[GHC] #12506: Compile time regression in GHC 8.

[GHC]

#12506: Compile time regression in GHC 8.
-------------------------------------+-------------------------------------
        Reporter:  deech             |                Owner:  bgamari
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by bgamari):

 The issue here seems to be the `Tree` module. With `-O1` compilation takes
 four minutes with much of the time being spent in the simplifier.

 However, deech said that his real concern was the compilation time with
 `-O0`. The culprit here appears to be desugaring, which takes over 10
 seconds of the 30 second total compilation time (with a profiled
 compiler).

 Looking into the profile, it seems that the culprit is `simpleOptPgm`. In
 particular, it appears that the majority of desugaring time is spent in
 `optCoercion` called from the `Cast` branch of `simple_opt_expr.go`.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12506#comment:8>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler