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

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Date:	Sat, 11 Apr 2015 20:57:52 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Ingo Molnar <mingo@...nel.org>,
	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

On Sat, 11 Apr 2015, Linus Torvalds wrote:
> On Apr 11, 2015 2:20 AM, "Ingo Molnar" <mingo@...nel.org> wrote:
> >
> > Another thing caught my attention (and I'm hijacking the RCU thread
> > again): GCC's notion of how to place branches seems somewhat random,
> > and rather bloaty.
> >
> > So I tried the experiment below on an x86 defconfig, turning off GCC's
> > branch heuristics, and it's rather surprising:
> >
> >      text           data     bss      dec         filename
> >  12566447        1617840 1089536 15273823         vmlinux.fguess-branch-probability
> >  11923593        1617840 1089536 14630969         vmlinux.fno-guess-branch-probability
> >
> > That's an 5.4% code size improvement!
> 
> Ugh. That's much larger than I would have expected. Is it just because
> gcc ends up turning
> 
>          if (a)
>                b
>          c
> 
> into
> 
>          if (a) goto out-of-line
>    return:
>          c
>          ..
>    out-of-line:
>          c
>          goto return;
> 
> a lot? Still, 5% sounds insanely big.
> 
> How much of that 5% comes from code alignment? Or was this on *top* of
> the 1-byte alignment testt?

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%

So with the no-guess applied the align-1 does not matter anymore.

Thanks,

	tglx
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