BLD Avoid misleading OpenMP warning with Apple Clang (scikit-learn#28839

)

sklearn/meson.build

@@ -60,6 +60,27 @@ np_dep = declare_dependency(include_directories: inc_np)
 openmp_dep = dependency('OpenMP', language: 'c', required: false)
 
 if not openmp_dep.found()
+ warn_about_missing_openmp = true
+ # On Apple Clang avoid a misleading warning if compiler variables are set.
+ # See https://github.com/scikit-learn/scikit-learn/issues/28710 for more
+ # details. This may be removed if the OpenMP detection on Apple Clang improves,
+ # see https://github.com/mesonbuild/meson/issues/7435#issuecomment-2047585466.
+ if host_machine.system() == 'darwin' and cc.get_id() == 'clang'
+ compiler_env_vars_with_openmp = run_command(py,
+ [
+ '-c',
+ '''
+import os
+
+compiler_env_vars_to_check = ["CPPFLAGS", "CFLAGS", "CXXFLAGS"]
+
+compiler_env_vars_with_openmp = [
+ var for var in compiler_env_vars_to_check if "-fopenmp" in os.getenv(var, "")]
+print(compiler_env_vars_with_openmp)
+'''], check: true).stdout().strip()
+ warn_about_missing_openmp = compiler_env_vars_with_openmp == '[]'
+ endif
+ if warn_about_missing_openmp
  warning(
 '''
  ***********
@@ -84,6 +105,13 @@ It seems that scikit-learn cannot be built with OpenMP.
 
  ***
 ''')
+ else
+ warning(
+'''It looks like compiler environment variables were set to enable OpenMP support.
+Check the output of "import sklearn; sklearn.show_versions()" after the build
+to make sure that scikit-learn was actually built with OpenMP support.
+''')
+ endif
 endif
 
 # For now, we keep supporting SKLEARN_ENABLE_DEBUG_CYTHON_DIRECTIVES variable
@@ -127,7 +155,6 @@ custom_target('write_built_with_meson_file',
  install: true,
  install_dir: py.get_install_dir() / 'sklearn'
 )
-# endif
 
 extensions = ['_isotonic']