Section: New Results

Verified compilation of Lustre

Participants : Timothy Bourke, Marc Pouzet.

Synchronous dataflow languages and their compilers are increasingly used to develop safety-critical applications, like fly-by-wire controllers in aircraft and monitoring software for power plants. A striking example is the SCADE Suite tool of ANSYS/Esterel Technologies which is DO-178B/C qualified for the aerospace and defense industries. This tool allows engineers to develop and validate systems at the level of abstract block diagrams that are automatically compiled into executable code.

Formal modelling and verification in an interactive theorem prover can potentially complement the industrial certification of such tools to give very precise definitions of language features and increased confidence in their correct compilation; ideally, right down to the binary code that actually executes.

This year we picked up on previous work in the PARKAS team to develop a verified compiler for a Lustre/SCADE-like synchronous language. We focused on the critical and until now unresolved compiler stage that transforms dataflow equations into imperative code. We developed, in Coq, a prototype compiler for the core language (without modular resets or tuples) and showed its correctness with respect to a dataflow semantics based on functions from natural numbers to present or absent values. This required the development of a novel intermediate model for relating delayed dataflow streams to imperative memories in such a way that a critical induction could be stated and proved. We further showed how to justify a post-transformation optimization that is essential for the efficiency of clock-directed code generation. We are preparing a paper describing these results. Work continues on both semantic questions (existence of a semantics for well-type and well-clocked programs, treatment of resets, etc.) and compilation issues (integration with the verified CompCert compiler).

In collaboration with Pierre-Évariste Dagand (CNRS) and Lionel Reig (Collège de France).