[GHC] #13228: Surprising inlining failure

2 Feb 2017


      #13228: Surprising inlining failure
-------------------------------------+-------------------------------------
           Reporter:  dfeuer         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.2.1
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Runtime
  Unknown/Multiple                   |  performance bug
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 When I fail to mark something seemingly unrelated `INLINE`, something else
 doesn't inline. In the below (cut down dramatically from my recent lazy ST
 work), if I don't mark `>>` inline in the `Monad` instance, then `*>`
 never inlines, and none of the rules fire when compiling `foom` and
 `basic`. If `>>` is marked inline, then everything works as expected.

 {{{#!hs
 module RulesLazy (ST, strictToLazyST, lazyToStrictST) where
 import qualified Control.Monad.ST as ST

 -- No, this is not really lazy ST
 newtype ST s a = ST (s -> (a, s))

 instance Functor (ST s) where
   fmap _ _ = undefined

 instance Applicative (ST s) where
     pure _ = undefined
     _ <*> _ = undefined
     m *> n = m `thenST` n

 {-# NOINLINE [1] thenST #-}
 thenST :: ST s a -> ST s b -> ST s b
 _ `thenST` _ = ST $ \_ -> undefined

 instance Monad (ST s) where
     {-# INLINE (>>) #-}  -- CRITICAL LINE
     m >> n = m `thenST` n

     _ >>= _ = undefined

 {-# NOINLINE [1] strictToLazyST #-}
 strictToLazyST :: ST.ST s a -> ST s a
 strictToLazyST _ = ST $ \_ -> undefined

 {-# NOINLINE [1] lazyToStrictST #-}
 lazyToStrictST :: ST s a -> ST.ST s a
 lazyToStrictST _ = undefined

 {-# RULES
 "then/S2L" forall m n . m `thenST` strictToLazyST n =
    strictToLazyST (lazyToStrictST m *> n)
 "L2S/S2L" forall m . lazyToStrictST (strictToLazyST m) = m
  #-}


 module RulesBurn where

 import RulesLazy
 import qualified Control.Monad.ST as SST

 {-# NOINLINE foom #-}
 foom :: SST.ST s a -> SST.ST s b -> SST.ST s c -> ST s c
 foom m n o = (strictToLazyST m *> strictToLazyST n) *> strictToLazyST o

 {-# NOINLINE basic #-}
 basic :: ST s a -> SST.ST s b -> ST s b
 basic m n = m *> strictToLazyST n
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13228
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