tl;dr: I'd like to deprecate and eventually remove the option to use 15-bit digits in the PyLong implementation. Before doing so, I'd like to find out whether there's anyone still using 15-bit PyLong digits, and if so, why they're doing so.
History: the use of 30-bit digits in PyLong was introduced for Python 3.1 and Python 2.7, to improve performance of int (Python 3) / long (Python 2) arithmetic. At that time, we retained the option to use 15-bit digits, for two reasons: - (1) use of 30-bit digits required C99 features (uint64_t and friends) at a time when we hadn't yet committed to requiring C99 - (2) it wasn't clear whether 30-bit digits would be a performance win on 32-bit operating systems Twelve years later, reason (1) no longer applies, and I suspect that: - No-one is deliberately using the 15-bit digit option. - There are few machines where using 15-bit digits is faster than using 30-bit digits. But I don't have solid data on either of these suspicions, hence this post. Removing the 15-bit digit option would simplify the code (there's significant mental effort required to ensure we don't break things for 15-bit builds when modifying Objects/longobject.c, and 15-bit builds don't appear to be exercised by the buildbots), remove a hidden compatibility trap (see b.p.o. issue 35037), widen the applicability of the various fast paths for arithmetic operations, and allow for some minor fast-path small-integer optimisations based on the fact that we'd be able to assume that presence of *two* extra bits in the C integer type rather than just one. As an example of the latter: if `a` and `b` are PyLongs that fit in a single digit, then with 15-bit digits and a 16-bit `digit` and `sdigit` type, `a + b` can't currently safely (i.e., without undefined behaviour from overflow) be computed with the C type `sdigit`. With 30-bit digits and a 32-bit `digit` and `sdigit` type, `a + b` is safe. Mark *References* Related b.p.o. issue: https://bugs.python.org/issue45569 MinGW compatibility issue: https://bugs.python.org/issue35037 Introduction of 30-bit digits: https://bugs.python.org/issue4258 _______________________________________________ Python-Dev mailing list -- python-dev@python.org To unsubscribe send an email to python-dev-le...@python.org https://mail.python.org/mailman3/lists/python-dev.python.org/ Message archived at https://mail.python.org/archives/list/python-dev@python.org/message/ZICIMX5VFCX4IOFH5NUPVHCUJCQ4Q7QM/ Code of Conduct: http://python.org/psf/codeofconduct/
