bpo-24076: Inline single digit unpacking in the integer fastpath of sum() (GH-28469)

scoder · web-flow · commit debd80403721 · 2021-09-21T11:01:18.000+02:00
diff --git a/Misc/NEWS.d/next/Core and Builtins/2021-09-20-10-02-12.bpo-24076.ZFgFSj.rst b/Misc/NEWS.d/next/Core and Builtins/2021-09-20-10-02-12.bpo-24076.ZFgFSj.rst
@@ -0,0 +1 @@
+sum() was further optimised for summing up single digit integers.
diff --git a/Python/bltinmodule.c b/Python/bltinmodule.c
@@ -2479,7 +2479,15 @@ builtin_sum_impl(PyObject *module, PyObject *iterable, PyObject *start)
                 return PyLong_FromLong(i_result);
             }
             if (PyLong_CheckExact(item) || PyBool_Check(item)) {
-                long b = PyLong_AsLongAndOverflow(item, &overflow);
+                long b;
+                overflow = 0;
+                /* Single digits are common, fast, and cannot overflow on unpacking. */
+                switch (Py_SIZE(item)) {
+                    case -1: b = -(sdigit) ((PyLongObject*)item)->ob_digit[0]; break;
+                    case  0: continue;
+                    case  1: b = ((PyLongObject*)item)->ob_digit[0]; break;
+                    default: b = PyLong_AsLongAndOverflow(item, &overflow); break;
+                }
                 if (overflow == 0 &&
                     (i_result >= 0 ? (b <= LONG_MAX - i_result)
                                    : (b >= LONG_MIN - i_result)))

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1 @@`
	`1`	`+sum() was further optimised for summing up single digit integers.`