The CLAW DSL: Abstractions for Performance Portable Weather and Climate Models

In order to profit from emerging high-performance computing systems, weather and climate models need to be adapted to run efficiently on different hardware architectures such as accelerators. This is a major challenge for existing community models that represent very large code bases written in Fortran. We introduce the CLAW domain-specific language (CLAW DSL) and the CLAW Compiler that allows the retention of a single code written in Fortran and achieve a high degree of performance portability. Specifically, we present the Single Column Abstraction (SCA) of the CLAW DSL that is targeted at the column-based algorithmic motifs typically encountered in the physical parameterizations of weather and climate models. Starting from a serial and non-optimized source code, the CLAW Compiler applies transformations and optimizations for a specific target hardware architecture and generates parallel optimized Fortran code annotated with OpenMP or OpenACC directives. Results from a state-of-the-art radiative transfer code, indicate that using CLAW, the amount of source code can be significantly reduced while achieving efficient code for x86 multi-core CPUs and GPU accelerators. The CLAW DSL is a significant step towards performance portable climate and weather model and could be adopted incrementally in existing code with limited effort.