[haberman]

I tried exposing the "cc 10" and "cc 11" calling conventions to Clang, but was unsuccessful. With "cc 10" I got crashes at runtime I could not explain. With "cc 11" I got a compile-time error:

> fatal error: error in backend: Can't generate HiPE prologue without runtime parameters

If "cc 10" would work from Clang I'd be happy to use that. Though I think reverse_cc could potentially still offer benefits by ordering arguments such that callee-save registers are assigned first. That is helpful when calling fallback functions that take arguments in the normal registers.

[obl]

If you didn't already, I'd recommend compiling llvm/clang in debug or release+assert mode when working on it. The codebase is quite heavy in debug-only assertions even for relatively trivial things (like missing implementation of some case, some combination of arguments being invalid, ...) which means that it's pretty easy to get into weird crashes down the line with assertions disabled.

[sillycross]

Yeah, you should do it at LLVM IR level.

[JonChesterfield]

Adding calling conventions to clang is fairly straightforward (e.g. https://reviews.llvm.org/D125970) but there's a limited size structure in clang somewhere that makes doing so slightly contentious. Partly because they're shared across all architectures. At some point we'll run out of bits and have to do something invasive and/or slow to free up space.

[haberman]

Yes, I've already experimented with doing this, see: https://github.com/haberman/llvm-project/commit/e8d9c75bb35c...

But when I tried it on my actual code, the results weren't quite as good as I hoped, due to sub-optimal register allocation.