There do exist ABIs that aren't the C ABI. But saying "use the C ABI" is far more portable than anything else. I can also point to the GCC Inline Assembler as

There do exist ABIs that aren't the C ABI. But saying "use the C ABI" is far more portable than anything else. I can also point to the GCC Inline Assembler as an excellent way to call arbitrary functions whether they implement the standard C procedure call standard or not. By providing the list of arguments and what register they correspond to, along with the clobber list, you know everything you need to know to call the function. So it's more suitable for "fastcall" type functions where you need the arguments to correspond to particular registers. But of course, ASM isn't portable.