Illegal instruction when I chroot into my Atom arch.

[Randy Andy]

Hi Folks,

the following problem treads me since a period of time I can't specifies exactly, must be during some month.

Every time, when I chroot on my Desktop PC (~x86_64 Intel(R) Core(TM)2 Quad CPU Q6600) into a local Backup of my Netbook (~i686 Intel(R) Atom(TM) CPU N270), I got an illegal instruction error.

I've done this thousand times before successfully, in the same way.
On the same machine resides some more backups of different ~x86 pentium- and athlon-systems.
To these, I can chroot successfully furthermore, to do upgrades for it into this kind of 32Bit chroot enviroment, similar to the procedure described here: http://www.gentoo.org/proj/en/base/amd64/howtos/chroot.xml

It already comes up while chrooting, when I enter this command:

[chithanh]

-march=atom can lead to illegal instructions on N270 too, because it is only intended for 64-bit atoms

If you want to be able to chroot from the Core2 into the Atom install, -march=prescott -mtune=atom would probably make sense.

[Randy Andy]

Thank you chithanh.

Now I see that I've chosen the wrong settings for my CPU, what must be the root cause for my hassle.

After reading some documentation to find out the most performance settings for my CPU, I found out some more facts:

Prescott supports:
MMX, SSE, SSE2, SSE3, Intel 64, XD-Bit, Hyper-Threading

My Atom N270 supports:
MMX, SSE, SSE2, SSE3, SSSE3, XD-Bit, Hyper-Threading


The missing support for Intel 64 arch is no problem, reasons are clear to me.
But I don't use the SSSE3 instruction set on the Atom if I only using this settings -march=prescott -mtune=atom, right?

Doc told me:
"simd’
Enable Advanced SIMD instructions. This implies floating-point instructions are enabled. This is the default for all current possible values for options -march and -mcpu=."

But I guess I found out that SIMD doesn't include SSSE3 instructions, so I have to activate it for the CFLAGS additionally, right?

If so, I'm not fully sure what's the right syntax for this, cause I'm eventually misinterpret the gcc doc (I'm no native).
So is it:


a) -march=prescott -ssse3 -mtune=atom

b) -march=prescott -mtune=atom -mfpmath=ssse3

c) -march=prescott -mtune=atom -ssse3



All of the settings above should work again on my Quad Core, cause it supports them all: MMX, SSE, SSE2, SSE3, SSSE3, Intel 64, EIST, XD-Bit, IVT


Thanks in advance, Andy.
_________________
If you want to see a Distro done right, compile it yourself!

[chithanh]

[Randy Andy]

[quote="chithanh"]

[krinn]

1/ you should just set your cpu as gcc told you
2/ I disagree totally with the chithanh suggest that -march=atom is for 64 bits atom, until i have read some gcc doc telling that clearly.
It's an old myth that came when gcc introduce prescott and nocona push by users that were thinking you can't use nocona because of the 64bits instruction set in it.
If anyone keep following this logic, it would imply that -march=corei7 is illegal to be use in a 32bits env, because that include 64bits instruction set.
3/ your gcc is just telling you truth, look at my output

[Randy Andy]

Thank you krinn,

for bringing more light into the dark.
Your comments are very helpful to me cause I did understand the gcc documentation partly in a different way.
With the help of your statements, I thought I were able to interpret the output of my gcc status information in the right way.

But when I compare your gcc lines, considering the different CPU, with mine, I'm again confused, cause the different behaviour of the output of

[krinn]

Yes i have check with my core2 and it always keep mmx, sse and the like disable, must be some kind of bug with core2.
But if you force it enable, it should confirm it, just try checking sse status with

[Josef.95]

This "gcc -Q --help=target -march=" Output is confusing
see http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39851

[Randy Andy]

What a bad awaking for me, krinn.

Now, after 6 (in terms six!) years of using and optimizing gentoo, I had to find out that (at least) two of my most performant machines, doesn't use any of the built in mmx and sse instruction sets. What a f... äh pity!
The older ones I will check later.

Not only when using gcc automatic detection (march=native) I'd never used, then I would know, automatisms could fail sometimes and that's exactly why I'm using gentoo and did the most things manually.

Now, also when following the Safe-Cflags documentation, which was recommended by the handbook, the fault was still the same. I guess there should be given a hint in future releases, how to check if the settings are in used or not, as it was in my case.

I found out that neither on my quad core, nor on my atom, the mmx and sse instruction sets are active, if I doesn't mentioned it explicit for my CFLAGS.
I tried out gcc-4.6.3 and 4.7.2, both with the same behaviour.
All of the documentation I read before, told me that it's included into the -march=core2 -mtune=core2 or into the -march=atom -mfpmath=sse ssettings I used before and there is no need to add it explicitly, or did I misinterpret it.

Actually I'm recompiling my atom's world (>1200 packages), the quad core is already done (>1600 packages!) activating all of it supported instruction sets, knowing that I lost the chroot compiling for the atom in this way.
I'm more interested in the additional performance. Now I need a good benchmark suite, to compare the before and after adjusted systems, which are apart from that, still the same.


When I had opened this thread, I'd never thought about this kind of findings, it would have brought to me.
So at the end, I'm happy to opened this thread.

Many thanks and best regards,

Andy.
_________________
If you want to see a Distro done right, compile it yourself!