(h);
      }
      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(struct hblk *h)
  {
      hdr * hhdr = HDR(h);
  
      for (;;) {
  	if (EXPECT(IS_FORWARDING_ADDR_OR_NIL(hhdr), FALSE)) {
            h = GC_next_used_block(h);
            if (h == 0) return(0);
            hhdr = GC_find_header((ptr_t)h);
  	}
  	if (hhdr -> hb_obj_kind == UNCOLLECTABLE) break;
          h += OBJ_SZ_TO_BLOCKS(hhdr -> hb_sz);
  	hhdr = HDR(h);
      }
      GC_push_marked(h, hhdr);
      return(h + OBJ_SZ_TO_BLOCKS(hhdr -> hb_sz));
  }
  