[llvm-dev] Some questions about lld with gdb-index option

[Sterling Augustine via llvm-dev]

Worth noting that both gnu gold and gnu ld parse the debug info and
generate entries for missing files. It is slower than one might want, but
with --gdb-index on the command line, it seems like one would be willing to
pay that cost.

On Sun, Feb 28, 2021 at 12:10 PM David Blaikie <dblaikie at gmail.com> wrote:

> On Sat, Feb 27, 2021 at 6:20 PM Fangrui Song <maskray at google.com> wrote:
>
>> On 2021-02-27, Sterling Augustine wrote:
>> >It's very common for some input files to have gnu-pubnames, and some not.
>> >So the warning would have to be somewhat smart.
>>
>> I agree with Sterling that this can be very common:
>>
>> * system libraries compiled with -g but not -ggnu-pubnames
>> * third-party libraries compiled with -g but not -ggnu-pubnames
>>
>
> Sure, but the resultant broken gdb behavior is pretty problematic to
> experience without some notification.
>
> If gdb-index had a way to say which CUs it covers (.debug_names has this,
> I believe), that'd be great - but without that, I think it's fundamentally
> invalid/incorrect to produce an incomplete gdb-index.
>
>
>> >On Sat, Feb 27, 2021 at 2:48 PM David Blaikie via llvm-dev <
>> >llvm-dev at lists.llvm.org> wrote:
>> >
>> >> I think the gdb-index support in lld only works with
>> debug_gnu_pubnames in
>> >> the input files - so you would need to compile with -ggnu-pubnames.
>> >>
>> >> Maybe lld could/should have a warning or something if the input files
>> have
>> >> debug_* sections but don't have debug_gnu_pubnames when
>> -Wl,--gdb-index is
>> >> specified.
>>
>> The issue is https://bugs.llvm.org/show_bug.cgi?id=34820
>> We were hesistant on whether ld.lld should learn more DWARF stuff.
>>
>
> I think it should be: either create a complete gdb-index, or error.
> Creating an incomplete gdb-index is/should be considered a fairly severe
> bug.
>
>
>> The problem is not that serious because it (`p &something` => $address,
>> no debug
>> info`) only affects objfiles which haven't been parsed by gdb (something
>> like a
>> lazy mode in gdb).  Once an objfile is loaded, everything works fine.
>>
>
> I'd say this is pretty serious - unreliable operations lead to lack of
> confidence/pretty deep frustration with tools.
>
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