Date: Sat, 11 Apr 2015 16:41:35 +0200
From: Markus Trippelsdorf <markus@trippelsdorf.de>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>, Ingo Molnar <mingo@kernel.org>,
        "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: <20150411144135.GB31416@x4>
References: <CA+55aFwZWi6ecDmVsMBQJTrgrW3GD2DaRtpiOspe=5amR1=dNg@mail.gmail.com>
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 <20150410090051.GA28549@gmail.com>
 <20150410091252.GA27630@gmail.com>
 <20150410092152.GA21332@gmail.com>
 <20150410111427.GA30477@gmail.com>
 <20150410112748.GB30477@gmail.com>
 <20150410120846.GA17101@gmail.com>
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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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