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 into 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 draw-backs in some of the verification techniques employed in several parts of the CakeML compiler. We provide a work-around for these drawbacks, and compare it to potential alternatives.
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