diff --git a/src/passes/OptimizeAddedConstants.cpp b/src/passes/OptimizeAddedConstants.cpp
index 50df848f657..142f1002491 100644
--- a/src/passes/OptimizeAddedConstants.cpp
+++ b/src/passes/OptimizeAddedConstants.cpp
@@ -55,7 +55,7 @@ struct GetParents {
   struct Inner : public ExpressionStackWalker<Inner> {
     void visitLocalGet(LocalGet* curr) { parentMap[curr] = getParent(); }
 
-    std::map<Expression*, Expression*> parentMap;
+    std::unordered_map<Expression*, Expression*> parentMap;
   } inner;
 };