Compiler Optimizations for OpenMP

摘要

Modern compilers support OpenMP as a convenient way to introduce parallelism into sequential languages like C/C++ and Fortran, however, its use also introduces immediate drawbacks. In many implementations, due to early outlining and the indirection though the OpenMP runtime, the front-end creates optimization barriers that are impossible to overcome by standard middle-end compiler passes. As a consequence, the OpenMP-annotated program constructs prevent various classic compiler transformations like constant propagation and loop invariant code motion. In addition, analysis results, especially alias information, is severely degraded in the presence of OpenMP constructs which can severely hurt performance. In this work we investigate to what degree OpenMP runtime aware compiler optimizations can mitigate these problems. We discuss several transformations that explicitly change the OpenMP enriched compiler intermediate representation. They act as stand-alone optimizations but also enable existing optimizations that were not applicable before. This is all done in the existing LLVM/Clang compiler toolchain without introducing a new parallel representation. Our optimizations do not only improve the execution time of OpenMP annotated programs but also help to determine the caveats for transformations on the current representation of OpenMP.