bpo-31415: Add -X importtime option.

Doc/using/cmdline.rst

@@ -407,6 +407,8 @@ Miscellaneous options
    * ``-X showalloccount`` to output the total count of allocated objects for
      each type when the program finishes. This only works when Python was built with
      ``COUNT_ALLOCS`` defined.
+   * ``-X importprofile`` to show how long each import takes. Nested imports are
+     indented. It can be used to optimize application's startup time.
 
    It also allows passing arbitrary values and retrieving them through the
    :data:`sys._xoptions` dictionary.
@@ -423,6 +425,9 @@ Miscellaneous options
    .. versionadded:: 3.6
       The ``-X showalloccount`` option.
 
+   .. versionadded:: 3.7
+      The ``-X importprofile`` option.
+
 
 Options you shouldn't use
 ~~~~~~~~~~~~~~~~~~~~~~~~~

Misc/NEWS.d/next/Core and Builtins/2017-09-11-14-28-56.bpo-31415.GBdz7o.rst

@@ -0,0 +1,2 @@
+Add ``-X importprofile`` option to show how long each import takes. It can
+be used to optimize application's startup time.

Python/import.c

@@ -1666,10 +1666,42 @@ PyImport_ImportModuleLevelObject(PyObject *name, PyObject *globals,
         }
     }
     else {
+        static int import_level;
+        _Py_IDENTIFIER(importprofile);
+        int import_profile = 0;
+        _PyTime_t t1=0, t2;
+
         Py_XDECREF(mod);
+
+        /* XOptions is initialized after first some imports.
+         * So we can't have negative cache.
+         * Anyway, importlib.__find_and_load is much slower than
+         * _PyDict_GetItemId()
+         */
+        PyObject *xoptions = PySys_GetXOptions();
+        if (xoptions) {
+            PyObject *value = _PyDict_GetItemId(xoptions, &PyId_importprofile);
+            import_profile = (value == Py_True);
+        }
+
+        if (import_profile) {
+            import_level++;
+            t1 = _PyTime_GetMonotonicClock();
+        }
+
         mod = _PyObject_CallMethodIdObjArgs(interp->importlib,
                                             &PyId__find_and_load, abs_name,
                                             interp->import_func, NULL);
+
+        if (import_profile) {
+            import_level--;
+            t2 = _PyTime_GetMonotonicClock();
+            fprintf(stderr, "%*s- %s %ld [us]\n",
+                    import_level*2, "",
+                    PyUnicode_AsUTF8(abs_name),
+                    (long)(t2 - t1) / 1000);
+        }
+
         if (mod == NULL) {
             goto error;
         }