[Bug middle-end/33816] New: gimplification before build_array_type re-set alias set of arrays to zero

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

The testcase from PR32921 shows that we end up with an alias set of zero for
real8[0:] because in both functions this type is used in casts.  When the
first function is gimplified the gimplifier asks if the cast is a useless
type conversion which in turn calls get_alias_set which sets the alias set.
Once the second function is lowered to GIMPLE, the array-type gets re-built
using build_array_type which calls layout_type:

  if (!COMPLETE_TYPE_P (t))
    layout_type (t);

[because the type looks incomplete]

which in turn globally re-sets the alias set to zero

  /* If an alias set has been set for this aggregate when it was incomplete,
     force it into alias set 0.
     This is too conservative, but we cannot call record_component_aliases
     here because some frontends still change the aggregates after
     layout_type.  */
  if (AGGREGATE_TYPE_P (type) && TYPE_ALIAS_SET_KNOWN_P (type))
    TYPE_ALIAS_SET (type) = 0;
}

causing effectively wrong gimplification of the first function and pessimizatio
of alias analysis.


-- 
           Summary: gimplification before build_array_type re-set alias set
                    of arrays to zero
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Keywords: wrong-code, missed-optimization
          Severity: blocker
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: rguenth at gcc dot gnu dot org
OtherBugsDependingO 32921
             nThis:


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33816