[Bug tree-optimization/112637] New: Bogus -Wmaybe-uninitialized warning

[Bug tree-optimization/112637] New: Bogus -Wmaybe-uninitialized warning

[dangelog at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112637

            Bug ID: 112637
           Summary: Bogus -Wmaybe-uninitialized warning
           Product: gcc
           Version: 13.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dangelog at gmail dot com
  Target Milestone: ---

Testcase from Qt: https://gcc.godbolt.org/z/6brn9Knra


    class QBenchmarkIterationController
    {
    public:
        QBenchmarkIterationController() noexcept;
        bool isDone() const noexcept;
        void next() noexcept;
    };

    class QBenchmarkIterationControllerWrapper {
        QBenchmarkIterationController controller;
        bool first = true;

    public:
        QBenchmarkIterationControllerWrapper() = default;
        bool isDone() const noexcept { return !first && controller.isDone(); }
        void next() noexcept { first = false; controller.next(); }
    };



    int f() noexcept;
    void check(bool);

    void testcase()
    {
        int result;

        for (QBenchmarkIterationControllerWrapper w; !w.isDone(); w.next())
            result = f();

        check(result == 42);
    }


With warnings enabled and any level of optimization enabled, results in:


    <source>: In function 'void testcase()':
    <source>:31:10: warning: 'result' may be used uninitialized
[-Wmaybe-uninitialized]
       31 |     check(result == 42);
          |     ~~~~~^~~~~~~~~~~~~~
    <source>:26:9: note: 'result' was declared here
       26 |     int result;
          |         ^~~~~~



But this is bogus, as the loop is always entered at least once. 

Removing the declaration of QBenchmarkIterationController's constructor makes
the warning disappear.



Upstream Qt patch discussion:
https://codereview.qt-project.org/c/qt/qtbase/+/518574

[Bug tree-optimization/112637] Bogus -Wmaybe-uninitialized warning

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112637

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2023-11-21
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
>and any level of optimization enabled,

No, it just at -O1, -Os and -Og. At -O2 and -O3 we are able to optimize the
first load of w.first out and end up with no uninitialized variable warning.

Basically it comes down to this:
```
  QBenchmarkIterationController::QBenchmarkIterationController (&w.controller);
  w.first = 1;
  goto <bb 4>; [100.00%]

....

  <bb 4> [local count: 1073741824]:
  # result_4 = PHI <result_8(D)(2), result_13(3)>
  _18 = w.first;
  if (_18 != 0)
    goto <bb 7>; [67.00%]
  else
    goto <bb 5>; [33.00%]
```
not being optimized ... I am not so sure if this is fixable, especially at -Og.
-Os might be. -O1 is in a similar situtation as -Os though. compile speed vs
runtime speed ...