Crash report

What happened?

Description

CPython's JIT optimizer crashes with an assertion failure probable_callable != NULL in optimize_uops (Python/optimizer_cases.c.h:4307) when a tracing function is installed and immediately removed via sys.settrace, followed by a loop that calls a method on type-unstable objects enough times to cross the JIT compilation threshold. The optimizer fails to find a non-null callable when processing a CALL uop whose receiver type varies across iterations, causing the assertion to fire.

Reproducer

import sys

D = True
A = True

class Tracer:
    def trace(self, frame, event, arg):
        return self.trace

sys.settrace(Tracer().trace)
sys.settrace(None)

class Cls:
    def __init__(self, val):
        self.val = val
    def method(self, x):
        return self.val

for i in range(200):
    try:
        o = Cls(D) if i % 3 == 0 else (int(A or 0) if i % 3 == 1 else str(A or ''))
        o.method(D) if hasattr(o, 'method') else str(o)
    except Exception:
        pass

Command

PYTHON_JIT=1 python reproduce.py

Expected behavior

The JIT optimizer should handle the case where no single callable type can be determined for a polymorphic call site, and should either bail out of optimization or emit a safe fallback. It should not crash with an assertion failure.

Actual behavior

python: ../Python/optimizer_cases.c.h:4307:
  int optimize_uops(...):
  Assertion `probable_callable != NULL' failed.
Aborted (core dumped)

Root cause

The loop calls o.method(D) where o alternates between a Cls instance, an int, and a str across iterations. After enough iterations to trigger JIT compilation, optimize_uops processes the CALL uop for this call site and attempts to determine the most likely callable via probable_callable. Because the receiver type is not stable across iterations, probable_callable remains NULL — but the optimizer unconditionally dereferences it without a null guard, triggering the assertion. The prior sys.settrace install-and-clear sequence appears to affect the interpreter's specialization state in a way that causes the JIT to attempt optimization of this polymorphic call site rather than bailing out early.

Environment

• CPython debug build
• Crash site: Python/optimizer_cases.c.h:4307 (optimize_uops)

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Output from running 'python -VV' on the command line:

No response