[GHC] #15503: interpreter: sequence_ (replicate 100000000 (return ())) gobbles up memory

[GHC]

#15503: interpreter: sequence_ (replicate 100000000 (return ()))  gobbles up memory
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        Reporter:  int-e             |                Owner:  osa1
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.8.1
       Component:  GHCi              |              Version:  8.5
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         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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Comment (by simonpj):

 Hmm.  Thanks for showing the full code that GHCi compiles
 to bytecode.  It looks like this:
 {{{
 let main_action = ...
 in bindIO main_action (\it -> return [it])
 }}}
 So GHCi will build a thunk for `main_action` and will pass it
 to the compiled code for `GHC.Base.bindIO`.

 But alas `main_action` is essentially this
 {{{
 main_action :: IO ()
 main_action = sequence_ (replicate ...)
 }}}
 (I have omitted the `ghcStepIO` stuff; it is essentially the
 identity function, and I don't think it affects things.)

 Now that `main_action` thunk will be updated, so if it
 remains live for any reason, we'll retain a huge PAP
 of the form described in an earlier comment `return () >> (return ()
 ...))`.

 Why is it being kept alive? I'm really not sure.  The interpreter
 should be ''tail-calling'' `bindIO`...

 I wonder if `main_action` could be floated to top level as a CAF
 before it gets to the bytecode generator?

 Anyway, I hope that may help a bit.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15503#comment:12>
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