log2

static VALUE
math_log2(VALUE obj, VALUE x)
{
    double d0, d;

    Need_Float(x);
    errno = 0;
    d0 = RFLOAT_VALUE(x);
    d = log2(d0);
    domain_check(d0, d, "log2");
    infinity_check(x, d, "log2");
    return DBL2NUM(d);
}