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);
}

Apparently P_ is the right type.