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

[Nico Weber via cfe-dev]

I don't have an opinion on this, but I note that we didn't expect the
current behavior in chromium and tripped over it (
https://source.chromium.org/chromium/chromium/src/+/master:build/config/compiler/BUILD.gn;l=2350?q=gsplit-dwarf%20file:%5C.gn).
So that's a data point suggesting that the current behavior is confusing at
least.

On Wed, May 13, 2020 at 6:32 PM Fangrui Song via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> -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.
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