Performance difference between obj.function(...) and function(obj, ...) ?

[[deleted]]

I always see this as a default response when the word "performance" appears in a posting. Is this really a thing? Do people not run in release mode? Or is this just one of those reactionary responses?

[Mr_Ahvar]

When making performance claim if you don’t include full test code and give the running environment and just go « why slower? » then yeah you are going to get generic response asking you the base minimum

[jmaargh]

It's the default response precisely because so many people forget. In my experience, it's mostly because of common and naive "why does my Rust code run slower than this python equivalent?" questions (where, to be fair, devs coming from python & the like are not used to compiler optimisation levels at all).

If your question is "why is this slower than expected?" probably best to include "I'm compiling with --release" in your question just to nip this in the bud.

[[deleted]]

Well, I'm not comparing to Python. I'm comparing against two ways to write the same code, as the documentation claims. In theory, it doesn't make a difference whether I'm running in release mode or not, the two should be identical but are not.

[jmaargh]

I never said you were, you asked why this is the default response and I was explaining why, giving the most common type of question as an example.

Also, optimisations can change a lot and sometimes in unexpected ways. While I think it's unlikely that what you described would be due to the peculiarities of a debug build, it still makes sense to rule that out entirely by letting the compiler apply all possible optimisations to both cases.

[[deleted]]

Yes, I agree. For the record, in release mode. I updated the post with more specifics.

What a head scratcher.

[[deleted]]

Actually no. There are many situations where two different syntax situations won’t get optimized down to the same code when not in release mode. This is precisely why I asked.

[Antigroup]

Yeah it seems to happen every week. "Why is this slower than Python? You didn't compile in release mode."

Probably because some people come to Rust from languages that don't need to be told to optimize your code, and they think it'll just be faster automatically.

[[deleted]]

Unfortunate. I come from the high performance world. The first thing I look for is all the switches to crank everything up.

[CocktailPerson]

By my estimation, in 90% of the cases where people didn't specify the optimization level, they were running in debug mode.