Better to turn off USE_OPENMP when build with AppleClang

[levyfan]

It is better to change

elseif(CMAKE_CXX_COMPILER_ID STREQUAL "AppleClang")
    message(FATAL_ERROR "AppleClang isn't supported. Please see https://github.com/Microsoft/LightGBM/blob/master/docs/Installation-Guide.rst#macos")

to

elseif(CMAKE_CXX_COMPILER_ID STREQUAL "AppleClang")
    SET(USE_OPENMP OFF CACHE BOOL "Enable OpenMP" FORCE)

Thus, macos with default xcode clang can build without openmp, and macos with gcc can build with openmp.

[Laurae2]

ping @guolinke

[guolinke]

I think building a single-thread LightGBM version isn't a good choice, since the algorithm is designed for high performance usage.

[levyfan]

Well, libomp can be installed >= el_capitan by homebrew https://github.com/Homebrew/homebrew-core/blob/master/Formula/libomp.rb

After install, it says

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 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.dylib
==> Summary
🍺  /usr/local/Cellar/libomp/5.0.1: 10 files, 1MB

I haven't figured out how to modify the cmake files, but I guess Apple Clang + OpenMP can be supported soon.

[levyfan]

When I run cmake command as suggested cmake -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.dylib ..

It fails with /usr/local/Cellar/cmake/3.9.1/share/cmake/Modules/FindOpenMP.cmake:402

CMake Error at /usr/local/Cellar/cmake/3.9.1/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:137 (message):
  Could NOT find OpenMP_C (missing: OpenMP_C_FLAGS OpenMP_C_LIB_NAMES) (found
  version "1.0")
Call Stack (most recent call first):
  /usr/local/Cellar/cmake/3.9.1/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:377 (_FPHSA_FAILURE_MESSAGE)
  /usr/local/Cellar/cmake/3.9.1/share/cmake/Modules/FindOpenMP.cmake:402 (find_package_handle_standard_args)
  CMakeLists.txt:67 (find_package)

[levyfan]

After some googling, I find FindOpenMP.cmake will support Apple Clang in cmake 3.12.0 release https://gitlab.kitware.com/cmake/cmake/merge_requests/1812/diffs

[chivee]

@levyfan what's version are you using for Apple Clang?

[levyfan]

@chivee

Apple LLVM version 9.1.0 (clang-902.0.39.2)
Target: x86_64-apple-darwin17.6.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

[guolinke]

@levyfan @chivee welcome to create a PR for the clang with openmp support

[StrikerRUS]

Fixed in #1501.