[Bug middle-end/94699] New: enhance -Wstring-compare to detect tautological or mutually exclusive strcmp expressions

[Bug middle-end/94699] New: enhance -Wstring-compare to detect tautological or mutually exclusive strcmp expressions

[msebor at gcc dot gnu.org]

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).