[GHC] #13174: Fix mismatch between unsafeDupablePerformIO and note

[GHC]

#13174: Fix mismatch between unsafeDupablePerformIO and note
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           Reporter:  dfeuer         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.2.1
          Component:  Core           |           Version:  8.1
  Libraries                          |
           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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 `ghc/libraries/base/GHC/IO/Unsafe.hs` contains the following note:

 {{{
 -- Note [unsafeDupablePerformIO is NOINLINE]
 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 -- Why do we NOINLINE unsafeDupablePerformIO?  See the comment with
 -- GHC.ST.runST.  Essentially the issue is that the IO computation
 -- inside unsafePerformIO must be atomic: it must either all run, or
 -- not at all.  If we let the compiler see the application of the IO
 -- to realWorld#, it might float out part of the IO.
 }}}

 However, `unsafeDupablePerformIO` does not actually have a `NOINLINE`
 pragma. So either the note is out of date and should be removed/modified,
 or someone messed up the code.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13174>
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