libomp 5.0.1 (new formula)

[henryiii]

Add OpenMP for Apple Clang. Apple Clang now has support for OpenMP, but
it has been disabled in the driver and is not included with High Sierra.
It can be built and used, though; this formula will build it and
provides hints on the correct usage.

There have been a few other formula in the past, such as clang-omp, that provided
special compilers and other tricks to add OpenMP. This is not like those; this is for
the built-in system Clang. On at least High Sierra, this is now possible due to the fact
that it is based on a version of Clang (4.0) that has OpenMP support; it's just not built and
included by Apple.

This is the message printed by the formula:

On Apple Clang, you need to add several options to use OpenMP's front end
instead of the standard driver option. This usually looks like:
  -Xpreprocessor -fopenmp -lomp

You might need to make sure the lib and include directories are discoverable if /usr/local is not searched:
  -L/usr/local/opt/libomp/lib -I/usr/local/opt/libomp/include

For CMake, the following will flags will cause the OpenMP::OpenMP_CXX target to be set up correctly:
  -DOpenMP_CXX_FLAGS="-Xpreprocessor -fopenmp -I/usr/local/opt/libomp/include" -DOpenMP_CXX_LIB_NAMES="omp" -DOpenMP_omp_LIBRARY=/usr/local/opt/libomp/lib/libomp.a

And, future thoughts:

• Support could be added to FindOpenMP for automatic discovery, at least if the OpenMP::* targets are used. I'm investigating a patch for CMake.

Note: Original idea from https://stackoverflow.com/questions/44380459/is-openmp-available-in-high-sierra-llvm/47230419#47230419

---

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[fxcoudert]

I do not think this is a good idea.

• It's not supported by Apple (the front-end option is there currently, but they might remove it in the future)
• The library version is not matching the compiler version exactly, and the mismatch might be problematic.
• We already ship llvm, which includes OpenMP support (GCC does as well, too)

[henryiii]

However, building with a completely different compiler can cause problems; for example, using Boost + llvm 5 vs. Boost + Apple llvm 9 vs. Boost + GCC. This does not require a completely different compiler chain.

[ilovezfs]

Agreed with @fxcoudert. This duplicates what's available in the llvm formula. It's already possible to use openmp from the llvm formula with Xcode's clang.

Sorry @henryiii! We look forward to your next contribution.

[henryiii]

@ilovezfs , how would you do that? Including /usr/local/opt/llvm\@4/lib/clang/4.0.1/include/ causes errors due to the other files in that directory (not omp.h itself). If I manually copy omp.h to the current directory, and use the line:

clang++ -Xpreprocessor -fopenmp simple.cxx -I. -L/usr/local/opt/llvm/lib -lomp

it works.

[henryiii]

If anyone needs it, I've moved the PR contents to a Tap, https://github.com/CLIUtils/homebrew-apple. A cmake include file that conditionally adds this if it's installed is in https://github.com/CLIUtils/cmake/blob/master/PatchOpenMPApple.cmake. I'm using it in GooFit successfully. I'll probably write a blog post about it eventually (which I'll link in the Stack Overflow answers above)

[ilovezfs]

@henryiii mind updating this for llvm 5? We'd like to experiment with using

depends_on "libomp"

instead of

depends_on "gcc"
fails_with :clang

for formulae that need openmp.

[henryiii]

Okay, though keep in mind, the build and linking line is a bit different than GCC. I can make a PR to CMake to help with that in the future.

[ilovezfs]

Okay, though keep in mind, the build and linking line is a bit different than GCC. I can make a PR to CMake to help with that in the future.

Right I think we need some superenv (see https://github.com/Homebrew/brew/tree/master/Library/Homebrew/shims/super) support for it. It would be really nice to be able to have

depends_on "libomp"

"just work"

[henryiii]

Done.

[ilovezfs]

@henryiii thanks! Have you been able to figure out what the minimum Xcode for this magic to work is?

[ilovezfs]

@henryiii the test block seems to pass even if I remove the -Xpreprocessor -fopenmp. It would be great if you could make the test do something that actually fails if that isn't passed.

[henryiii]

Okay, will fix that in a minute. Working on adding a travis test on multiple versions of XCode (running, unfortunately it seems, that test block...)

[henryiii]

Is there something obvious missing from the test block? I can get that to fail or pass manually, just not when it's part of the formula; then it always passes.

[ilovezfs]

I would think the test should fail if the -Xpreprocessor -fopenmp is not passed.

[henryiii]

Oh, I thought the rest ran when install ran. Nevermind, I now have a test that fails properly if those parts are removed. I'll commit it in a second.

[ilovezfs]

Ah, I see. Yeah you have to manually run brew test --verbose --debug foo

[henryiii]

@ilovezfs , it works with XCode 7+, from a simple test. XCode 6 fails. See https://travis-ci.org/henryiii/test_omp . (Build is still finishing for XCode 6, but it will eventually fail)

Is it better to have the caveat print -L$(brew --prefix libomp)/lib -I$(brew --prefix libomp)/include or the actual expanded value of the path (as it does now)? (For me, the unexpanded line is incorrect because I use fish shell)

[ilovezfs]

It should use #{opt_lib} and #{opt_include}.

Also, I think we want

depends_on :macos => :yosemite

since that's the minimum version for Xcode 7.

[ilovezfs]

I think by default we'd want to suggest the dylib not the .a

[henryiii]

That's a typo, I don't think it even makes a .a.

[henryiii]

Fixed. (Actually, this one does make .a and .dylib; the one I had in my tap only made the dylib)

[henryiii]

What about adding the bottles? I can see the hashes from Jenkins, should I copy and paste them in?

[ilovezfs]

@henryiii no that is handled automatically by brew pull

[henryiii]

Oh, okay, thanks. Wasn't sure how that was setup.

[ilovezfs]

If you're curious https://github.com/Homebrew/brew/blob/master/Library/Homebrew/dev-cmd/pull.rb#L19-L20

[ilovezfs]

Thanks for your first contribution to Homebrew, @henryiii! Without people like you submitting PRs we couldn't run this project. You rock!

[henryiii]

I've got a patch I'm working on for CMake's FindOpenMP: https://gitlab.kitware.com/cmake/cmake/merge_requests/1812