------- Comment #2 from rguenth at gcc dot gnu dot org 2010-03-15 11:27 -------
(In reply to comment #1)
> In GCC 4.3.x you need optimization enabled for Wuninitialized.
>
> In GCC 4.4.1 and GCC 4.5 (trunk) I get warnings in line 6 with -O1 -O2 and
> -O3.
>
> We do not get a warning with -O0 because the default definition appears in a
> PHI node:
>
> <bb 2>:
> [pr43361.C : 6 : 3] goto <bb 4>;
>
> <bb 3>:
> [pr43361.C : 7:19] std::basic_ostream<char>::operator<< ([pr43361.C : 7]
> &cout, i_1);
> [pr43361.C : 8:18] [pr43361.C : 8] array[i_1] = i_1;
> [pr43361.C : 6:3] i_4 = i_1 + 1;
>
> <bb 4>:
> # i_1 = PHI <i_2(D)(2), [pr43361.C : 6:3] i_4(3)>
> [pr43361.C : 6:3] D.21125_3 = i_1 <= 9;
> [pr43361.C : 6:3] if (D.21125_3 != 0)
> goto <bb 3>;
> else
> goto <bb 5>;
>
> <bb 5>:
> [pr43361.C : 0:0] D.21127_5 = 0;
> return D.21127_5;
>
>
> This can be seen as a natural limitation of the analysis at -O0 or a problem
> with the way GCC represents loops:
>
> for (init; test; next) { for-body }; is transformed to:
>
> init:
> init;
> goto eval
> body:
> for-body;
> next;
> goto eval;
> eval:
> (test) ? goto body : goto finish;
> finish:
>
> So, although we know what "test" is evaluated at least once after init, GCC
> doesn't know that with -O0.
>
> In any case, there is no trivial fix (but there is a workaround: use
> optimization).
The trivial fix would be to compute post-dominator info and check if the
edge with the uninitialized use is executed on all paths from function
entry to exit (its source and destination post-dominate the entry bb).
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43361
