#4830: Simplifier does case-to-let too eagerly
---------------------------------+------------------------------------------
Reporter: simonpj | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.0.1
Keywords: | Testcase:
Blockedby: | Difficulty:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
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Roman found an example where the simplifier was turning a case expression
to a let binding when it shouldn't
{{{
foo :: Int -> Maybe (Double,Double) -> Double
foo _ Nothing = 0
foo 0 (Just (x,y)) = x+y
foo n (Just (x,y)) = let r = f x y in r `seq` foo (n-1) (Just r)
where
f x y | x <= y = (x,y)
| otherwise = (y,x)
}}}
GHC 7.0.1 generates this
{{{
foo =
\ (ds_dks :: Int) (ds_dkt :: Maybe (Double, Double)) ->
case ds_dkt of _ {
Nothing -> lvl_sly;
Just ipv_skO ->
case ds_dks of _ { I# ds_dku ->
case ds_dku of ds_XkC {
__DEFAULT ->
case ipv_skO of _ { (x_aax, y_aay) ->
case x_aax of wild_akY { D# x_al0 ->
case y_aay of wild1_al2 { D# y_al4 ->
foo
(I# (-# ds_XkC 1))
(Just
@ (Double, Double)
(case <=## x_al0 y_al4 of _ {
False -> (wild1_al2, wild_akY); True -> (wild_akY,
wild1_al2)
}))
}
}
};
0 -> case ipv_skO of _ { (x_aau, y_aav) -> plusDouble x_aau
y_aav }
}
}
}
}}}
That `<=##` should be outside the call to `foo` not inside. (The code
isn't wrong, it's just less efficient than it should be.)
I know what's wrong. Patch coming.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4830>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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