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

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Message-ID: <20150411144135.GB31416@x4>
Date:	Sat, 11 Apr 2015 16:41:35 +0200
From:	Markus Trippelsdorf <markus@...ppelsdorf.de>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	"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

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).

-- 
Markus
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