OpenMP on macOS with Xcode tools

Warning! Everything described on this page is strictly experimental and not officially supported by CRAN, R-core or R Foundation. In may break at any time. The information is provided in the hope of being useful to some tech-savvy people. It is not intended for the regular R user.

For those impatient, skip to how to enable OpenMP in packages.

OpenMP support in Xcode

Apple has explicitly disabled OpenMP support in compilers that they ship in Xcode:

   $ clang -c omp.c -fopenmp
   clang: error: unsupported option '-fopenmp'
even though clang had OpenMP support for quite a long time now (great thanks to the folks at Intel providing their library as open source!). In fact, the clang compiler in Xcode can generate all the necessary code for OpenMP. It can be tricked into performing its designed function by using -Xclang -fopenmp flags.

The unfortunate part about this is that Apple is not shipping the necesssary libomp.dylib run-time library needed for OpenMP support. To make things worse, the version of the library you need depends on the clang version used, which Apple obfuscates so that it's non-trivial to reverse-engineer it. Fortunately, some clever folks were able to find the traces in Apple's released source so we can build the binaries that correspond to the clang version used. It is sometimes possible to use a more recent version of the runtime than the version of Apple clang.

OpenMP run-time downloads

The follwing are links to libomp OpenMP run-time built from official LLVM release sources using Xcode compilers. They are signed and support macOS 10.13 (High Sierra) and higher. All tar-balls contain the system tree usr/local/lib and usr/local/include so the recommended installation is:
    curl -O https://mac.r-project.org/openmp/openmp-9.0.1-darwin17-Release.tar.gz
    sudo tar fvx openmp-9.0.1-darwin17-Release.tar.gz -C /
The contained set of files is the same in all tar balls:
    usr/local/lib/libomp.dylib
    usr/local/include/ompt.h
    usr/local/include/omp.h
    usr/local/include/omp-tools.h
so you can simply remove those to uninstall. Note that any package you compile against libomp.dylib will need that run-time so you have to ship it with your package or have users install it.

BuildDownloadSHA1 checksum
LLVM 10.0.0
Xcode 12+
openmp-10.0.0-darwin17-Release.tar.gz (Release)
openmp-10.0.0-darwin17-Debug.tar.gz (Debug)
9bf16a64ab747528c5de7005a1ea1a9e318b3cf0
d4508d3f0c2952c3f984393b088e0b4beab33b58
LLVM 9.0.1
Xcode 11.4 and up (Apple clang 1103.x)
openmp-9.0.1-darwin17-Release.tar.gz (Release)
openmp-9.0.1-darwin17-Debug.tar.gz (Debug)
e5bd8501a3f957b4babe27b0a266d4fa15dbc23f
c4c8491631504fb060f7c25ec14324d02d617d5b
LLVM 8.0.1
Xcode 11.0-11.3.1 (Apple clang 1100.x)
openmp-8.0.1-darwin17-Release.tar.gz (Release)
openmp-8.0.1-darwin17-Debug.tar.gz (Debug)
e4612bfcb1bf520bf22844f7db764cadb7577c28
d6c83918b28405d43950d4b864ca8d1687eed4d1
LLVM 7.1.0
Xcode 10.2-10.3 (Apple clang 1001.x)
openmp-7.1.0-darwin17-Release.tar.gz (Release)
openmp-7.1.0-darwin17-Debug.tar.gz (Debug)
6891ff6f83f2ed83eeed42160de819b50cf643cd
34456adde62b9a1047f906e1d7f54990a1c15a34

How to enable OpenMP in packages

How you do the latter depends on the package, but if the package does not set these environment variables itself, you can try
    PKG_CPPFLAGS='-Xclang -fopenmp' PKG_LIBS=-lomp R CMD INSTALL myPackage
If that doesn't work, please consult the package's documentation, and liaise with its maintainer. It is also possible to add those flags globally by adding the following to ~/.R/Makevars:
    CPPFLAGS += -Xclang -fopenmp
    LDFLAGS += -lomp
but be very careful when doing this, always check your ~/.R/Makevars whenever you upgrade R, macOS or Xcode.

Side notes

It may be possible in principle to build static version of the run-time. That can be done via -DLIBOMP_ENABLE_SHARED=OFF, but has not been tested. There is a potential for chaos when more OMP run-times get loaded into one process. Another interesting test would be to try -DLIBOMP_FORTRAN_MODULES=ON and see which versions are compatible with the GNU Fortran used in R. We are intentionally removing the gomp symlink from the binary so that iomp and gomp don't conflict.

License and sources

All sources were obtained directly from the LLVM releases and copies are also available in the src directory (including the build script). See LICENSE.txt in the sources for the corresponding license and https://openmp.llvm.org/ for details on the OpenMP run-time.

Acknowledgements

Thanks to John Clayden, Kevin Ushey and others who contributed to the discussion and testing.

Trademark notices

Last modified on 2020/05/01 by Simon Urbanek