+ // Once this BB is encountered, Val's value for this BB will not be Undefined
+ // any longer. When we encounter this BB again, if Val's value is Overdefined,
+ // we need to compute its value again.
+ //
+ // For example, considering this control flow,
+ // BB1->BB2, BB1->BB3, BB2->BB3, BB2->BB4
+ //
+ // Suppose we have "icmp slt %v, 0" in BB1, and "icmp sgt %v, 0" in BB3. At
+ // the very beginning, when analyzing edge BB2->BB3, we don't know %v's value
+ // in BB2, and the data flow algorithm tries to compute BB2's predecessors, so
+ // then we know %v has negative value on edge BB1->BB2. And then we return to
+ // check BB2 again, and at this moment BB2 has Overdefined value for %v in
+ // BB2. So we should have to follow data flow propagation algorithm to get the
+ // value on edge BB1->BB2 propagated to BB2, and finally %v on BB2 has a
+ // constant range describing a negative value.
+
+ if (!BBLV.isUndefined() && !BBLV.isOverdefined()) {