[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]

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).