ORAQL - Optimistic Responses to Alias Queries in LLVM

Alias analysis (AA) is a prerequisite for many compiler optimizations, which are crucial for performance especially for parallel and scientific software. AA is the subject of ongoing research, and compilers can in practice only approximate the alias information of a given program. In this paper we investigate the extent to which performance in high-performance computing (HPC) applications could be improved if better AA were available in LLVM, one of the most widely used compilers today. To this end we present ORAQL, an optimistic (rather than conservative) AA pass for LLVM that determines AA queries that cannot be answered conclusively by existing techniques, and systematically explores which queries can be answered no-alias without breaking user-provided tests. While ORAQL does not result in provably correct programs and therefore should not be used to compile production code, it allows us to estimate the gap between current and ideal performance. By determining the AA queries that cause the majority of this gap, ORAQL may also guide developers toward beneficial modifications to AA or to HPC programs. Our results showthat the performance of HPC proxy applications across multiple programming languages and parallel programming models is not severely limited by AA when compiled with LLVM, although we show performance gains for some applications.