[GHC] #7520: Implement cardinality analysis

[GHC]

#7520: Implement cardinality analysis
---------------------------------+------------------------------------------
    Reporter:  simonpj           |       Owner:                  
        Type:  bug               |      Status:  new             
    Priority:  normal            |   Milestone:                  
   Component:  Compiler          |     Version:  7.6.1           
    Keywords:                    |          Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown    
  Difficulty:  Unknown           |    Testcase:                  
   Blockedby:                    |    Blocking:                  
     Related:                    |  
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Comment(by nfrisby):

 SPJ determined that this cardinality analysis could make some fragile
 fusion opportunities more robust. My experimentation with -fdicts-strict
 in `nofib/shootout/spectral-norm` was disrupting such a fusion.

 {{{
 let x_aPi = build (\b c n -> eftIntFB c n 0 (...)) -- from "[0..n-1]"
 in Library.allocaArray n $ \ptr ->
   foldr (mapFB ...) (...) x_aPi -- from "forM_ ..."
 }}}

 If `allocaArray` is not inlined early enough (which my -fdicts-strict
 tinkering prevented), the `ptr` lambda is not identified as one-shot, so
 `x_aPi` will not be inlined under it and `foldr/build` won't fire.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7520#comment:2>
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