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

[Ingo Molnar]

* Markus Trippelsdorf <markus@xxxxxxxxxxxxxxx> wrote:

> On 2015.04.12 at 07:47 +0200, Ingo Molnar wrote:
> > 
> > * Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> > 
> > > On Sat, Apr 11, 2015 at 11:57 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> 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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/