To support most REAL(16) (i.e. 128-bit float) math intrinsics Flang relies on third-party libraries providing the implementation.
-DFLANG_RUNTIME_F128_MATH_LIB=libquadmath CMake option can be used to build FortranFloat128Math library that has unresolved references to GCC libquadmath library. A Flang driver built with this option will automatically link FortranFloat128Math and libquadmath libraries to any Fortran program. This implies that libquadmath library has to be available in the standard library paths, so that linker can find it. The libquadmath library installation into Flang project distribution is not automatic in CMake currently.
Testing shows that libquadmath versions before GCC-9.3.0 have accuracy issues, so it is recommended to distribute the Flang package with later versions of libquadmath.
Care must be taken by the distributors of a Flang package built with REAL(16) support via libquadmath because of its licensing under the GNU Library General Public License. Moreover, static linking of libquadmath to the Flang users' programs may imply some restrictions/requirements. This document is not intended to give any legal advice on distributing such a Flang compiler.
Flang compiler targeting systems with LDBL_MANT_DIG == 113 may provide REAL(16) math support without a libquadmath dependency, using standard libc APIs for the long double data type. It is not recommended to use the above CMake option for building Flang compilers for such targets.