[GHC] #14626: No need to enter a scrutinised value

[GHC]

#14626: No need to enter a scrutinised value
-------------------------------------+-------------------------------------
        Reporter:  heisenbug         |                Owner:  heisenbug
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.2
      Resolution:                    |             Keywords:  performance
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #13861            |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by heisenbug):

 >
 > > There is only one place in the code generator namely
 `StgCmmCon.buildDynCon'`.  You could perhaps add (runtime) assertions
 there to see if any strict fields had un-tagged pointers.

 After adding a runtime assertion here, I found one smoking gun. Banged
 `IORef`s (a.k.a `newtype`d `STRef`s) should be tagged, but end up untagged
 e.g. in `Handle__`. I am investigating why this happens.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14626#comment:28>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler