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Date: Sun, 12 Apr 2015 12:15:51 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Markus Trippelsdorf <markus@trippelsdorf.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>, Jakub Jelinek <jakub@redhat.com>,
        Denys Vlasenko <dvlasenk@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Andy Lutomirski <luto@amacapital.net>, Jason Low <jason.low2@hp.com>,
        Brian Gerst <brgerst@gmail.com>,
        Aswin Chandramouleeswaran <aswin@hp.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        "H. Peter Anvin" <hpa@zytor.com>, LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH] x86: Turn off GCC branch probability heuristics
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* 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