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

Ingo Molnar
2015-05-17T05:34:41Z

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

> 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.

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