[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.