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FindOpenMP doesn't find OpenMP on current macOS

    • Closed Issue created by Henry Schreiner @henryiii

    It looks like the tests in FindOpenMP.cmake are not running and finding OpenMP on macOS because they are missing the /usr/local/include directory.

    The proper fix would be to add a search for omp.h and then use that include directory in the targets, and that would also fix #18098 (closed) and could fix #17474.

    Edited
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    • Brad King
      Brad King @brad.king ·
      Owner

      Cc: @ChrisTX @rolandschulz

    • Brad King added os:macos triage:mr-welcome labels

      added os:macos triage:mr-welcome labels

    • Henry Schreiner changed the description

      changed the description

    • Craig Scott mentioned in merge request !3916 (merged)

      mentioned in merge request !3916 (merged)

    • Kitware Robot closed via merge request !3916 (merged)

      closed via merge request !3916 (merged)

    • Craig Scott closed via commit 409891ba

      closed via commit 409891ba

      • Moritz Moeller
        Moritz Moeller @virtualritz ·

        FindOpenMP still doesn't work for me on macOS Catalina (10.15.7). Using CMake 3.20.1.

        It does not work in Homebrew

        I have libomp installed via Homebrew.

        brew install libomp

        Which installs:

        /usr/local/opt/llvm/lib/libomp.dylib

        Supposedly this should be enough to build OpenMP code even with Apple's Clang. But that doesn't work. I get:

          clang: error: unsupported argument 'libomp' to option 'fopenmp='

        So I also installed Homebrew's Clang which has OpenMP support and set CMAKE_C_COMPILER and CMAKE_CXX_COMPILER to the location of Homebrew's clang/clang++.

        After this I get:

          -- Could NOT find OpenMP_C (missing: OpenMP_C_FLAGS OpenMP_C_LIB_NAMES)
  -- Could NOT find OpenMP_CXX (missing: OpenMP_CXX_FLAGS OpenMP_CXX_LIB_NAMES)
  -- Could NOT find OpenMP (missing: OpenMP_C_FOUND OpenMP_CXX_FOUND)

        So I tried a bunch of stuff from here.

        if(APPLE)
    if(CMAKE_C_COMPILER_ID MATCHES "Clang")
        set(OpenMP_C "${CMAKE_C_COMPILER}")
        set(OpenMP_C_FLAGS "-fopenmp=libomp -Wno-unused-command-line-argument")
        set(OpenMP_C_LIB_NAMES "libomp" "libgomp" "libiomp5")
        set(OpenMP_libomp_LIBRARY ${OpenMP_C_LIB_NAMES})
        set(OpenMP_libgomp_LIBRARY ${OpenMP_C_LIB_NAMES})
        set(OpenMP_libiomp5_LIBRARY ${OpenMP_C_LIB_NAMES})
    endif()
    if(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
        set(OpenMP_CXX "${CMAKE_CXX_COMPILER}")
        set(OpenMP_CXX_FLAGS "-fopenmp=libomp -Wno-unused-command-line-argument")
        set(OpenMP_CXX_LIB_NAMES "libomp" "libgomp" "libiomp5")
        set(OpenMP_libomp_LIBRARY ${OpenMP_CXX_LIB_NAMES})
        set(OpenMP_libgomp_LIBRARY ${OpenMP_CXX_LIB_NAMES})
        set(OpenMP_libiomp5_LIBRARY ${OpenMP_CXX_LIB_NAMES})
    endif()
endif()
find_package(OpenMP)

        This the makes the compile work but the lib path somehow doesn't make it through as I get:

          ld: library not found for -lomp

        So then I tried this (next answer from above's StackOverflow thread):

        if(APPLE)
    if(CMAKE_C_COMPILER_ID MATCHES "Clang\$")
        set(OpenMP_C_FLAGS "-Xpreprocessor -fopenmp")
        set(OpenMP_C_LIB_NAMES "omp")
        set(OpenMP_omp_LIBRARY omp)
    endif()
    if(CMAKE_CXX_COMPILER_ID MATCHES "Clang\$")
        set(OpenMP_CXX_FLAGS "-Xpreprocessor -fopenmp")
        set(OpenMP_CXX_LIB_NAMES "omp")
        set(OpenMP_omp_LIBRARY omp)
    endif()
endif
find_package(OpenMP)

        This also doesn't work.

        Undefined symbols for architecture x86_64:
    "___kmpc_for_static_fini", referenced from:
        _.omp_outlined._debug__ in ompEvaluator.cpp.o
        _.omp_outlined._debug__.1 in ompEvaluator.cpp.o
        _.omp_outlined._debug__.3 in ompEvaluator.cpp.o
        _.omp_outlined._debug__ in ompKernel.cpp.o
        _.omp_outlined._debug__.3 in ompKernel.cpp.o
        _.omp_outlined._debug__.5 in ompKernel.cpp.o
    "___kmpc_for_static_init_4", referenced from:
        _.omp_outlined._debug__ in ompEvaluator.cpp.o
        _.omp_outlined._debug__.1 in ompEvaluator.cpp.o
        _.omp_outlined._debug__.3 in ompEvaluator.cpp.o
        _.omp_outlined._debug__ in ompKernel.cpp.o
        _.omp_outlined._debug__.3 in ompKernel.cpp.o
        _.omp_outlined._debug__.5 in ompKernel.cpp.o
    "___kmpc_fork_call", referenced from:
    [...]
        Edited by Brad King
      • Brad King
        Brad King @brad.king ·
        Owner

        @virtualritz we have a passing test for OpenMP on macOS:

        The C compiler identification is AppleClang 12.0.0.12000032
...
Check for working C compiler: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/cc - skipped
...
The CXX compiler identification is AppleClang 12.0.0.12000032
...
Check for working CXX compiler: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++ - skipped
...
Found OpenMP_C: -Xclang -fopenmp (found version "4.5") 
Found OpenMP_CXX: -Xclang -fopenmp (found version "4.5") 
Found OpenMP: TRUE (found version "4.5")

        Your problem is with Homebrew's compiler and OpenMP package. Please open a new issue for that.

      • Please register or sign in to reply
      • Moritz Moeller
        Moritz Moeller @virtualritz ·

        Your problem is with Homebrew's compiler and OpenMP package. Please open a new issue for that. @brad.king

        Nope. As I wrote, I tried with Apple Clang first.

        I also uninstalled the Homebrew libomp package to be sure and tried again:

        ❯ cmake -DNO_EXAMPLES=1 -DNO_TUTORIALS=1 -DNO_REGRESSION=1 -DNO_PTEX=1 -DNO_DOC=1 -DNO_OPENCL=1  -DNO_CLEW=1 -DNO_TBB=1 -DNO_METAL=1 -DNO_CUDA=1 ..                                                        2.204.229.42   100% 
-- The C compiler identification is AppleClang 12.0.0.12000032
-- The CXX compiler identification is AppleClang 12.0.0.12000032
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/cc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Compiling OpenSubdiv version v3_4_4
-- Using cmake version 3.20.1
-- Could NOT find OpenMP_C (missing: OpenMP_C_FLAGS OpenMP_C_LIB_NAMES)
-- Could NOT find OpenMP_CXX (missing: OpenMP_CXX_FLAGS OpenMP_CXX_LIB_NAMES)
-- Could NOT find OpenMP (missing: OpenMP_C_FOUND OpenMP_CXX_FOUND)
      • Brad King
        Brad King @brad.king ·
        Owner

        -DNO_EXAMPLES=1 -DNO_TUTORIALS=1 -DNO_REGRESSION=1 -DNO_PTEX=1 -DNO_DOC=1 -DNO_OPENCL=1 -DNO_CLEW=1 -DNO_TBB=1 -DNO_METAL=1 -DNO_CUDA=1

        That looks like some complex project. Please try a simple CMakeLists.txt file. For example:

        $ cat ../CMakeLists.txt
cmake_minimum_required(VERSION 3.19)
project(Issue18470)
find_package(OpenMP COMPONENTS C CXX)

$ cmake --version
cmake version 3.20.1
...

$ cmake ..
-- The C compiler identification is AppleClang 12.0.0.12000032
-- The CXX compiler identification is AppleClang 12.0.0.12000032
...
-- Found OpenMP_C: -Xclang -fopenmp (found version "4.5")
-- Found OpenMP_CXX: -Xclang -fopenmp (found version "4.5")
-- Found OpenMP: TRUE (found version "4.5") found components: C CXX
...

        If that works, please try to narrow down the failing project to a minimal example.

        BTW, please use the "Reply..." box in this discussion thread instead of posting a separate comment to reply.

      • Moritz Moeller
        Moritz Moeller @virtualritz ·

        Hey, sorry about the reply thingy. Missed that. So I tried your minimal example and I get the same.

        -- The C compiler identification is AppleClang 12.0.0.12000032
-- The CXX compiler identification is AppleClang 12.0.0.12000032
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/cc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Could NOT find OpenMP_C (missing: OpenMP_C_FLAGS OpenMP_C_LIB_NAMES)
-- Could NOT find OpenMP_CXX (missing: OpenMP_CXX_FLAGS OpenMP_CXX_LIB_NAMES)
-- Could NOT find OpenMP (missing: OpenMP_C_FOUND OpenMP_CXX_FOUND C CXX)
-- Configuring done
-- Generating done
-- Build files have been written to: /Users/moritz/code/test_cmake

        P.S.: the complex project also just uses a one liner (and nothing else related to OpenMP set before/after):

        find_package(OpenMP)
      • Brad King
        Brad King @brad.king ·
        Owner

        Please attach the CMakeFiles/CMake{Error,Output}.log files from the build tree of the tiny test project.

        Also please run again in a fresh/empty build tree with cmake .. --trace-expand 2>trace.log and attach the trace.log file.

      • Moritz Moeller
        Moritz Moeller @virtualritz ·

        Original CMakeOutput.log (there wasn't any CMakeError.log).

        And trace.log from --trace-expand run.

      • Brad King
        Brad King @brad.king ·
        Owner

        Ah. I think Apple may not be providing OpenMP libraries with their Clang. The nightly test I linked above is enabled only on a specific machine that has Homebrew's libomp installed. I have it on my local machine too. I end up with:

        $ grep OpenMP_libomp_LIBRARY: CMakeCache.txt
OpenMP_libomp_LIBRARY:FILEPATH=/usr/local/lib/libomp.dylib

        Please restore Homebrew's libomp package, and try again with the simple example and the AppleClang compilers.

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    • cedric gestes
      cedric gestes @cedric.gestes ·

      Same issue on my system.

      Fixed using the following before running cmake: export CMAKE_PREFIX_PATH=/opt/homebrew/opt/openblas:/opt/homebrew/opt/libomp:/opt/homebrew

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