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

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Date:	Fri, 10 Apr 2015 21:23:19 +0200
From:	Daniel Borkmann <daniel@...earbox.net>
To:	Denys Vlasenko <dvlasenk@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
CC:	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 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
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