Automatically Introducing Tail Recursion in CakeML

Abstract

In this thesis, we describe and implement an optimizing compiler transformation which turns non–tail-recursive functions into equivalent tail-recursive functions in an intermediate language of the CakeML compiler. CakeML is a strongly typed functional language based on Standard ML with call-by-value semantics and a fully verified compiler. We integrate our implementation with the existing structure of the CakeML compiler, and provide a machine-checked proof verifying that the observational semantics of programs is preserved under the transformation. To the best of our knowledge, this is the first fully verified implementation of this transformation in any modern compiler. Moreover, our verification efforts uncover surprising drawbacks in some of the verification techniques currently employed in several parts of the CakeML compiler. We analyze these drawbacks and discuss potential remedies.