ignore -flto= options recognized by GCC

[doko]

Summary

as requested in https://bugs.llvm.org/show_bug.cgi?id=49553, submitting the proposed changes to just ignore the -flto= options which are recognized by GCC ("auto" and "jobserver").

GCC supports -flto=<auto|jobserver|<N> to select the parallelity for LTO builds. LLVM also has -flto-jobs=<N>, which only seems to have a meaning when used with -flto=thin?

The attached patch just ignores the values "auto" and "jobserver". that doesn't change anything in functionality. Another option would be to map these values to either "thin" or "full", maybe in presence of the -ffat-lto-objects option?

-flto=<n> could also be translated to -flto-jobs=<N>.

[serge-sans-paille]

Please submit a patch with context. @mehdi_amini this looks good to me, any thoughts?

[mehdi_amini]

I don't know what's the best practice for gcc compatibility with respect to clang command line flags, adding Teresa and Richard here.

[tejohnson]

It looks like typically Clang will warn for unsupported gcc options. See clang_ignored_gcc_optimization_f_Group and its handling in Clang.cpp. I think it should be possible to simply mark these as clang_ignored_gcc_optimization_f_Group in Options.td. I.e. have an entry specifically for each of these, like:

def : Flag<["-"], "flto=auto">, Group<clang_ignored_gcc_optimization_f_Group>;
def : Flag<["-"], "flto=jobserver">, Group<clang_ignored_gcc_optimization_f_Group>;

[doko]

do you mean, just discard the current patch, and just list those as unsupported?

[tejohnson]

In D99501#2664253, @doko wrote:

do you mean, just discard the current patch, and just list those as unsupported?

Yes as an alternate approach that results in these being warned about (and ignored) as we seem to do for other unsupported gcc options.

[doko]

discarded the first approach, and used the proposed approach. I added this close to the ignoring of the -ffat-lto-objects option. Please let me know if there's a better location. Builds and works as expected:

$ clang -flto=auto hello.c
clang: warning: optimization flag '-flto=auto' is not supported [-Wignored-optimization-argument]
$ echo $?
0

$ clang -flto=auto -ffat-lto-objects hello.c
clang: warning: optimization flag '-flto=auto' is not supported [-Wignored-optimization-argument]
clang: warning: optimization flag '-ffat-lto-objects' is not supported [-Wignored-optimization-argument]

$ clang -flto=jobserver -ffat-lto-objects hello.c
clang: warning: optimization flag '-flto=jobserver' is not supported [-Wignored-optimization-argument]
clang: warning: optimization flag '-ffat-lto-objects' is not supported [-Wignored-optimization-argument]

[tejohnson]

lgtm

[lebedev.ri]

Tests missing.
This somehow wasn't sent to cfe-commits list

[fhahn]

In D99501#2668774, @lebedev.ri wrote:

Tests missing.
This somehow wasn't sent to cfe-commits list

@doko could you add a test? e.g. in clang/test/Driver/clang_f_opts.c, which already contains a check for -ffat-lto-objects

[doko]

here in the same place, or in a different merge request?

[fhahn]

In D99501#2679394, @doko wrote:

here in the same place, or in a different merge request?

This one is closed now I think, so a new review would be ideal.

[doko]

that's now https://reviews.llvm.org/D100484

[tejohnson]

Just approved the test case patch. Sorry I missed the lack of test on this!

[fhahn]

In D99501#2689090, @tejohnson wrote:

Just approved the test case patch. Sorry I missed the lack of test on this!

Great, thanks!

clang/include/clang/Driver/Options.td

 def fdiagnostics_show_location_EQ : Joined<["-"], "fdiagnostics-show-location=">, Group<clang_ignored_f_Group>;
 
 defm fcheck_new : BooleanFFlag<"check-new">, Group<clang_ignored_f_Group>;
 defm caller_saves : BooleanFFlag<"caller-saves">, Group<clang_ignored_gcc_optimization_f_Group>;
 defm reorder_blocks : BooleanFFlag<"reorder-blocks">, Group<clang_ignored_gcc_optimization_f_Group>;
 defm branch_count_reg : BooleanFFlag<"branch-count-reg">, Group<clang_ignored_gcc_optimization_f_Group>;
 defm default_inline : BooleanFFlag<"default-inline">, Group<clang_ignored_gcc_optimization_f_Group>;
 defm fat_lto_objects : BooleanFFlag<"fat-lto-objects">, Group<clang_ignored_gcc_optimization_f_Group>;
+def : Flag<["-"], "flto=auto">, Group<clang_ignored_gcc_optimization_f_Group>;
+def : Flag<["-"], "flto=jobserver">, Group<clang_ignored_gcc_optimization_f_Group>;
 defm float_store : BooleanFFlag<"float-store">, Group<clang_ignored_gcc_optimization_f_Group>;
 defm friend_injection : BooleanFFlag<"friend-injection">, Group<clang_ignored_f_Group>;
 defm function_attribute_list : BooleanFFlag<"function-attribute-list">, Group<clang_ignored_f_Group>;
 defm gcse : BooleanFFlag<"gcse">, Group<clang_ignored_gcc_optimization_f_Group>;
 defm gcse_after_reload: BooleanFFlag<"gcse-after-reload">, Group<clang_ignored_gcc_optimization_f_Group>;
 defm gcse_las: BooleanFFlag<"gcse-las">, Group<clang_ignored_gcc_optimization_f_Group>;
 defm gcse_sm: BooleanFFlag<"gcse-sm">, Group<clang_ignored_gcc_optimization_f_Group>;
 defm gnu : BooleanFFlag<"gnu">, Group<clang_ignored_f_Group>;