[GHC] #7219: Reinstate constant propagation in some form

[GHC]

#7219: Reinstate constant propagation in some form
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    Reporter:  simonmar          |       Owner:  simonmar               
        Type:  bug               |      Status:  new                    
    Priority:  highest           |   Milestone:  7.8.1                  
   Component:  Compiler          |     Version:  7.7                    
    Keywords:                    |          Os:  Unknown/Multiple       
Architecture:  Unknown/Multiple  |     Failure:  Runtime performance bug
  Difficulty:  Unknown           |    Testcase:                         
   Blockedby:                    |    Blocking:                         
     Related:                    |  
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 The new codegen doesn't have a constant propagation pass.  This used to be
 done by the mini-inliner which was ad-hoc at best; instead of the mini-
 inliner we now have `CmmSink` which is much less ad-hoc but doesn't do
 constant folding or propagation.

 This is a regression that will affect performance of things like
 `copyArray#`, so we should fix it before the release.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7219>
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