Re: [GHC] #8763: forM_ [1..N] does not get fused (allocates 50% more)

6 Sep 2018

      #8763: forM_ [1..N] does not get fused (allocates 50% more)
-------------------------------------+-------------------------------------
        Reporter:  nh2               |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  7.6.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #7206             |  Differential Rev(s):  Phab:D5131
       Wiki Page:                    |
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Comment (by simonpj):

 Here's a longer, and to me more comprehensible, Note
 {{{
 Note [List fusion and continuations in 'c']
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Suppose we define
    mapM_ f = foldr ((>>) . f) (return ())
 (this is the way it used to be).

 Now suppose we want to optimise the call

   mapM_ <big> (build g)
     where
   g c n = ...(c x1 y1)...(c x2 y2)....n...

 GHC used to proceed like this:

   mapM_ <big> (build g)

   = { Defintion of mapM_ }
     foldr ((>>) . <big>) (return ()) (build g)

   = { foldr/build rule }
     g ((>>) . <big>) (return ())

   = { Inline g ]
     let c = (>>) . <big>
         n = return ()
     in ...(c x1 y1)...(c x2 y2)....n...

 The trouble is that `c`, being big, will not be inlined.  And that can
 be absolutely terrible for performance, as we saw in Trac #8763.

 It's much better to define

    mapM_ f = foldr c (return ())
      where
        c x k = f x >> k
        {-# INLINE c #-}

 Now we get
   mapM_ <big> (build g)

   = { inline mapM_ }
     foldr c (return ()) (build g)
       where c x k = f x >> k
             {-# INLINE c #-}
             f = <big>

 Notice that `f` does not inine into the RHS of `c`, because the
 ININE pragma stops it; see Note [How INLINE pragmas /prevent/ inlining].
 Continuing:

   = { foldr/build rule }
     g c (return ())
       where ...
          c x k = f x >> k
          {-# INLINE c #-}
             f = <big>

   = { inline g }
     ...(c x1 y1)...(c x2 y2)....n...
       where c x k = f x >> k
             {-# INLINE c #-}
             f = <big>
             n = return ()

       Now, crucially, `c` does inline

   = { inline c }
     ...(f x1 >> y1)...(f x2 >> y2)....n...
       where f = <big>
             n = return ()

 And all is well!  The key thing is that the fragment
 `(f x1 >> y1)` is inlined into the body of the builder
 `g`.
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8763#comment:81
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