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

[Josh Triplett]

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@xxxxxxxxxx> 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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