[cfe-dev] -gsplit-dwarf implies -g

[Fangrui Song via cfe-dev]

-gsplit-dwarf takes part in the computation of amount of debugging information
(clang::codegenoptions::DebugInfoKind).

// https://github.com/llvm/llvm-project/blob/master/clang/lib/Driver/ToolChains/Clang.cpp#L3700
   if (const Arg *A =
           Args.getLastArg(options::OPT_g_Group, options::OPT_gsplit_dwarf,
                           options::OPT_gsplit_dwarf_EQ)) {
     DebugInfoKind = codegenoptions::LimitedDebugInfo;

     // If the last option explicitly specified a debug-info level, use it.
     if (checkDebugInfoOption(A, Args, D, TC) &&
         A->getOption().matches(options::OPT_gN_Group)) {
       DebugInfoKind = DebugLevelToInfoKind(*A);
       // For -g0 or -gline-tables-only, drop -gsplit-dwarf. This gets a bit more
       // complicated if you've disabled inline info in the skeleton CUs
       // (SplitDWARFInlining) - then there's value in composing split-dwarf and
       // line-tables-only, so let those compose naturally in that case.
       if (DebugInfoKind == codegenoptions::NoDebugInfo ||
           DebugInfoKind == codegenoptions::DebugDirectivesOnly ||
           (DebugInfoKind == codegenoptions::DebugLineTablesOnly &&
            SplitDWARFInlining))
         DwarfFission = DwarfFissionKind::None;
     }
   }

This order dependency with other g_Group options (-g0, -g1, -g2, -ggdb3, -gdwarf-5, etc)
makes it somewhat inconvenient to use in a build system:

* -g0 -gsplit-dwarf -> level 2
   -gsplit-dwarf "upgrades" the amount of debugging information despite the previous intention (-g0) to drop debugging information
* -g1 -gsplit-dwarf -> level 2
   -gsplit-dwarf "upgrades" the amount of debugging information.

I guess using "-g0 -gsplit-dwarf" as a whole might be able to get rid of some
order dependency in clang but it will not work greatly in gcc which has another
level: -g3.

What do people think we should do to make -gsplit-dwarf less confusing?

Add another -f flag (-fsplit-dwarf? -fdebug-*?)
Update -gsplit-dwarf to not imply -g? (If we coordinate well with GCC people, I think this is still doable
https://gcc.gnu.org/pipermail/gcc-patches/2020-May/545646.html )

There is a whole group (g_flags_Group) of -g options which do not takes part in the computation of amount of debugging information
(-gz, -grecord-command-line, -gstrict-dwarf, etc).

Honestly I would hope -gdwarf-5 did not affect DebugInfoKind (we would not need -fdebug-default-version=5)
but the -gdwarf- ship has sailed.