diff --git a/vyper/codegen/ir_node.py b/vyper/codegen/ir_node.py
index 9d6770ce81..a253a9278d 100644
--- a/vyper/codegen/ir_node.py
+++ b/vyper/codegen/ir_node.py
@@ -405,6 +405,8 @@ def gas(self):
     def is_complex_ir(self):
         # list of items not to cache. note can add other env variables
         # which do not change, e.g. calldatasize, coinbase, etc.
+        # reads (from memory or storage) should not be cached because
+        # they can have side effects.
         do_not_cache = {"~empty", "calldatasize", "callvalue"}
 
         return (