[GHC] #11284: Lambda-lifting fails in simple Text example

[GHC]

#11284: Lambda-lifting fails in simple Text example
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        Reporter:  bgamari           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by bgamari):

 * failure:  None/Unknown => Runtime performance bug


Old description:

> Consider the example,
> {{{#!hs
> import Data.Char (isSpace)
> import Data.List (foldl')
> import GHC.Exts (build)
> import qualified  Data.Text as T
>
> longestWord :: T.Text -> Int
> longestWord t = foldl' max 0 $ map T.length $ fusedWords t
>
> fusedWords :: T.Text -> [T.Text]
> fusedWords t0 = build $ \cons nil ->
>   let go !t
>         | T.null t  = nil
>         | otherwise = let (w, rest) = T.span (not . isSpace) t
>                       in cons w (go $ T.dropWhile isSpace rest)
>   in go t0
> }}}

New description:

 Consider the example (which uses `Text`; I'm working on finding a more
 minimal example),
 {{{#!hs
 import Data.Char (isSpace)
 import Data.List (foldl')
 import GHC.Exts (build)
 import qualified  Data.Text as T

 longestWord :: T.Text -> Int
 longestWord t = foldl' max 0 $ map T.length $ fusedWords t

 fusedWords :: T.Text -> [T.Text]
 fusedWords t0 = build $ \cons nil ->
   let go !t
         | T.null t  = nil
         | otherwise = let (w, rest) = T.span (not . isSpace) t
                       in cons w (go $ T.dropWhile isSpace rest)
   in go t0
 }}}

 `longestWord` here produces the simplified Core`,

 {{{#!hs
 Rec {
 Ticket.$wgo1 :: [T.Text] -> GHC.Prim.Int# -> GHC.Prim.Int#
 Ticket.$wgo1 =
   \ (w_s4GJ :: [T.Text]) (ww_s4GN :: GHC.Prim.Int#) ->
     case w_s4GJ of _ {
       [] -> ww_s4GN;
       : y_a4vC ys_a4vD ->
         case y_a4vC
         of _ { Data.Text.Internal.Text dt_a4jP dt1_a4jQ dt2_a4jR ->
         let {
           a_a4jO :: GHC.Prim.Int#
           a_a4jO = GHC.Prim.+# dt1_a4jQ dt2_a4jR } in
         letrec {
           -- Why must you allocate? For the love of all that is good, why?
           $wloop_length_s4GI :: GHC.Prim.Int# -> GHC.Prim.Int# ->
 GHC.Prim.Int#
           $wloop_length_s4GI =
             \ (ww1_s4Gz :: GHC.Prim.Int#) (ww2_s4GD :: GHC.Prim.Int#) ->
               case GHC.Prim.tagToEnum# @ Bool (GHC.Prim.>=# ww2_s4GD
 a_a4jO) -- bounds check
               of _ {
                 False -> {
                   ...
                   -- in this body there are few cases analyses with
                   -- recursive calls of the form
                   $wloop_length_s4GI (GHC.Prim.+# ww1_s4Gz 1) (GHC.Prim.+#
 ww2_s4GD 1)
                   ...
                 True -> ww1_s4Gz
               }; } in
         case $wloop_length_s4GI 0 dt1_a4jQ of ww1_s4GH { __DEFAULT ->
         case GHC.Prim.tagToEnum# @ Bool (GHC.Prim.<=# ww_s4GN ww1_s4GH)
         of _ {
           False -> Ticket.$wgo1 ys_a4vD ww_s4GN;
           True -> Ticket.$wgo1 ys_a4vD ww1_s4GH
         }
         }
         }
     }
 end Rec }

 longestWord :: T.Text -> Int
 longestWord =
   \ (w_s4GT :: T.Text) ->
     case w_s4GT
     of _ { Data.Text.Internal.Text ww1_s4GW ww2_s4GX ww3_s4GY ->
     case Ticket.$wgo1 (Ticket.$wgo ww1_s4GW ww2_s4GX ww3_s4GY) 0
     of ww4_s4H2 { __DEFAULT ->
     GHC.Types.I# ww4_s4H2
     }
     }
 }}}

 Notice `$wloop_length_s4GI`: It should be a nice tight loop traversing
 `dt_a4jP :: ByteArray#` until it finds whitespace. However, GHC fails to
 lambda-lift this closure, thereby turning it into an allocating operation!
 Oh no!

--

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11284#comment:1>
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