[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:  8.6.1
       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):

 IIRC originally we had just the `bd->gen`, and adding `bd->gen_no` was a
 small optimisation because it saved a few instructions in the inner loop
 (`evacuate()`).  I'm not sure I tried removing `bd->gen`, but I can always
 be persuaded of anything by data :)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15414#comment:2>
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