gh-95173: Revert commit 51ed2c5 (GH-95176)

(cherry picked from commit 9007dec)

Co-authored-by: Pablo Galindo Salgado <[email protected]>

Author: pablogsal

Doc/whatsnew/3.11.rst

@@ -1478,13 +1478,6 @@ Changes in the Python API
   the ``'utf-8'`` encoding.
   (Contributed by Srinivas Reddy Thatiparthy (శ్రీనివాస్  రెడ్డి తాటిపర్తి) in :issue:`41137`.)
 
-* When sorting using tuples as keys, the order of the result may differ
-  from earlier releases if the tuple elements don't define a total
-  ordering (see :ref:`expressions-value-comparisons` for
-  information on total ordering).  It's generally true that the result
-  of sorting simply isn't well-defined in the absence of a total ordering
-  on list elements.
-
 * :mod:`calendar`: The :class:`calendar.LocaleTextCalendar` and
   :class:`calendar.LocaleHTMLCalendar` classes now use
   :func:`locale.getlocale`, instead of using :func:`locale.getdefaultlocale`,

Objects/listobject.c

@@ -1226,13 +1226,6 @@ struct s_MergeState {
      * of tuples. It may be set to safe_object_compare, but the idea is that hopefully
      * we can assume more, and use one of the special-case compares. */
     int (*tuple_elem_compare)(PyObject *, PyObject *, MergeState *);
-
-    /* Used by unsafe_tuple_compare to record whether the very first tuple
-     * elements resolved the last comparison attempt. If so, next time a
-     * method that may avoid PyObject_RichCompareBool() entirely is tried.
-     * 0 for false, 1 for true.
-     */
-    int first_tuple_items_resolved_it;
 };
 
 /* binarysort is the best method for sorting small arrays: it does
@@ -2205,24 +2198,7 @@ unsafe_float_compare(PyObject *v, PyObject *w, MergeState *ms)
  * using the same pre-sort check as we use for ms->key_compare,
  * but run on the list [x[0] for x in L]. This allows us to optimize compares
  * on two levels (as long as [x[0] for x in L] is type-homogeneous.) The idea is
- * that most tuple compares don't involve x[1:].
- * However, that may not be right. When it is right, we can win by calling the
- * relatively cheap ms->tuple_elem_compare on the first pair of elements, to
- * see whether v[0] < w[0] or w[0] < v[0]. If either are so, we're done.
- * Else we proceed as in the tuple compare, comparing the remaining pairs via
- * the probably more expensive PyObject_RichCompareBool(..., Py_EQ) until (if
- * ever) that says "no, not equal!". Then, if we're still on the first pair,
- * ms->tuple_elem_compare can resolve it, else PyObject_RichCompareBool(...,
- * Py_LT) finishes the job.
- * In any case, ms->first_tuple_items_resolved_it keeps track of whether the
- * most recent tuple comparison was resolved by the first pair. If so, the
- * next attempt starts by trying the cheap tests on the first pair again, else
- * PyObject_RichCompareBool(..., Py_EQ) is used from the start.
- * There are cases where PyObject_RichCompareBool(..., Py_EQ) is much cheaper!
- * For example, that can return "almost immediately" if passed the same
- * object twice (it special-cases object identity for Py_EQ), which can,
- * potentially, be unboundedly faster than ms->tuple_elem_compare.
- */
+ * that most tuple compares don't involve x[1:]. */
 static int
 unsafe_tuple_compare(PyObject *v, PyObject *w, MergeState *ms)
 {
@@ -2238,52 +2214,26 @@ unsafe_tuple_compare(PyObject *v, PyObject *w, MergeState *ms)
 
     vt = (PyTupleObject *)v;
     wt = (PyTupleObject *)w;
-    i = 0;
-    if (ms->first_tuple_items_resolved_it) {
-        /* See whether fast compares of the first elements settle it. */
-        k = ms->tuple_elem_compare(vt->ob_item[0], wt->ob_item[0], ms);
-        if (k) /* error, or v < w */
-            return k;
-        k = ms->tuple_elem_compare(wt->ob_item[0], vt->ob_item[0], ms);
-        if (k > 0) /* w < v */
-            return 0;
-        if (k < 0) /* error */
-            return -1;
-        /* We have
-         *     not (v[0] < w[0]) and not (w[0] < v[0])
-         * which implies, for a total order, that the first elements are
-         * equal. So skip them in the loop.
-         */
-        i = 1;
-        ms->first_tuple_items_resolved_it = 0;
-    }
-    /* Now first_tuple_items_resolved_it was 0 on entry, or was forced to 0
-     * at the end of the `if` block just above.
-     */
-    assert(! ms->first_tuple_items_resolved_it);
 
     vlen = Py_SIZE(vt);
     wlen = Py_SIZE(wt);
-    for (; i < vlen && i < wlen; i++) {
+
+    for (i = 0; i < vlen && i < wlen; i++) {
         k = PyObject_RichCompareBool(vt->ob_item[i], wt->ob_item[i], Py_EQ);
-        if (!k) { /* not equal */
-            if (i) {
-                return PyObject_RichCompareBool(vt->ob_item[i], wt->ob_item[i],
-                                                Py_LT);
-            }
-            else {
-                ms->first_tuple_items_resolved_it = 1;
-                return ms->tuple_elem_compare(vt->ob_item[0], wt->ob_item[0],
-                                              ms);
-            }
-        }
         if (k < 0)
             return -1;
+        if (!k)
+            break;
     }
-    /* all equal until we fell off the end */
-    return vlen < wlen;
 
- }
+    if (i >= vlen || i >= wlen)
+        return vlen < wlen;
+
+    if (i == 0)
+        return ms->tuple_elem_compare(vt->ob_item[i], wt->ob_item[i], ms);
+    else
+        return PyObject_RichCompareBool(vt->ob_item[i], wt->ob_item[i], Py_LT);
+}
 
 /* An adaptive, stable, natural mergesort.  See listsort.txt.
  * Returns Py_None on success, NULL on error.  Even in case of error, the
@@ -2466,7 +2416,6 @@ list_sort_impl(PyListObject *self, PyObject *keyfunc, int reverse)
             }
 
             ms.key_compare = unsafe_tuple_compare;
-            ms.first_tuple_items_resolved_it = 1; /* be optimistic */
         }
     }
     /* End of pre-sort check: ms is now set properly! */