GH-131798: Narrow the return type of _GET_LEN to int (GH-133345)

Lib/test/test_capi/test_opt.py

@@ -1925,6 +1925,50 @@ class TestUopsOptimization(unittest.TestCase):
         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

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.

Python/optimizer_bytecodes.c

@@ -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) {

Python/optimizer_cases.c.h

@@ -1463,8 +1463,29 @@
         }
 
         case _GET_LEN: {
+            JitOptSymbol *obj;
             JitOptSymbol *len;
-            len = sym_new_not_null(ctx);
+            obj = stack_pointer[-1];
+            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);
+                stack_pointer[0] = len;
+                stack_pointer += 1;
+                assert(WITHIN_STACK_BOUNDS());
+                Py_DECREF(temp);
+                stack_pointer += -1;
+            }
             stack_pointer[0] = len;
             stack_pointer += 1;
             assert(WITHIN_STACK_BOUNDS());