What I’ve come to believe is this: you should work at a level of abstraction you’re comfortable with, but you should also understand the layer beneath it.
If you’re a C programmer, you should have some idea of how the C runtime works, and how it interacts with the operating system. You don’t need every detail, but you need enough to know what’s going on when something breaks. Because one day printf won’t work, and if the layer below is a total mystery, you won’t even know where to start looking.
So: know one layer well, have working knowledge of the layer under it, and, most importantly, be aware of the shape of the layer below that.
Also this article in acmqueue by Matt is not new at all, but super great introduction to these types of optimizations.
GCC version: 11.3 target: Cortex-A9 Qt version: 5.15
I think we tested single core and quad core, also possibly a newer GCC version, but I'm not sure. Just wanted to add my two cents.
Many thanks to him for that.
Between that and compiler explorer, it is fair to say he made the world a better place for many of us, developers.
-O2 is basically all you usually need. As you update your compiler, it'll end up tweaking exactly what that general optimization does based on what they know today.
Because that's the thing about these flags, you'll generally set them once at the beginning of a project. Compiler authors will reevaluate them way more than you will.
Also, a trap I've observed is setting flags based on bad benchmarks. This applies more to the JVM than a C++ compiler, but never the less, a system's current state is somewhat random. 1->2% fluctuations in performance for even the same app is normal. A lot of people won't realize that and ultimately add flags based on those fluctuations.
But further, how code is currently layed out can affect performance. You may see a speed boost not because you tweaked the loop unrolling variable, but rather your tweak may have relocated a hot path to be slightly more cache friendly. A change in the code structure can eliminate that benefit.
To be able to support multiple arch levels in the same binary I think you still need to do manual work of annotating specific functions where several versions should be generated and dispatched at runtime.
If you know the architecture and oldest CPU model, we're better served with added a bunch more flags, no?
I wish I could compile my server code to target CPU released on/after a particular date like:
-O2 -cpu-newer-than=2019-O2 in gcc has vectorization flags set which will use avx if the target CPU supports it. It is less aggressive on vectorization than -O3.
Flags from -O3 often flow down into -O2 as they are proven generally beneficial.
That said, I don't think -O3 has the problems it once did.
In my experience a team of 200 developers will see 1 compiler bug affect them every 10 years. This isn't scientific, but it is a good rule of thumb and may put the above in perspective.
In the case of open source compilers the bug was generally fixed upstream and we just needed to get on a newer release.
You generally avoid O3 because it's slower. Slower to compile, and slower to run. Aggressively unrolling loops and larger inlining windows bloat code size to the degree it impacts icache.
The optimization levels aren't "how fast do you want to code to go", they're "how aggressive do you want the optimizer to be." The most aggressive optimizations are largely unproven and left in O3 until they are generally useful, at which point they move to O2.
where is the problem to be solved?