Comment by pron
4 hours ago
This has absolutely nothing to do with what I said. I wasn't referring to people who think that program performance doesn't matter (although I'm sure there are many of those) but to people working on code that either doesn't impact the overall program's performance much or it does but not due to layout. The number of developers working on code where layout is a major contributor to performance is relatively low, and this includes people working on programs where layout does impact performance significantly (because even in such a program, that particular hot path is not touched by every developer).
> but to people working on code that either doesn't impact the overall program's performance much or it does but not due to layout
And that's the problem. Who decides that? How do you know and that's my problem with it. Things always change. It's always temporary, not in the hot path, doesn't matter etc until it does.
So what is considered "doesn't impact" often comes back to bite.
That is why profiling is the only way to good performance. It's what lets you know what matters, and it's the only thing that does or can. I've been doing low-level (as well as high level) programming for more than 25 years, and I don't know in advance what is more efficient than what. An operation that was inefficient in the program I wrote yesterday under high contention or bad branch prediciton could be efficient in the program I'll write tomorrow. I can only know that if I profile my specific program (and when writing code for different architectures, I need to profile my program on all of them, because what's efficient on x86-64 may be inefficient on Aarch64 or vice-versa). The days we could tell that something is efficient or not, except for the obvious cases, are gone. Computers, at both the hardware and software infrastructure layers, don't work like that anymore.
If your profile shows you a hot path that's responsible for 90% of the time your program spends, any second optimising anything outside of it harms your performance, as it's a second spent on low ROI instead of high ROI.