[GHC] #12370: Implement LetUp in DmdAnal (or document why we do not do it)

[GHC]

#12370: Implement LetUp in DmdAnal (or document why we do not do it)
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        Reporter:  nomeata           |                Owner:
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      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 nomeata):

 Work in progress in branch `wip/T12370`.

 It already works in this simple case:
 {{{
 foo :: (Int, Int) -> Int
 foo (x,y) = x + y
 {-# NOINLINE foo #-}

 bar n m =
     let p = (n,m)
         {-# NOINLINE p #-}
     in foo p
 }}}
 Here, bar now gets the very detailed signature
 `<S(S),1*U(U)><S(S),1*U(U)>m` instead of just `<L,U><L,U>m`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12370#comment:1>
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