garbage_collect(int testing)
{
    int		copyID;
    int		abort = FALSE;
    buf_T	*buf;
    win_T	*wp;
    int		did_free = FALSE;
    tabpage_T	*tp;

    if (!testing)
    {
	// Only do this once.
	want_garbage_collect = FALSE;
	may_garbage_collect = FALSE;
	garbage_collect_at_exit = FALSE;
    }

    // The execution stack can grow big, limit the size.
    if (exestack.ga_maxlen - exestack.ga_len > 500)
    {
	size_t	new_len;
	char_u	*pp;
	int	n;

	// Keep 150% of the current size, with a minimum of the growth size.
	n = exestack.ga_len / 2;
	if (n < exestack.ga_growsize)
	    n = exestack.ga_growsize;

	// Don't make it bigger though.
	if (exestack.ga_len + n < exestack.ga_maxlen)
	{
	    new_len = (size_t)exestack.ga_itemsize * (exestack.ga_len + n);
	    pp = vim_realloc(exestack.ga_data, new_len);
	    if (pp == NULL)
		return FAIL;
	    exestack.ga_maxlen = exestack.ga_len + n;
	    exestack.ga_data = pp;
	}
    }

    // We advance by two because we add one for items referenced through
    // previous_funccal.
    copyID = get_copyID();

    /*
     * 1. Go through all accessible variables and mark all lists and dicts
     *    with copyID.
     */

    // Don't free variables in the previous_funccal list unless they are only
    // referenced through previous_funccal.  This must be first, because if
    // the item is referenced elsewhere the funccal must not be freed.
    abort = abort || set_ref_in_previous_funccal(copyID);

    // script-local variables
    abort = abort || garbage_collect_scriptvars(copyID);

    // buffer-local variables
    FOR_ALL_BUFFERS(buf)
	abort = abort || set_ref_in_item(&buf->b_bufvar.di_tv, copyID,
								  NULL, NULL);

    // window-local variables
    FOR_ALL_TAB_WINDOWS(tp, wp)
	abort = abort || set_ref_in_item(&wp->w_winvar.di_tv, copyID,
								  NULL, NULL);
    if (aucmd_win != NULL)
	abort = abort || set_ref_in_item(&aucmd_win->w_winvar.di_tv, copyID,
								  NULL, NULL);
#ifdef FEAT_PROP_POPUP
    FOR_ALL_POPUPWINS(wp)
	abort = abort || set_ref_in_item(&wp->w_winvar.di_tv, copyID,
								  NULL, NULL);
    FOR_ALL_TABPAGES(tp)
	FOR_ALL_POPUPWINS_IN_TAB(tp, wp)
		abort = abort || set_ref_in_item(&wp->w_winvar.di_tv, copyID,
								  NULL, NULL);
#endif

    // tabpage-local variables
    FOR_ALL_TABPAGES(tp)
	abort = abort || set_ref_in_item(&tp->tp_winvar.di_tv, copyID,
								  NULL, NULL);
    // global variables
    abort = abort || garbage_collect_globvars(copyID);

    // function-local variables
    abort = abort || set_ref_in_call_stack(copyID);

    // named functions (matters for closures)
    abort = abort || set_ref_in_functions(copyID);

    // function call arguments, if v:testing is set.
    abort = abort || set_ref_in_func_args(copyID);

    // funcstacks keep variables for closures
    abort = abort || set_ref_in_funcstacks(copyID);

    // loopvars keep variables for loop blocks
    abort = abort || set_ref_in_loopvars(copyID);

    // v: vars
    abort = abort || garbage_collect_vimvars(copyID);

    // callbacks in buffers
    abort = abort || set_ref_in_buffers(copyID);

    // 'completefunc', 'omnifunc' and 'thesaurusfunc' callbacks
    abort = abort || set_ref_in_insexpand_funcs(copyID);

    // 'operatorfunc' callback
    abort = abort || set_ref_in_opfunc(copyID);

    // 'tagfunc' callback
    abort = abort || set_ref_in_tagfunc(copyID);

    // 'imactivatefunc' and 'imstatusfunc' callbacks
    abort = abort || set_ref_in_im_funcs(copyID);

#ifdef FEAT_LUA
    abort = abort || set_ref_in_lua(copyID);
#endif

#ifdef FEAT_PYTHON
    abort = abort || set_ref_in_python(copyID);
#endif

#ifdef FEAT_PYTHON3
    abort = abort || set_ref_in_python3(copyID);
#endif

#ifdef FEAT_JOB_CHANNEL
    abort = abort || set_ref_in_channel(copyID);
    abort = abort || set_ref_in_job(copyID);
#endif
#ifdef FEAT_NETBEANS_INTG
    abort = abort || set_ref_in_nb_channel(copyID);
#endif

#ifdef FEAT_TIMERS
    abort = abort || set_ref_in_timer(copyID);
#endif

#ifdef FEAT_QUICKFIX
    abort = abort || set_ref_in_quickfix(copyID);
#endif

#ifdef FEAT_TERMINAL
    abort = abort || set_ref_in_term(copyID);
#endif

#ifdef FEAT_PROP_POPUP
    abort = abort || set_ref_in_popups(copyID);
#endif

    if (!abort)
    {
	/*
	 * 2. Free lists and dictionaries that are not referenced.
	 */
	did_free = free_unref_items(copyID);

	/*
	 * 3. Check if any funccal can be freed now.
	 *    This may call us back recursively.
	 */
	free_unref_funccal(copyID, testing);
    }
    else if (p_verbose > 0)
    {
	verb_msg(_("Not enough memory to set references, garbage collection aborted!"));
    }

    return did_free;
}