[GHC] #8763: forM_ [1..N] does not get fused (10 times slower than go function)

[GHC]

#8763: forM_ [1..N] does not get fused (10 times slower than go function)
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        Reporter:  nh2               |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       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):
       Wiki Page:                    |
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Comment (by sgraf):

 This definition of `efdtIntUpFB` only has a single occurence of `c` and
 `n` and consequently fixes th issue.

 But this probably doesn't have the same performance for the non-fused
 case.

 {{{
 data CounterState
   = More
   | Last
   | End

 -- Requires x2 >= x1
 {-# INLINE [0] efdtIntUpFB #-} -- See Note [Inline FB functions] in
 GHC.List
 efdtIntUpFB :: (Int -> r -> r) -> r -> Int# -> Int# -> Int# -> r
 efdtIntUpFB c n x1 x2 y =
    -- Be careful about overflow!
    let !first_state
          | isTrue# (y <# x2) = if isTrue# (y <# x1) then End else Last
          | otherwise         = More -- Common case: x1 <= x2 <= y

        !delta = x2 -# x1 -- >= 0
        !y' = y -# delta  -- x1 <= y' <= y; hence y' is representable

        next_state End  _     = End
        next_state Last _     = End
        next_state More x
          | isTrue# (x ># y') = Last
          | otherwise         = More

        -- Invariant: x <= y
        -- Note that: z <= y' => z + delta won't overflow
        -- so we are guaranteed not to overflow if/when we recurse
        emit End _ = n
        emit st  x
          | let x' = x +# delta
          = I# x `c` emit (next_state st x') x'

    in emit first_state x1
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8763#comment:52>
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