What Ofast means for Fortran developers

[CRquantum]

Is -Ofast in gfortran or -fast flag in Intel Fortran safe to use?

Dear all, I just wonder, has anyone use -Ofast in gfortran or -fast flag in Intel Fortran? I sometimes find that some results with -fast are a little different from -O3. Is -Ofast or -fast safe to use? Thank you very much in advance!