[GHC] #15414: Why both gen and gen_no in bdescr?

[GHC]

#15414: Why both gen and gen_no in bdescr?
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        Reporter:  bgamari           |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Runtime System    |              Version:  8.4.3
      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 simonmar):

 Yes that's right. I had planned to do the experiment of removing `start`
 at some point and measuring the difference, but never got around to it.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15414#comment:5>
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