[Bug fortran/94769] Possible use of uninitialized variable num

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94769

--- Comment #9 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Stefan Schulze Frielinghaus
<stefansf@gcc.gnu.org>:

https://gcc.gnu.org/g:27594524d8a93cddb197ad8c9d4075c5870f1473

commit r10-8053-g27594524d8a93cddb197ad8c9d4075c5870f1473
Author: Stefan Schulze Frielinghaus <stefansf@linux.ibm.com>
Date:   Tue Apr 28 13:14:28 2020 +0200

    fortran/io.c: Fix use of uninitialized variable num [PR94769]

    While bootstrapping GCC on S/390 the following warning occurs:

    gcc/fortran/io.c: In function 'bool gfc_resolve_dt(gfc_code*, gfc_dt*,
locus*)':
    gcc/fortran/io.c:3857:7: error: 'num' may be used uninitialized in this
function [-Werror=maybe-uninitialized]
     3857 |       if (num == 0)
          |       ^~
    gcc/fortran/io.c:3843:11: note: 'num' was declared here
     3843 |       int num;

    Since gfc_resolve_dt is a non-static function we cannot assume anything
about
    argument DT.  Argument DT gets passed to function check_io_constraints
which
    passes values depending on DT, namely
dt->asynchronous->value.character.string
    to function compare_to_allowed_values as well as argument warn which is
true as
    soon as DT->dterr is true.  Thus both arguments depend on DT.

    If function compare_to_allowed_values is called with
    dt->asynchronous->value.character.string not being an allowed value, and
    ALLOWED_F2003 as well as ALLOWED_GNU being NULL (which is the case at the
    particular call side), and WARN equals true, then the function returns with
a
    non-zero value and leaves num uninitialized which renders the warning true.

    Initialized num to -1 and added an assert statement.

    gcc/fortran/ChangeLog:

    2020-04-29  Stefan Schulze Frielinghaus  <stefansf@linux.ibm.com>

            PR fortran/94769
            * io.c (check_io_constraints): Initialize local variable num to
            -1 and assert that it receives a meaningful value by function
            compare_to_allowed_values.