Re: [PATCH] Fix gdb printers for std::string

[Jonathan Wakely <jwakely@redhat.com>]

On Sat, 1 Oct 2022 at 11:43, François Dumont <frs.dumont@gmail.com> wrote:
>
> On 01/10/22 12:06, Jonathan Wakely wrote:
> > On Sat, 1 Oct 2022 at 08:20, François Dumont via Libstdc++
> > <libstdc++@gcc.gnu.org> wrote:
> >> I had forgotten to re-run tests after I removed the #define
> >> _GLIBCXX_USE_CXX11_ABI 0.
> >>
> >> The comment was misleading, it could also impact output of std::list.
> >>
> >> I am also restoring the correct std::string alias for
> >> std::__cxx11::basic_string, even if with my workaround it doesn't really
> >> matter as the one for std::basic_string will be used.
> >>
> >> I also restored the printer for std::__cxx11::string typedef. Is it
> >> intentional to keep this ?
> > Yes, I kept that intentionally. There can be programs where some
> > objects still use that typedef, if those objects were compiled with
> > GCC 8.x or older.
> >
> >>       libstdc++: Fix gdb pretty printers when dealing with std::string
> >>
> >>       Since revision 33b43b0d8cd2de722d177ef823930500948a7487 std::string
> >> and other
> >>       similar typedef are ambiguous from a gdb point of view because it
> >> matches both
> >>       std::basic_string<char> and std::__cxx11::basic_string<char>
> >> symbols. For those
> >>       typedef add a workaround to accept the substitution as long as the
> >> same regardless
> >>       of __cxx11 namespace.
> > Thanks for figuring out what was going wrong here, and how to fix it.
> >
> >
> >>       Also avoid to register printers for types in std::__cxx11::__8::
> >> namespace, there is
> >>       no such symbols.
> >>
> >>       libstdc++-v3/ChangeLog:
> >>
> >>               * libstdc++-v3/python/libstdcxx/v6/printers.py
> >> (Printer.add_version): Do not add
> >>               version namespace for __cxx11 symbols.
> >>               (add_one_template_type_printer): Likewise.
> >>               (add_one_type_printer): Likewise.
> >>               (FilteringTypePrinter._recognizer.recognize): Add a
> >> workaround for std::string & al
> >>               ambiguous typedef matching both std:: and std::__cxx11::
> >> symbols.
> >>               (register_type_printers): Refine type registration to limit
> >> false positive in
> >>               FilteringTypePrinter._recognize.recognize requiring to look
> >> for the type in gdb.
> > I don't really like this part of the change though:
>
> I'll check what you are proposing but I don't think it is necessary to
> fix the problem.

Most of my patch is an alternative way to make the filter match on
"basic_string<char", but there's also an alternative way to check for
the ambiguous string typedefs, by using match.split('::')[-1] to get
the class template name without namespaces and then compare that to
"basic_string".

> I did this on my path to find out what was going wrong. Once I
> understood it I consider that it was just a good change to keep. If you
> think otherwise I can revert this part.

Yeah it looks like it's just an optimization to fail faster without
having to do gdb.lookup_type.

Please revert the changes to register_type_printers then, and we can
consider that part later if we want to revisit it. I'm not opposed to
making that fail-fast optimization, as long as we keep the property
that FilteringTypePrinter.match is the class template name. Maybe it
should be renamed to something other than "match" to make that clear.

The rest of the patch is OK for trunk, thanks.

> I also noted that gdb consider the filters as a filo list, not fifo. And
> I think that the 1st filters registered are the most extensively used. I
> can propose to invert all the registration if you think it worth it.

I've not noticed any performance problems with the printers, but I
have wondered how many printers is too many. That's an interesting
observation about the order they're checked. I'll talk to some of the
GDB devs to find out if they think it's something we should worry
about. Let's not try make premature optimizations until we know if it
matters.

>
>
> >
> > @@ -2105,29 +2120,29 @@ def register_type_printers(obj):
> >           return
> >
> >       # Add type printers for typedefs std::string, std::wstring etc.
> > -    for ch in ('', 'w', 'u8', 'u16', 'u32'):
> > -        add_one_type_printer(obj, 'basic_string', ch + 'string')
> > -        add_one_type_printer(obj, '__cxx11::basic_string', ch + 'string')
> > +    for ch in (('char', ''), ('wchar_t', 'w'), ('char8_t', 'u8'),
> > ('char16_t', 'u16'), ('char32_t', 'u32')):
> > +        add_one_type_printer(obj, 'basic_string<' + ch[0], ch[1] + 'string')
> > +        add_one_type_printer(obj, '__cxx11::basic_string<' + ch[0],
> > ch[1] + 'string')
> >
> >
> > As the docs for FilteringTypePrinter says, the first argument is
> > supposed to be the class template name:
> >
> > class FilteringTypePrinter(object):
> >      r"""
> >      A type printer that uses typedef names for common template specializations.
> >
> >      Args:
> >          match (str): The class template to recognize.
> >          name (str): The typedef-name that will be used instead.
> >
> >      Checks if a specialization of the class template 'match' is the same type
> >      as the typedef 'name', and prints it as 'name' instead.
> >
> >      e.g. if an instantiation of std::basic_istream<C, T> is the same type as
> >      std::istream then print it as std::istream.
> >      """
> >
> > With this change, the "class template" is sometimes just a string
> > prefix of a particular specialization, e.g. "basic_string<char"
> > The class template is "basic_string", and that's how the
> > FilteringTypePrinter was intended to work.
> >
> > How about something like the attached (untested) change instead. which
> > keeps the 'match' argument as the class template name, but optionally
> > supports passing the first template argument? Then you can use
> > match.split('::')[-1] is 'basic_string' to check if we are dealing
> > with one of the possibly ambiguous typedefs.
>
>