gh-131798: Narrow the return type of _FORMAT_SIMPLE and _FORMAT_WITH_SPEC to str

Include/internal/pycore_optimizer.h

@@ -393,6 +393,7 @@ extern JitOptRef _Py_uop_sym_new_type(
 
 extern JitOptRef _Py_uop_sym_new_const(JitOptContext *ctx, PyObject *const_val);
 extern JitOptRef _Py_uop_sym_new_const_steal(JitOptContext *ctx, PyObject *const_val);
+extern bool _Py_uop_sym_is_safe_type(JitOptRef sym);
 bool _Py_uop_sym_is_safe_const(JitOptContext *ctx, JitOptRef sym);
 _PyStackRef _Py_uop_sym_get_const_as_stackref(JitOptContext *ctx, JitOptRef sym);
 extern JitOptRef _Py_uop_sym_new_null(JitOptContext *ctx);

Lib/test/test_capi/test_opt.py

@@ -2263,6 +2263,44 @@ def testfunc(n):
         self.assertNotIn("_GUARD_TOS_UNICODE", uops)
         self.assertIn("_BINARY_OP_ADD_UNICODE", uops)
 
+    def test_format_simple_narrows_to_str(self):
+        def testfunc(n):
+            x = []
+            for _ in range(n):
+                v = 42
+                s = f"{v}"
+                t = "hello" + s
+                x.append(t)
+            return x
+
+        res, ex = self._run_with_optimizer(testfunc, TIER2_THRESHOLD)
+        self.assertEqual(res, ["hello42"] * TIER2_THRESHOLD)
+        self.assertIsNotNone(ex)
+        uops = get_opnames(ex)
+
+        self.assertIn("_FORMAT_SIMPLE", uops)
+        self.assertNotIn("_GUARD_TOS_UNICODE", uops)
+        self.assertIn("_BINARY_OP_ADD_UNICODE", uops)
+
+    def test_format_with_spec_narrows_to_str(self):
+        def testfunc(n):
+            x = []
+            for _ in range(n):
+                v = 3.14
+                s = f"{v:.2f}"
+                t = "pi=" + s
+                x.append(t)
+            return x
+
+        res, ex = self._run_with_optimizer(testfunc, TIER2_THRESHOLD)
+        self.assertEqual(res, ["pi=3.14"] * TIER2_THRESHOLD)
+        self.assertIsNotNone(ex)
+        uops = get_opnames(ex)
+
+        self.assertIn("_FORMAT_WITH_SPEC", uops)
+        self.assertNotIn("_GUARD_TOS_UNICODE", uops)
+        self.assertIn("_BINARY_OP_ADD_UNICODE", uops)
+
     def test_binary_op_subscr_str_int(self):
         def testfunc(n):
             x = 0

Misc/NEWS.d/next/Core_and_Builtins/2026-03-30-17-01-34.gh-issue-131798.WSefcr.rst

@@ -0,0 +1,2 @@
+Allow the JIT to remove unicode guards after ``_FORMAT_SIMPLE`` and
+``_FORMAT_WITH_SPEC`` when the input type is a known built-in type.

Python/optimizer_analysis.c

@@ -250,6 +250,7 @@ add_op(JitOptContext *ctx, _PyUOpInstruction *this_instr,
 /* Shortened forms for convenience, used in optimizer_bytecodes.c */
 #define sym_is_not_null _Py_uop_sym_is_not_null
 #define sym_is_const _Py_uop_sym_is_const
+#define sym_is_safe_type _Py_uop_sym_is_safe_type
 #define sym_is_safe_const _Py_uop_sym_is_safe_const
 #define sym_get_const _Py_uop_sym_get_const
 #define sym_new_const_steal _Py_uop_sym_new_const_steal

Python/optimizer_bytecodes.c

@@ -1551,6 +1551,22 @@ dummy_func(void) {
         set = sym_new_type(ctx, &PySet_Type);
     }
 
+    op(_FORMAT_SIMPLE, (value -- res)) {
+        if (sym_is_safe_type(value)) {
+            res = sym_new_type(ctx, &PyUnicode_Type);
+        } else {
+            res = sym_new_not_null(ctx);
+        }
+    }
+
+    op(_FORMAT_WITH_SPEC, (value, fmt_spec -- res)) {
+        if (sym_is_safe_type(value)) {
+            res = sym_new_type(ctx, &PyUnicode_Type);
+        } else {
+            res = sym_new_not_null(ctx);
+        }
+    }
+
     op(_SET_UPDATE, (set, unused[oparg-1], iterable -- set, unused[oparg-1], i)) {
         (void)set;
         i = iterable;

Python/optimizer_symbols.c

@@ -264,6 +264,25 @@ _Py_uop_sym_get_const_as_stackref(JitOptContext *ctx, JitOptRef sym)
     return PyStackRef_FromPyObjectBorrow(const_val);
 }
 
+/*
+ Indicates whether the type is a known built-in type
+ that is safe to narrow.
+ */
+bool
+_Py_uop_sym_is_safe_type(JitOptRef sym)
+{
+    PyTypeObject *typ = _Py_uop_sym_get_type(sym);
+    if (typ == NULL) {
+        return false;
+    }
+    return (typ == &PyLong_Type) ||
+           (typ == &PyUnicode_Type) ||
+           (typ == &PyFloat_Type) ||
+           (typ == &_PyNone_Type) ||
+           (typ == &PyBool_Type) ||
+           (typ == &PyFrozenDict_Type);
+}
+
 /*
  Indicates whether the constant is safe to constant evaluate
  (without side effects).