Disallow combination of gcc > 9 and IBM_VSX

The compiler flag is not detected for newer versions of gcc and can lead
to confusing error messages.

Fixes #3380

Change-Id: I0c5311325b2bf2bc08ab727eabd670115720b4a6

cmake/gmxManageSimd.cmake

 #
 # This file is part of the GROMACS molecular simulation package.
 #
-# Copyright (c) 2012,2013,2014,2015,2016,2017,2018,2019, by the GROMACS development team, led by
+# Copyright (c) 2012,2013,2014,2015, The GROMACS development team.
+# Copyright (c) 2016,2017,2018,2019,2020, by the GROMACS development team, led by
 # Mark Abraham, David van der Spoel, Berk Hess, and Erik Lindahl,
 # and including many others, as listed in the AUTHORS file in the
 # top-level source directory and at http://www.gromacs.org.
@@ -263,6 +264,13 @@ elseif(GMX_SIMD_ACTIVE STREQUAL "IBM_VMX")
 
 elseif(GMX_SIMD_ACTIVE STREQUAL "IBM_VSX")
 
+    # IBM_VSX and gcc > 9 do not work together, so we need to prevent people from
+    # choosing a combination that might fail. Issue #3380.
+    if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU" AND CMAKE_CXX_COMPILER_VERSION VERSION_GREATER "9")
+        message(FATAL_ERROR "IBM_VSX does not work together with gcc > 9. Disable SIMD support (slower), or use an older version of the GNU compiler")
+    endif()
+
+
     gmx_find_simd_ibm_vsx_flags(SIMD_IBM_VSX_C_SUPPORTED SIMD_IBM_VSX_CXX_SUPPORTED
                                 SIMD_IBM_VSX_C_FLAGS SIMD_IBM_VSX_CXX_FLAGS)
 

docs/release-notes/2020/2020.2.rst

@@ -83,6 +83,13 @@ The tool would fail with a segmentation fault.
 Fixes that affect portability
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+Give clearer message about not detecting IBM_VSX support in gcc > 9
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+CMake would fail with a confusing error message.
+
+:issue:`3380`
+
 Miscellaneous
 ^^^^^^^^^^^^^