diff --git a/llvm/cmake/modules/CheckAtomic.cmake b/llvm/cmake/modules/CheckAtomic.cmake
--- a/llvm/cmake/modules/CheckAtomic.cmake
+++ b/llvm/cmake/modules/CheckAtomic.cmake
@@ -43,7 +43,7 @@
 # Check for (non-64-bit) atomic operations.
 if(MSVC)
   set(HAVE_CXX_ATOMICS_WITHOUT_LIB True)
-elseif(LLVM_COMPILER_IS_GCC_COMPATIBLE)
+elseif(LLVM_COMPILER_IS_GCC_COMPATIBLE OR CMAKE_CXX_COMPILER_ID MATCHES "XLClang")
   # 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.
@@ -64,7 +64,7 @@
 # Check for 64 bit atomic operations.
 if(MSVC)
   set(HAVE_CXX_ATOMICS64_WITHOUT_LIB True)
-elseif(LLVM_COMPILER_IS_GCC_COMPATIBLE)
+elseif(LLVM_COMPILER_IS_GCC_COMPATIBLE OR CMAKE_CXX_COMPILER_ID MATCHES "XLClang")
   # First check if atomics work without the library.
   check_working_cxx_atomics64(HAVE_CXX_ATOMICS64_WITHOUT_LIB)
   # If not, check if the library exists, and atomics work with it.