COGENT: Certified Compilation for a Functional Systems Language

Abstract

We present a self-certifying compiler for the Cogent systems language. Cogent is a restricted, polymorphic, higher-order, and purely functional language with linear types and without the need for a trusted runtime or garbage collector. It compiles to efficient C code that is designed to interoperate with existing C functions. The language is suited for layered systems code with minimal sharing such as file systems or network protocol control code.For a well-typed Cogent program, the compiler produces C code, a high-level shallow embedding of its semantics in Isabelle/HOL, and a proof that the C code correctly implements this embedding. The aim is for proof engineers to reason about the full semantics of real-world systems code productively and equationally, while retaining the interoperability and leanness of C.We describe the formal verification stages of the compiler, which include automated formal refinement calculi, a switch from imperative update semantics to functional value semantics formally justified by the linear type system, and a number of standard compiler phases such as type checking and monomorphisation. The compiler certificate is a series of language-level meta proofs and per-program translation validation phases, combined into one coherent top-level theorem in Isabelle/HOL.

Translation Certification for Smart Contracts

“…The Cogent certifying compiler [22] has shown that it is possible to use translation validation for lowering the cost of functional verification of low-level code: a program can be written and reasoned about in a high-level functional language, which is compiled down to C. The generated certificate then proves a refinement relation, capable of transporting the verification results to the corresponding C code. The situation is different from ours: the Cogent compiler goes through a range of languages with different semantic models and uses the forward-simulation technique as a consequence.…”