log2

static VALUE
math_log2(VALUE unused_obj, VALUE x)
{
    size_t numbits;
    double d = get_double_rshift(x, &numbits);

    /* check for domain error */
    if (d < 0.0) domain_error("log2");
    /* check for pole error */
    if (d == 0.0) return DBL2NUM(-INFINITY);

    return DBL2NUM(log2(d) + numbits); /* log2(d * 2 ** numbits) */
}