[GHC] #5916: runST isn't free

[GHC]

#5916: runST isn't free
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        Reporter:  tibbe             |            Owner:
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:  7.6.2
       Component:  Compiler          |          Version:  7.4.1
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by nomeata):

 Using
 {{{#!haskell
 {-# INLINE runSTRep #-}
 runSTRep :: (forall s. STRep s a) -> a
 runSTRep st_rep = case st_rep (stWorld# st_rep) of
                         (# _, r #) -> r

 {-# NOINLINE stWorld# #-}
 stWorld# :: a -> State# RealWorld
 stWorld# _ = realWorld#
 }}}
 it validates. But do we actually have a test for this?

 Here would be one:
 {{{#!haskell
 import Control.Monad.ST
 import Data.STRef

 main =
     let f n = runST $ do
         ref <- newSTRef 0
         modifySTRef ref (+n)
         readSTRef ref
     in print (f 1 + f 2)
 }}}
 returns 3 in master, returns 4 if I inline `runSTRep`, and returns 3 with
 the `stWorld#`-trick.

 So unless someone says that this is not worth it (which would be strange,
 given that this bug is about an unexpected real-world performance issue),
 I suggest to give `runSTRep` a custom unfolding in `MkId` that replaces
 `runSTRep e` by `case e (stWorld# $(all free local variables of e)) of (#
 _, r #) -> r`.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/5916#comment:12>
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