Date: Mon, 13 Apr 2015 18:23:08 +0200
From: Markus Trippelsdorf <markus@trippelsdorf.de>
To: Ingo Molnar <mingo@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>, Denys Vlasenko <dvlasenk@redhat.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Jason Low <jason.low2@hp.com>, Peter Zijlstra <peterz@infradead.org>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Aswin Chandramouleeswaran <aswin@hp.com>,
        LKML <linux-kernel@vger.kernel.org>, Borislav Petkov <bp@alien8.de>,
        Andy Lutomirski <luto@amacapital.net>, Brian Gerst <brgerst@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [PATCH] x86: Align jump targets to 1 byte boundaries
Message-ID: <20150413162308.GB398@x4>
References: <20150410090051.GA28549@gmail.com>
 <20150410091252.GA27630@gmail.com>
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 <20150410111427.GA30477@gmail.com>
 <20150410112748.GB30477@gmail.com>
 <20150410120846.GA17101@gmail.com>
 <5527C700.3030405@redhat.com>
 <5527CD92.1080901@zytor.com>
 <20150411144135.GB31416@x4>
 <20150412101422.GA2862@gmail.com>
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On 2015.04.12 at 12:14 +0200, Ingo Molnar wrote:
> 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.

I did some measurements with gcc-5.1-RC on X86_64 using Andi's latest
LTO kernel patch for 4.0. With my simple monolithic .config the code
size savings are below 1%. That is lower than I've expected.

-- 
Markus
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