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FizzyWidget
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PostPosted: Fri Oct 29, 2010 9:15 am    Post subject: Cflags -march=native or -march=core2 ? Reply with quote

While looking at the wiki for cflags i notice in the big red box at the top of the page it says

Quote:
Warning: GCC 4.2 and above support -march=native, which automatically detects the optimum settings to use based on your processor. -march=native also detects and applies additional settings beyond -march specific to your CPU, (eg. -msse4). Unless you have a specific reason not to (e.g. distcc cross-compiling), you should probably be using -march=native rather than anything listed below.


Yet if i click the link for my CPU it states to use

Quote:
CFLAGS="-march=core2 -O2 -pipe"
CXXFLAGS="${CFLAGS}"


which should i use? will -march=native use the -core2 switch or should i just go ahead and use the -core2 switch?

I dont wish to use the incorrect one, if either could be called that, as i'm installing onto a new disk and it would me recompiling a lot of packages if i need to swap, could do it overnight i know but its still a PITA either way
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mv
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PostPosted: Fri Oct 29, 2010 9:37 am    Post subject: Reply with quote

If >=gcc-4.2 is your active compiler and you compile only on your machine for your machine (not using distcc or building packages for other machines), it is preferrable to use -march=native.
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PostPosted: Fri Oct 29, 2010 9:38 am    Post subject: Reply with quote

I think -march=native is a safe choice. Also, CFLAGS are not as important as many people think. :wink:
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PostPosted: Fri Oct 29, 2010 9:46 am    Post subject: Reply with quote

Thanks, i decided to take a chance and select native :-) rebuilding world now as im using ~amd64
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PostPosted: Fri Oct 29, 2010 12:29 pm    Post subject: Reply with quote

I use this config for a long time :

Code:

CFLAGS="-march=native -O2 -pipe"
CXXFLAGS="${CFLAGS}"

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PostPosted: Fri Oct 29, 2010 1:46 pm    Post subject: Reply with quote

mv wrote:
If >=gcc-4.2 is your active compiler and you compile only on your machine for your machine (not using distcc or building packages for other machines), it is preferrable to use -march=native.


My impression was that "-march=native" really only worked if gcc was newer than your CPU. I just got a core-i7-based Thinkpad, and was told to use "-march=core2 -mtune=generic" for it. Of course we just got a new gcc the other day, so I'll have to check if it's time for "-march=native" again.
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PostPosted: Fri Oct 29, 2010 1:59 pm    Post subject: Reply with quote

it's in fact the contrary, as march=native guess your cpu, if your gcc is older than your cpu, it should be smarter to let gcc detect for you what cpu is compatible and having it picking up the right set for you.

for your core-i7 example, if your gcc support march=native but not march=core2 you'll get more benefits because march=native will pickup a compatible and close cpu to yours.
It's better than let the user pickup one that might be less accurate (ie: the wiki stupid nocona/prescott case for core2)

if your gcc support your cpu, march=native and march=yourcpu are just equivalent. That's why i think march=native strengh will be come up when your gcc is older than your cpu
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PostPosted: Fri Oct 29, 2010 4:13 pm    Post subject: Reply with quote

krinn wrote:
It's better than let the user pickup one that might be less accurate (ie: the wiki stupid nocona/prescott case for core2)


Back when it was written, niether "-march=core2" nor "-march=native" existed, so nocona/prescott was in fact the best option ;)
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PostPosted: Fri Oct 29, 2010 4:20 pm    Post subject: Reply with quote

it's not about native or core2 presence.

It's about picking nocona because 64bits and prescott when 32bits.
It was dumb, it is dumb, it's still dumb

So, no, it wasn't the best option, even it was harmless.
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PostPosted: Fri Oct 29, 2010 4:44 pm    Post subject: Reply with quote

Back when I first got the Core-I7 machine, the "Gentoo safe CFLAGS" page recommended what I used over native. I can see how gcc could attempt some heuristics to guess how to optimize for an unknown CPU, but I'm betting that they don't do that. I'd guess that they just do a table-lookup based on CPUID - for a resource-constrained effort I wouldn't put a lot more into this part of it.
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PostPosted: Fri Oct 29, 2010 6:00 pm    Post subject: Reply with quote

depontius wrote:
Back when I first got the Core-I7 machine, the "Gentoo safe CFLAGS" page recommended what I used over native. I can see how gcc could attempt some heuristics to guess how to optimize for an unknown CPU, but I'm betting that they don't do that. I'd guess that they just do a table-lookup based on CPUID - for a resource-constrained effort I wouldn't put a lot more into this part of it.


Considering that the difference between flags set by "-march=core2" and "-march=native" on a Core2 machine is a pair of flags that identify the processor cache sizes, I'm pretty sure you're wrong. If it can get the cache sizes, it could just as well check which other features it supports (sse, 3dnow, etc.), which is probably what it does.
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PostPosted: Mon Nov 01, 2010 12:17 pm    Post subject: Reply with quote

From http://en.gentoo-wiki.com/wiki/Safe_Cflags/Intel#Core_i7_and_Core_i5.2C_Xeon_55xx
Code:
Core i7 and Core i5, Xeon 55xx

vendor_id       : GenuineIntel
cpu family      : 6
model           : 26
model name      : Intel(R) Core(TM) i7 CPU         920  @ 2.67GHz

vendor_id       : GenuineIntel
cpu family      : 6
model           : 37
model name      : Intel(R) Core(TM) i5 CPU         650  @ 3.20GHz / 660 @ 3.33 GHz

vendor_id       : GenuineIntel
cpu family      : 6
model           : 26
model name      : Intel(R) Xeon(R) CPU          X5570  @ 2.93GHz

32 bit profile (x86)

CHOST="i686-pc-linux-gnu"
CFLAGS="-march=prescott -O2 -fomit-frame-pointer -pipe"
CXXFLAGS="${CFLAGS}"

64 bit profile (amd64) for < GCC 4.3

CHOST="x86_64-pc-linux-gnu"
CFLAGS="-march=nocona -O2 -pipe"
CXXFLAGS="${CFLAGS}"

64 bit profile (amd64) for >= GCC 4.3

CHOST="x86_64-pc-linux-gnu"
CFLAGS="-march=core2 -mtune=generic -O2 -pipe"
CXXFLAGS="${CFLAGS}"

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PostPosted: Mon Nov 01, 2010 7:08 pm    Post subject: Reply with quote

Not sure what you're getting at...
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PostPosted: Mon Nov 01, 2010 7:27 pm    Post subject: Reply with quote

I had been reporting what I saw on the Gentoo safe CFLAGS page, yet people here disagree. I figured I'd give a direct quote.
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PostPosted: Tue Nov 02, 2010 11:29 am    Post subject: Reply with quote

Code:
Core i7 and Core i5, Xeon 55xx
32 bit profile (x86)

CHOST="i686-pc-linux-gnu"
CFLAGS="-march=nocona -O2 -fomit-frame-pointer -pipe"
CXXFLAGS="${CFLAGS}"

64 bit profile (amd64) for < GCC 4.3

CHOST="x86_64-pc-linux-gnu"
CFLAGS="-march=nocona -O2 -pipe"
CXXFLAGS="${CFLAGS}"

Here's how it should have been set.
And thinking that nocona is for 64bits (because of 64bits instructions) is as dumb as thinking core2 is also for 64bits.

And there's a 2nd error :
-march=core2 -mtune=generic is a dumb optimization that will slow down your code
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PostPosted: Tue Nov 02, 2010 6:43 pm    Post subject: Reply with quote

Then I suggest you update the "safe CFLAGS" people, and get the web page changed.
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PostPosted: Tue Nov 02, 2010 7:18 pm    Post subject: Reply with quote

I knew I seen it somewhere on these forums. Check this thread;

http://forums.gentoo.org/viewtopic-t-602834.html

for the magic formula. The relevant section of the test on my phenom x6 I get;
Code:


echo 'int main(){return 0;}' > test.c && gcc -v -Q -march=native -O2 test.c -o test && rm test.c test

COLLECT_GCC_OPTIONS='-v' '-Q'  '-O2' '-o' 'test'
 /usr/libexec/gcc/x86_64-pc-linux-gnu/4.4.5/cc1 -v test.c -D_FORTIFY_SOURCE=2 -march=amdfam10 -mcx16 -msahf -mpopcnt --param l1-cache-size=64 --param l1-cache-line-size=64 --param l2-cache-size=512 -mtune=amdfam10 -dumpbase test.c -auxbase test -O2 -version -o


If I use "-march-amdfam10 -O2 -pipe" I get;

Code:


echo 'int main(){return 0;}' > test.c && gcc -v -Q -march=amdfam10 -O2 test.c -o test && rm test.c test

COLLECT_GCC_OPTIONS='-v' '-Q' '-march=amdfam10' '-O2' '-o' 'test'
 /usr/libexec/gcc/x86_64-pc-linux-gnu/4.4.5/cc1 -v test.c -D_FORTIFY_SOURCE=2 -dumpbase test.c -march=amdfam10 -auxbase test -O2 -version -o /home/joededman/tmp/ccpiJBe9.s


I don't know that all the extra options you get from gcc by selecting "native" actually produces better binaries, but it saves me from adding more entries to me CFLAGS in /etc/make.conf

Later,

joe
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PostPosted: Tue Nov 02, 2010 7:21 pm    Post subject: Reply with quote

This
Code:
gcc -march=native -E -v - </dev/null 2>&1 | sed -n 's/.* -v - //p'
will show what flags will be applied with native



On my system I get
Code:
-D_FORTIFY_SOURCE=2 -march=amdfam10 -mcx16 -msahf -mpopcnt --param l1-cache-size=64 --param l1-cache-line-size=64 --param l2-cache-size=512 -mtune=amdfam10

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PostPosted: Tue Nov 02, 2010 7:32 pm    Post subject: Reply with quote

Anon-E-moose wrote:
This


Ahh, there we go. The simplified version. And so by extension;

Code:

~ $ gcc -march=amdfam10 -E -v - </dev/null 2>&1 | sed -n 's/.* -v - //p'
-D_FORTIFY_SOURCE=2 -march=amdfam10


Thanks, Anon-E-moose

joe
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depontius
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PostPosted: Wed Nov 03, 2010 2:00 pm    Post subject: Reply with quote

For Core I7 : http://gcc.gnu.org/ml/gcc-patches/2010-10/msg01670.html
Code:

PATCH: Update -march=native for Cora e2 and Core i7

    * From: "H.J. Lu" <hongjiu dot lu at intel dot com>
    * To: gcc-patches at gcc dot gnu dot org
    * Cc: Richard Henderson <rth at redhat dot com>, Uros Bizjak <ubizjak at gmail dot com>
    * Date: Tue, 19 Oct 2010 13:23:13 -0700
    * Subject: PATCH: Update -march=native for Cora e2 and Core i7
    * Reply-to: "H.J. Lu" <hjl dot tools at gmail dot com>

After this patch:

http://gcc.gnu.org/ml/gcc-patches/2010-10/msg01616.html

is checked in, we can use core2/corei7 for -march=native on Core 2 and
Core i7.  OK for trunk?

Thanks.


H.J.
---
2010-10-19  H.J. Lu  <hongjiu.lu@intel.com>

   * config/i386/driver-i386.c (host_detect_local_cpu): Updated
   for Core 2 and Core i7.

diff --git a/gcc/config/i386/driver-i386.c b/gcc/config/i386/driver-i386.c
index 8a76857..998214b 100644
--- a/gcc/config/i386/driver-i386.c
+++ b/gcc/config/i386/driver-i386.c
@@ -554,21 +554,21 @@ const char *host_detect_local_cpu (int argc, const char **argv)
    case 0x1e:
    case 0x1f:
    case 0x2e:
-     /* FIXME: Optimize for Nehalem.  */
-     cpu = "core2";
+     /* Nehalem.  */
+     cpu = "corei7";
      break;
    case 0x25:
    case 0x2f:
-     /* FIXME: Optimize for Westmere.  */
-     cpu = "core2";
+     /* Westmere.  */
+     cpu = "corei7";
      break;
    case 0x17:
    case 0x1d:
-     /* Penryn.  FIXME: -mtune=core2 is slower than -mtune=generic  */
+     /* Penryn.  */
      cpu = "core2";
      break;
    case 0x0f:
-     /* Merom.  FIXME: -mtune=core2 is slower than -mtune=generic  */
+     /* Merom.  */
      cpu = "core2";
      break;
    default:

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