C ABI with LLVM

I can't say I understand your question 100%, but it's worth noting that LLVM IR simply can not represent all the subtleties of platform ABIs. Therefore, in the Clang toolchain, it is the frontend that's responsible for performing ABI lowering, such as properly passing objects by value to functions, etc.

Take a look at lib/Basic/Targets.cpp in the Clang source tree for the definitions. The gory details are further in lib/CodeGen/TargetInfo.cpp

I ended up hacking Clang's CodeGen internals to perform C ABI calling for me (C++ ABI support was already done). Thus instead of having to re-implement (and re-test) their code, I simply re-used their work. Officially the CodeGen APIs aren't public and aren't meant to be used by anyone, but in this case, I managed to make it work. It turns out that it's a lot less scary than it looks- many of the classes like LValue/RValue/ReturnValueSlot are just wrappers on llvm::Value* with a couple extra optional semantics tacked on.

More problematic will be creating trampolines from C ABI to my own ABI. The CodeGenFunction interface doesn't seem quite as amenable to that. But I think I can make it work.