[R-SIG-Mac] Clarifying the status of OMP in CRAN binaries on Mac

[Simon Urbanek]

See my explanation below - to guarantee a common run-time for CRAN packages that wish to use OpenMP even if neither R nor Apple support it. The CRAN R binary is designed specifically to support CRAN R package binaries so it "just works" for Mac users. If each package that wishes to go that route shipped their own (or linked statically) then chaos would ensue breaking all of them.

Cheers,
Simon


> On Apr 25, 2025, at 11:38 AM, bill+rsigmac using 8pawexpress.com wrote:
> 
> If it’s not compiled in, why is |libomp.dylib| distributed in R for Mac?
> 
> Bill
> 
> On 4/24/25 17:35, Simon Urbanek wrote:
> 
>> Gavin,
>> 
>> 
>>> On Apr 24, 2025, at 9:58 PM, Gavin Simpson<ucfagls using gmail.com> wrote:
>>> 
>>> Dear list,
>>> 
>>> I'm trying to clarify my understanding of what is and is not implemented / expected in terms of openMP support in R and CRAN package *binaries* for MacOS on apple silicon.
>>> 
>>> I have the CRAN's R binary for MacOS X (apple silicon) and also the CRAN binaries for two packages, glmmTMB and mgcv, both of which seem to be instrumented (following WRE) to use OpenMP if available. And from discussions here I understand that thanks to Simon and  others, OpenMP support for MacOS X is in R and so should I expect OpenMP to work in these two packages?
>>> 
>> No, OpenMP is not supported by Apple compilers, so R does not enable OpenMP support. You can see that in the logs, e.g:
>> 
>> https://mac.r-project.org/logs/log-R-4.5-branch.big-sur.arm64.html
>> [...]
>> checking for clang -arch arm64 -std=gnu2x option to support OpenMP... unsupported
>> 
>> That said, packages are free to run their own tests if they desire, so anything further is package-dependent. Most packages simply use R's settings and thus don't use OpenMP on macOS.
>> 
>> In cases where the package authors decided that OpenMP is critical for the package they can choose to add a check for '-Xclang -fopenmp' which will force Apple compilers to generate OpenMP code *if* a run-time is available (which is not the case for Apple tools) even if it is not supported by Apple. In that case the package's CRAN binary will have OpenMP support, because the run time is available on the CRAN build machines to support such packages. However, that only applies to the CRAN binaries, it is not supported for source installation due to the lack of support by Apple.
>> 
>> Cheers,
>> Simon
>> 
>> 
>>> If I am, neither of them is working on various of my Mac systems; model fitting is single threaded and the test functions that glmmTMB and mgcv use to detect OMP support both return FALSE:
>>> 
>>> R version 4.5.0 (2025-04-11) -- "How About a Twenty-Six"
>>> Platform: aarch64-apple-darwin20 (64-bit)
>>> rMDHR-4e07c29ad14bbec3188329bad0dc7b261e96d29b46dcecbe592890e60a53f114gt; glmmTMB::omp_check()
>>> [1] FALSE
>>> rMDHR-4e07c29ad14bbec3188329bad0dc7b261e96d29b46dcecbe592890e60a53f114gt; mgcv:::mgcv.omp()
>>> [1] FALSE
>>> 
>>> Is this reproducible on other's systems? (Could someone run those two bits of code on their systems if they use the MacOS X R binary from CRAN and the CRAN Mac binaries for the packages - running mgcv's test should be easier as this pkg is shipped with the R binary).
>>> 
>>> I am happy to provide further system details, but at this point I'm mostly hoping to clarify what I should expect and whether what I'm seeing is typical?
>>> 
>>> Thanks in advance
>>> 
>>> Gavin
>>> 
>>> -- 
>>> Gavin Simpson, PhD [he/him/his]
>>> [tw] @ucfagls [OrciD] 0000-0002-9084-8413
>>> [web] fromthebottomoftheheap.net
>>> • Assistant Professor
>>> • Department of Animal & Veterinary Sciences, Aarhus University
>>> • Adjunct Professor, Department of Biology, University of Regina.
>>> 
>>> _______________________________________________
>>> R-SIG-Mac mailing list
>>> R-SIG-Mac using r-project.org
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>>> 
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