Re: issue with GDB python pretty-printers for libstdc++

[Jonathan Wakely <jwakely dot gcc at gmail dot com>]

On Wed, 26 Feb 2020 at 17:01, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
>
> On Wed, 26 Feb 2020 at 14:32, Paul Smith <paul@mad-scientist.net> wrote:
> >
> > Hi all.  I was seeing a strange error in GDB (8.2.1) debugging some C++
> > code while trying to print a value.  The pretty printer was throwing Python
> > exceptions.
> >
> > Debugging it I discovered the problem, which is here (from GCC 9.2):
> >
> > libstdc++-v3/python/libstdcxx/v6/printers.py:
> >   # Starting with the type ORIG, search for the member type NAME.  This
> >   # handles searching upward through superclasses.  This is needed to
> >   # work around http://sourceware.org/bugzilla/show_bug.cgi?id=13615.
> >   def find_type(orig, name):
> >       typ = orig.strip_typedefs()
> >       while True:
> >           # Strip cv-qualifiers.  PR 67440.
> > -->       search = '%s::%s' % (typ.unqualified(), name)
> >           try:
> >               return gdb.lookup_type(search)
> >           except RuntimeError:
> >               pass
> >           # The type was not found, so try the superclass.  We only need
> >           # to check the first superclass, so we don't bother with
> >           # anything fancier here.
> >           field = typ.fields()[0]
> >
> > (First that GDB bug was fixed in 2012 so I'm not sure if we still need this
> > method, but anyway...)
> >
> > The issue is on the marked line above.  Here we are using the __str__()
> > method on a gdb.Type to obtain the string name of the type.  However, I've
> > discovered that (at least on my system) the __str__() representation of a
> > gdb.Type prepends the keyword "class " or "struct " (as appropriate) to the
> > output.  So the above will result in a string like:
> >
> >   search = 'class std::unordered_map...::...'
>
> I don't think I've seen that problem before. Are you sure that's the
> cause, and not something like
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91997 ?

I compiled this program:

#include <unordered_map>

int main()
{
  std::unordered_map<int, int> m;
  m[3] = 5;
  return m[1];
}

And with Fedora 31's GDB 8.3.50 I get this behaviour:

(gdb) py m = gdb.parse_and_eval('m')
(gdb) py print(m.type)
std::unordered_map<int, int, std::hash<int>, std::equal_to<int>,
std::allocator<std::pair<int const, int> > >
(gdb) py print(str(m.type))
std::unordered_map<int, int, std::hash<int>, std::equal_to<int>,
std::allocator<std::pair<int const, int> > >

There's no 'class' being added. But I also tried changing the
printers.py script to use .name and all our tests still pass, so I'm
not opposed to that change.