Re: [PATCH 1/2] x86_64/memset: simple optimizations

[Rich Felker <dalias@libc.org>]

On Tue, Feb 10, 2015 at 05:36:48PM -0500, Rich Felker wrote:
> On Tue, Feb 10, 2015 at 04:37:56PM -0500, Rich Felker wrote:
> > On Tue, Feb 10, 2015 at 10:08:29PM +0100, Denys Vlasenko wrote:
> > > On Tue, Feb 10, 2015 at 9:50 PM, Rich Felker <dalias@libc.org> wrote:
> > > > On Tue, Feb 10, 2015 at 06:30:56PM +0100, Denys Vlasenko wrote:
> > > >> "and $0xff,%esi" is a six-byte insn (81 e6 ff 00 00 00), can use
> > > >> 4-byte "movzbl %sil,%esi" (40 0f b6 f6) instead.
> > > >> [...]
> > > >
> > > > Do you want to go ahead with these patches as-is, or consider some of
> > > > the other ideas we discussed off-list like avoiding the 64-bit imul
> > > > entirely in the small-n case? If you think that's easy as another
> > > > incremental change I'll go ahead with these
> > > 
> > > I think you can apply these patches without waiting
> > > for potential future improvements.
> > 
> > OK. Based on some casual testing on my Celeron 847:
> > 
> > - For small sizes, your patches make significant improvement, 20-30%.
> > 
> > - For rep stosq path, the improvement is minimal (roughly 1-2 cycles).
> > 
> > - Using 32-bit imul instead of 64-bit makes no difference at all.
> > 
> > I'll review the patches again for correctness, but so far they look
> > good, and it doesn't look like these are things we'd want to back out
> > or rewrite for subsequent improvements anyway.
> > 
> > Thanks!
> 
> One more trivial change I might do: since the non-rep-stosq path is
> faster for small sizes, changing the jb 1f to jbe 1f significantly
> improves 16-byte memsets with no additional code changes.

A few more observations:

- I think we shoul swap the order of the small/rep-stos code paths so
  that the small case doesn't branch. In practice this seems to save a
  few cycles and it makes sense that it should.

- The small path strategy seems to be optimal for sizes much larger
  than 16. We could easily extend it up to 32 with only one more
  branch step, but should probably extend it up to 64 or so.

- At very small sizes, memset.c beats the asm. It seems to be a result
  of deferring the movbzl/imul until larger-than-byte stores at
  needed. But once the size gets large performance is asymtotically
  half the speed of the asm (rep stosq).

These can all of course be done as separate patches later.

Also, similar changes should be considered for the i386 asm, and .sub
files should be added for x32 to use the x86_64 asm.

Rich