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

[GHC]

#14737: Improve performance of Simplify.simplCast
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        Reporter:  tdammers          |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       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:                    |
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Comment (by simonpj):

 Try getting rid of the first equation for `puchCoTyArg`
 {{{
 pushCoTyArg co ty
   | tyL `eqType` tyR
   = Just (ty, mkRepReflCo (piResultTy tyR ty))
 }}}
 This is another big pile of type-equalities, rather like calling
 `isReflexiveCo` at the wrong moment.

 Claim: if it happens that `tyL` = `tyR`, but we go ahead with all that
 `mkCoherenceLeftCo` stuff anyway, then the coercion optimiser will get rid
 of it later.  '''Richard''': will it?

 But try that change anyway.  NO WAY should `pushCoTyArg` take 54% of
 compile time!

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14737#comment:10>
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