--- a/tests/check-perf-code.py	Fri May 20 09:47:35 2016 +0900
+++ b/tests/check-perf-code.py	Fri May 20 09:47:35 2016 +0900
@@ -16,7 +16,50 @@
   ]
 ]
 
+def modulewhitelist(names):
+    replacement = [('.py', ''), ('.c', ''), # trim suffix
+                   ('mercurial%s' % (os.sep), ''), # trim "mercurial/" path
+                  ]
+    ignored = set(['__init__'])
+    modules = {}
+
+    # convert from file name to module name, and count # of appearances
+    for name in names:
+        name = name.strip()
+        for old, new in replacement:
+            name = name.replace(old, new)
+        if name not in ignored:
+            modules[name] = modules.get(name, 0) + 1
+
+    # list up module names, which appear multiple times
+    whitelist = []
+    for name, count in modules.items():
+        if count > 1:
+            whitelist.append(name)
+
+    return whitelist
+
 if __name__ == "__main__":
+    # in this case, it is assumed that result of "hg files" at
+    # multiple revisions is given via stdin
+    whitelist = modulewhitelist(sys.stdin)
+    assert whitelist, "module whitelist is empty"
+
+    # build up module whitelist check from file names given at runtime
+    perfpypats[0].append(
+        # this matching pattern assumes importing modules from
+        # "mercurial" package in the current style below, for simplicity
+        #
+        #    from mercurial import (
+        #        foo,
+        #        bar,
+        #        baz
+        #    )
+        ((r'from mercurial import [(][a-z0-9, \n#]*\n(?! *%s,|^[ #]*\n|[)])'
+          % ',| *'.join(whitelist)),
+         "import newer module separately in try clause for early Mercurial"
+         ))
+
     # import contrib/check-code.py as checkcode
     assert 'RUNTESTDIR' in os.environ, "use check-perf-code.py in *.t script"
     contribpath = os.path.join(os.environ['RUNTESTDIR'], '..', 'contrib')