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

[GHC]

#15348: Enable two-step allocator on FreeBSD
-------------------------------------+-------------------------------------
        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):

 From `kern_mmap()` in `/usr/src/sys/vm/vm_mmap.c`:
 {{{
         if ((flags & MAP_GUARD) != 0 && (prot != PROT_NONE || fd != -1 ||
             pos != 0 || (flags & (MAP_SHARED | MAP_PRIVATE | MAP_PREFAULT
 |
             MAP_PREFAULT_READ | MAP_ANON | MAP_STACK)) != 0))
                 return (EINVAL);
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15348#comment:5>
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