[Bug c/48267] New: incorrect signed overflow warning when a pointer cannot possibly overflow

[Bug c/48267] New: incorrect signed overflow warning when a pointer cannot possibly overflow

[eggert at gnu dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48267

           Summary: incorrect signed overflow warning when a pointer
                    cannot possibly overflow
           Product: gcc
           Version: 4.5.2
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: eggert@gnu.org


I ran into this problem when compiling the GNU Emacs trunk with a GCC
4.5.2 that I built on RHEL 5.5 (x86-64).  I narrowed it down to the
following stripped-down test case.  This smells different from the previous
bug report I filed in this area (PR48228) on the same platform.

When I compile the following program with "gcc -S -Wstrict-overflow -O2"
GCC reports "warning: assuming pointer wraparound does not occur when comparing
P +- C1 with P +- C2".  This warning is incorrect, since
signed overflow is obviously impossible in this function: the only
pointers computed are head_table and head_table + 1, which are both
in range.  Changing the "+ 1" to "+ 7" generates even more warnings,
though the program is still correct.

int head_table[7];

int
foo (void)
{
  const int *p;
  int x = 0;

  for (p = head_table; p < head_table + 1; p++)
    x ^= *p;
  return x;
}

[Bug middle-end/48267] incorrect signed overflow warning when a pointer cannot possibly overflow

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48267

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2011.03.24 12:14:46
          Component|c                           |middle-end
     Ever Confirmed|0                           |1
      Known to fail|                            |4.5.2, 4.7.0

--- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-03-24 12:14:46 UTC ---
Confirmed.  Happens from

#1  0x00000000007307e7 in fold_undefer_overflow_warnings (issue=1 '\001', 
    stmt=0x7ffff5b373c0, code=2)
    at /space/rguenther/src/svn/trunk/gcc/fold-const.c:281
281       warning_at (locus, OPT_Wstrict_overflow, "%s", warnmsg);
(gdb) 
#2  0x0000000000afdfdf in evaluate_stmt (stmt=0x7ffff5b373c0)
    at /space/rguenther/src/svn/trunk/gcc/tree-ssa-ccp.c:2117
2117          fold_undefer_overflow_warnings (is_constant, stmt, 0);
(gdb)
#3  0x0000000000aff7c5 in visit_cond_stmt (stmt=0x7ffff5b373c0, 
    taken_edge_p=0x7fffffffda48)
    at /space/rguenther/src/svn/trunk/gcc/tree-ssa-ccp.c:2443
2443      val = evaluate_stmt (stmt);
(gdb) 
#4  0x0000000000aff8f8 in ccp_visit_stmt (stmt=0x7ffff5b373c0, 
    taken_edge_p=0x7fffffffda48, output_p=0x7fffffffda40)
    at /space/rguenther/src/svn/trunk/gcc/tree-ssa-ccp.c:2501
2501            return visit_cond_stmt (stmt, taken_edge_p);

(gdb) call debug_gimple_stmt (stmt)
if (p_1 < &head_table[1])

which calls fold_binary with &head_table < &head_table[1] and in the end warns
here:

8688                  if (code != EQ_EXPR
8689                      && code != NE_EXPR
8690                      && bitpos0 != bitpos1
8691                      && (pointer_may_wrap_p (base0, offset0, bitpos0)
8692                          || pointer_may_wrap_p (base1, offset1, bitpos1)))
8693                    fold_overflow_warning (("assuming pointer wraparound
does not "
8694                                            "occur when comparing P +- C1
with "
8695                                            "P +- C2"),
8696                                          
WARN_STRICT_OVERFLOW_CONDITIONAL)

because we call pointer_may_wrap_p with base0 which isn't an address
but an object which is bogus.  It's argument has to depend on
indirect_base in which case we stripped an ADDR_EXPR.

[Bug middle-end/48267] incorrect signed overflow warning when a pointer cannot possibly overflow

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=48267

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED
      Known to work|                            |6.1.0
      Known to fail|                            |5.5.0
   Target Milestone|---                         |6.0

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Fixed in GCC 6.