  Gas clumps formed within massive gravitationally unstable circumstellar discs
are potential seeds of gas giant planets, brown dwarfs and companion stars.
Simulations show that competition between three processes -- migration, gas
accretion and tidal disruption -- establishes what grows from a given seed.
Here we investigate the robustness of numerical modelling of clump migration
and accretion with the codes PHANTOM, GADGET, SPHINX, SEREN, GIZMO-MFM, SPHNG
and FARGO. The test problem comprises a clump embedded in a massive disc at an
initial separation of 120 AU. There is a general qualitative agreement between
the codes, but the quantitative agreement in the planet migration rate ranges
from $\sim 10$% to $\sim 50$%, depending on the numerical setup. We find that
the artificial viscosity treatment and the sink particle prescription may
account for much of the differences between the codes. In order to understand
the wider implications of our work, we also attempt to reproduce the planet
evolution tracks from our hydrodynamical simulations with prescriptions from
three previous population synthesis studies. We find that the disagreement
amongst the population synthesis models is far greater than that between our
hydrodynamical simulations. The results of our code comparison project are
therefore encouraging in that uncertainties in the given problem are probably
dominated by the physics not yet included in the codes rather than by how
hydrodynamics is modelled in them.
