[Bug tree-optimization/66975] New: parloops to handle signed int reductions

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66975

            Bug ID: 66975
           Summary: parloops to handle signed int reductions
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vries at gcc dot gnu.org
  Target Milestone: ---

With this ( https://gcc.gnu.org/ml/gcc-cvs/2015-07/msg00896.html ) fix we
introduce xfails for signed int reductions:
...
        * gcc.dg/autopar/outer-4.c: Add xfail.
        * gcc.dg/autopar/outer-5.c: Same.
        * gcc.dg/autopar/outer-6.c: Same.
        * gcc.dg/autopar/reduc-2.c: Same.
        * gcc.dg/autopar/reduc-2char.c: Same.
        * gcc.dg/autopar/reduc-2short.c: Same.
        * gcc.dg/autopar/reduc-8.c: Same.
...

The problem is that when parallelizing reductions in parloops, we change the
order of evaluation, which can mean that we introduce an overflow, where there
was none before. This only yields the same result if the overflow behaviour can
be assumed to wrap. The fix adds this missing check.

However, we can convert the signed reduction to an unsigned one, correctly
parallelize it, and cast the result back to signed.