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From: "eggert at gnu dot org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c/48267] New: incorrect signed overflow warning when a pointer cannot possibly overflow
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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;
}