Considerable research has been directed towards efficient test stimuli generation for Register Transfer Level (RTL) circuits. However, stimuli generation frameworks are still not capable of generating effective stimuli for all circuits. Some of the limiting factors are 1) It is hard to ascertain if a branch in the RTL code is reachable, and 2) Some hard-to-reach branches require intelligent algorithms to reach them.

Since unreachable branches cannot be reached by any test sequence, we propose a method to deduce unreachability of a branch by looking for the possible values which a signal can take in an RTL code without explicit unrolling of the design. To the best of our knowledge, this method has been able to identify more unreachable branches than any method published in this domain, while being computationally less expensive.

Moreover, some branches require very specific values on input signals in specific cycles to reach them. Conventional symbolic execution can generate those values but is computationally expensive. We propose a cycle-by-cycle restrictive symbolic execution that analyzes only a selected subset of program statements to reduce the computational cost. Our proposed method gathers information from an initial execution trace generated by any technique, to intelligently decide specific cycles where the application of this method will be helpful. This method can hybrid with simulation-based test stimuli generation methods to reduce the cost of formal verification. With this method, we were able to reach some previously unreached branches in ITC99 benchmark circuits.