[GHC] #10613: Mechanism for checking that we only enter single-entry thunks once

[GHC]

#10613: Mechanism for checking that we only enter single-entry thunks once
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        Reporter:  bgamari           |                   Owner:
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.10.1
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Other             |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by rwbarton):

 For what it's worth, while investigating #10414, I ran the whole test
 suite with `-feager-blackholing`, which (at the time) caused all single-
 entry thunks to be overwritten with a black hole on entry. There were no
 test failures, which gives me a reasonably high degree of confidence that
 the "single-entry thunks are entered only once" invariant is maintained,
 outside of the situations involving multiple threads entering a closure
 simultaneously discussed in #10414.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10613#comment:2>
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