python · brandtbucher · May 20, 2025 · May 3, 2025 · May 3, 2025 · May 3, 2025
diff --git a/Lib/test/test_capi/test_opt.py b/Lib/test/test_capi/test_opt.py
@@ -1925,6 +1925,50 @@ def testfunc(n):
        self.assertNotIn("_GUARD_NOS_INT", uops)
        self.assertNotIn("_GUARD_TOS_INT", uops)

+    def test_get_len_with_const_tuple(self):
+        def testfunc(n):
+            x = 0.0
+            for _ in range(n):
+                match (1, 2, 3, 4):
+                    case [_, _, _, _]:
+                        x += 1.0
+            return x
+        res, ex = self._run_with_optimizer(testfunc, TIER2_THRESHOLD)
+        self.assertEqual(int(res), TIER2_THRESHOLD)
+        uops = get_opnames(ex)
+        self.assertNotIn("_GUARD_NOS_INT", uops)
+        self.assertNotIn("_GET_LEN", uops)
+        self.assertIn("_LOAD_CONST_INLINE_BORROW", uops)
+
+    def test_get_len_with_non_const_tuple(self):
+        def testfunc(n):
+            x = 0.0
+            for _ in range(n):
+                match object(), object():
+                    case [_, _]:
+                        x += 1.0
+            return x
+        res, ex = self._run_with_optimizer(testfunc, TIER2_THRESHOLD)
+        self.assertEqual(int(res), TIER2_THRESHOLD)
+        uops = get_opnames(ex)
+        self.assertNotIn("_GUARD_NOS_INT", uops)
+        self.assertNotIn("_GET_LEN", uops)
+        self.assertIn("_LOAD_CONST_INLINE_BORROW", uops)
+
+    def test_get_len_with_non_tuple(self):
+        def testfunc(n):
+            x = 0.0
+            for _ in range(n):
+                match [1, 2, 3, 4]:
+                    case [_, _, _, _]:
+                        x += 1.0
+            return x
+        res, ex = self._run_with_optimizer(testfunc, TIER2_THRESHOLD)
+        self.assertEqual(int(res), TIER2_THRESHOLD)
+        uops = get_opnames(ex)
+        self.assertNotIn("_GUARD_NOS_INT", uops)
+        self.assertIn("_GET_LEN", uops)
+
    def test_binary_op_subscr_tuple_int(self):
        def testfunc(n):
            x = 0

diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2025-05-03-22-31-53.gh-issue-131798.fQ0ato.rst b/Misc/NEWS.d/next/Core_and_Builtins/2025-05-03-22-31-53.gh-issue-131798.fQ0ato.rst
@@ -0,0 +1,2 @@
+Allow the JIT to remove int guards after ``_GET_LEN`` by setting the return
+type to int.
@@ -1116,6 +1116,25 @@ dummy_func(void) {
        res = sym_new_type(ctx, &PyLong_Type);
    }

+    op(_GET_LEN, (obj -- obj, len)) {
+        int tuple_length = sym_tuple_length(obj);
+        if (tuple_length == -1) {
+            len = sym_new_type(ctx, &PyLong_Type);
+        }
+        else {
+            assert(tuple_length >= 0);
+            PyObject *temp = PyLong_FromLong(tuple_length);
+            if (temp == NULL) {
+                goto error;
+            }
+            if (_Py_IsImmortal(temp)) {
+                REPLACE_OP(this_instr, _LOAD_CONST_INLINE_BORROW, 0, (uintptr_t)temp);
+            }
+            len = sym_new_const(ctx, temp);
+            Py_DECREF(temp);
+        }
+    }
+
    op(_GUARD_CALLABLE_LEN, (callable, unused, unused -- callable, unused, unused)) {
        PyObject *len = _PyInterpreterState_GET()->callable_cache.len;
        if (sym_get_const(ctx, callable) == len) {

diff --git a/Python/optimizer_cases.c.h b/Python/optimizer_cases.c.h
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,2 @@
		Allow the JIT to remove int guards after ``_GET_LEN`` by setting the return
		type to int.