[openmp] [test] Try to link -latomic to provide atomics when available

[mgorny]

Summary

When using -rtlib=libgcc, the fallback implementation of __atomic_*
builtins is provided via libatomic (included in GCC). However, neither
GCC itself nor clang link libatomic implicitly, and it seems that GCC
upstream expects projects to link it explicitly as necessary.

Since compiler-rt provides __atomic_* builtins directly in the main
library, check if they are provided by the default libraries first.
If they are not, check if -latomic is available to provide them
and add explicit -latomic for tests in this case.

This fixes unresolved __atomic_load() references when running openmp
tests on i386 with libgcc backend.

[mgorny]

Le gentle ping.

[jlpeyton]

LGTM

openmp/trunk/runtime/test/CMakeLists.txt

 # CMakeLists.txt file for unit testing OpenMP Library
 include(FindPythonInterp)
 include(CheckTypeSize)
+include(CheckFunctionExists)
 include(CheckLibraryExists)
 
 if(NOT PYTHONINTERP_FOUND)
   libomp_warning_say("Could not find Python.")
   libomp_warning_say("The check-libomp target will not be available!")
   return()
 endif()
 
 # Some tests use math functions
 check_library_exists(m sqrt "" LIBOMP_HAVE_LIBM)
+# When using libgcc, -latomic may be needed for atomics
+# (but when using compiler-rt, the atomics will be built-in)
+# Note: we can not check for __atomic_load because clang treats it
+# as special built-in and that breaks CMake checks
+check_function_exists(__atomic_load_1 LIBOMP_HAVE_BUILTIN_ATOMIC)
+if(NOT LIBOMP_HAVE_BUILTIN_ATOMIC)
+  check_library_exists(atomic __atomic_load_1 "" LIBOMP_HAVE_LIBATOMIC)
+else()
+  # not needed
+  set(LIBOMP_HAVE_LIBATOMIC 0)
+endif()
 
 macro(pythonize_bool var)
   if (${var})
     set(${var} True)
   else()
     set(${var} False)
   endif()
 endmacro()
 
 pythonize_bool(LIBOMP_USE_HWLOC)
 pythonize_bool(LIBOMP_OMPT_SUPPORT)
 pythonize_bool(LIBOMP_OMPT_BLAME)
 pythonize_bool(LIBOMP_OMPT_TRACE)
 pythonize_bool(LIBOMP_HAVE_LIBM)
+pythonize_bool(LIBOMP_HAVE_LIBATOMIC)
 
 set(LIBOMP_TEST_CFLAGS "" CACHE STRING
   "Extra compiler flags to send to the test compiler")
 
 if(${LIBOMP_STANDALONE_BUILD})
   # Make sure we can use the console pool for recent cmake and ninja > 1.5
   if(CMAKE_VERSION VERSION_LESS 3.1.20141117)
     set(cmake_3_2_USES_TERMINAL)

openmp/trunk/runtime/test/lit.cfg

config.test_cflags = config.test_openmp_flag + \
     " -I " + config.omp_header_directory + \
     " -L " + config.library_dir + \
     " " + config.test_extra_cflags
 
 # extra libraries
 libs = ""
 if config.has_libm:
     libs += " -lm"
+if config.has_libatomic:
+    libs += " -latomic"
 
 # Allow XFAIL to work
 config.target_triple = [ ]
 if re.search('gcc', config.test_compiler) is not None:
     config.available_features.add('gcc')
 
 # Setup environment to find dynamic library at runtime
 append_dynamic_library_path(config.library_dir)

openmp/trunk/runtime/test/lit.site.cfg.in

 @AUTO_GEN_COMMENT@
 
 config.test_compiler = "@LIBOMP_TEST_COMPILER@"
 config.test_filecheck = "@FILECHECK_EXECUTABLE@"
 config.test_openmp_flag = "@LIBOMP_TEST_OPENMP_FLAG@"
 config.test_extra_cflags = "@LIBOMP_TEST_CFLAGS@"
 config.libomp_obj_root = "@CMAKE_CURRENT_BINARY_DIR@"
 config.library_dir = "@LIBOMP_LIBRARY_DIR@"
 config.omp_header_directory = "@LIBOMP_BINARY_DIR@/src"
 config.operating_system = "@CMAKE_SYSTEM_NAME@"
 config.hwloc_library_dir = "@LIBOMP_HWLOC_LIBRARY_DIR@"
 config.using_hwloc = @LIBOMP_USE_HWLOC@
 config.has_ompt = @LIBOMP_OMPT_SUPPORT@ and @LIBOMP_OMPT_BLAME@ and @LIBOMP_OMPT_TRACE@
 config.has_libm = @LIBOMP_HAVE_LIBM@
+config.has_libatomic = @LIBOMP_HAVE_LIBATOMIC@
 
 # Let the main config do the real work.
 lit_config.load_config(config, "@LIBOMP_BASE_DIR@/test/lit.cfg")
-