Add functions to check if OpenMP is working

[hsinfan1996]

I haven't written in C language for a while. This is how I check if OpenMP is working on the compiled pyccl. Checked on macOS 10.15.7 with Apple Clang with OpenMP disabled and homebrew-installed Clang 15, the function is working as expected even if the environment variable "OMP_NUM_THREAD" is set to 0.

[coveralls]

Pull Request Test Coverage Report for Build 3645632564

• 2 of 4 (50.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.04%) to 97.524%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pyccl/pyutils.py	2	4	50.0%

Totals
Change from base Build 3481038780:	-0.04%
Covered Lines:	4844
Relevant Lines:	4967

---

💛 - Coveralls

[hsinfan1996]

@tilmantroester I wonder if add_link_options("LINKER:-lomp") will prevent you from compiling on your machine, since it looks like the link option is not needed for Apple Clang on macOS 12.

[tilmantroester]

Is the add_link_options("LINKER:-lomp") necessary? This should be automatically added if openmp is found. Can you add file(GENERATE OUTPUT "omp_lib_generator.txt" CONTENT $<$<BOOL:${OpenMP_C_FOUND}>:OpenMP::OpenMP_C>) to the CMakeLists.txt and check if there's any output? Setting set( CMAKE_VERBOSE_MAKEFILE on ) also shows the full build command and should show if the openmp library is added to the linker.

[hsinfan1996]

Is the add_link_options("LINKER:-lomp") necessary? This should be automatically added if openmp is found. Can you add file(GENERATE OUTPUT "omp_lib_generator.txt" CONTENT $<$<BOOL:${OpenMP_C_FOUND}>:OpenMP::OpenMP_C>) to the CMakeLists.txt and check if there's any output? Setting set( CMAKE_VERBOSE_MAKEFILE on ) also shows the full build command and should show if the openmp library is added to the linker.

This is what I got in the file

OpenMP::OpenMP_C

[nikfilippas]

This is the 1000th tag! 🎉🎉🎉

[hsinfan1996]

The c unit tests in ci on ubuntu did not run any tests (or the log did not show it?). Is this behaviour expected?
https://github.com/LSSTDESC/CCL/actions/runs/3237240260/jobs/5304053146#step:10:18

[hsinfan1996]

Just want to leave a note for Apple Clang users with homebrew-installed libomp on macOS 10.15. If the installation failed or there are errors when python tries to import pyccl, in the shell, execute export LDFLAGS="-L/usr/local/lib -lomp" and export CFLAGS="-I/usr/local/opt/libomp/include" before pip install . (or python setup.py install).

[damonge]

@hsinfan1996 @tilmantroester can I check where we stand on this PR?

[hsinfan1996]

@hsinfan1996 @tilmantroester can I check where we stand on this PR?

The functions checking OpenMP version and threads are done. However, some Apple Clang compilers (e.g. on my machine running MacOS 10.15) cannot link homebrew-installed libomp. The latest commit (d892a30) solves this issue by forcing libomp to link with all compiled library. I will revert the commit if this is not a proper way to solve the problem. I am happy with the changes to CMakeLists.txt that @tilmantroester made. If there is no more to add to CMakeLists.txt, then this PR is ready for review.

[damonge]

OK, can we run the tests and check they pass?

[damonge]

@tilmantroester you've paid more attention than me to this. Are you happy to approve?

[tilmantroester]

I think it would be preferable to have functions that return the numerical values with the expected values documented in a doc string, rather than print messages to stdout.
Right now they can't be used in automated scripts, for example.

[tilmantroester]

LGTM!