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Massimo B.
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PostPosted: Tue Jan 27, 2015 8:16 am    Post subject: gcc versions 4.8 to 4.9, changing CFLAGS Reply with quote

Hi,

while comparing the different -march keywords on different gcc versions here...

...I found some major differences:
Lot of marchs are dropped from 4.8 to 4.9. Currently using 4.8.4 and facing the next 4.9.2 upgrade, I wonder why all "core*" marchs have been dropped for these new items:

  • ‘core2’
  • ‘nehalem’
  • ‘westmere’
  • ‘sandybridge’
  • ‘ivybridge’
  • ‘haswell’
  • ‘broadwell’

Are there still aliases? Beside the "native" item, do I need to adapt fixed -march CFLAGS?
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xaviermiller
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PostPosted: Tue Jan 27, 2015 8:32 am    Post subject: Reply with quote

I you don't use distcc, using native for -march and -mtune works for all GCC versions and architectures, from your microwave to your local server ;)

If you don't like native, find the new native flags with
Code:
#cc -O2 -pipe -fomit-frame-pointer -march=native -mtune=native -E -v - </dev/null 2>&1 | grep cc1
(adapt "-O2 -pipe" & co with your CFLAGS)
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Massimo B.
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PostPosted: Tue Jan 27, 2015 12:28 pm    Post subject: -march and -mtune for the same arch? Reply with quote

For some VMware ESX virtual machine I just need to stay compatible with different archs in order to move the VM from server to local archs. There I set the compatible -march together with -mtune=native. Usually these machine are building themselves with the option to be moved temporarily to a different VMware Workstation without rebuilding all.

btw. for the -march=native machines, what does it mean to set -march and -mtune to the same arch? Isn't -mtune redundant or even worse? If architecture features are natively built in why to add some more runtime checks the the same optimizations?
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chithanh
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PostPosted: Tue Jan 27, 2015 3:23 pm    Post subject: Reply with quote

The compiler flags are still supported, just the man page no longer mentions them.
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krinn
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PostPosted: Tue Jan 27, 2015 3:25 pm    Post subject: Reply with quote

march is the familly opitmize, mtune is the cpu
if you don't set one or the other, the missing one met the other (so redundant in this case), but mtune can let you tune more your cpu optimization than its familly.

a coreX familly may say : a coreX must have sse, but some coreX may not have sse2
so if you have a coreX-33 cpu: setting only familly (-march=coreX) will not enable sse2 because not all cpu of the familly have sse2
and setting -mtune=coreX-33 will enable coreX set + the set only coreX-33 cpu could handle (in the example, sse2)
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Massimo B.
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PostPosted: Tue Jan 27, 2015 7:09 pm    Post subject: Reply with quote

So as final conclusion on native platforms not being portable, setting both -march=native and -mtune=native is the optimum?
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mreff555
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PostPosted: Tue Jan 27, 2015 10:01 pm    Post subject: Reply with quote

krinn wrote:
march is the familly opitmize, mtune is the cpu
if you don't set one or the other, the missing one met the other (so redundant in this case), but mtune can let you tune more your cpu optimization than its familly.

a coreX familly may say : a coreX must have sse, but some coreX may not have sse2
so if you have a coreX-33 cpu: setting only familly (-march=coreX) will not enable sse2 because not all cpu of the familly have sse2
and setting -mtune=coreX-33 will enable coreX set + the set only coreX-33 cpu could handle (in the example, sse2)



Bottom line, and this is from the gcc docs, -march will strictly build based on that platform and may compile software that isn't very portable. -mtune is more likely to be portable at the cost of less tuning.

In my opinion. if you are certain about your architecture, use -march. This should be much less ambiguous with the flag changes in gcc 4.9.1+.
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Hu
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PostPosted: Wed Jan 28, 2015 2:27 am    Post subject: Reply with quote

Massimo B. wrote:
So as final conclusion on native platforms not being portable, setting both -march=native and -mtune=native is the optimum?
You do not need to set -mtune if you have set -march. Only use -mtune when you are building with a portable -march, but want code optimized to run best on a particular platform that is newer than your minimum supported hardware.
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