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

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Message-ID: <20150517053429.GA17002@gmail.com>
Date:	Sun, 17 May 2015 07:34:29 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: Align jump targets to 1 byte boundaries


* Josh Triplett <josh@...htriplett.org> wrote:

> On Mon, Sep 17, 2001 at 07:00:00AM +0000, Ingo Molnar wrote:
> > * Denys Vlasenko <dvlasenk@...hat.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.

It's three separate patches, in case there are any regressions.

> 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.)

First things first - 64-bit is getting far more testing these days 
than 32-bit.

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

So isn't the default on x86 for -falign-labels already 1?

Thanks,

	Ingo
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