[GHC] #5997: GHC fails to make function tail recursive

[GHC]

#5997: GHC fails to make function tail recursive
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 Reporter:  Khudyakov         |          Owner:                  
     Type:  bug               |         Status:  new             
 Priority:  normal            |      Component:  Compiler        
  Version:  7.4.1             |       Keywords:                  
       Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
  Failure:  None/Unknown      |       Testcase:                  
Blockedby:                    |       Blocking:                  
  Related:                    |  
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 Sometimes GHC does not compile function as tail recursive despite it looks
 like tail recursive. Here is simplified example.

 {{{
 {-# LANGUAGE BangPatterns #-}

 incompleteBetaWorker :: Double -> Double
 incompleteBetaWorker _ = loop 1 1 1 1
   where
     -- Constants
     eps = 1e-15
     -- Loop
     loop :: Double -> Int -> Double -> Double -> Double
     loop !psq !ns !term !betain
       | done      = betain'
       | otherwise = loop psq' (ns - 1) term' betain'
       where
         -- New values
         term'   = term
         betain' = betain
         psq'    = if ns < 0 then psq + 1 else psq
         -- This condition cause stack overflow
         done = db <= eps && db <= eps*betain' where db = abs term'
         -- With this it loops endlessly
         -- done = db <= eps * betain' where db = abs term'

 main :: IO ()
 main = print $ incompleteBetaWorker 0
 }}}

 If second equation for done is used function is tail recursive and works
 in constant space. if first one (uncommented) is used it isn't. From
 looking at the it looks loop is converted to several corecursive
 functions.

 Such loops appear frequently in numerical code and it's important to
 ensure that worker function is tail recursive since getting answer may
 require a lot of iteration.


 I've tested it with GHC 7.2.2 and 7.4.1. Both are affected. Example was
 compiled with -O2.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5997>
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