On Thu, 22 Jul 2021 at 10:23, Stephan Bergmann via Libstdc++
<libstd...@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.
This should fix it:
--- a/libstdc++-v3/include/bits/std_function.h
+++ b/libstdc++-v3/include/bits/std_function.h
@@ -237,7 +237,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
{ __functor._M_access<_Functor*>() = new _Functor(std::move(__f)); }
};
- _Function_base() : _M_manager(nullptr) { }
+ _Function_base() = default;
~_Function_base()
{
@@ -250,8 +250,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
typedef bool (*_Manager_type)(_Any_data&, const _Any_data&,
_Manager_operation);
- _Any_data _M_functor;
- _Manager_type _M_manager;
+ _Any_data _M_functor{};
+ _Manager_type _M_manager{};
};
template<typename _Signature, typename _Functor>
@@ -634,8 +634,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
private:
using _Invoker_type = _Res (*)(const _Any_data&, _ArgTypes&&...);
- _Invoker_type _M_invoker;
- };
+ _Invoker_type _M_invoker = nullptr;
+ };
#if __cpp_deduction_guides >= 201606
template<typename>
