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Date: Fri, 10 Apr 2015 21:23:19 +0200
From: Daniel Borkmann <daniel@iogearbox.net>
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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>,
        "H. Peter Anvin" <hpa@zytor.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 02:50 PM, Denys Vlasenko wrote:
...
> New-ish versions of gcc allow people to specify optimization
> options per function:
>
> https://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html#Function-Attributes
>
> optimize
>      The optimize attribute is used to specify that a function is to be compiled
>      with different optimization options than specified on the command line.
>      Arguments can either be numbers or strings. Numbers are assumed to be an
>      optimization level. Strings that begin with O are assumed to be an
>      optimization option, while other options are assumed to be used with
>      a -f prefix.
>
> How about not aligning code by default, and using
>
>      #define hot_func __attribute__((optimize("O2","align-functions=16","align-jumps=16")))

I stumbled over that some time ago in a different context. Apparently,
that's being considered broken by gcc folks [1]. ;)

   [1] http://gcc.gnu.org/ml/gcc/2012-07/msg00211.html