diff --git a/llvm/cmake/config-ix.cmake b/llvm/cmake/config-ix.cmake
index 731071ef85b2..5833119e26fd 100755
--- a/llvm/cmake/config-ix.cmake
+++ b/llvm/cmake/config-ix.cmake
@@ -76,6 +76,9 @@ if( LLVM_USING_GLIBC )
   add_llvm_definitions( -D_GNU_SOURCE )
 endif()
 
+# Define LLVM_MULTITHREADED if gcc atomic builtins exists.
+include(CheckAtomic)
+
 include(CheckCXXCompilerFlag)
 check_cxx_compiler_flag("-fPIC" SUPPORTS_FPIC_FLAG)
 
diff --git a/llvm/cmake/modules/CheckAtomic.cmake b/llvm/cmake/modules/CheckAtomic.cmake
new file mode 100644
index 000000000000..27bbaba6998c
--- /dev/null
+++ b/llvm/cmake/modules/CheckAtomic.cmake
@@ -0,0 +1,18 @@
+# atomic builtins are required for threading support.
+
+INCLUDE(CheckCXXSourceCompiles)
+	
+CHECK_CXX_SOURCE_COMPILES("
+int main() {
+        volatile unsigned long val = 1;
+        __sync_synchronize();
+        __sync_val_compare_and_swap(&val, 1, 0);
+        __sync_add_and_fetch(&val, 1);
+        __sync_sub_and_fetch(&val, 1);
+        return 0;
+      }
+" LLVM_MULTITHREADED)
+
+if( NOT LLVM_MULTITHREADED )
+  message(STATUS "Warning: LLVM will be built thread-unsafe because atomic builtins are missing")
+endif()
diff --git a/llvm/include/llvm/Config/config.h.cmake b/llvm/include/llvm/Config/config.h.cmake
index 051114053da8..8a52f92503b4 100644
--- a/llvm/include/llvm/Config/config.h.cmake
+++ b/llvm/include/llvm/Config/config.h.cmake
@@ -467,6 +467,9 @@
 /* Installation directory for man pages */
 #undef LLVM_MANDIR
 
+/* Build multithreading support into LLVM */
+#cmakedefine LLVM_MULTITHREADED ${LLVM_MULTITHREADED}
+
 /* Define if this is Unixish platform */
 #cmakedefine LLVM_ON_UNIX ${LLVM_ON_UNIX}