[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 simonpj):

 Looks plausible to me, but needs a careful Note to explain the issues.

 But before we go too far with this, I'd like to point to [wiki:LateLamLift
 late lambda lifting].  In the core reported in comment:44, all we need to
 do is lambda-lift `$wc` and `go_up` to top level, and all will be well, I
 claim.  And that is precisely what late-lambda lifting does.  And the
 result might be faster than the very careful code above, because of the
 extra argument passing and case-tesing it has to do.

 To me LLF is low-hanging fruit. There are promising results described on
 the wiki page, and the whole join-point business eliminates its principal
 shortcoming.

 I wonder if, before going ahead with this somewhat-delicate `efdtIntFB`
 business, it might be fun to re-awaken LLF?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8763#comment:61>
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