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

[Fangrui Song via cfe-dev]

On 2020-05-20, Nico Weber wrote:
>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.

Thanks for the data point.
Created https://reviews.llvm.org/D80391 [Driver] Don't make -gsplit-dwarf imply -g2

>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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