For AMDGPU, we have been using the operating system component of the triple
for specifying the low-level runtime that is being used. The rationale for
this is that the host operating system (e.g. Linux) is irrelevant for GPU code,
since its execution enviroment will be mostly controled by the low-level runtime
being used to execute the code.
In most cases, higher level languages have their own runtime which is
implemented on top of the low-level runtime. The kernel ABIs of each
language mostly depend on the low-level runtime, but there may be some
slight differences between languages. OpenCL for example, may append
additional arguments to the kernel in order to pass values like global
offsets or buffers for printf. OpenMP, HCC, or other languages may want
to add their own values which differ from OpenCL.
The reason for adding a new opencl environment type is to make it possible for the backend
to distinguish between the ABIs of the higher-level languages and handle them correctly.
It seems cleaner to use the enviroment component for this rather than creating a new
OS type for every combination of low-level runtime / high-level language.