-pba_mode后什么都不加=不用PBA, 跑GBA。
path 先跑一遍GBA,再给有违例的path单独跑PBA; 足以应对绝大多数情况。
exhaustive - Performs an exhaustive path-based analysis to determine the truly worst-case paths in the design. This is the most accurate and most computation-intensive mode. You cannot use the exhaustive mode together with the -start_end_pair, -cover_design, or path_collection options.