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

[Daniel Berlin via cfe-dev]

To explain to others who didn't follow that thread:

GDB currently does something amazingly stupid (and has since i wrote it) to
find the RTTI type.  There were no other good options at the type.

What it does is find the vtable for the object, find the symbol that
represents the vtable, demangle it, , chops off "vtable for", and tries to
find the symbol for the string that results.

If you don't emit the linkage name, there are cases it won't find it,
because this is a really dumb way of trying to find the answer :)

It also wont' find it depending on what demangler you use, etc.

Here's a more direct way:
For each vtable DIE, link to the concrete type it represents.

Now you just go from vtable object to concrete type with no string lookup,
which is faster, doesnt' require linkage names, doesn't depend on
demanglers matching, etc.

As an added bonus: This is what Tom Tromey already added to Rust to do
this. So it's even been implemented before.






On Sat, Mar 3, 2018 at 8:20 PM, Daniel Berlin <dberlin at dberlin.org> wrote:

>
>
> On Fri, Mar 2, 2018 at 3:58 PM, Roman Popov via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hi all,
>>
>> As you may know modern C++ debuggers (GDB and LLDB) support dynamic type
>> identification for polymorphic objects, by utilizing C++ RTTI.
>> Unfortunately this feature does not work with Clang and GDB >= 7.x .  The
>> last compiler that worked well was G++ 6.x
>>
>> I've asked about this issue both on GDB and LLDB maillists. Unfortunately
>> it's hard or impossible to fix it on debugger side.
>>
>
> Errr, i posited a solution on the gdb mailing list that i haven't seen
> shot down so far, that doesn't require linkage names, it only requires one
> new attribute that is a DW_FORM_ref, and very cheap.
>
> I also wrote the RTTI code for GDB :)
>
> 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?
>>
>
> Please, no.
>
> This is completely unneeded and wastes a huge amount of space.
>
> As you can see from the replies to my solution on the gdb mailing list, it
> is used by other languages (rust, for example) *anyway*, so we might as
> well use it for C++ too.
>
>
>
>
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