It all boils down to calling CUDA runtime or driver APIs. Compiler just sprinkles fairly trivial amount of syntactic sugar, and under-the-hood glue code. One can launch GPU kernel from a pure C source file compiled with gcc.

> The GPU kernel code itself needs to be compiled and for CUDA that is done with the proprietary nvcc, which is clang-based.

nvcc is not the only option. Clang itself can compile most of existing CUDA code just fine, and the rest usually needs minimal porting.

Also, nvcc is not based on clang. It may use it as the host compiler, but that's the extent of its involvement with clang. IIRC, their front-end used to be based on EDG.

> HIP is better because it's an open-source llvm backend/frontend

CUDA and HIP share most of the front-end code in clang, so CUDA compilation with clang shares the same benefits. What's missing is PTX to SASS assembler, which creates the actual GPU binary, and that part is proprietary to NVIDIA.