Re: Dubious "'foo' might be used uninitialized in this function"message

[Dmitry Antipov <antipov at dev dot rtsoft dot ru>]

Nathan Sidwell wrote:

> Dmitry Antipov wrote:
>
> > w.c: In function `f':
> > w.c:5: warning: 'z' might be used uninitialized in this function
> >
> > which is not true.
>
> 'tis true too, just here.
>    return x + y + z;

Oops. That's my mistake, both examples are incorrect :-(. It should be
(with line numbers, for referencing lines)

    1  #include <unistd.h>
    2
    3  int f (int x, int y)
    4  {
    5   int z;
    6
    7   if (x)
    8     z = getppid ();
    9   y = getpid ();
   10   if (x)
   11     y += z;
   12   return x + y;
   13  }

in the first case and

    1  #include <unistd.h>
    2
    3  int f (int x, int y)
    4  {
    5   int z;
    6
    7   if (x)
    8     z = getppid ();
    9   y = getpid ();
   10   y += z;
   11   return x + y;
   12  }

in the second, respectively.

> read the documentation for -Wuninitialized, which will explain
> why it can't get all the cases correct.

I've taken a look through 3.4.3. docs online. The docs describes
two different cases:

>           {
>             int x;
>             switch (y)
>               {
>               case 1: x = 1;
>                 break;
>               case 2: x = 4;
>                 break;
>               case 3: x = 5;
>               }
>             foo (x);
>          }
>    
> If the value of y is always 1, 2 or 3, then x is always initialized, 
but GCC doesn't know this.

This sample probably means that we have something like:

void bar (int y)
{
 int x;
 switch (y)
 {
   case 1: x = 1;
     break;
   case 2: x = 4;
     break;
   case 3: x = 5;
 }
 foo (x);
}

If 'bar()' has the global scope (can be called from another modules), it's
impossible to predict the value of 'y', and we should warn always. But, if
the bar is 'static', we can look through all calls of the 'bar()' and do
the following check for each call:

if (argument of 'bar()' is a compile-time constant) {
  if (argument of bar is not 1, 2, or 3)
     warn ();
} else
  warn ();

IMHO, this kind of check may be really useful.

The second example from documentation

> {
>  int save_y;
>  if (change_y) save_y = y, y = new_y;
>  ...
>  if (change_y) y = save_y;
> }

is similar to my sample #1 (except that this example implicitly assumes
that 'change_y' isn't changed in the middle code marked as '...').

Probably that's the case which is described with "GCC is not smart enough to
see all the reasons why the code might be correct" words in the docs :-).

> > Is it reasonable to learn GCC do more analysis in attempt to avoid
> > warning in this case ? How is it complex ?
>
>
> it gets as complex as the halting problem :)

Really ? Why ?

Dmitry