[GHC] #9864: Need realloc/resize feature for mallocForeignPtrBytes allocated memory

[GHC]

#9864: Need realloc/resize feature for mallocForeignPtrBytes allocated memory
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       Reporter:  slomo              |                   Owner:  ekmett
           Type:  feature request    |                  Status:  new
       Priority:  low                |               Milestone:
      Component:  Core Libraries     |                 Version:  7.8.3
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:
     Blocked By:                     |  None/Unknown
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
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 Hi,

 it would be great if there would be something to reallocate or resize the
 memory allocated by
 - mallocPlainForeignPtrBytes or
 - mallocForeignPtrBytes

 Currently this is only possible with memory allocated via mallocBytes by
 using reallocBytes, but that's using the system's normal
 malloc()/realloc() and will not return memory in the GC heap.

 My proposal for a reallocForeignPtrBytes would have the same effects as
 reallocBytes. I.e.
 - if the new size is bigger than the old size, make enough free space
 available. This probably might result in copying the previous memory area
 to a newly allocated one
 - if the new size is smaller than the old size, mark mark the memory area
 as smaller. It should not result in immediate copying of the data, but it
 would e.g. allow the GC during a compacting phase to release the
 additional memory (note: I don't know how the GHC GC works internally)
 - if the new size is the same, do nothing


 One possible use case for this would be in ByteString. Currently
 Data.ByteString.Internal.createAndTrim will copy the completely memory if
 the actual size was less than the expected maximum size. This should be
 unnecessary and only wastes CPU cycles.

 It would also be useful for bindings to native libraries, e.g. for
 allocating the output buffer for iconv().

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9864>
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