amd64 and core 2 duo

alshain · Posted: Fri Dec 08, 2006 5:21 pm Post subject: amd64 and core 2 duo

Hello, I'm just about to upgrade my machine from an Athlon X2 to a Core 2 Duo. Until recently I'd assumed that this would be a matter of compiling a new kernel and away we go, but I've been thinking about how I've compiled my system. Everything is compiled as march=amd64 what with the C2D not supporting 3dNow! I'm wondering whether any packages would even run. I'm not familiar about how gcc goes about optimizing for amd64 architecture and if it uses 3dNow functions for everything so has anyone had any experience of this?

Some searching has suggested that the ideal march settings for the C2D (until gcc 4.2 comes along) is nocona but there don't appear to be any generic 64-bit options like there is for x86. I heard that simply leaving out the march option and setting -m64 disables all usage of 3dNow, SSE and the like. Can anyone confirm, and whether the 64-bit liveCD is compiled with basic settings? I see it boots on (some) Core 2 Duo machines, so it would be useful to know how. If march=amd64 software won't run, then I could simply build a new kernel with generic 64-bit options, emerge system, and recompile everything with march=nocona when I've got the new machine running.

Thanks for listening to the rambling!

Andrew

erik258 · Posted: Fri Dec 08, 2006 8:40 pm Post subject:

kill · Apprentice Joined: 25 Dec 2004 Posts: 179

thedude0001 · Arch/Herd Tester Joined: 16 Jan 2005 Posts: 69

If you build a new kernel configured for generic x86_64 systems and remerge system without any -march you should be safe. From there you should be able to boot that disk on a Core2 and pick everything up. You will have to "emerge -e world" though.

With current gcc versions the best thing you can do is -march=nocona (I read something about that on one of our MLs with a reference to gcc devs and what they say to use for now, but I just can't find it any more, sorry...). If you don't use any -march your binaries won't be optimized for mmx & sse, but you can enable that by adding "-mmmx -mmmxext -msse -msse2 -msse3" to your CFLAGS. -march=nocona implies everything but the -msse3. However that has no influence on the use of special asm code in programs like mplayer.

There won't be a -march for core2 in gcc-4.2 (that much I remember from the email I can't find any longer), but it has been added to the development trunk and should be present in gcc-4.3.

-m64 is bad, don't do it, it will break your system.

The live CDs are built without any optimization, so they should be running fine on any core2 box. The problems people are experiencing are mostly due to the whole Core2 architecture being new, with new chipsets etc. That always causes problems until all (or at least most) quirks have been evened out and new installation media have been released.

As a final note: IA64 (Itanium) systems aren't "older" server chips by intel, they are aimed at a completely different target group than core2/athlon64 CPUs.
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alshain · Posted: Sat Dec 09, 2006 10:20 am Post subject:

Thanks everyone, that pretty much confirmed what I've been thinking/hoping. Emerge system without the march=amd64, upgrade with new kernel and emerge -e world. Should be a good test of the new machine!

Andrew

kill · Apprentice Joined: 25 Dec 2004 Posts: 179

thedude0001 · Arch/Herd Tester Joined: 16 Jan 2005 Posts: 69

thedude0001 · Arch/Herd Tester Joined: 16 Jan 2005 Posts: 69

I just found the post I was referring to earlier. It wasn't on a ML I was on (that's why I couldn't find it) but rather a blog post on http://planet.gentoo.org.

For the lazy ones: