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


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