I have tried several attempts but without success. AMDGPU-PRO 17.40, 17.50, ROCm, Oibaf PPA, Kernel 4.16 and something more, but nothing makes OpenCL working with the AMD card and clinfo never gives right results.
Can I only wait the next AMDGPU-PRO driver compatible with Ubuntu 18.04 or could I do something different?