[GHC] #2439: Missed optimisation with dictionaries and loops

[GHC]

#2439: Missed optimisation with dictionaries and loops
---------------------------------+------------------------------------------
    Reporter:  rl                |       Owner:  simonpj                
        Type:  bug               |      Status:  new                    
    Priority:  lowest            |   Milestone:  7.6.2                  
   Component:  Compiler          |     Version:  6.9                    
    Keywords:                    |          Os:  Unknown/Multiple       
Architecture:  Unknown/Multiple  |     Failure:  Runtime performance bug
  Difficulty:  Unknown           |    Testcase:                         
   Blockedby:                    |    Blocking:                         
     Related:                    |  
---------------------------------+------------------------------------------

Comment(by morabbin):

 For the original example, `-fdicts-strict` seems to make no difference now
 (i.e., it seems the "right" code is now produced even without `-fdicts-
 strict`):
 {{{
 Foo.sum' :: forall a_aeH. GHC.Num.Num a_aeH => [a_aeH] -> a_aeH
 [GblId, Arity=2, Caf=NoCafRefs]
 Foo.sum' =
   \ (@ a_c) ($dNum_afi :: GHC.Num.Num a_c) (xs_afb :: [a_c]) ->
     let {
       f_af2 :: a_c -> a_c -> a_c
       [LclId]
       f_af2 = GHC.Num.+ @ a_c $dNum_afi } in
     case GHC.Num.fromInteger @ a_c $dNum_afi (__integer 0)
     of z_Xf9 { __DEFAULT ->
     letrec {
       loop_aff [Occ=LoopBreaker] :: a_c -> [a_c] -> a_c
       [LclId, Arity=2]
       loop_aff =
         \ (z1_af6 :: a_c) (ds_dfG :: [a_c]) ->
           case z1_af6 of z2_Xfg { __DEFAULT ->
           case ds_dfG of _ {
             [] -> z2_Xfg;
             : x_af8 xs1_af9 -> loop_aff (f_af2 z2_Xfg x_af8) xs1_af9
           }
           }; } in
     loop_aff z_Xf9 xs_afb
     }
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2439#comment:16>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler