[R] Detecting OpenMP support does not work correctly on macOS when building with GCC

[barracuda156]

Configure assumes Clang’s libomp, while GCC uses libgomp.

checking whether OpenMP will work in a package... no
***********************************************************************************************
 OpenMP is unavailable on this macOS system. GPBoost code will run single-threaded as a result.
 To use all CPU cores for training jobs, you should install OpenMP by running

     brew install libomp
***********************************************************************************************

[fabsig]

Thanks a lot for reporting this.

I have not yet had a lot of time to look into this.

Which version of macOS are you using and which chip? arm64 Apple silicon, e.g., M1/M2, or older x86 Intel ones?

Have you read these requirements? https://github.com/fabsig/GPBoost/tree/master/python-package#requirements

In particular the ones concerning arm64 (M1/M2) if you have one of those (see glibc, proper installation of libomp / homebrew)

[barracuda156]

@fabsig Thank you for responding. I should have made clear I refer to building R package, I added that to issue title now.

libomp is Clang-specific library. GCC has libgomp, but normally it needs only -fopenmp flag passed, not explicit linking to the dylib.
(I build for ppc, but this issue is not arch-specific: GCC does not use libomp on any arch.)

I switched my PR to draft, because while I fixed OpenMP detection with GCC, I am getting one symbol undefined: #97
Apparently emulated TLS still needs some fix.

[fabsig]

Is it an arm64 Apple silicon, e.g., M1/M2, or x86 Intel processor?

I suggest that you still pay attention to the Python package requirements. It might be that the OpenMP / libgomp library is not correctly installed for ARM processors (-> uninstall homebrew, reinstall homebrew, install libgomp). Just a guess...

FWIW: if it is the R package, why don't you just take it from CRAN? There are many precompiled versions there.

[barracuda156]

@fabsig This is on PowerPC :)

libgomp is installed perfectly fine, it is configure script which is used here that is broken and fails to find it, assuming that macOS = Clang, which is obviously needs not be the case.

Anyway, I have fixed everything, will update the PR (symbol error mentioned above is specific to emulation in Rosetta, for w/e reason; native build is fine).

UPD. All tests pass now.

[fabsig]

Very nice! Thanks a lot for your contribution.