[GHC] #15703: Significant compilation time blowup when refactoring singletons-heavy code

[GHC]

#15703: Significant compilation time blowup when refactoring singletons-heavy code
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  8.6.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #15725            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 After some digging, I've found the commit that made compiling this program
 become so much faster. It's commit
 55a3f8552c9dc9b84e204ec6623c698912795347 (`Refactor coercion rule`). In
 the commit prior, it took this long to build:

 {{{
 $ time ~/Software/ghc2/inplace/bin/ghc-stage2 -O1 -fforce-recomp Lib.hs
 [1 of 2] Compiling Lib2             ( Lib2.hs, Lib2.o )
 [2 of 2] Compiling Lib              ( Lib.hs, Lib.o )

 real    3m17.647s
 user    3m17.776s
 sys     0m0.168s
 }}}

 But if I apply that commit, it then takes this long to build:

 {{{
 $ time ~/Software/ghc2/inplace/bin/ghc-stage2 -O1 -fforce-recomp Lib.hs
 [1 of 2] Compiling Lib2             ( Lib2.hs, Lib2.o )
 [2 of 2] Compiling Lib              ( Lib.hs, Lib.o )

 real    0m26.102s
 user    0m26.032s
 sys     0m0.108s
 }}}

 This is quite a humorous coincidence, since up to this point I had been
 under the impression that that commit had made compilation performance
 slightly //worse// overall. But this shows I was completely wrong!

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15703#comment:13>
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