Pragmatic Performance Portability with OpenMP 4.x

In this paper we investigate the current compiler technologies supporting OpenMP 4.x features targeting a range of devices, in particular, the Cray compiler 8.5.0 targeting an Intel Xeon Broadwell and NVIDIA K20x, IBM’s OpenMP 4.5 Clang branch (clang-ykt) targeting an NVIDIA K20x, the Intel compiler 16 targeting an Intel Xeon Phi Knights Landing, and GCC 6.1 targeting an AMD APU. We outline the mechanisms that they use to map the OpenMP model onto their target architectures, and conduct performance testing with a number of representative data parallel kernels. Following this we present a discussion about the current state of play in terms of performance portability and propose some straightforward guidelines for writing performance portable code, derived from our observations. At the time of writing, developers will likely have to rely on the pre-processor for certain kernels to achieve functional portability, but we expect that future homogenisation of required directives between compilers and architectures is feasible.