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

Ingo Molnar
2015-04-12T10:15:59Z

* Markus Trippelsdorf <markus@trippelsdorf.de> wrote:

> 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...

Well, I had a look, they are rather incomplete (at least as far as
branch optimizations go), and I wouldn't want to rely on them for
production kernel patches in any case, only on the more well-known
compiler options.

Thanks,

Ingo