Re: [PATCH] x86_64/memset: use "small block" code for blocks up to 30 bytes long

[Rich Felker <dalias@libc.org>]

On Tue, Feb 17, 2015 at 05:51:11PM +0100, Denys Vlasenko wrote:
> On Tue, Feb 17, 2015 at 5:12 PM, Rich Felker <dalias@libc.org> wrote:
> > On Tue, Feb 17, 2015 at 02:08:52PM +0100, Denys Vlasenko wrote:
> >> >> Please see attached file.
> >> >
> >> > I tried it and it's ~1 cycle slower for at least sizes 16-30;
> >> > presumably we're seeing the cost of the extra compare/branch at these
> >> > sizes but not at others. What does your timing test show?
> >>
> >> See below.
> >> First column - result of my2.s
> >> Second column - result of vda1.s
> >>
> >> Basically, the "rep stosq" code path got a bit faster, while
> >> small memsets stayed the same.
> >
> > Can you post your test program for me to try out? Here's what I've
> > been using, attached.
> 
> With your program I see similar results:
> 
> ....
> size 50: min=10, avg=10           min=10, avg=10
> size 52: min=10, avg=10           min=10, avg=10

The ... was the part where mine seemed better. :)

Anyway thanks; I'll give your test program a run and see what comes
out. I don't think the difference is going to be big either way, but I
suspect mine is slightly faster for small sizes (~1-30) and slightly
slower for large sizes (>126).

BTW I appreciate your work and interest in improving this. I just
don't like string-ops optimization in general because determining that
changes are actually a net gain for a wide range of cpus and usage
cases and not just for one benchmark turns into a big time sink. :-(
But at least it's fun...

Rich