[GHC] #8456: Control flow optimisations duplicate blocks

[GHC]

#8456: Control flow optimisations duplicate blocks
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        Reporter:  jstolarek                |            Owner:  jstolarek
            Type:  bug                      |           Status:  new
        Priority:  highest                  |        Milestone:  7.8.1
       Component:  Compiler                 |          Version:  7.7
      Resolution:                           |         Keywords:
Operating System:  Unknown/Multiple         |     Architecture:
 Type of failure:  Runtime performance bug  |  Unknown/Multiple
       Test Case:                           |       Difficulty:  Unknown
        Blocking:  8275                     |       Blocked By:
                                            |  Related Tickets:
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Comment (by simonmar):

 `lookup` looks up in the domain, not the range.  You would need to do
 something like `elem b' (mapElems shortcut_map)`.  This is O(n).

 But I still think you don't need this test.  You're keeping `backEdges` up
 to date, so there's no reason to believe that you have something "hidden"
 in the `shortcut_map` that would represent more predecessors, right?  The
 `shortcut_map` is for replacing labels inside expressions after the pass
 is complete.  It used to update calls, but as you point out, the pass now
 does this as it goes along.

 There are no extra invariants.  The `shortcut_map` doesn't represent any
 extra edges in the graph.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8456#comment:14>
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