JIT: Remove guard for slicing known type list

Author: fluhus

Lib/test/test_capi/test_opt.py

@@ -2217,6 +2217,22 @@ def f(n):
         self.assertNotIn("_LOAD_ATTR_METHOD_NO_DICT", uops)
         self.assertNotIn("_LOAD_ATTR_METHOD_LAZY_DICT", uops)
 
+    def test_remove_guard_for_slice_list(self):
+        def f(n):
+            for i in range(n):
+                false = i == TIER2_THRESHOLD
+                sliced = [1, 2, 3][:false]
+                if sliced:
+                    return 1
+            return 0
+
+        res, ex = self._run_with_optimizer(f, TIER2_THRESHOLD)
+        self.assertEqual(res, 0)
+        self.assertIsNotNone(ex)
+        uops = get_opnames(ex)
+        self.assertIn("_TO_BOOL_LIST", uops)
+        self.assertNotIn("_GUARD_TOS_LIST", uops)
+
 
 def global_identity(x):
     return x

Python/optimizer_bytecodes.c

@@ -1217,11 +1217,10 @@ dummy_func(void) {
     }
 
     op(_BINARY_SLICE, (container, start, stop -- res)) {
-        // Slicing a string always returns a string.
-        // TODO: We can apply this to lists and tuples as well.
-        //       We'll start with string to simplify the process.
+        // Slicing a string/list always returns the same type.
         PyTypeObject *type = sym_get_type(container);
-        if (type == &PyUnicode_Type) {
+        if (type == &PyUnicode_Type ||
+            type == &PyList_Type) {
             res = sym_new_type(ctx, type);
         } else {
             res = sym_new_not_null(ctx);