#13144: FoatOut is not floating bottoming expressions properly -------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Consider part of Eric's program in #13143, and pretty much exactly what is in `GHC.Arr`. {{{ {-# INLINE index #-} index :: (Int, Int) -> Int -> Int index b@(l,h) i | l <= i && i < h = 0 | otherwise = indexError b i 0 {-# NOINLINE indexError #-} indexError :: (Int, Int) -> Int -> Int -> b indexError rng i tp = error (show rng) }}} Before float-out we have {{{ index = \ (b_ay8 :: (Int, Int)) (i_ayb :: Int) -> case b_ay8 of wild_Xe { (l_ay9, h_aya) -> case && (leInt l_ay9 i_ayb) (ltInt i_ayb h_aya) of { False -> indexError @ Int wild_Xe i_ayb (I# 0#); True -> GHC.Types.I# 0# } } }}} and after float-out we see {{{ index = \ (b_ay8 :: (Int, Int)) (i_ayb :: Int) -> case b_ay8 of wild_Xe { (l_ay9, h_aya) -> case && (leInt l_ay9 i_ayb) (ltInt i_ayb h_aya) of { False -> indexError @ Int b_ay8 i_ayb lvl_s2cd; True -> GHC.Types.I# 0# } } }}} We've floated the `(I# 0#)`. ''But we should have floated the entire call to `indexError` thus'': {{{ False -> lvl_xxx b_ay8 i_ayb }}} with {{{ lvl_xxx b i = indexError @Int b i (I# 0#) }}} Why? Because it makes the expression smaller and moves the error handling code out of the way. Float-out makes some attempt to to this, but it's not right yet. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13144 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler