[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 goldfire):

 Interestingly, the coercion optimizer will '''not''' work on `InstCo ...
 (Refl ...)`.

 Other points of interest here:
  - The first line of the `opt_co4` case for `InstCo` looks plain wrong,
 extending the substitution with a use of `kind_co`, where `kind_co` has
 not, itself, been substituted.

  - `opt_co4` uses `splitForAllCo_maybe`, which doesn't look for `Refl`s.
 Perhaps it should.

  - The ''only'' way `InstCo`s can come into being is in the coercion
 optimizer. There is no call to `mkInstCo` beyond it. So perhaps we can
 take that into account when designing these functions.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14737#comment:15>
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