  The rise of exascale supercomputers has fueled competition among GPU vendors,
driving lattice QCD developers to write code that supports multiple APIs.
Moreover, new developments in algorithms and physics research require frequent
updates to existing software. These challenges have to be balanced against
constantly changing personnel. At the same time, there is a wide range of
applications for HISQ fermions in QCD studies. This situation encourages the
development of software featuring a HISQ action that is flexible,
high-performing, open source, easy to use, and easy to adapt. In this technical
paper, we explain the design strategy, provide implementation details, list
available algorithms and modules, and show key performance indicators for
SIMULATeQCD, a simple multi-GPU lattice code for large-scale QCD calculations,
mainly developed and used by the HotQCD collaboration. The code is publicly
available on GitHub.
