Fortran/gfortran

How do I force gfortran to stop program at first NaN?
 * -ffpe-trap=invalid

How do I force gfortran to stop at a first floating point exception?
 * -ffpe-trap=zero,overflow

What optimization does gfortran support?
 * O0, O1, O2, O3, Os, Ofast, Og.

How do I debug?
 * Use gdb.