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 <mi...@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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
