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

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Date:	Sun, 12 Apr 2015 12:15:51 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Markus Trippelsdorf <markus@...ppelsdorf.de>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jakub Jelinek <jakub@...hat.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Andy Lutomirski <luto@...capital.net>,
	Jason Low <jason.low2@...com>, Brian Gerst <brgerst@...il.com>,
	Aswin Chandramouleeswaran <aswin@...com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Davidlohr Bueso <dave@...olabs.net>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"H. Peter Anvin" <hpa@...or.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] x86: Turn off GCC branch probability heuristics


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

> On 2015.04.12 at 07:47 +0200, Ingo Molnar wrote:
> > 
> > * Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> > 
> > > On Sat, Apr 11, 2015 at 11:57 AM, Thomas Gleixner <tglx@...utronix.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
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