modulus question

[Pat Haugen]

Wondering if someone could explain why I'm seeing the following difference
when doing a mod by 2.  The final generated code ends up with the 'xor/and'
sequence for mod by 2 but the simpler 'and' for mod by 4 (or 8, or 16,
...).  This is from mainline, powerpc64-unknown-linux-gnu.

int i;
void test1(unsigned int u, unsigned int u2)
{
  if ((u % 2) == 0)
    i = 1;


  if ((u2 % 4) == 0)
    i = 2;

}


>From dump file t04.generic:

test1 (u, u2)
{
  unsigned int T.0;
  int T.1;
  int T.2;
  _Bool T.3;
  unsigned int T.4;

  T.0 = u ^ 1;
  T.1 = (int)T.0;
  T.2 = T.1 & 1;
  T.3 = (_Bool)T.2;
  if (T.3)
    {
      i = 1;
    }
  else
    {

    }
  T.4 = u2 & 3;
  if (T.4 == 0)
    {
      i = 2;
    }
  else
    {

    }
}