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

[SjoerdMeijer]

Summary

This is a doc change documenting that option -mtune does not perform any CPU type specific tuning but exists for compatability reasons with GCC.

[• Quuxplusone]

clang/docs/ClangCommandLineReference.rst

2738

"Accepts any value, for compatibility with GCC. Does not perform any CPU-specific tuning."

In email I said:

there is no practical difference, but there may be a psychological difference, between saying "Clang permanently treats -mtune as a no-op" versus "Clang has a remarkably low quality of implementation for -mtune, and has no immediate plans to either improve or regress it."

To me, this patch's wording implies that Clang is promising to keep -mtune as a no-op forever. If Bob puts -mtune=supercalifragilistic in his Makefile today, and then in the future Clang starts emitting an "unknown target" error, will Bob point to this documentation as evidence that Clang promised to accept -mtune=supercalifragilistic as a no-op?

So I would rather say:

"Accepted for compatibility with GCC. Currently has no effect."

[SjoerdMeijer]

Cheers, that's probably what I wanted to say.

[psmith]

That wording looks good to me. I've accepted the change, but worth waiting a day or so to see if there are any objections or suggestions.

[SjoerdMeijer]

Thank you both for reviewing!
And I will wait a day.

clang/docs/ClangCommandLineReference.rst

 .. option:: -mtls-size=<arg>
 
 Specify bit size of immediate TLS offsets (AArch64 ELF only): 12 (for 4KB) \| 24 (for 16MB, default) \| 32 (for 4GB) \| 48 (for 256TB, needs -mcmodel=large)
 
 .. program:: clang1
 .. 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>
 
 .. option:: -mvx, -mno-vx
 
 .. option:: -mwarn-nonportable-cfstrings, -mno-warn-nonportable-cfstrings
 

clang/include/clang/Driver/Options.td

def mno_relax_pic_calls : Flag<["-"], "mno-relax-pic-calls">,
   Group<m_mips_Features_Group>,
   HelpText<"Do not produce relaxation hints for linkers to try optimizing PIC "
            "call sequences into direct calls (MIPS only)">, Flags<[HelpHidden]>;
 def mglibc : Flag<["-"], "mglibc">, Group<m_libc_Group>, Flags<[HelpHidden]>;
 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">;
 def mwarn_nonportable_cfstrings : Flag<["-"], "mwarn-nonportable-cfstrings">, Group<m_Group>;
 def no_canonical_prefixes : Flag<["-"], "no-canonical-prefixes">, Flags<[HelpHidden, CoreOption]>,
   HelpText<"Use relative instead of canonical paths">;
 def no_cpp_precomp : Flag<["-"], "no-cpp-precomp">, Group<clang_ignored_f_Group>;
 def no_integrated_cpp : Flag<["-", "--"], "no-integrated-cpp">, Flags<[DriverOption]>;