[Bug tree-optimization/106922] New: [12 Regression] Bogus uninitialized warning on boost::optional<<std::vector<std::string>>>

[jan.zizka at nokia dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106922

            Bug ID: 106922
           Summary: [12 Regression] Bogus uninitialized warning on
                    boost::optional<<std::vector<std::string>>>
           Product: gcc
           Version: 12.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jan.zizka at nokia dot com
  Target Milestone: ---

Created attachment 53568
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=53568&action=edit
Reproducer for maybe uninitialized warning with gcc 12

In our production code when upgrading to gcc 12 from gcc 11 we have faced bogus
warning
"may be used uninitialized" on bit complex use of boost::optional vector of
strings used
in a loop with unrelated string stream redirect.

> In file included from <..>/include/c++/12.0.0/vector:67,
>                  from reproduce.cpp:13:
> In destructor ‘std::vector<_Tp, _Alloc>::~vector() [with _Tp = std::__cxx11::basic_string<char>; _Alloc = std::allocator<std::__cxx11::basic_string<char> >]’,
>     inlined from ‘void boost::optional_detail::optional_base<T>::destroy_impl() [with T = std::vector<std::__cxx11::basic_string<char> >]’ at /usr/include/boost/optional/optional.hpp:771:50,
>     inlined from ‘void boost::optional_detail::optional_base<T>::destroy() [with T = std::vector<std::__cxx11::basic_string<char> >]’ at /usr/include/boost/optional/optional.hpp:757:21,
>     inlined from ‘void boost::optional_detail::optional_base<T>::assign(const boost::optional_detail::optional_base<T>&) [with T = std::vector<std::__cxx11::basic_string<char> >]’ at /usr/include/boost/optional/optional.hpp:264:21,
>     inlined from ‘boost::optional_detail::optional_base<T>& boost::optional_detail::optional_base<T>::operator=(const boost::optional_detail::optional_base<T>&) [with T = std::vector<std::__cxx11::basic_string<char> >]’ at /usr/include/boost/optional/optional.hpp:241:19,
>     inlined from ‘boost::optional<T>& boost::optional<T>::operator=(const boost::optional<T>&) [with T = std::vector<std::__cxx11::basic_string<char> >]’ at /usr/include/boost/optional/optional.hpp:1040:15, 
>     inlined from ‘void test()’ at reproduce.cpp:55:13:
> <..>/include/c++/12.0.0/bits/stl_vector.h:680:22: error: ‘*(std::vector<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > >*)((char*)&external + offsetof(boost::Optional<std::vector<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > > >,boost::optional<std::vector<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > > >::<unnamed>.boost::optional_detail::optional_base<std::vector<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > > >::m_storage.boost::optional_detail::aligned_storage<std::vector<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > > >::dummy_)).std::vector<std::__cxx11::basic_string<char> >::<anonymous>.std::_Vector_base<std::__cxx11::basic_string<char>, std::allocator<std::__cxx11::basic_string<char> > >::_M_impl.std::_Vector_base<std::__cxx11::basic_string<char>, std::allocator<std::__cxx11::basic_string<char> > >::_Vector_impl::<anonymous>.std::_Vector_base<std::__cxx11::basic_string<char>, std::allocator<std::__cxx11::basic_string<char> > >::_Vector_impl_data::_M_finish’ may be used uninitialized [-Werror=maybe-uninitialized]
>   680 |         std::_Destroy(this->_M_impl._M_start, this->_M_impl._M_finish,
>       |         ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>   681 |                       _M_get_Tp_allocator());
>       |                       ~~~~~~~~~~~~~~~~~~~~~~
> reproduce.cpp: In function ‘void test()’:
> reproduce.cpp:45:40: note: ‘external’ declared here
>    45 |     Optional<std::vector<std::string>> external;
>       |                                        ^~~~~~~~

I have tested this with gcc 12 (12.1.1, 12.2.1 and release/gcc-12 commit
https://gcc.gnu.org/git/?p=gcc.git&a=commit;h=4ce316ca54c863cf0fd4257ba0ab768ab83c62e5") 
with boost 1.69.0, 1.75.0, 1.76.0 and 1.80.0. All fails.

With any boost version and gcc 11 (11.3.1 and release/gcc-11 tip commit
https://gcc.gnu.org/git/?p=gcc.git&a=commit;h=7e356c3083c79473c941bc92d61f755e923bc86c)
the warning doesn't appear.

To reproduce:

> $ g++ -Werror -std=c++20 -O2 -Wall -o reproduce.o -c reproduce.cpp

I was not able to bump the reproducer to simpliear case. In all modification
where loop
is removed the warning disappears.

The problem doesn't occur when std::optional is used.

Only workaround I found is to add following pragma at top of the module:

> #pragma GCC diagnostic push
> #pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
> #include <vector>
> #pragma GCC diagnostic pop

I have bisected the problem to start with 
https://gcc.gnu.org/git/?p=gcc.git&a=commit;h=5b8b1522e04adc20980f396571be1929a32d148a

commit 5b8b1522e04adc20980f396571be1929a32d148a (HEAD, refs/bisect/bad)
Author: Richard Biener <rguenther@suse.de>
Date:   Mon Sep 27 12:01:38 2021 +0200

    tree-optimization/100112 - VN last_vuse and redundant store elimination

    This avoids the last_vuse optimization hindering redundant store
    elimination by always also recording the original VUSE that was
    in effect on the load.

    In stage3 gcc/*.o we have 3182752 times recorded a single
    entry and 903409 times two entries (that's ~20% overhead).

    With just recording a single entry the number of hashtable lookups
    done when walking the vuse->vdef links to find an earlier access
    is 28961618.  When recording the second entry this makes us find
    that earlier for donwnstream redundant accesses, reducing the number
    of hashtable lookups to 25401052 (that's a ~10% reduction).

    2021-09-27  Richard Biener  <rguenther@suse.de>

            PR tree-optimization/100112
            * tree-ssa-sccvn.c (visit_reference_op_load): Record the
            referece into the hashtable twice in case last_vuse is
            different from the original vuse on the stmt.

            * gcc.dg/tree-ssa/ssa-fre-95.c: New testcase.