[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        |           Version:  8.4.3
  System                             |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Block descriptors currently contain both `gen` (a `generation *`) and
 `gen_no` (a `uint16_t, which is `gen->no` cached). I wonder whether it
 would be worthwhile to drop the former and always use
 `generations[bd->gen_no]`. Afterall, this would save an entire word per
 descriptor which may improve our caches' coverage of the bdescr region
 appreciably. Presumably the `generations` array would be hot enough to
 stay in cache in such a scheme, so I would guess that the cost would be
 small-ish.

 Simon, have you measured this in the past?

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