Comment by Pannoniae

Yeah, that's what MSVC and GCC did on x86, called "custom calling conventions" on MSVC and the regparm attribute on GCC.

All of these were dropped on x64, on x64 (and ARM) you get standard calling conventions for just about everything with proper unwind tables for functions.

It doesn't really "cheat" on the registers unless it inlines a function entirely. LLVM has support for custom calling conventions and pragmas to specify them, this is used by GHC on Haskell and other things, but it's practically unheard of in "normal" C/C++ code.