[Bug c++/26729] [3.3/3.4 Regression] bogus 'comparison is always 0' message

["apl at alum dot mit dot edu" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #6 from apl at alum dot mit dot edu  2006-03-28 15:21 -------
I was wrong, this is still broken in GCC 4.0.2 and 4.1.0 although the original
test case doesn't demonstrate the bug.

/tools/linux/gcc-4.1.0/bin/g++ -c -Werror -Wall -Wextra b.cxx -O2
cc1plus: warnings being treated as errors
b.cxx: In function 'void f()':

The slightly simpler test is attached


I did debug this in gcc-3.4.3 sources to discover that the problem was in
dealing with having set overflow on a complemented constant mask extended from
16 to 32 bits in fold_truthop() inside fold_const.c

This code fragment shows where 'integer_zerop() fails because while
const_binop() returned a ZERO value, it set the OVERFLOW flag causing
the predicate to reject it....

      if (! integer_zerop (const_binop (BIT_AND_EXPR, l_const,
                                        fold (build1 (BIT_NOT_EXPR,
                                                      lntype, ll_mask)),
                                        0)))
        {
          warning ("comparison is always %d", wanted_code == NE_EXPR);

I haven't looked inside the 4.1 sources to establish where the problem is
now...


-- 

apl at alum dot mit dot edu changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
      Known to fail|3.4.3 3.3.3                 |3.4.3 3.3.3 4.0.2 4.1.0
      Known to work|4.1.0 4.0.0 3.2.3           |
         Resolution|FIXED                       |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26729