[CFLAA] Make CFLAnders conservative when it sees newly created values

[grievejia]

Summary

Currently CFLAnders only keeps track of values that do aliases. As a result, if it sees a value that's created after the analysis gets executed, it will automatically assume that value does not alias anything since the value is not in its record.

This patch tries to fix the problem by modifying attribute lookup process to let it shield the alias lookups. Now CFLAnders's AttrMap will try to store every value it has seen before, and if it sees a new value, return AttrUnknown immediately to guarantee that no AliasMap lookup will be conducted.

[george.burgess.iv]

LGTM, thanks!

llvm/trunk/lib/Analysis/CFLAndersAliasAnalysis.cpp

 class AliasAttrMap {
   typedef DenseMap<InstantiatedValue, AliasAttrs> MapType;
   MapType AttrMap;
 
 public:
   typedef MapType::const_iterator const_iterator;
 
   bool add(InstantiatedValue V, AliasAttrs Attr) {
-    if (Attr.none())
-      return false;
     auto &OldAttr = AttrMap[V];
     auto NewAttr = OldAttr | Attr;
     if (OldAttr == NewAttr)
       return false;
     OldAttr = NewAttr;
     return true;
   }
 
getInterfaceValue(InstantiatedValue IValue,
   return None;
 }
 
 static void populateAttrMap(DenseMap<const Value *, AliasAttrs> &AttrMap,
                             const AliasAttrMap &AMap) {
   for (const auto &Mapping : AMap.mappings()) {
     auto IVal = Mapping.first;
 
+    // Insert IVal into the map
+    auto &Attr = AttrMap[IVal.Val];
     // AttrMap only cares about top-level values
     if (IVal.DerefLevel == 0)
-      AttrMap[IVal.Val] |= Mapping.second;
+      Attr |= Mapping.second;
   }
 }
 
 static void
 populateAliasMap(DenseMap<const Value *, std::vector<OffsetValue>> &AliasMap,
                  const ReachabilitySet &ReachSet) {
   for (const auto &OuterMapping : ReachSet.value_mappings()) {
     // AliasMap only cares about top-level values
CFLAndersAAResult::FunctionInfo::FunctionInfo(
   populateExternalAttributes(Summary.RetParamAttributes, Fn, RetVals, AMap);
   populateAliasMap(AliasMap, ReachSet);
   populateExternalRelations(Summary.RetParamRelations, Fn, RetVals, ReachSet);
 }
 
 AliasAttrs CFLAndersAAResult::FunctionInfo::getAttrs(const Value *V) const {
   assert(V != nullptr);
 
-  AliasAttrs Attr;
+  // Return AttrUnknown if V is not found in AttrMap. Sometimes V can be created
+  // after the analysis gets executed, and we want to be conservative in
+  // those cases.
+  AliasAttrs Attr = getAttrUnknown();
   auto Itr = AttrMap.find(V);
   if (Itr != AttrMap.end())
     Attr = Itr->second;
   return Attr;
 }
 
 bool CFLAndersAAResult::FunctionInfo::mayAlias(const Value *LHS,
                                                uint64_t LHSSize,