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

[John McCall via cfe-dev]

> On Mar 3, 2018, at 11:30 PM, Daniel Berlin via cfe-dev <cfe-dev at lists.llvm.org> wrote:
> 
> 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.

Glorious. :)

Do any of the common C++ demangler implementations provide any sort of API to get at the demangler tree?  We did this in Swift, and even though our tree design isn't real great, it's been a huge help for implementing various reflection / debugging features.

John.

> 
> 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 <mailto: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 <mailto: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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