Talk:Safe CFLAGS

The aim of this page is to create an alternative to this. Since we can not directly copy, I would suggest that users instead check their own CFLAGS and put them into this page so that we can build up a solid list of CPUs and their suggested CFLAGS.

--march=native
I think this page should include a chapter explaining why --march= would be better than --march=native, because it's confusing for new users to find these two different recommendations without any explanation which one is the "proper" way.


 * Although the page now mentions march=native, it still doesn't explain, why march=corei7 would be preferred (what are the benefits) over march=native, which may confuse many people. Smartass 15:54, 11 December 2012 (UTC)


 * The use of distcc (particularly in support of *cross*-compiling) is a significant motivation for using a specific setting for -march= other than native. I have found this very useful and have referred to it for years.  However, I've recently noticed that the Handbook no longer links to this and instead now refers users to GCC_optimization which uses a different procedure than that prescribed in this page.  I suppose that may reflect he difference between "safe" and "optimum"  See also this forum thread. brendlefly62 (talk) 17:59, 23 January 2017 (UTC)

I just added a section for the Core 2 Duo, and ran into a problem that touched on your comment. Apparently setting the "-march" explicitly vs using native might effect your ability to cross compile, and it may effect what processor optimizations are enabled:

Setting -march=core2 tells gcc to enable MMX, SSE, SSE2, SSE3, SSSE3 instruction set support. (Citation: http://gcc.gnu.org/onlinedocs/gcc-4.4.4/gcc/i386-and-x86_002d64-Options.html#i386-and-x86_002d64-Options  ) Setting the USE variable to include "MMX, SSE, SSE2, SSE3" would then be redundant; I believe.

Some core 2 solo/duo/quad processors support sse4, sse4a, and sse4_1. I'd like to find a safe and correct way of enabling support for sse4 sse4a, and sse4.1 in the CFLAGS and/or USE for a core 2 duo that supports them.

I don't think seting these in the USE flags is the correct way. Unless I'm mistaken, only packages that support those use flags would benefit; or am I wrong about that? Also I noticed that the sse4, sse4a, and sse4_1 USE flags are not included in the global use flag list at: http://www.gentoo.org/dyn/use-index.xml  Does there abscence from that list mean those use flags would have no effect?

One person who made a nice page about optimizing for the Core 2 solo/duo/quad (Citation: http://wiki.gentoo.org/wiki/Intel_Core_2 ) suggested just setting -march=native. GCC seems to recognize the sse4 instruction sets when I run "gcc -march=native -E -v - &1 | grep cc1" so setting -march=native might solve the problem. However sometimes I cross compile; and according to the Gentoo GCC optimization guide (  http://www.gentoo.org/doc/en/gcc-optimization.xml ) using native will cause problems when trying to cross compile. So many questions raised.... I should turn this into a forum post - It kind of got out of hand... To reiterate: I'd like to add to this page a safe and correct way of enabling support for sse4 sse4a, and sse4.1 in the CFLAGS/USE for a core 2 duo that supports them. Any one know a definitive way of identifying processor support for those processor optimizations; and how to enable them in GCC? Thank you! Aries97 (talk) 04:45, 15 May 2013 (UTC)

Update: Core2 processors support EM64T - aka AMD64. (Wikipedia: http://en.wikipedia.org/wiki/EM64T#Intel_64 ) For whatever reason; I had an i686 CHOST set earlier. I've tested this on my processor and changed the entry to reflect a corrected CHOST="x86_64-pc-linux-gnu". Aries97 (talk) 06:23, 28 May 2013 (UTC)

Add Intel Suggested i7 Optimizations
Suggest adding Intel's suggested optimizations someplace for i7 processors. http://software.intel.com/en-us/blogs/2012/09/26/gcc-x86-performance-hints

ie. CFLAGS="-m64 -Ofast -flto -march=native -funroll-loops"

Granted this is Safe CFLAGS, and some reference should be made to the manufacturer's suggestions.

Section Core i3/i5/i7 & Xeon E3/E5/E7 *V2 seems to be incorrect
CFLAGS="-march=core-avx2 -O2 -pipe" is not working for my Xeon-E5 2620 which seems to be sandybridge based.

Please correct -- Tazinblack (talk)‎ 09:39, 25. Nov. 2016‎

Hi, I corrected the values. Since GCC 4.9 you should use: CFLAGS="-march=sandybridge -O2 -pipe" -- JackDelik (talk) 10:59, 25 November 2016 (UTC)

It's still not correct by now.

The headline says 'Haswell' and in the next line 'Core i3/i5/i7 & Xeon E3/E5/E7 *V2'. But Haswell is not V2 but V3. So this should be V3.

As far as I see V1 should be SandyBridge, V2 IvyBridge and V3 Haswell. -- Tazinblack (talk)‎ 16:41, 25. Nov. 2016‎ (UTC)
 * You're right, I corrected this. Be bold: you can edit the pages too :) JackDelik (talk) 16:57, 25 November 2016 (UTC)