Configure CMake in windows to use clang from command line to get modern OpenMP support

Question

I have a small test project that uses OpenMP for parallelization. My target is to compile it so it produces .dll and .lib for libraries (because my real project links to external libraries distributed using these types), with support for OpenMP 4.5 or newer and do it from the command line so it can be done on a docker for testing and checking (the docker part is out of the scope of this question, it is just for reference on why I need it to work from the command line). I can compile this project with different compilers with which I am not happy:

• MSVC: mkdir build-msvc, then cmake .. (from the newly created folder) and finally cmake --build . --config Release. This compiles fine but only supports OpenMP 2.0, therefore is not a good option for my real project.
• Intel Parallel Studio: mkdir build-intel, then cmake .. -T "Intel C++ Compiler 19.0" (from the newly created folder) and finally cmake --build . --config Release. This supports OpenMP 5.0 but its licenses are pretty expensive for me.
• MinGW x64 g++: mkdir build-g++, then cmake .. -G "MinGW Makefiles" (from the newly created folder) and finally cmake --build .. It supports OpenMP 4.5 but this compiler is not compatible wiht .lib (as far as I know) which I already mentioned is necessary for me.

I have tried without success to use clang:

• CLANG from MSVC: mkdir build-clang-msvc, then cmake -G Ninja -DCMAKE_CXX_COMPILER=clang-cl .. (from the newly created folder), but it fails with the following error:

-- The CXX compiler identification is Clang 8.0.1 with MSVC-like command-line
-- Check for working CXX compiler: C:/Program Files (x86)/Microsoft Visual Studio/2019/Community/VC/Tools/Llvm/bin/clang-cl.exe
-- Check for working CXX compiler: C:/Program Files (x86)/Microsoft Visual Studio/2019/Community/VC/Tools/Llvm/bin/clang-cl.exe -- broken
CMake Error at C:/Program Files/CMake/share/cmake-3.15/Modules/CMakeTestCXXCompiler.cmake:53 (message):
  The C++ compiler

    "C:/Program Files (x86)/Microsoft Visual Studio/2019/Community/VC/Tools/Llvm/bin/clang-cl.exe"

  is not able to compile a simple test program.

  It fails with the following output:

    Change Dir: C:/Users/[USER NAME]/source/repos/test_openmp/build-clang-msvc/CMakeFiles/CMakeTmp

    Run Build Command(s):C:/PROGRA~2/MICROS~1/2019/COMMUN~1/Common7/IDE/COMMON~1/MICROS~1/CMake/Ninja/ninja.exe cmTC_bd131 && [1/2] Building CXX object CMakeFiles\cmTC_bd131.dir\testCXXCompiler.cxx.obj
    [2/2] Linking CXX executable cmTC_bd131.exe
    FAILED: cmTC_bd131.exe
    cmd.exe /C "cd . && "C:\Program Files\CMake\bin\cmake.exe" -E vs_link_exe --intdir=CMakeFiles\cmTC_bd131.dir --rc=rc --mt=CMAKE_MT-NOTFOUND --manifests  -- C:\PROGRA~1\MINGW-~1\X86_64~1.0-P\mingw64\bin\ld.exe /nologo CMakeFiles\cmTC_bd131.dir\testCXXCompiler.cxx.obj  /out:cmTC_bd131.exe /implib:cmTC_bd131.lib /pdb:cmTC_bd131.pdb /version:0.0  /machine:X86  /debug /INCREMENTAL /subsystem:console  kernel32.lib user32.lib gdi32.lib winspool.lib shell32.lib ole32.lib oleaut32.lib uuid.lib comdlg32.lib advapi32.lib && cd ."
    RC Pass 1: command "rc /fo CMakeFiles\cmTC_bd131.dir/manifest.res CMakeFiles\cmTC_bd131.dir/manifest.rc" failed (exit code 0) with the following output:
    The system cannot find the file specified
    ninja: build stopped: subcommand failed.





  CMake will not be able to correctly generate this project.
Call Stack (most recent call first):
  CMakeLists.txt:2 (project)


-- Configuring incomplete, errors occurred!
See also "C:/Users/[USER NAME]/source/repos/test_openmp/build-clang-msvc/CMakeFiles/CMakeOutput.log".
See also "C:/Users/[USER NAME]/source/repos/test_openmp/build-clang-msvc/CMakeFiles/CMakeError.log".

• Clang and Ninja apart from MSVC: mkdir build-clang-ninja, then cmake -G Ninja -DCMAKE_CXX_COMPILER=clang-cl .. (from the newly created folder and changing the PATH environmental variable so the non-msvc are found first), but it fails with the following error:

-- The CXX compiler identification is Clang 9.0.0 with MSVC-like command-line
-- Check for working CXX compiler: C:/Program Files/LLVM/bin/clang-cl.exe
-- Check for working CXX compiler: C:/Program Files/LLVM/bin/clang-cl.exe -- broken
CMake Error at C:/Program Files/CMake/share/cmake-3.15/Modules/CMakeTestCXXCompiler.cmake:53 (message):
  The C++ compiler

    "C:/Program Files/LLVM/bin/clang-cl.exe"

  is not able to compile a simple test program.

  It fails with the following output:

    Change Dir: C:/Users/[USER NAME]/source/repos/test_openmp/buid-clang-ninja/CMakeFiles/CMakeTmp

    Run Build Command(s):C:/Ninja/ninja.exe cmTC_50b73 && [1/2] Building CXX object CMakeFiles\cmTC_50b73.dir\testCXXCompiler.cxx.obj
    [2/2] Linking CXX executable cmTC_50b73.exe
    FAILED: cmTC_50b73.exe
    cmd.exe /C "cd . && "C:\Program Files\CMake\bin\cmake.exe" -E vs_link_exe --intdir=CMakeFiles\cmTC_50b73.dir --rc=rc --mt=CMAKE_MT-NOTFOUND --manifests  -- CMAKE_LINKER-NOTFOUND /nologo CMakeFiles\cmTC_50b73.dir\testCXXCompiler.cxx.obj  /out:cmTC_50b73.exe /implib:cmTC_50b73.lib /pdb:cmTC_50b73.pdb /version:0.0  /machine:x64  /debug /INCREMENTAL /subsystem:console  kernel32.lib user32.lib gdi32.lib winspool.lib shell32.lib ole32.lib oleaut32.lib uuid.lib comdlg32.lib advapi32.lib && cd ."
    RC Pass 1: command "rc /fo CMakeFiles\cmTC_50b73.dir/manifest.res CMakeFiles\cmTC_50b73.dir/manifest.rc" failed (exit code 0) with the following output:
    The system cannot find the file specified
    ninja: build stopped: subcommand failed.





  CMake will not be able to correctly generate this project.
Call Stack (most recent call first):
  CMakeLists.txt:2 (project)


-- Configuring incomplete, errors occurred!
See also "C:/Users/[USER NAME]/source/repos/test_openmp/buid-clang-ninja/CMakeFiles/CMakeOutput.log".
See also "C:/Users/[USER NAME]/source/repos/test_openmp/buid-clang-ninja/CMakeFiles/CMakeError.log".

Any ideas on how to proceed with the clang? I think it is my best option for what I want to achieve (compile my test program with support for OpenMP 4.5 or newer and producing .lib and .dll).

Related posts/webpages that I have checked but have been non-useful to solve this:

• Building c++ project on Windows with CMake, Clang and Ninja
• Building with CMake, Ninja and Clang on Windows
• Ways to Compile with Clang on Windows, specifically setting the linker: -DCMAKE_LINKER="C:/Program Files/LLVM/bin/lld-link.exe"
• CMake building for Windows (clang-cl) using Ninja Generator
• Clang-cl user manual

Answer 1

我能够在Clang+MSVC案例中重复此错误。因为我们试图从 Visual Studio 外部（即命令行）使用 Visual-Studio 定制的编译器，所以在使用编译器之前有必要在命令行中初始化VS 构建环境。这些文件VCVarsXX.bat完成了这个任务；它们是 VS 命令提示符工具的一部分。因此，通过选择您的架构（x86、x64等）并运行脚本，这应该允许 CMake 构建简单的测试程序并clang-cl继续。这是 VS 2019 的位置：

>"C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Auxiliary\Build\vcvarsall.bat" x86

>cmake -G Ninja -DCMAKE_CXX_COMPILER=clang-cl ..
-- The CXX compiler identification is Clang 8.0.1
-- Check for working CXX compiler: C:/Program Files (x86)/Microsoft Visual Studio/2019/Community/VC/Tools/Llvm/bin/clang-cl.exe
-- Check for working CXX compiler: C:/Program Files (x86)/Microsoft Visual Studio/2019/Community/VC/Tools/Llvm/bin/clang-cl.exe -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
...

为了扩展 OpenMP 问题，我个人发现使用它find_package(OpenMP REQUIRED)是毫无结果的。正如你提到的，我也收到了这个 CMake 错误：

Could NOT find OpenMP_CXX (missing: OpenMP_CXX_FLAGS OpenMP_CXX_LIB_NAMES)

该网站上有一些建议（例如此处和此处），声称手动填充所有变量可以FindOpenMP.cmake成功找到库。我用你的示例 CMake 文件尝试了这个，并取得了一些成功：

cmake_minimum_required (VERSION 2.8)
project(test_openmp LANGUAGES CXX)

set(OpenMP_CXX "${CMAKE_CXX_COMPILER}")
set(OpenMP_CXX_FLAGS "-Xclang -fopenmp")
set(OpenMP_CXX_LIB_NAMES "libomp" "libiomp5")
set(OpenMP_libomp_LIBRARY libomp)
set(OpenMP_libiomp5_LIBRARY libiomp5)

#OPENMP
find_package(OpenMP REQUIRED)
if(OPENMP_FOUND)
    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OpenMP_CXX_FLAGS}")
    set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} ${OpenMP_EXE_LINKER_FLAGS}")
endif()

# Library
add_library(example_lib SHARED example_lib.h example_lib.cpp)
target_link_libraries(example_lib PUBLIC ${OpenMP_CXX_LIBRARIES})
target_compile_definitions(example_lib PRIVATE EXEMPLE_LIB_EXPORT)

# Executable
add_executable(test_openmp test_openmp.cpp)
target_link_libraries(test_openmp example_lib)

这在第二次 CMake 配置尝试中成功“找到”了库（第一次总是产生Could NOT find OpenMP_CXX与之前相同的错误）：

>cmake -G Ninja -DCMAKE_CXX_COMPILER=clang-cl ..
-- Found OpenMP_CXX: -Xclang -fopenmp (found version "3.1")
-- Found OpenMP: TRUE (found version "3.1")
-- Configuring done
-- Generating done

但是，这无法构建可执行文件，因为没有任何 CMake OpenMP 变量（特别是OpenMP_CXX_LIBRARIES）实际指向库位置。CMake 的缺点FindOpenMP.cmake已在 CMake 问题网站上提出，并且 LLVM/Clang 端似乎也存在相关限制。

不管怎样，我能让这个例子工作的最干净的方法就是find_package()完全放弃。以下内容使我能够成功生成构建系统，并编译并运行可执行文件：

cmake_minimum_required (VERSION 2.8)
project(test_openmp LANGUAGES CXX)

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(example_lib SHARED example_lib.h example_lib.cpp)
# Link in the OpenMP libraries.
target_link_libraries(example_lib PUBLIC libomp libiomp5md)
target_compile_definitions(example_lib PRIVATE EXEMPLE_LIB_EXPORT)

# Executable
add_executable(test_openmp test_openmp.cpp)
target_link_libraries(test_openmp example_lib)