#2439: Missed optimisation with dictionaries and loops
-----------------------------------------+----------------------------------
Reporter: rl | Owner:
Type: run-time performance bug | Status: new
Priority: normal | Component: Compiler
Version: 6.9 | Severity: normal
Keywords: | Testcase:
Architecture: Unknown | Os: Unknown
-----------------------------------------+----------------------------------
{{{
{-# LANGUAGE BangPatterns #-}
module Foo (sum') where
foldl' :: (a -> b -> a) -> a -> [b] -> a
{-# INLINE foldl' #-}
foldl' f !z xs = loop z xs
where
loop !z [] = z
loop !z (x:xs) = loop (f z x) xs
sum' :: Num a => [a] -> a
sum' xs = foldl' (+) 0 xs
}}}
This is the code before !LiberateCase:
{{{
Foo.sum' =
\ (@ a_a9T) ($dNum_aa1 [ALWAYS Just L] :: GHC.Num.Num a_a9T) ->
let {
lit_scm [ALWAYS Just L] :: a_a9T
[Str: DmdType]
lit_scm =
case $dNum_aa1
of tpl_B1 [ALWAYS Just A]
{ GHC.Num.:DNum tpl_B2 [ALWAYS Just A]
tpl_B3 [ALWAYS Just A]
tpl_B4 [ALWAYS Just A]
tpl_B5 [ALWAYS Just A]
tpl_B6 [ALWAYS Just A]
tpl_B7 [ALWAYS Just A]
tpl_B8 [ALWAYS Just A]
tpl_B9 [ALWAYS Just A]
tpl_Ba [ALWAYS Just C(S)] ->
tpl_Ba lvl_sbH
} } in
letrec {
loop_sck [ALWAYS LoopBreaker Nothing] :: a_a9T -> [a_a9T] -> a_a9T
[Arity 2
Str: DmdType SS]
loop_sck =
\ (z_a6Y :: a_a9T) (ds_db7 :: [a_a9T]) ->
case z_a6Y of z_X7h [ALWAYS Just L] { __DEFAULT ->
case ds_db7 of wild_B1 [ALWAYS Just A] {
[] -> z_a6Y;
: x_a72 [ALWAYS Just L] xs_a74 [ALWAYS Just S] ->
case $dNum_aa1
of tpl_Xl [ALWAYS Just A]
{ GHC.Num.:DNum tpl_B2 [ALWAYS Just A]
tpl_B3 [ALWAYS Just A]
tpl_B4 [ALWAYS Just C(C(S))]
tpl_B5 [ALWAYS Just A]
tpl_B6 [ALWAYS Just A]
tpl_B7 [ALWAYS Just A]
tpl_B8 [ALWAYS Just A]
tpl_B9 [ALWAYS Just A]
tpl_Ba [ALWAYS Just A] ->
loop_sck (tpl_B4 z_a6Y x_a72) xs_a74
}
}
}; } in
\ (xs_a76 :: [a_a9T]) -> loop_sck lit_scm xs_a76
}}}
Note that the Num dictionary is scrutinised in the loop even though sum'
is actually strict in the dictionary (by virtue of being strict in
lit_scm) and it would make sense to take it apart before entering the
loop. !LiberateCase does nail this but only if the loop is small enough
and at the expense of code size.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2439>
GHC <http://www.haskell.org/ghc/>
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