Crash report

We like to pretend that _Py_Dealloc (and, by extension, Py_DECREF, Py_XDECREF, Py_CLEAR, PyStackRef_CLOSE, PyStackRef_CLEAR, etc...) isn't an escaping call in tier two. However, it's perfectly capable of invalidating the world. For example, the following code crashes when run under the JIT:

import sys

class Crashy:
    def __del__(self):
        ns = sys._getframe(1).f_locals
        if ns["i"] == 999:
            ns["i"] = None

def crashy():
    for i in range(1000):
        n = Crashy()
        i + i  # Remove guards for i.
        n = None  # Change i.
        i + i  # Crash!

crashy()

Longer term, we're hoping that removing refcounting operations in tier two will help us out here. Fortunately, the _SET_IP and _CHECK_VALIDITY instructions necessary to fix this issue aren't expensive enough to show up on benchmarks when added back (for now).

Linked PRs

• GH-124483: Treat _Py_Dealloc as an escaping call in tier two #124615
• GH-124483: Mark Py_DECREF, etc. as escaping for the JIT #128678