[GHC] #10626: Missed opportunity for SpecConstr

[GHC]

#10626: Missed opportunity for SpecConstr
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        Reporter:  simonpj           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by dsc):

 Just tried with GHC 8.0.2 and 7.10.3; similar problem.

 By the way, the original source looks like this:

 {{{
 intersperseTest =
   $$(maybeSimplify $

         S.enumFromToNum [|| 1 ||] [|| 1000000 :: Int ||]
       & S.concatMap (\a -> yield a S.++ yield a)
       & S.intersperse [|| 42 ||]
       & S.concatMap yield
       & S.sum_
       & runIdentityE_static
     )
 }}}

 (WIP library/experiment, trying for something like the
 [https://hackage.haskell.org/package/streaming streaming] package with
 robust fusion using typed TH :))

 If I change the first `concatMap` to just `concatMap yield`  or remove
 either the `intersperse` or the second `concatMap`, the problem is not
 present.

 I've had similar instances where the problem disappears after removing a
 random part of the pipeline (with that part fusing correctly if I remove a
 different part). Feels like I'm hitting some size limit rather than a
 fundamental problem (?) But as mentioned above, I already tried setting
 various GHC flags to large numbers.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10626#comment:4>
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