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

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Message-ID: <20150412101422.GA2862@gmail.com>
Date:	Sun, 12 Apr 2015 12:14:22 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Markus Trippelsdorf <markus@...ppelsdorf.de>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	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


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

> On 2015.04.10 at 06:18 -0700, H. Peter Anvin wrote:
> > On 04/10/2015 05:50 AM, Denys Vlasenko wrote:
> > > 
> > > However, I'm an -Os guy. Expect -O2 people to disagree :)
> > > 
> > 
> > The problem with -Os is that the compiler will make *any* tradeoffs to
> > save a byte.  It is really designed to squeeze as much code into a
> > fixed-size chunk, e.g. a ROM, as possible.
> > 
> > We have asked for an -Okernel mode from the gcc folks forever.  It
> > basically would mean "-Os except when really dumb."
> 
> If you want the best of both worlds perhaps you should reconsider Andy's
> LTO patch? With -flto gcc automatically optimizes all functions that it
> considers cold for size. So you could expect some code size savings even
> with -O2 (or -O3).

In my (past) experience the main win from -flto is not due to better 
hot/cold decisions, but simply due to more aggressive dead code 
elimination. -flto has less of an effect on code that is actually 
being executed.

Which isn't to be sneered at, but it's far less of a direct effect as 
branch probabilities are, which cut to the core of most hotpaths in 
the kernel.

Thanks,

	Ingo
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