mark.c

A garbage collector for C and C

/* 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) {