Proving JDK's Dual Pivot Quicksort Correct

Sorting is a fundamental functionality in libraries, for which efficiency is crucial. Correctness of the highly optimized implementations is often taken for granted. De Gouw et al. have shown that this certainty is deceptive by revealing a bug in the Java Development Kit (JDK) implementation of TimSort. We have formally analysed the other implementation of sorting in the JDK standard library: A highly efficient implementation of a dual pivot quicksort algorithm. We were able to deductively prove that the algorithm implementation is correct. However, a loop invariant which is annotated to the source code does not hold. This paper reports on how an existing piece of non-trivial Java software can be made accessible to deductive verification and successfully proved correct, for which we use the Java verification engine KeY.