[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:
            Type:  bug                      |           Status:  new
        Priority:  normal                   |        Milestone:
       Component:  Compiler                 |          Version:  7.6.3
      Resolution:                           |         Keywords:
Operating System:  Unknown/Multiple         |     Architecture:
 Type of failure:  Runtime performance bug  |  Unknown/Multiple
       Test Case:                           |       Difficulty:  Unknown
        Blocking:                           |       Blocked By:
                                            |  Related Tickets:
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Comment (by nh2):

 @simonpj: Summary:

 1) I reported that my manually written loop is much faster than `forM_
 [1..n]` in some cases, suggesting that in some cases optimizing the list
 away doesn't work well.

 2) nomeata said some technical things that are a bit beyond me.

 3) I submit two benchmarks in the gist at
 ​https://gist.github.com/nh2/11333427, a "matmult" benchmark where there
 is a big difference between `forM_` and the hand-written `loop`, and a
 "simple" benchmark where they are equally fast.

 4) Daniel suspects the slow case comes from using the same syntactical
 list twice, and that in this case GHC floats it out to share it, which
 breaks eliminating it. He suggests we might special-case `enumFromTo` when
 used with `forM_` to prevent it.

 5) I give a counter example for his suspicion, by changing my "simple"
 benchmark, where using the same list twice gives the same good performance
 as using it once.

 I get the same behaviour for 7.6 and 7.8.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8763#comment:14>
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