Unknown value (Xs) may exist in a design due to uninitialized registers or blocks that are powered down. Such Xs may cause X bugs known as X-pessimism and X-optimism. When we are solving X-pessimism problems, we have to trace along the fan-ins of a register's input. By reconstructing cell functions, the procedure above will get extra information of the cells encountered and thus can reduce its search space. To analyze the cell properly and provide the exact cell functions information, we proposed a methodology including pattern matching and symmetry detecting techniques which reveals the functional information about SUDP inputs. Our experimental results on commercial designs show that the proposed method is effective and efficient.