gh-102221: speed up math.lcm by swapping numbers

kevin1kevin1k · kevin1kevin1k · commit 2e0bcbbe38e3 · 2025-06-15T00:57:12.000+08:00
diff --git a/Misc/NEWS.d/next/Library/2023-10-29-15-35-57.gh-issue-102221.fQnOaT.rst b/Misc/NEWS.d/next/Library/2023-10-29-15-35-57.gh-issue-102221.fQnOaT.rst
@@ -0,0 +1 @@
+Speed up math lcm
diff --git a/Modules/mathmodule.c b/Modules/mathmodule.c
@@ -769,7 +769,15 @@ long_lcm(PyObject *a, PyObject *b)
     if (_PyLong_IsZero((PyLongObject *)a) || _PyLong_IsZero((PyLongObject *)b)) {
         return PyLong_FromLong(0);
     }
-    g = _PyLong_GCD(a, b);
+
+    /* Make sure a <= b to speed up (a // g) * b. */
+    if (PyObject_RichCompareBool(b, a, Py_LT)) {
+        g = a;
+        a = b;
+        b = g;
+    }
+
+    g = _PyLong_GCD(b, a);
     if (g == NULL) {
         return NULL;
     }
