http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60165
--- Comment #9 from Marc Glisse <glisse at gcc dot gnu.org> ---
(In reply to Vincent Lefèvre from comment #8)
> The -Wmaybe-uninitialized rule given in the gcc man page is (for GCC 4.8):
>
> -Wmaybe-uninitialized
> For an automatic variable, if there exists a path from the function
> entry to a use of the variable that is initialized, but there exist
> some other paths for which the variable is not initialized, the
> compiler emits a warning if it cannot prove the uninitialized paths
> are not executed at run time. [...]
>
> This rule does *not* depend on the optimization level, except for the "if it
> cannot prove the uninitialized paths are not executed at run time" part.
> Indeed, the fact that a path exists or not is not something that depends on
> the optimizations.
The definition of a function changes with inlining ;-)
> Concerning the "if it cannot prove the uninitialized paths are not executed
> at run time" part, GCC should be able to prove more things with -O3 than
> with -O2, meaning that -Wmaybe-uninitialized warnings could disappear with
> -O3 compared to -O2, but generally not the opposite.
>
> In the original example, GCC cannot prove anything about run time, so that
> according to the gcc man page, one should get a warning whatever the
> optimization level.
f(&i) is considered as an initialization of i. This heuristic is necessary,
otherwise the number of false positives would make the warning useless.
> I've another example (similar to the MPFR code):
>
> int fn3 (void);
> void fn2 (int *p)
> {
> if (fn3 ())
> *p = 0;
> }
> int fn1 (void)
> {
> int c;
> fn2 (&c);
> return c;
> }
>
> GCC doesn't give any "may be used uninitialized" warning (whether -O2 or -O3
> is used). This is incorrect according to the above rule. BTW, for -O3, this
> is surprising because it is similar to the original example.
A different optimization pass (CCP) seems to notice that the value returned is
either 0 or something uninitialized and thus must be 0. Maybe there is an
opportunity for a warning there (though again we need to make sure it won't
cause too many false positives).
I don't know why you think warnings should be so well defined. They have always
been a heuristic compromise.
