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    SubjectRe: [PATCH] x86: Align jump targets to 1 byte boundaries
    On Fri, Apr 10, 2015 at 07:10:08AM -0700, Paul E. McKenney wrote:
    > On Fri, Apr 10, 2015 at 02:08:46PM +0200, Ingo Molnar wrote:
    > > * Ingo Molnar <mingo@kernel.org> wrote:
    [...]
    > > Btw., totally off topic, the following NOP caught my attention:
    > >
    > > > 5a: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1)
    > >
    > > That's a dead NOP that boats the function a bit, added for the 16 byte
    > > alignment of one of the jump targets.
    > >
    > > I realize that x86 CPU manufacturers recommend 16-byte jump target
    > > alignments (it's in the Intel optimization manual), but the cost of
    > > that is very significant:
    > >
    > > text data bss dec filename
    > > 12566391 1617840 1089536 15273767 vmlinux.align.16-byte
    > > 12224951 1617840 1089536 14932327 vmlinux.align.1-byte
    > >
    > > By using 1 byte jump target alignment (i.e. no alignment at all) we
    > > get an almost 3% reduction in kernel size (!) - and a probably similar
    > > reduction in I$ footprint.
    > >
    > > So I'm wondering, is the 16 byte jump target optimization suggestion
    > > really worth this price? The patch below boots fine and I've not
    > > measured any noticeable slowdown, but I've not tried hard.
    >
    > Good point, adding Josh Triplett on CC. I suspect that he might be
    > interested. ;-)

    Quite interested, yes. Even if there *are* benchmarks to support
    keeping the optimization (which wouldn't surprise me), it'd be nice to
    have a Kconfig option to enable the jump-target optimization. (With 'y'
    meaning "pad jump targets to 16 bytes", so that allnoconfig and
    tinyconfig automatically don't.)

    - Josh Triplett


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