[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: unassig...@gcc.gnu.org

        ReportedBy: egg...@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;

}