[Bug middle-end/55481] [4.8 regression] -O2 generates a wrong-code infinite loop in C++Benchmark's simple_types_constant_folding int8 xor test

["rguenth at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55481

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Depends on|                            |35634

--- Comment #7 from Richard Biener <rguenth at gcc dot gnu.org> 2012-11-27 11:12:03 UTC ---
Note that the frontends warn with

lucky13x.c: In function 'int main(int, char**)':
lucky13x.c:24:64: warning: overflow in implicit constant conversion
[-Woverflow]
       int8_t temp = (int8_t)SIZE * ((int8_t)(23) | (int8_t)(10));

when manually inlining do_shift at the checking point:

      int8_t temp = (int8_t)SIZE * ((int8_t)(23) | (int8_t)(10));
      if (result != temp)
        abort ();

this computes 13 * 31 which is 403 so the warning is correct.  Fixed with
an extra cast.  This also means that

      for (n = 0; n < SIZE; ++n)
        result += do_shift( first[n] );

triggers the bug about doing the addition in the wrong type and the bug
is fixed by rewriting this as

        result = result + do_shift (first[n]);

that is, self-modify is not correctly doing the addition in 'int'.  And thus
this is a dup of PR35634 I believe (-fwrapv also fixes this).