[GHC] #10728: Add e.g. "-N<=4" in addition to the fixed "-N4" and variable "-N" RTS options

[GHC]

#10728: Add e.g. "-N<=4" in addition to the fixed "-N4" and variable "-N" RTS
options
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        Reporter:  rrnewton          |                   Owner:  simonmar
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Runtime System    |                 Version:  7.10.2
      Resolution:                    |                Keywords:  newcomer
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:  #9221, #8224      |  Differential Revisions:
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Comment (by hvr):

 Another use-case that comes up often is to have `-N<cores-1>`, but I'm not
 sure what kind of expression syntax to use. For instance, allow simple
 arithmetic expressions w/ a couple of binary functions such as `max(_,_)`,
 and a constant `n` denoting the number of cores detected:

   - `-Nmax(n,4)`
   - `-Ndiv(n,2)`
   - `-Nadd(n,-1)`

 or something other that's cheap/trivial to parse.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10728#comment:6>
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