Re: [BUG] -Wuninitialized: initialize variable with itself

[Andrew Pinski <pinskia@gmail.com>]

On Sun, Nov 13, 2022 at 10:41 AM Andrew Pinski <pinskia@gmail.com> wrote:
>
> 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.

When I says years, it was 19 years ago even, see
https://gcc.gnu.org/PR5582
and
https://gcc.gnu.org/PR10538
and
https://gcc.gnu.org/r0-52301-g3390f9c9bef0be

Thanks,


>
> >
> > 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/>