Computing summaries of string loops in C for better testing and refactoring

Analysing and comprehending C programs that use strings is hard: using standard library functions for manipulating strings is not enforced and programs often use complex loops for the same purpose. We introduce the notion of memoryless loops that capture some of these string loops and present a counterexample-guided synthesis approach to summarise memoryless loops using C standard library functions, which has applications to testing, optimisation and refactoring. We prove our summarisation is correct for arbitrary input strings and evaluate it on a database of loops we gathered from thirteen open-source programs. Our approach can summarise over two thirds of memoryless loops in less than five minutes of computation time per loop. We then show that these summaries can be used to (1) improve symbolic execution (2) optimise native code, and (3) refactor code.