[GHC] #4065: Inconsistent loop performance

[GHC]

#4065: Inconsistent loop performance
---------------------------------+------------------------------------------
    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
---------------------------------+------------------------------------------
 Here are two small benchmarks:

 {{{
 import Criterion.Main

 foo :: Int -> Int -> Int
 foo n k | n <= 0    = k
         | otherwise = foo (n-1) (k+1)

 bar :: Int -> Int -> Int
 bar n k | n == 0    = k
         | otherwise = bar (n-1) (k+1)

 main :: IO ()
 main = defaultMain [ bench "foo" $ nf (uncurry foo) (20000000,0)
                    , bench "bar" $ nf (uncurry bar) (20000000,0)
                    ]
 }}}

 On my laptop, I consistently get a mean running time of about 36ms for
 `foo` and about 45ms for `bar`. This is a rather big difference. The Core
 looks about the same and going via C gives me about 42ms for both loops so
 this is certainly a code generator problem.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4065>
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