[GHC] #14248: GHC misses optimization opportunity

[GHC]

#14248: GHC misses optimization opportunity
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        Reporter:  vagarenko         |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Alas, your proposed optimsation changes the semantics of the function.  As
 it stands, it's not strict in `eta`, but after your transformation it has
 become strict.

 If you make it strict yourself, I think it'll probably optimise right.
 This seems to do ths trick
 {{{
 unzipVec ps =
     let (es1, es2) = unzip @n ps
         !a1 = fromList es1
         !a2 = fromList es2
     in (a1, a2)
 }}}
 gives
 {{{
 unzipVecSpecialized
   = \ (eta_B1 :: [(Float, Float)]) ->
       case eta_B1 of {
         [] -> case lvl20_r2SX of wild1_00 { };
         : x_a14H xs_a14I ->
           case x_a14H of { (a_a14J, b_a14K) ->
           case xs_a14I of {
             [] -> case lvl20_r2SX of wild3_00 { };
             : x1_X18m xs1_X18o ->
               case x1_X18m of { (a1_X18u, b1_X18w) ->
               case a_a14J of { GHC.Types.F# dt1_a15E ->
               case a1_X18u of { GHC.Types.F# dt3_a15F ->
               case b_a14K of { GHC.Types.F# dt5_X17Q ->
               case b1_X18w of { GHC.Types.F# dt7_X17W ->
               ((Unzip.Vector2f dt1_a15E dt3_a15F)
                `cast` (Sym (Unzip.D:R:Vector2Float0[0])
                        :: (Unzip.R:Vector2Float :: *) ~R# (Vector 2 Float
 :: *)),
                (Unzip.Vector2f dt5_X17Q dt7_X17W)
                `cast` (Sym (Unzip.D:R:Vector2Float0[0])
                        :: (Unzip.R:Vector2Float :: *) ~R# (Vector 2 Float
 :: *)))
               }
               }
               }
               }
               }
           }
           }
       }
 }}}
 Does that make sense?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14248#comment:1>
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