cmm call format for foreign primop (integer-gmp example)

I have been checking out integer-gmp source code to understand how foreign primops can be implemented in terms of cmm as documented on GHC Primops page. I am aware of techniques to implement them using llvm hack or fvia-C/gcc - this is more of a learning experience for me to understand this third approach that interger-gmp library uses.

So, I looked up CMM tutorial on MSFT page (pdf link), went through GHC CMM page, and still there are some unanswered questions (hard to keep all those concepts in head without digging into CMM which is what I am doing now). There is this code fragment from integer-bmp cmm file:

integer_cmm_int2Integerzh (W_ val)
{
   W_ s, p; /* to avoid aliasing */

   ALLOC_PRIM_N (SIZEOF_StgArrWords + WDS(1), integer_cmm_int2Integerzh, val);

   p = Hp - SIZEOF_StgArrWords;
   SET_HDR(p, stg_ARR_WORDS_info, CCCS);
   StgArrWords_bytes(p) = SIZEOF_W;

   /* mpz_set_si is inlined here, makes things simpler */
   if (%lt(val,0)) {
        s  = -1;
        Hp(0) = -val;
   } else {
     if (%gt(val,0)) {
        s = 1;
        Hp(0) = val;
     } else {
        s = 0;
     }
  }

   /* returns (# size  :: Int#,
                 data  :: ByteArray#
               #)
   */
   return (s,p);
}

As defined in ghc cmm header:

W_ is alias for word.
ALLOC_PRIM_N is a function for allocating memory on the heap for primitive object.
Sp(n) and Hp(n) are defined as below (comments are