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

[David Blaikie via llvm-dev]

Is that putting linkage name on every class that has an identifier in the
IR?

I'm guessing that's not necessary/excessive - only RTTI types would need
it? Perhaps without changing Clang, LLVM could be changed to only attach
the linkage name to a type that it was emitting the virtual parts of a
class?

Also: The original example with "template<auto T>" (I don't know what that
feature is called, sorry) does present some problems that probably need to
be fixed regardless (two different instantiations having the same name's
clearly a problem, for example - and I might be convinced that the enum
case should change so it can produce a consistent type even in the face of
a forward-declared enum (with an explicit underlying type specified), for
example) - but even with those things fixed, I agree it's still not a great
idea to have debuggers need to match mangled names to pretty printed names.

On Fri, Mar 2, 2018 at 7:55 PM Roman Popov via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Here is result of experiment:
>
> (Original size , with DW_AT_linkage_name for composites, % increase)
> clang-7.0 1926574256 1952846192   1.4%
> clang-tidy 1220980360 1238498112   1.4%
> llvm-mt  7404728 7525328 1.6 %
> std::cout << "hello world!" 21552 22080  2.4 %
>
> IMO, not that big price for reliable dynamic type identification (Full
> disclosure: I need depend on this feature, since I'm writing Python pretty
> printers for GDB )
>
> -Roman
>
> 2018-03-02 18:12 GMT-08:00 Roman Popov <ripopov at gmail.com>:
>
>> Mangled names can be long indeed, but pretty-printed types are also long.
>> I can evaluate effect on size on clang codebase itself.
>>
>> If you disable RTTI, than obviously you can't use it. So if RTTI is
>> disabled, we can disable mangled names in DWARF. Clang is compiled without
>> standard C++ RTTI because it has it's own RTTI. In general, however, many
>> libraries use standard RTTI.
>>
>>
>>
>> 2018-03-02 17:43 GMT-08:00 <paul.robinson at sony.com>:
>>
>>> > Currently what debugger has to do is to demangle RTTI name and try to
>>> > match it to DW_AT_name attribute to find type. As you can see it does
>>> > not work for any of 3 examples.
>>> >
>>> > I've asked about the problem on G++ maillist, and one of the proposed
>>> > solutions is to emit DW_AT_linkage_name for types.
>>> >
>>> > Can this solution be also implemented in LLVM?
>>>
>>> It could, but mangled names can be very long and we need to consider
>>> whether the additional size cost is worth it under various conditions.
>>> For example, does this type matching work when a program is compiled
>>> with `-fno-rtti`?  (Clang itself is compiled this way by default.)
>>> Thanks,
>>> --paulr
>>>
>>>
>>
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