[PATCH] Repair cmake libatomic check.

[James Y Knight]

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.

http://reviews.llvm.org/D10453

Files:
  cmake/modules/CheckAtomic.cmake

Index: cmake/modules/CheckAtomic.cmake
===================================================================
--- cmake/modules/CheckAtomic.cmake
+++ 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. */

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