[Bug c/16302] New: gcc fails to warn about some common logic errors

["trt at acm dot org" <gcc-bugzilla at gcc dot gnu dot org>]

A "gcc -Wextra -O" misses the 4 bugs in this test program:

int
main (int argc, char *argv[])
{
   if (argc != 1 || argc != 2) return 1;
   if (argc < 0 && argc > 10) return 1;
   if (argc || !argc) return 1;
   if (argc && !argc) return 1;
   return 0;
}

With a tweak to fold-const.c (below), they will be caught:

t.c:4: warning: `or' of collectively exhaustive tests is always 1
t.c:5: warning: `and' of mutually exclusive tests is always 0
t.c:6: warning: `or' of collectively exhaustive tests is always 1
t.c:7: warning: `and' of mutually exclusive tests is always 0

A bootstrap of gcc-3.5-20040627 reports such a warning in fold-const.c itself,
the patch below includes the obvious fix.

====
Comment: The -O is needed due to this code in fold-const.c:

      /* We only do these simplifications if we are optimizing.  */
      if (!optimize)
        return t;

This is probably from when fold-const.c was much simpler,
but I think is no longer appropriate and recommend it be deleted.
====

*** fold-const.c.orig   Sun Jun 27 11:23:44 2004
--- fold-const.c        Wed Jun 30 12:43:39 2004
***************
*** 4377,4379 ****
                                         in_p, low, high))))
!     return or_op ? invert_truthvalue (tem) : tem;
   
--- 4377,4388 ----
                                         in_p, low, high))))
!     {
!       if (TREE_CODE (tem) == INTEGER_CST && extra_warnings)
!         {
!           if (or_op)
!             warning ("`or' of collectively exhaustive tests is always 1");
!           else
!             warning ("`and' of mutually exclusive tests is always 0");
!         }
!       return or_op ? invert_truthvalue (tem) : tem;
!     }
   
***************
*** 9998,10000 ****
          || (TREE_CODE (op0) == REAL_CST
!             && TREE_CODE (op0) != REAL_CST))
        {
--- 10007,10009 ----
          || (TREE_CODE (op0) == REAL_CST
!             && TREE_CODE (op1) != REAL_CST))
        {

-- 
           Summary: gcc fails to warn about some common logic errors
           Product: gcc
           Version: 3.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: trt at acm dot org
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16302