GH-131798: Remove guard for known type in TO_BOOL_STR

Lib/test/test_capi/test_opt.py

@@ -1581,6 +1581,24 @@ def testfunc(n):
         self.assertNotIn("_COMPARE_OP_INT", uops)
         self.assertIn("_POP_TWO_LOAD_CONST_INLINE_BORROW", uops)
 
+    def test_remove_guard_for_known_type_str(self):
+        def f(n):
+            for i in range(n):
+                false = i == TIER2_THRESHOLD
+                empty = "X"[:false]
+                empty += ""  # Make JIT realize this is a string.
+                if empty:
+                    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_STR", uops)
+        self.assertNotIn("_GUARD_TOS_UNICODE", uops)
+
+
 def global_identity(x):
     return x
 

Misc/NEWS.d/next/Core_and_Builtins/2025-03-28-19-02-55.gh-issue-131798.fNZ5-2.rst

@@ -0,0 +1 @@
+Allow JIT to omit str guard in truthiness test when str type is known.

Python/bytecodes.c

@@ -512,10 +512,14 @@ dummy_func(
             res = PyStackRef_False;
         }
 
-        inst(TO_BOOL_STR, (unused/1, unused/2, value -- res)) {
+        op(_GUARD_TOS_UNICODE, (value -- value)) {
             PyObject *value_o = PyStackRef_AsPyObjectBorrow(value);
             EXIT_IF(!PyUnicode_CheckExact(value_o));
+        }
+
+        op(_TO_BOOL_STR, (value -- res)) {
             STAT_INC(TO_BOOL, hit);
+            PyObject *value_o = PyStackRef_AsPyObjectBorrow(value);
             if (value_o == &_Py_STR(empty)) {
                 assert(_Py_IsImmortal(value_o));
                 DEAD(value);
@@ -528,6 +532,9 @@ dummy_func(
             }
         }
 
+        macro(TO_BOOL_STR) =
+            _GUARD_TOS_UNICODE + unused/1 + unused/2 + _TO_BOOL_STR;
+
         op(_REPLACE_WITH_TRUE, (value -- res)) {
             PyStackRef_CLOSE(value);
             res = PyStackRef_True;