[GHC] #14684: combineIdenticalAlts is only partially implemented

[GHC]

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

 In `CoreUtils`, `Note [Combine identical alternatives]` acknowledges this
 problem, saying
 {{{
 To avoid an expensive test, we just merge branches equal to the *first*
 alternative; this picks up the common cases
      a) all branches equal
      b) some branches equal to the DEFAULT (which occurs first)
 }}}
 And indeed you are right that it'd be better to combine
 {{{
 foo x = case x of   ==>   foo x = case x of
           A -> 0                    DEFAULT -> 2
           B -> 1                    A -> 0
           C -> 2                    B -> 1
           D -> 2
 }}}
 (Reminder: in Core the DEFAULT alternative always comes first, if it
 exists.)

 Fortunately, we now have `TrieMap.CoreMap`, an efficient finite map whose
 keys are `CoreExprs`.  Using this I think we can efficiently ask (as we
 iterate oover the alternatives) "have I seen this RHS in an earlier
 alternative?".  More advanced: find the RHS that occurs most often.  Take
 care that in the common case the RHSs are (a) large and (b) different,
 then the test does not exhaustively traverse the RHSs; just looks far
 enough to establish they are different.

 This a nice well-contained task, if someone would like to have a go.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14684#comment:2>
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