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

[GHC]

#12506: Compile time regression in GHC 8.
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        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:                    |
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Comment (by tdammers):

 OK, got it compiled against GHC head. The output of `-ddump-timings` shows
 two obvious suspects:

 {{{
 [5 of 5] Compiling Main             ( app/Main.hs, app/Main.o )
 *** Parser [Main]:
 Parser [Main]: alloc=14721280 time=9.927
 *** Renamer/typechecker [Main]:
 Renamer/typechecker [Main]: alloc=585898184 time=956.617
 *** Desugar [Main]:
 Desugar [Main]: alloc=76966472 time=214.242
 *** Simplifier [Main]:
 Simplifier [Main]: alloc=3469053424 time=2430.489
 *** CoreTidy [Main]:
 CoreTidy [Main]: alloc=220130032 time=371.644
 *** CorePrep [Main]:
 CorePrep [Main]: alloc=7744 time=0.030
 *** CodeGen [Main]:
 CodeGen [Main]: alloc=3436008728 time=4730.477
 Linking app/Main ...
 }}}

 In other words, while compiling `Main`, we get very long timings on the
 Simplifier and CodeGen steps. I suspect that the latter is caused by the
 former.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12506#comment:20>
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