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What CFLAGS should i use?
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lea0014
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Joined: 09 Mar 2010
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PostPosted: Wed Aug 14, 2013 6:08 pm    Post subject: What CFLAGS should i use? Reply with quote

Hi there, i'm installing a new box with an AMD FX-6300 processor. Of course, im following the handbook to install the system, but when i have to set the CFLAGS variable, the handbook is not handy at all. So, as far i could see, -march=native is what should i take.
Here comes the deal; i want to run a distcc service at the FX system, and the wiki says:
Quote:
Starting with GCC 4.3.0, the compiler supports the -march=native switch which turns on CPU autodetection and optimizations that are worth being enabled on the processor the GCC is running at. This is a problem with distcc as it allows mixing of code optimized for different processors (like AMD Athlon and Intel Pentium). Don't use -march=native or -mtune=native in your CFLAGS or CXXFLAGS when compiling with distcc .

Help! (probably i need some explanations about distcc?)

Thanks.
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BradN
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PostPosted: Wed Aug 14, 2013 7:21 pm    Post subject: Reply with quote

I believe distcc uses the target system's -march setting when doing remote compile. So, it is okay to use -march=native on the big machine, but you cannot use it on the smaller target machines (assuming lesser instruction set) as that would tell distcc to build binaries with instructions they cannot use (since it is gcc itself deciding what 'native' means, not portage).

If you do "man gcc" you will find the possible choices for -march= on x86 somewhere around line 9000 or so.
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Hu
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PostPosted: Wed Aug 14, 2013 8:22 pm    Post subject: Reply with quote

Last time I checked, distcc uses the source system's -march setting when using -march=native by way of refusing to distribute the work to remote nodes if the arguments include -march=native.
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Ant P.
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PostPosted: Fri Aug 16, 2013 2:43 am    Post subject: Reply with quote

Here's a script I have that outputs the actual flags -march=native expands to; I use this for distcc on my own machines:
Code:
#!/bin/bash
echo $(
    gcc -v -march=native -x c /dev/null 2>&1 | \
    grep -- '-march' | \
    egrep -o -- '-+(m|param )[-_=.a-zA-Z0-9]+' | \
    sort -u
)
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CrUpf
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PostPosted: Sat Aug 24, 2013 11:16 pm    Post subject: Reply with quote

As the FX-6300 is a Piledriver cpu, and Piledriver is based on Bulldozer, I would suggest something like:
CFLAGS="-O2 -march=bdver1 -mtune=bdver1"

See here:
http://en.wikipedia.org/wiki/Piledriver_%28microarchitecture%29
http://en.wikipedia.org/wiki/Bulldozer_%28microarchitecture%29

http://gcc.gnu.org/gcc-4.6/changes.html
Quote:
Support for AMD Bulldozer (family 15) processors is now available through the -march=bdver1 and -mtune=bdver1 options.


Actually I would be quite interested if you could encrypt one of your drives and test how fast your CPU/drive is working due to the AES instruction set compared to a comparable unencrypted drive.
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Schnulli
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PostPosted: Sat Aug 31, 2013 9:57 pm    Post subject: Reply with quote

you shouldnt use march=native with gcc 4.3 because its only supported above gcc >4.6 ;) its dangerous then .....
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