[GHC] #8326: Place heap checks common in case alternatives before the case

[GHC]

#8326: Place heap checks common in case alternatives before the case
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        Reporter:  jstolarek         |                   Owner:
            Type:  task              |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.7
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |               Test Case:
      Blocked By:                    |                Blocking:  8317
 Related Tickets:  #1498             |  Differential Revisions:  Phab:D343
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Comment (by rwbarton):

 I ran a nofib benchmark with these changes and there was mostly no effect,
 aside from a few changes in the plus-or-minus 5-10% range that I could not
 explain. When I find a larger block of free time I'll return to this
 subject (should be more pleasant now that ghcspeed will benchmark wip
 branches).

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8326#comment:37>
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