[GHC] #5344: CSE should look through coercions

[GHC]

#5344: CSE should look through coercions
---------------------------------+------------------------------------------
    Reporter:  reinerp           |       Owner:                         
        Type:  feature request   |      Status:  new                    
    Priority:  normal            |   Component:  Compiler               
     Version:  7.0.3             |    Keywords:  cse                    
    Testcase:                    |   Blockedby:                         
          Os:  Unknown/Multiple  |    Blocking:                         
Architecture:  Unknown/Multiple  |     Failure:  Runtime performance bug
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 This is probably a known limitation of the CSE pass, but there doesn't
 appear to be a ticket so I'll make one.

 Consider the module:
 {{{
 module M where

 newtype Id a = Id a

 f (a, b) = (Id a, b)

 g (a, b) = (a, b)
 }}}

 Compiling this with {{{ghc -ddump-simpl -dsuppress-all -O2}}}, we get

 {{{
 g = \ (@ t_adf) (@ t1_adg) (ds_ddl :: (t_adf, t1_adg)) -> ds_ddl

 f =
   \ (@ t_adi) (@ a_adj) (ds_ddn :: (a_adj, t_adi)) ->
     case ds_ddn of _ { (a_aaW, b_aaX) -> (a_aaW `cast` ..., b_aaX) }
 }}}

 We see that {{{g}}} shares its argument tuple, but {{{f}}} allocates a new
 copy of it. Ideally {{{f}}} would also share its argument tuple, and would
 look like this:

 {{{
 f = \ (@ t_adi) (@ a_adj) (ds_ddn :: (a_adj, t_adi)) -> ds_ddn `cast` ...
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5344>
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