[GHC] #15348: Enable two-step allocator on FreeBSD

[GHC]

#15348: Enable two-step allocator on FreeBSD
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           Reporter:  bgamari           |             Owner:  (none)
               Type:  feature request   |            Status:  new
           Priority:  normal            |         Milestone:  8.6.1
          Component:  Compiler          |           Version:  8.4.3
           Keywords:                    |  Operating System:  FreeBSD
       Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
          Test Case:                    |        Blocked By:
           Blocking:                    |   Related Tickets:
Differential Rev(s):                    |         Wiki Page:
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 Currently the two-step allocator is disabled on FreeBSD as the
 `MEM_NORESERVE` macro is undefined. It seems that FreeBSD provided this
 macro until 2014, when it was [[https://reviews.freebsd.org/D848|removed]]
 as it wasn't implemented in the kernel. Regardless, Viktor Dukhovni
 reports empirical evidence on `ghc-devs` that just plain `mmap` does what
 we want.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15348>
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