[Driver][docs] Document option -mtune as a no-op.

This documents that option -mtune is accepted for compatibility with GCC,
currently it has no effect, and thus does not currently perform any CPU type
specific tuning.

Corresponding discussion on the cfe dev list:
http://lists.llvm.org/pipermail/cfe-dev/2020-April/065169.html

Differential Revision: https://reviews.llvm.org/D78511

diff --git a/clang/docs/ClangCommandLineReference.rst b/clang/docs/ClangCommandLineReference.rst
index c7afcf7..a10e747 100644 (file)
--- a/clang/docs/ClangCommandLineReference.rst
+++ b/clang/docs/ClangCommandLineReference.rst
@@ -2735,6 +2735,8 @@ Specify bit size of immediate TLS offsets (AArch64 ELF only): 12 (for 4KB) \| 24
 .. 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>

diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
index 2ec7269..f9850c6 100644 (file)
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -2721,7 +2721,8 @@ def muclibc : Flag<["-"], "muclibc">, Group<m_libc_Group>, Flags<[HelpHidden]>;
 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">;