[GHC] #15998: GHC.Event.Thread.eventManager has a lot of indirections

[GHC]

#15998: GHC.Event.Thread.eventManager has a lot of indirections
-------------------------------------+-------------------------------------
           Reporter:  chessai        |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:  8.6.3
          Component:                 |           Version:  8.6.2
  libraries/base                     |
           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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 Current `eventManager`:

 {{{#!hs
 eventManager :: IORef (IOArray Int (Maybe (ThreadId, EventManager)))
 eventManager = unsafePerformIO $ do
 ...
 }}}

 That's a lot of indirections just to grab your thread's event manager.

 Consider the following, which I believe would improve the performance of
 this:

 {{{#!hs
 data UnliftedIORef :: TYPE 'UnliftedRep -> Type where
   UnliftedIORef :: MutVar# RealWorld a -> UnliftedIORef a

 eventManager :: UnliftedIORef (MutableArray# RealWorld Things)

 data Things = Things !ThreadId !EventManager
 }}}

 I think the Maybe can be eliminated. I'm unsure. What makes me think it
 can be is the following snippet:

 {{{#!hs
 getSystemEventManager :: IO (Maybe EventManager)
 getSystemEventManager = do
   t <- myThreadId
   (cap, _) <- threadCapability t
   eventManagerArray <- readIORef eventManager
   mmgr <- readIOArray eventManagerArray cap
   return $ fmap snd mmgr

 getSystemEventManager_ :: IO EventManager
 getSystemEventManager_ = do
   Just mgr <- getSystemEventManager
   return mgr
 {-# INLINE getSystemEventManager_ #-}
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15998>
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