All current (2001) S/390 and z/Architecture machines are single-issue,

but some newer machines have a deep pipeline.  Software-pipelining is

therefore beneficial.

* mpn_add_n, mpn_sub_n: Use code along the lines below.  Two-way unrolling

  would be adequate.

  mp_limb_t

  mpn_add_n (mp_ptr rp, mp_srcptr up, mp_srcptr vp, mp_size_t n)

  {

    mp_limb_t a, b, r, cy;

    mp_size_t i;

    mp_limb_t mm = -1;

    cy = 0;

    up += n;

    vp += n;

    rp += n;

    i = -n;

    do

      {

	a = up[i];

	b = vp[i];

	r = a + b + cy;

	rp[i] = r;

	cy = (((a & b) | ((a | b) & (r ^ mm)))) >> 31;

	i++;

      }

    while (i < 0);

    return cy;

  }

* mpn_lshift, mpn_rshift: Use SLDL/SRDL, and two-way unrolling.

* mpn_mul_1, mpn_addmul_1, mpn_submul_1: For machines with just signed

  multiply (MR), use two loops, similar to the corresponding VAX or

  POWER functions.  Handle carry like for mpn_add_n.