Flacc: Towards OpenACC support for Fortran in the LLVM Ecosystem

OpenACC is a directive-based programming model for heterogeneous accelerators initially launched in 2010 to provide a portable solution at a level of abstraction above OpenCL, CUDA, and other lower-level programming models. Various implementations of OpenACC for C, C++, and Fortran exist; however, only one open-source, production implementation of OpenACC for Fortran does exist. Moreover, most contemporary compiler tool chains for heterogeneous computing are based on LLVM. This lack of support poses a serious risk for high-performance computing application developers targeting GPUs and other accelerators, and it limits the ability of the community to experiment with, extend, and contribute to the OpenACC specification and open-source implementation itself. To address this gap, we have designed and begun implementing Flacc: an effort funded by the US Exascale Computing Project to develop production OpenACC compiler support for Fortran based on Flang within the LLVM ecosystem. In this paper, we describe the Flacc goals, initial design and prototype, and challenges that we have encountered so far in our prototyping efforts. Flacc is implemented as a MLIR dialect in the Flang Fortran front end in LLVM. The Flacc front end currently supports OpenACC version 3.1, and the Flacc run time is currently under development and relies on contributions from the Clacc project. Current contributions to Flacc are available in the main ${\color{Green}{\mathbf{LLVM}}\;{\mathbf{repository}}}$.1