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Re: CPU optimization

:: Mircea Luca writes:

>> I guess that recompiling every package is not worth the
>> trouble, but recompiling multimedia and CPU intensive ones may
>> be a major gain (say, MPEG players and such).

> Yes,I was thinking more like X ,KDE ,Gnome and multimedia so I guess
> the few things left won't take that much time.:-).

Is it safe to recompile X with different flags? IIRC, X 3.3.6
shouldn't even be compiled with gcc 2.95.2 (the Makefile would look
for gcc272), because 2.95 would miscompile it. (I could be wrong).

>> BTW, you can use -O6 for your programs. If I remember
>> correctly, it will fallback to whatever is the highest mode of
>> optimization (somebody please correct me if I'm wrong -- I
>> read this a long time ago and didn't find it listed in the
>> current documentation now; the funny thing is that it's not
>> mentioned in the current potato manpage).
>> I also don't know if -O6 does the same thing as -O9.
>> []s, Roger...

> I've read something on a archive of a gcc mailing list but apparently it
> doesn't relate to the gcc we use-don't remember the link ,I
> researched this for a few hours.I got stuck at the debian/rules when
> I decided to ask for help.

Yes... I saw a few Makefiles using -O9 and -O10.

I've made some tests here modifying pentium-builder. I think the
prolem with changing it is that, if I call

gcc -O2 -O3

That'll be the same as "gcc -O3"

So it'll work if you set an env. variable to "O3" and override the
-O2, but will not work the other way araound (you can't force it to
use a lower optimization level than the one in the command line).

That's because -O2 will just enable a set of optimizations, and -O3
will enable a superset of -O2.

Unless the code was changed in order to strip the previous "-O"
setting from the arguments (Joey, if you're reading this: is it

Anyway, the difference between -O2 and -O3 seems to be loop-unrolling;
I'm not sure if it's worth recompiling everything with -O3...


Jeronimo Pellegrini
Institute of Computing - Unicamp - Brazil
mailto:jeronimo@ic.unicamp.br    mailto:pellegrini@iname.com

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