Optimize fill_time for typical timestamps

While file timestamps can be anything the file system can store, most
lie between the recent past and the near future.  Optimize fill_time for
typical timestamps in three ways:

- When possible, convert to nanoseconds with C arithmetic.
- When using C arithmetic and the seconds member is not required (for
  st_birthtime), avoid creating a long object.
- When using C arithmetic, reorder the code to avoid the null checks
  implied in Py_XDECREF.

Author: jbosboom

Modules/posixmodule.c

@@ -2588,12 +2588,43 @@ static int
 fill_time(PyObject *module, PyObject *v, int s_index, int f_index, int ns_index, time_t sec, unsigned long nsec)
 {
     assert(!PyErr_Occurred());
+#define S_TO_NS (1000000000LL)
+    assert(nsec < S_TO_NS);
+
+    if (f_index >= 0) {
+        PyObject *float_s = PyFloat_FromDouble(sec + 1e-9*nsec);
+        if (!float_s) {
+            return -1;
+        }
+        PyStructSequence_SET_ITEM(v, f_index, float_s);
+    }
+
+    /* 1677-09-21 00:12:44 to 2262-04-11 23:47:15 UTC inclusive */
+    if ((LLONG_MIN / S_TO_NS) <= sec && sec <= (LONG_MAX / S_TO_NS - 1)) {
+        if (s_index >= 0) {
+            PyObject *s = _PyLong_FromTime_t(sec);
+            if (!s) {
+                return -1;
+            }
+            PyStructSequence_SET_ITEM(v, s_index, s);
+        }
+
+        if (ns_index >= 0) {
+            PyObject *ns_total = PyLong_FromLongLong(sec * S_TO_NS + nsec);
+            if (!ns_total) {
+                return -1;
+            }
+            PyStructSequence_SET_ITEM(v, ns_index, ns_total);
+        }
+
+        assert(!PyErr_Occurred());
+        return 0;
+    }
+#undef S_TO_NS
 
     int res = -1;
     PyObject *s_in_ns = NULL;
     PyObject *ns_total = NULL;
-    PyObject *float_s = NULL;
-
     PyObject *s = _PyLong_FromTime_t(sec);
     PyObject *ns_fractional = PyLong_FromUnsignedLong(nsec);
     if (!(s && ns_fractional)) {
@@ -2606,22 +2637,14 @@ fill_time(PyObject *module, PyObject *v, int s_index, int f_index, int ns_index,
     }
 
     ns_total = PyNumber_Add(s_in_ns, ns_fractional);
-    if (!ns_total)
-        goto exit;
-
-    float_s = PyFloat_FromDouble(sec + 1e-9*nsec);
-    if (!float_s) {
+    if (!ns_total) {
         goto exit;
     }
 
     if (s_index >= 0) {
         PyStructSequence_SET_ITEM(v, s_index, s);
         s = NULL;
     }
-    if (f_index >= 0) {
-        PyStructSequence_SET_ITEM(v, f_index, float_s);
-        float_s = NULL;
-    }
     if (ns_index >= 0) {
         PyStructSequence_SET_ITEM(v, ns_index, ns_total);
         ns_total = NULL;
@@ -2635,7 +2658,6 @@ fill_time(PyObject *module, PyObject *v, int s_index, int f_index, int ns_index,
     Py_XDECREF(ns_fractional);
     Py_XDECREF(s_in_ns);
     Py_XDECREF(ns_total);
-    Py_XDECREF(float_s);
     return res;
 }