What processor instructions should be in march and mtune?

[Oleksa75]

Hello there!

Please, I need some advice.

I've got a new processor and ran for detailed instruction set next command:

[pietinger]

Take the simple:

[Oleksa75]

[logrusx]

You can try app-misc/resolve-march-native if you want it "expanded", but it only makes sense if you're configuring distcc or if you're curious.

Best Regards,
Georgi

[CaptainBlood]

pietinger is right keeping it simple.

AFAIK, for amd64 arch here -mtune=native -march=core2 does compile for both native and core2 within same generated binaries.

if mtune(native here) part of the code isn't supported, system will attempt to run -march(core2 here) part of the code: e.g. this would allow to bring HD to a lower system (down to core2) for a chroot and a fix.(Remember bash is the first executable called by chroot)

Beware there is a cost however in binary size.

In arm context -march -mtune may have a slightly different meaning.

Thks 4 ur attention, interest & support.
_________________
USE="-* ..." in /etc/portage/make.conf here.
LT: "I've been doing a passable imitation of the Fontana di Trevi, except my medium is mucus. Sooo much mucus. "

[Anon-E-moose]

Depends on what you're trying to do.

If the code will always and only run on your current processor then -march=native is fine, and leave mtune alone as it dupes march.

If you want to possibly run it on a variety of (modern) processors then set march to something like x86-64-v3 and set mtune to native.
That will allow the code to run on most modern x86 processors while tuning the code for your particular processor.
_________________
PRIME x570-pro, 3700x, 6.1 zen kernel
gcc 13, profile 17.0 (custom bare multilib), openrc, wayland

[logrusx]

[CaptainBlood]

I meant at start.

app-portage/cpuid2cpuflags should help setting e.g. here:

[Goverp]

[CaptainBlood]

Goverp,

You're likely to be right.
I tried to discuss the point years ago on the forum, but never get a answer clear to my understanding.
So my statement is indeed more of a personal view than a knowledge

Never took time to write, compile and bisect dummy but good enough C code in this respect.

Maybe it's time to focus as I consider this a serious matter.

Thks 4 ur attention, interest & support.
_________________
USE="-* ..." in /etc/portage/make.conf here.
LT: "I've been doing a passable imitation of the Fontana di Trevi, except my medium is mucus. Sooo much mucus. "

[Anon-E-moose]

March sets the base level to compile for, mtune sets optimizations at whatever level you set (generic is default)

So march core2 will run at that level of cpu, most modern processors could also run that binary.
But if you don't set mtune, then you get base level optimizations, ie practically none.
Setting march core2 and mtune skylake (for example) will optimize the core2 level code to run the best on the skylake.
_________________
PRIME x570-pro, 3700x, 6.1 zen kernel
gcc 13, profile 17.0 (custom bare multilib), openrc, wayland

[CaptainBlood]

As a result

[Anon-E-moose]

Core 2 is kind of old, I would shoot for the level of Haswell, at least.

But x86_64-v2 to v4 works for more modern processors.
https://en.wikipedia.org/wiki/X86-64 -- scroll down to see more about x85_64-v*

v2 is Intel Nehalem or Amd Bulldozer level

v3 is Intel Haswell or Amd Excavator level

v4 is Intel Skylake or Amd Zen4 level

setting these at the march level gets you general levels of performance, then tune on top of this.
_________________
PRIME x570-pro, 3700x, 6.1 zen kernel
gcc 13, profile 17.0 (custom bare multilib), openrc, wayland

[pietinger]

[CaptainBlood]

-march=core2 here, because alternate system to the skylake one is core2.
I want 2 be able to chroot one HD on another system for fixes.
skylake is headless, so prone to hard 2 fix possible issues, e.g. grub.

But this is a local case.

From a strict performance pov -march=native or equivalent seems king.

Thks 4 ur attention, interest & support.
_________________
USE="-* ..." in /etc/portage/make.conf here.
LT: "I've been doing a passable imitation of the Fontana di Trevi, except my medium is mucus. Sooo much mucus. "

[Anon-E-moose]

[logrusx]

[figueroa]

This isn't definitive but I've wasted a lot of energy looking for the right answer for me. Apparently I didn't quite hit it.

I'm not looking for ultimate optimization. I'm not sure I would notice the difference. I'm running all older hardware from circa 2008-2012. What's in my /etc/portage/make.conf is the following:

[christoph_peter_s]