Implementing OpenMP's SIMD Directive in LLVM's GPU Runtime

GPUs support three levels of parallelism: thread blocks, warps (or wavefronts) within a block, and threads within a warp. Some GPU programming models allow the use of all three of these levels, such as OpenMP offloading with the teams, parallel, and simd directives. However LLVM/OpenMP does not support simd and only uses two levels, thread blocks and all threads within a block. For codes with three explicit layers of parallelism this can decrease performance and potentially require restructuring of the application. In this work we present our design and implementation of the OpenMP simd directive in LLVM's OpenMP GPU runtime, which includes both CPU-centric and GPU-centric execution models. We evaluate our prototype using kernels and a few proxy applications showing a performance improvement ranging from 1.3x to 3.5x depending on the benefit the kernels receives from such an optimization. Thus, this work enables real-world applications with three explicit layers of parallelism to expose to better exploit the full benefits of GPU architecture.