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Date: Fri, 10 Apr 2015 06:18:10 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Denys Vlasenko <dvlasenk@redhat.com>, Ingo Molnar <mingo@kernel.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
CC: 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
References: <CA+55aFz6KKxGVxPAbsmw9GsKJfy85P2C0EmYBrGpn+aJDjZJWw@mail.gmail.com> <20150409175652.GI6464@linux.vnet.ibm.com> <CA+55aFzXMDjQQ7jTjsPdh1RikXfgV7OCd-+13cz06MOmDBA33w@mail.gmail.com> <CA+55aFwZWi6ecDmVsMBQJTrgrW3GD2DaRtpiOspe=5amR1=dNg@mail.gmail.com> <20150409183926.GM6464@linux.vnet.ibm.com> <20150410090051.GA28549@gmail.com> <20150410091252.GA27630@gmail.com> <20150410092152.GA21332@gmail.com> <20150410111427.GA30477@gmail.com> <20150410112748.GB30477@gmail.com> <20150410120846.GA17101@gmail.com> <5527C700.3030405@redhat.com>
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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."

As far as the 16-byte alignment, my understanding is not that it is
related to the I$ but rather is the decoder datum.

	-hpa