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Malik222
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Joined: 15 Aug 2012
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PostPosted: Wed Aug 15, 2012 4:05 am    Post subject: CFLAGS with an Ivy Bridge (tuning compiler) Reply with quote

Hi,

I just got an Intel Ivy Bridge I7 CPU. I was emerging -e system with the following CFLAGS:

-march=corei7-avx-i -mtune=corei7-avx-i -O3

It seems lots of packages can't compile with these CFLAGS.

I installed GCC 4.7.1 to be able to get the optimization for the Ivy Bridge (thus the ...avx-i). Although, so far, I checked at the benchmarks from Phoronix (http://www.phoronix.com/scan.php?page=article&item=intel_ivy_tuning&num=4) and it didn't seem worth it trying to set the compiler to use the new Ivy Bridge functions.



Has anyone tried to use the -march with ...avx (Sandy Bridge) or ...avx-i (Ivy) ? What are the differences in terms of performance for instance? If you have any suggestions?

Should I even bother or leave it like this:
CFLAGS="-march=native -O2 -pipe"

Thank you
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Veldrin
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Joined: 27 Jul 2004
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PostPosted: Wed Aug 15, 2012 10:41 am    Post subject: Reply with quote

Short answer:
Quote:
Should I even bother or leave it like this:
CFLAGS="-march=native -O2 -pipe"

Don't bother, and stick to -march=native.

and -march=native will revert automatically to the best suited -march, which will be core-avx-i.

Long answer:
the improvements between sandy bridge and ivy bridge mainly concern the avx (or sse) part of the cpu, which their vector processing unit for floating point. The only programms that make extensive use of that are scientific ones (number crunching), multimedia applications (steam conversion, image manipulation) and games (though they are rarely self-compiled).

If you are using one of those programs you might see a difference in speed, but it hard to say if that improvement is measurable, let alone noticeable for a human being (you need ~30% of speed improvement to be noticed as human)


HTH
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Malik222
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PostPosted: Wed Aug 15, 2012 1:19 pm    Post subject: compiling Reply with quote

Thank you!

I'll stick with -march=native as you said. Everything is compiling fine.
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