diff --git a/llvm/lib/Target/SystemZ/SystemZISelLowering.cpp b/llvm/lib/Target/SystemZ/SystemZISelLowering.cpp
--- a/llvm/lib/Target/SystemZ/SystemZISelLowering.cpp
+++ b/llvm/lib/Target/SystemZ/SystemZISelLowering.cpp
@@ -129,6 +129,8 @@
   setBooleanContents(ZeroOrOneBooleanContent);
   setBooleanVectorContents(ZeroOrNegativeOneBooleanContent);
 
+  setMaxAtomicSizeInBitsSupported(64);
+
   // Instructions are strings of 2-byte aligned 2-byte values.
   setMinFunctionAlignment(Align(2));
   // For performance reasons we prefer 16-byte alignment.