> Never ever build in '-03 -march=native' by default. This is always a red flag and a sign of immaturity.
Perhaps you can see how there are some assumptions baked into that statement.
Perhaps you can see how there are some assumptions baked into that statement.
Shipping anything built with -march=native is a horrible idea. Even on homogeneous targets like one of the clouds, you never know if they'll e.g. switch CPU vendors.
The correct thing to do is use microarch levels (e.g. x86-64-v2) or build fully generic if the target architecture doesn't have MA levels.
I am willing to hear arguments for other approaches.
it certainly has scale issues when you need to support larger deployments.
[P.S.: the way I understand the words, "shipping" means "passing it off to someone else, likely across org boundaries" whereas what you're doing I'd call "deploying"]
> when you need to support larger deployments
> shipping
> passing it off to someone else
-mtune says "generate code that is optimised for this architecture" but it doesn't trigger arch specific features.
Whether these are right or not depends on what you are doing. If you are building gentoo on your laptop you should absolutely -mtune=native and -march=native. That's the whole point: you get the most optimised code you can for your hardware.
If you are shipping code for a wide variety of architectures and crucially the method of shipping is binary form then you want to think more about what you might want to support. You could do either: if you're shipping standard software pick a reasonable baseline (check what your distribution uses in its cflags). If however you're shipping compute-intensive software perhaps you load a shared object per CPU family or build your engine in place for best performance. The Intel compiler quite famously optimised per family, included all the copies in the output and selected the worst one on AMD ;) (https://medium.com/codex/fixing-intel-compilers-unfair-cpu-d...)
Or on newer CPUs of the same vendor (e.g. AMD dropped some instructions in Zen that Intel didn't pick up) or even in different CPUs of the same generation (Intel market segmenting shenanigans with AVX512).
I’ve never heard of anyone doing that.
If you use a cloud provider and use a remote development environment (VSCode remote/Jetbrains Gateway) then you’re wrong: cloud providers swap out the CPUs without telling you and can sell newer CPUs at older prices if theres less demand for the newer CPUs; you can’t rely on that.
To take an old naming convention, even an E3-Xeon CPU is not equivalent to an E5 of the same generation. I’m willing to bet it mostly works but your claim “I build on the exact hardware I ship on” is much more strict.
The majority of people I know use either laptops or workstations with Xeon workstation or Threadripper CPUs— but when deployed it will be a Xeon scalable datacenter CPU or an Epyc.
Hell, I work in gamedev and we cross compile basically everything for consoles.
Some people, gasp, run physical hardware, that they bought.
If you're deploying the binary to more than one machine, you quickly run into issues where the CPUs are different and you would need to rebuild for each of them. This is feasible if you have a couple of machines that you generally upgrade together, but quickly falls apart at just slightly more than 2 machines.
And you use them for remote development?
I think this is highly unusual.
Nobody is saying dev machines are building code that ships to their servers though… quite the opposite, a dev machine builds software for local use… a server builds software for running on other servers. And yes, often build machines are the same spec as the production ones, because they were all bought together. It’s not really rare. (Well, not using the cloud in general is “rare” but, that’s what we’re discussing.)
The size of your L1/L2/L3 cache or the number of TLB misses doesn't matter too much if your python web service is just waiting for packets.
And then, the binary that the end user receives will actually have been built on one of the CI systems. I bet they don't all have quite the same spec. And the above argument applies anyway.
> I build on the exact hardware I intend to deploy my software to and ship it to another machine with the same specs as the one it was built on.
This is exactly the use case in HPC. We always build -march=native and go to some trouble to enable all the appropriate vectorization flags (e.g., for PowerPC) that don't come along automatically with the -march=native setting.
Every HPC machine is a special snowflake, often with its own proprietary network stack, so you can forget about binaries being portable. Even on your own machine you'll be recompiling your binaries every time the machine goes down for a major maintenance.
Still, they are all the same breed.
Quite hard to build on the exact hardware for those scenarios.
However I'm not sure about -O3. I know it can make the binary larger, not sure about other downsides.
IME -O3 should only be used if you have benchmarks that show -O3 actually produces a speedup for your specific codebase.
It is completely fine to use -march=native, just do not make it the default for someone building your project.
That should always be something to opt-in.
The main reason is that software are a composite of (many) components. It becomes quickly a pain in the ass of maintainability if any tiny library somewhere try to sneak in '-march=native' that will make the final binary randomly crash with an illegal instruction error if executed on any CPU that is not exactly the same than the host.
When you design a build system configuration, think for the others first (the users of your software), and yourself after.
I fully concur with that whole post as someone who also maintained a C++ codebase used in production.