AFAICT, it seems that win64 x64 gcc alloca works the same (Don't remember though if the difference was alloca return value=sp+8 or sp+16).

So x64 win64 FFI only works because we force sp before calling externalFunction with:

    if ((0 + (cStackAlignment())) > 0) {
        setsp((calloutState->argVector));
    }