Re: [GHC] #13080: Constant values are not floated out of the loop

7 Jan 2017

      #13080: Constant values are not floated out of the loop
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        Reporter:  Feuerbach         |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by rwbarton):

 Floating out `return ()` won't help much. Even if you do that, the program
 looks like
 {{{
 worker = let l = poll ()
          in forever l

 r = return ()
 poll a = r >> poll a
 }}}
 so executing `l` will cause `l` to be evaluated as `r >> r >> r >> r >>
 ...` and we still have a live reference to `l` from within `forever`.

 If you instead define
 {{{
 poll a = r >>= \_ -> poll a
 }}}
 and compile ''without optimization'' then `l` now looks like `r >>= (\_ ->
 ...)` and the space leak is gone. But turning on optimizations seems to
 cause GHC to rewrite the above to
 {{{
 poll a = r >>= (let s = poll a in \_ -> s)
 }}}
 which now has the original problem again.

 (BTW I am always compiling with `-fno-state-hack`, since that just adds
 another layer of confusion to an already confusing situation.)

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13080#comment:1
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