linux-kernel - Re: [PATCH] x86: Align jump targets to 1 byte boundaries

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Date:	Sat, 11 Apr 2015 15:48:57 +0200
From:	Markus Trippelsdorf <markus@...ppelsdorf.de>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jason Low <jason.low2@...com>,
	Peter Zijlstra <peterz@...radead.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Aswin Chandramouleeswaran <aswin@...com>,
	LKML <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Andy Lutomirski <luto@...capital.net>,
	Brian Gerst <brgerst@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] x86: Align jump targets to 1 byte boundaries

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
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