I think it's incorrect assumption. If you want to develop a compiler for this new ISA, you'll develop a compiler into Clang's IL (intermediate language, that's

I think it's incorrect assumption. If you want to develop a compiler for this new ISA, you'll develop a compiler into Clang's IL (intermediate language, that's probably incorrect name, I'm not a specialist in compilers) and then Apple's Clang will do the rest. Or may be even Apple will develop that part of Clang and make it open source then this is your documentation effectively.