[GHC] #9198: large performance regression in type checker speed in 7.8

[GHC]

#9198: large performance regression in type checker speed in 7.8
-------------------------------------+-------------------------------------
              Reporter:  carter      |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  high        |        Milestone:  7.10.1
             Component:  Compiler    |          Version:  7.8.2
  (Type checker)                     |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:  Compile-    |  Related Tickets:
  time performance bug               |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
-------------------------------------+-------------------------------------

Old description:

> it was recently demonstrated on the haskell-cafe mailing list that  the
> following code takes measurably longer to type check in GHC 7.8.2 than in
> GHC  7.6.
>
> http://www.haskell.org/pipermail/haskell-cafe/2014-June/114562.html
>

> the program example is as follows
> {{{
> begin cont = cont (return ())
> a m =cont (m >> putstrn "a")
> end m = m
> main = begin a a a a a a a a a a a a a a a a a end
>
> }}}
> takes about a second to type check in ghc 7.8, and is "instant" in 7.6 ()
>
> each additional a doubles the time to type check the program
>
> {{{
> main = begin a a a a a a a a a a a a a a a a a a a a a a a a end
> }}}
>
> takes longer than I have the patience to wait :) (so more than 5 seconds)
>
> the author of the email notes that a related program
>
> {{{
>   main = id id id id id id id id id id id
>          id id id id id id id id id id id (return ())
> }}}
> has the exponential complexity in both 7.8.2 and 7.6.2
>
> This It could very well be that some of the type checker changes between
> 7.8 and 7.6 enlarged the space of programs that trip up exponential worst
> case behavior, but one way or another understanding *why* this happened

New description:

 it was recently demonstrated on the haskell-cafe mailing list that  the
 following code takes measurably longer to type check in GHC 7.8.2 than in
 GHC  7.6.

 http://www.haskell.org/pipermail/haskell-cafe/2014-June/114562.html


 the program example is as follows
 {{{
 begin :: Monad m => (m () -> t) -> t
 begin cont = cont (return ())

 a :: IO a -> (IO () -> t) -> t
 a m cont = cont (m >> putStrLn "a")

 end :: t -> t
 end m = m

 main = begin a a a a a a a a a a a a a a a a a a end
 }}}
 takes about a second to type check in ghc 7.8, and is "instant" in 7.6 ()

 each additional a doubles the time to type check the program

 {{{
 main = begin a a a a a a a a a a a a a a a a a a a a a a a a end
 }}}

 takes longer than I have the patience to wait :) (so more than 5 seconds)

 the author of the email notes that a related program

 {{{
   main = id id id id id id id id id id id
          id id id id id id id id id id id (return ())
 }}}
 has the exponential complexity in both 7.8.2 and 7.6.2

 This It could very well be that some of the type checker changes between
 7.8 and 7.6 enlarged the space of programs that trip up exponential worst
 case behavior, but one way or another understanding *why* this happened

--

Comment (by thomie):

 For what it's worth: crude timing results for the example from the
 description with 18 `a`'s:

 ||= GHC =||= Time =||
 ||= 7.4.2 =||  2.2s  ||
 ||= 7.6.3 =||  2.2s  ||
 ||= 7.8.3 =||  3.4s  ||
 ||= 7.9.20141113 =||  3.8s  ||

 Command: `time ghc -c -fforce-recomp test.hs`

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9198#comment:4>
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