Originally posted by: Nicole Trudeau

When compiling with XLC for Linux 13.1.6 and up, you may have noticed strange output from CMake identifying it as Clang, for example:

-- The C compiler identification is Clang 4.0.1
-- The CXX compiler identification is Clang 4.0.1
...
-- CMAKE_CXX_COMPILER_ID Clang

You may have also noticed strange behaviour in your program if you have conditionals checking if CMAKE_CXX_COMPILER_ID STREQUAL/MATCHES XL or Clang.

CMake has fixed the issue in CMake v3.11.0-rc3 (released Mar 9, 2018) and later - please upgrade CMake if you notice this behaviour.

If you use Summit, the OLCF admins made CMake 3.11.3 available on both summit and summitdev, so there should not be any issues there.

Related:

• If your CMake output is showing XL, but when you make it uses GCC, check the cc symlink on your system to make sure it points to XL and not GCC. Do a which cc and then ls -l that. If this doesn't work, follow the 3 methods outlined in the CMake FAQ "How do I use a different compiler?" section.
• CMake 3.12 has a bug where you can request that XL 16.1.1 be used, with OMP compiler flags, yet OMP will be disabled. This only happens with mixed compilation of Fortran and C++. There is a workaround available, see details at Issue 18518: IBM XL incompatibility when linking mixed C++/Fortran project with OpenMP. This was discovered on Oct 27, 2018.