[Bug c++/116742] New: -Wuninitialized failed to issue uninitialized variable

[Bug c++/116742] New: -Wuninitialized failed to issue uninitialized variable

[xiaohuba2021 at 163 dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116742

            Bug ID: 116742
           Summary: -Wuninitialized failed to issue uninitialized variable
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: xiaohuba2021 at 163 dot com
  Target Milestone: ---

The following code WON'T generate any warning with -O2 -Wuninitialized
-Wmaybe-uninitialized:

```
#include <iostream>
using namespace std;

signed main()
{
        ios_base::sync_with_stdio(false);
        cin.tie(0);

        int n, s = n; // n is uninitialized

        cin >> n;
        cout << s << endl;

        return 0;
}

```

Turning on -Wall -Wextra does not help.

However, with `ios_base::sync_with_stdio(false)` and `cin.tie(0)` commented,
the compiler will notice that n is uninitialized.

g++ version:
```
Using built-in specs.
COLLECT_GCC=/opt/compiler-explorer/gcc-snapshot/bin/g++
Target: x86_64-linux-gnu
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 15.0.0 20240916 (experimental)
(Compiler-Explorer-Build-gcc-4f2cd256aace732bff511c59f5871456a564a3a5-binutils-2.42) 
```


compiler explorer: https://godbolt.org/z/dWbcjq7vn

[Bug tree-optimization/116742] -Wuninitialized failed to issue uninitialized variable

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116742

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Blocks|                            |24639
          Component|c++                         |tree-optimization
           Keywords|                            |diagnostic

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
There might be a dup of this one already.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=24639
[Bug 24639] [meta-bug] bug to track all Wuninitialized issues

[Bug tree-optimization/116742] -Wuninitialized failed to issue uninitialized variable

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116742

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2024-09-17
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
The issue is that 'n' is address taken and escapes:

  <bb 2> :
  std::ios_base::sync_with_stdio (0);

  <bb 3> :
  std::basic_ios<char>::tie (&cin.D.48191, 0B);
  s_6 = n;
  std::basic_istream<char>::operator>> (&cin, &n);

  <bb 4> :
  _9 = std::basic_ostream<char>::operator<< (&cout, s_6);

we thus consider that for example std::ios_base::sync_with_stdio might
initialize 'n' since we lose the fact that 'n' only becomes live after that
call.  The C++ frontend does not add explicit scopes for

  int n, s = n;

so it doesn't get CLOBBER treatment.  Adding scopes doesn't help either
(the gimplifier doesn't add CLOBBERs for start-of-live).

[Bug tree-optimization/116742] -Wuninitialized failed to issue uninitialized variable

[xiaohuba2021 at 163 dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116742

--- Comment #3 from xiaohuba2021 <xiaohuba2021 at 163 dot com> ---
So is there any workaround or patch?