See https://github.com/alibaba/PhotonLibOS/blob/2fb4e979a4913e68... for GNU C++ example. It's a tiny bit more limited, because of how the compilation works, but the concept is the same.

To be fair, this can be done in GNU C as well. Like the Zig implementation, you'd still have to use inline assembly.

> If this was done the classical C way, you would always have to stack-save a number of registers

I see, so you're saying that GCC can be coaxed into gathering only the relevant registers to stack and unstack not blindly do all of them?

Yes, you write inline assembly that saves the frame pointer, stack pointer, and instruction pointer to the stack, and list every other register as a clobber. GCC will know which ones its using at the call-site (assuming the function gets inlined; this is more likely in Zig due to its single unit of compilation model), and save those to the stack. If it doesn't get inlined, it'll be treated as any other C function and only save the ones needed to be preserved by the target ABI.