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diff --git a/testsuite/tests/perf/should_run/T15226.hs b/testsuite/tests/perf/should_run/T15226.hs
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+-- T15226
+import Control.Exception (evaluate)
+
+-- Just in case Prelude.repeat changes for some reason.
+import Prelude hiding (repeat)
+
+-- We want to be sure that the compiler *doesn't* know that
+-- all the elements of the list are in WHNF, because if it
+-- does, PrelRules may erase the seq#'s altogether.
+repeat :: a -> [a]
+repeat a = res
+  where res = a : res
+{-# NOINLINE repeat #-}  -- Belt *and* suspenders
+
+silly :: [Int] -> IO ()
+silly = foldr go (pure ())
+  where
+    go x r = do
+      x' <- evaluate x
+      evaluate (x' + 3)  -- GHC should know that x' has been evaluated,
+                         -- so this calculation will be erased entirely.
+                         -- Otherwise, we'll create a thunk to pass to
+                         -- evaluate.
+      r
+
+main :: IO ()
+-- 10,000,000 repetitions take only a twentieth of a second,
+-- but allocations go up dramatically if the result is not
+-- known evaluated.
+main = silly $ take 10000000 $ repeat 1