Question about CFLAGS and CXXFLAGS

gexarcha · n00b Joined: 01 May 2010 Posts: 3

I am a new user of gentoo and I noticed something that made me a little curious.
I installed gentoo on an samsung n130 netbook with an atom processor.
I looked at the gcc manual an I saw that the -march option could be changed with the -mtune option for some architectures.
And atom processors are only mentioned in the as a value for the -mtune option.
so I used this:
CFLAGS="-O2 -mtune=atom"
in my make.conf file.
I didn't have a problem compiling the kernel like that but I after I compiled the kernel I couldn't emerge any other package (it crashed when compilation started) unless I changed the CFLAGS to this:
CFLAGS="-O2 -march=i686"

I used the same CXXFLAGS in both cases and I don't use -pipe because I only have 1 GB RAM.
I used a lot of variations of USE flags and it is still the same.

I am writing this post to understand the purpose of this behavior?
What would happen if I wrote -march=atom?
Why does the gentoo handbook recommend -march and the gcc manual focuses so much on -mtune?
I don't know where to look for information on this so please recommend some websites or books?

chithanh · Posted: Fri Jun 04, 2010 10:44 am Post subject:

The kernel build system ignores make.conf because the CFLAGS are chosen by the Processor Type and Features in menuconfig.

You should have checked config.log as emerge told you. -march implies -mtune, and -march=atom is supported since gcc 4.5.0. Use -march=native so gcc automatically chooses the optimal march. Unless you use distcc, that is.
Using -pipe on a 1 GB system should be ok.

gexarcha · n00b Joined: 01 May 2010 Posts: 3

thank you for your quick response chithanh.

I just tried -march=atom and it doesn't work but -march=native does.

Do you know why -march=atom doesn't work?
My system works now but I am still curious.

I am not using -pipe because I think it only matters in compile time not the result.
Is that right?

I took every step that emerge suggests but I couldn't figure out what the problem was.
I still don't see the reason why -mtune=atom doesn't work.
I couldn't figure out the config.log.
(I think it was a file with something like compiler errors and warnings
I don't remember very well because the last time I looked was a few of weeks ago).
It was very big and it was constantly mentioning the CFLAGS.
That's why I focused there and not something else.

This is my second gentoo installation.
The first one was on virtual box and I didn't use it very much so I don't know a lot about gentoo.
I can use any information that you can provide.

chithanh · Posted: Fri Jun 04, 2010 7:06 pm Post subject:

-march=atom is supported since gcc-4.5.0, earlier versions do not recognize this option. Check "gcc-config -l" and the gcc manpage for supported -march values if in doubt.

gexarcha · n00b Joined: 01 May 2010 Posts: 3

I see thank you

phajdan.jr · Posted: Sat Jun 05, 2010 8:00 am Post subject:

Please consider running "emerge -e world" after playing with CFLAGS. I've seen you mentioned some crashes/errors. Re-emerging all packages with now known to be working CFLAGS may save you some mysterious trouble later. :wink:

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bendeguz · Apprentice Joined: 10 Feb 2010 Posts: 189

Hi!

Does the -Os CFLAGS option have some security risks? Or it can cause "only" instability?

Is it wise to use -fstack-protector-all on a hardened system? So far glibc doesn't like this option. Should I compile glibc without it and the rest with the option enabled?

(This is just my home machine, but I wouldn't like to reinstall from scratch

)

Edit: I've read that -fstack-protector-all is enabled by default on hardened. So this means I have a working SSP by default?