[llvm-dev] DWARF: Preferred names in templates

[David Blaikie via llvm-dev]

On Wed, Aug 11, 2021 at 4:14 PM Shafik Yaghmour <syaghmour at apple.com> wrote:

>
>
> 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
>
> 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.
>

Yep, finally coming back to this - I've disabled the use of preferred names
in debug info here:
https://github.com/llvm/llvm-project/commit/2ff049b12ee3fb60581835a28bf9d0acc1723f23


>
>
> 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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