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Date: Sat, 11 Apr 2015 15:48:57 +0200
From: Markus Trippelsdorf <markus@trippelsdorf.de>
To: Denys Vlasenko <dvlasenk@redhat.com>
Cc: 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>,
        "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
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On 2015.04.10 at 14:50 +0200, 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")))
>     ...
> 
>     void hot_func super_often_called_func(...) {...}
> 
> in hot code paths?

__attribute__((optimize)) is meant for compiler debugging only (to help
folks to reduce their testcases to a single function).
It should _not_ be used in production code, because the implementation
is very buggy.

-- 
Markus