[Answered] "native" Cflag question

MaximeG · l33t Joined: 15 Apr 2008 Posts: 722 Location: Belgium

Hi,

I was wondering whether the '-march=native' Cflag works well for an Intel Core i7 CPU ?

Regards,
Maxime
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Veldrin · Posted: Wed May 06, 2009 8:58 am Post subject:

Without having hands on experience, consider the following.

the -march=native flags looks at the cpu specs and then decide from a table what optimisations should be taken (or are considered useful form the developers point of view). It basically simplifies the selection process for the "best" -march=XXX option.
IIRC gcc-4.3 has no profiles for corei7, thus the best optimisation you will get is (probably) core2.

and you can test it for yourself. (To be honest, I do not remember where the code comes from, but I guess from somewhere in this forum)

MaximeG · l33t Joined: 15 Apr 2008 Posts: 722 Location: Belgium

All right.

I'm currently using 'core2' with sse4, sse4.1, sse4.2.
I'll do the command for both core2 and native, trying to spot the differences

Thanks,
Maxime
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d2_racing · Posted: Wed May 06, 2009 11:35 am Post subject:

For my concern, I use -march=native, because if Gcc release a new Cflags that is valid for my CPU, then I will use it automagically.

Veldrin · Posted: Wed May 06, 2009 11:51 am Post subject:

fair enough

@MaximeG:
I completely forgot about the new instrucation sets in Corei7 - thus native could simplify the your config (resp CFLAGS line)

@d2_racing:
true - I sort of start to wonder why I stopped using it. (IIRC native can cause some ugly problems when used in cross-compiling over different archs [i.e core2 and athlon64])

Note to myself: switch to native.

cheers
V.

d2_racing · Posted: Wed May 06, 2009 12:09 pm Post subject:

I think that GCC 4.4 will be optimised for CoreI7 or I wish that a release of it will include that CPU.

Right now, I think that -march=native will see a CoreI7 as a Core2 arch.

MaximeG · l33t Joined: 15 Apr 2008 Posts: 722 Location: Belgium

Yes, that's what I tought.

However I was wondering if native would behave as if it was Core2, or wouldn't behave at all

Thanks,
Maxime
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jmartos · Tux's lil' helper Joined: 23 Mar 2008 Posts: 76

I tried the above code with both "core2" and "native" on my system with dual Xeon E5530's and the only additional parameters that "native" added were the following.

d2_racing · Posted: Thu May 07, 2009 1:31 pm Post subject:

And right now, I have no idea if core2 is faster then native or the opposite.

hielvc · Posted: Sat May 09, 2009 5:49 pm Post subject:

If you use "native" or in my case k8-see3 and your running a 32bit system built with march=athlon-xp, does this change whats compiled to 64bits :?:

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Martux · Veteran Joined: 04 Feb 2005 Posts: 1917

hielvc · Posted: Sun May 10, 2009 5:41 pm Post subject:

Thanks that what I thought, but I didnt want to spend the day rebuilding and then reboot into a nightmare :lol:

Still it set off one my pet pives. The last thing "emerge system" built was gcc. So if you want save time build gcc first, check gcc-config and select gcc-4.3.3 . now emerge -1 linux-headers glibc binutils. Now do a emerge -e world . Im being a good boy running portage 2.1.16. If you using portage-2.2 then then a --emptyree system and the world will work without recompileing a bunch of stuff. Or third choise would be emerge gcc and do you normal -uDN and after a couple of months most everything rebuilt with "native"
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