[GHC] #8814: 7.8 optimizes attoparsec improperly

[GHC]

#8814: 7.8 optimizes attoparsec improperly
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        Reporter:  joelteon                 |            Owner:
            Type:  bug                      |           Status:  new
        Priority:  normal                   |        Milestone:
       Component:  Compiler                 |          Version:  7.8.1-rc1
      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:  #8763
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Comment (by simonpj):

 What would be really helpful would be if you, or someone else, could
 diagnose exactly why the slow-down is happening.  If you compile with
 `-ticky` you can very quickly zero in on the code that is taking more
 time, and compare one with the other.

 It would be remarkable if fusion was really responsible for such an
 enormous change in time, but perhpas it is.  Maybe it would be worth
 commenting out RULES one at a time, to see which (if any) is responsible,
 and to reduce accidental differences between the two.  Maybe the test case
 can be boiled down quite a lot further.

 You can switch off ALL rule rewrites with `-fno-enable-rewrite-rules`.

 Switching off full laziness might also be a good thing to try `-fno-full-
 laziness`.

 Reducing the inlining threshold (which doesn't affect INLINE pragmas)
 would mean less looking at inlined code.  `-funfolding-use-threshold=N`
 (default is 60)

 It's a bit fiddly and time consuming, which is why I was appealing for
 help.  I'll gladly explain anything you (or whoever) wants to know about
 the GHC end of things.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8814#comment:16>
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