Re: [gentoo-user] How to properly change CFLAGS ?

[Alan Mckinnon]

On Mon, 2006-08-21 at 22:51 +0200, Stefan G. Weichinger wrote:
> I *don't* want to start some flamewar her, I am a newbie in this group,
> and I just would like some info on how to do it right, as my extensive
> rtfm'ing/googling/etc. has still not given me a satisfying answer.
>
> I started my Gentoo-installation with CFLAGS containing -O3, believing
> to do it right ...
>
> Now I read about the fact that -O3 results in bigger binaries and isn't
> at all guaranteed to give me a faster system. The bigger files result in
> more load on IO, so this tells me that it puts the load on the
> (relatively slow) 5400 rpm HDD I have in my laptop. OTOH I have "only"
> 512 MB RAM in there so it seems interesting to me to go the -O2 way of
> doing Gentoo ...

-O3 tells gcc to do some wild and wonderful optimizations, none of which
are guaranteed to perform well, or even to work at all. They have a use
and a place, but are not intended for general use.

-O2 is correct for a general purpose machine

> Now the question:
>
> Do I have to do "emerge -e --newuse world" on my system or what else
> would be needed?

modify CFLAGS in /etc/make.conf
emerge -e system
emerge -s world

This will rebuild your toolchain (gcc, glibc and friends) to use -O2
then rebuild the entire system, including the toolchain again, with -O2.
Your current compiler was built with -O3, and you want to rebuild the
system using a compiler compiled as -O2, hence the 2 step process.

Be warned, this will take a LONG time to complete - consider yourself
lucky if you get away with 36 hours if you have all OOo and of KDE or
Gnome... So you should only consider doing it if you have specific
problems caused by -O3


alan


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