FWIW, on MSVC the cutoff size is 8 bytes before structs are passed on the stack. It's an ABI detail that shouldn't be relied upon in portable code. ...but also

FWIW, on MSVC the cutoff size is 8 bytes before structs are passed on the stack. It's an ABI detail that shouldn't be relied upon in portable code. ...but also don't stress it too much for functions that are not called frequently - and for frequently called small functions like in the example, just make sure that the compiler can inline the code (for instance via LTO) this will unlock much more useful optimizations beyond just passing args in registers.