I was previously using the AMD fork of LLVM, but I've switched to using the Gentoo version. This may or may not work with any given version. Caveat emptor.
For GCC/Offloading support, use this overlay in conjunction with my "gentoo-gpu" overlay.
HIP is tested and working with AMD RX Vega 64 (gfx900) and Vega8/Raven Ridge APU (gfx902), this includes OpenCL, tested with "hashcat". rocm-opencl-runtime currently provides much better support for OpenCL2 than RustiCL.
I would appreciate any testing with other hardware.
gfx902 and gfx900 with provided patches now appear to be fully functional for both HIP and OpenCL.