[BUG] -Wuninitialized: initialize variable with itself

[Andrew Pinski]

On Sun, Nov 13, 2022 at 10:40 AM Andrew Pinski <pinskia@gmail.com> wrote:
>
> On Sun, Nov 13, 2022 at 10:36 AM Alejandro Colomar via Gcc
> <gcc@gcc.gnu.org> wrote:
> >
> > Hi,
> >
> > While discussing some idea for a new feature, I tested the following example
> > program:
> >
> >
> >      int main(void)
> >      {
> >          int i = i;
> >          return i;
> >      }
>
> This is NOT a bug but a documented way of having the warning not being there.
> See https://gcc.gnu.org/onlinedocs/gcc-12.2.0/gcc/Warning-Options.html#index-Winit-self
> https://gcc.gnu.org/onlinedocs/gcc-12.2.0/gcc/Warning-Options.html#index-Wuninitialized
> "If you want to warn about code that uses the uninitialized value of
> the variable in its own initializer, use the -Winit-self option."

I should note the main reason why I Know about this is because I fixed
this feature years ago (at least for C front-end)
and added the option to disable the feature.

>
> Thanks,
> Andrew Pinski
>
> >
> >
> > It seems obvious that it should give a warning, and in Clang it does:
> >
> >
> >      $ clang --version | head -n1
> >      Debian clang version 14.0.6
> >
> >      $ clang -Wall -Wextra foo.c
> >      foo.c:3:10: warning: variable 'i' is uninitialized when used within its own
> > initialization [-Wuninitialized]
> >              int i = i;
> >                  ~   ^
> >      1 warning generated.
> >
> >
> > But for GCC it looks fine:
> >
> >      $ gcc --version | head -n1
> >      gcc (Debian 12.2.0-9) 12.2.0
> >
> >      $ gcc -Wall -Wextra foo.c
> >      $
> >
> >
> > Until you enable the analyzer, which catches the uninitialized use:
> >
> >
> >      $ gcc -fanalyzer foo.c
> >      foo.c: In function ‘main’:
> >      foo.c:3:13: warning: use of uninitialized value ‘i’ [CWE-457]
> > [-Wanalyzer-use-of-uninitialized-value]
> >          3 |         int i = i;
> >            |             ^
> >        ‘main’: events 1-2
> >          |
> >          |    3 |         int i = i;
> >          |      |             ^
> >          |      |             |
> >          |      |             (1) region created on stack here
> >          |      |             (2) use of uninitialized value ‘i’ here
> >          |
> >
> >
> >
> > I expect that GCC should be able to detect this bug with a simple warning.  The
> > analyzer is quite unreadable compared to normal warnings.
> >
> > Cheers,
> > Alex
> >
> > --
> > <http://www.alejandro-colomar.es/>