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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.
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