bpo-41902: Micro optimization for compute_item of range (GH-22492)

Author: corona10

Misc/NEWS.d/next/Core and Builtins/2020-10-02-13-32-05.bpo-41902.ZKTxzW.rst

@@ -0,0 +1,3 @@
+Micro optimization when compute :c:member:`~PySequenceMethods.sq_item` and
+:c:member:`~PyMappingMethods.mp_subscript` of :class:`range`. Patch by
+Dong-hee Na.

Objects/rangeobject.c

@@ -254,11 +254,17 @@ compute_item(rangeobject *r, PyObject *i)
     /* PyLong equivalent to:
      *    return r->start + (i * r->step)
      */
-    incr = PyNumber_Multiply(i, r->step);
-    if (!incr)
-        return NULL;
-    result = PyNumber_Add(r->start, incr);
-    Py_DECREF(incr);
+    if (r->step == _PyLong_One) {
+        result = PyNumber_Add(r->start, i);
+    }
+    else {
+        incr = PyNumber_Multiply(i, r->step);
+        if (!incr) {
+            return NULL;
+        }
+        result = PyNumber_Add(r->start, incr);
+        Py_DECREF(incr);
+    }
     return result;
 }