[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:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:              |       Blocked By:
  None/Unknown                       |  Related Tickets:  #1498
             Test Case:              |
              Blocking:  8317        |
Differential Revisions:              |
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Comment (by carter):

 @dfeuer, (my interpretation is) this ticket is articulating how we'd
 *like* to write isTrue# and noting some of the optimization engineering
 thats needed to support writing things in the desired high level fashion.

 Your safety concern is at least in some small part spurious, because some
 of this optimization/engineering is about how to transform high level type
 safe code into that branch free form as optimizations in Core, STG and
 CMM.  That is, end users should not have to deal with this tomfoolery
 ever, but we'd still like to give them branch free code when its safe!

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8326#comment:4>
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