diff --git a/src/mapleall/maple_me/include/alias_class.h b/src/mapleall/maple_me/include/alias_class.h
index 8257766244cbb416a7f5fba4391ec229c364748a..a93314af0f1342b0ceef22165cbc7bb54ee3bff9 100644
--- a/src/mapleall/maple_me/include/alias_class.h
+++ b/src/mapleall/maple_me/include/alias_class.h
@@ -220,6 +220,7 @@ class AliasClass : public AnalysisResult {
   void UpdateNextLevelNodes(std::vector<OriginalSt*> &nextLevelOsts, const AliasElem &aliasElem);
   void UnionNodes(std::vector<OriginalSt*> &nextLevelOsts);
   int GetOffset(const Klass &super, const Klass &base) const;
+  void UnionAllNodes(MapleVector<OriginalSt *> *nextLevOsts);
 
   MemPool &acMemPool;
   MapleAllocator acAlloc;
diff --git a/src/mapleall/maple_me/src/alias_class.cpp b/src/mapleall/maple_me/src/alias_class.cpp
index 5f381bb1846ec1f4c59426172e7928be4d51ec54..93c286a34d8bf68cca21be439d5b6e6f3ba46b06 100644
--- a/src/mapleall/maple_me/src/alias_class.cpp
+++ b/src/mapleall/maple_me/src/alias_class.cpp
@@ -38,29 +38,29 @@ inline bool IsPotentialAddress(PrimType primType, MIRModule *mirModule) {
 // return true if this expression opcode can result in a valid address
 static bool OpCanFormAddress(Opcode op) {
   switch (op) {
-  case OP_dread:
-  case OP_regread:
-  case OP_iread:
-  case OP_ireadoff:
-  case OP_ireadfpoff:
-  case OP_ireadpcoff:
-  case OP_addrof:
-  case OP_addroffunc:
-  case OP_addroflabel:
-  case OP_addroffpc:
-  case OP_iaddrof:
-  case OP_constval:
-  case OP_conststr:
-  case OP_conststr16:
-  case OP_alloca:
-  case OP_malloc:
-  case OP_add:
-  case OP_sub:
-  case OP_select:
-  case OP_array:
-  case OP_intrinsicop:
-    return true;
-  default: ;
+    case OP_dread:
+    case OP_regread:
+    case OP_iread:
+    case OP_ireadoff:
+    case OP_ireadfpoff:
+    case OP_ireadpcoff:
+    case OP_addrof:
+    case OP_addroffunc:
+    case OP_addroflabel:
+    case OP_addroffpc:
+    case OP_iaddrof:
+    case OP_constval:
+    case OP_conststr:
+    case OP_conststr16:
+    case OP_alloca:
+    case OP_malloc:
+    case OP_add:
+    case OP_sub:
+    case OP_select:
+    case OP_array:
+    case OP_intrinsicop:
+      return true;
+    default: ;
   }
   return false;
 }
@@ -147,7 +147,6 @@ AliasElem *AliasClass::FindOrCreateAliasElem(OriginalSt &ost) {
   }
   if (ost.GetIndirectLev() > 1) {
     aliasElem->SetNotAllDefsSeen(true);
-    aliasElem->SetNextLevNotAllDefsSeen(true);
   }
   id2Elem.push_back(aliasElem);
   osym2Elem[ostIdx] = aliasElem;
@@ -600,28 +599,51 @@ AliasElem *AliasClass::FindOrCreateDummyNADSAe() {
   return osym2Elem[dummyOst->GetIndex()];
 }
 
+void AliasClass::UnionAllNodes(MapleVector<OriginalSt *> *nextLevOsts) {
+  if (nextLevOsts->size() < 2) {
+    return;
+  }
+
+  auto it = nextLevOsts->begin();
+  OriginalSt *ostA = *it;
+  AliasElem *aeA = FindAliasElem(*ostA);
+  ++it;
+  for (; it != nextLevOsts->end(); ++it) {
+    OriginalSt *ostB = *it;
+    AliasElem *aeB = FindAliasElem(*ostB);
+    unionFind.Union(aeA->GetClassID(), aeB->GetClassID());
+  }
+}
+
 // This is applicable only for C language.  For each ost that is a struct,
 // union all fields within the same struct
 void AliasClass::ApplyUnionForStorageOverlaps() {
   // iterate through all the alias elems
   for (AliasElem *ae : id2Elem) {
     OriginalSt *ost = &ae->GetOriginalSt();
-    OriginalSt *prevLevOst = GetAliasAnalysisTable()->GetPrevLevelNode(*ost);
-    if (prevLevOst == nullptr) {
+    MIRType *mirType = GlobalTables::GetTypeTable().GetTypeFromTyIdx(ost->GetTyIdx());
+    if (mirType->GetKind() != kTypePointer) {
       continue;
     }
-    MIRType *prevLevType = GlobalTables::GetTypeTable().GetTypeFromTyIdx(prevLevOst->GetTyIdx());
-    if (prevLevType->GetKind() != kTypePointer) {
+    auto nextLevOsts = GetAliasAnalysisTable()->GetNextLevelNodes(*ost);
+    if (nextLevOsts == nullptr) {
       continue;
     }
-    MIRType *prevLevPointedType = static_cast<MIRPtrType *>(prevLevType)->GetPointedType();
-    MIRType *ostType = GlobalTables::GetTypeTable().GetTypeFromTyIdx(ost->GetTyIdx());
-    if (!ostType->HasFields() && prevLevPointedType->GetKind() != kTypeUnion) {
+
+    MIRType *pointedType = static_cast<MIRPtrType *>(mirType)->GetPointedType();
+    if (pointedType->GetKind() == kTypeUnion) {
+      // union all fields of union
+      UnionAllNodes(nextLevOsts);
       continue;
     }
-    for (OriginalSt *sameLevOst : *GetAliasAnalysisTable()->GetNextLevelNodes(*prevLevOst)) {
-      AliasElem *ae1 = FindAliasElem(*sameLevOst);
-      unionFind.Union(ae->GetClassID(), ae1->GetClassID());
+
+    for (auto *nextLevOst : *nextLevOsts) {
+      MIRType *nextLevType = GlobalTables::GetTypeTable().GetTypeFromTyIdx(nextLevOst->GetTyIdx());
+      if (nextLevType->HasFields()) {
+        // union all fields if one next-level-ost has fields
+        UnionAllNodes(nextLevOsts);
+        break;
+      }
     }
   }
 }
@@ -1331,8 +1353,6 @@ void AliasClass::InsertMayDefUseIntrncall(StmtNode &stmt, BBId bbid) {
     for (int32 i = 0; i < stmt.NumOpnds(); i++) {
       InsertMayUseExpr(*stmt.Opnd(i));
     }
-  } else {
-    CollectMayUseFromNADS(mayDefUseOsts);
   }
   // 2. collect mayDefs and mayUses caused by not_all_defs_seen_ae
   CollectMayUseFromNADS(mayDefUseOsts);