Optimizing Java Programs in the Presence of Exceptions

Abstract

The support for precise exceptions in Java, combined with frequent checks for runtime exceptions, leads to severe limitations on the compiler’s ability to perform program optimizations that involve reordering of instructions. This paper presents a novel framework that allows a compiler to relax these constraints. We first present an algorithm using dynamic analysis, and a variant using static analysis, to identify the subset of program state that need not be preserved if an exception is thrown. This allows many spurious dependence constraints between potentially excepting instructions (PEIs) and writes into variables to be eliminated. Our dynamic algorithm is particularly suitable for dynamically dispatched methods in object-oriented languages, where static analysis may be quite conservative. We then present the first software-only solution that allows dependence constraints among PEIs to be completely ignored while applying program optimizations, with no need to execute any additional instructions if an exception is not thrown. With a preliminary implementation, we show that for many benchmark programs, a large percentage of methods can be optimized (while honoring the precise exception requirement) without any constraints imposed by frequent runtime exceptions. Finally, we show that relaxing these reordering constraints can lead to substantial improvements (up to a factor of 7 on small codes) in the performance of programs.