[question] Handling OpenMP in recipes

[Morwenn]

Several libraries optionally handles OpenMP and generally require specific compiler and linker options in order to do so, including when one needs to link against them. What would be the best way to handle such packages in CII?

[danimtb]

if the OpenMP support is done with an already supported compiler in CCI, I'd say that adding a recipe option and the required flags will be enough to enable openmp support in the recipes

[Morwenn]

Most recipes I've seen add the following flags unconditionally to package_info:

self.cpp_info.cppflags = ["-fopenmp"]
self.cpp_info.sharedlinkflags = ["-fopenmp"]

However it fails with some configurations and I learnt recently that cpp_info.cppflags is going away, even very soon for the future users of CONAN_V2_MODE.

[danimtb]

self.cpp_info.cppflags is deprecated in favor of self.cpp_info.cxxflags that is already available. So you can still use that possibility.

Should OpenMP be a "system requirement recipe" like the ones discussed in #641 or is it just a compiler optimization?

[jasal82]

I think that -fopenmp is a special case because it does multiple things and it's not even consistent across the platforms. Usually it will enable linkage against libgomp and omp pragma processing during compilation. So it's actually both, a system requirement and a compiler optimization, dependent on one another. The cxxflags/sharedlinkflags approach is a good start but one might want to add an option for it to enable/disable it.

[philsuess]

Stumbling on this when porting to conan v2. What is the conanical (see what I did there?) way in conan 2?

[valgur]

Basically this:

def requirements(self):
    if self.options.with_openmp and self.settings.compiler in ["clang", "apple-clang"]:
        self.requires("llvm-openmp/17.0.6", transitive_headers=True, transitive_libs=True)

@property
def _openmp_flags(self):
    if self.settings.compiler == "clang":
        return ["-fopenmp=libomp"]
    elif self.settings.compiler == "apple-clang":
        return ["-Xclang", "-fopenmp"]
    elif self.settings.compiler == "gcc":
        return ["-fopenmp"]
    elif self.settings.compiler == "intel-cc":
        return ["-Qopenmp"]
    elif self.settings.compiler == "sun-cc":
        return ["-xopenmp"]
    elif is_msvc(self):
        return ["-openmp"]
    return None

def package_info(self):
    ...
    if self.options.with_openmp:
        if self.settings.compiler in ["clang", "apple-clang"]:
            self.cpp_info.requires.append("llvm-openmp::llvm-openmp")
        openmp_flags = self._openmp_flags
        self.cpp_info.cflags = openmp_flags
        self.cpp_info.cxxflags = openmp_flags
        self.cpp_info.sharedlinkflags = openmp_flags
        self.cpp_info.exelinkflags = openmp_flags

Only set the cflags and cxxflags if #pragma omp is used somewhere in the public headers of the library.

For more details about OpenMP and Conan/CCI see:

• A case for improved OpenMP support on CCI #24577
• openmp/system: new recipe #22360