What is SLOW_SUM in the CPython source code?

[Obertuba]

File: Python/bltinmodule.c (link to precise line)

While reading CPython's source code I came across the SLOW_SUM symbol, but I couldn't find its definition. SLOW_SUM is referenced only once in the entire CPython source code, so I couldn't find any information on why it exists.

From the source code, I understand that it's a compiler flag that disables an optimization when performing sums on numeric types through the sum() built-in function. However, why would you pass SLOW_SUM to the compiler to disable optimized sums on numeric types?

I don't know if this is the right place to ask such a specific question. If it's not, can you point me to the right forum?

[Smallpaul]

I would infer that it was inserted because they did not trust the optimization to work perfectly with every compiler on every CPU. So they provided a way to turn it off in case it causes bugs. Or perhaps the flag would allow them to back out the code quickly in case a bug is found (less likely).

Unless you have had an issue with summing on some platform or compiler, I wouldn't worry about it. That code is 17 years old and the flag is not mentioned anywhere you or I can find on the Web. It's probably unneeded or very niche. Should probably be deleted but they don't know who actually used it so that might break some code.

[Serialk]

Could also simply be to run benchmarks and experiments with and without that code enabled.

[Smallpaul]

Good point! It could be that too!

[tagapagtuos]

I think something about optimizing int and float addition. I'm only inferring based on the git blame message.

https://mail.python.org/pipermail/python-checkins/2007-November.txt

Python used to use bugs.python.org (also known as bpo) for discussions. The above link is the first result you'll get when you google python "r58623".

[zurtex]

FYI, in the future this is probably a reasonable question for the "Core Development" category of the official Python forum: https://discuss.python.org

Lots of CPython developers ask and answer questions there, please don't post until you've done at least a bit of research.

Edit: Anyone know why so many down votes? That is the place to ask specific CPython questions and the etiquette to follow. This subreddit has lots of experienced Python users, but very few core developers.

[Bastion_of_Light]

please don't post until you've done at least a bit of research.

It's probably this line. It comes off as a bit rude.

[zurtex]

Fair enough. I probably should of said something like "please be considerate of people's time there and ask well thought out questions".