[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:  patch
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:
Operating System:  FreeBSD           |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D4939
       Wiki Page:                    |
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Comment (by vdukhovni):

 I found the OSMem.c code a bit of an `#ifdef` maze. The latest patch
 attached should improve clarity, by better separating the logic from CPP
 conditionals.  One side-effect is that `madvise(MADV_WILLNEED)` will now
 be called on `COMMIT` not only for Linux, but also for FreeBSD and other
 non-Darwin `mmap()` API platforms. Will test this shortly...

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15348#comment:10>
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