Repair cmake libatomic check.

The cmake check for whether libatomic could be used had been
unconditionally setting the result to false. Which was somewhat
fortunate, because the prerequisite check for whether it was *needed*
was always claiming it was, even if it was not.

However, this made platforms where libatomic is actually necessary
fail to link.

Differential Revision: http://reviews.llvm.org/D10453

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239819 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 36 insertions, 6 deletions

diff --git a/cmake/modules/CheckAtomic.cmake b/cmake/modules/CheckAtomic.cmake
index a03788ec9f9..551de6ade84 100644
--- a/cmake/modules/CheckAtomic.cmake
+++ b/cmake/modules/CheckAtomic.cmake
@@ -2,15 +2,45 @@
 
 INCLUDE(CheckCXXSourceCompiles)
 
-check_function_exists(__atomic_fetch_add_4 HAVE___ATOMIC_FETCH_ADD_4)
-if( NOT HAVE___ATOMIC_FETCH_ADD_4 )
-  check_library_exists(atomic __atomic_fetch_add_4 "" HAVE_LIBATOMIC)
-  set(HAVE_LIBATOMIC False)
-  if( HAVE_LIBATOMIC )
-    list(APPEND CMAKE_REQUIRED_LIBRARIES "atomic")
+# Sometimes linking against libatomic is required for atomic ops, if
+# the platform doesn't support lock-free atomics.
+
+function(check_working_cxx_atomics varname)
+  set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
+  set(CMAKE_REQUIRED_FLAGS "-std=c++11")
+  CHECK_CXX_SOURCE_COMPILES("
+#include <atomic>
+std::atomic<int> x;
+int main() {
+  return x;
+}
+" ${varname})
+  set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
+endfunction(check_working_cxx_atomics)
+
+# This isn't necessary on MSVC, so avoid command-line switch annoyance
+# by only running on GCC-like hosts.
+if (LLVM_COMPILER_IS_GCC_COMPATIBLE)
+  # First check if atomics work without the library.
+  check_working_cxx_atomics(HAVE_CXX_ATOMICS_WITHOUT_LIB)
+  # If not, check if the library exists, and atomics work with it.
+  if(NOT HAVE_CXX_ATOMICS_WITHOUT_LIB)
+    check_library_exists(atomic __atomic_fetch_add_4 "" HAVE_LIBATOMIC)
+    if( HAVE_LIBATOMIC )
+      list(APPEND CMAKE_REQUIRED_LIBRARIES "atomic")
+      check_working_cxx_atomics(HAVE_CXX_ATOMICS_WITH_LIB)
+      if (NOT HAVE_CXX_ATOMICS_WITH_LIB)
+	message(FATAL_ERROR "Host compiler must support std::atomic!")
+      endif()
+    else()
+      message(FATAL_ERROR "Host compiler appears to require libatomic, but cannot find it.")
+    endif()
   endif()
 endif()
 
+## TODO: This define is only used for the legacy atomic operations in
+## llvm's Atomic.h, which should be replaced.  Other code simply
+## assumes C++11 <atomic> works.
 CHECK_CXX_SOURCE_COMPILES("
 #ifdef _MSC_VER
 #include <Intrin.h> /* Workaround for PR19898. */