[Bug tree-optimization/49234] [4.4/4.5/4.7/4.7 Regression] -Wstrict-overflow gives obviously unwarranted warning

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49234

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> 2011-05-31 09:02:29 UTC ---
We hit:
163724    rguenth       /* Similarly, if the new maximum is smaller or larger
than
163724    rguenth        the previous one, go all the way to +INF.  */
163724    rguenth       if (cmp_max < 0 || cmp_max > 0)
163724    rguenth       {
163724    rguenth         if (!needs_overflow_infinity (TREE_TYPE
(vr_result.max))
163724    rguenth             || !vrp_var_may_overflow (lhs, phi))
163724    rguenth           vr_result.max = TYPE_MAX_VALUE (TREE_TYPE
(vr_result.max));
163724    rguenth         else if (supports_overflow_infinity (TREE_TYPE
(vr_result.max)))
163724    rguenth           vr_result.max =
163724    rguenth               positive_overflow_infinity (TREE_TYPE
(vr_result.max));
163724    rguenth       }

As the IV (i) might overflow, vrp_var_may_overflow returns true.
In particular, chrec is SCEV_NOT_KNOWN.  Thus it just in case sets
vr_result.max to +INF(OVF) and later on we warn about it.
Before hitting this code, vr_result contains the right range [0, 2], but it
doesn't know it can safely use that.