[GHC] #12847: ghci -fobject-code -O2 doesn't do the same optimisations as ghc --make -O2

[GHC]

#12847: ghci -fobject-code -O2 doesn't do the same optimisations as ghc --make -O2
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           Reporter:  nh2            |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Other
       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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 From IRC:

 I have a module that takes around 10 seconds to compile on ghc --make -O2,
 but only around 3 seconds on ghci -fobject-code -O2.

 > bgamari: that is surprising

 I've put the -v timing output of the two cases into
 https://gist.github.com/nh2/e9bc4732111bfe422d8e606c10f8f0ac

 I'm comparing the 2 files with `meld` for spotting differences easily.

 Check e.g. `Result size of Tidy Core` which seems to have 3x as many terms
 for ghc, and `Simplifier [Mymodule]` doesn't seem to run in ghci

 The first time terms are different is at `Desugar, the first time a factor
 3 comes in is after `Specialize`.

 > bgamari: wow, it sounds like the simplifier isn't getting run as
 aggressively in GHCi


 I'm running this on Ubuntu 16.04 inside nix (but happens the same way
 outside nix).

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12847>
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