[Bug middle-end/110534] New: confusing -Wuninitialized when strict aliasing is violated

[Bug middle-end/110534] New: confusing -Wuninitialized when strict aliasing is violated

[vanyacpp at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110534

            Bug ID: 110534
           Summary: confusing -Wuninitialized when strict aliasing is
                    violated
           Product: gcc
           Version: 13.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vanyacpp at gmail dot com
  Target Milestone: ---

GCC gives -Wuninitialized on this code:

#include <cstdint>
uint16_t test()
{
    uint32_t foo32[4] = {0, 0, 0, 0};
    uint16_t* foo16 = reinterpret_cast<uint16_t*>(&foo32[0]);
    return foo16[0];
}

<source>:7:19: warning: 'foo32' is used uninitialized [-Wuninitialized]
    7 |     return foo16[0];
      |                   ^
<source>:5:14: note: 'foo32' declared here
    5 |     uint32_t foo32[4] = {0, 0, 0, 0};
      |              ^~~~~

This issue was originally published on reddit:
https://www.reddit.com/r/cpp/comments/14lc9w9/gcc_warnings_for_uninitialized_variables_is/

The poster found the warning quite confusing and I agree with them.

I believe the ideal behavior would be to show -Wstrict-aliasing on this code
and avoid showing -Wuninitialized.

[Bug middle-end/110534] confusing -Wuninitialized when strict aliasing is violated

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110534

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
There are different levels of -Wstrict-aliasing and iirc level 3 will warn.

Note I don't think the uninitialized warning is a bad thing here. Because it
does point out gcc is thinking it is uninitialized due to the alias violation.

[Bug middle-end/110534] confusing -Wuninitialized when strict aliasing is violated

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110534

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |DUPLICATE

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Oh and there is already a bug recording this, PR 99768 (still open too).

*** This bug has been marked as a duplicate of bug 99768 ***

[Bug middle-end/110534] confusing -Wuninitialized when strict aliasing is violated

[egallager at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110534

Eric Gallager <egallager at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |egallager at gcc dot gnu.org

--- Comment #3 from Eric Gallager <egallager at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #1)
> There are different levels of -Wstrict-aliasing and iirc level 3 will warn.
>

Remember that -Wstrict-aliasing's numerical levels are weird compared to other
warnings with numerical levels

> Note I don't think the uninitialized warning is a bad thing here. Because it
> does point out gcc is thinking it is uninitialized due to the alias
> violation.

The wording that gets printed is still a bit confusing, though, since the
variable that gets the first caret isn't the same one that gets named in the
first line of the warning. The note helps a bit, but I can still see how it
might be confusing to some.