Remove redundant decref from _PyLong_Negate

[eendebakpt]

Feature or enhancement

Proposal:

In _PyLong_Negate there is a fast path for unique referenced longs. The slow path assumes the argument is a small int. We can add an assert for this condition, and remove the final decref (small ints are immortal).

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

Linked PRs

• gh-143050: Remove redundant decref in _PyLong_Negate (gh-143051) #143051
• Revert "gh-143050: Remove redundant decref in _PyLong_Negate (gh-143051)" #145891
• gh-143050: correct PyLong_FromString() to use _PyLong_Negate() #145901
• [3.14] gh-143050: Correct PyLong_FromString() to use _PyLong_Negate() (GH-145901) #147331
• [3.13] gh-143050: Correct PyLong_FromString() to use _PyLong_Negate() (#145901) #147437
• gh-143050: add helper _PyLong_InitTag() #147956

[serhiy-storchaka]

Even if they are immortal, I prefer to keep incref/decref balanced. This saves time when you read code.

[sergey-miryanov]

Maybe it is worth to replace Py_DECREF with something like Py_DECREF_IMMORTAL?

[eendebakpt]

Maybe it is worth to replace Py_DECREF with something like Py_DECREF_IMMORTAL?

I would be in favor of that. There is a related discussion here #124074

[colesbury]

Thanks

[StanFromIreland]

This introduced a regression that was caught by our OSS Fuzzer (https://oss-fuzz.com/testcase-detail/6342648498814976).

$ ./python -c "
import sys
sys.set_int_max_str_digits(10000)

while True:
    try:
        int('-' + '0' * 7000, 10)
    except:
        pass
"
python: Objects/longobject.c:371: _PyLong_Negate: Assertion `_long_is_small_int((PyObject *)x)' failed.
Aborted                    (core dumped)

full reproducer

To reproduce, download the testcase: https://drive.google.com/file/d/1Ain_DfzbodyKU_YiwtZPaT4ViAOssvOt/view?usp=sharing

And run it to trigger the assert, note that it requires a few iterations:

$ ./python -c "
import sys
sys.set_int_max_str_digits(10000)
with open('/tmp/inttestcase','rb') as f:
    data = f.read().decode('utf-8')

while True:
    try:
        int(data, 10)
    except:
        pass
"
python: Objects/longobject.c:371: _PyLong_Negate: Assertion `_long_is_small_int((PyObject *)x)' failed.
Aborted                    (core dumped)

[eendebakpt]

Thanks for reporting, I will have a look at the reproducer tomorrow.