https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94699
Bug ID: 94699
Summary: enhance -Wstring-compare to detect tautological or
mutually exclusive strcmp expressions
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: msebor at gcc dot gnu.org
Target Milestone: ---
The article referenced in pr94629
(https://habr.com/en/company/pvs-studio/blog/497640/) points out the following
suspicious-looking code in GCC:
static bool
dw_val_equal_p (dw_val_node *a, dw_val_node *b)
{
....
case dw_val_class_vms_delta:
return (!strcmp (a->v.val_vms_delta.lbl1, b->v.val_vms_delta.lbl1)
&& !strcmp (a->v.val_vms_delta.lbl1, b->v.val_vms_delta.lbl1));
....
}
and goes on to say:
PVS-Studio warning: V501 There are identical sub-expressions
'!strcmp(a->v.val_vms_delta.lbl1, b->v.val_vms_delta.lbl1)' to the left and to
the right of the '&&' operator. dwarf2out.c 1481
Two strcmp functions compare the same pointers. That is, a clearly redundant
check is performed.
The -Wstring-compare warning introduced in GCC 10 is designed to detect likely
erroneous uses of strcmp and strncmp that evaluate to a constant result due to
the sizes of the pointed-to arrays. Detecting also this pattern seems like a
natural and useful extension. (The warning is implemented in the strlen pass
while these issues might perhaps be better diagnosed earlier.)
A simple test case:
int f0 (const char *a, const char *b)
{
// second strcmp call is redundant and can be eliminated; should
// be diagnosed by -Wstring-compare (ditto with ||)
return 0 == __builtin_strcmp (a, b) && 0 == __builtin_strcmp (b, a);
}
int f1 (const char *a)
{
// results of calls are mutually exclusive and the whole expression
// can be folded to zero; should be diagnosed by -Wstring-compare
return 0 == __builtin_strcmp (a, "123") && 0 == __builtin_strcmp (a, "456");
}
Another similar warning designed to catch some of these likely bugs is
-Wtautological-compare (see also pr70181).
