Re: FYI -ffreestanding shrinks kernel by 2% on x86_64

Alexey Dobriyan
2019-05-12T17:56:01Z

On Sun, May 12, 2019 at 11:32:28AM +0200, Ingo Molnar wrote:
>
> * Alexey Dobriyan <adobriyan@gmail.com> wrote:
>
> > On Sat, May 11, 2019 at 11:02:24PM +0300, Alexey Dobriyan wrote:
> > > I compiled current F29 kernel config on x86_64 (5.0.13-200.fc29.x86_64)
> > > with -ffreestanding. The results are interesting :^):
> > >
> > > add/remove: 30/22 grow/shrink: 1290/46867 up/down: 33658/-1778055 (-1744397)
> > > Total: Before=83298859, After=81554462, chg -2.09% (!)
> > >
> > > That's original config with modules compiled built-in.
> >
> > Argh, it's the other way: adding -ffreestanding shrinks kernel by 2%.
>
> This is a very interesting finding, as we've seen numerous code
> generation artifacts from GCC assuming libgcc things.
>
> Has anyone investigated by any chance where the -ffreestanding space
> savings come from mostly - is it mostly in cold paths, or does it make or
> hot codepaths more efficient as well?
>
> If it's the latter then the kernel would be directly faster as well
> (fewer instructions executed), not just indirectly from better cache
> packing, I suppse?

Turns out -ffreestanding completely disables stack protector :-\

F29 83298859
-ffreestanding -1744397 -2.09%
STACKPROTECTOR_STRONG=n -1369949 -1.64%

Builtin function are in noise, e.g -fno-builtin-sprintf is only -14KB.