Three files, one with only main, one with main + non-referenced IO object, one with main + referenced IO object (technically, one with a larger main IO object). The non-referenced IO object is never compiled, as the resulting object files have the same md5sum.

dead-code-tests.hs

-- test1.hs
-- No dead code: compiles to 1.22M executable
--               with stripped .o 1320 byes
--               .o file md5sum: 35ccd1647c772a5757fba3ae8bfa6542

main :: IO ()
main = getLine >>= print

-- test2.hs
-- With dead code: compiles to 1.22M executable
--                 with stripped .o 1320 byes
--                 .o file md5sum: 35ccd1647c772a5757fba3ae8bfa6542

main :: IO ()
main = getLine >>= print

unreached :: IO ()
unreached = putStrLn "What are you doing here?"

-- test3.hs
-- "Control": compiles to 1.23M executable
--            with stripped .o 1448 byes
--            .o file md5sum: 09b15c76ecae08c9b5be7564808602f1

main :: IO ()
main = getLine >>= reached

reached :: String -> IO ()
reached s = print (tail s)