Soft error has become an important reliability issue in advanced technologies. To tolerate soft errors, solutions suggested in previous works incur significant performance and power penalties, especially when a design with fault-tolerant structures is overprotected. In this paper, we present a soft-error-tolerant design methodology to tradeoff performance, power, and reliability for different applications. First, four novel detection and correction flip-flop (FF) structures are proposed to provide different levels of tolerance capability against soft errors. Second, architecture-level vulnerability and logic-level susceptibility analyses are employed to identify weak FFs that can easily cause program execution errors. Third, an optimization framework is developed to synthesize the proposed four novel FF structures into weak and highly observable storage bits with the flexibility of trading off performance, power, and reliability. A five-stage pipeline RISC core (UniRISC) is adopted to demonstrate the usefulness of our methodology. Experimental results show that the proposed method can accomplish design goals by balancing performance, power, and reliability. For example, we can not only satisfy the reliability requirement that no more than five errors occur per one billion hours in a design but also reduce up to 87% performance overhead and 91% power overhead when compared with previous works.