#4081: Strict constructor fields inspected in loop
---------------------------------+------------------------------------------
Reporter: rl | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 6.13 | Keywords:
Os: Unknown/Multiple | Testcase:
Architecture: Unknown/Multiple | Failure: Runtime performance bug
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Here is a small example to illustrate the problem:
{{{
module T where
data S a b = S !a !b
class C a where
make :: a -> S a a
instance C Int where
{-# NOINLINE make #-}
make n = S n n
foo :: (C a, Num a) => a -> Int -> a
{-# INLINE foo #-}
foo x k = k `seq` m `seq` go k 0
where
S m n = make x
go 0 i = i
go k i = go (k-1) (i + m)
}}}
{{{
module U where
import T
bar :: Int -> Int -> Int
bar s k = foo s k + 1
}}}
Relying on !LiberateCase seems to be the only way to unbox `m` outside of
the loop in `bar`. The seq in `foo` doesn't help because it gets
eliminated immediately.
GHC does have enough information to do this:
{{{
U.bar =
\ (s_aaw [Dmd=Just S(A)] :: GHC.Types.Int)
(k_aax [Dmd=Just U(L)] :: GHC.Types.Int) ->
case k_aax
of k1_ajh [Dmd=Just U(L)] { GHC.Types.I# ipv_ajj [Dmd=Just A] ->
case T.$fCInt_$cmake s_aaw of _ { T.S m_ajy [Dmd=Just U(T)] _ ->
...
}}}
Note the demand on `m`. If it was an argument instead of a local binding,
it would be unboxed by w/w.
Also, the seq does help if we use lazy pairs instead of strict ones.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4081>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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