[GHC] #11372: Loopification does not trigger for IO even if it could

[GHC]

#11372: Loopification does not trigger for IO even if it could
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        Reporter:  jscholl           |                Owner:
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.3
  (CodeGen)                          |             Keywords:  cmm,
      Resolution:                    |  loopification, code generation
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D1767
       Wiki Page:                    |
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Changes (by jscholl):

 * status:  new => patch
 * differential:   => Phab:D1767


Comment:

 The problem with loopification for loops in {{{IO}}} / any monad of the
 form {{{State# s -> (# State s, a #)}}} is indeed caused by the extra
 {{{State#}}} tokens. While they have no runtime representation and thus
 are not passed to a self-recursive call, they are still counted as
 arguments when considering a self-recursive call for loopification.

 So I tried to fix this by adding an additional parameter to
 {{{getCallMethod}}}, namely the number of void arguments, and subtract
 that number before checking if the correct number of arguments is present
 (I could not just remove void arguments from the arguments as the number
 of arguments is used in other places as well and I think there these void
 arguments have to generate {{{stg_ap_v}}} calls or something similar). I
 also tried to add a note describing the situation with these void
 arguments and loopification.

 I also did a nofib-run. Most things changed only a little bit, but most of
 the code seems to run a little bit faster, though in praxis it is even
 less than in my benchmark, but this is not suprising as most code does not
 (only) consist of such loops in IO.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11372#comment:2>
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