Typecasting std::complex<double> to __complex128

I guess you're using GCC, in which case you can use the __real__ and __imag__ extensions to set the individual components of your __complex128:

__complex128 y;
__real__ y = x.real();
__imag__ y = x.imag();

This works in Clang for __complex64, too (Clang doesn't yet support __complex128).