Never mind, I see it in the other commit. Could you please update the table of relocation types in AMDGPUUsage.rst? It has an R_AMDGPU_RELATIVE64, I don't know what that's about, but it doesn't mention R_AMDGPU_REL64.
The AMDGPU_REL64 is defined in AMDGPUUsage as:
Relocation Type Kind Value Field Calculation
R_AMDGPU_REL64 Static 5 word64 S + A - P
It is a static relocation used to create PC relative values between sections. For example, there is a field in the kernel descriptor that has to be the delta from the kernel descriptor to the kernel entry point. So if the kernel descriptor is put in a different section to the code, a static relocation record is need. It will be resolved when the executable is linked.
The R_AMDGPU_RELATIVE64 is a dynamic relocation record implemented by most targets to support having absolute addresses in data sections that need to be initialized to the address of a symbol in the same shared object. For example, a variable initialized to the address of another variable which is considered a compile time constant in languages such as C/C++.