Imagine running Clear Linux for a while and suddenly clang++ -fopenmp stops working. No announcement was made at the time, OpenMP removal in CL 39970.
LLVM-17 Installation
Building LLVM requires the c-basic and git bundles.
sudo swupd bundle-add c-basic git
Ensure adequate disk space 11.1+ GB. After git clone, the llvm-17 folder consumes 2.0 GB on disk. Which grows to 7.2 GB after compilation, and 3.9 GB for the installation folder /opt/llvm-17. I have the NVIDIA RTX 3070 GPU and default to sm_86. Note that 75 is included for compute capabilities.
The LLVM base version must match the system clang --version if you want to restore OpenMP functionality, described later.
#!/bin/bash
sudo mkdir -p /opt/llvm-17
sudo mkdir -p /usr/local/share/gdb
sudo chown $USER:$USER /opt/llvm-17
sudo chown $USER:$USER /usr/local/share/gdb
unset CFLAGS
unset CXXFLAGS
export CC=clang
export CXX=clang++
git clone --depth 1 -b "release/17.x" https://github.com/llvm/llvm-project llvm-17
rm -fr llvm-17/build
cmake -S llvm-17/llvm -B llvm-17/build -G Ninja \
-DCMAKE_BUILD_TYPE=Release \
-DLLVM_INCLUDE_TESTS=OFF \
-DLLVM_LIBDIR_SUFFIX=64 \
-DLLVM_HOST_TRIPLE="x86_64-generic-linux" \
-DLLVM_BINUTILS_INCDIR=/usr/include \
-DLLVM_BUILD_RUNTIME:BOOL=ON \
-DLLVM_BUILD_TOOLS:BOOL=ON \
-DLLVM_ENABLE_RTTI=ON \
-DLLVM_ENABLE_ZLIB=OFF \
-DLLVM_ENABLE_TERMINFO=OFF \
-DLLVM_ENABLE_PROJECTS="lld;lldb;clang;clang-tools-extra;compiler-rt;openmp;polly" \
-DLLVM_TARGETS_TO_BUILD="AMDGPU;NVPTX;WebAssembly;X86" \
-DCLANG_OPENMP_NVPTX_DEFAULT_ARCH="sm_86" \
-DLIBOMPTARGET_NVPTX_COMPUTE_CAPABILITIES="75;86;89"
[[ $? -ne 0 ]] && exit $?
cmake --build llvm-17/build && \
cmake --install llvm-17/build --prefix=/opt/llvm-17
OpenMP Support
Using clang++ from the /opt/llvm-17 folder is simply setting two environment variables.
export PATH=/opt/llvm-17/bin:$PATH
export LD_LIBRARY_PATH=/opt/llvm-17/lib64:$LD_LIBRARY_PATH
clang++ -fopenmp ...
System LLVM Workaround
Another option is restoring OpenMP functionality for /usr/bin/clang++ and friends. I do this with a script. Run with sudo privilege. The OpenMP GPU targets may be copied as well, but not needed for my use case.
#!/bin/bash -x
if [[ -d /opt/llvm-17/lib64/clang/17 && -d /usr/lib64/clang/17 ]]; then
cd /usr/lib64/cmake
cp -a /opt/llvm-17/lib64/cmake/openmp .
cd /usr/lib64/clang/17/include
cp -a /opt/llvm-17/lib64/clang/17/include/omp*.h .
cd /usr/lib64
cp -a /opt/llvm-17/lib64/libarcher.so .
# cp -a /opt/llvm-17/lib64/libomptarget.* .
# cp -a /opt/llvm-17/lib64/libomptarget-* .
cp -a /opt/llvm-17/lib64/libompd.so .
cp -a /opt/llvm-17/lib64/libomp.so .
ln -sf libomp.so libiomp5.so
fi
Running clang++ -fopenmp on Clear Linux is again, fun, consuming many CPU cores. 