GH-133231: Add JIT utilities in sys._jit

Doc/library/sys.rst

@@ -1282,6 +1282,64 @@ always available. Unless explicitly noted otherwise, all variables are read-only
 
  .. versionadded:: 3.5
 
+.. data:: _jit
+
+ Utilities for observing just-in-time compilation.
+
+ .. impl-detail::
+
+ JIT compilation is an *experimental implementation detail* of CPython.
+ ``sys._jit`` is not guaranteed to exist or behave the same way in all
+ Python implementations, versions, or build configurations.
+
+ .. versionadded:: next
+
+ .. function:: _jit.is_available()
+
+ Return ``True`` if the current Python executable supports JIT compilation,
+ and ``False`` otherwise. This can be controlled by building CPython with
+ the ``--experimental-jit`` option on Windows, and the
+ :option:`--enable-experimental-jit` option on all other platforms.
+
+ .. function:: _jit.is_enabled()
+
+ Return ``True`` if JIT compilation is enabled for the current Python
+ process (implies :func:`sys._jit.is_available`), and ``False`` otherwise.
+ If JIT compilation is available, this can be controlled by setting the
+ :envvar:`PYTHON_JIT` environment variable to ``0`` (disabled) or ``1``
+ (enabled) at interpreter startup.
+
+ .. function:: _jit.is_active()
+
+ Return ``True`` if the topmost Python frame is currently executing JIT
+ code (implies :func:`sys._jit.is_enabled`), and ``False`` otherwise.
+
+ .. note::
+
+ This function is intended for testing and debugging the JIT itself.
+ It should be avoided for any other purpose.
+
+ .. note::
+
+ Due to the nature of tracing JIT compilers, repeated calls to this
+ function may give surprising results. For example, branching on its
+ return value will likely lead to unexpected behavior (if doing so
+ causes JIT code to be entered or exited):
+
+ .. code-block:: pycon
+
+ >>> for warmup in range(BIG_NUMBER):
+ ... # This line is "hot", and is eventually JIT-compiled:
+ ... if sys._jit.is_active():
+ ... # This line is "cold", and is run in the interpreter:
+ ... assert sys._jit.is_active()
+ ...
+ Traceback (most recent call last):
+ File "<stdin>", line 5, in <module>
+ assert sys._jit.is_active()
+ ~~~~~~~~~~~~~~~~~~^^
+ AssertionError
+
 .. data:: last_exc
 
  This variable is not always defined; it is set to the exception instance

Doc/using/cmdline.rst

@@ -1279,6 +1279,14 @@ conflict.
 
  .. versionadded:: 3.14
 
+.. envvar:: PYTHON_JIT
+
+ On builds where experimental just-in-time compilation is available, this
+ variable can force the JIT to be disabled (``0``) or enabled (``1``) at
+ interpreter startup.
+
+ .. versionadded:: 3.13
+
 Debug-mode variables
 ~~~~~~~~~~~~~~~~~~~~
 

Lib/test/libregrtest/utils.py

@@ -335,43 +335,11 @@ def get_build_info():
  build.append('with_assert')
 
  # --enable-experimental-jit
- tier2 = re.search('-D_Py_TIER2=([0-9]+)', cflags)
- if tier2:
- tier2 = int(tier2.group(1))
-
- if not sys.flags.ignore_environment:
- PYTHON_JIT = os.environ.get('PYTHON_JIT', None)
- if PYTHON_JIT:
- PYTHON_JIT = (PYTHON_JIT != '0')
- else:
- PYTHON_JIT = None
-
- if tier2 == 1: # =yes
- if PYTHON_JIT == False:
- jit = 'JIT=off'
- else:
- jit = 'JIT'
- elif tier2 == 3: # =yes-off
- if PYTHON_JIT:
- jit = 'JIT'
+ if sys._jit.is_available():
+ if sys._jit.is_enabled():
+ build.append("JIT")
  else:
- jit = 'JIT=off'
- elif tier2 == 4: # =interpreter
- if PYTHON_JIT == False:
- jit = 'JIT-interpreter=off'
- else:
- jit = 'JIT-interpreter'
- elif tier2 == 6: # =interpreter-off (Secret option!)
- if PYTHON_JIT:
- jit = 'JIT-interpreter'
- else:
- jit = 'JIT-interpreter=off'
- elif '-D_Py_JIT' in cflags:
- jit = 'JIT'
- else:
- jit = None
- if jit:
- build.append(jit)
+ build.append("JIT (disabled)")
 
  # --enable-framework=name
  framework = sysconfig.get_config_var('PYTHONFRAMEWORK')

Lib/test/support/__init__.py

@@ -2648,13 +2648,9 @@ def exceeds_recursion_limit():
 
 Py_TRACE_REFS = hasattr(sys, 'getobjects')
 
-try:
- from _testinternalcapi import jit_enabled
-except ImportError:
- requires_jit_enabled = requires_jit_disabled = unittest.skip("requires _testinternalcapi")
-else:
- requires_jit_enabled = unittest.skipUnless(jit_enabled(), "requires JIT enabled")
- requires_jit_disabled = unittest.skipIf(jit_enabled(), "requires JIT disabled")
+_JIT_ENABLED = sys._jit.is_enabled()
+requires_jit_enabled = unittest.skipUnless(_JIT_ENABLED, "requires JIT enabled")
+requires_jit_disabled = unittest.skipIf(_JIT_ENABLED, "requires JIT disabled")
 
 
 _BASE_COPY_SRC_DIR_IGNORED_NAMES = frozenset({

Lib/test/test_capi/test_misc.py

@@ -306,7 +306,7 @@ def test_getitem_with_error(self):
  CURRENT_THREAD_REGEX +
  r' File .*, line 6 in <module>\n'
  r'\n'
- r'Extension modules: _testcapi, _testinternalcapi \(total: 2\)\n')
+ r'Extension modules: _testcapi\(total: 1\)\n')
  else:
  # Python built with NDEBUG macro defined:
  # test _Py_CheckFunctionResult() instead.

Lib/test/test_dis.py

@@ -1336,7 +1336,7 @@ def test_loop_quicken(self):
  # Loop can trigger a quicken where the loop is located
  self.code_quicken(loop_test)
  got = self.get_disassembly(loop_test, adaptive=True)
- jit = import_helper.import_module("_testinternalcapi").jit_enabled()
+ jit = sys._jit.is_enabled()
  expected = dis_loop_test_quickened_code.format("JIT" if jit else "NO_JIT")
  self.do_disassembly_compare(got, expected)
 

Lib/test/test_sys.py

@@ -2196,6 +2196,64 @@ def test_remote_exec_in_process_without_debug_fails_xoption(self):
  self.assertIn(b"Remote debugging is not enabled", err)
  self.assertEqual(out, b"")
 
+class TestSysJIT(unittest.TestCase):
+
+ def test_jit_is_available(self):
+ available = sys._jit.is_available()
+ script = f"import sys; assert sys._jit.is_available() is {available}"
+ assert_python_ok("-c", script, PYTHON_JIT="0")
+ assert_python_ok("-c", script, PYTHON_JIT="1")
+
+ def test_jit_is_enabled(self):
+ available = sys._jit.is_available()
+ script = "import sys; assert sys._jit.is_enabled() is {enabled}"
+ assert_python_ok("-c", script.format(enabled=False), PYTHON_JIT="0")
+ assert_python_ok("-c", script.format(enabled=available), PYTHON_JIT="1")
+
+ def test_jit_is_active(self):
+ available = sys._jit.is_available()
+ script = textwrap.dedent(
+ """
+ import _testcapi
+ import _testinternalcapi
+ import sys
+
+ def frame_0_interpreter() -> None:
+ assert sys._jit.is_active() is False
+
+ def frame_1_interpreter() -> None:
+ assert sys._jit.is_active() is False
+ frame_0_interpreter()
+ assert sys._jit.is_active() is False
+
+ def frame_2_jit(expected: bool) -> None:
+ # Inlined into the last loop of frame_3_jit:
+ assert sys._jit.is_active() is expected
+ # Insert C frame:
+ _testcapi.pyobject_vectorcall(frame_1_interpreter, None, None)
+ assert sys._jit.is_active() is expected
+
+ def frame_3_jit() -> None:
+ # JITs just before the last loop:
+ for i in range(_testinternalcapi.TIER2_THRESHOLD + 1):
+ # Careful, doing this in the reverse order breaks tracing:
+ expected = {enabled} and i == _testinternalcapi.TIER2_THRESHOLD
+ assert sys._jit.is_active() is expected
+ frame_2_jit(expected)
+ assert sys._jit.is_active() is expected
+
+ def frame_4_interpreter() -> None:
+ assert sys._jit.is_active() is False
+ frame_3_jit()
+ assert sys._jit.is_active() is False
+
+ assert sys._jit.is_active() is False
+ frame_4_interpreter()
+ assert sys._jit.is_active() is False
+ """
+ )
+ assert_python_ok("-c", script.format(enabled=False), PYTHON_JIT="0")
+ assert_python_ok("-c", script.format(enabled=available), PYTHON_JIT="1")
 
 
 if __name__ == "__main__":

Misc/NEWS.d/next/Core_and_Builtins/2025-04-30-19-07-11.gh-issue-133231.H9T8g_.rst

@@ -0,0 +1,3 @@
+Add new utilities of observing JIT compilation:
+:func:`sys._jit.is_available`, :func:`sys._jit.is_enabled`, and
+:func:`sys._jit.is_active`.

Modules/_testinternalcapi.c

@@ -1206,13 +1206,6 @@ verify_stateless_code(PyObject *self, PyObject *args, PyObject *kwargs)
  Py_RETURN_NONE;
 }
 
-
-static PyObject *
-jit_enabled(PyObject *self, PyObject *arg)
-{
- return PyBool_FromLong(_PyInterpreterState_GET()->jit);
-}
-
 #ifdef _Py_TIER2
 
 static PyObject *
@@ -2337,7 +2330,6 @@ static PyMethodDef module_functions[] = {
  METH_VARARGS | METH_KEYWORDS, NULL},
  {"verify_stateless_code", _PyCFunction_CAST(verify_stateless_code),
  METH_VARARGS | METH_KEYWORDS, NULL},
- {"jit_enabled", jit_enabled, METH_NOARGS, NULL},
 #ifdef _Py_TIER2
  {"add_executor_dependency", add_executor_dependency, METH_VARARGS, NULL},
  {"invalidate_executors", invalidate_executors, METH_O, NULL},