[GHC] #10260: last uses too much space with optimizations disabled

[GHC]

#10260: last uses too much space with optimizations disabled
-------------------------------------+-------------------------------------
        Reporter:  rwbarton          |                   Owner:  nomeata
            Type:  bug               |                  Status:  patch
        Priority:  normal            |               Milestone:
       Component:  Core Libraries    |                 Version:  7.10.1
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:  Phab:D844
-------------------------------------+-------------------------------------

Comment (by nomeata):

 I had a closer look, and the problem is the following:

 When we added the implementation

 {{{
 last = foldl (\_ x -> x) (error "..."))
 }}}

 to `GHC.List` in #9339, we assumed (and I thought we checked, but maybe
 that not true) that GHC would optimize  the core will look something like
 this:

 {{{
 last :: forall a_ask. [a_ask] -> a_ask
 [GblId,
  Arity=1,
  Str=DmdType,
  Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True,
          WorkFree=True, Expandable=True, Guidance=IF_ARGS [] 30 60}
          Tmpl=\ (@ a_aTE) -> foldl @ a_aTE @ a_aTE (GHC.List.last2 @
 a_aTE) (GHC.List.last1 @ a_aTE)]
 last = .. efficient implementation derived from foldl ... ..
 }}}

 This way, when using `last` in code compiled without `-O`, the efficient
 variant would be called, while with `-O` the unfolding would be used and
 optimized on the spot, possibly fusing, and producing good code if Call
 Arity kicks in.

 Unfortunately, this is what we see:
 {{{
 last :: forall a_ask. [a_ask] -> a_ask
 [GblId,
  Arity=1,
  Str=DmdType,
  Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True,
          WorkFree=True, Expandable=True, Guidance=IF_ARGS [] 30 60}]
 last =
   \ (@ a_aTE) ->
     foldl
       @ a_aTE @ a_aTE (GHC.List.last2 @ a_aTE) (GHC.List.last1 @ a_aTE)
 }}}

 Now the question is: Why is the code not optimized in `GHC.List`? My guess
 is that we should have written

 {{{
 last xs = foldl (\_ x -> x) (error "...")) xs
 }}}

 And indeed, this way, we get:

 {{{
 Rec {
 GHC.List.last2 [Occ=LoopBreaker]
   :: forall a_aTF. [a_aTF] -> a_aTF -> a_aTF
 [GblId, Arity=2, Caf=NoCafRefs, Str=DmdType <S,1*U><L,1*U>]
 GHC.List.last2 =
   \ (@ a_aTF) (ds_a1Es :: [a_aTF]) (eta_B1 :: a_aTF) ->
     case ds_a1Es of _ [Occ=Dead] {
       [] -> eta_B1;
       : y_a1Ex ys_a1Ey -> GHC.List.last2 @ a_aTF ys_a1Ey y_a1Ex
     }
 end Rec }

 last :: forall a_ask. [a_ask] -> a_ask
 [GblId,
  Arity=1,
  Str=DmdType <S,1*U>,
  Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True,
          WorkFree=True, Expandable=True, Guidance=IF_ARGS [0] 30 0}]
 last =
   \ (@ a_aTF) (xs_ast :: [a_aTF]) ->
     GHC.List.last2 @ a_aTF xs_ast (GHC.List.last1 @ a_aTF)
 }}}

 which his nice, but now we lost the original definition.

 Next try, also adding `{-# INLINEABLE  last #-}`, which yields.

 {{{
 Rec {
 poly_go_r2F3 :: forall a_aTF. [a_aTF] -> a_aTF -> a_aTF
 [GblId, Arity=2, Caf=NoCafRefs, Str=DmdType <S,1*U><L,1*U>]
 poly_go_r2F3 =
   \ (@ a_aTF) (ds_a1Es :: [a_aTF]) (eta_B1 :: a_aTF) ->
     case ds_a1Es of _ [Occ=Dead] {
       [] -> eta_B1;
       : y_a1Ex ys_a1Ey -> poly_go_r2F3 @ a_aTF ys_a1Ey y_a1Ex
     }
 end Rec }

 last [InlPrag=INLINABLE[ALWAYS]] :: forall a_ask. [a_ask] -> a_ask
 [GblId,
  Arity=1,
  Str=DmdType <S,1*U>,
  Unf=Unf{Src=InlineStable, TopLvl=True, Value=True, ConLike=True,
          WorkFree=True, Expandable=True, Guidance=IF_ARGS [0] 160 0
          Tmpl= \ (@ a_aTF) (xs_ast [Occ=Once] :: [a_aTF]) ->
                  foldr
                    @ a_aTF
                    @ (a_aTF -> a_aTF)
                    (\ (ds_d1DA [Occ=Once] :: a_aTF)
                       (ds1_d1DB [Occ=Once!, OS=OneShot] :: a_aTF -> a_aTF)
                       _ [Occ=Dead, OS=OneShot] ->
                       ds1_d1DB ds_d1DA)
                    (id @ a_aTF)
                    xs_ast
                    (errorEmptyList
                       @ a_aTF
                       (build
                          @ Char (\ (@ b_a1Fd) -> unpackFoldrCString# @
 b_a1Fd "last"#)))}]
 last =
   \ (@ a_aTF) (xs_ast :: [a_aTF]) ->
     poly_go_r2F3 @ a_aTF xs_ast (lvl8_r2F2 @ a_aTF)
 }}}

 That looks better. The unfolding is a bit more complicated, as `foldl` is
 being lined, but that is presumably ok.

 I’ll create a pull request with this in a small while, for review and
 discussion.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10260#comment:6>
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