  Recently, there has been growing interest in using standard language
constructs (e.g. C++'s Parallel Algorithms and Fortran's do concurrent) for
accelerated computing as an alternative to directive-based APIs (e.g. OpenMP
and OpenACC). These constructs have the potential to be more portable, and some
compilers already (or have plans to) support such standards. Here, we look at
the current capabilities, portability, and performance of replacing directives
with Fortran's do concurrent using a mini-app that currently implements OpenACC
for GPU-acceleration and OpenMP for multi-core CPU parallelism. We replace as
many directives as possible with do concurrent, testing various configurations
and compiler options within three major compilers: GNU's gfortran, NVIDIA's
nvfortran, and Intel's ifort. We find that with the right compiler versions and
flags, many directives can be replaced without loss of performance or
portability, and, in the case of nvfortran, they can all be replaced. We
discuss limitations that may apply to more complicated codes and future
language additions that may mitigate them. The software and Singularity
containers are publicly provided to allow the results to be reproduced.
