Particle physics has an ambitious and broad experimental programme for the
coming decades. This programme requires large investments in detector hardware,
either to build new facilities and experiments, or to upgrade existing ones.
Similarly, it requires commensurate investment in the R&D of software to
acquire, manage, process, and analyse the shear amounts of data to be recorded.
In planning for the HL-LHC in particular, it is critical that all of the
collaborating stakeholders agree on the software goals and priorities, and that
the efforts complement each other. In this spirit, this white paper describes
the R&D activities required to prepare for this software upgrade.