OpenMP 4.5 compiler optimization for GPU offloading

Ability to efficiently offload computational workloads to graphic processing units (GPUs) is critical for the success of hybrid CPU–GPU architectures, such as the Summit and Sierra supercomputing systems. OpenMP 4.5 is a high-level programming model that enables the development of architecture- and accelerator-independent applications. This article describes aspects of the OpenMP implementation in the IBM XL C/C++ and XL Fortran OpenMP compilers that aid programmers to achieve performance objectives. This includes an interprocedural static analysis the XL optimizer uses to specialize code generation of the OpenMP distribute parallel do loop within the dynamic context of a target region, and other compiler optimizations designed to reduce the overhead of data transferred to an offloaded target region. We introduce the heuristic used at runtime to select optimal grid sizes for offloaded target team constructs. These tuned heuristics lead to an average improvement of 2× in the runtime of several target regions in the SPEC ACCEL V1.2 benchmark suite. In addition to performance enhancement, this article also presents an advanced diagnostic feature implemented in the XL Fortran compiler to aid in debugging OpenMP applications offloaded to accelerators.