GH-131798: Optimize _GUARD_TOS_SLICE (GH-144470)

Author: Sacul0457

Lib/test/test_capi/test_opt.py

@@ -1986,6 +1986,23 @@ def f(n):
         self.assertNotIn("_GUARD_NOS_TUPLE", uops)
         self.assertIn("_BINARY_OP_SUBSCR_TUPLE_INT", uops)
 
+    def test_remove_guard_for_known_type_slice(self):
+        def f(n):
+            x = 0
+            for _ in range(n):
+                l = [1, 2, 3]
+                slice_obj = slice(0, 1)
+                x += l[slice_obj][0] # guarded
+                x += l[slice_obj][0] # unguarded
+            return x
+        res, ex = self._run_with_optimizer(f, TIER2_THRESHOLD)
+        self.assertEqual(res, TIER2_THRESHOLD * 2)
+        uops = get_opnames(ex)
+
+        count = count_ops(ex, "_GUARD_TOS_SLICE")
+        self.assertEqual(count, 1)
+        self.assertIn("_BINARY_OP_SUBSCR_LIST_INT", uops)
+
     def test_remove_guard_for_tuple_bounds_check(self):
         def f(n):
             x = 0

Misc/NEWS.d/next/Core_and_Builtins/2026-02-04-12-19-48.gh-issue-131798.My5jLy.rst

@@ -0,0 +1 @@
+Optimise ``_GUARD_TOS_SLICE`` in the JIT.

Python/optimizer_bytecodes.c

@@ -1374,6 +1374,13 @@ dummy_func(void) {
         }
     }
 
+    op(_GUARD_TOS_SLICE, (tos -- tos)) {
+        if (sym_matches_type(tos, &PySlice_Type)) {
+            ADD_OP(_NOP, 0, 0);
+        }
+        sym_set_type(tos, &PySlice_Type);
+    }
+
     op(_GUARD_NOS_NULL, (null, unused -- null, unused)) {
         if (sym_is_null(null)) {
             ADD_OP(_NOP, 0, 0);