1 files changed, 46 insertions, 0 deletions
diff --git a/testsuite/tests/perf/should_run/T21839r.hs b/testsuite/tests/perf/should_run/T21839r.hs
new file mode 100644
index 0000000000..594459da03
--- /dev/null
+++ b/testsuite/tests/perf/should_run/T21839r.hs
@@ -0,0 +1,46 @@
+-- For in depth details see the ticket #21839. The short version:
+
+-- We noticed that GHC got slower compiling Cabal the libary.
+-- Eventually I narrowed it down to the pattern below of deriving Generics
+-- for a Enum, and then deriving a Binary instance for that Enum via Generics.
+-- A pattern very frequently used in Cabal.
+-- However this turned out to be a classic compile vs runtime tradeoff.
+-- In benchmarks I found the resulting code for the Binary instance was running
+-- more than twice as fast!
+-- So we decided to merely document this change and add a test representing this behaviour
+-- rather than trying to coax ghc back into its old behaviour.
+
+{-# LANGUAGE DeriveGeneric #-}
+
+{-# OPTIONS_GHC #-}
+module Main
+  ( main
+  ) where
+
+import GHC.Generics
+import Data.Typeable
+import Data.Binary
+import qualified Data.ByteString.Lazy as BL
+import qualified Data.ByteString as BS
+
+data PathTemplateVariable =
+
+       Var0
+     | Var1
+     | Var2
+     | Var3
+     | Var4
+     | Var5
+     | Var6
+     | Var7
+     | Var8
+     | Var9
+  deriving (Generic,Enum)
+
+instance Binary PathTemplateVariable
+
+main :: IO ()
+main = do
+  let lists = replicate 10000 Var0
+      lbs = encode lists
+  print $ BS.length $ BS.toStrict lbs