[GHC] #9339: last is not a good consumer

20 Jul 2014


      #9339: last is not a good consumer
-------------------------------------+-------------------------------------
        Reporter:  dfeuer            |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  libraries/base    |                 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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 The profiler indicates that `print $ last [(1::Int)..10^7]` (compiled with
 -O2) allocates around 8*10^8 bytes. Using the Henning Thienemann-inspired
 {{{
 myLast = fromJust . foldr (\x -> Just . maybe x id) Nothing
 }}}
 (based on his code for viewR/unsnoc) reduces allocation by about half at
 the cost of some extra work. What we really want, I believe, is for `last`
 to fuse with the producer in a fashion that allows the Ints to be unboxed,
 eliminating all the allocation. I have no idea if this will fall out of
 the general fusion work planned for 7.9.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9339
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