[Bug other/49942] Wrong code generated for -O2 with target s390 on Linux

["jphartmann at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49942

--- Comment #5 from jphartmann at gmail dot com <jphartmann at gmail dot com> 2011-08-02 09:59:44 UTC ---
Because the unoptimised code has a je at that place.  And putting anything
after the assignment generates correct code.  And debug code after the loop
shows that it falls through where it shouldn't.  Maybe it will help to show the
actual C rather than what comes out of the preprocessor, but this is the one
that works.  Remove the assignment to dummy and it fails:

      for (i = 0; NUMD - FUZZ > i; i++)
      {
         int diff = (0xff & op0->digits[i]) - (0xff & op1->digits[i]);

         DDDEBUG("i %d op %c op1 %c diff %d\n", i,
            op0->digits[i], op1->digits[i], diff);

         dummy = diff; /* cures a gcc bug */
         if (diff) return '-' == op0->sign
               ? (0 < diff ? -1 : 1)
               : (0 > diff ? -1 : 1);
      }
      DDEBUG("Fell through. %s vs %s\n", op0->digits, op1->digits);

The DDDEBUG is discarded by the preprocessor.  FUZZ is 0 and NUMD is 9 in the
case at hand.

In the case at hand the leading digits are different; that may have a bearing
since it looks like some of the loop is unrolled.