[GHC] #5642: Deriving Generic of a big type takes a long time and lots of space

[GHC]

#5642: Deriving Generic of a big type takes a long time and lots of space
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        Reporter:  basvandijk        |                Owner:  bgamari
            Type:  bug               |               Status:  merge
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.3
      Resolution:                    |             Keywords:  deriving-
                                     |  perf, Generics
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:  T5642
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D2304
       Wiki Page:                    |
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Comment (by rrnewton):

 One more small experiment.  Before diving into the compiler internals, I
 thought it might be possible to do standard test case minimization.  I.e.
 can we create some stub definitions for the relevant bits of
 `GHC.Generics`, while preserving the bad behavior?

 [https://gist.github.com/rrnewton/df17230b748df4ce1f06b49831348000 The
 file in this gist] attempts to do so, using the following stub
 definitions:

 {{{
 class Generic a where
      from :: a -> M1 a

 data M1 a where
   M1 :: b -> M1 a

 data U1 p = U1
 data (:+:) (f :: * -> *) (g :: * -> *) p = L1 (f p) | R1 (g p)
 }}}

 This is a lower bound -- it fails to produce the really slow behavior.  It
 is simplified enough that the time for compiling just the "from" case
 drops from 5.7s all the way down to 0.85s.  Getting back to the bad
 behavior will require putting more of the complexity of "M1" back into
 play...

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/5642#comment:46>
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