[GHC] #11735: Optimize coercionKind

[GHC]

#11735: Optimize coercionKind
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        Reporter:  goldfire          |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             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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Comment (by goldfire):

 OK. I looked at `pushCoTyArg` and friends, and I have a very simple
 solution: just move the `isReflexiveCo` case in `addCoerce` (a local
 function within `Simplify.simplCast`) to the top. That should do it. Then
 `pushCoTyArg` is never called with a reflexive coercion, and so the
 `piResultTy` case won't happen.

 Now, `pushCoArgs` might still call `pushCoTyArg` with a reflexive
 coercion, but it can be taught not to as well: Have `pushCoArgs` return a
 `Maybe ([CoreArg], Maybe Coercion)` and `pushCoArg` return a `Maybe
 (CoreArg, Maybe Coercion)`. If the second return values are `Nothing`,
 that means that there is no cast (i.e., that the cast would have been
 reflexive). The only client of `pushCoArg(s)` is `exprIsConApp_maybe`,
 which simply omits a cast if `pushCoArgs` returns `Nothing`. Then, we
 never have to bother creating the reflexive coercions.

 This should be an easy win all around.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11735#comment:41>
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