Offloading support for Openmp in clang and LLVM

Abstract

OpenMP 4.5 allows performance portability by enabling users to write a single application code and run it on multiple types of accelerators. Our goal is to deliver a high-performance implementation of OpenMP into the Clang/LLVM project. This paper describes our initial work to fully support code generation for OpenMP device offloading constructs. We describe a new driver implementation to handle compilation for multiple host and device types, which generalizes the current Clang CUDA implementation and supports OpenMP. It can also be extended to any offloading based language including OpenCL and OpenACC. We describe an implementation of the OpenMP offloading constructs in the runtime library, giving details on two critical aspects. First, how data mapping is implemented. Second, how different device code sections in the binaries are handled to enable application execution on different devices without recompilation. We report initial performance on a prototype that extends current LLVM trunk repositories with all our proposed patches plus future ones, showing near-CUDA performance of our solution.