[GHC] #12754: Adding an explicit export list halves compilation time.

[GHC]

#12754: Adding an explicit export list halves compilation time.
-------------------------------------+-------------------------------------
        Reporter:  mpickering        |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            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 mpickering):

 Bartosz game me the command he was using.

 {{{
 rm A.{hs,o,hi}; python gen12754.py > A.hs && time ghc A.hs
 }}}

 and I ran the test again, here are my 4 runs.

 {{{
 [1 of 1] Compiling A                ( A.hs, A.o )

 real    0m26.460s
 user    0m24.643s
 sys     0m0.711s
 ~/Documents/haskell/ghc:vim gen.py
 ~/Documents/haskell/ghc:rm A.{hs,o,hi}; python gen.py > A.hs && time ghc
 A.hs
 [1 of 1] Compiling A                ( A.hs, A.o )

 real    0m15.743s
 user    0m15.100s
 sys     0m0.402s
 ~/Documents/haskell/ghc:vim gen.py
 ~/Documents/haskell/ghc:rm A.{hs,o,hi}; python gen.py > A.hs && time ghc
 A.hs
 [1 of 1] Compiling A                ( A.hs, A.o )

 real    0m24.858s
 user    0m24.283s
 sys     0m0.392s
 ~/Documents/haskell/ghc:vim gen.py
 ~/Documents/haskell/ghc:rm A.{hs,o,hi}; python gen.py > A.hs && time ghc
 A.hs
 [1 of 1] Compiling A                ( A.hs, A.o )

 real    0m12.098s
 user    0m11.672s
 sys     0m0.321s
 }}}

 so I do see a big difference.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12754#comment:4>
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