[GHC] #9434: GHC.List.reverse does not fuse

[GHC]

#9434: GHC.List.reverse does not fuse
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              Reporter:  dfeuer      |            Owner:
                  Type:  bug         |           Status:  patch
              Priority:  normal      |        Milestone:
             Component:              |          Version:  7.9
  libraries/base                     |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Easy (less than 1
  Unknown/Multiple                   |  hour)
       Type of failure:  Runtime     |       Blocked By:
  performance bug                    |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by dfeuer):

 I've improved the rules for writing `reverse` back to a recursive form. It
 appears to be good to fuse in most cases. The main exceptions seem to be
 `filter p . reverse` (although `reverse . filter p` is good) and
 `takeWhile . reverse` (although `reverse . takeWhile` is good). The latter
 was investigated by Dan Doel. The problem seems to be that the closures
 built up in the bad cases end up allocating more than the conses they
 replace. Even the worst case for `nofib` (a `filter . reverse` form) isn't
 really too terrible. I think we're probably better off making this change
 than not.

 Note: rules to try to convert `filter p . reverse` back to a (slightly)
 better version seem inherently too fragile to want to bother with.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9434#comment:7>
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