Function diff

strings/dtoa.c:1129–1180

static Bigint *diff(Bigint *a, Bigint *b, Stack_alloc *alloc)
{
  Bigint *c;
  int i, wa, wb;
  ULong *xa, *xae, *xb, *xbe, *xc;
  ULLong borrow, y;

  i= cmp(a,b);
  if (!i)
  {
    c= Balloc(0, alloc);
    c->wds= 1;
    c->p.x[0]= 0;
    return c;
  }
  if (i < 0)
  {
    c= a;
    a= b;
    b= c;
    i= 1;
  }
  else
    i= 0;
  c= Balloc(a->k, alloc);
  c->sign= i;
  wa= a->wds;
  xa= a->p.x;
  xae= xa + wa;
  wb= b->wds;
  xb= b->p.x;
  xbe= xb + wb;
  xc= c->p.x;
  borrow= 0;
  do
  {
    y= (ULLong)*xa++ - *xb++ - borrow;
    borrow= y >> 32 & (ULong)1;
    *xc++= (ULong) (y & FFFFFFFF);
  }
  while (xb < xbe);
  while (xa < xae)
  {
    y= *xa++ - borrow;
    borrow= y >> 32 & (ULong)1;
    *xc++= (ULong) (y & FFFFFFFF);
  }
  while (!*--xc)
    wa--;
  c->wds= wa;
  return c;
}


static double ulp(U *x)