[llvm-dev] Emiting linkage names for Types to Debuginfo (C++ RTTI support in GDB/LLDB)

[Daniel Berlin via llvm-dev]

On Tue, Mar 6, 2018 at 9:46 AM, David Blaikie <dblaikie at gmail.com> wrote:

>
>
> On Tue, Mar 6, 2018 at 9:28 AM Daniel Berlin <dberlin at dberlin.org> wrote:
>
>> On Tue, Mar 6, 2018 at 9:22 AM, David Blaikie <dblaikie at gmail.com> wrote:
>>
>>>
>>>
>>> On Tue, Mar 6, 2018 at 8:39 AM Daniel Berlin <dberlin at dberlin.org>
>>> wrote:
>>>
>>>> On Mon, Mar 5, 2018 at 11:55 PM, Roman Popov <ripopov at gmail.com> wrote:
>>>>
>>>>> I don't understand how extra vtable ref DIE will help in case on
>>>>> non-polymorphic classes. If you remove virtual destructor from example,
>>>>> vtable won't be generated for class, but DWARF will still have incorrect
>>>>> ambiguous names for types.
>>>>>
>>>> 1. Calling them incorrect is ... not right.  As Andrew quoted on the
>>>> gdb mailing list, this is what DWARF specifies should happen,
>>>>
>>>
>>> Might be helpful to point to/include any details cited here for the
>>> purpose of this conversation - a bit hard for the rest of us to follow
>>> along.
>>>
>>>
>>
>> "
>> Reading http://wiki.dwarfstd.org/index.php?title=Best_Practices:
>> the DW_AT_name attribute should contain the name of the corresponding
>> program object as it appears in the source code, without any
>> qualifiers such as namespaces, containing classes, or modules (see
>> Section 2.15). A consumer can easily reconstruct the fully-qualified
>> name from the DIE hierarchy. In general, the value of DW_AT_name
>> should be such that a fully-qualified name constructed from the
>> DW_AT_name attributes of the object and its containing objects will
>> uniquely represent that object in a form natural to the source
>> language."
>>
>>
>>
>>> so they are correct by spec. If you believe the spec is wrong, file an
>>>> issue on the DWARF web site and discuss it on the mailing list, and bring
>>>> back the consensus of the committee as to what to do :)
>>>>
>>>
>>> The ambiguous names are probably incorrect - having two distinct types
>>> that have the same name's not really going to work out well for a consumer.
>>> (so having the distinct types foo<11u> and foo<11> in source both produce a
>>> DWARF type named "foo<11>" I'd say is a bug that ought to be fixed - as is
>>> any other case where the names become ambiguous, otherwise matching up
>>> types between TUs would become impossible, which would be not good)
>>>
>>
>> I'm sure the spec needs to be updated, i'm just saying "it's not wrong by
>> what the spec and best practices say to do right now".
>>
>
> Looks wrong to me. It doesn't "uniquely represent" the object nor is it
> natural to the source language (foo<11> gets you the signed one, you'd have
> to write foo<11u> or foo<(unsigned)11> to get the unsigned one - yet
> Clang's DWARF currently names them both foo<11>).
>
>

Yes, this is the gdb/gcc bug i cited, and ther eis probably a clang bug
somewhere too.



>
>>
>>>
>>>
>>>> 2. The failure that was cited on the gdb mailing list only occurs on
>>>> polymorphic classes.   If you have it occurring on non-polymorphic classes,
>>>> that seems like a very different problem, and probably related to the fact
>>>> that GDB does not know how to assemble or parse C++ names properly in some
>>>> cases.  Otherwise, this would occur on literally every class you saw in
>>>> GDB, and that's definitely not the case:)
>>>>
>>>
>>> Sounds like Roman's talking about other use cases apart from GDB.
>>>
>>
>> Yes.
>>
>>
>>>
>>>
>>>> The only reason linkage names would fix that issue is because they
>>>> provide an exact match to GDB's parsing failure.
>>>>
>>>
>>> Not sure I follow this - providing linkage names would provide a
>>> reliable mechanism to match the vtable symbol. There wouldn't need to be
>>> any parsing, or any failure of parsing involved.
>>>
>>> But, yes, addresses would be potentially a better description rather
>>> than having to match names in the object's symbol table.
>>>
>>
>> I'm saying the only reason it would fix non-polymorphic classes is if gdb
>> is failing to parse names so that it can do die lookup properly.
>>
>> GDB gives up in some cases and incorrectly says "lookup foo::bar::fred in
>> the global symbol namespace"  instead of "lookup fred inside class bar
>> symbol namespace".
>>
>> In those cases, the linkage name would fix it because it will appear in
>> the global symbol namespace.
>> But it would also work if you just fixed the name parsing.
>>
>
> Can't say I'm following this part.. well, sort of following. But doesn't
> seem relevant to Roman's situation, which isn't about GDB.
>

He did in fact claim non-polymorphic gdb lookup would fail (it won't), then
later talked about different use cases .
I'm just pointing out it will not.




>
> I think the only problem being addressed for GDB is the polymorphic case.
> The ability to match non-polymorphic types (with what, I'm not sure - not
> vtables in any case) is motivated by Roman's other examples of IR, etc, not
> GDB's dynamic type discovery.
>
>
Sure.
In those cases, there are a host of other problems
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