From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S934809AbbEOPpu (ORCPT <rfc822;w@1wt.eu>);
	Fri, 15 May 2015 11:45:50 -0400
Received: from relay6-d.mail.gandi.net ([217.70.183.198]:36713 "EHLO
	relay6-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S934709AbbEOPps (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Fri, 15 May 2015 11:45:48 -0400
X-Originating-IP: 50.43.43.179
Date: Fri, 15 May 2015 08:45:42 -0700
From: Josh Triplett <josh@joshtriplett.org>
To: Ingo Molnar <mingo@kernel.org>
Cc: linux-kernel@vger.kernel.org
Subject: Re: [PATCH] x86: Align jump targets to 1 byte boundaries
Message-ID: <20150515154540.GA12500@x>
References: <20150514181707.GA21728@gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20150514181707.GA21728@gmail.com>
User-Agent: Mutt/1.5.23 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Mon, Sep 17, 2001 at 07:00:00AM +0000, Ingo Molnar wrote:
> * Denys Vlasenko <dvlasenk@redhat.com> wrote:
> > > What do you guys think about this? I think we should seriously 
> > > consider relaxing our alignment defaults.
> > 
> > Looks like nobody objected. I think it's ok to submit
> > this patch for real.
> 
> Yeah, so my plan is to apply the following three changes from that 
> discussion:
> 
> --- tip.orig/arch/x86/Makefile
> +++ tip/arch/x86/Makefile
>  <at>  <at>  -77,6 +77,15  <at>  <at>  else
>          KBUILD_AFLAGS += -m64
>          KBUILD_CFLAGS += -m64
> 
> +        # Pack jump targets tightly, don't align them to the default 16 bytes:
> +        KBUILD_CFLAGS += -falign-jumps=1
> +
> +        # Pack functions tightly as well:
> +        KBUILD_CFLAGS += -falign-functions=1
> +
> +        # Pack loops tightly as well:
> +        KBUILD_CFLAGS += -falign-loops=1
> +
>          # Don't autogenerate traditional x87 instructions
>          KBUILD_CFLAGS += $(call cc-option,-mno-80387)
>          KBUILD_CFLAGS += $(call cc-option,-mno-fp-ret-in-387)

It looks like the patch you applied to the tip tree only included one of
these (-falign-junmps=1), not the other two.

Also, you've only applied these to 64-bit; could you please apply them
to both 32-bit and 64-bit, since many embedded systems aiming for small
code size use 32-bit?  (Unless 32-bit already defaults to these.)

Have you considered including -falign-labels=1 as well?  Does that make
a difference on top of the other three.

Finally, it looks like -Os already implies all four of those, as well
as a few others, so unfortunately the code size benefits don't actually
apply to the tiniest kernels, which already effectively incorporate this
change.  Oh well.

- Josh Triplett