linux-kernel - Re: [PATCH] x86: Align jump targets to 1 byte boundaries

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Message-ID: <20150413190914.GA398@x4>
Date:	Mon, 13 Apr 2015 21:09:14 +0200
From:	Markus Trippelsdorf <markus@...ppelsdorf.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Jason Low <jason.low2@...com>,
	Peter Zijlstra <peterz@...radead.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Aswin Chandramouleeswaran <aswin@...com>,
	LKML <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Andy Lutomirski <luto@...capital.net>,
	Brian Gerst <brgerst@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] x86: Align jump targets to 1 byte boundaries

On 2015.04.13 at 11:31 -0700, Linus Torvalds wrote:
> On Mon, Apr 13, 2015 at 10:26 AM, Markus Trippelsdorf
> <markus@...ppelsdorf.de> wrote:
> >
> > I must have made a measurement mistake above, because the actual code
> > size savings are roughly 5%:
> 
> Can you check against the -fno-guess-branch-probability output?

   text    data     bss     dec     filename
   8746230  970072  802816 10519118 ./vmlinux gcc-5 (lto) 
   9202488  978512  811008 10992008 ./vmlinux gcc-5
   8036915  970296  802816 9810027  ./vmlinux gcc-5 (lto -fno-guess-branch-probability)
   8593615  978512  811008 10383135 ./vmlinux gcc-5 (-fno-guess-branch-probability)

> Does lto (without pgo) perhaps end up undoing a lot of the random
> "branch out and back" noise?

As Honza wrote somewhere else in this thread, gcc uses
-fguess-branch-probability to get a profile estimate, that is then used
throughout the whole optimization chain. So disabling this option
really makes no sense and will result in significant performance loss.

On the other hand the LTO code size savings should not affect
performance negatively at all.

-- 
Markus
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