[Bug middle-end/55863] New: Fails to fold (i + 2) - (i + 1) to 1

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

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55863

             Bug #: 55863
           Summary: Fails to fold (i + 2) - (i + 1) to 1
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: rguenth@gcc.gnu.org


For

(analyze_overlapping_iterations
  (chrec_a = {pretmp_1529 + 1, +, 1}_20)
  (chrec_b = {pretmp_1529 + 2, +, 1}_20)

The SIV test currently fails because when computing the difference
between the CHRECs we compute pretmp_1529 + 2 + ~pretmp15_29.  This
is because when computing A - B we check if B is easy to negate - which
X + 1 is - and compute it as A + ~X which association then fails to
optimize.

This makes dependence tests of this kind fail.