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

[GHC]

#13080: Constant values are not floated out of the loop
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        Reporter:  Feuerbach         |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.0.1
      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 dfeuer):

 The original program leaks memory even without optimization. In this case,
 the trouble is that

 {{{#!hs
 poll () = return () >> return () >> ...
 }}}

 `forever` always holds on to its argument, which prevents any part of
 `poll ()` from being collected as it's run. You can fix this (with or
 without optimization) by defining

 {{{#!hs
 poll _ = fix (return () >>)
 }}}

 which makes `poll ()` finite in size.

 While you and I both know that `forever` doesn't need to hold on to `poll
 ()` in this case, I don't see how GHC can be expected to see that.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13080#comment:6>
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