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In the default LLVM provided by Apple in macOS 10.13, High Sierra, is OpenMP finally available? It has been available in main LLVM for more than a year now.

(another way to ask the question might be what version of LLVM is the new Apple LLVM based on)

Henry Schreiner
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2 Answers2

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Standard Apple's clang supports OpenMP. They just disabled the driver option. But you can use the frontend option instead this way: clang -Xclang -fopenmp <you_program> -I <path to omp.h> -L <path to libomp.dylib> -lomp

Also, you need to set DYLD_LIBRARY_PATH environmental variable: export DYLD_LIBRARY_PATH=<path to libomp.dylib>

How to get/build libomp.

$ cd
$ svn co http://llvm.org/svn/llvm-project/openmp/trunk libomp
$ cd libomp
$ mkdir build && cd build
$ cmake -DCMAKE_INSTALL_PREFIX=~/libomp/openmp
$ make && make install

After this directory ~/libomp/openmp will have 2 subdirs: include and lib, which should be used as the path to omp.h and the path to libomp.dylib correspondingly.

See also my answer to Is C++ compilable with OpenMP and boost on MacOS?

Alexey Bataev
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  • Two things would greatly help this answer: The link to the other thread, and a mention (or link) to the way one should get libomp.a. I've tried to do what you suggest before, but was missing one of the steps (or was using an incorrect libomp). Edit: Link here: https://stackoverflow.com/questions/47081991/is-c-compilable-with-openmp-and-boost-on-macos/47225639#47225639 – Henry Schreiner Nov 11 '17 at 21:22
  • How to get/build libomp is added – Alexey Bataev Nov 13 '17 at 14:55
  • Or vote to close as duplicates if the answers are identical. It is generally frowned-upon to post the same answer to multiple questions (not threads, there are no threads on StackOverflow). – Vladimir F Героям слава Nov 13 '17 at 14:58
  • I believe another way to put that flag is `-Xpreprocessor -fopenmp`, since `-Xclang` seems to be undocumented. I've added a homebrew formula PR based on your suggestions here: https://github.com/Homebrew/homebrew-core/pull/20589. – Henry Schreiner Nov 13 '17 at 15:11
  • Valadimir F, my question is 5 months older than that question, and he's asking for any way to make OpenMP work with Boost on macOS, not my specific question of how to get OpenMP in the system default Apple Clang, so I think they are different enough to have different correct answers. – Henry Schreiner Nov 13 '17 at 15:13
  • I don't think `-Xpreprocessor` is a replacement for `-Xclang`. This suboption allows you to specify options for preprocessor, while `-Xclang` affects the frontend itself – Alexey Bataev Nov 13 '17 at 16:17
  • It seems setting the environment variable `DYLD_FALLBACK_LIBRARY_PATH` still requires flags `-L/path/to/libomp.dylib` to be set on compile command. Otherwise, it says `-lomp` is not found. – MeadowMuffins Nov 28 '17 at 07:01
  • Yes, you right. This what I said. But you can try to set environment variable LIBRARY_PATH instead of using L/path/to/libomp.dylib – Alexey Bataev Nov 28 '17 at 16:35
  • You didn't specify the cmake source: you need to write "cmake ../ --DCMAKE_INSTALL_PREFIX=~/libomp/openmp" – cgreen Oct 15 '18 at 20:07
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No, it is not (officially). The Apple LLVM is roughly based on 4.0, but the OpenMP module is left out. You can even see a remaining OpenMP option in the help to set offloading targets, but it does not support OpenMP.

$ clang++ --help | grep openmp
  -fopenmp-targets=<value>

Apple, adding OpenMP would be a huge boon to science researchers!

Note: Brew has support for building the OpenMP library! Use:

brew install libomp

A patch has been accepted into CMake that will allow users to get the correct compile and linking line OpenMP in CMake 3.12 if they use the targets system.

Henry Schreiner
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  • +1 this is better (for me) than building from source. Brew also includes detailed instructions on the compiler flags to get this to work. – Ahmed Fasih Aug 07 '18 at 18:01