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

[Daniel Berlin via cfe-dev]

On Tue, Mar 6, 2018 at 9:48 AM, David Blaikie <dblaikie at gmail.com> wrote:

>
>
> On Tue, Mar 6, 2018 at 9:36 AM Daniel Berlin <dberlin at dberlin.org> 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".
>>>
>>>
>>>>
>>>>
>>>>> 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.
>>>
>>
>>
>> If you want an example, gdb's parser understands that Foo<unsigned int>
>> and Foo<unsigned> are the same because it parses them properly.
>> It does not understand that Foo<2> and Foo<2u> are the same because it
>> parses them incorrectly.
>>
>
> Ah, but they aren't necessarily the same type. Clang (& GCC?) are
> producing two different types but naming them both Foo<2>. That's the
> ambiguity Roman's referring to.
>
> That's a bug in Clang (& GCC?) that ought to be fixed.
>

Agreed.
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