https://wiki.archlinux.org/index.php/AMDGPU_PRO
Or… you can just follow these very simple instructions…
For proprietary OpenGL implementation, install amdgpu-pro-libgl (AUR).
For proprietary OpenCL implementation, install opencl-amdgpu-pro-orca (AUR) or opencl-amdgpu-pro-pa (AUR)l.
For proprietary Vulkan implementation, install vulkan-amdgpu-pro (AUR).
Curently (as of version 20.20_1098277-1) you can use proprietary OpenGL with workaround. You need to install amdgpu-pro-libglAUR only before launching application that requires it (DaVinci Resolve in most cases). Remove it, after your are done. Otherwise, your Display Manager (sddm, for example) could not be loaded at next system boot. In case you are already in such situation, just switch to the tty2 (Crtl+Alt+F2), login to the system and remove amdgpu-pro-libglAUR. Then run
systemctl restart sddm.service, then switch to tty1.
Instead of using the “alternative”