[GHC] #9326: Minor change to list comprehension structure leads to poor performance

[GHC]

#9326: Minor change to list comprehension structure leads to poor performance
-------------------------------------+-------------------------------------
        Reporter:  dfeuer            |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.8.3
        Keywords:                    |  Differential Revisions:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |              Difficulty:  Unknown
       Test Case:                    |              Blocked By:
        Blocking:                    |         Related Tickets:
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 According to [http://stackoverflow.com/questions/24690406/haskell-list-
 comprehension-speed-inconsistencies] ''and my own testing'', the code in
 compspeed.hs reliably runs several times faster than the code in
 compspeedslow.hs with ghc 7.8.3, although this apparently does not happen
 with 7.6.3. The basic distinction is between a list comprehension of the
 form

 {{{
 [(a,h) | ..., let h = expr]
 }}}

 and

 {{{
 [(a,expr) | ...]
 }}}

 The core produced by -ddump-simpl is completely different in each case, so
 I can't figure out what's going on.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9326>
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