[GHC] #16126: Make -threaded the default

[GHC]

#16126: Make -threaded the default
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           Reporter:  mpickering     |             Owner:  (none)
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.3
           Keywords:  newcomer       |  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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 Simon M writes back in 2016

 > Related to this, I think it's about time we made -threaded the default.
 We could add a -single-threaded option to get back the old behaviour.
 >
 > There is a small overhead to using -threaded, but -threaded is also
 required to make a lot of things work (e.g. waitForProcess in a
 multithreaded program, not to mention parallelism).

 All that needs doing is to add a flag `-single-threaded` to enable the old
 behaviour and a deprecation warning alerting uses that `-threaded` is the
 new default.

 https://mail.haskell.org/pipermail/ghc-devs/2016-October/013101.html

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16126>
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