gc.c

static void
gc_mark(ptr, lev)
    VALUE ptr;
    int lev;
{
    register RVALUE *obj;

    obj = RANY(ptr);
    if (rb_special_const_p(ptr)) return; /* special const not marked */
    if (obj->as.basic.flags == 0) return;       /* free cell */
    if (obj->as.basic.flags & FL_MARK) return;  /* already marked */
    obj->as.basic.flags |= FL_MARK;

    if (lev > GC_LEVEL_MAX || (lev == 0 && ruby_stack_check())) {
	if (!mark_stack_overflow) {
	    if (mark_stack_ptr - mark_stack < MARK_STACK_MAX) {
		*mark_stack_ptr = ptr;
		mark_stack_ptr++;		
	    }
	    else {
		mark_stack_overflow = 1;
	    }
	}
	return;
    }
    gc_mark_children(ptr, lev+1);
}

ruby.h

#define IMMEDIATE_MASK 0x03
#define IMMEDIATE_P(x) ((VALUE)(x) & IMMEDIATE_MASK)
#define SPECIAL_CONST_P(x) (IMMEDIATE_P(x) || !RTEST(x))

static inline int
#if defined(HAVE_PROTOTYPES)
rb_special_const_p(VALUE obj)
#else
rb_special_const_p(obj)
    VALUE obj;
#endif
{
    if (SPECIAL_CONST_P(obj)) return Qtrue;
    return Qfalse;
}