[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 modified the proposed OSMem.c patch as attached ,rebuilt GHC-8.4.3 and
 am now running my DANE scanner (runs for ~8 hours, using around 300MB of
 VM space, and doing a total of around 4TB of allocations).  So far so
 good.  And it did allocate a 1TB hole in its address space, partly
 replaced with actually allocated pages.  Because this is a reserved hole,
 and not a mapping, the reported virtual size is still modest.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15348#comment:7>
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