How to compile Linux kernel with -O0 flag

[greg@kroah.com (Greg KH)]

On Sun, Aug 14, 2016 at 07:11:01PM +0800, Hao Lee wrote:
> > Just out of curiosity - is there a technical reason why -O0 couldn't
> > be used in Linux kernel? I don't know, spinlocks would not work in this
> > case because it's how GCC was written or something. Or just nobody
> > compiles and tests kernel like this so it most likely would not work?
> >
> >> But why do you want to use a debugger on the kernel?  That's not a
> >> normal task unless you are bringing up a new hardware platform.
> >
> > It's just something I always do when I learn new things. Trying to
> > figure out how to debug something in this new environment. No real task
> > so far.
> 
> Hi, I also like figuring out what's happening in the OS underlying. So
> I have some ideas about reducing optimization. Although you couldn't
> turn off optimization completely, you can use both -O2 and other
> options to reducing optimization as far as possible.
> You can use -O2 -Q -v to find out which options are enabled when using
> -O2. Then you can try -O2 -fno-defer-pop -fno-thread-jumps etc. to
> disable some options. I once used this approach to debug kernel-2.4 in
> bochs simulator. Unfortunately, this approach could not counteract the
> effects of -O2 completely, but it's worth a try.

No, please don't do that.  If you do, you will end up with a completly
unsuported and unknown system and no one will be able to help you out
with any sort of problem solving.

Just use the default build options, and all should be fine.  We know
they work.  Anything else is a total unknown, and not something that
anyone trying to learn about the kernel should be messing with.

thanks,

greg k-h