[GHC] #12781: Significantly higher allocation with INLINE vs NOINLINE

29 Oct 2016


      #12781: Significantly higher allocation with INLINE vs NOINLINE
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           Reporter:                 |             Owner:
  MikolajKonarski                    |
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Linux
       Architecture:  x86_64         |   Type of failure:  Runtime
  (amd64)                            |  performance bug
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The following program allocates much more memory ("bytes allocated in the
 heap") in its current form than with INLINE replaced by NOINLINE, as in
 the comment. That may be normal, but virtually every feature of this
 example is needed to trigger the behaviour and so I have no clear idea
 which language feature is responsible and should be avoided and,
 consequently, how to optimize my much more complex codebase (in which
 INLINES of once used functions tend to help and sometimes significantly
 so). As a comment to this ticket I will post another example, where the
 difference in allocation is pumped to a factor of 2000, as a proof that
 the issue is serious.

 {{{
 {-# LANGUAGE RankNTypes #-}
 import Control.Monad.ST.Strict
 import qualified Data.IntMap.Strict as IM

 -- ghc --make -O1 InlineBloat.hs; ./InlineBloat +RTS -s

 data P = P Int

 instance Enum P where
   fromEnum (P x) = x
   toEnum n = undefined

 main = do
   let {-# NOINLINE z #-}
       z = 44
       dis :: Int -> ()
       {-# INLINE dis #-}  -- change here to NOINLINE and observe lower
 alloc
       dis pI =
         let p0 = let (_, x) = pI `quotRem` z in P x
             p1 = let (_, x) = pI `quotRem` z in P x
             m = IM.lookup (fromEnum p0) IM.empty
             b = IM.member (fromEnum p1) IM.empty
         in m == Just 'c' `seq` b `seq` ()
       {-# NOINLINE l #-}
       l = [0..10000000]
       mapVT :: forall s. () -> ST s ()
       {-# INLINE mapVT #-}
       mapVT _ = mapM_ (\x -> return $! dis x) l
   return $! runST (mapVT ())
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12781
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