[GHC] #14287: Early inlining causes potential join points to be missed

27 Sep 2017

      #14287: Early inlining causes potential join points to be missed
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           Reporter:  jheek          |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
           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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 While trying to make stream fusion work with recursive step functions I
 noticed that the following filter implementation did not fuse nicely.

 {{{#!haskell

 data Stream s a = Stream (s -> Step s a) s
 data Step s a = Done | Yield a s

 sfilter :: (a -> Bool) -> Stream s a -> Stream s a
 sfilter pred (Stream step s0) = Stream filterStep s0 where
   filterStep s = case step s of
     Done -> Done
     Yield x ns
       | pred x    -> Yield x ns
       | otherwise -> filterStep ns

 fromTo :: Int -> Int -> Stream Int Int
 {-# INLINE fromTo #-}
 fromTo from to = Stream step from where
   step i
     | i > to    = Done
     | otherwise = Yield i (i + 1)

 sfoldl :: (b -> a -> b) -> b -> Stream s a -> b
 {-# INLINE sfoldl #-}
 sfoldl acc z (Stream !step s0) = oneShot go z s0 where
   go !y s = case step s of
     Done       -> y
     Yield x ns -> go (acc y x) ns

 ssum :: (Num a) => Stream s a -> a
 ssum = sfoldl (+) 0

 filterTest :: Int
 filterTest = ssum $ sfilter even (fromTo 1 101)
 }}}

 For this code to work nicely, GHC should detect that filterStep is a join
 point. However, in the definition of sfilter it is not because not all
 references are tail-called & saturated.

 After inlining of sfilter and some trivial case-of-case transformations
 filterStep should become a join point. But it seems like the simplifier
 never gets the change to do this because float-out optimization makes
 filterStep a top level binding. With -fno-full-laziness filterStep does
 become a join point at the call site, but of course this is not really a
 solution.

 Then I found that the following also works:

 {{{#!haskell
 sfilter :: (a -> Bool) -> Stream s a -> Stream s a
 sfilter pred (Stream step s0) = Stream filterStep s0 where
   {-# INLINE [2] filterStep #-}
   filterStep s = case step s of
     Done -> Done
     Yield x ns
       | pred x    -> Yield x ns
       | otherwise -> filterStep ns
 }}}

 Simply adding an INLINE [2] pragma disables the inlining in the early run
 of the simplifier. Therefore, the float out pass does not get the change
 to float-out before the filterStep is recognized as a joint point.
 Or at least that is my interpretation of what is going on.

 What surprises me about this issue is that the gentle run seems to perform
 inlining while the wiki mentions that inlining is not performed in this
 stage:
 https://ghc.haskell.org/trac/ghc/wiki/Commentary/Compiler/Core2CorePipeline

 Intuitively, I would think that floating-out is sub-optimal when the
 simplifier did not use all its tricks yet, because inlining typically
 opens up possibilities for simplification while floating-out typically
 reducing these possibilities.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14287
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