[GHC] #10918: Float once-used let binding into a recursive function

[GHC]

#10918: Float once-used let binding into a recursive function
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        Reporter:  nomeata           |                Owner:
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
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Comment (by nomeata):

 Looking at the code, maybe this is a viable course of action:

 In `FloatIn`, in the equation of `fiExpr` for recursive bindings, where
 `extra_fvs` are calculated, exclude any variables that are free variables
 of a right-hand-side of the binding if they have a Dmd signature that
 indicates that they are used at most once. This allows them to float
 inside the `let`

 Then we still need to ensure that they pass past the lambda. So the lambda
 case of `fiExpr` needs to be improved to separate the used-once floats
 from the others and float only those in.

 I’m not sure if the float in pass is the right place to do this, though.
 Shouldn’t the simplifier be able to do these things? So maybe
 `simplRecBind` should not zap all floats, but rather distinguish between
 those that may float into a recursive group and the others?

 This needs advise from one of the experts on the simplifier.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10918#comment:1>
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