[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:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.3
  (CodeGen)                          |
           Keywords:  cmm,           |  Operating System:  Unknown/Multiple
  loopification, code generation     |
       Architecture:                 |   Type of failure:  Runtime
  Unknown/Multiple                   |  performance bug
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The loopification optimization, as I understand it, allows a self-
 recursive function to jump to a local label instead of the beginning of
 the function, thus skipping a potential stack check. However, I only
 observe it triggering for pure functions while IO functions do not get
 that benefit, even when it would be possible.

 I discovered this in #8793 after looking into an unexpected speedup by
 removing the IO context from an otherwise pure loop and while such a loop
 can simply be changed to a pure version (the {{{IOLoop}}} examples), other
 functions can not, but the optimization could be applied to them (see
 {{{MapM.hs}}}).

 I tried to benchmark the differences between a naive loop in IO and some
 horrible {{{inlinePerformIO}}} hacks to get the loopification to fire and
 the "optimized" version performs 3-5% faster on my machine.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11372>
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