[GHC] #11443: SPECIALIZE pragma does not work + performance drop in GHC 8.0-rc1

16 Jan 2016


      #11443: SPECIALIZE pragma does not work + performance drop in GHC 8.0-rc1
-------------------------------------+-------------------------------------
           Reporter:  danilo2        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:
          Component:  Compiler       |           Version:  8.0.1-rc1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Hello!
 I've just hit a strange issue. I might missinterpret how the `SPECIALIZE`
 pragma works, but if I understand correctly, then there is a bug in GHC.
 Lets consider this simple code:

 module `A`:

 {{{

 {-# LANGUAGE UndecidableInstances #-}
 {-# LANGUAGE TypeOperators        #-}
 {-# LANGUAGE KindSignatures       #-}
 {-# LANGUAGE DataKinds            #-}
 {-# LANGUAGE TypeFamilies         #-}
 {-# LANGUAGE FlexibleInstances    #-}

 module A where

 import Prelude
 import GHC.TypeLits

 -- TF utils

 type family (a :: Nat) :== (b :: Nat) where
     a :== a = 'True
     a :== b = 'False

 type family If cond (a :: Nat) (b :: Nat) where
     If 'True  a b = a
     If 'False a b = b

 -- Heavy TF computations

 type family HeavyTF (n :: Nat) (i :: Nat) :: Nat where HeavyTF n 0 = 0
                                                        HeavyTF n i = If
 (HeavyTF' n :== 0) (HeavyTF n (i - 1)) 1


 type family HeavyTF' (n :: Nat) :: Nat where HeavyTF' 0 = 0
                                              HeavyTF' n = HeavyTF' (n - 1)


 -- Params for tests (bigger numbers = longer compile times)

 type family NatOf a :: Nat
 type instance NatOf Int    = 12000
 type instance NatOf String = 12000

 -- Type class to check GHC behavior
 class PerfC1 a where perfc1 :: a -> String
 instance CheckOk (HeavyTF 10 (NatOf a)) => PerfC1 a where perfc1 _ = "oh"
 ; {-# INLINABLE perfc1 #-}

 class CheckOk (n :: Nat)
 instance CheckOk 0 -- where

 main_cache :: IO ()
 main_cache = do
     print $ perfc1 (1 :: Int)
     print $ perfc1 ("a" :: String)


 perfc1_Int :: Int -> String
 perfc1_Int = perfc1


 perfc1_String :: String -> String
 perfc1_String = perfc1


 {-# SPECIALIZE perfc1 :: Int -> String #-}
 {-# SPECIALIZE perfc1 :: String -> String #-}


 -----

 perfc1' :: PerfC1 a => a -> String
 perfc1' = perfc1
 {-# INLINABLE perfc1' #-}


 {-# SPECIALIZE perfc1' :: Int -> String #-}
 {-# SPECIALIZE perfc1' :: String -> String #-}

 }}}

 module `Test1`:

 {{{
 import A

 main = do
     print $ perfc1 (1 :: Int)
     print $ perfc1 ("a" :: String)
 }}}

 module `Test2`:

 {{{
 import A

 main = do
     print $ perfc1' (1 :: Int)
     print $ perfc1' ("a" :: String)
 }}}

 module `Test3`:

 {{{
 import A

 main = do
     print $ perfc1_Int (1 :: Int)
     print $ perfc1_String ("a" :: String)
 }}}

 Compile with:
 `ghc 7.10.3` : `ghc -O2 -fenable-rewrite-rules Test<n>.hs`
 `ghc 8.0-rc1` : `ghc -O2 -fenable-rewrite-rules -freduction-depth=0
 Test<n>.hs`

 If module `A` was already compiled the compilation times for `ghc 7.10.3`
 were as follow:
 - `Test1`: ~ 16s
 - `Test2`: ~ 16s
 - `Test3`: almost instant

 And for `ghc 8.0-rc1` were as follow:
 - `Test1`: ~ 28s
 - `Test2`: ~ 28s
 - `Test3`: almost instant

 Here are 2 bugs to note:

    1) the compilation times are much longer with new GHC

    2) the specialize pragmas do not work

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11443
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