#14248: GHC misses optimization opportunity -------------------------------------+------------------------------------- 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: | -------------------------------------+------------------------------------- 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 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler