Sentinel PRE: hoisting beyond exception dependency with dynamic deoptimization

Abstract

Many excepting instructions cannot be removed by existing partial redundancy elimination (PRE) algorithms because the ordering constraints must be preserved between the excepting instructions, which we call exception dependencies. In this work, we propose Sentinel PRE, a PRE algorithm that overcomes exception dependencies and retains program semantics. Sentinel PRE first hoists excepting instructions without considering exception dependencies, and then detects exception reordering by fast analysis. If an exception occurs at a reordered instruction, it deoptimizes the code into the one before hoisting. Since we rarely encounter exceptions in real programs, the optimized code is executed in almost all cases. We implemented Sentinel PRE in a Java just-in-time compiler and conducted experiments. The results show 9.0% performance improvement in the LU program in the Java Grande Forum Benchmark Suite.