[GHC] #8766: length [Integer] is twice as slow but length [Int] is 10 times faster

[GHC]

#8766: length [Integer] is twice as slow but length [Int] is 10 times faster
------------------------------+--------------------------------------------
       Reporter:  George      |             Owner:
           Type:  bug         |            Status:  new
       Priority:  normal      |         Milestone:
      Component:  Compiler    |           Version:  7.8.1-rc1
       Keywords:              |  Operating System:  Unknown/Multiple
   Architecture:              |   Type of failure:  Runtime performance bug
  Unknown/Multiple            |         Test Case:
     Difficulty:  Unknown     |          Blocking:
     Blocked By:              |
Related Tickets:              |
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 Compared to 7.6.3 length in 7.8.1-rc1 has a performance regression:
 {{{
  ghc --version
 The Glorious Glasgow Haskell Compilation System, version 7.8.20140130
 bash-3.2$ ghc -O2 LengthIntegerList.hs
 [1 of 1] Compiling Main             ( LengthIntegerList.hs,
 LengthIntegerList.o )
 Linking LengthIntegerList ...
 bash-3.2$ time ./LengthIntegerList
 1073741824

 real    0m45.344s
 user    0m44.230s
 sys     0m0.494s
 bash-3.2$ /usr/bin/ghc --version
 The Glorious Glasgow Haskell Compilation System, version 7.6.3
 bash-3.2$ /usr/bin/ghc -O2 LengthIntegerList.hs
 [1 of 1] Compiling Main             ( LengthIntegerList.hs,
 LengthIntegerList.o )
 Linking LengthIntegerList ...
 bash-3.2$ time ./LengthIntegerList
 1073741824

 real    0m22.769s
 user    0m22.042s
 sys     0m0.385s
 bash-3.2$ cat LengthIntegerList.hs
 {-# OPTIONS_GHC -Wall #-}

 module Main where

 main :: IO()
 main = print $ length [1..(2^(30::Int)::Integer)]


 }}}
 thus length of [Integer] is twice as slow in rc1 but length of [Int] is 10
 times faster:
 {{{
  ghc --version
 The Glorious Glasgow Haskell Compilation System, version 7.8.20140130
 bash-3.2$ ghc -O2 LengthIntList.hs
 [1 of 1] Compiling Main             ( LengthIntList.hs, LengthIntList.o )
 Linking LengthIntList ...
 bash-3.2$ time ./LengthIntList
 1073741824

 real    0m0.723s
 user    0m0.693s
 sys     0m0.003s
 bash-3.2$ /usr/bin/ghc --version
 The Glorious Glasgow Haskell Compilation System, version 7.6.3
 bash-3.2$ /usr/bin/ghc -O2 LengthIntList.hs
 [1 of 1] Compiling Main             ( LengthIntList.hs, LengthIntList.o )
 Linking LengthIntList ...
 bash-3.2$ time ./LengthIntList
 1073741824

 real    0m11.805s
 user    0m10.900s
 sys     0m0.351s
 bash-3.2$ cat LengthIntList.hs
 {-# OPTIONS_GHC -Wall #-}


 module Main where

 main :: IO()
 main = print $ length [1..(2^(30::Int)::Int)]


 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8766>
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