Re: [PATCH] x86: Turn off GCC branch probability heuristics

Markus Trippelsdorf
2015-04-12T06:20:21Z

On 2015.04.12 at 07:47 +0200, Ingo Molnar wrote:
>
> * Linus Torvalds <torvalds@linux-foundation.org> wrote:
>
> > On Sat, Apr 11, 2015 at 11:57 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
> > >
> > > I thinks its just the no-guess one:
> > >
> > > text data dec patch reduction
> > > 7563475 1781048 10302987
> > > 7192973 1780024 9931461 no-guess -4.8%
> > > 7354819 1781048 958464 align-1 -2.7%
> > > 7192973 1780024 9931461 no-guess + align-1 -4.8%
> >
> > Yeah, a 5% code expansion is a big deal. Sadly, it looks like
> > 'no-guess' also disables our explicit likely/unlikely handling.
>
> So I spent some time trying to get as much code size reduction as
> possible via GCC optimization options, and the total savings possible
> are 10.1%:
>
> text data bss dec filename
> 12566391 1617840 1089536 15273767 vmlinux.vanilla
> 11416805 1617840 1089536 14124181 vmlinux.combo
> 10532552 1596080 1089536 13218168 vmlinux.Os

If you like to play with more knobs you could explore the various
--param options that are listed in the gcc man page...

--
Markus