On Thu, Dec 30, 2021 at 4:47 AM Mark Dickinson <dicki...@gmail.com> wrote:
> 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 > tying the thread together: this is https://bugs.python.org/issue45569 Check 32-bit builds. When I pushed for the 30-bit digit implementation, I wanted it for all builds but if I recall correctly it *might* have changed the minimum structure size for PyLong which could've been an ABI issue? double check that. 32-bit is still important. Raspbian. rpi, rpi zero, and first rev rpi2 are 32-bit arm architectures so even with 64-bit raspbian on the horizon, that won't be the norm. and for those, memory matters so a 32-bit userspace on 64-bit capable hardware is still preferred for small pointer sizes on the majority which have <=4GiB ram. I believe performance was the other concern, 30-bit happens to perform great on 32-bit x86 as it has 32*32->64 multiply hardware. Most 32-bit architectures do not AFAIK, making 30 bit digit multiplies less efficient. And 32-bit x86 was clearly on its way out by the time we adopted 30-bit so it was simpler to just not do it on that dying snowflake of a platform. (test it on raspbian - it's the one that matters) Regardless of possible issues to work out, I'd love us to have a simpler 30-bit only implementation. Granted, modern 64-bit hardware often has 64*64->128 bit multiply hardware so you can imagine going beyond 30 and winding up in complexity land again. at least the extra bits would be >=2 at that point. The reason for digits being a multiple of 5 bits should be revisited vs its original intent and current state of the art "bignum optimized for mostly small numbers" at some point as well. -gps > > > *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/ >
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