FindOpenMPMacOS.cmake

# Find OpenMP library on MacOS
# Prefer libomp from the active Conda environment and fall back to Homebrew.

# lint_cmake: -package/consistency

macro(find_openmp_macos)
  if(NOT APPLE)
    message(FATAL_ERROR "${CMAKE_CURRENT_FUNCTION}() must only be used on MacOS")
  endif()

  if(DEFINED ENV{CONDA_PREFIX} AND EXISTS "$ENV{CONDA_PREFIX}/lib/libomp.dylib")
    set(CONDA_LIBOMP_PREFIX "$ENV{CONDA_PREFIX}")
    set(OpenMP_C_FLAGS
      "-Xpreprocessor -fopenmp -I${CONDA_LIBOMP_PREFIX}/include")
    set(OpenMP_CXX_FLAGS
      "-Xpreprocessor -fopenmp -I${CONDA_LIBOMP_PREFIX}/include")
    set(OpenMP_C_LIB_NAMES omp)
    set(OpenMP_CXX_LIB_NAMES omp)
    set(OpenMP_omp_LIBRARY ${CONDA_LIBOMP_PREFIX}/lib/libomp.dylib)
    find_package(OpenMP)
  endif()

  if(NOT OpenMP_FOUND)
    find_package(OpenMP)
  endif()
  if(NOT OpenMP_FOUND)
    # Try again with extra path info. This step is required for libomp 15+ from Homebrew,
    # as libomp 15.0+ from brew is keg-only
    # See https://github.com/Homebrew/homebrew-core/issues/112107#issuecomment-1278042927.
    execute_process(COMMAND brew --prefix libomp
                    OUTPUT_VARIABLE HOMEBREW_LIBOMP_PREFIX
                    OUTPUT_STRIP_TRAILING_WHITESPACE)
    set(OpenMP_C_FLAGS
      "-Xpreprocessor -fopenmp -I${HOMEBREW_LIBOMP_PREFIX}/include")
    set(OpenMP_CXX_FLAGS
      "-Xpreprocessor -fopenmp -I${HOMEBREW_LIBOMP_PREFIX}/include")
    set(OpenMP_C_LIB_NAMES omp)
    set(OpenMP_CXX_LIB_NAMES omp)
    set(OpenMP_omp_LIBRARY ${HOMEBREW_LIBOMP_PREFIX}/lib/libomp.dylib)
    find_package(OpenMP REQUIRED)
  endif()
endmacro()

# Patch libxgboost.dylib so that it depends on @rpath/libomp.dylib instead of
# /opt/homebrew/opt/libomp/lib/libomp.dylib or other hard-coded paths.
# Doing so enables XGBoost to interoperate with multiple kinds of OpenMP
# libraries. See https://github.com/lightgbm-org/LightGBM/pull/6391 for detailed
# explanation. Adapted from https://github.com/lightgbm-org/LightGBM/pull/6391
# by James Lamb.
# MacOS only.
function(patch_openmp_path_macos target target_default_output_name)
  if(NOT APPLE)
    message(FATAL_ERROR "${CMAKE_CURRENT_FUNCTION}() must only be used on MacOS")
  endif()
  # Get path to libomp found at build time
  get_target_property(
    __OpenMP_LIBRARY_LOCATION
    OpenMP::OpenMP_CXX
    INTERFACE_LINK_LIBRARIES
  )
  # Get the base name of the OpenMP lib
  # Usually: libomp.dylib, libgomp.dylib, or libiomp.dylib
  get_filename_component(
    __OpenMP_LIBRARY_NAME
    ${__OpenMP_LIBRARY_LOCATION}
    NAME
  )
  # Get the directory containing the OpenMP lib
  get_filename_component(
    __OpenMP_LIBRARY_DIR
    ${__OpenMP_LIBRARY_LOCATION}
    DIRECTORY
  )
  # Get the name of the XGBoost lib, e.g. libxgboost
  get_target_property(
    __LIBXGBOOST_OUTPUT_NAME
    ${target}
    OUTPUT_NAME
  )
  if(NOT __LIBXGBOOST_OUTPUT_NAME)
    set(__LIBXGBOOST_OUTPUT_NAME "${target_default_output_name}")
  endif()

  # Get the file name of the XGBoost lib, e.g. libxgboost.dylib
  if(CMAKE_SHARED_LIBRARY_SUFFIX_CXX)
    set(
      __LIBXGBOOST_FILENAME_${target} "${__LIBXGBOOST_OUTPUT_NAME}${CMAKE_SHARED_LIBRARY_SUFFIX_CXX}"
      CACHE INTERNAL "Shared library filename ${target}"
    )
  else()
    set(
      __LIBXGBOOST_FILENAME_${target} "${__LIBXGBOOST_OUTPUT_NAME}.dylib"
      CACHE INTERNAL "Shared library filename ${target}"
    )
  endif()

  message(STATUS "Creating shared lib for target ${target}: ${__LIBXGBOOST_FILENAME_${target}}")

  # Override the absolute path to OpenMP with a relative one using @rpath.
  #
  # This also ensures that if a libomp.dylib has already been loaded, it'll just use that.
  if(KEEP_BUILD_ARTIFACTS_IN_BINARY_DIR)
    set(__LIB_DIR ${xgboost_BINARY_DIR}/lib)
  else()
    set(__LIB_DIR ${xgboost_SOURCE_DIR}/lib)
  endif()
  add_custom_command(
    TARGET ${target}
    POST_BUILD
      COMMAND
        install_name_tool
        -change
        ${__OpenMP_LIBRARY_LOCATION}
        "@rpath/${__OpenMP_LIBRARY_NAME}"
        "${__LIBXGBOOST_FILENAME_${target}}"
      WORKING_DIRECTORY ${__LIB_DIR}
  )
  message(STATUS
    "${__LIBXGBOOST_FILENAME_${target}}: "
    "Replacing hard-coded OpenMP install_name with '@rpath/${__OpenMP_LIBRARY_NAME}'..."
  )
  # Add RPATH entries to ensure the loader looks in the following, in the following order:
  #
  #   - $CONDA_PREFIX/lib             (when building inside a Conda environment)
  #   - /opt/homebrew/opt/libomp/lib  (where 'brew install' / 'brew link' puts libomp.dylib)
  #   - ${__OpenMP_LIBRARY_DIR}       (wherever find_package(OpenMP) found OpenMP at build time)
  #
  # Note: This list will only be used if libomp.dylib isn't already loaded into memory.
  #       So Conda users will likely use ${CONDA_PREFIX}/libomp.dylib
  execute_process(COMMAND brew --prefix libomp
                  OUTPUT_VARIABLE HOMEBREW_LIBOMP_PREFIX
                  OUTPUT_STRIP_TRAILING_WHITESPACE)
  set(__OPENMP_RPATHS "${HOMEBREW_LIBOMP_PREFIX}/lib;${__OpenMP_LIBRARY_DIR}")
  if(DEFINED ENV{CONDA_PREFIX} AND EXISTS "$ENV{CONDA_PREFIX}/lib/libomp.dylib")
    set(__OPENMP_RPATHS "$ENV{CONDA_PREFIX}/lib;${__OPENMP_RPATHS}")
  endif()
  set_target_properties(
    ${target}
    PROPERTIES
      BUILD_WITH_INSTALL_RPATH TRUE
      INSTALL_RPATH "${__OPENMP_RPATHS}"
      INSTALL_RPATH_USE_LINK_PATH FALSE
  )
endfunction()