Verifying Efficient Function Calls in CakeML

Scott Owens University of Kent, UK, gives the third talk in the first panel, Low-level and Systems Programming, on the 2nd day of the ICFP conference. Co-written Michael Norrish, Ramana Kumar (Data61 at CSIRO, Australia and UNSW, Australia), Magnus O. Myreen (Chalmers University of Technology, Sweden), Yong Kiam Tan (Carnegie Mellon University, USA). We have designed an intermediate language (IL) for the CakeML compiler that supports the verified, efficient compilation of functions and calls. Verified compilation steps include batching of multiple curried arguments, detecting calls to statically known functions, and specialising calls to known functions with no free variables. Finally, we verify the translation to a lower-level IL that only supports closed, first-order functions. These compilation steps resemble those found in other compilers (especially OCaml). Our contribution here is the design of the semantics of the IL, and the demonstration that our verification techniques over this semantics work well in practice at this scale. The entire development was carried out in the HOL4 theorem prover.