diff --git a/emscripten.py b/emscripten.py
index 15405124d..000a50d81 100644
--- a/emscripten.py
+++ b/emscripten.py
@@ -852,8 +852,19 @@ def get_all_implemented(forwarded_json, metadata):
 
 
 def check_all_implemented(all_implemented, pre):
-  for requested in shared.Settings.ORIGINAL_EXPORTED_FUNCTIONS:
-    if not is_already_implemented(requested, pre, all_implemented):
+  # we are not checking anyway, so just skip this
+  if not shared.Settings.ERROR_ON_UNDEFINED_SYMBOLS and not shared.Settings.WARN_ON_UNDEFINED_SYMBOLS:
+    return
+  # the initial list of missing functions are those we expected to export, but were not implemented in compiled code
+  missing = list(set(shared.Settings.ORIGINAL_EXPORTED_FUNCTIONS) - set(all_implemented))
+  # special-case malloc, EXPORTED by default for internal use, but we bake in a
+  # trivial allocator and warn at runtime if used in ASSERTIONS
+  if '_malloc' in missing:
+    missing.remove('_malloc')
+
+  for requested in missing:
+    in_pre = ('function ' + asstr(requested)) in pre
+    if not in_pre:
       # could be a js library func
       if shared.Settings.ERROR_ON_UNDEFINED_SYMBOLS:
         exit_with_error('undefined exported function: "%s"', requested)
@@ -861,15 +872,6 @@ def check_all_implemented(all_implemented, pre):
         logger.warning('undefined exported function: "%s"', requested)
 
 
-def is_already_implemented(requested, pre, all_implemented):
-  is_implemented = requested in all_implemented
-  # special-case malloc, EXPORTED by default for internal use, but we bake in a
-  # trivial allocator and warn at runtime if used in ASSERTIONS
-  is_exception = requested == '_malloc'
-  in_pre = ('function ' + asstr(requested)) in pre
-  return is_implemented or is_exception or in_pre
-
-
 def get_exported_implemented_functions(all_exported_functions, all_implemented, metadata):
   funcs = set(metadata['exports'])
   export_bindings = shared.Settings.EXPORT_BINDINGS