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I've been trying to build an OpenMP sample program with an LLVM compiler integrated to Visual Studio 2019.

LLVM compiler was downloaded from here (version 10.0, win64), C:\Program Files\LLVM\bin added to the PATH environment variable. LLVM Compiler Toolchain extension was installed from Visual Studio Marketplace.

It successfully builds a hello world program, but when I try to use OpenMP the linker fails with the following errors:

1>clang version 10.0.0 1>Target: x86_64-pc-windows-msvc 1>Thread model: posix 1>InstalledDir: C:\Program Files\LLVM\bin 1> (in-process) 1> "C:\\Program Files\\LLVM\\bin\\clang-cl.exe" -cc1 -triple x86_64-pc-windows-msvc19.26.28805 -emit-obj -mrelax-all -mincremental-linker-compatible -disable-free -disable-llvm-verifier -discard-value-names -main-file-name llvmtest.cpp -mrelocation-model pic -pic-level 2 -mthread-model posix -mframe-pointer=none -relaxed-aliasing -fmath-errno -fno-rounding-math -masm-verbose -mconstructor-aliases -munwind-tables -target-cpu x86-64 -mllvm -x86-asm-syntax=intel -D_DEBUG -D_MT -D_DLL --dependent-lib=msvcrtd --dependent-lib=oldnames -stack-protector 2 -fcxx-exceptions -fexceptions -fexternc-nounwind -fms-volatile -fdefault-calling-conv=cdecl -fdiagnostics-format msvc -gcodeview -debug-info-kind=limited -v -resource-dir "C:\\Program Files\\LLVM\\lib\\clang\\10.0.0" -D _DEBUG -D _CONSOLE -D _UNICODE -D UNICODE -internal-isystem "C:\\Program Files\\LLVM\\lib\\clang\\10.0.0\\include" -internal-isystem "C:\\Program Files (x86)\\Microsoft Visual Studio\\2019\\Community\\VC\\Tools\\MSVC\\14.26.28801\\include" -internal-isystem "C:\\Program Files (x86)\\Microsoft Visual Studio\\2019\\Community\\VC\\Tools\\MSVC\\14.26.28801\\atlmfc\\include" -internal-isystem "C:\\Program Files (x86)\\Microsoft Visual Studio\\2019\\Community\\VC\\Auxiliary\\VS\\include" -internal-isystem "C:\\Program Files (x86)\\Windows Kits\\10\\Include\\10.0.18362.0\\ucrt" -internal-isystem "C:\\Program Files (x86)\\Windows Kits\\10\\Include\\10.0.18362.0\\um" -internal-isystem "C:\\Program Files (x86)\\Windows Kits\\10\\Include\\10.0.18362.0\\shared" -internal-isystem "C:\\Program Files (x86)\\Windows Kits\\10\\Include\\10.0.18362.0\\winrt" -internal-isystem "C:\\Program Files (x86)\\Windows Kits\\10\\Include\\10.0.18362.0\\cppwinrt" -internal-isystem "C:\\Program Files (x86)\\Windows Kits\\NETFXSDK\\4.6.1\\Include\\um" -O0 -Wall -Wno-error -fdeprecated-macro -fdebug-compilation-dir "Z:\\llvmtest" -ferror-limit 19 -fmessage-length 0 -fopenmp -fno-use-cxa-atexit -fms-extensions -fms-compatibility -fms-compatibility-version=19.26.28805 -std=c++14 -fdelayed-template-parsing -fobjc-runtime=gcc -fno-caret-diagnostics -fdiagnostics-show-option -faddrsig -o "Debug\\llvmtest.obj" -x c++ llvmtest.cpp 1>clang -cc1 version 10.0.0 based upon LLVM 10.0.0 default target x86_64-pc-windows-msvc 1>#include "..." search starts here: 1>#include <...> search starts here: 1> C:\Program Files\LLVM\lib\clang\10.0.0\include 1> C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.26.28801\include 1> C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.26.28801\atlmfc\include 1> C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Auxiliary\VS\include 1> C:\Program Files (x86)\Windows Kits\10\Include\10.0.18362.0\ucrt 1> C:\Program Files (x86)\Windows Kits\10\Include\10.0.18362.0\um 1> C:\Program Files (x86)\Windows Kits\10\Include\10.0.18362.0\shared 1> C:\Program Files (x86)\Windows Kits\10\Include\10.0.18362.0\winrt 1> C:\Program Files (x86)\Windows Kits\10\Include\10.0.18362.0\cppwinrt 1> C:\Program Files (x86)\Windows Kits\NETFXSDK\4.6.1\Include\um 1>End of search list. 1>lld-link : error : undefined symbol: __kmpc_global_thread_num 1>>>> referenced by Z:\llvmtest\llvmtest.cpp:9 1>>>> Debug\llvmtest.obj:(main) 1> 1>lld-link : error : undefined symbol: __kmpc_push_num_threads 1>>>> referenced by Z:\llvmtest\llvmtest.cpp:15 1>>>> Debug\llvmtest.obj:(main) 1> 1>lld-link : error : undefined symbol: __kmpc_fork_call 1>>>> referenced by Z:\llvmtest\llvmtest.cpp:15 1>>>> Debug\llvmtest.obj:(main) 1> 1>lld-link : error : undefined symbol: omp_get_thread_num 1>>>> referenced by Z:\llvmtest\llvmtest.cpp:19 1>>>> Debug\llvmtest.obj:(.omp_outlined._debug__) 1> 1>lld-link : error : undefined symbol: omp_get_num_threads 1>>>> referenced by Z:\llvmtest\llvmtest.cpp:25 1>>>> Debug\llvmtest.obj:(.omp_outlined._debug__) 1>Done building project "llvmtest.vcxproj" -- FAILED.

The code is rather trivial:

 #include <omp.h> #include <stdio.h> int main() { int nthreads, tid; //omp_set_num_threads(4); #pragma omp parallel private(tid) num_threads(3) { tid = omp_get_thread_num(); printf("Hello World from thread = %d\n", tid); if (tid == 0) { nthreads = omp_get_num_threads(); printf("Number of threads = %d\n", nthreads); } } /* All threads join master thread and terminate */ }

Project settings shown below: enter image description here enter image description here

I have tried different options: -fopenmp, -Xclang -fopenmp compiler flags, passing -openmp and -fopenmp as linker flags, changing x64 to x86, adding C:\Program Files\LLVM\lib to the PATH. Nothing helped so far.

Interestingly enough, I can build the same code from the command line by running the following from x64 Native Tools Command Prompt for VS 2019:

clang-cl -openmp llvmtest.cpp

So, basically, -openmp flag was enough... Does anyone have an idea how to make it work in Visual Studio?

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  • These are all missing symbols from the OpenMP runtime library. Somewhere in the LLVM's lib directory there should be a libomp.lib file. You need to add it to the additional libraries in the linker settings. The problem is that VS does a separate link step and the linker doesn't know that you need the OpenMP runtime too. When you compile with clang-cl -openmp ..., the compiler driver automatically adds the OpenMP runtime library when calling the linker. Commented Jun 1, 2020 at 14:24
  • @HristoIliev I was thinking of that, but I don't know how to properly add those libs to the linker. It seems to me 'usual' VS linker settings are irrelevant in this case as I am using LLVM, hence lld-link is used instead of the Microsoft linker. There is 'Additional Linker Options' field under LLVM (see the image), but I have no idea what to pass there. Can you give me a hint on that? Commented Jun 1, 2020 at 14:51
  • I guess -lomp. Commented Jun 1, 2020 at 16:03
  • @HristoIliev lld-link : warning : ignoring unknown argument '-lomp' Commented Jun 1, 2020 at 16:27
  • Assuming lld-link is command-line compatible with link, then the additional option should be libomp.lib. You may also need to add the path to LLVM's lib directory somewhere too. Commented Jun 1, 2020 at 17:33

3 Answers 3

Reset to default
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Finally, I have managed to make it work (thanks to Marek Aniola, the author of the LLVM Toolchain extension, who helped me out).

First, I passed the /openmp flag to LLVM -> Additional Compiler Options (it works with -openmp too).

Second, it turns out that OpenMP is not automatically linked when building from Visual Studio (in contrast to compiling from the command line by means of clang-cl). Linker settings for LLVM are specified the same way as for the Microsoft linker.

The path to libraries C:\Program Files\LLVM\lib has to be added to Linker -> General -> Additional Library Directories.

It is also necessary to list libomp.lib in Linker -> Input -> Additional Dependencies.

Linker -> Optimization -> Link Time Code Generation must be set to Default.

These steps let me successfully build the code (64-bit application). Although the 64-bit LLVM compiler can build a 32-bit program, it includes only x64 version of OpenMP libraries, so in order to build a 32-bit application I had to link libomp.lib and copy libomp.dll from another LLVM distribution. If there is a better way to do that, please let me know.

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Visual Studio 2019 16.6.0 comes Clang 10.0.0 that targets both x86 and x64. You can install it using the Visual Studio Installer.
@HristoIliev Yes, I've tried it first. But it seems to work in CMake projects only. And it also has some issues with finding OpenMP. I think it's discussed here: stackoverflow.com/questions/57209524/…
@mentalmushroom, it works if you switch off the error on multi-threading in Microsoft.Cpp.ClangCl.Common.targets like this <!-- <VCMessage Code="MSB8055" Type="Error" Condition="('%(ClCompile.OpenMPSupport)' == 'true')" /> -->.
BTW: Which OpenMP version is supported by Clang in VS2019?
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In addition to @mentalmushroom answer, it seems only step two is now necessary:

"...It is also necessary to list libomp.lib in Linker -> Input -> Additional Dependencies..."

Visual Studio 2019 16.10.3 using LLVM tool-set.

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I also spent some time on getting Clang + OpenMP working in Visual Studio 2019. Here's my approach, hopefully it's useful to someone. It largely mimics the approach you would take on Linux with gcc.

Right-click Project in Visual Studio's Solution Explorer > Properties:
(1) General > Platform Toolset > LLVM (clang-cl)
(2) C/C++ > General > Additional Include Directories > C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Tools\Llvm\lib\clang\10.0.0\include //Tells compiler where to find omp.h (in gcc: -I)
//The 64-bit version is located in C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Tools\Llvm\x64\lib\clang\10.0.0\include
(3) C/C++ > Command Line > Additional Options > -openmp //Tells the compiler to use OpenMP (gcc: -fopenmp)
(4) Linker > General > Additional Library Directories > C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Tools\Llvm\lib //Tells linker where to look for the OpenMP library (in gcc: -L)
//The 64-bit .lib file is located in C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Tools\Llvm\x64\lib
(5) Linker > Input > Additional Dependencies > libomp.lib (or whatever the library file is called in C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Tools\Llvm\lib) //Tells linker which library to link to (N.B. libomp.lib depends on libomp.dll) (in gcc: -l)
(6) Add to PATH system environment variable: C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Tools\Llvm\bin //Here libomp.dll is located, so Windows can dynamically link the built executable to libomp.dll (in Linux: LD_LIBRARY_PATH)
//The 64-bit .dll file is located in C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Tools\Llvm\x64\bin
//Restart CommandPrompt/PowerShell or computer to reload the environment variables
(7) Additionally, for 64 bit (fatal error LNK1112: module machine type 'x64' conflicts with target machine type 'X86'):
(7.1) Linker > Target Machine > MachineX64 (/MACHINE:x64)
(7.2) VC++ Directories > Executable Directories > C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Tools\Llvm\x64\bin

The above process needs to be repeated for each combination of Release/Debug and x86(or Win32)/x64 (why do people prefer GUIs in a build system...)

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