diff --git a/Lib/test/test_capi/test_opt.py b/Lib/test/test_capi/test_opt.py
index 651148336f7f2c..73ce55400adcdf 100644
--- a/Lib/test/test_capi/test_opt.py
+++ b/Lib/test/test_capi/test_opt.py
@@ -1959,6 +1959,121 @@ def testfunc(n):
self.assertNotIn("_GUARD_THIRD_NULL", uops)
self.assertNotIn("_GUARD_CALLABLE_ISINSTANCE", uops)
+ def test_call_isinstance_is_true(self):
+ def testfunc(n):
+ x = 0
+ for _ in range(n):
+ y = isinstance(42, int)
+ if y:
+ x += 1
+ return x
+
+ res, ex = self._run_with_optimizer(testfunc, TIER2_THRESHOLD)
+ self.assertEqual(res, TIER2_THRESHOLD)
+ self.assertIsNotNone(ex)
+ uops = get_opnames(ex)
+ self.assertIn("_CALL_ISINSTANCE", uops)
+ self.assertNotIn("_TO_BOOL_BOOL", uops)
+ self.assertNotIn("_GUARD_IS_TRUE_POP", uops)
+
+ def test_call_isinstance_is_false(self):
+ def testfunc(n):
+ x = 0
+ for _ in range(n):
+ y = isinstance(42, str)
+ if not y:
+ x += 1
+ return x
+
+ res, ex = self._run_with_optimizer(testfunc, TIER2_THRESHOLD)
+ self.assertEqual(res, TIER2_THRESHOLD)
+ self.assertIsNotNone(ex)
+ uops = get_opnames(ex)
+ self.assertIn("_CALL_ISINSTANCE", uops)
+ self.assertNotIn("_TO_BOOL_BOOL", uops)
+ self.assertNotIn("_GUARD_IS_FALSE_POP", uops)
+
+ def test_call_isinstance_subclass(self):
+ def testfunc(n):
+ x = 0
+ for _ in range(n):
+ y = isinstance(True, int)
+ if y:
+ x += 1
+ return x
+
+ res, ex = self._run_with_optimizer(testfunc, TIER2_THRESHOLD)
+ self.assertEqual(res, TIER2_THRESHOLD)
+ self.assertIsNotNone(ex)
+ uops = get_opnames(ex)
+ self.assertIn("_CALL_ISINSTANCE", uops)
+ self.assertNotIn("_TO_BOOL_BOOL", uops)
+ self.assertNotIn("_GUARD_IS_TRUE_POP", uops)
+
+ def test_call_isinstance_unknown_object(self):
+ def testfunc(n):
+ x = 0
+ for _ in range(n):
+ # The optimizer doesn't know the return type here:
+ bar = eval("42")
+ # This will only narrow to bool:
+ y = isinstance(bar, int)
+ if y:
+ x += 1
+ return x
+
+ res, ex = self._run_with_optimizer(testfunc, TIER2_THRESHOLD)
+ self.assertEqual(res, TIER2_THRESHOLD)
+ self.assertIsNotNone(ex)
+ uops = get_opnames(ex)
+ self.assertIn("_CALL_ISINSTANCE", uops)
+ self.assertNotIn("_TO_BOOL_BOOL", uops)
+ self.assertIn("_GUARD_IS_TRUE_POP", uops)
+
+ def test_call_isinstance_tuple_of_classes(self):
+ def testfunc(n):
+ x = 0
+ for _ in range(n):
+ # A tuple of classes is currently not optimized,
+ # so this is only narrowed to bool:
+ y = isinstance(42, (int, str))
+ if y:
+ x += 1
+ return x
+
+ res, ex = self._run_with_optimizer(testfunc, TIER2_THRESHOLD)
+ self.assertEqual(res, TIER2_THRESHOLD)
+ self.assertIsNotNone(ex)
+ uops = get_opnames(ex)
+ self.assertIn("_CALL_ISINSTANCE", uops)
+ self.assertNotIn("_TO_BOOL_BOOL", uops)
+ self.assertIn("_GUARD_IS_TRUE_POP", uops)
+
+ def test_call_isinstance_metaclass(self):
+ class EvenNumberMeta(type):
+ def __instancecheck__(self, number):
+ return number % 2 == 0
+
+ class EvenNumber(metaclass=EvenNumberMeta):
+ pass
+
+ def testfunc(n):
+ x = 0
+ for _ in range(n):
+ # Only narrowed to bool
+ y = isinstance(42, EvenNumber)
+ if y:
+ x += 1
+ return x
+
+ res, ex = self._run_with_optimizer(testfunc, TIER2_THRESHOLD)
+ self.assertEqual(res, TIER2_THRESHOLD)
+ self.assertIsNotNone(ex)
+ uops = get_opnames(ex)
+ self.assertIn("_CALL_ISINSTANCE", uops)
+ self.assertNotIn("_TO_BOOL_BOOL", uops)
+ self.assertIn("_GUARD_IS_TRUE_POP", uops)
+
def global_identity(x):
return x
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2025-04-26-17-50-01.gh-issue-131798.XiOgw5.rst b/Misc/NEWS.d/next/Core_and_Builtins/2025-04-26-17-50-01.gh-issue-131798.XiOgw5.rst
new file mode 100644
index 00000000000000..45ab1bea6b1dd7
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2025-04-26-17-50-01.gh-issue-131798.XiOgw5.rst
@@ -0,0 +1,2 @@
+Narrow the return type and constant-evaluate ``CALL_ISINSTANCE`` for a
+subset of known values in the JIT. Patch by Tomas Roun
diff --git a/Python/optimizer_bytecodes.c b/Python/optimizer_bytecodes.c
index 7c160cdcb0c149..ad25c68e62aab2 100644
--- a/Python/optimizer_bytecodes.c
+++ b/Python/optimizer_bytecodes.c
@@ -890,6 +890,26 @@ dummy_func(void) {
}
}
+ op(_CALL_ISINSTANCE, (unused, unused, instance, cls -- res)) {
+ // the result is always a bool, but sometimes we can
+ // narrow it down to True or False
+ res = sym_new_type(ctx, &PyBool_Type);
+ PyTypeObject *inst_type = sym_get_type(instance);
+ PyTypeObject *cls_o = (PyTypeObject *)sym_get_const(ctx, cls);
+ if (inst_type && cls_o && sym_matches_type(cls, &PyType_Type)) {
+ // isinstance(inst, cls) where both inst and cls have
+ // known types, meaning we can deduce either True or False
+
+ // The below check is equivalent to PyObject_TypeCheck(inst, cls)
+ if (inst_type == cls_o || PyType_IsSubtype(inst_type, cls_o)) {
+ sym_set_const(res, Py_True);
+ }
+ else {
+ sym_set_const(res, Py_False);
+ }
+ }
+ }
+
op(_GUARD_IS_TRUE_POP, (flag -- )) {
if (sym_is_const(ctx, flag)) {
PyObject *value = sym_get_const(ctx, flag);
diff --git a/Python/optimizer_cases.c.h b/Python/optimizer_cases.c.h
index deb912662e4b0b..5d469765203833 100644
--- a/Python/optimizer_cases.c.h
+++ b/Python/optimizer_cases.c.h
@@ -2124,8 +2124,22 @@
}
case _CALL_ISINSTANCE: {
+ JitOptSymbol *cls;
+ JitOptSymbol *instance;
JitOptSymbol *res;
- res = sym_new_not_null(ctx);
+ cls = stack_pointer[-1];
+ instance = stack_pointer[-2];
+ res = sym_new_type(ctx, &PyBool_Type);
+ PyTypeObject *inst_type = sym_get_type(instance);
+ PyTypeObject *cls_o = (PyTypeObject *)sym_get_const(ctx, cls);
+ if (inst_type && cls_o && sym_matches_type(cls, &PyType_Type)) {
+ if (inst_type == cls_o || PyType_IsSubtype(inst_type, cls_o)) {
+ sym_set_const(res, Py_True);
+ }
+ else {
+ sym_set_const(res, Py_False);
+ }
+ }
stack_pointer[-4] = res;
stack_pointer += -3;
assert(WITHIN_STACK_BOUNDS());
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