[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           |  Unknown/Multiple
  performance bug                    |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by George):

 With 7.10.1 I see a factor of 2 difference in performance:

 benchmarking matmultForM_
 time                 10.90 μs   (10.89 μs .. 10.91 μs)
                      1.000 R²   (1.000 R² .. 1.000 R²)
 mean                 10.89 μs   (10.89 μs .. 10.91 μs)
 std dev              32.72 ns   (18.98 ns .. 65.42 ns)

 benchmarking matmultLoop
 time                 5.404 μs   (5.387 μs .. 5.419 μs)
                      1.000 R²   (1.000 R² .. 1.000 R²)
 mean                 5.409 μs   (5.398 μs .. 5.420 μs)
 std dev              37.99 ns   (33.64 ns .. 44.26 ns)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8763#comment:18>
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