OpenMP target flags depend on CMake version

[rhornung67]

In RAJA, we required CMake 3.14.5 as the min cmake version (default on LC systems). Then, we specified OpenMP compiler flags for OpenMP target offload builds using, for example for clang:

-DOpenMP_CXX_FLAGS="-fopenmp;-fopenmp-targets=nvptx64-nvidia-cuda"

In the last release, we bumped the min cmake version requirement to 3.20. Now, to build and link with OpenMP target offload, we need to use:

-DBLT_OPENMP_COMPILE_FLAGS="-fopenmp;-fopenmp-targets=nvptx64-nvidia-cuda" -DBLT_OPENMP_LINK_FLAGS="-fopenmp;-fopenmp-targets=nvptx64-nvidia-cuda"

That is, we need to over-ride BLT logic for setting OpenMP complier and linker flags and force the flag settings to get the code to build and link. Fortunately, BLT gave us a way to do that.

Anyway, is this expected? Is there anything that can be done in BLT to simplify this cmake version dependency? Or, is there a better way we can handle this in our code?

[white238]

We are going to revamp the OpenMP targets very soon to use the true CMake targets. Sadly yes this is expected. The CMake openmp targets didn't exist originally (or didn't work properly I can't remember). The current generator expressions are less than ideal but that is how you override it.

[skyreflectedinmirrors]

I will add: right now, if you pass in e.g.:

-DOpenMP_CXX_FLAGS:STRING="-fopenmp=libomp --offload-arch=gfx90a"

to your configure line to cmake (at least, on v3.24.2), a target depending on openmp will end up using this raw string, which fails with:

[ 25%] Building CXX object blt/tests/smoke/CMakeFiles/blt_openmp_smoke.dir/blt_openmp_smoke.cpp.o
cd ~arghdos/build/blt/tests/smoke && /opt/rocm-5.4.3//llvm/bin/clang++   -Wall -Wextra      -O3 -DNDEBUG -fPIE "-fopenmp=libomp --offload-arch=gfx90a" -std=c++11 -MD -MT blt/tests/smoke/CMakeFiles/blt_openmp_smoke.dir/blt_openmp_smoke.cpp.o -MF CMakeFiles/blt_openmp_smoke.dir/blt_openmp_smoke.cpp.o.d -o CMakeFiles/blt_openmp_smoke.dir/blt_openmp_smoke.cpp.o -c ~arghdos/blt/tests/smoke/blt_openmp_smoke.cpp
clang-15: error: unsupported argument 'libomp --offload-arch=gfx90a' to option '-fopenmp='
clang-15: error: unsupported argument 'libomp --offload-arch=gfx90a' to option '-fopenmp='
clang-15: error: unsupported argument 'libomp --offload-arch=gfx90a' to option '-fopenmp='

but is parsed correctly, at least for the target defined by cmake in this version:

I learned through this issue that the way to get around this is to semi-colon seperate the flags :)