Index: clang/docs/ClangCommandLineReference.rst
===================================================================
--- clang/docs/ClangCommandLineReference.rst
+++ clang/docs/ClangCommandLineReference.rst
@@ -2735,6 +2735,8 @@
 .. option:: -mtune=<arg>
 .. program:: clang
 
+Accepted for compatibility with GCC. Currently has no effect.
+
 .. option:: -mtvos-version-min=<arg>, -mappletvos-version-min=<arg>
 
 .. option:: -municode<arg>
Index: clang/include/clang/Driver/Options.td
===================================================================
--- clang/include/clang/Driver/Options.td
+++ clang/include/clang/Driver/Options.td
@@ -2710,7 +2710,8 @@
 def module_file_info : Flag<["-"], "module-file-info">, Flags<[DriverOption,CC1Option]>, Group<Action_Group>,
   HelpText<"Provide information about a particular module file">;
 def mthumb : Flag<["-"], "mthumb">, Group<m_Group>;
-def mtune_EQ : Joined<["-"], "mtune=">, Group<m_Group>;
+def mtune_EQ : Joined<["-"], "mtune=">, Group<m_Group>,
+  HelpText<"Accepted for compatibility with GCC. Currently has no effect.">;
 def multi__module : Flag<["-"], "multi_module">;
 def multiply__defined__unused : Separate<["-"], "multiply_defined_unused">;
 def multiply__defined : Separate<["-"], "multiply_defined">;