#3138: Returning a known constructor: GHC generates terrible code for cmonad
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Reporter: simonpj | Owner:
Type: run-time performance bug | Status: new
Priority: normal | Milestone: 6.12 branch
Component: Compiler | Version: 6.10.1
Severity: normal | Resolution:
Keywords: | Difficulty: Unknown
Testcase: | Os: Unknown/Multiple
Architecture: Unknown/Multiple |
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Comment (by augustss):
I expect the code to be the same as the code for
{{{
main = do
rs <- newIORef 0
ri <- newIORef 0
writeIORef ri 1
let loop = do
i <- readIORef ri
if i < 1e3 then do
s <- readIORef s
writeIORef rs (s + 1/i)
writeIORef ri (i+1)
loop
else
return ()
loop
s <- readIORef rs
putStrLn $ "Almost infinity is " ++ show s
}}}
Because that's what the code does. Getting that far would require
specializing the recursive while function (why doesn't ghc specialize
recursive functions?).
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3138#comment:3>
GHC <http://www.haskell.org/ghc/>
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