OpenCL vs OpenACC: Lessons from Development of Lattice QCD Simulation Code

OpenCL and OpenACC are generic frameworks for heterogeneous programming using CPU and accelerator devices such as GPUs. They have contrasting features: the former explicitly controls devices through API functions, while the latter generates such procedures along a guide of the directives inserted by a programmer. In this paper, we apply these two frameworks to a general-purpose code set for numerical simulations of lattice QCD, which is a computational physics of elementary particles based on the Monte Carlo method. The fermion matrix inversion, which is usually the most time-consuming part of the lattice QCD simulations, is offloaded to the accelerator devices. From a viewpoint of constructing reusable components based on the object-oriented programming and also tuning the code to achieve high performance, we discuss feasibility of these frameworks through the practical implementations