We present a method based on program analysis and formal verification to identify conditionally relevant variables (CRVs) - variables which could lead to violation of safety properties in control software when affected by single event upsets (SEUs). Traditional static analysis can distinguish between relevant and irrelevant variables. However, it would fail to take into account the conditions specific to the control software in question. This can lead to false positives. Our algorithm employs formal verification to avoid false positives. We have conducted experiments that demonstrate that CRVs indeed are fewer in number than what traditional static analysis can detect and that our algorithm is able to identify this fact. The information provided by our algorithm could prove helpful to a compiler while it does register allocation during the compilation of the control software. In turn, this could cause significant reduction in the cost of controller chips.