Re: [GHC] #5075: CPR optimisation for sum types if only one constructor is used

20 Mar 2013

      #5075: CPR optimisation for sum types if only one constructor is used
---------------------------------+------------------------------------------
    Reporter:  batterseapower    |       Owner:  simonpj         
        Type:  feature request   |      Status:  patch           
    Priority:  normal            |   Milestone:  7.6.2           
   Component:  Compiler          |     Version:  7.0.3           
    Keywords:                    |          Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown    
  Difficulty:  Unknown           |    Testcase:                  
   Blockedby:                    |    Blocking:                  
     Related:                    |  
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Comment(by nfrisby):

 Confirmed: the Just gets ww'd-away with this core2core pipeline:
 SpecConstr, simpl, stranal, wwsplit, simpl.

 {{{
 Rec {
 $w$s$wloop
 $w$s$wloop =
   \ w_sqX w1_sqY ->
     case w_sqX of ds_Xng {
       __DEFAULT ->
         $w$s$wloop
           (-# ds_Xng 1) (case w1_sqY of _ { I# x_anM -> I# (+# x_anM 1)
 });
       0 -> (# w1_sqY #)
     }
 end Rec }

 loop_$s$wloop [InlPrag=INLINE[0]]
   :: ...
 [...
  Str=DmdType m2,
  Unf=Unf{Src=Worker=$w$s$wloop,...}]
 loop_$s$wloop =
   \ w_sqX w1_sqY ->
     case $w$s$wloop w_sqX w1_sqY of _ { (# ww1_sr4 #) -> Just ww1_sr4 }

 Rec {
 loop_$s$wloop1
 loop_$s$wloop1 =
   \ sc_sqe ->
     case sc_sqe of ds_Xng {
       __DEFAULT -> loop_$s$wloop1 (-# ds_Xng 1);
       0 -> Nothing
     }
 end Rec }
 }}}

 That's an impressive prefix, "$w$s$w".

 In case any reader is wondering, the second argument to the loop gets
 unboxed if a strict variant of Maybe is used.

 Thanks again for the compact example.

-- 
Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/5075#comment:13
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