On Thu, 22 Jul 2021 at 11:03, Jonathan Wakely wrote:
>
> On Thu, 22 Jul 2021 at 10:23, Stephan Bergmann via Libstdc++
> <libstdc++@gcc.gnu.org> wrote:
> >
> > Compared to GCC 11 (at least gcc-c++-11.1.1-3.fc34.x86_64), recent GCC
> > 12 trunk emits two "unhelpful" -Wmaybe-uninitialized for
> >
> > > $ cat test.cc
> > > #include <functional>
> > > using fn = std::function<void()>;
> > > fn f(fn x) {
> > >     fn a;
> > >     a = x;
> > >     return x;
> > > }
> >
> > > $ ~/gcc/trunk/inst/bin/g++ -c -Wmaybe-uninitialized -O2 test.cc
> > > In file included from ~/gcc/trunk/inst/include/c++/12.0.0/bits/stl_function.h:60,
> > >                  from ~/gcc/trunk/inst/include/c++/12.0.0/functional:49,
> > >                  from test.cc:1:
> > > In function ‘std::_Require<std::__not_<std::__is_tuple_like<_Tp> >, std::is_move_constructible<_Tp>, std::is_move_assignable<_Tp> > std::swap(_Tp&, _Tp&) [with _Tp = void (*)(const std::_Any_data&)]’,
> > >     inlined from ‘void std::function<_Res(_ArgTypes ...)>::swap(std::function<_Res(_ArgTypes ...)>&) [with _Res = void; _ArgTypes = {}]’ at ~/gcc/trunk/inst/include/c++/12.0.0/bits/std_function.h:529:11,
> > >     inlined from ‘std::function<_Res(_ArgTypes ...)>& std::function<_Res(_ArgTypes ...)>::operator=(const std::function<_Res(_ArgTypes ...)>&) [with _Res = void; _ArgTypes = {}]’ at ~/gcc/trunk/inst/include/c++/12.0.0/bits/std_function.h:442:20,
> > >     inlined from ‘fn f(fn)’ at test.cc:5:9:
> > > ~/gcc/trunk/inst/include/c++/12.0.0/bits/move.h:204:11: warning: ‘<unnamed>.std::function<void()>::_M_invoker’ may be used uninitialized [-Wmaybe-uninitialized]
> > >   204 |       _Tp __tmp = _GLIBCXX_MOVE(__a);
> > >       |           ^~~~~
> > > In file included from ~/gcc/trunk/inst/include/c++/12.0.0/functional:59,
> > >                  from test.cc:1:
> > > ~/gcc/trunk/inst/include/c++/12.0.0/bits/std_function.h: In function ‘fn f(fn)’:
> > > ~/gcc/trunk/inst/include/c++/12.0.0/bits/std_function.h:442:9: note: ‘<anonymous>’ declared here
> > >   442 |         function(__x).swap(*this);
> > >       |         ^~~~~~~~~~~~~
> > > In file included from ~/gcc/trunk/inst/include/c++/12.0.0/bits/stl_function.h:60,
> > >                  from ~/gcc/trunk/inst/include/c++/12.0.0/functional:49,
> > >                  from test.cc:1:
> > > In function ‘std::_Require<std::__not_<std::__is_tuple_like<_Tp> >, std::is_move_constructible<_Tp>, std::is_move_assignable<_Tp> > std::swap(_Tp&, _Tp&) [with _Tp = std::_Any_data]’,
> > >     inlined from ‘void std::function<_Res(_ArgTypes ...)>::swap(std::function<_Res(_ArgTypes ...)>&) [with _Res = void; _ArgTypes = {}]’ at ~/gcc/trunk/inst/include/c++/12.0.0/bits/std_function.h:527:11,
> > >     inlined from ‘std::function<_Res(_ArgTypes ...)>& std::function<_Res(_ArgTypes ...)>::operator=(const std::function<_Res(_ArgTypes ...)>&) [with _Res = void; _ArgTypes = {}]’ at ~/gcc/trunk/inst/include/c++/12.0.0/bits/std_function.h:442:20,
> > >     inlined from ‘fn f(fn)’ at test.cc:5:9:
> > > ~/gcc/trunk/inst/include/c++/12.0.0/bits/move.h:204:11: warning: ‘*(std::_Any_data*)((char*)&<unnamed> + offsetof(std::function, std::function<void()>::<unnamed>))’ may be used uninitialized [-Wmaybe-uninitialized]
> > >   204 |       _Tp __tmp = _GLIBCXX_MOVE(__a);
> > >       |           ^~~~~
> > > In file included from ~/gcc/trunk/inst/include/c++/12.0.0/functional:59,
> > >                  from test.cc:1:
> > > ~/gcc/trunk/inst/include/c++/12.0.0/bits/std_function.h: In function ‘fn f(fn)’:
> > > ~/gcc/trunk/inst/include/c++/12.0.0/bits/std_function.h:442:9: note: ‘<anonymous>’ declared here
> > >   442 |         function(__x).swap(*this);
> > >       |         ^~~~~~~~~~~~~
> >
> > This appears to be an issue with more aggressive -Wmaybe-uninitialized
> > in GCC 12 vs. 11, rather than an issue with changes to <functional> in
> > libstdc++ 12 vs. 11, as effectively the same warnings are emitted when I
> > use GCC 12 with libstdc++ 11 with
> >
> > > $ ~/gcc/trunk/inst/bin/g++ -c -Wmaybe-uninitialized -O2 -nostdinc++ -isystem /usr/include/c++/11 -isystem /usr/include/c++/11/x86_64-redhat-linux test.cc
> >
> > The warnings may technically be correct, and I'm not sure whether this
> > is something that should be addressed in the GCC code emitting the
> > warnings or in the libstdc++ <functional> implementation.
> >
> > (I found this when building LibreOffice with recent GCC 12 trunk.)
>
> The problem is that the _Function_base default constructor is
> user-provided, so when std::function value-initializes its base class,
> that doesn't do zero-init first. It just calls the default ctor, which
> doesn't initialize the _M_fucntor member.

I've pushed the attached patch to trunk, after testing on powerpc64le-linux.