GeezerVison

I'm slightly worry that this pass will non-debug builds.

@corona10 (author of #103962), are you sure there is no other way to implement this helper? Return a different value, raise an error?

That's okay as long as it catches the regression in a debug build.

_testcapi is always built with assertions, even in release mode.

I built Python in release mode without PyLong_FromString() fix. _PyLong_FlipSign() assertion didn't catch the bug since Python was built without assertions. But at least, test_immortal_small_ints() was able to detect that small integers were corrupted:

test_bug_143050 (test.test_capi.test_long.LongTests.test_bug_143050) ...
python: ./Modules/_testcapi/immortal.c:35: test_immortal_small_ints: Assertion `has_int_immortal_bit' failed.
Fatal Python error: Aborted

Current thread 0x00007f617b91b780 [python] (most recent call first):
  File "/home/vstinner/python/main/Lib/test/test_capi/test_long.py", line 811 in test_bug_143050

That's okay as long as it catches the regression in a debug build.

IMO the intent here is to check if an integer is a small integer, rather than checking for a tag. So I would prefer to rename the function to _PyLong_IsSmallInt().

I wanted to suggest referring to _PY_IS_SMALL_INT() but I noticed the macro is wrong. I wrote #146631 to fix the macro.

There are two notions for "small int"'s. One - by value and other is - this. If @colesbury is ok - I'm fine with renaming..

_PyLong_HasImmortalTag() and _PY_IS_SMALL_INT() should be true or false for the same values.

For example, you can add the following check for _PyLong_HasImmortalTag():

+    assert((_PyLong_IsCompact(op) && _PY_IS_SMALL_INT(_PyLong_CompactValue(op)))
+           || (!is_small_int));

(You need to update your branch to retrieve my _PY_IS_SMALL_INT() change.)

For me, testing IMMORTALITY_BIT_MASK is just an implementation detail to check if an integer is "a small int".

I've added an assert. BTW, what do you think on #145901 (comment)?

I don't think we should mention _PY_IS_SMALL_INT in the description of the function, assert is readable enough.

For me, testing IMMORTALITY_BIT_MASK is just an implementation detail to check if an integer is "a small int".

For now, we use _PY_IS_SMALL_INT() to check this. But eventually this could be just testing a flag.

I would prefer to rename the function to _PyLong_IsSmallInt().

I've added suggestions.