[llvm-dev] DWARF: Preferred names in templates

[Shafik Yaghmour via llvm-dev]

> On Aug 8, 2021, at 3:40 PM, David Blaikie via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Context: https://reviews.llvm.org/D91311 <https://reviews.llvm.org/D91311>
> 
> So, this preferred name feature is designed to print names in a more user-centric way (eg: "std::vector<std::string, ...>" instead of "std::vector<std::basic_string<char, ...>, ...>".
> 
> But this causes some divergence in the DWARF - the textual string says std::string, but the DW_TAG_template_type_parameter says std::basic_string<char... 
> 
> This isn't fundamentally problematic, kind of - there's a bunch of ways the full string name of a template won't match perfectly between producers and so consumers basically have to do some structural equivalence testing so far as I know. Though I'm not sure exactly how much - they could do it by normalizing the string (with GCC and LLVM's default debug info don't include structural descriptions of template parameters on template declarations - so consumers would have to do string normalization, rather than discarding the string argument representation and relying solely on the structural representation) in which case only a very advanced normalization that parsed std::string, did a lookup, resolved through typedefs and alias templates and then used the resulting string would succeed here. I haven't tested gdb or lldb to see if/how they cope with this situation - but I would assume it's not good.
> 
> So I think the only good solution here is to suppress use of preferred names when printing type names for debug info?

I agree that it seems like the solution is to not use preferred names for debug info.

David and I chatted offline and he was able to come up with a scenario that simulates the mixed debug info case where one compiler support preferred name and the other does not and indeed LLDB has problems in this case. From what I can tell this is because we are using the DW_AT_name from the parent, we don't attempt to reconstruct the template parameters from the children's DW_TAG_template_type_parameter.

Besides the fact that LLDB does not handle the mixed case well, it just seems more desirable to have consistent naming.

> 
> It might be nice to have use of preferred names (& maybe take it further - I have a prototype patch - and use the preferred names/types in the structural representation as well (which presumably would break mixed clang/gcc debug info with most consumers, I'd imagine - maybe it'd fall out OK for lldb when building ASTs)) under a flag? If you're building the codebase with one compiler and/or you just want to do more experimentation with the feature? Not sure it's worth it, but I think I have some reasonable attempt at this... (there's one issue around cases of template declarations not carrying preferred names - discussed on the review itself)
> 
> Thoughts, feelings, perspectives?
> 
> - Dave
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