[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 tdammers):

 Replying to [comment:13 simonpj]:
 > On to `mkInstCo`.  I see that it does an inefficient single-variable
 substitution.  So if we have
 > {{{
 > mkInstCo (mkInstCo (mkInstCo co (Refl t1))
 >                    (Refl t2))
 >          (Refl t3)
 > }}}
 > we will traverse `co` three times (and `t1` twice etc).  Bad bad.
 >
 > Idea: leave that to the coercion optimiser.  Try simply removing the
 first equation for `mkInstCo` leaving
 > {{{
 > mkInstCo = InstCo
 > }}}

 That doesn't seem to work well at all - I don't know how long exactly it
 takes, but it's been compiling Grammar.hs for about half an hour now, so I
 think it's safe to say that it's much slower now than the 8 seconds we got
 previously. Unfortunately it's still running, so I can't say where the
 time is going yet.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14737#comment:14>
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