[GHC] #14684: combineIdenticalAlts is only partially implemented

[GHC]

#14684: combineIdenticalAlts is only partially implemented
-------------------------------------+-------------------------------------
        Reporter:  mpickering        |                Owner:  sjakobi
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.2.2
      Resolution:                    |             Keywords:  newcomer, CSE
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D4542
       Wiki Page:                    |
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Comment (by sjakobi):

 As I haven't made any progress with this in a while, and as I'm not sure
 I'll make any soon, I'm recording where I'm currently stuck:

 While `combineIdenticalAlts` uses

 {{{
 cheapEqExpr' :: (Tickish Id -> Bool) -> Expr b -> Expr b -> Bool
 }}}

 `CSE.combineAlts` uses

 {{{
 eqExpr :: InScopeSet -> CoreExpr -> CoreExpr -> Bool
 }}}

 What I haven't figured out, is how to translate the use of `eqExpr` into
 using a `CoreMap` where `CoreExpr`s end up as the same key if they are
 equal according to `eqExpr in_scope`.

 I wanted to look at how the rest of `CSE` uses `CoreMap`s, but I didn't
 get around to that yet.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14684#comment:17>
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