Bit-Precise Reasoning with Parametric Bit-Vectors

The SMT-LIB theory of bit-vectors is restricted to bit-vectors of fixed width. However, several important applications can benefit from reasoning about bit-vectors of symbolic widths, i.e., parametric bit-vectors. Recent work has introduced an approach for solving formulas over parametric bit-vectors, via an eager translation to quantified integer arithmetic with uninterpreted functions. The approach was shown to be successful for several applications, including the bit-width independent verification of compiler optimizations, invertibility conditions, and rewrite rules. We extend and improve that approach in several aspects. Theoretically, we improve expressiveness by defining a new theory of parametric bit-vectors that supports more operators and allows reasoning about the bit-widths themselves. Algorithmically, we introduce a lazy algorithm that avoids the use of uninterpreted functions and quantified axioms for them. Empirically, we show a significant improvement by implementing and evaluating our approach, and comparing it experimentally to the previous one.