[GHC] #12765: Don't optimize coercions with -O0

[GHC]

#12765: Don't optimize coercions with -O0
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           Reporter:  bgamari        |             Owner:
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:  8.2.1
          Component:  Compiler       |           Version:  8.0.1
           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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 During the call today Simon and I discussed the possibility of omitting
 coercion optimization from the desugarer's simple optimization
 (`simpleOptPgm`) when compiling with `-O0`. The justification here was to
 perhaps speed-up unoptimized compilation times for programs like that of
 #12506, which has very large coercions (presumably due to #8095).

 The rationale was that while this first pass of coercion simplification
 during desugaring may reduce the size of some of these coercions, we only
 walk over them once more during code generation, so there's little benefit
 given the cost.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12765>
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