[GHC] #14737: Improve performance of Simplify.simplCast

[GHC]

#14737: Improve performance of Simplify.simplCast
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        Reporter:  tdammers          |                Owner:  goldfire
            Type:  bug               |               Status:  patch
        Priority:  highest           |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.2.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #11735 #14683     |  Differential Rev(s):  Phab:D4385
       Wiki Page:                    |  Phab:D4568
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Comment (by simonpj):

 OK, well commit the patch, and let's investigate the regressions.  The
 places I'd start would be (distinct alternatives)

 * Try a `cheapEqType`, so we get something in between `isReflCo` (dirt
 cheap) and `isReflexiveCo` (which uses `eqType`, can be expensive).

 * Take a regression and see what the (presumably) bigger coercion look
 like.  Then we can see if there's an easy way to kill them off.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14737#comment:29>
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