[Dwarf Fission] Implement Fission Proposal (issue6305113)

[Cary Coutant]

>>>> +      if (dwarf_version >= 4 || dwarf_split_debug_info)
>>>
>>> Shouldn't -gsplit-debug require -gdwarf-4+ anyway?
>>
>> Seems better to check explicitly than to rely on an implied relationship.
>
> I think Jason is right here. Although they are theoretically separate,
> there has been no testing of -gsplit-dwarf without dwarf4, and I don't
> think we plan to test it that way.
>
> I think the best thing to do is make -gspit-debug-info imply dwarf4.
> I'll fix it up.

OK, sounds good to me.

>>>> -  add_AT_pubnames (comp_unit_die ());
>>>> +  if (!dwarf_split_debug_info)
>>>> +    add_AT_pubnames (comp_unit_die ());
>>>
>>> Why not emit AT_pubnames with DWO?
>>
>> In the dwarf_split_debug_info case, the attribute gets added by
>> add_top_level_skeleton_die_attrs. Looks like both cases should be
>> handled in the same place -- Sterling, can you remove the call in
>> add_top_level_skeleton_die_attrs and make the one in dwarf2out_finish
>> unconditional (using main_comp_unit_die)?
>
> AT_pubnames needs to go in all the skeleton dies, including the TUs,
> so moving the call from add_top_level_skeleton_die_attrs would cause a
> problem. Adding it here also would make me need to special case the
> comp_unit_die there. So that's pretty confusing.
>
> Still, you are absolutely right that it is confusing here. I'll add a
> comment to sort it all out.

Right, I should have remembered about the TUs.

-cary