Here's another big question. How can you tell if your motherboard supports PCI-e atomics?

I did find this thread link. But it raised more questions than answers.

Useless. If you managed to get this compiler bridge working, you'll find the performance dragging. Right now you'll have to use AMDGPU-PRO drivers for good openCL usage.

For arch there is also "official" OpenCL (1.2) for Mesa (AMDGPU). opencl-amd

There's something called "ROCm" but I haven't tried it yet.

Quoting: crt0megaThere's something called "ROCm" but I haven't tried it yet.

Only needed for the Vega cards, Others will use mesa openCL fine. Though apparently you can use ROCm on others.

Still not managed to get it to run with my rx64 :(

Hi I'm on opensuse and done the archway. Use amdgpu mesa drivers and opencl on top (from amdgpu-pro). You only need to install some opencl rpm or debs, copy some libraries and then sed them like it's written on the archlinux PKGBUILD.
Download the full package from amd.com .I think the rpm/debs you need to install are opencl-amdgpu-pro-icd_17.XXXXXX_amd64.deb and extra one, they are two I think (can't find out know the name of the other but it has opencl on it's name).Just install those with rpm/deb package manager.

Then follow the instructions on the PKGBUILD

extract opencl-amdgpu-pro-icd_${major}-${minor}_amd64.deb"
tar xJf data.tar.xz

find this library and sed it (libamdocl64.so)
sed -i "s|libdrm_amdgpu|libdrm_amdgpo|g" libamdocl64.so

extract libdrm-amdgpu-amdgpu1_2.4.82-${minor}_amd64.deb"
tar xJf data.tar.xz

cd into it
rm "libdrm_amdgpu.so.1"
mv "libdrm_amdgpu.so.1.0.0" "libdrm_amdgpo.so.1.0.0"
ln -s "libdrm_amdgpo.so.1.0.0" "libdrm_amdgpo.so.1"

then copy libamdocl64.so libamdocl12cl64.so libdrm_amdgpo.so.1.0.0 libdrm_amdgpo.so.1 to /usr/lib

finally link the ids file
ln -s /usr/share/libdrm/amdgpu.ids /opt/amdgpu-pro/share/libdrm/amdgpu.ids

PS: I don't know what distro you are running I've used the Suse packages from AMD not deb files, but if you are using an uncommon distro you might run into problems, if it crashes and that stuff you need to try with the other packages for other distros from amd.com if there's one that works for you.