When using MemorySanitizer, maybe_small_long() in Objects/longobject.c is flagged as using uninitialized memory when passed certain values, which means Python main, 312 and 3.11 can't even be built correctly with --with-memorysanitizer:

% CC=clang-13 CXX=clang++-13 ./configure --with-memory-sanitizer
[...]
% make
[...]
./_bootstrap_python Programs/_freeze_module.py abc Lib/abc.py Python/frozen_modules/abc.h
==21198==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x7c5058 in maybe_small_long Objects/longobject.c:62:13
    #1 0x7c5058 in long_bitwise Objects/longobject.c:5267:24
    #2 0x7b81a6 in long_and Objects/longobject.c:5279:12
[...]
  Uninitialized value was created by a heap allocation
    #0 0x45288d in __interceptor_malloc (_bootstrap_python+0x45288d)
    #1 0x878da6 in _PyMem_RawMalloc Objects/obmalloc.c:45:12
    #2 0x87bb6f in PyObject_Malloc Objects/obmalloc.c:801:12
    #3 0x7bf9fc in _PyLong_New Objects/longobject.c:158:14
    #4 0x7bf9fc in long_bitwise Objects/longobject.c:5225:9
    #5 0x7b81a6 in long_and Objects/longobject.c:5279:12
[...]
SUMMARY: MemorySanitizer: use-of-uninitialized-value Objects/longobject.c:62:13 in maybe_small_long
Exiting
make: *** [Makefile:1332: Python/frozen_modules/abc.h] Error 1

The source of the problem is _PyLong_CompactValue (from Include/cpython/longintrepr.h) multiplying ob_digit[0] by size, where ob_digit[0] may be uninitialized when size is 0 (the simplest reproducer for that is int('0')). (In Python 3.11 this same logic happens in medium_value() in longobject.c instead. Python 3.10 had similar logic in the MEDIUJM_VALUE macro in longobject.c, but it had an explicit check for size being 0.)

(I believe, but I'm not entirely sure, the code relies on undefined behaviour, which we should avoid -- although it's only undefined for very esoteric reasons. Even so, the simple solution is to add back the check for sign being 0. If that really has a noticeable impact, we could add the branch only when using MemorySanitizer.)

I believe Greg was working on getting us to a point where we could have a MSan buildbot (#79395), although I don't know what the state of that is now.