placeByteArray# CMM primop

bytearray.cmm

#include "Cmm.h"

/* Place a ByteArray# object header so that its payload is at the given address.

   The payload pointer we're given points to the first byte of the payload,
   so we have to place the object header immediately before it. The caller
   must ensure that the space before the payload is appropriately allocated
   so that we can use it.
 */
stg_placeByteArrayzh ( W_ payload, W_ n )
{
    gcptr p;
    p = payload - SIZEOF_StgArrBytes;
    SET_HDR(p, stg_ARR_WORDS_info, CCCS);
    StgArrBytes_bytes(p) = n;
    return (p);
}

I'm not sure if W_ payload is the right CMM type for Addr#:

foreign import prim "stg_placeByteArrayzh" placeByteArray#
  :: Addr#
  -> Int#
  -> State# s
  -> (# State# s, ByteArray# #)

placeByteArray :: Ptr () -> Int -> IO ByteArray
placeByteArray (Ptr addr#) (I# sz#) =
  IO (\s -> case placeByteArray# addr# sz# s of
              (# s', ba# #) -> (# s', ByteArray ba# #))

Apparently P_ is the right type.