Positive supercompilation for a higher-order call-by-value language

Abstract: Intermediate structures such as lists and higher-order functions are very common in most styles of functional programming. While allowing the programmer to write clear and concise programs, the creation and destruction of these structures impose a run time overhead which is not negligible. Deforestation algorithms is a family of program transformations that remove these intermediate structures in an automated fashion, thereby improving program performance. While there has been plenty of work on deforestation-like transformations that remove intermediate structures for languages with call-by-name semantics, no investigations have been performed or call-by-value languages. It has been suggested that existing call-by-name algorithms could be applied to call-by-value programs, possibly introducing termination in the program. This hides looping bugs from the programmer, and changes the behaviour of a program depending on whether it is optimized or not. We present a transformation, positive supercompilation, for a higher-order call-by-value language that preserves termination properties of the programs it is applied to. We prove the algorithm correct and compare it to existing call-by-name transformations. Our results show that deforestation-like transformations are both possible and useful for call-by-value languages, with speedups up to an order of magnitude for certain benchmarks. Our algorithm is particularly important in the context of embedded systems where resources are scarce. By both removing intermediate structures and performing program specialization the footprint of programs can shrink considerably without any manual intervention by the programmer.