The application of correctness preserving transformations to software maintenance

The size and complexity of hardware and software systems continues to grow, making the introduction of subtle errors a more likely possibility. A major goal of software engineering is to enable developers to construct systems that operate reliably despite increased size and complexity. One approach to achieving this goal is through formal methods: mathematically based languages, techniques and tools for specifying and verifying complex software systems. The authors apply a theoretical tool (that is supported by many formal methods), the correctness preserving transformation (CPT), to a real software engineering problem: the need for optimization during the maintenance of code. We present four program transformations and a model that forms a framework for proof of correctness. We prove the transformations correct and then apply them to a cryptography application implemented in C++. Our experience shows that CPTs can facilitate generation of more efficient code while guaranteeing the preservation of original behavior.