Different legal addressing modes - global variables

[blackgeorge-boom]

int main(int argc, char *argv[])
{
  int i, j, k, it;

  double rnorm;

  firstrow = 0;
  lastrow  = NA-1;
  lastcol  = NA-1;

  printf(" Size: %11d\n", NA);

  naa = NA;
  nzz = NZ;

  //---------------------------------------------------------------------
  // set starting vector to (1, 1, .... 1)
  //---------------------------------------------------------------------
  for (i = 0; i < NA+1; i++) {
    x[i] = 1.0;
  }
  for (j = 0; j < lastcol - firstcol + 1; j++) {
    q[j] = 0.0;
    z[j] = 0.0;
    r[j] = 0.0;
    p[j] = 0.0;
  }

  for (it = 1; it <= 1; it++) {
    conj_grad(colidx, rowstr, x, z, a, p, q, r, &rnorm);
  }


  return 0;
}

Since lastcol is a global variable, the comparison in the 2nd loop requires addressing a global address. X86 allows that:

501076:	mov    QWORD PTR [rax+rcx*1+0x927c8],rdx <- the hex number is the global address

but AArch64 doesn't, so it leverages an extra register:

  501088:      	mov	w19, #0x27c0
  50108c:      	movk	w19, #0x9, lsl #16