Re: PATCH][RFC][resend] CC_OPTIMIZE_FOR_SIZE should default to N

Ingo Molnar
2011-03-22T10:27:55Z

* Pekka Enberg <penberg@kernel.org> wrote:

> Hi Jesper,
>
> On Mon, Mar 21, 2011 at 10:08 PM, Jesper Juhl <jj@chaosbits.net> wrote:
> > I believe that the majority of systems we are built on want a -O2 compiled
> > kernel. Optimizing for size (-Os) is mainly benneficial for embedded
> > systems and systems with very small CPU caches (correct me if I'm wrong).
>
> Please take a look at commit 0910b44 ("Expose "Optimize for size"
> option for everybody") for the reasoning behind defaulting to -Os.

If that situation has changed - if GCC has regressed in this area then a commit
changing the default IMHO gains a lot of credibility if it is backed by careful
measurements using perf stat --repeat or similar tools.

See the hard numbers in this upstream commit for example:

ea7145477a46: x86: Separate out entry text section

there we were able to prove the positive effects of a pretty subtle change to
the layout of the instruction cache, with a measurement noise in the 0.1%
range.

Thanks,

Ingo