[GHC] #8900: unordered-containers 16% slower in HEAD vs 7.6.3

[GHC]

#8900: unordered-containers 16% slower in HEAD vs 7.6.3
--------------------------------------------+------------------------------
        Reporter:  tibbe                    |            Owner:
            Type:  bug                      |           Status:  new
        Priority:  normal                   |        Milestone:
       Component:  Compiler                 |          Version:  7.9
      Resolution:                           |         Keywords:
Operating System:  MacOS X                  |     Architecture:  x86_64
 Type of failure:  Runtime performance bug  |  (amd64)
       Test Case:                           |       Difficulty:  Unknown
        Blocking:                           |       Blocked By:
                                            |  Related Tickets:
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Comment (by tibbe):

 While the extra `case` is definitely a regression, it doesn't seem to be
 the cause of the time difference. Changing `A.index` to:

 {{{
 #!haskell
 index :: Array a -> Int -> (# a #)
 index ary _i@(I# i#) =
         indexArray# (unArray ary) i#
 }}}

 and the snippet from `Data.HashMap.Base` to:

 {{{
 #!haskell
     go h k x s t@(Full ary) =
         let !(# st #) = A.index ary i
             !st' = go h k x (s+bitsPerSubkey) st

 }}}

 makes the difference in the Core go away, but the time difference remains.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8900#comment:8>
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