Compilation error w/ open_mp backend

[bgranzow]

Many errors of the form:

In file included from /scratch/bngranz/nightly/jughead-openmp/src/omega_h/src/Omega_h_adj.cpp:7:0:
/scratch/bngranz/nightly/jughead-openmp/src/omega_h/src/Omega_h_for.hpp:42:0: error: ignoring #pragma omp parallel [-Werror=unknown-pragmas]
 #pragma omp parallel for

pop up with gcc/7.2.0 with the cmake command

/projects/sems/install/rhel6-x86_64/sems/utility/cmake/3.12.2/bin/cmake /scratch
/bngranz/nightly/jughead-openmp/src/omega_h -DCMAKE_INSTALL_PREFIX=/scratch/bngr
anz/nightly/jughead-openmp/install/omega_h -DCMAKE_PREFIX_PATH=";/projects/sems/
install/rhel6-x86_64/sems/utility/cmake/3.12.2" -DBUILD_SHARED_LIBS:BOOL=ON -DBU
ILD_TESTING:BOOL=ON -DOmega_h_USE_MPI:BOOL=ON -DOmega_h_USE_OpenMP:BOOL=ON

Here is the output from a file with make VERBOSE=1. Presumably -fopenmp was stripped out of the compile flags during one of the recent CMake changes?

[  1%] Building CXX object src/CMakeFiles/omega_h.dir/Omega_h_adj.cpp.o
cd /scratch/bngranz/nightly/jughead-openmp/build/omega_h/src && /projects/sems/install/rhel6-x86_64/sems/compiler/gcc/7.2.0/openmpi/1.10.1/bin/mpicxx  -Domega_h_EXPORTS -I/scratch/bngranz/nightly/jughead-openmp/src/omega_h/src -I/scratch/bngranz/nightly/jughead-openmp/build/omega_h/src -I/scratch/bngranz/nightly/jughead-openmp/src/omega_h/tpl  -fPIC   -W -Wall -Wextra -Werror -std=c++11 -o CMakeFiles/omega_h.dir/Omega_h_adj.cpp.o -c /scratch/bngranz/nightly/jughead-openmp/src/omega_h/src/Omega_h_adj.cpp

[bgranzow]

This 7547cb2 commit fixed the issue.

[bgranzow]

At 44ed745

with gcc/7.2.0

linking of tests fails with the error:

[ 55%] Building CXX object src/CMakeFiles/hypercube_test.dir/hypercube_test.cpp.o
cd /scratch/bngranz/nightly/jughead-openmp/build/omega_h/src && /projects/sems/install/rhel6-x86_64/sems/compiler/gcc/7.2.0/openmpi/1.10.1/bin/mpicxx   -I/scratch/bngranz/nightly/jughead-openmp/src/omega_h/src -I/scratch/bngranz/nightly/jughead-openmp/build/omega_h/src  -fopenmp -W -Wall -Wextra -Werror -std=c++11 -o CMakeFiles/hypercube_test.dir/hypercube_test.cpp.o -c /scratch/bngranz/nightly/jughead-openmp/src/omega_h/src/hypercube_test.cpp
[ 56%] Linking CXX executable hypercube_test
cd /scratch/bngranz/nightly/jughead-openmp/build/omega_h/src && /projects/sems/install/rhel6-x86_64/sems/utility/cmake/3.12.2/bin/cmake -E cmake_link_script CMakeFiles/hypercube_test.dir/link.txt --verbose=1
/projects/sems/install/rhel6-x86_64/sems/compiler/gcc/7.2.0/openmpi/1.10.1/bin/mpicxx     CMakeFiles/hypercube_test.dir/hypercube_test.cpp.o  -o hypercube_test -Wl,-rpath,/scratch/bngranz/nightly/jughead-openmp/build/omega_h/src libomega_h.so /usr/lib64/libz.so 
libomega_h.so: undefined reference to `omp_get_thread_num'
libomega_h.so: undefined reference to `omp_get_num_threads'
libomega_h.so: undefined reference to `GOMP_atomic_end'
libomega_h.so: undefined reference to `GOMP_parallel'
libomega_h.so: undefined reference to `omp_get_max_threads'
libomega_h.so: undefined reference to `GOMP_task'
libomega_h.so: undefined reference to `GOMP_atomic_start'
libomega_h.so: undefined reference to `GOMP_taskwait'
libomega_h.so: undefined reference to `GOMP_barrier'
collect2: error: ld returned 1 exit status

I've tried adding a line like:

target_compile_options(${EXE_NAME} PUBLIC -fopenmp)

here but that didn't fix the issue. make VERBOSE=1 shows omega_h is built with -fopenmp, I think the command just doesn't get carried through to the link line, even though from reading the CMake documentation I think it is supposed to

[a-jp]

Hi,

Seem to have a similar issue compiling with OpenMP support on OSX using llvm8 compilers (from macports, not stock compilers, so do include openmp support). If I add '-fopenmp' to 'CMAKE_CXX_FLAGS' all my linking errors go away. Not sure if this is related?

[penguinpee]

Apologies for replying to an old issue. Since I ran into this trying to build with Omega_h_USE_OpenMP, I thought I add to it here since it inspired my workaround.

If I add '-fopenmp' to 'CMAKE_CXX_FLAGS' all my linking errors go away. Not sure if this is related?

Since CMAKE_CXX_FLAGS is preset in our build environment, I didn't want to mess with that, instead I added

target_link_options(omega_h PUBLIC -fopenmp)

just below

omega_h/src/CMakeLists.txt

Line 191 in e889123

target_compile_options(omega_h PUBLIC -fopenmp)

to pass the flag to the linker as well.