[GHC] #12916: GHC 8.0.1 vs GHC HEAD (8.1.20161202) doesn't do inlining and unboxing very well

[GHC]

#12916: GHC 8.0.1 vs GHC HEAD (8.1.20161202) doesn't do inlining and unboxing very
well
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        Reporter:  alexbiehl         |                Owner:
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:  invalid           |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by alexbiehl):

 Ok, thanks for the explanation!

 But one last question: Why is GHC 8.0.1 and HEAD even in perf flavour not
 able to inline the length function on the vector?

 {{{#!hs
              ...
             Just a ->
               case length $fVectorVectora a of { I# y ->
               case y of {
                 ...
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12916#comment:10>
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