Re: [GHC] #12603: INLINE and manually inlining produce different code

24 Oct 2016

      #12603: INLINE and manually inlining produce different code
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        Reporter:  bgamari           |                Owner:  bgamari
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by MikolajKonarski):

 I have an example that may or may not capture the original case 2
 (allocation bloat due to INLINE, here by a factor of 2000). Perhaps it's
 just INLINE pushing the subexpression `mapVT n` over the threshold where
 some kind of simplification and/or floating out is not done any more. If
 it's interesting, please let me know and I will file a new bug report.

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

 -- 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
 --
 -- with INLINE:
 --           384,409,080 bytes allocated in the heap
 -- with NOINLINE:
 --               169,080 bytes allocated in the heap
       dis pI =
         let p0 = let (_, x) = pI `quotRem` z in P x
             p1 = let (y, _) = pI `quotRem` z in P y
             !_ = isJust $ IM.lookup (fromEnum p0) IM.empty
             !_ = isJust $ IM.lookup (fromEnum p1) IM.empty
         in ()
       mapVT :: forall s. Int -> ST s ()
       {-# NOINLINE l #-}
       l = [0..1600]
       {-# INLINE mapVT #-}
       mapVT _ = mapM_ (\x -> return $! dis x) l
       !runRes = foldl' (\() n -> runST (mapVT n)) () [1..10000]
   return ()
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12603#comment:17
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