  The LHCb experiment at CERN is undergoing an upgrade in preparation for the
Run 3 data taking period of the LHC. As part of this upgrade the trigger is
moving to a fully software implementation operating at the LHC bunch crossing
rate. We present an evaluation of a CPU-based and a GPU-based implementation of
the first stage of the High Level Trigger. After a detailed comparison both
options are found to be viable. This document summarizes the performance and
implementation details of these options, the outcome of which has led to the
choice of the GPU-based implementation as the baseline.
