#4301: Optimisations give bad core for foldl' (flip seq) ()
----------------------------------+-----------------------------------------
Reporter: daniel.is.fischer | Owner:
Type: bug | Status: new
Priority: normal | Milestone: 7.0.2
Component: Compiler | Version: 6.12.3
Keywords: | Testcase:
Blockedby: | Difficulty:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: Runtime performance bug
----------------------------------+-----------------------------------------
Comment(by daniel.is.fischer):
Looks closely related.
Experimenting a bit more, we get good core if we manually provide the
worker:
{{{
fseq :: [a] -> ()
fseq = lgo ()
where
lgo z [] = z
lgo z (x:xs) = (x `seq` z) `seq` lgo z xs
}}}
produces
{{{
Rec {
FSeq.fseq_lgo :: forall a_ag2 t_ag9 a1_age.
a1_age -> [t_ag9] -> a1_age
GblId
[Arity 2
NoCafRefs
Str: DmdType SS]
FSeq.fseq_lgo =
\ (@ a_ag2)
(@ t_ag9)
(@ a1_age)
(z_afd :: a1_age)
(ds_dgC :: [t_ag9]) ->
case ds_dgC of _ {
[] -> z_afd;
: x_aff xs_afg ->
case x_aff of _ { __DEFAULT ->
FSeq.fseq_lgo @ a_ag2 @ t_ag9 @ a1_age z_afd xs_afg
}
}
end Rec }
}}}
with a phantom type argument a_ag2. Presumably, that's to prevent the
elimination of the constant argument? But if we give the worker the more
restricted type,
{{{
spseq :: [a] -> ()
spseq = sgo ()
where
sgo :: () -> [a] -> ()
sgo z [] = z
sgo z (x:xs) = (x `seq` z) `seq` sgo z xs
}}}
we get the non-tail-call worker again:
{{{
Rec {
FSeq.$wsgo :: forall a_afS a1_afr. [a1_afr] -> (##)
GblId
[Arity 1
NoCafRefs
Str: DmdType S]
FSeq.$wsgo =
\ (@ a_afS) (@ a1_afr) (w_shQ :: [a1_afr]) ->
case case w_shQ of _ {
[] -> GHC.Unit.();
: x_afu xs_afv ->
case x_afu of _ { __DEFAULT ->
case FSeq.$wsgo @ a_afS @ a1_afr xs_afv of _ { (# #) ->
GHC.Unit.()
}
}
}
of _ { () ->
GHC.Prim.(##)
}
end Rec }
}}}
making the worker strict in z,
{{{
okseq :: [a] -> ()
okseq = okgo ()
where
okgo :: () -> [a] -> ()
okgo !z [] = z
okgo z (x:xs) = (x `seq` z) `seq` okgo z xs
}}}
we get perfect core again:
{{{
Rec {
FSeq.$wokgo :: forall a_afI a1_afx. [a1_afx] -> ()
GblId
[Arity 1
NoCafRefs
Str: DmdType S]
FSeq.$wokgo =
\ (@ a_afI) (@ a1_afx) (w_shJ :: [a1_afx]) ->
case w_shJ of _ {
[] -> GHC.Unit.();
: ipv_sgL ipv1_sgM ->
case ipv_sgL of _ { __DEFAULT ->
FSeq.$wokgo @ a_afI @ a1_afx ipv1_sgM
}
}
end Rec }
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4301#comment:2>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
_______________________________________________
Glasgow-haskell-bugs mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
