diff --git a/mlir/lib/Analysis/FlatLinearValueConstraints.cpp b/mlir/lib/Analysis/FlatLinearValueConstraints.cpp
--- a/mlir/lib/Analysis/FlatLinearValueConstraints.cpp
+++ b/mlir/lib/Analysis/FlatLinearValueConstraints.cpp
@@ -1054,37 +1054,46 @@
 }
 
 /// Merge and align symbols of `this` and `other` such that both get union of
-/// of symbols that are unique. Symbols in `this` and `other` should be
-/// unique. Symbols with Value as `None` are considered to be inequal to all
-/// other symbols.
+/// symbols that are unique. The symbols will be ordered with the symbols from
+/// `this` appearing first, then the symbols from `other`. Symbols in `this` and
+/// `other` should be unique. Symbols with Value as `None` are considered to be
+/// inequal to all other symbols.
 void FlatLinearValueConstraints::mergeSymbolVars(
     FlatLinearValueConstraints &other) {
 
   assert(areVarsUnique(*this, VarKind::Symbol) && "Symbol vars are not unique");
   assert(areVarsUnique(other, VarKind::Symbol) && "Symbol vars are not unique");
 
-  SmallVector<Value, 4> aSymValues;
-  getValues(getNumDimVars(), getNumDimAndSymbolVars(), &aSymValues);
+  auto maybeSymValues = getMaybeValues(presburger::VarKind::Symbol);
 
   // Merge symbols: merge symbols into `other` first from `this`.
   unsigned s = other.getNumDimVars();
-  for (Value aSymValue : aSymValues) {
-    unsigned loc;
-    // If the var is a symbol in `other`, then align it, otherwise assume that
-    // it is a new symbol
-    if (other.findVar(aSymValue, &loc) && loc >= other.getNumDimVars() &&
-        loc < other.getNumDimAndSymbolVars())
-      other.swapVar(s, loc);
-    else
-      other.insertSymbolVar(s - other.getNumDimVars(), aSymValue);
+  for (std::optional<Value> maybeValue : maybeSymValues) {
+    if (maybeValue.has_value()) {
+      unsigned loc;
+      // If the var is a symbol in `other`, then align it, otherwise assume that
+      // it is a new symbol
+      if (other.findVar(*maybeValue, &loc) && loc >= other.getNumDimVars() &&
+          loc < other.getNumDimAndSymbolVars())
+        other.swapVar(s, loc);
+      else
+        other.insertSymbolVar(s - other.getNumDimVars(), *maybeValue);
+    } else
+      // if the symbol has no Value attached, we have to add it as a new symbol
+      // to `other`
+      other.insertSymbolVar(s - other.getNumDimVars());
     s++;
   }
 
   // Symbols that are in other, but not in this, are added at the end.
   for (unsigned t = other.getNumDimVars() + getNumSymbolVars(),
                 e = other.getNumDimAndSymbolVars();
-       t < e; t++)
-    insertSymbolVar(getNumSymbolVars(), other.getValue(t));
+       t < e; t++) {
+    if (other.hasValue(t))
+      insertSymbolVar(getNumSymbolVars(), other.getValue(t));
+    else
+      insertSymbolVar(getNumSymbolVars());
+  }
 
   assert(getNumSymbolVars() == other.getNumSymbolVars() &&
          "expected same number of symbols");