[GHC] #830: Compiler performance bug: large "do" expression

#830: Compiler performance bug: large "do" expression
-------------------------+--------------------------------------------------
    Reporter:  simonmar  |        Owner:         
        Type:  bug       |       Status:  new    
    Priority:  normal    |    Milestone:         
   Component:  Compiler  |      Version:  6.4.2  
    Severity:  normal    |     Keywords:         
          Os:  Unknown   |   Difficulty:  Unknown
Architecture:  Unknown   |  
-------------------------+--------------------------------------------------
From Neil Mitchell:

 Using GHC 6.4.2 on Windows and 6.4.1 on Linux, I get really poor
 compile time performance for large do blocks - for example:

 {{{
 main = do
   return ()
   return ()
   ....
   return ()
 }}}

 The following are the compile times in seconds for a given number of
 return () lines, GHC 6.4.1 on Linux, with a very beefy processor (my
 department server) and loads of RAM.

 {{{
 100    1.444
 200    4.054
 300    7.977
 400    13.967
 500    21.265
 600    30.71
 700    43.997
 }}}

 For comparison on my desktop machine I get out of heap space with
 268Mb of heap, in a few minutes for 700. Compared to Hugs and Yhc,
 which both deal with all of these files instantly.

 By splitting the 700 line do block into 7 separate do blocks, each 100
 lines, I get a compile time of 8.530.

 The particular reason I was running into this issue was with an auto
 generated file of tests, about 360 lines of reasonably lines in a do
 block, which exhausted the memory in GHCi. I have modified my test
 generator to split the files into 50 line blocks.

 Thanks

 Neil

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/830>
GHC <http://www.haskell.org/ghc/>
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