On path with known exact float, extract the double with the fast macr…

…o. (pythonGH-21072)

Modules/mathmodule.c

@@ -1256,9 +1256,15 @@ static PyObject *
 math_floor(PyObject *module, PyObject *number)
 /*[clinic end generated code: output=c6a65c4884884b8a input=63af6b5d7ebcc3d6]*/
 {
+ double x;
+
  _Py_IDENTIFIER(__floor__);
 
- if (!PyFloat_CheckExact(number)) {
+ if (PyFloat_CheckExact(number)) {
+ x = PyFloat_AS_DOUBLE(number);
+ }
+ else
+ {
  PyObject *method = _PyObject_LookupSpecial(number, &PyId___floor__);
  if (method != NULL) {
  PyObject *result = _PyObject_CallNoArg(method);
@@ -1267,11 +1273,10 @@ math_floor(PyObject *module, PyObject *number)
  }
  if (PyErr_Occurred())
  return NULL;
+ x = PyFloat_AsDouble(number);
+ if (x == -1.0 && PyErr_Occurred())
+ return NULL;
  }
- double x = PyFloat_AsDouble(number);
- if (x == -1.0 && PyErr_Occurred())
- return NULL;
-
  return PyLong_FromDouble(floor(x));
 }