[GHC] #149: missed CSE opportunity

[GHC]

#149: missed CSE opportunity
------------------------------+---------------------------------------------
    Reporter:  nobody         |        Owner:  nobody  
        Type:  bug            |       Status:  assigned
    Priority:  low            |    Milestone:  _|_     
   Component:  Compiler       |      Version:  5.04.2  
    Severity:  normal         |   Resolution:  None    
    Keywords:  optimisations  |   Difficulty:  Unknown 
Architecture:  Unknown        |           Os:  Unknown 
------------------------------+---------------------------------------------
Changes (by igloo):

  * architecture:  => Unknown
  * difficulty:  => Unknown
  * milestone:  => _|_
  * keywords:  => optimisations
  * os:  => Unknown

Old description:

> {{{
> Don't know if this is a bug, but it was at least
> _surprising_ to find that
>
> playerMostOccur [a] = a
> playerMostOccur (x:xs)| numOccur x (x:xs)
>                                   >
>                                   numOccur
> (playerMostOccur xs) xs
>                                   = x
>                                 | otherwise =
> playerMostOccur xs
>
> was exponentially slower when compiled with ghc-5.04.2
> -O than:
>
> playerMostOccur [a] = a
> playerMostOccur (x:xs)| numOccur x (x:xs)
>                                   >
>                                   numOccur pmo xs
>                                   = x
>                                 | otherwise = pmo
>                                   where pmo =
> playerMostOccur xs
>
> Although the student responsible for the code couldn't
> spot the
> obvious optimisation, I was expecting that GHC's
> optimiser would. :)
> If it's not a bug, could you explain it to me?
>
> -Greg(gregm.at.cs.uwa.edu.au)
> }}}

New description:

 {{{
 Don't know if this is a bug, but it was at least
 _surprising_ to find that

 playerMostOccur [a] = a
 playerMostOccur (x:xs)| numOccur x (x:xs)
                                   >
                                   numOccur
 (playerMostOccur xs) xs
                                   = x
                                 | otherwise =
 playerMostOccur xs

 was exponentially slower when compiled with ghc-5.04.2
 -O than:

 playerMostOccur [a] = a
 playerMostOccur (x:xs)| numOccur x (x:xs)
                                   >
                                   numOccur pmo xs
                                   = x
                                 | otherwise = pmo
                                   where pmo =
 playerMostOccur xs

 Although the student responsible for the code couldn't
 spot the
 obvious optimisation, I was expecting that GHC's
 optimiser would. :)
 If it's not a bug, could you explain it to me?

 -Greg(gregm.at.cs.uwa.edu.au)
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/149>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler