[GHC] #15980: unpin a mutable byte array

[GHC]

#15980: unpin a mutable byte array
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           Reporter:  andrewthad     |             Owner:  (none)
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:  8.6.3
          Component:  Compiler       |           Version:  8.6.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Sometimes, it necessary to allocate a mutable byte array as pinned. This
 commonly happens when the mutable byte array will immidiately be handed
 over to a `safe` FFI routine to be initialized. However, after it has been
 initialized, it might not be useful for it to be pinned anymore. If it's a
 little byte array, it may contribute to a fragmented heap. Even if it's
 large, the fact that the user explicitly asked pin it prohibits it from
 ever being added to a compact region (pinned bytearrays that were pinned
 simply because they are over 3KB can be added to compact regions). A
 workaround for either of these problems is to copy the pinned bytearray
 into an unpinned byte array afterwards. But that's kind of wasteful. Is it
 possible to have a primop

 {{{
 unpinMutableByteArray :: MutableByteArray s -> State# s -> State# s
 }}}

 After this, the mutable byte array could be moved in memory. This would
 require `isMutableByteArrayPinned#` to undergo a similar transition as
 `sizeofMutableByteArray#` (which was superceeded by
 `getSizeofMutableByteArray#`) underwent.

 Is this possible? Or are pinned things allocated in a special place that
 makes unpinning impossible?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15980>
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