Index: llvm/lib/CodeGen/AssignmentTrackingAnalysis.cpp
===================================================================
--- llvm/lib/CodeGen/AssignmentTrackingAnalysis.cpp
+++ llvm/lib/CodeGen/AssignmentTrackingAnalysis.cpp
@@ -1630,17 +1630,20 @@
   unsigned IntersectSize = Join.size();
   (void)IntersectSize;
 
-  // Add the elements in B with variables that are not in A into
-  // SymmetricDifference.
-  for (const auto &Pair : B) {
-    VariableID Var = Pair.first;
-    if (A.count(Var) == 0)
-      SymmetricDifference.push_back(Var);
-  }
+  // Check if B can possibly contain variables not in A.
+  if (!SymmetricDifference.empty() || A.size() != B.size()) {
+    // Add the elements in B with variables that are not in A into
+    // SymmetricDifference.
+    for (const auto &Pair : B) {
+      VariableID Var = Pair.first;
+      if (A.count(Var) == 0)
+        SymmetricDifference.push_back(Var);
+    }
 
-  // Add SymmetricDifference elements to Join and return the result.
-  for (const auto &Var : SymmetricDifference)
-    Join.insert({Var, LocKind::None});
+    // Add SymmetricDifference elements to Join and return the result.
+    for (const auto &Var : SymmetricDifference)
+      Join.insert({Var, LocKind::None});
+  }
 
   assert(Join.size() == (IntersectSize + SymmetricDifference.size()));
   assert(Join.size() >= A.size() && Join.size() >= B.size());
@@ -1721,17 +1724,20 @@
   unsigned IntersectSize = Join.size();
   (void)IntersectSize;
 
-  // Add the elements in B with variables that are not in A into
-  // SymmetricDifference.
-  for (const auto &Pair : B) {
-    VariableID Var = Pair.first;
-    if (A.count(Var) == 0)
-      SymmetricDifference.push_back(Var);
-  }
+  // Check if B can possibly contain variables not in A.
+  if (!SymmetricDifference.empty() || A.size() != B.size()) {
+    // Add the elements in B with variables that are not in A into
+    // SymmetricDifference.
+    for (const auto &Pair : B) {
+      VariableID Var = Pair.first;
+      if (A.count(Var) == 0)
+        SymmetricDifference.push_back(Var);
+    }
 
-  // Add SymmetricDifference elements to Join and return the result.
-  for (auto Var : SymmetricDifference)
-    Join.insert({Var, Assignment::makeNoneOrPhi()});
+    // Add SymmetricDifference elements to Join and return the result.
+    for (auto Var : SymmetricDifference)
+      Join.insert({Var, Assignment::makeNoneOrPhi()});
+  }
 
   assert(Join.size() == (IntersectSize + SymmetricDifference.size()));
   assert(Join.size() >= A.size() && Join.size() >= B.size());