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JIT: Assertion failure in _PyOptimizer_Optimize #143123
Description
Crash report
What happened?
It's possible to abort a patched debug JIT build by running the code below. The interpreter must be called from a venv in which lafleur is installed as editable, likely due to that priming the GC into a state suitable for the crash.
Sorry about the convoluted reproduction steps.
The necessary patch is:
diff --git a/Include/internal/pycore_backoff.h b/Include/internal/pycore_backoff.h
index 23ca7299e0d..0289b5fee7c 100644
--- a/Include/internal/pycore_backoff.h
+++ b/Include/internal/pycore_backoff.h
@@ -124,7 +124,7 @@ trigger_backoff_counter(void)
// For example, 4095 does not work for the nqueens benchmark on pyperformance
// as we always end up tracing the loop iteration's
// exhaustion iteration. Which aborts our current tracer.
-#define JUMP_BACKWARD_INITIAL_VALUE 4000
+#define JUMP_BACKWARD_INITIAL_VALUE 63
#define JUMP_BACKWARD_INITIAL_BACKOFF 6
static inline _Py_BackoffCounter
initial_jump_backoff_counter(void)
@@ -137,7 +137,7 @@ initial_jump_backoff_counter(void)
* Must be larger than ADAPTIVE_COOLDOWN_VALUE,
* otherwise when a side exit warms up we may construct
* a new trace before the Tier 1 code has properly re-specialized. */
-#define SIDE_EXIT_INITIAL_VALUE 4000
+#define SIDE_EXIT_INITIAL_VALUE 63
#define SIDE_EXIT_INITIAL_BACKOFF 6
static inline _Py_BackoffCounterTo install lafleur as editable, first create a venv and activate it, then clone the lafleur repo and run pip install -e /path/to/lafleur/clone.
This MRE must be invoked like this:
/path/to/venv/with/editable/lafleur/install/bin/python mre.py
MRE:
import asyncio
any_v2 = None
tuple_v4 = (1, "")
def f1():
class A:
def _():...
def __(): ...
def ___():...
class B: ...
class C:
def _(): ...
class D:
def _(): ...
class E:
def _(): ...
class F:
def _(): ...
class G:
def _(): ...
class H:
def _(): ...
l = []
for i in range(5):
def _(): ...
class I:
async def _(): ...
try:
c = _()
except Exception: ...
def __(): ...
class J:
def _(): ...
for _ in range(300):
try:
try: ValueError("")
except E: ...
except Exception: ...
(
any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2,
any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2,
any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2,
any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2,
any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2,
any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2,
any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2, any_v2,
any_v2, any_v2,
)
class K:
def _():...
def __(): ...
def ___():...
class AsyncCtx_9078:
def _(self): ...
async def __():
try:
for _ in K(10): ...
except Exception: ...
try:
c = __()
except Exception: ...
x, *y = tuple_v4
class L:
def _():...
def __(): ...
def ___():...
class M:
def _(self): ...
async def __():
try:
for _ in L(10): ...
except Exception: ...
try:
c = __()
except Exception: ...
x, *y = x, *y = tuple_v4
class M:
def _():...
def __(): ...
def ___():...
class N:
def _():...
def __(): ...
def ___():...
class O:
def _(self): ...
async def __():
try:
for _ in N(10): ...
except Exception: ...
try:
c = __()
except Exception: ...
class P:
def _():...
def __(): ...
def ___():...
class Q:
def _(self): ...
async def __():
try:
with Q(): ...
except Exception: ...
try:
c = __()
except Exception: ...
class R:
def _(self): ...
async def __():
try:
with R(): ...
except Exception: ...
try:
c = __()
except Exception: ...
x, *y = x, *y = tuple_v4
class S:
def __init__(self, lit):
selit = self.count = 0
def __aiter__(self):return self
async def __anext__(self):
if self.count: n
self.count += 1
class T:
def _(self): ...
async def __():
try:
async for _ in S(10): ...
except Exception: ...
try:
c = __()
while True:
try: c.send(None)
except Exception: break
except Exception: ...
for i in range(300):
print(i)
f1()Traceback:
1
[...]
200
201
python: Python/optimizer.c:144: int _PyOptimizer_Optimize(_PyInterpreterFrame *, PyThreadState *): Assertion `_tstate->jit_tracer_state.initial_state.func != NULL' failed.
Program received signal SIGABRT, Aborted.
#0 __pthread_kill_implementation (threadid=<optimized out>, signo=6, no_tid=0) at ./nptl/pthread_kill.c:44
#1 __pthread_kill_internal (threadid=<optimized out>, signo=6) at ./nptl/pthread_kill.c:89
#2 __GI___pthread_kill (threadid=<optimized out>, signo=signo@entry=6) at ./nptl/pthread_kill.c:100
#3 0x00007ffff7c45e2e in __GI_raise (sig=sig@entry=6) at ../sysdeps/posix/raise.c:26
#4 0x00007ffff7c28888 in __GI_abort () at ./stdlib/abort.c:77
#5 0x00007ffff7c287f0 in __assert_fail_base (fmt=<optimized out>, assertion=<optimized out>, file=<optimized out>, line=<optimized out>, function=<optimized out>) at ./assert/assert.c:118
#6 0x00007ffff7c3c19f in __assert_fail (assertion=<optimized out>, file=<optimized out>, line=<optimized out>, function=<optimized out>) at ./assert/assert.c:127
#7 0x00005555563c0bd0 in _PyOptimizer_Optimize (frame=frame@entry=0x7e8ff6de5298, tstate=tstate@entry=0x555556e62830 <_PyRuntime+358864>) at Python/optimizer.c:144
#8 0x0000555555ed2e64 in stop_tracing_and_jit (tstate=0x555556e62830 <_PyRuntime+358864>, frame=frame@entry=0x7e8ff6de5298) at Python/ceval.c:1405
#9 0x0000555555e8eeed in _PyEval_EvalFrameDefault (tstate=<optimized out>, frame=<optimized out>, throwflag=<optimized out>) at Python/generated_cases.c.h:11628
#10 0x0000555555e6bf98 in _PyEval_EvalFrame (tstate=0x555556e62830 <_PyRuntime+358864>, frame=0x7e8ff6de5220, throwflag=0) at ./Include/internal/pycore_ceval.h:119
#11 _PyEval_Vector (tstate=<optimized out>, func=<optimized out>, locals=<optimized out>, args=<optimized out>, argcount=<optimized out>, kwnames=0x0) at Python/ceval.c:2483
#12 0x0000555555e6b9b5 in PyEval_EvalCode (co=<optimized out>, globals=<optimized out>, locals=0x7c7ff6e86440) at Python/ceval.c:1008
Output from running with PYTHON_LLTRACE=4 PYTHON_OPT_DEBUG=4:
830_abort_lltrace_opt_debug.txt
Found using lafleur.
CPython versions tested on:
CPython main branch
Operating systems tested on:
Linux
Output from running 'python -VV' on the command line:
Python 3.15.0a3+ (heads/main-dirty:3cc57505e53, Dec 23 2025, 19:04:15) [Clang 21.1.2 (2ubuntu6)]
Activity
Fidget-Spinner commentedon Dec 23, 2025
Can you try on current main please? Mark just fixed a GC problem with the executors a few hours ago.
devdanzin commentedon Dec 23, 2025
Yup, it's fixed on current main.devdanzin commentedon Dec 23, 2025
Nope, I tested an unpatched build. It still aborts on current main with a correctly patched build.
Fidget-Spinner commentedon Dec 23, 2025
@devdanzin can you see if it's fixed by this #143126 please ?
devdanzin commentedon Dec 23, 2025
Yes, that fixes it. I've double checked that I ran the right build this time and confirmed the fix.
Fidget-Spinner commentedon Dec 25, 2025
@devdanzin sorry to bother you, could you please check again on the branch of the latest fix? I updated the PR to try a slightly different approach. Thanks!
devdanzin commentedon Dec 25, 2025
No bother at all, happy to help :)
The new approach works too, no assertion failure with the PR applied.
_tstate->jit_tracer_state.initial_state.func != NULLfailed in_PyOptimizer_Optimize#143610 as a duplicate of this issuedevdanzin commentedon Jan 9, 2026
#143126 also fixes the same issue with the MRE from #143610, an obvious duplicate I reported by mistake.
Fidget-Spinner commentedon Jan 9, 2026
@devdanzin i just updated the PR, could you please try again?
devdanzin commentedon Jan 9, 2026
@Fidget-Spinner: It fixes the assertion failure after you update too.
gh-143123: Protect against recursive tracer calls/finalization (GH-14…
pythongh-143123: Protect against recursive tracer calls/finalization (p…