[GHC] #5970: Type checker hangs

[GHC]

#5970: Type checker hangs
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 Reporter:  Lemming                       |          Owner:                         
     Type:  bug                           |         Status:  new                    
 Priority:  normal                        |      Component:  Compiler (Type checker)
  Version:  7.4.1                         |       Keywords:                         
       Os:  Linux                         |   Architecture:  x86                    
  Failure:  Compile-time performance bug  |       Testcase:                         
Blockedby:                                |       Blocking:                         
  Related:                                |  
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 When compiling my synthesizer-llvm package, GHC-7.4.1 hangs in the type
 checking phase for the module
 http://code.haskell.org/synthesizer/llvm/src/Synthesizer/LLVM/Server/Packed/Instrument.hs

 This module always needed more than a minute for type-checking in
 GHC-7.2.2 and before. I guess this was because it is a module where
 functions with a lot of type-level arithmetic from the type-level package
 are applied to values of concrete types. Maybe with GHC-7.4.1 the
 compilation time simply became even longer - at least several minutes.

 I tried to simplify the example and reduce the dependencies. This is not
 so simple because reducing the module size also decreases type checking
 time and when drastically simplified the compilation time for that module
 is ok. Now I am trying to construct an example where I repeat the same
 (simple) function definition. I already observed that repeating a function
 definition n times does not multiply the type checking by n but it needs
 considerably more time. I think this can be considered a bug.

 I am investigating further. If you have some advice, I like to know. Maybe
 you remember a 'foldl' that should be a 'foldr' in the type checker, that
 may cause a quadratic type checking time ... If so then this would be a
 good opportunity to fix it. ;-)

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5970>
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