I tried to compile software with some legacy math functions, in these case you need to use -fdec-math option (see https://gcc.gnu.org/onlinedocs/gfortran/Option-Summary.html for it). I got error message gfortran: error: unrecognized command line option '-fdec-math'; did you mean '-ffast-math'?
USE flags: Installed versions: 6.4.0-r1(6.4.0)^s(12时52分59秒 2018年03月27日)(cxx fortran multilib nls nptl openmp pch pie sanitize ssp vtv -altivec -awt -cilk -debug -doc -fixed-point -gcj -go -graphite -hardened -jit -libssp -mpx -objc -objc++ -objc-gc -pgo -regression-test -vanilla)
The manual you refer to describes GCC 7, wherein -fdec-math was introduced. https://gcc.gnu.org/gcc-7/changes.html Support for a number of extensions for compatibility with legacy code with new flags: -fdec-structure Support for DEC STRUCTURE and UNION -fdec-intrinsic-ints Support for new integer intrinsics with B/I/J/K prefixes such as BABS, JIAND... -fdec-math Support for additional math intrinsics, including COTAN and degree-valued trigonometric functions such as TAND, ASIND... -fdec Enable the -fdec-* family of extensions.
