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

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
new file mode 100644 (file)
index 0000000..b680884
--- /dev/null
+++ 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
index 76c9f87fdf18a8ffe0c5ac8145d589a4e04ce852..38bdfb2766f3df0e362e560dc3d5111ad84b848f 100644 (file)
--- 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)))