Using OpenMP with Clang and CMake in Visual Studio - c++

Question

I'm trying to compile a simple app to test a few libraries I might be using in the future. Because of some problems I had with msvc I tried Clang, which made a strange error I got disappear.
The problem I have now is that the libraries I want to test use OpenMP. They import it using the FindOpenMP module CMake privides. However the module doesn't find it with Clang.
cmake_minimum_required(VERSION 3.14.0)
project(blaze-test VERSION 0.1.0)
set(CMAKE_CXX_STANDARD 14)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
find_package(OpenMP)
I got this output :
1> CMake generation started for configuration: 'x64-Debug'.
1> Environment settings:
1> CXXFLAGS=-m64 -fdiagnostics-absolute-paths
1> CFLAGS=-m64 -fdiagnostics-absolute-paths
1> Command line: "cmd.exe" /c ""C:\PROGRAM FILES (X86)\MICROSOFT VISUAL STUDIO\2019\COMMUNITY\COMMON7\IDE\COMMONEXTENSIONS\MICROSOFT\CMAKE\CMake\bin\cmake.exe" -G "Ninja" -DCMAKE_INSTALL_PREFIX:PATH="PATH\blaze-test\install\x64-Debug" -DCMAKE_CXX_COMPILER:FILEPATH="C:/PROGRAM FILES (X86)/MICROSOFT VISUAL STUDIO/2019/COMMUNITY/VC/Tools/Llvm/8.0.0/bin/clang-cl.exe" -DCMAKE_C_COMPILER:FILEPATH="C:/PROGRAM FILES (X86)/MICROSOFT VISUAL STUDIO/2019/COMMUNITY/VC/Tools/Llvm/8.0.0/bin/clang-cl.exe" -DCMAKE_BUILD_TYPE="Debug" -DCMAKE_MAKE_PROGRAM="C:\PROGRAM FILES (X86)\MICROSOFT VISUAL STUDIO\2019\COMMUNITY\COMMON7\IDE\COMMONEXTENSIONS\MICROSOFT\CMAKE\Ninja\ninja.exe" "PATH\blaze-test" 2>&1"
1> Working directory: PATH\blaze-test\build\x64-Debug
1> [CMake] -- The C compiler identification is Clang 8.0.0
1> [CMake] -- The CXX compiler identification is Clang 8.0.0
1> [CMake] -- Check for working C compiler: C:/PROGRAM FILES (X86)/MICROSOFT VISUAL STUDIO/2019/COMMUNITY/VC/Tools/Llvm/8.0.0/bin/clang-cl.exe
1> [CMake] -- Check for working C compiler: C:/PROGRAM FILES (X86)/MICROSOFT VISUAL STUDIO/2019/COMMUNITY/VC/Tools/Llvm/8.0.0/bin/clang-cl.exe -- works
1> [CMake] -- Detecting C compiler ABI info
1> [CMake] -- Detecting C compiler ABI info - done
1> [CMake] -- Detecting C compile features
1> [CMake] -- Detecting C compile features - done
1> [CMake] -- Check for working CXX compiler: C:/PROGRAM FILES (X86)/MICROSOFT VISUAL STUDIO/2019/COMMUNITY/VC/Tools/Llvm/8.0.0/bin/clang-cl.exe
1> [CMake] -- Check for working CXX compiler: C:/PROGRAM FILES (X86)/MICROSOFT VISUAL STUDIO/2019/COMMUNITY/VC/Tools/Llvm/8.0.0/bin/clang-cl.exe -- works
1> [CMake] -- Detecting CXX compiler ABI info
1> [CMake] -- Detecting CXX compiler ABI info - done
1> [CMake] -- Detecting CXX compile features
1> [CMake] -- Detecting CXX compile features - done
1> [CMake] -- Could NOT find OpenMP_C (missing: OpenMP_C_FLAGS OpenMP_C_LIB_NAMES)
1> [CMake] -- Could NOT find OpenMP_CXX (missing: OpenMP_CXX_FLAGS OpenMP_CXX_LIB_NAMES)
1> [CMake] -- Could NOT find OpenMP (missing: OpenMP_C_FOUND OpenMP_CXX_FOUND)
Based on this I added
set(OpenMP_CXX_FLAGS "-Xclang -fopenmp" CACHE STRING "" FORCE)
set(OpenMP_C_FLAGS "-Xclang -fopenmp" CACHE STRING "" FORCE)
right before the find_package call. It removed the part about the compiler flags from the error message. I added libomp.lib the same way. Then I get
1> [CMake] -- Could NOT find OpenMP_C (missing: OpenMP_C:/Program Files (x86)/Microsoft Visual Studio/2019/Community/VC/Tools/Llvm/8.0.0/lib/libomp.lib_LIBRARY)
1> [CMake] -- Could NOT find OpenMP_CXX (missing: OpenMP_C:/Program Files (x86)/Microsoft Visual Studio/2019/Community/VC/Tools/Llvm/8.0.0/lib/libomp.lib_LIBRARY)
1> [CMake] -- Could NOT find OpenMP (missing: OpenMP_C_FOUND OpenMP_CXX_FOUND)
I hope someone can tell me how to get CMake to find it. These unreliable Find Modules are really annoying.
ps: I'm using Clang 8 (clang-cl.exe) with CMake 3.14 and Visual Studio 2019.

Answer 1

When using find_package(OpenMP REQUIRED), I have also received similar errors when using clang-cl from Visual Studio. There are known limitations with CMake's FindOpenMP.cmake module, as well as some lack of support the clang-cl side. There are other answers on this site suggesting to populate the CMake OpenMP_ variables manually, but this seems backwards. When I tried it, CMake claimed that OpenMP was FOUND, but the paths to the libraries was still unknown and compilation was unsuccessful. I was able to successfully run CMake then compile my applications with MSVC+Clang by avoiding the use of find_package and (unfortunately) hard-coding the path to the OpenMP libraries for now:
cmake_minimum_required(VERSION 3.14.0)
project(blaze-test VERSION 0.1.0)
set(CMAKE_CXX_STANDARD 14)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(OpenMP_LIBRARY_DIR "C:/Program Files (x86)/Microsoft Visual Studio/2019/Community/VC/Tools/Llvm/lib")
set(OpenMP_CXX_FLAGS "-Xclang -fopenmp")
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OpenMP_CXX_FLAGS}")
# Tell CMake where to find the OpenMP libraries.
link_directories(${OpenMP_LIBRARY_DIR})
# Library
add_library(MyLibrary SHARED MyExample.h MyExample.cpp)
# Link in the OpenMP libraries.
target_link_libraries(MyLibrary PUBLIC libomp libiomp5md)
# Executable
add_executable(MyOpenMpTest MyOpenMpTest .cpp)
target_link_libraries(MyOpenMpTest MyLibrary)
Hopefully, the folks at CMake and LLVM will get this issue worked out soon and FindOpenMP.cmake can be used more reliably.