Mostly Sound Type System Improves a Foundational Program Verifier

We integrate a verified typechecker with a verified program logic for the C language, proved sound with respect to the operational semantics of the CompCert verified optimizing C compiler. The C language is known to not be type-safe but we show the value of a provably mostly sound type system: integrating the typechecker with the program logic makes the logic significantly more usable. The computational nature of our typechecker (within Coq) makes program proof much more efficient. We structure the system so that symbolic execution—even tactical (nonreflective) symbolic execution—can keep the type context and typechecking always in reified form, to avoid expensive re-reification.